Somatosensory - senses.docx

8/21/2019 Somatosensory - senses.docx

1/56

Chapter 2: Somatosensory Systems

Patrick Dougherty, Ph.D., Department of Anesthesiology and Pain Medicine, MD AndersonCancer Center(content proided !y Chieyeko "suchitani, Ph.D.#

The somatosensory systems inform us about objects in our external environmentthrough touch (i.e., physical contact with skin) and about the position andmovement of our body parts (proprioception) through the stimulation of muscle and joints. The somatosensory systems also monitor the temperature of the body,external objects and environment, and provide information about painful, itchy andtickling stimuli. The sensory information processed by the somatosensory systemstravels along different anatomical pathways depending on the information carried.

For example, the posterior columnmedial lemniscal pathway carries discriminativetouch and proprioceptive information from the body, and the main sensorytrigeminal pathway carries this information from the face. !hereas, thespinothalamic pathways carry crude touch, pain and temperature information fromthe body, and the spinal trigeminal pathway carries this information from the face.

This first series of chapters on somatosensory systems concentrates on thesomatosensory systems that provide accurate information about the location andtemporal features of stimuli and about sharp pain, tactile stimuli and the positionand movement of body parts. This chapter describes somatosensory stimuli, thesensations produced when they are applied, and the cutaneous, muscle, and joint

receptors that are responsible for initiating the perceived somatic sensations."ubse#uent chapters describe the pathways processing other pain, temperature,crude touch and visceral sensations.

2.1 Somatic Stimuli

Modality Specificity in the Somatosensory System. The somatosensorysystems process information about, and represent, several modalities of somaticsensation (i.e., pain, temperature, touch, proprioception). Each of these modalitiescan be divided into sub-modalities, as shown in Table 1 (e.g., pain into sharp,pricking, cutting pain dull, burning pain and deep aching pain). Discriminative

touch is also subdivided into touch, pressure, flutter and vibration. Each of thesesensations (i.e., sub-modalities) is represented b! neurons that e"hibit modalityspecificity. That is, when a somatosensor! neuron is stimulated naturall! (e.g., b!skin warming) or artificiall! (e.g., b! electrical stimulation of the neuron), the

sensation perceived is specific to the information normall! processed b! the neuron(i.e., warm skin). #onse$uentl!, a %warm% somatosensor! neuron will not respond tocooling of the skin or to a touch stimulus that does not %warm% the skin. Thesomatosensor! receptor and its central connections determine the modalit!specificit! of the neurons forming a somatosensor! pathwa!.

Table I

http://faculty.mdanderson.org/Patrick_Dougherty/Default.asp?SNID=948746342http://www.mdanderson.org/education-and-research/departments-programs-and-labs/departments-and-divisions/pain-medicine/index.htmlhttp://www.mdanderson.org/http://www.mdanderson.org/http://www.mdanderson.org/education-and-research/departments-programs-and-labs/departments-and-divisions/pain-medicine/index.htmlhttp://www.mdanderson.org/http://www.mdanderson.org/http://faculty.mdanderson.org/Patrick_Dougherty/Default.asp?SNID=948746342


2/56

The Sensory Modalities Represented by the Somatosensory S

Modality Sub Modality Sub-Sub Modality Somatosensory Pathway (Body)

Painsharp cutting pain Neospinothalamicdull burning pain Paleospinothalamic

deep aching pain Archispinothalamic

Temperaturewarm/hot Paleospinothalamic

cool/cold Neospinothalamic

Touch

itch/tickle & crude touch Paleospinothalamic

discriminative touch

touch

Medial Lemniscal

pressure

flutter

vibration

Proprioception

Position: Static orces

muscle length

muscle tension

!oint pressure

Movement: "#namic orces

muscle length

muscle tension

!oint pressure

!oint angle

Tactile Stimuli. Tactile stimuli are e"ternal forces in ph!sical contact with the skinthat give rise to the sensations of touch, pressure, flutter, or vibration. &e normall!think of touch as involving minimal force on-or-b! an ob'ect that produces ver! littledistortion of the skin. n contrast, pressure involves a greater force that displacesthe skin and underl!ing tissue. Time var!ing tactile stimuli produce more comple"

sensations such as ob'ect movement or ob'ect flutter (2 to * +) or vibration (1to +). n initial clinical e"amination of discriminative touch often involvestesting the vibrator! sense b! appl!ing a 12/ + tuning fork over a bon!prominence.

Proprioceptive Stimuli.1 Proprioceptive stimuli are internal forces that aregenerated b! the position or movement of a bod! part. 0tatic forces on the 'oints,muscles and tendons, which maintain limb position against the force of gravit!,indicate the position of a limb. The movement of a limb is indicated b! d!namicchanges in the forces applied to muscles, tendons and 'oints. n initial clinicale"amination of proprioception often involves testing the position sense b! having thepatient, with e!es closed, touch one finger with another after the target finger hasbeen moved.

Proprioception is critical for maintaining posture andbalance. 0omatosensor! proprioceptive cues are combined with vestibularproprioceptive cues and visual cues to control motor responses to changes inbod!head position. uring a clinical e"amination, the 3omberg test re$uires the

patient to maintain balance while standing with feet together and e!es closed. t

http://neuroscience.uth.tmc.edu/s2/chapter02.htmlhttp://neuroscience.uth.tmc.edu/s2/chapter02.html


3/56

tests whether the proprioceptive components are working properl! when the visualcues are missing and proprioceptive cues are the ma'or sources of information.

Sharp Cutting Pain Stimuli. 4ainful (nociceptive) stimuli are tissue-damagingsources of energ! that ma! be e"ternal or internal to the bod! surface. 0harp,

cutting pain is the sensation elicited on initial contact with the painful stimulus. Thesensation of dull, burning pain ma! follow as a conse$uence of tissue inflammation.n initial clinical e"amination of the pain sense often involves testing sharp, cuttingpain sensitivit! b! asking the patient, who has herhis e!es closed, what the! feelwhen pricked with a pin. 4ain mechanisms and pathwa!s are described in detail inlater chapters.

2.2 Introduction to Peripheral Organization ofSomatosensory Systems

Peripheral Somatosensory Neurons. The cell bodies of the first-order (15)

somatosensor! afferent neurons2

are located in posterior root or cranial root ganglia(i.e., are part of the peripheral nervous s!stem, 6igure 2.1). The 15 afferents arepseudounipolar cells. The cell bod! gives rise to a single process that divides to forma peripheral a"on and a central a"on. The peripheral a"on travels to and ends in theskin, muscle, tendon or 'oint and the central a"on travels to and ends in the centralnervous s!stem.

Somatosensory Receptor Organ. The receptors of most sensor! s!stems arelocated in specialied sensor! receptor organs (e.g., the photoreceptors in the e!eand the auditor! and vestibular hair cells in the inner ear) or within a restricted partof the bod! (e.g., the taste buds in the mouth and the olfactor! receptors in theolfactor! mucosa of the nose). 6or the tactile component of the somatosensor!s!stem, the skin covering the entire bod!, head and face functions as the touchreceptor organ, whereas 'oint tissues, muscles and tendons act as the proprioceptionreceptor organs. These sensor! receptor organs %house% the somatosensor!receptors and deliver the somatosensor! stimuli to the receptors.

Sensory Receptors. 0pecialied sensor! receptor cells (e.g., the photoreceptorsof the e!e) are located in specialied receptor organs, produce receptor potentials,contain s!naptic specialiations, and release neural transmitters (6igure 2.2).0pecialied sensor! receptors ma! be modified neurons (e.g., the photoreceptorsand olfactor! receptors) or modified epithelial cells (e.g., taste receptors and theauditor! and vestibular hair cells).



4/56

Figure 2.1The somatosensory frst-order (1°) aerent is a

pseudounipolar neuron, which has a single process thatdiides into a peripheral process and a central process. Theperipheral process is part o! the peripheral nerous system("#$) and terminates to !orm or end on a somatosensory

receptor in s%in, muscle or &oint. The central process traelsso the central nerous system ('#$) where it terminates on

a spinal cord or rain stem neuron.

Figure 2.2

The specialied sensory receptors o! the auditory and isual systems.

These cells are specialied neurons (*.isual receptors) or specialied

epithelial cells (+. auditory receptors)that generate receptor potentials and

contain synaptic esicles.

There is onl! one t!pe of sensor! receptor cell in the somatosensor! s!stem,the Merkel cells, and the! are found onl! in skin. The vast ma'orit! ofsomatosensor! receptors are not specialied receptor cells. That is, the! are formedb! the endings of the somatosensor! 15 afferent peripheral a"on and ad'acent tissue(6igure 2.). There is no s!naptic specialiation or neurotransmitter within thead'acent tissue. The ad'acent tissue also does not generate receptor potentials.


5/56

Figure 2.(*) hen stimulated, the auditoryreceptor cell generates a receptorpotential (1), which results in therelease o! neurotransmitter at its

synapse with the auditory 1°aerent. The neurotransmitter

depolaries the 1° aerent, whichgenerates action potentials (2 )

that trael to the 1° aerent synapticterminals on 2° aerents in thecentral nerous system. The 2°

aerent generates action potentials(/) in response to the transmitter

release y the 1° aerent.

(+) 0ost somatosensory receptorsare not specialied receptor cells andare !ormed y the terminal endings

o! the somatosensory 1° aerents. tis the 1° aerent terminal that

produces a generator potential (1)which, in turn, initiates action

potentials (2 ) in the 1° aerentaon. The 1° aerent releases

neurotransmitter on 2° aerents inthe central nerous system. The 2°aerent generates action potentials(/) in response to the transmitter y

the 1° aerent.

nstead of ending on specialied receptors, most peripheral a"ons of somatosensor!15 afferents travel to skin, muscle or 'oint, branch near their terminal sites, and endin the skin (6igure 2.7), muscle, tendon or 'oint tissue.


6/56

Figure 2./The primary

(1°)somatosensory aerentneuron. The1° aerent3scell ody islocated in

the gangliono! a cranialor posterior

(spinal)nere root.

The 1°aerent3speripheral

processtraels to

s%in, muscleor &oint -where itranches

intoterminal

fers. 4achterminal

fer !orms,or ends on,

asomatosensory receptor.

The 1°aerent3s


7/56

centralprocess &oins a

cranial orspinal nere

and entersthe rainstem or

spinal cord -where itsynapseswith a 2°

somatosensory neuron.

ll the peripheral terminal branches of a 15 somatosensor! a"on end in a specific

t!pe of tissue (e.g., skin) and not in multiple t!pes of tissue (i.e., not in skin andmuscle). ll the peripheral terminal branches of a 15 a"on form onl! one t!pe ofsomatosensor! receptor.


8/56

Figure 2.5The

locations o! somatosens

ory

receptors inthe ody.

8an! of the 15 somatosensor! afferent terminals are enveloped in a connectivetissue capsule along with surrounding muscle, tendon or cutaneous cells, or end onhair follicles. The hair follicles and the encapsulated tissue ad'acent to the 15afferent terminals (i.e., skin, muscle, tendon, and 'oint tissues) contain no s!napticspecialiations and do not generate receptor potentials or release neuraltransmitters. The comple" of encapsulated tissue and afferent endings and thecomple" of hair follicle and afferent endings pla! a role in the receptor transduction

process, and each comple" is considered to form a %somatosensor! receptor%. 8an!other 15 somatosensor! a"ons branch and terminate in skin, muscle, or 'oint as freenerve endings. These endings are bare of m!elin, are not encapsulated and are notassociated with a specific t!pe of tissue.

The sensitivit! of the receptors to specific stimuli (e.g., touch verses muscle stretch)is determined b! the location of the receptor and b! the non-neural tissuesurrounding the 15 afferent terminal (6igure 2.9).

Figure 2.6The

locations o! cutaneous

(somatosensory)

receptors inhairy andnon-hairy(glarous)

s%in.


9/56

2. Sensory Transduction

The de!uate Stimulus. The ade$uate somatosensor! stimulus (i.e., thestimulus to which a somatosensor! neuron is most sensitive) is either a mechanicalforce, a temperature change, tissue damage, or a chemical action. The

discriminative touch and proprioceptive s!stems are most sensitive to mechanicalforce. #onse$uentl!, their sensor! receptors are of the mechanoreceptor categor!.

Sensory Transduction. The non-neural tissue surrounding the peripheral endingof the somatosensor! 15 afferent helps concentrate and deliver the stimulus (e.g.,mechanical force) onto the 15 afferent terminal membrane.0omatosensor!mechanoreceptors function to transduce the applied mechanicalforce into an electrical potential change in the 15 afferent neuron.

The mechanoreceptor 15 afferent terminal membrane contains ion channels thatrespond to mechanical distortion b! increasing sodium and potassium conductance

(i.e., the channels are stress gated). :enerator potentials are produced as sodiumand potassium flow down their electrochemical gradients to depolarie the terminalending (see 6igure 2.;). n most cases, the magnitude and duration of thegenerator potentials are related to the applied mechanical force< the greater themechanical force, the greater is the depolariation, and the longer the mechanicalforce is applied, the longer the terminal remains depolaried (6igure 2.=). Terminalsthat do not sustain the depolariation for the duration of the mechanical distortionare called rapidl! adapting. Terminals that sustain the depolariation with minimaldecrease in amplitude for the duration of a stimulus are called slowl! adapting.

Figure 2.7*t the T8"

o! thisfgure, two

1°somatosens

http://neuroscience.uth.tmc.edu/s2/chapter02.html#fig2_3http://neuroscience.uth.tmc.edu/s2/chapter02.html#fig2_3


10/56

ory neuronsare

illustrated.*

mechanical

!orce (*) isapplied and

theresponses

aremeasured ya recordingelectrode in

thesomatosensory receptor

(+), and arecording

electrode inthe aon (').+498 The

responses o! somatosens

ory 1°aerent

neurons tostimulation

o! thereceptorwith a

sustainedstimulus areillustrated!or rapidlyadaptingaerents

(94FT panel)and slowlyadaptingaerents(:;


11/56

aon (') areillustrated.#otice thatthe :u>nicorpuscle

and 0er%eldis% andtheir 1°aerent

responsesare estsuited totransduce

and transmitin!ormationaout long-

lasting(maintained

orsustained)stimuli thatdo not aryoer time.

The generator potential spreads passivel! along the 15 terminal fiber to the a"ontrigger one - that part of the 15 afferent a"on containing voltage-sensitive sodiumand potassium channels (see 6igure 2.;). f the depolariation reaches threshold atthese voltage-sensitive sites, action potentials are generated b! the 15 afferent

peripheral a"on. &hen the action potentials reach the central terminals of the 15afferent, the! initiate the release neurotransmitters on 25 afferents within spinalcord or brain stem nuclei. f, as in the e"ample in 6igure 2./, the generator potentialis slowl! adapting, the 15 afferent produces a sustained discharge of actionpotentials that continue for the duration of the stimulus.

Figure 2.?$tretching the :u>ni corpuscle

produces a slowly adapting(sustained) generator potential in

the 1° aerent terminal that

degrades slowly !or the duration o! the stretch. ! the !orce applied tothe 1° aerent terminal produces a

generator potential that is o! su>cient amplitude at the aontrigger one, a train o! action

potentials is generated that traelalong the aon to the terminals o! the its central process. The actionpotentials in the central terminals

initiate the release o! neurotransmitters on 2°

somatosensory aerent neurons

within the central nerous system,which results in a discharge o! the 2°



12/56

aerent.

f the generator potential is rapidl! adapting (6igure 2.>), the 15 afferent produces a

transient, short burst of action potentials and falls silent even in the continuedpresence of the stimulus.

Figure 2.@+ending a hair produces

a rapidly adaptingdischarge o! actionpotentials in the 1°

aerent aon that doesnot last the duration o! the ending !orce. ! the

!orce applied to the 1°aerent terminalproduces a generator

potential that is o! su>cient amplitude atthe aon trigger one,

one or more actionpotentials are generated

that trael to theterminals o! the 1°

aerent central process.The action potentials in

the central terminals

initiate the release o! neurotransmitters on 2°somatosensory aerent

neurons within thecentral nerous system.

The 1° aerent aonresponse is rapidlyadapting and actionpotentials are only

generated when the hairis ent.

The rapidl! adapting receptors produce generator potentials and action potentialdischarges that follow the time-var!ing waveform of pressure changes produced b!a vibrating stimulus (6igure 2.1, left panel). n contrast, the slowing adaptingreceptors produce generator potentials and action potential discharges that aresustained and unable to mimic the time-var!ing pattern of the stimulus (6igure 2.1,right panel). #onse$uentl!, the responses of rapidl! adapting 15 afferents are bestsuited for representing time var!ing (e.g., vibrating or moving) stimuli, whereasslowl! adapting 15 afferents better represent static stimuli (e.g., sustainedpressure).


13/56

Figure 2.1A*t the T8" o! this fgure, two

1° somatosensory neurons areillustrated= each in contact

with a mechanical !orce (*), a

recording electrode in thesomatosensory receptor (+),and a recording electrode in

the aon ('). +498 Theresponses o! the

somatosensory 1° aerents tostimulation o! the receptor

with a irating stimulus areillustrated !or rapidly adapting

aerents (94FT panel) andslowly adapting aerents

(:;


14/56

The locations o! cutaneous receptors. 'lic% on the somatosensoryreceptor name (in green shaded area) to iew a detailed drawing o! the receptor. The location o! the receptor will e circled in the larger

drawing o! the s%in.

Cutaneous Receptors

0ome of the somatosensory receptors in skin (i.e., the cutaneous receptors) areclassified as encapsulated receptors as the 15 afferent terminal and surroundingcutaneous tissue are encapsulated b! a thin sheath (Table ). The encapsulatedcutaneous receptors include 8eissner corpuscles, 4acinian corpuscles and 3uffinicorpuscles (0ee 6igure 2.11). ?ther cutaneous receptors are unencapsulated andinclude the hair follicle receptor (the 15 afferent ends on hair follicles) and the8erkel comple" (the 15 afferent ends at the base of a specialied receptor cell calledthe 8erkel cell). The sensor! receptors of the crude touch, pain and temperature

senses are bare or free nerve endings. That is, the! are unencapsulated, do not endon or near specialied tissue, and ma! be mechanoreceptors, nociceptors orthermoreceptors.

s was noted earlier, the sensitivit! (modalit! specificit!) of the somatosensor!receptor is determined b! its location and b! the structure of the non-neural tissuesurrounding the 15 afferent terminal. The following describes the most commonl!observed cutaneous receptors.

Meissner Corpuscle. The Meissner corpuscle is found in glabrous (i.e.,hairless) skin, within the dermal papillae (6igure 2.11). t consists of an elongated,

encapsulated stack of flattened epithelial (laminar) cells with 15 afferent terminalfibers interdigitated between the cells (6igure 2.12).

Figure 2.12The 0eissner

corpuscle consistso! an encapsulatedstac% o! Battened

epithelial (laminar)cells with 1°

aerent terminals

interdigitatedetween thesecells. The 0eissnercorpuscle is locatedwithin the dermalpapilla, near the

sur!ace o! the s%in,with its long aisperpendicular tothe s%in sur!ace.

force applied to non-hair! skin (6igure 2.1) causes the laminar cells in the8eissner corpuscle to slide past one another, which distorts the membranes of thea"on terminals located between these cells. f the force is maintained, the laminar

http://neuroscience.uth.tmc.edu/s2/chapter02.html#table2http://neuroscience.uth.tmc.edu/s2/chapter02.html#table2


15/56

cells remain in a fi"ed, albeit, displaced position, and the shearing force on the a"onterminals@ membranes disappears. #onse$uentl!, the 15 afferent a"ons produce atransient, rapidl! adapting response to a sustained mechanical stimulus.

Figure 2.1hen a !orce is applied to thedermal papilla containing the

0eissner corpuscle, the laminars in the corpuscle slide past onether. This shearing !orce distortsthe memranes o! the aon

erminals located etween theminar cells, which depolaries the

aon terminals. ! the !orce istained on the dermal papilla, thelaminar cells remain in their

splaced positions and no longer

roduce a shearing !orce on theon terminals. 'onseCuently, asustained !orce on the dermalpapilla is trans!ormed into atransient !orce on the aon

minals o! the 0eissner corpuscle.e 1° aerent aon response o! a0eissner corpuscle is rapidlyapting and action potentials arenly generated when the !orce is

frst applied.

The 8eissner 15 afferent discharges %follow% low fre$uenc! vibrating ( -* +)stimuli, which produces the sensation of %flutter% (6igure 2.1, left panel). ;ecause asingle 15 afferent a"on forms man!, dispersed (-7 mm) 8eissner corpuscles, the 15afferent can detect and signal small movements across the skin. 0timulation of ase$uence of 8eissner corpuscles have been described to produce the perception oflocalied movement along the skin.

$onse#uently, %eissner corpuscles are considered to be the discriminative touchsystem&s flutter and movement detecting receptors in nonhairy skin.

Pacinian Corpuscle. Pacinian corpuscles are found in subcutaneous tissuebeneath the dermis (6igure 2.>) and in the connectivetissues of bone, the bod! walland bod! cavit!. Therefore, the! can be cutaneous, proprioceptive or visceralreceptors, depending on their location.

Figure 2.1/The "acinian corpuscle

consists o! a single,centrally placed 1° aerentterminal that is surrounded

y concentrically layeredepithelial (laminar) cells

that are all encapsulatedwithin a sheath. n s%in, the

http://neuroscience.uth.tmc.edu/s2/chapter02.html#fig2_10http://neuroscience.uth.tmc.edu/s2/chapter02.html#fig2_9http://neuroscience.uth.tmc.edu/s2/chapter02.html#fig2_10http://neuroscience.uth.tmc.edu/s2/chapter02.html#fig2_9


16/56

"acinian corpuscle is locateddeep in the sucutaneous

adipose tissue.

The 4acinian corpuscle is football-shaped, encapsulated, and contains concentricall!la!ered epithelial (laminar) cells (6igure 2.17). n cross section, the 4aciniancorpuscle looks like a slice of onion, with a single 15 afferent terminal fiber located inits center. The outer la!ers of laminar cells contain fluid that is displaced when aforce is applied on the corpuscle.

&hen a force is first applied on the 4acinian corpuscle (6igure 2.1*), it initiall!displaces the laminar cells and distorts the a"on terminal membrane. f the e"ternalpressure is maintained on the corpuscle, the displacement of fluid in the outerlaminar cells dissipates the applied force on the a"on terminal. #onse$uentl!, asustained force on the corpuscle is transformed into a transient force on the a"on

terminal, and the 4acinian corpuscle 15 afferent produces a fast adapting response.

Figure 2.15hen a !orce is appliedto the tissue oerlyingthe "acinian corpuscle(press "9*D), its outer

laminar cells, whichcontain Buid, are

displaced and distortthe aon terminal

memrane. ! thepressure is sustainedon the corpuscle, the

Buid is displaced,which dissipates theapplied !orce on the

aon terminal.'onseCuently, a

sustained !orce on the"acinian corpuscle istrans!ormed into a

transient !orce on itsaon terminal. The

"acinian corpuscle 1°aerent aon responseis rapidly adapting andaction potentials areonly generated when

the !orce is frstapplied.

4acinian corpuscles 15 afferent a"ons are most sensitive to vibrating stimuli (e.g., atuning fork vibrating at 1 to +, 6igure 2.1, left) and unresponsive to stead!pressure. The sensation elicited when cutaneous 4acinian corpuscles are stimulated

is of vibration or tickle.


17/56

'acinian corpuscles in skin are considered to be the vibration sensitive receptors ofthe discriminative touch system.

Ruffini Corpuscle. The Ruffini corpuscles are found deep in the skin (6igure2.11), as well as in 'oint ligaments and 'oint capsules and can function as cutaneous

or proprioceptive receptors depending on their location. The 3uffini corpuscle (6igure2.19) is cigar-shaped, encapsulated, and contains longitudinal strands of collagenousfibers that are continuous with the connective tissue of the skin or 'oint. &ithin thecapsule, the 15 afferent fiber branches repeatedl! and its branches are intertwinedwith the encapsulated collagenous fibers.

Figure 2.16The :u>ni corpuscleconsists o! 1° aerent

terminal fers that areintertwined with

collagenous fers andtogether with the

collagenous fers areencapsulated in a

frous sheath. The:u>ni corpuscles are

oriented parallel to thes%in sur!ace and

situated deep within thedermis.

The 3uffini corpuscles are oriented with their long a"es parallel to the surface of theskin and are most sensitive to skin stretch. 0tretching the skin (6igure 2.1=)stretches the collagen fibers within the 3uffini corpuscle, which compresses the a"onterminals. s the collagen fibers remain stretched and the a"on terminals remaincompressed during the skin stretch, the 3uffini corpuscle@s 15 afferent a"onproduces a sustained slowl! adapting discharge to maintained stimuli.

Figure 2.17hen the s%in is

stretched, the collagenfers in the :u>ni

corpuscles are alsostretched and

compress their 1°aerent terminals. *s

the collagen fersremain stretched and

the aon terminalsremain compressed

during the s%instretch, the :u>ni

corpuscle 1° aerentaon produces a

sustained generator

potential and a slowlyadapting discharge to


18/56

maintained stimuli.

uffini corpuscles in skin are considered to be skin stretch sensitive receptors of the

discriminative touch system. They also work with the proprioceptors in joints andmuscles to indicate the position and movement of body parts.

"air #ollicle. The hair follicle receptor is an unencapsulated cutaneous receptor(6igure 2.1). The 15 afferent terminal a"ons spiral around the hair follicle base orrun parallel to the hair shaft forming a lattice-like pattern (6igure 2.1/).

Figure 2.1?

The hair !ollicle 1°aerent terminalfers enter the

!ollicle to encircleor to !orm a

lattice patternaround the hair

sha!t.

8ost hair follicle 15 afferents are the fast-adapting t!pe displacement of the hairproduces a transient discharge of action potentials at the onset of the displacementand a maintained displacement of the hair often fails to produce a sustaineddischarge (6igure 2.1>). The hair follicle afferents respond best to moving ob'ectsand signal the direction and velocit! of the movement of a stimulus brushing against

hair! skin.

Figure 2.1@+ending a hair producesa transient !orce on the

hair !ollicle ase as theentire !ollicle isdisplaced y the

ending !orce. The 1°aerent terminal may

produce a rapidlyadapting generatorpotential and the 1°

aerent aon atransient discharge o!

action potentials E eento sustained ending o!

the hair.



19/56

s %eissner corpuscles are absent from hairy skin, the hair follicle endings are

considered to be the discriminative touch system&s movement sensitive receptors inhairy skin.

Mer$el Comple%. The Merkel complex is found in both hair! and non-hair! skinand is located in the basal la!er of the epidermis (6igure 2.11). The 8erkel comple"is unencapsulated and consists of a specialied receptor cell, the 8erkel cell, and a15 afferent terminal ending, the 8erkel disk (6igure 2.2). Thick, short, finger-likeprotrusions of the 8erkel cell couple it tightl! to the surrounding tissue. The 8erkelcell is a modified epithelial cell, which contains s!naptic vesicles that appear torelease neuropeptides that modulate the activit! of the 15 afferent terminal. Each 15afferent a"on often innervates onl! a few 8erkel cells in a discrete patch of skin(6igure 2.1/).

Figure 2.2AThe 0er%el comple

consists o! aspecialied 0er%el cell,

which containssynaptic esicles, andthe 0er%el dis% ending

o! a 1° aerentterminal fer. * single1° aerent aon o!ten

innerates only a !ew0er%el cells within a

discrete patch o! s%in.

force applied to the skin overl!ing the 8erkel cell distorts it (6igure 2.21), whichstimulates its release of a neuropeptide at its s!naptic 'unctions with the 8erkeldisk. s the 8erkel cell is mechanicall! coupled to the surrounding skin, it remainsdistorted for the duration of the force applied on the overl!ing skin. #onse$uentl!,the 8erkel comple" 15 afferent a"on responds to small forces applied to a discretepatch of skin with a slowl! adapting, sustained discharge.



20/56

Figure 2.21The 0er%el cell is

coupled to thesurrounding tissueand cannot shi!t its

position relatie tothe surrounding

tissue.'onseCuently, a

!orce applied to theoerlying s%in(press"9*D),

distorts the 0er%elcell !or the durationo! the applied !orce.

The distortion o! the 0er%el cellresults in the

release o! a streamo! neuropeptides at

its synaptic &unctions with the1° 0er%el dis%. *s a

result the actionpotential

dischargesproduced y the

0er%el comple 1°aerent is slowly

adapting.

%erkel cells are considered to be the fine tactile receptors of the discriminativetouch system that provide cues used to locali*e tactile stimuli and to perceive theedges (shape or form) of objects.

#ree Nerve &ndings. ree nerve endings are found throughout the bod!, inskin (6igure 2.11), muscles, tendons, 'oints, mucous membranes, cornea, bod!mesenter!, the dura, the viscera, etc. The free nerve endings in skin are stimulatedb! tissue-damaging (nociceptive) stimuli that produce the sensation of pain or b!cooling of the skin or the warming of skin or b! touch. Aotice that although all

cutaneous free nerve endings appear ver! similar morphologicall!, there aredifferent functional t!pes of free nerve endings, with each responding to specifict!pes of cutaneous stimuli (e.g., nociceptive, cooling, warming or touch).

Free nerve endings are considered to be the somatosensory receptors for pain,temperature and crude touch.

Table II Cutaneous Receptors

Receptor Type Sensation Signals

Meissner

corpuscle

$ncapsulated

& la#ered %ouch: lutter & Movement reuenc#/'elocit# & "irectio


21/56

Pacinian

corpuscle

$ncapsulated

& la#ered%ouch: 'ibration reuenc#: ())*+)) ,-

Ruffini

corpuscle

$ncapsulated

collagen%ouch: Skin Stretch "irection & orce

Hair follicle .nencapsulated %ouch: Movement "irection &'elocit#

Merel

comple!

Speciali-ed

epithelial cell%ouch Pressure orm Location & Magnitude

"ree #er$e

%nding.nencapsulated Pain %ouch or %emperature

%issue damage 0ontact or %emper

change

2.* Proprioceptive Receptors

4roprioceptors are located in muscles, tendons, 'oint ligaments and in 'oint capsules.

There are no specialied sensor! receptor cells for bod! proprioception

7

. n skeletal(striated) muscle, there are two t!pes of encapsulated proprioceptors, musclespindles and :olgi tendon organs (6igure 2.22), as well as numerous free nerveendings. &ithin the 'oints, there are encapsulated endings similar to those in skin,as well as numerous free nerve endings.

Figure 2.22* muscle spindle

receptor and ;olgitendon organ in

the icep muscle.

Muscle Spindles. Muscle spindles are found in nearl! all striated muscles. muscle spindle is encapsulated and consists of small muscle fibers, called intrafusalmuscle fibers, and afferent and efferent nerve terminals (6igure 2.2).



22/56

Figure 2.2* muscle spindle withits sensory and motor

inneration. Theprimary muscle spindle

aerent terminates asannulospiral endings inthe central area o! the

intra!usal muscleswhereas the secondarymuscle spindle aerent

terminates as Bowerspray endings in more

polar regions o! intra!usal muscles. The

motor endplates o! gamma motor neuronsare located in the polar

regions. The musclespindle is attached to

the surroundingetrastriate musclesand lays with its long

ais in parallel with thelong aes o! the

surrounding muscle.

ntrafusal muscles are found e"clusivel! in muscle spindle receptors and aredistributed throughout the bod! among the ordinar! e"trafusal muscle fibers ofskeletal muscles. The intrafusal fibers are attached to the larger, surroundinge"trafusal muscle fibers. The! are oriented in parallel with the e"trafusal fibers butdo not contribute directl! to muscle strength when the! contract because of theirsmall sie.

There are two t!pes of afferent terminals in the muscle spindle (6igure 2.2). Theannulospiral endings wrap around the central region of the intrafusal fibers, whereasthe flower-spra! endings terminate predominantl! in more polar regions (awa! fromthe central area) of the intrafusal fibers. The 15 afferents forming the annulospiralendings are called the primar! muscle spindle afferents, whereas those forming theflower-spra! endings are called the secondar! muscle spindle afferents.

n addition to afferent terminals, the terminals (motor endplates) of gamma motorneurons end on intrafusal muscle fibers. The! will be described in detail in thechapters covering motor s!stems.

n summar!, the muscle spindles are proprioceptors specialied to monitor musclelength (stretch) and signal the rate of change in muscle length b! changing thedischarge rate of afferent action potentials. 8uscle spindles are most numerous inmuscles that carr! out fine movements, such as the e"traocular muscles and theintrinsic muscles of the hand. There are fewer spindles in large muscles that controlgross movements of the bod! (e.g., the muscles of the back).


23/56

Figure 2.2/The ;olgi

tendon organ islocated at the &unction o!

muscle andtendon. The;olgi tendon

organsresemle the

:u>nicorpuscles.

That is, the 1°aerent

terminal fersare intertwined

withcollagenousfers o! the

tendon and theentire organ isencapsulatedin a frous

sheath.

'olgi Tendon Organs. !olgi tendon organs are found in the tendons ofstriated e"trafusal muscles near the muscle-tendon 'unction (6igure 2.22). :olgitendon organs resemble 3uffini corpuscles. 6or e"ample, the! are encapsulated and

contain intertwining collagen bundles, which are continuous with the muscle tendon,and fine branches of afferent fibers that weave between the collagen bundles (6igure2.27). The! are functionall! %in series% with striated muscle.

The :olgi tendon organ collagen fibers are continuous with the e"trafusal muscle atone end and with the muscle tendon at its opposite end. #onse$uentl!, themechanical force on the organ is ma"imal when the e"trafusal muscles contract,shorten, and increase the tension on the tendon. &hen the muscles contract, the 15afferent terminals are compressed and remain compressed as long as the muscleremains contracted. The :olgi tendon organ 15 afferent response to sustainedisometric muscle contraction is slowl! adapting, and the 15 afferent generates actionpotentials as long as the tension is maintained. The responses of the :olgi tendon

organ 15 afferent a"on is ma"imal when the contracted muscle bears a load, e.g.,when lifting a heav! ob'ect.

The 'olgi tendon organ is a proprioceptor that monitors and signals musclecontraction against a force (muscle tension), whereas the muscle spindle is a proprioceptor that monitors and signals muscle stretch (muscle length).

(oint Receptors. Boint receptors are found within the connective tissue, capsuleand ligaments of 'oints (6igure 2.2*). The encapsulated endings resemble the 3uffiniand 4acinian corpuscles and the :olgi tendon organs.


24/56

Figure 2.25The &oint receptors

are !ree nereendings andencapsulated

endings in the &ointcapsule and &ointligaments. Theencapsulated

receptors in the &oint capsule

resemle "acinianand :u>ni endingswhereas those in

the ligamentsresemle ;olgitendon organs.

The 'oint 15 afferents respond to changes in the angle, direction, and velocit! ofmovement in a 'oint. The responses are predominantl! rapidl! adapting with few 'oint 15 afferents signaling the resting (static) position of the 'oint. t has beensuggested that information from muscles, tendons, skin and 'oints are combined toprovide estimates of 'oint position and movement. 6or e"ample, when the hip 'oint isreplaced C removing all 'oint receptors C the abilit! to detect the position of thethigh relative to the pelvis is not lost.

#ree Nerve &ndings. s mentioned above, free nerve endings of 15 afferents areabundant in muscles, tendons, 'oints, and ligaments. These free nerve endings areconsidered to be the somatosensor! receptors for pain resulting from muscle,tendon, 'oint, or ligament damage and are not considered to be part of theproprioceptive s!stem.

Table III

Receptor Type Sensation Signals &daptation

Muscle

Spindle

$ncapsulated

annulospiral

and flower spra#

endings

Muscle

stretch

Muscle

length &velocit#

1apid initial

transient and

slowsustained

Muscle'

olgi

Tendon

rgan

$ncapsulated

collagen

Muscle

tension

Muscle

contractionSlow

*oint'

Pacinian

$ncapsulated

& la#ered

2oint

Movement

"irection &

velocit#1apid

*oint' $ncapsulated 2oint Pressure & Slow


25/56

Ruffini collagen pressure Angle

*oint'

olgi

rgan

$ncapsulated

collagen2oint torue

%wisting

forceSlow

2.9 Summary

n this chapter, !ou have learned about somatosensor! stimuli and the receptors ofthree components of the somatosensor! s!stems. These three components provideaccurate information about the location, shape, te"ture, and movement of tactilestimuli, (discriminative touch), the position and movement of bod! parts(proprioception) and the application and location of painful stimuli (nociception).Tactile and proprioceptive stimuli are the mechanical forces produced when skincontacts e"ternal ob'ects (discriminative touch), limbs oppose the force of gravit!

(bod! position) and muscles contract and bod! parts move. 4ainful stimuli aretissue-damaging forces. The sensations produced are those of touch, pressure,flutter, and vibrationmovement (discriminative touch), bod! position and movement(proprioception), and sharp cutting pain. The discriminative touch receptors areencapsulated 15 afferent terminals (8eissner, 4acinian and 3uffini corpuscles), hairfollicle endings and 8erkel comple"es in skin. The proprioceptive receptors in muscleare also encapsulated and include the muscle spindle and :olgi tendon organ. The 'oint receptors are similar to the encapsulated endings in skin and tendon and arefound in the 'oint capsule and ligaments. The sharp cutting nociceptors are freenerve endings.

lthough it is convenient to subdivide somatosensor! receptors and pathwa!s for

didactic, clinical and research purposes, it is important to keep in mind that mostsomatosensor! stimuli act simultaneousl! and in var!ing degrees on allsomatosensor! receptors in the bod! part stimulated. 6or e"ample, placing a heav!,cold ob'ect in an outstretched hand produces tactile, thermal, and proprioceptivesensations that allow us to appreciate the presence (touch, pressure), temperature,and weight of the ob'ect and provide proprioceptive information for finger, wrist andarm ad'ustments so we do not drop the ob'ect.

•

$omatosensationWritten by: Andrea Alenda from UCL, Greta Santagata from Manchester University,

ntroduction to the $omatosensory $ystem

How many senses do humans have? Five? You have known this since you were a kid, but, after

reading this article, you might want to reconsider.

The somatosensory system detects, relays and processes somatosensory sensations, like touch,

vibration, pressure, itch, nociception (information about painful stimuli), temperature, proprioception

(information about the position and movement of our joints and muscles) and interoception

(information about internal organs). So, without taking into account what other sensory systems do,

the somatosensory system is already detecting more than five senses by itself.


26/56

The somatosensory system has been linked to the sense of self awareness as it provides an internal

representation of the body. In pathologies affecting the somatosensory system, such as the phantom

limb or the hemineglect syndrome, this awareness of the body is challenged.

This article will look at the basic anatomy and physiology of the somatosensory system, from

receptors in the periphery to the cerebral cortex in the brain.

'haracteristics o! somatosensory receptors

To explain the properties of the somatosensory receptors we are going to use two important concepts:

• Receptive field: the physical space where a stimulus has to

appear in order to be detected by the receptor. This physical

space can have different sizes.

• Adaptation is a physiological property of the neural response to

stimuli measured in the number and timing of action potentials.

Commonly a neuron that shows adaptation will respond with a

large number of action potentials when being stimulated but will

gradually decrease the number of action potentials over time.

This is because it quite literally gets "used" to the stimulus after

a while. Each type of mechanoreceptor can be either rapidly or

slowly adapting (RA/SA). The rapidly adapting

mechanoreceptors fire when they detect a change in their

receptive field. For example they can get activated when a

stimulus is delivered or withdrawn, but they won't fire during the

period of time the stimulus is present in the receptive field.

Slowly adapting mechanoreceptors will respond throughout the

whole duration of the stimulus in the receptive field.

ierent receptors !or dierent modalities

The somatosensory system is generally considered to have four

modalities:mechanoreception,propioception,temperature andnociception. Each of thesemodalities has its specialised receptors, which will transmit their information through specific

anatomical pathways to the brain.

The mechanoreceptors

Of the four modalities we will concentrate on touch.

Mechanoreceptors deal with mechanical stimuli, such as pressure and vibration. Most

mechanoreceptors are found in the skin, although some are also found in the muscles, tendons and

joints.

There are four types of skin mechanoreceptors:


27/56

• Meissner's corpuscules convey information regarding stroking

and fluttering. They are characterized by a small receptive field

and a rapid adapating response (RA).

• Merkel's disks convey information regarding pressure and

texture. It is characterized by a small receptive field and a slow

adapting response (SA)

• Ruffini's capsules respond to stretching of the skin. They have a

big receptive field and and have the slowest adapting response of

all mechanoreceptors (SA)

• Pacinian corpuscules:respond to vibration. Their receptive field

is large and they show an extremely rapid adapting response (RA)

$peed o! in!ormation traelling to the rain

Anatomical pathways are composed by populations of neurons. The speed by which information will

reach the brain will depend on the type neurons that compose the anatomical pathway. To be more

precise, by the size and myelination of the axons of these neurons. The bigger and the more

myelinated* the axon is, the faster the signal will travel. Depending on these characteristics, axons are

classified as:

• A alpha: biggest diameter and fully insulated by the myelin

sheath. These fibres are the quickest to transmit information.

Proprioceptores of skeletal muscle are innervated by this type of

fibre.

• A beta: medium size diameter and myelined. Mechanoreceptors

in the skin are innervated by this type of fibre.

• A delta: small diameter with little myelination. They relay

temperature and information on pain.


28/56

• C : small diameter and non-myelinated fibres, they relay

information on temperature, pain and itch.

* Myelin is a sheath that covers the axons of some neurons. The more myelinated an axon is, the

faster the signal will travel through it, because the myelin sheath increases the speed of the electrical

signal travelling through the axon by decreasing capacitance and increasing electrical resistance.

Experiment yourself

You fall on a pool (with water) belly first. What will you experience? First you will perceive the position

of your body followed by the temperature of the water. Then you will have a quick acute pain in the

stomach, the first area that contacted the water. Finally, a pain that starts a few miliseconds later,

which makes your body curl. What happened? The A alpha fibers have transmitted information about

the position of your body faster, then the A delta transmit the information of temperature and acutepain. Finally information carried through the C fibers reached the brain with the slow and more intense

pain.

The somatosensory pathways

Axons from sensory receptors are segregated into parallel anatomical pathways that reach different

processing areas in the brain. Each class of sensory receptor is capable of activating a corresponding

cortical area. This is known asmodality segregation, which allows the brain to reconstruct signals by

time, modality and location. This is true for all sensory systems.

Modality specific information travels from the receptors to the brain through different anatomical

pahtways:

• Two somatosensory pathways for the body: the anterolateral

pathway and the dorsal column/medial lemnical pathway

• Two somatosensory pathways for the head: the spinotrigeminal

pathway and the trigeminal pathway.

The ascending pathwaysG !rom somatosensory receptor to

cereral corte

This is a summary of the ascending pathways carrying somatosensory information to the brain, the

descending pathways will not be covered here.

Information from the body:


29/56

• Discriminative touch - the dorsal column/medial lemniscal

system

Large-diameter afferents (A-alpha and A-beta fibres) synapse in the dorsal column nuclei of the

medulla (gracile and cuneate), then cross the midline and ascend to the ventro posterolateral (VPL)

thalamic nucleus, via the medial lemniscus. They then finally reach the somatosensory cortex.

Information of discriminative touch and proprioception travels through fast conducting fibres. The

afferents that compose this pathway cross the midline, which means that the right hemisphere of the

brain will process the information regarding the left side of the body and viceversa.

• Nociception and temperature – the anterolateral system

Smaller afferents (A-delta and C fibres) synapse in the spinal cord, then cross midline and ascend via

the spinal cord and brainstem to the VPL and other nuclei of the thalamus. Collaterals of these axons

terminate in the reticular formation of the pons and medulla. Information about temperature, deep

touch and nociception travels through slow conducting fibres. The anterolateral system is complex, itsplits into three pathways, each of which ends in different brain areas with several relay synapses.

Awareness of pain, temperature and deep touch is distributed through different brain areas. The

afferents that compose this pathway also cross the midline, this means that the right hemisphere of

the brain will process the information regarding the left side of the body and viceversa.

Information from the head:

• Discriminative touch - the trigeminal system

Large, fast axons (A-alpha and A-beta) innervate the pars oralis and principal sensory nucleus. Thesecond order axons cross the midline, ascend in the trigeminothalamic tract and terminate in the

ventro posteromedial nucleus (VPM). From the thalamus information is conducted to the

somatosensory cortex. The afferents that compose this pathway cross the midline, which means that

the right hemisphere of the brain will process the information regarding the left side of the head and

viceversa.

• Nociception and temperature - the spinotrigeminal system

Small, slowly conducting axons (A-delta and C fibres) descend in the spinal trigeminal tract and

terminate in the pars caudalis of the spinal nucleus of the brainstem. The second-order axons cross

the midline and ascend to the VPM and intralaminar nuclei of the thalamus. From the thalamus

information reaches the somatosensory cortex. The afferents that compose this pathway cross the

midline, which means that the right hemisphere of the brain will process the information regarding the

left side of the head and viceversa.

The dorsal columnHmedial lemniscal pathway


30/56

The anterolateral system


31/56

The trigeminal pathway


32/56

The $pinotrigeminal pathway


33/56

The thalamus

The thalamus is a structure with the shape of two attached beetles that belongs to the diencephalon. It

is essential for gating, processing and transferring information to and from the cerebral cortex. The

majority of sensory information coming from the periphery of the body is filtered in the thalamus

before reaching the cortex.

The thalamus is divided in multiple specialised nuclei. The thalamic nuclei which process

somatosensory information are the ventro posterolateral nucleus (VPL), which processes information

from the body, and the ventro posteromedial nucleus (VPM), which processes information from the

head. Somatosensory information also reaches the intralaminar nuclei in the thalamus which are

inespecific: they process mixed type of information and they reach several different cortical areas.

The somatosensory corte


34/56

In 1909 Korbinian Brodmann defined different areas of the cerebral cortex based on

their cytoarchitecture (structure and shape of the cells). He discovered that different areas of the

cortex have different types, density and organization of neurons and labelled each area with a

number. The majority of the areas that Brodmann defined have been correlated with specific

physiological functions. His terminology is still in current use today.

In the case of the somatosensory cortices, Brodmann’s areas 1,2 and 3 correspond to the primary

somatosensory cortex (SI) and area 7 corresponds to the associative somatosensory cortex.

The somatosensory cortex is divided into:

• Primary somatosensory cortex (SI) where somatosensory

information is still segreated. It is arranged by receptor type and

location of origin.

•

Secondary somatosensory cortex (SII) where somatosensoryinformation is integrated.

• Somatosensory associative areas, where information from

different sensory modalities is further integrated. The

somatosensory associative cortices are found in the parietal

cortex. The posterior parietal cortex is an area where several

segregated streams of sensory information converge to produce

a complex representation. The posterior parietal cortex

processes somatic sensation together with visual information

and a person’s state of attentiveness.

All the somatosensory cortices are part of the neocortex. Like most of the neocortex, they too are

divided in six layers, layer I being at the surface and layer VI the deepest one. Each cortical layer is

characterised by different types of neurons and specific connectivity.

Thalamic input goes into layer IV and a minor input goes to layer VI. Information from layer IV is then

transmitted to layer II and III where some connections travel horizontally, or intracortically, to

neighbouring cortical areas. Apical dentrites from layer V cells contact layer III small pyramidal

neurons and send outputs from the somatosensory cortex to subcortical areas. Finally informationfrom layer VI that comes from layer V and the thalamus, sends projections back to the thalamus. This

forms a loop of inputs and outputs of information coming into the cortex from deeper nuclei and being

sent back to the thalamus and other neighbouring cortical areas.

The somatosensory cortices


35/56

ierent types o! neurons in the primary somatosensory corte


36/56

$ cortical connections


37/56

* topographic map in $

The termsomatotopy refers to the correspondence between a receptors in the body and thededicated area of the cortex that is activated by it. In the case of the somatosensory system, this

means that for every part of the body there is a corresponding area in the brain. Atopographic

map of the whole body can therefore be found in the somatosensory cortex. The sensory topographic

map of our body is known as thesensory homunculus, and it represents the location and the

amount of cortical area dedicated to a particular part of the body or the head.

As you can see from the figure, the representation of some parts of the body and head are distorted in

the somatosensory cortex. Certain body parts, like the lips and the hands, have more cortical area

dedicated to them than some others. This is because spatial resolution in the cortex is correlated with

the innervation density of the skin, so the more sensitive a region of the body is, the more space it will

take in the cortex. You could look at the sensory homunculus as a map of how sensitive our bodies


38/56

are: hands and lips are clearly the most sensitive areas, whereas legs and torso are much less

sensitive.

The sensory homunculus is different in each species. Rabbits have a broad cortical area dedicated to

the representation of their face and especially their teeth. Cats have a great representation of theirheads and their four paws. Most remarkably rats and mice have a large cortical area completely

dedicated only to their whiskers, known as the barrel cortex. This is such a sophisticated example of

cortical specialisation that it has become one of the principal models for the investigation of the

somatosensory system.

Somatosensory maps are plastic and can be modified by training or use. They are shaped by

experience during development and can be modified during adulthood. As reported by Mezernich in

1984, if a monkey looses a finger of the hand, the receptive field of the monkey's brain adjusts, so the

somatosensory homunculus allocates more of its receptive field to the adjacent fingers.

The humunculus


39/56

"athologies o! the somatosensory systemG the phantom lim

Early reported cases:

Ambroise Pare (1510-1590) was a barber who became a French military surgeon. He observed thatamputees reported strange sympthoms. In the 16th century he wrote: "...the patients who many

months after the cutting away of the leg grievously complained that they yet felt exceeding great pain

of the leg so cut off..."

Silas Weir Mitchell (1871) who was a surgeron in the American Civil War wrote: "...nearly every man

who loses a limb, carries about with him a constant or inconstant phantom of the missing member, a

sensory ghost of that much of himself and sometimes a most inconvenient presence faintly felt at

times, but ready to be called up to his perceptios by a blow, touch or a wind of change...".

What we know today about the phantom limb

Phantom limbs occur in nearly every case of amputees who lose a limb.

The phantom is usually described as resembling the normal somatosensory experience of having the

physical limb, the same way as it was before being amputated. It might move, the same way as


40/56

before, with normal shape and size. As time passes after the amputation, the shape and size of the

phantom can become distorted. This is termed as the telescopic phenomenon of the phantom limb.

Phantom limbs can sometimes be painful and the sensation can range from itch to a strong muscular

contraction to burning pain. Not only this, but pain to the missing limb can be elicited by stimulation of

a different part of the body.

Why do amputees feel sensations on a limb that doesn't exist?

When a limb is amputated, changes occur both in the peripheral and the central nervous system:

The fibres belonging to the somatic receptors that innervated the amputated limb are cut, turning into

free nerve endings that will eventually loose the myelination. This doesn't mean that they stop

transmitting pulses, quite the opposite, the free nerve endings are activated and fire every time the

stump is stimulated.

However, the brain codes information coming from those fibers as if they were still coming from the

missing limb. So the brain constructs a percept with that sensation that corresponds to the phantom.

In the somatosensory cortex, the areas dedicated to the missing limb do not receive as much

stimulation anymore. When an area in the brain stops receiving information it eventually degenerates.

As explained earlier, adjacent areas in the homunculus take over the unused area to make the most of

the cortical space. It can therefore happen that stimulation of one body part can provoke sensations

on the phantom limb.

If you would like to find out more about the phantom limb, we recommend the book by V.S.

Ramanchandran and S Blakeslee: "Phantoms in the brain".

$ummary

• The somatosensory system processes mechanoreceptive,

proprioceptive, nociceptive and information related to

temperature.

• The somatosensory system carries information from the

periphery to the cerebral cortex through the lemniscal,

anterolateral and trigeminal pathways. All sensory inputs cross

to the midline, so the information coming from the left part of the

body is processed by the right hemisphere of the brain and

viceversa.

• The somatosensory system maintains its somatotopy along its

pathways from the periphery to the cerebral cortex.

• The somatosensory cortex is part of the neocortex. It is divided

in six layers, each of which have different types of neurons

forming different connections.

• There is a sensory homunculus in the somatosensory cortex

that represents our body and head. The amount of cortical area

dedicated to each part will determine how sensitive that area is.


41/56

Further readings

References

• Phillips, J. R., Johansson, R.S., Johnson, K.O. 1990.

Representation of braille characters in human nerve fibres.

Experimental brain research, 81:589-592.

• Mountcastle, V. 1957. Modality and topographic properties of

single neurons of cat's somatic sensory cortex. J

Neurosci,20:408-434.

• Lübke, J., Feldmeyer, D. 2007. Excitatory signal flow and

connectivity in a cortical column: focus on the barrel

cortex. Brain Struct Funct 212(1):3-17.

• Buonomano, D.V., Merzenich, M.M. 1998. Cortical plasticity:

from synapses to maps. Annual reviews neuroscience. 21:149-

186.

• Merzenich, M.M., Nelson, R.J., Stryker, M.P., Cynader, M.S.,

Schoppmann, A., Zook, J.M. 1984. Somatosensory cortical map

changes following digit amputation in adult monkeys. J Comp

Neurol. 224(4):591-605.

• Woodhouse, A. 2005. The Phantom limb sensation. Clin Exp

Pharmacol Physiol. 32(1-2):132-4

• Flor, H., Nikolajsen, L., Jensen, T.S.. 2006. The Phantom limb

pain: a case of maladaptive CNS plasticity?. Nat Neurosci Rev.

7(11):873-81.

Bibliography

• Chapter 12: “The Somatic Sensory System” In: Neuroscience:

Exploring the Brain (Bear MF, Connors BW, Paradiso BW., eds).

Williams & Wilkins.

• Chapter 18: “The Functional Organization of Perception and

Movement”; Chapter 22: “The Body Senses” In: Principles of

Neuroscience (Kandel ER, Schwartz JH and Jessel TM., eds).

McGraw-Hill.

• Chapter 23: “Touch”. In: Principles of Neuroscience (Kandel ER,

Schwartz JH and Jessel TM., eds). McGraw-Hill.


42/56

• Chapter 26: “The Somatic Sensation”. In: The Fundamental

Neuroscience Ed. (Zigmond MJ, Bloom FE, Landis SC, Roberts

JL and Squire LR., eds). Academic Press.

CELLULAR & MOLECULAR

Action potentials and synaptic transmission

Ion channels and membrane potential

Neurotransmitters and their receptors

Overview of the neuron

NS ANATOMY

Central nervous system

Peripheral nervous system

DEVELOPMENT OF NS

Axonal growth

Cell differentiation

Induction and pattern formation

Neural Crest

Programmed cell death

Synapse formation

NEUROPHARMACOLOGY

Anaesthetics and Analgesics

Antidepressants

Antipsychotics

Anxiolytics

Drug abuse

Neurobiology of Pain

http://www.fastbleep.com/biology-notes/39/117/741http://www.fastbleep.com/biology-notes/39/117/739http://www.fastbleep.com/biology-notes/39/117/740http://www.fastbleep.com/biology-notes/39/117/738http://www.fastbleep.com/biology-notes/39/140/882http://www.fastbleep.com/biology-notes/39/140/881http://www.fastbleep.com/biology-notes/39/141/885http://www.fastbleep.com/biology-notes/39/141/884http://www.fastbleep.com/biology-notes/39/141/883http://www.fastbleep.com/biology-notes/39/141/1030http://www.fastbleep.com/biology-notes/39/141/887http://www.fastbleep.com/biology-notes/39/141/886http://www.fastbleep.com/biology-notes/39/142/1234http://www.fastbleep.com/biology-notes/39/142/891http://www.fastbleep.com/biology-notes/39/142/890http://www.fastbleep.com/biology-notes/39/142/889http://www.fastbleep.com/biology-notes/39/142/893http://www.fastbleep.com/biology-notes/39/142/888http://www.fastbleep.com/biology-notes/39/117/741http://www.fastbleep.com/biology-notes/39/117/739http://www.fastbleep.com/biology-notes/39/117/740http://www.fastbleep.com/biology-notes/39/117/738http://www.fastbleep.com/biology-notes/39/140/882http://www.fastbleep.com/biology-notes/39/140/881http://www.fastbleep.com/biology-notes/39/141/885http://www.fastbleep.com/biology-notes/39/141/884http://www.fastbleep.com/biology-notes/39/141/883http://www.fastbleep.com/biology-notes/39/141/1030http://www.fastbleep.com/biology-notes/39/141/887http://www.fastbleep.com/biology-notes/39/141/886http://www.fastbleep.com/biology-notes/39/142/1234http://www.fastbleep.com/biology-notes/39/142/891http://www.fastbleep.com/biology-notes/39/142/890http://www.fastbleep.com/biology-notes/39/142/889http://www.fastbleep.com/biology-notes/39/142/893http://www.fastbleep.com/biology-notes/39/142/888


43/56

Stimulants

LEARNING AND MEMORY

Declarative memory

Emotional memory

Explicit and implicit memory

Memory consolidation

Molecular basis of memory

Overview of the hippocampus

Procedural memory

Spatial learning

NEUROENDOCRINOLOGY

Oxytocin and vasopressin

Sexual behaviour and competition

Sexual development & dimorphism

Sleep and hunger

Stress

The brain and it's hormones

SENSORY SYSTEMS

Audition

Gustation

Olfaction

Somatosensation

Vision

NEUROIMMUNOLOGY

Brain repair after acute inflammation

http://www.fastbleep.com/biology-notes/39/142/892http://www.fastbleep.com/biology-notes/39/143/897http://www.fastbleep.com/biology-notes/39/143/899http://www.fastbleep.com/biology-notes/39/143/894http://www.fastbleep.com/biology-notes/39/143/901http://www.fastbleep.com/biology-notes/39/143/895http://www.fastbleep.com/biology-notes/39/143/896http://www.fastbleep.com/biology-notes/39/143/898http://www.fastbleep.com/biology-notes/39/143/900http://www.fastbleep.com/biology-notes/39/144/904http://www.fastbleep.com/biology-notes/39/144/907http://www.fastbleep.com/biology-notes/39/144/903http://www.fastbleep.com/biology-notes/39/144/906http://www.fastbleep.com/biology-notes/39/144/905http://www.fastbleep.com/biology-notes/39/144/902http://www.fastbleep.com/biology-notes/39/145/910http://www.fastbleep.com/biology-notes/39/145/908http://www.fastbleep.com/biology-notes/39/145/909http://www.fastbleep.com/biology-notes/39/145/911http://www.fastbleep.com/biology-notes/39/145/912http://www.fastbleep.com/biology-notes/39/146/918http://www.fastbleep.com/biology-notes/39/142/892http://www.fastbleep.com/biology-notes/39/143/897http://www.fastbleep.com/biology-notes/39/143/899http://www.fastbleep.com/biology-notes/39/143/894http://www.fastbleep.com/biology-notes/39/143/901http://www.fastbleep.com/biology-notes/39/143/895http://www.fastbleep.com/biology-notes/39/143/896http://www.fastbleep.com/biology-notes/39/143/898http://www.fastbleep.com/biology-notes/39/143/900http://www.fastbleep.com/biology-notes/39/144/904http://www.fastbleep.com/biology-notes/39/144/907http://www.fastbleep.com/biology-notes/39/144/903http://www.fastbleep.com/biology-notes/39/144/906http://www.fastbleep.com/biology-notes/39/144/905http://www.fastbleep.com/biology-notes/39/144/902http://www.fastbleep.com/biology-notes/39/145/910http://www.fastbleep.com/biology-notes/39/145/908http://www.fastbleep.com/biology-notes/39/145/909http://www.fastbleep.com/biology-notes/39/145/911http://www.fastbleep.com/biology-notes/39/145/912http://www.fastbleep.com/biology-notes/39/146/918


44/56

Clinical assessment of neuroinflammation

Early response to infection and injury

Immune cells and mediators in the brain

Neural control of host defense

System inflammation and the development of neurode

The role of the immune system in the healthy brain

COMPUTATIONAL NS

Artificial neural nets

Neural noise

Noisy but reliable computation

Population codes

Single neuron codes

Spike triggered average

The neural code

DISEASES OF THE CNS

Alzheimer's disease

Dementia

Depression

Epilepsy

Motor neuron disease

Multiple sclerosis

Parkinson's disease

Schizophrenia

Stroke

RELATED NEWS

http://www.fastbleep.com/biology-notes/39/146/919http://www.fastbleep.com/biology-notes/39/146/916http://www.fastbleep.com/biology-notes/39/146/913http://www.fastbleep.com/biology-notes/39/146/915http://www.fastbleep.com/biology-notes/39/146/917http://www.fastbleep.com/biology-notes/39/146/914http://www.fastbleep.com/biology-notes/39/147/926http://www.fastbleep.com/biology-notes/39/147/923http://www.fastbleep.com/biology-notes/39/147/924http://www.fastbleep.com/biology-notes/39/147/922http://www.fastbleep.com/biology-notes/39/147/921http://www.fastbleep.com/biology-notes/39/147/925http://www.fastbleep.com/biology-notes/39/147/920http://www.fastbleep.com/biology-notes/39/148/927http://www.fastbleep.com/biology-notes/39/148/935http://www.fastbleep.com/biology-notes/39/148/934http://www.fastbleep.com/biology-notes/39/148/929http://www.fastbleep.com/biology-notes/39/148/932http://www.fastbleep.com/biology-notes/39/148/930http://www.fastbleep.com/biology-notes/39/148/928http://www.fastbleep.com/biology-notes/39/148/933http://www.fastbleep.com/biology-notes/39/148/931http://www.fastbleep.com/biology-notes/39/146/919http://www.fastbleep.com/biology-notes/39/146/916http://www.fastbleep.com/biology-notes/39/146/913http://www.fastbleep.com/biology-notes/39/146/915http://www.fastbleep.com/biology-notes/39/146/917http://www.fastbleep.com/biology-notes/39/146/914http://www.fastbleep.com/biology-notes/39/147/926http://www.fastbleep.com/biology-notes/39/147/923http://www.fastbleep.com/biology-notes/39/147/924http://www.fastbleep.com/biology-notes/39/147/922http://www.fastbleep.com/biology-notes/39/147/921http://www.fastbleep.com/biology-notes/39/147/925http://www.fastbleep.com/biology-notes/39/147/920http://www.fastbleep.com/biology-notes/39/148/927http://www.fastbleep.com/biology-notes/39/148/935http://www.fastbleep.com/biology-notes/39/148/934http://www.fastbleep.com/biology-notes/39/148/929http://www.fastbleep.com/biology-notes/39/148/932http://www.fastbleep.com/biology-notes/39/148/930http://www.fastbleep.com/biology-notes/39/148/928http://www.fastbleep.com/biology-notes/39/148/933http://www.fastbleep.com/biology-notes/39/148/931


45/56

a touchy-feely job to do - Futurity: Research News (05 Jan 15)

lty member to focus on sensorimotor integration - Penn State News (02 Feb 15)

eurons which can help to better understand 'sense of touch' - Daily News & Analysis (07 Jan 15)

es to high-end gyms - Boston Globe (24 Apr 14)

ste? - Gizmodo (18 Feb 14)

how emotions are mapped in the body - Science a Gogo (01 Jan 14)

macaques 'feel' when prosthetic hand is touched - Wired.co.uk (15 Oct 13)

eaf Brains - Scientist (blog) (12 Feb 13)

um nitride fractal coatings - Today's Medical Developments (30 Apr 14)

chanism repatterns developing brain regions - Eureka! Science News (23 Jul 13)

• About What we do and who we work with.

• T&Cs Terms and conditions of use.

• Powered by Software development at Fastbleep.

• Get published Write an article for Fastbleep Notes.

• Volunteer For our Schools Programme.

• Contact us We like to talk.

Keep in touch byfollowing us on Twitter andfinding us on Facebook.

Copyright © 2015 Fastbleep Ltd.

All rights reserved.

Happy Tuesday.

http://www.futurity.org/neurons-touch-whiskers-831382/http://news.psu.edu/story/342928/2015/01/30/research/new-penn-state-faculty-member-focus-sensorimotor-integrationhttp://www.dnaindia.com/scitech/report-scientists-discover-neurons-which-can-help-to-better-understand-sense-of-touch-2050604http://www.bostonglobe.com/lifestyle/style/2014/04/23/balance-training-moves-from-assisted-living-facilities-high-end-gyms/iroifKdhU792zMTQ0hIowN/story.htmlhttp://gizmodo.com/what-causes-aftertaste-1524996679http://www.scienceagogo.com/news/20140001165108.shtmlhttp://www.wired.co.uk/news/archive/2013-10/15/feeling-prosthetic-limbshttp://www.the-scientist.com/?articles.view/articleNo/34363/title/Dual-Adaptation-in-Deaf-Brains/http://www.onlinetmd.com/denton-vacuum-titanium-nitride-medical-implant-coatings-43014.aspxhttp://esciencenews.com/articles/2013/07/22/novel.top.down.mechanism.repatterns.developing.brain.regionshttp://www.fastbleep.com/blog/abouthttp://www.fastbleep.com/blog/abouthttp://www.fastbleep.com/termshttp://www.fastbleep.com/devhttp://www.fastbleep.com/revision-notes/get-involvedhttp://www.fastbleep.com/schools/medical-studentshttp://www.fastbleep.com/contacthttp://www.fastbleep.com/contacthttp://twitter.com/fastbleephttp://www.facebook.com/fastbleephttp://www.facebook.com/fastbleephttp://www.futurity.org/neurons-touch-whiskers-831382/http://news.psu.edu/story/342928/2015/01/30/research/new-penn-state-faculty-member-focus-sensorimotor-integrationhttp://www.dnaindia.com/scitech/report-scientists-discover-neurons-which-can-help-to-better-understand-sense-of-touch-2050604http://www.bostonglobe.com/lifestyle/style/2014/04/23/balance-training-moves-from-assisted-living-facilities-high-end-gyms/iroifKdhU792zMTQ0hIowN/story.htmlhttp://gizmodo.com/what-causes-aftertaste-1524996679http://www.scienceagogo.com/news/20140001165108.shtmlhttp://www.wired.co.uk/news/archive/2013-10/15/feeling-prosthetic-limbshttp://www.the-scientist.com/?articles.view/articleNo/34363/title/Dual-Adaptation-in-Deaf-Brains/http://www.onlinetmd.com/denton-vacuum-titanium-nitride-medical-implant-coatings-43014.aspxhttp://esciencenews.com/articles/2013/07/22/novel.top.down.mechanism.repatterns.developing.brain.regionshttp://www.fastbleep.com/blog/abouthttp://www.fastbleep.com/termshttp://www.fastbleep.com/devhttp://www.fastbleep.com/revision-notes/get-involvedhttp://www.fastbleep.com/schools/medical-studentshttp://www.fastbleep.com/contacthttp://twitter.com/fastbleephttp://www.facebook.com/fastbleephttps://www.boundless.com/


46/56

Subjects

• Accounting• Algebra• Art History• Biology• Business• Calculus• Chemistry• Communications• Computer Science• Economics• Education• Finance• Management• Marketing• Microbiology• Music• Philosophy• Physics• Physiology• Political Science• Psychology• Sociology• Statistics

• US History• !orld History• !riting

Contribute

Sign Up "ogin

• Accounting• Algebra• Art History• Biology• Business•

Calculus• Chemistry

https://www.boundless.com/subjects/https://www.boundless.com/subjects/https://www.boundless.com/accounting/https://www.boundless.com/algebra/https://www.boundless.com/art-history/https://www.boundless.com/biology/https://www.boundless.com/business/https://www.boundless.com/calculus/https://www.boundless.com/chemistry/https://www.boundless.com/communications/https://www.boundless.com/computer-science/https://www.boundless.com/economics/https://www.boundless.com/education/https://www.boundless.com/finance/https://www.boundless.com/management/https://www.boundless.com/marketing/https://www.boundless.com/microbiology/https://www.boundless.com/music/https://www.boundless.com/philosophy/https://www.boundless.com/physics/https://www.boundless.com/physiology/https://www.boundless.com/political-science/https://www.boundless.com/psychology/https://www.boundless.com/sociology/https://www.boundless.com/statistics/https://www.boundless.com/u-s-history/https://www.boundless.com/world-history/https://www.boundless.com/writing/https://www.boundless.com/issues/https://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/somatosensation-206/somatosensory-receptors-778-12012/#registerhttps://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/somatosensation-206/somatosensory-receptors-778-12012/#registerhttps://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/somatosensation-206/somatosensory-receptors-778-12012/#loginhttps://www.boundless.com/accounting/https://www.boundless.com/algebra/https://www.boundless.com/art-history/https://www.boundless.com/biology/https://www.boundless.com/business/https://www.boundless.com/calculus/https://www.boundless.com/chemistry/https://www.boundless.com/subjects/https://www.boundless.com/subjects/https://www.boundless.com/accounting/https://www.boundless.com/algebra/https://www.boundless.com/art-history/https://www.boundless.com/biology/https://www.boundless.com/business/https://www.boundless.com/calculus/https://www.boundless.com/chemistry/https://www.boundless.com/communications/https://www.boundless.com/computer-science/https://www.boundless.com/economics/https://www.boundless.com/education/https://www.boundless.com/finance/https://www.boundless.com/management/https://www.boundless.com/marketing/https://www.boundless.com/microbiology/https://www.boundless.com/music/https://www.boundless.com/philosophy/https://www.boundless.com/physics/https://www.boundless.com/physiology/https://www.boundless.com/political-science/https://www.boundless.com/psychology/https://www.boundless.com/sociology/https://www.boundless.com/statistics/https://www.boundless.com/u-s-history/https://www.boundless.com/world-history/https://www.boundless.com/writing/https://www.boundless.com/issues/https://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/somatosensation-206/somatosensory-receptors-778-12012/#registerhttps://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/somatosensation-206/somatosensory-receptors-778-12012/#loginhttps://www.boundless.com/accounting/https://www.boundless.com/algebra/https://www.boundless.com/art-history/https://www.boundless.com/biology/https://www.boundless.com/business/https://www.boundless.com/calculus/https://www.boundless.com/chemistry/


47/56

• Communications• Computer Science• Economics• Education• Finance•

Management• Marketing• Microbiology• Music• Philosophy• Physics• Physiology• Political Science• Psychology• Sociology• Statistics• US History• !orld History• !riting

+iology

#e$tbooks

Boundless Biology

Sensory Systems

Somatosensation

'oncept %ersion &

Created by Boundless

Fa'orite

(

!atch

(

https://www.boundless.com/communications/https://www.boundless.com/computer-science/https://www.boundless.com/economics/https://www.boundless.com/education/https://www.boundless.com/finance/https://www.boundless.com/management/https://www.boundless.com/marketing/https://www.boundless.com/microbiology/https://www.boundless.com/music/https://www.boundless.com/philosophy/https://www.boundless.com/physics/https://www.boundless.com/physiology/https://www.boundless.com/political-science/https://www.boundless.com/psychology/https://www.boundless.com/sociology/https://www.boundless.com/statistics/https://www.boundless.com/u-s-history/https://www.boundless.com/world-history/https://www.boundless.com/writing/https://www.boundless.com/biology/https://www.boundless.com/biology/textbooks/https://www.boundless.com/biology/textbooks/boundless-biology-textbook/https://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/https://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/somatosensation-206/https://www.boundless.com/communications/https://www.boundless.com/computer-science/https://www.boundless.com/economics/https://www.boundless.com/education/https://www.boundless.com/finance/https://www.boundless.com/management/https://www.boundless.com/marketing/https://www.boundless.com/microbiology/https://www.boundless.com/music/https://www.boundless.com/philosophy/https://www.boundless.com/physics/https://www.boundless.com/physiology/https://www.boundless.com/political-science/https://www.boundless.com/psychology/https://www.boundless.com/sociology/https://www.boundless.com/statistics/https://www.boundless.com/u-s-history/https://www.boundless.com/world-history/https://www.boundless.com/writing/https://www.boundless.com/biology/https://www.boundless.com/biology/textbooks/https://www.boundless.com/biology/textbooks/boundless-biology-textbook/https://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/https://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/somatosensation-206/


48/56

Somatosensory )eceptors )EA*

E*+# FEE*BAC, %E)S+-. H+S#-)/ USA0E

$


49/56

• !Pacinian corpuscles are rapidly!adapting, deep receptors that respond to

deep pressure and high!fre$uency "ibration.

TERMS[ EDIT ]

• glabrous

smooth, hairless, bald

• dendrite

branched projections of a neuron that conduct the impulses recei"ed from otherneural cells to the cell body

Give us feedback on this content:

)egister 1or F)EE to stop seeing ads

FULL TEXT[ EDIT ]

Somatosensory )eceptors

%ensory receptors are classified into fi"e

categories:mechanoreceptors, thermoreceptors, proprioceptors, pain

receptors, and chemoreceptors. These categories are based on the nature of

the stimuli that each receptor class transduces. Mechanoreceptors in the

skin are described as encapsulated or unencapsulated. & free ner"e ending

is an unencapsulated dendrite of a sensory neuron# they are the most

common ner"e endings in skin. ree ner"e endings are sensiti"e to painful

stimuli, to hot and cold, and to light touch. They are slo to adjust to a

stimulus and so are less sensiti"e to abrupt changes in stimulation.

Mechanoreceptors

https://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/somatosensation-206/somatosensory-receptors-778-12012/issues/new/https://www.boundless.com/definition/glabrous/https://www.boundless.com/definition/dendrite/https://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/somatosensation-206/somatosensory-receptors-778-12012/https://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/somatosensation-206/somatosensory-receptors-778-12012/issues/new/https://www.boundless.com/definition/somatosensory/https://www.boundless.com/definition/receptor/https://www.boundless.com/definition/mechanoreceptor/https://www.boundless.com/definition/thermoreceptor/https://www.boundless.com/definition/dendrite/https://www.boundless.com/definition/neuron/https://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/somatosensation-206/somatosensory-receptors-778-12012/issues/new/https://www.boundless.com/definition/glabrous/https://www.boundless.com/definition/dendrite/https://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/somatosensation-206/somatosensory-receptors-778-12012/https://www.boundless.com/biology/textbooks/boundless-biology-textbook/sensory-systems-36/somatosensation-206/somatosensory-receptors-778-12012/issues/new/https://www.boundless.com/definition/somatosensory/https://www.boundless.com/definition/receptor/https://www.boundless.com/definition/mechanoreceptor/https://www.boundless.com/definition/thermoreceptor/https://www.boundless.com/definition/dendrite/https://www.boundless.com/definition/neuron/


50/56

There are three classes of mechanoreceptors: tactile, proprioceptors, and

baroreceptors. Mechanoreceptors sense stimuli due to physical

deformation of theirplasma membranes. They contain mechanically!

gated ion channels hose gates open or close in response to pressure,

touch, stretching, and sound. There are four primary tactile

mechanoreceptors in human skin: Merkel's disks, Meissner's corpuscles,

Ruffini endings, and Pacinian corpuscle# to are located toard the surface

of the skin and to are located deeper . & fifth type of mechanoreceptor,

(rause end bulbs, are found only in speciali)ed regions.

Primary mechanore

Documents

Somatosensory - senses.docx