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05 The Somatosensory System

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Introduction

Somatic SensationA. Touch, sharp pain, ache, chill/burnB. Proprioception: Body position, location of limbs.

Interacts with vestibular sense and vision; interacts with touch for “haptics”. Baroreceptors.

C. Somatic sensory system: Different from other systems: several sense systems

1. Receptors: Distributed throughout body2. Responds to widest variety of stimulus types

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Touch

Types and layers of skinA. Hairy and glabrous (hairless)B. Epidermis (outer) and dermis (inner)

I. Functions of skinA. Protective functionB. Prevents evaporation of body fluidsC. Provides direct contact with world

II. MechanoreceptorsA. Most somatosensory receptors are

mechanoreceptors

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Mechanoreceptors A. Pacinian corpusclesB. Ruffini's endingsC. Meissner's corpuscles D. Merkel's disks

Free nerve endingsKrause end bulbs

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MechanoreceptorsÅke Vallbo and colleagues

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MechanoreceptorsA. Åke Vallbo and colleagues

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MechanoreceptorsTwo Point Threshold

Braille letters consist of dots 2.5 mm apart andcan be read at ~600characters / minute.

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Mechanoreceptors

I. Rapidly Adapting (RA) -respond to changes in stimulation, but do not continue to respond to constant stimulation

II. Slowly Adapting (SA) -respond to constant stimulationIII. Punctate - small receptive fields with distinct boundariesIV. Diffuse - large receptive fields with non-distinct boundaries

Punctate Diffuse

RA RA-PunctateMeissner Corpuscles

RA-DiffusePacinian Corpuscles

SA SA-PunctateMerkel Disks

SA-DiffuseRuffini Endings

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MechanoreceptorsI. Meissner Corpuscles (RA-punctate) responds best to

active touch involved in object exploration; textureII. Pacinian Corpuscles (RA-diffuse) extremely sensitive

over a large receptive field to high frequency changes in pressure; its membrane is deformed – e.g., a breeze on the palm of your hand; vibrations

III. Merkel Disks (SA-punctate) constant sources of stimulation over a small area – e.g., carrying a pebble

IV. Ruffini Endings (SA-diffuse) constant stimulation over a larger area – e.g., the feeling of clothing; skin stretch

V. Free nerve endings - pain fibers & thermal conductance fibers

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MechanoreceptorsReceptive field size and adaptation rate

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Mechanoreceptorsadaptation rate: Pacinian Corpuscle

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Primary Afferent AxonsA. AProprioception

B. ATouch

C. AFast Pain, temperature

D. C, Slow Pain, Temperature, Itch

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Primary Afferent AxonsEnter spinal cord through the dorsal root

Dorsal Root

Dorsal RootGanglion Cell

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I. The Spinal cordA. Spinal segments (30)- spinal nerves within 4

divisions of spinal cordB. Dermatomes- 1-to-1 correspondence with

segments with overlap.What happens when a dorsal root is cut?Shingles

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The Spinal cord Sensory Organization

I. Cervical (C)II. Thoracic (T)III. Lumbar (L)IV. Sacral (S)

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The Spinal cord Sensory Organization I. Cervical (C)II. Thoracic (T)III. Lumbar (L)IV. Sacral (S)

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The Spinal cord Sensory Organization I. Cervical (C)II. Thoracic (T)III. Lumbar (L)IV. Sacral (S)

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Division of spinal gray matter:1. Dorsal horn

Myelinated A axons (mechano)

2. Intermediate zone 3. Ventral horn

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Dorsal Column–Medial Lemniscal PathwayIpsilateral without synapses along spinal cord

Touch information ascends through dorsal column, dorsal nuclei, medial lemniscus, and ventral posterior nucleus

to primary somatosensory cortex

Cerebral Cortex

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Dorsal Column–Medial Lemniscal Pathway

dorsal column, dorsal nuclei, medial lemniscus, and ventral posterior nucleus

to primary somatosensory cortex

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The Trigeminal Touch PathwayA. Trigeminal nerves: Face, mouth, tongue

(cranial V)B. Other cranial nerves (throat and abdominal

cavity: cranial IX, X)

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Somatosensory CortexA. Primary: BA 3bB. Other areas

1. Postcentral gyrusBA 1, 2

2. Posterior ParietalCortex

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Somatosensory Cortex A. Brodmann’s Area 3b (or S1): Primary

somatosensory cortex1. Receives dense input from VP nucleus of the

thalamus2. Neurons: Responsive to stimuli3. Lesions impair somatic sensations4. Electrical stimulation evokes sensory

experiences

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Somatosensory Cortex : 3bColumnar OrganizationSomatotopic Organization

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Somatosensory CortexCortical Somatotopy

1. Homunculus2. Importance of mouth

i. Tactile sensations: Important for speechii. Lips and tongue: Last line of defenseiii. Infants and non-primate mammals touch

primarily with their mouths3. Fingers and Thumb

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Somatosensory CortexCortical Somatotopy

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Somatosensory CortexCortical Magnification

The receptive fields and cortical representations give more acuity to fingers, mouth, nose and tongue

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Somatosensory CortexS1: Rat

1. Vibrissae2. “Barrel cortex”

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Somatosensory CortexS1: Rat

1. Vibrissae2. “Barrel cortex”

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Somatosensory Cortex

S1 – Owl monkey

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Somatosensory Cortex A. Cortical Map PlasticityB. Remove digits or overstimulate – examine

somatotopy before and after1. Conclusions of experiments

Reorganization of cortical maps

a. Dynamic

b. Adjust depending on the amount of sensory experience

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TouchSomatosensory Cortex

A. Cortical Map PlasticityB. Remove digits or

overstimulate – examine somatotopy before and after

1. Conclusions of experimentsReorganization of

cortical maps

a. Dynamic

b. Adjust depending on the amount of sensory experience

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Somatosensory CortexSquirrel Monkey

Cortical Map Plasticity

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The Posterior Parietal Cortex1. Involved in somatic sensation, visual stimuli,

and movement planning2. Astereoagnosia 3. Neglect syndrome

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Pain

Nociception: No pain, no gain? Hansen’s Disease.A. Pain - feeling of sore, aching, throbbing B. Nociception - sensory process, provides signals

that trigger pain1. Mechanical2. Thermal3. Chemical4. Polymodal

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PainTransduction of Pain

A. Mechanically-gated ion channelsB. Damage to cells release proteases (an enzyme which

digests proteins) which cause kaninogen to break down to form bradykinin which binds to certain nociceptors:

Hyperaglesia: Bradykinin – increases the sensitivity of nociceptors and

thermoreceptorsProstaglandins – caused by enzymatic breakdown of

membrane lipids – cause increased sensitivity of nociceptors

Substance P – Released by nociceptors - Causes swelling & Mast cell activation: release of histamine

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I. Primary Afferents and Spinal mechanismsA. First pain and second painB. Referred pain: Angina

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Ascending Pain PathwaysDifferences between touch and pain pathway

1. Nerve endings in the skin2. Diameter of axons3. Connections in spinal cord

i. Touch – Ascends Ipsilaterallyii. Pain – Ascends Contralaterally

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Ascending Pain Pathways Spinothalamic Pain Pathway

Dorsal Root – immediactly decussates

Spinothalamic Tract

Spinothalamic tract

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Spinothalamic tract

Ascending Pain Pathways Spinothalamic Pain Pathway

Dorsal Root – immediactly decussates

Spinothalamic Tract

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ComparingPathways

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Reflex Arc

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I. The Regulation of PainA. Afferent RegulationB. Descending RegulationC. The endogenuos opiates

1. Opioids and endomorphins

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The Regulation of Pain

Descending regulation

Periaquaductal gray matter

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Phantom Pain

I. After surgical removal of a limb, sensations resume in the limb

II. In 90% of patients, the sensations are very painful

III. In 60% the pain is excruciating: described sometimes as an arm on fire, being torn or punctured, great pressure

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Phantom Pain: Strange Facts

I.Stimulating certain areas of skin (e.g., face) may aggrevate phantom pain.

II.Severing the nerve doesn’t help. Blocking the nerve doesn’t help. Removing the portion of the thalamus that relays the information to the brain doesn’t help

III.Stimulating the nerve does help. Electric or manual stimulation of the stump helps tremendously electric more so).

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Phantom Pain: A TheoryI. Recall that the cortex is plastic and may

reorganize.II. Normally this involves annexing juvenile or

unused neurons (indicated by low activity level)

III. In amputation the entire area of say an arm is no longer active in the brain

IV. Other areas attempt to annex these neuronsV. Because the neurons already had a

specialization (e.g., sharp pain) and are no being stimulated by adjacent areas of cortex, the subject feels pain.

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The cortical areas for the face annex the cortical areas for the arm and fingers.

Some of those neurons were previously specialized for pain.

Phantom Pain

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Temperature

ThermoreceptorsI. A uniquely tactile object property II. The rate at which heat is gained or lost between the skin

and an object - we do not detect absolute temperatureIII. Metal objects, fluids etc. create a more extreme

sensation of temperature than do other objects (despite no differences in absolute temperature) because heat energy is transferred more easily to and from them

IV. If a metal and a wooden block are both 150°, the metal block will feel hotter than the wooden block.Likewise for the same blocks at 0° the metal block will feel colder

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Temperature

The Temperature PathwayA. Organization of temperature pathway

Identical to pain pathwayB. Cold receptors coupled to A and CC. Hot receptors coupled to C

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Concluding Remarks

I. Sensory systems exhibit similar organization and function

II. Sensory types are segregated within the spinal cord and cerebral cortex

III. Repeated themesParallel processing of informationColumnar OrganizationCortical Magnification

IV. Perception of objects (haptics) involves the seamless coordination of somatic sensory information

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End of Presentation