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Sensory Function of the Nervous
SystemPrepared By Prof.Hussein
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About this Chapter
l What are the senses
l How sensory systems work
l Body sensors and homeostatic maintenance
l Sensing the external environment
l Mechanisms and pathways to perception
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Classification of Sensory Systemby Structural Complexity
Somatic (= general)senses
1. Touch2. Temperature
3. Nociception
4.Itch
5. Proprioception
Special senses
1. Vision
2. Hearing
3. Taste
4. Smell5. Equilibrium
Conscious vs.Unconscious
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Somatic (= general) senses
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General Principles
Sensory system = receptor cells + afferent neurons + part of brain thatprocesses the information.
Sensation = sensory information that reaches the level of consciousness
Perception = an individuals understanding and interpretation of sensory
information
Sensory transduction process by which a stimulus is transformed into an electrical response.
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General Senses
senses associated with skin, muscles, joints, and viscera
three groups
exteroceptivesenses senses associated with body surface;touch, pressure, temperature, pain
visceroceptive senses senses associated with changes in
viscera; blood pressure stretching blood vessels, ingesting ameal
proprioceptive senses senses associated with changes inmuscles and tendons
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Classes of receptors
l Exteroceptors
l Proprioceptors
l Interoceptors
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Exteroceptors
l Detect stimuli near the outer surface of thebody and include those from the skin that
respond to cold, warmth, touch, pressureand vibration. May also include specialreceptors for hearing and vision.
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Exteroceptors
l Mechanoreceptors mechanosensitive ionchannels
l Thermoreceptorsl Nociceptors
l Teloceptors - receptors of electromagnetic
radiationl Chemoceptors (taste and smell)
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Mechanoreceptors
l Pacinian corpuscle vibrationl Meissners corpuscle touch
l Hair-follicle receptor touchl Merkels disk pressurel Tactile disk pressure
l Ruffinis corpuscle pressure
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Proprioceptors
l Located in skeletal muscles, tendons,ligaments and joint capsules. Sensitive to
muscle stretch, muscle tone, position andangle of joints. Provide a sense of bodyposition self receptors.
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Proprioceptors
l Muscle spindle
l Golgi tendon organ
l Joint receptorl Vestibular hair cell
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Interoceptors
l Detect stimuli from inside the body andinclude receptors that respond to pH,
oxygen level in arterial blood, carbondioxide concentration, osmolality of bodyfluids, distention and spasm (e.g., gut), andflow (e.g., urethra)
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Sensory Receptors
l are transducers convert stimuli into graded potential(receptor potential)
l are of various complexity
l react to particular forms of stimuli Chemoreceptors
_____________
_____________
_____________
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Sensory Transduction
l Converts Stimulus into graded potential = receptor potential.
Threshold
lIf receptor potential above threshold AP Adequate Stimulus
l Receptor potential in non-neural receptors change in membranepotential influences NT release
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l Simple receptors
l Complex neural
l Special sensesl Chemoreceptors
l Mechanoreceptors
l Thermoreceptorsl Photoreceptors
Sensory Receptor Types
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Sensory Receptor Types
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General Classes of Sensory receptors:
Mechanoreceptors: - pressure, stretch, touch, blood pressure
- muscle tension
Thermoreceptors: - warm and cold sensations
Photoreceptors: - particular wavelengths
Chemoreceptors: - binding of particular chemicals to receptive
membrane
- smell, taste, blood pH, blood oxygen levels
Nociceptors: - harmful stimuli
- heat or tissue damage
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General PrinciplesSensory receptors are eitherspecialized endings ofafferent neurons, or separate cellsthat signal the afferentneuron.
Transduction in all sensory neuronsinvolve the opening or closing of ion
channels in response to a stimulus.
The influx of ions will lead to agraded potential in the receptorcalled a receptor potential.
The receptor potential may or maynot lead to an action potential.
G l P i i l
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General Principles
Variable stimulusintensity
Produces variableReceptor potentials
Produces variablepatterns of
action potentialsin the CNS
A simplified model showing a possiblerelationship between the intensity of
peripheral stimuli and CNS activity.
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General Principles
Factors that control magnitude of the receptor potentialinclude:
1) Stimulus strength
2) Change of stimulus strength
3) Temporal summation of successive receptor potentials
4) Adaptation: a decrease in receptor activity
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General Principles
Coding- converting sensoryinformation into an action
potential. Depends on:1. Type of energy(modality)
2. Intensity&duration3. Location
Sensory unit- a single afferentneuron with all its receptorendings.
Receptive field- area of bodythat leads to activity of a
particular sensory unit whenstimulated.
Activity in a sensory unit is altered by peripheral events
and communicates information into the CNS.
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General PrinciplesStimulus Intensity:
The number of action potentials generated by a hypothetical, pressure-sensitive,sensory afferent neuron, as shown here, is directly proportional to stimulusintensity. The calling in of receptors on additional afferent neurons is knownas recruitment.
G l P i i l
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General PrinciplesStimulus Location:
Acuity precision with which we can
locate and differentiate one stimulusfrom an adjacent one. Depends on:
1) The amount ofconvergence of neuronalinput in specific
ascending pathways.2) Size of receptive field
covered by a single sensoryunit.
3) Density of sensory units
4) Amount of overlap betweennearby sensory fields.
Sensory afferent neuron (a) has a finer, more spatially limited
receptive field than sensory afferent neuron (b).
G l P i i l
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General PrinciplesAcuity:
The greater amount of convergence,
the less the acuity.
The smaller the receptive field size,the greater the acuity.
G l P i i l
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General Principles
Acuity:
Referred pain is an example of how
convergence leads to a decrease inacuity.
Referred pain results when bothvisceral and somatic afferents
converge on the same neurons in thespinal cord.
G l P i i l
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General PrinciplesAcuity:
The higher the density of receptor
fields and the more overlapping, thehigher the acuity.
The overlapping of receptor fieldsincrease acuity through a process
called lateral inhibition.
G l P i i l
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General Principles
Becauseneuron Bis firing at
thehighest freq.,it inhibits Aand C viainhibitorypathways toa greater
extent thanA and Cinhibit B.
Lateral inhibition sharpens contrast in the pattern of actionpotentials received by the CNS, allowing a finer resolution of
stimulus location.
L t l i hibiti
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Lateral inhibition
General Principles
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General Principles
Lateral inhibition allows theCNS to more accurately locate
the source of stimulation, whichcan help guidenecessary or beneficialresponses.
Lateral inhibition is used inpathways involving the mostaccurate localization, such ashair movements, or retinal
processing for visual acuity.
Poorly localized sensations suchas pain and temperature uselateral inhibition to a lesserdegree.
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Receptive Fields
l Each 1 sensory neuron picks up informationfrom a receptive field
l Often convergence onto 2 sensory neuronsummation of multiple stimuli
l Size of receptive field determines sensitivity to
stimulus Two point discrimination test
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Receptive Fields
l Area of skin whose stimulation results in changesin the firing rate of the neuron. Area of each receptor field varies inversely with the
density of receptors in the region.
l Back and legs have few sensory endings. Receptive field is large.
l Fingertips have large # of cutaneous receptors.
Receptive field is small.
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On receptive fields on the skin
Excitatoryregion
Inhibitoryregion
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On receptive fields on the skin
1 stimulus inexcitatoryregion:
Feels like 1strong point
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On receptive fields on the skin
2 stimuli inexcitatoryregions, farapart:
Feels like 2separate strongpoints
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On receptive fields on the skin
2 stimuli inexcitatoryregions, closetogether (bothinhibiting andexciting eachother)
Feels like 1 point
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On receptive fields on the skin
2 stimuli inexcitatoryregions,intermediatedistance (eachinhibits theother)
Feels like 2 weakpoints
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Relative sensitivity of body
partsl This rationale (measuring the two-point-
threshold) was used to elicit the absolute
sensitivity of different regions of skin.l It allows to infer the spacing of receptivefields and in turn the spatial resolution ofa given skin region for a given body part.
l This was done very early (as early as 1880),using a compass.
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2-point Thresholds on skin
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Receptive Fields of Sensory
Neurons
Figure 10-2
T i t di i i ti
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Two-point discrimination
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Two-Point Touch Threshold
l Minimum distance atwhich 2 points of touchcan be perceived as
separate. Measures of distance
between receptive fields.
l Indication of tactile acuity. If distance between 2 points
is less than minimumdistance, only 1 point will
be felt.
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Sensory pathways
l Sensory systems allow us to detect, analyze andrespond to our environment
l ascending pathwaysl Carry information from sensory receptors to the
brain
l Conscious: reach cerebral cortex
l Unconscious: do not reach cerebral cortex
l Sensations from body reach the opposite side of thebrain
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Receptors
Free Nerve Endings
- Nerve endings without special structural
organization
-pain and temperature receptor
Expanded Tip Endings
- Merkels Touch Corpuscle
Merkel cells in basal layer of epidermis-Type I Hair cells of Vestibular Labyrinth
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Receptors
Encapsulated Endings
- Meissners Corpuscle
- Pacinian Corpuscle(Corpuscle of Vater-Pacini)
- Genital Corpuscle
- Ruffinis Ending
- End Bulb of Krause- Golgi tendon organ:Proprioceptor
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ReceptorEndingsFree nerve
ending
Expanded tip
ending
Encapsulated
ending
Sensory receptors
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Sensory receptors
A: Free nerve endings (pain, temperature)
B: Pacinian corpuscle (pressure)
C: Meissners corpuscle (touch)
D: Muscle spindle (stretch)
A
B C
D
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Ruffini's endings respond to tension and stretch in the skin
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Sensory Receptors
l Perceptions of the world are created by the brainfrom AP sent from sensory receptors.
l Sensory receptors respond to a particular modality(differences in neural pathways and synapticconnections) of environmental stimuli.
l Receptorstransduce(change) different forms of
sensation to nerve impulses that are conducted toCNS.
Functional Categories of Sensory
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Functional Categories of SensoryReceptors
lGrouped according to type ofstimulus energy they transduce. Chemoreceptors:
l Chemical stimuli in environment orblood (pH, C02).
Photoreceptors:l Rods and cones.
Thermoreceptors:l Temperature.
Mechanoreceptors:l Touch and pressure.
Nociceptors:
l Pain.
Proprioceptors:l Body position.
nCategorized according to type ofsensory information delivered tobrain:
n Cutaneous receptors:n Touch, pressure,
temperature,
pain.n Special senses:
n Sight, hearing,equilibrium.
General Principles
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General Principles
Adaptation = a decrease inreceptor sensitivity to a stimulus
resulting in a decrease in theaction potential frequency in anafferent neuron.
Rapidly adapting receptors
Slowly adapting receptors.
**All receptors undergoadaptation to varying degrees
with the exception ofnociceptors.
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Sensory Adaptation
l Tonic receptors: Produce constant rate of
firing as long as stimulusis applied.
l
Pain.l Phasic receptors:
Burst of activity butquickly reduce firingrate (adapt) if stimulusmaintained.
Sensory adaptation:l Cease to pay attention
to constant stimuli.
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Tonic Receptors
l Slow or no adaptation
l Continuous signal
transmission for duration ofstimulus
l Monitoring of parametersthat must be continuallyevaluated, e.g.:
baroreceptors
l Rapid adaptation
lCease firing if strength of acontinuous stimulusremains constant
l Allow body to ignoreconstant unimportantinformation, e.g.:
Smell
Phasic
Receptors
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Law of Specific Nerve Energies
l Sensation characteristic of each sensory neuron isthat produced by its normal or adequate stimulus.
l Adequate stimulus:
Requires least amount of energy to activate a receptor.l Regardless of how a sensory neuron is stimulated,
only one sensory modality will be perceived. Allows brain to perceive the stimulus accurately under
normal conditions.
Generator Potentials
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Generator Potentials
l In response to stimulus,sensory nerve endings
produce a local gradedchange in membrane
potential.
l Potential changes are calledreceptor or generator
potential.
nPhasic response:n Generator potential increases with increased stimulus, then as
stimulus continues, generator potential size diminishes.nTonic response:
n Generator potential proportional to intensity of stimulus.
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SomatosensationTouch
TemperaturePain
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Touch and Pressure Senses
Free nerve endings common in epithelial
tissues simplest receptors
sense itching Ex ( nociciptor)
Meissners corpuscles abundant in hairless portions of
skin; lips detect fine touch; distinguish
between two points on the skin
Pacinian corpuscles common in deeper subcutaneous tissues,
tendons, and ligaments detect heavy pressure and vibrations
Touch and Pressure Receptors
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Touch and Pressure Receptors
Touch (pressure)
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Touch (pressure)
Temperature Senses
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Temperature Senses-Free nerve endings act as thermoreceptors
-There are separate classes of thermoreceptors: Cold fibers and
Warm fibers-Cold fibers respond to a decrease in temperature, while warmfibers respond to an increase in temperature.
Warm receptors sensitive to temperatures above 25oC (77o F) unresponsive to temperature above 45oC (113oF)
Cold receptors
sensitive to temperature between 10oC (50oF) and 20oC (68oF)
Pain receptors (nociceptor) respond to temperatures below 10oC respond to temperatures above 45oC
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Thermoreceptors
l Both cold and warm fibers dont respond tomechanical pressure.
l Preferred temperatures of cold fibers range from
20 C to 45 C, while preferred temperatures ofwarm fibers range from 30 C to 48 C.
l This response to a preferred temperature is a
sustained response, it doesnt adapt like theadditional increase/decrease response discussedpreviously.
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Thermoreceptor tuning curves
l Explains paradoxical cold
l Jumping into a very hot bath transiently activatesboth hot and cold fibers.
l For a short time, the bath feels both hot and cold
Cold
Hot
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Pain as a special perceptual
qualityl A quality that has complex phenomenological
facets (behavioral, sensory, emotional)l
Pain perception can be modulated by all kindsof factors, including behavioral states (stress,sex), cognitive states (hypnosis), mental states(trance), social norms and drugs.
l Its significance for society is enormous. E.g.:Should fishing be legal?l Evolutionary significance to organism obvious.
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Pain is not a purely sensory
experiencel It shares components of an emotion (distress
that is associated with pain).
l But it also shares sensory characteristics. Itsignals the presence of stimuli that are harmfulto the organism.
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Pain receptors
l Free nerve endings in the skin
l They are connected to various fibers:
l A small, myelinated. High conductance speed
l C small, unmyelinated. Slow conductance speed
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Pain receptors
l A fast, sensitive to mechanical noxious stimuli.
l C slow, sensitive to many noxious stimuli(chemical, etc.)
l This distinction has been used to explain thephenomenon ofdouble-pain:
l One noxious stimulus causes first a quick, sharp
pain (mediated by A fibers) and is followed by adull and burning pain (mediated by C fibers)
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Ascending Pathways
l For conscious perception:
Spinothalamic system
Medial Lemniscal systeml For unconscious perception:
Spinocerebellar
Spino-olivary
Spinotectal
Spinoreticular
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Sensory pathways: 3 neurons
l 1st: enters spinal cord from periphery
l2nd: crosses over (decussates), ascends
in spinal cord to thalamus
l 3rd: projects to somatosensory cortex
1 Spinothalamic pathway
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1. Spinothalamic pathway
l Carries pain, temperature,touch and pressure signals
l 1st neuron enters spinalcord through dorsal root
l 2nd neuron crosses over inspinal cord; ascends tothalamus
l 3rd neuron projects fromthalamus to somatosensorycortex
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spinothalamicpathway
Spinothalamic Pathway
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Small sensory fibres:
Pain, temperature, sometouch
Primary somatosensorycortex (S1)
Thalamus
Medulla
Spinal cord
Spinothalamictract
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Spinothalamic System
l Lateral spinothalamic tract
l Anterior spinothalamic tract
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Lateral Spinothalamic Tract
l Carries pain and temperature
l Primary fibers ascend or descend 1-2 spinal
cord segments before synapsing withsecondary fibers.
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Lateral Spinothalamic Tract
l Secondary fibers are joined in brainstem byfibers of the trigeminothalamic tract:
(Pain and temperature from face and teeth.)
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Lateral Spinothalamic Tract
l Secondary fiber collaterals project toreticular formation:
Stimulate wakefulness and consciousness.l Secondary fibers project to ventral
posterolateral (VPL) nucleus of thalamus.
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Lateral Spinothalamic Tract
l Secondary fibers synapse with tertiaryfibers in VPL.
l Tertiary fibers (corticopetal fibers) synapsein postcentral gyrus:
Somatic sensory areas 3, 1, 2
l Tertiary fibers form part of internal capsule.
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Anterior Spinothalamic Tract
l Carries light touch (crude touch), pressure,tickle, itch
lPrimary neurons may ascend 8-10 spinalcord segments before synapsing withsecondary neurons.
l Secondary fibers decussate in anterior gray
or white commissures.
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Anterior Spinothalamic Tract
l Secondary fibers ascend to synapse withtertiary fibers in VPL nucleus of thalamus.
lTertiary fibers ascend through internalcapsule to primary sensory cortex.
Trigeminothalamic Tract
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Modality: General Sensation from face
Receptor: Most receptors in the face
1st Neuron: Trigeminal (Semilunar) Ganglia
2nd Neuron: Trigeminal Sensory Nucleus pain & temperature ---- pars caudalis of
spinal tract nucleus of V
Ventral and Dorsal Trigeminothalamic Tract
3rd Neuron: Thalamus (VPM)Internal Capsule ----- Corona Radiata
Termination: Primary Somesthetic Area (S I)
g
T i i th l i
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TrigeminothalamicTract
A. trigeminal ganglionB. trigeminal sensory
nucleus
C. thalamus (VPM)
D. cerebral cortex (S I)
1. spinal tract of
trigeminal nerve
2. ventral
trigeminothalamic tract
3. dorsal
trigeminothalamic tract4. corona radiata
V. trigeminal nerve
Spinothalamic damage
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p g
spinothalamic pathway
Leftspinal cord injury
Loss of sense of:TouchPainWarmth/coldin right leg
2. Dorsal column pathway
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p y
l Carries fine touch, vibrationand conscious proprioceptionsignals
l 1st neuron enters spinal cord
through dorsal root; ascendsto medulla (brain stem)
l 2nd neuron crosses over inmedulla; ascends to thalamus
l 3rd neuron projects tosomatosensory cortex
Two-Point Discrimination
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Two Point Discrimination
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dorsalcloumnpathway
Dorsal column pathway
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Large sensory nerves:
Touch, vibration, two-pointdiscrimination, proprioception
Primary somatosensorycortex (S1) in parietallobe
Thalamus
Medulla
Mediallemniscus
Spinal cord
Dorsal column
Dorsalcolumnnuclei
Dorsaldorsal columnpathway
Leftspinal cord injury
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Dorsalcolumndamage
pathway
Loss of sense of:touchproprioceptionvibrationin left leg
Dorsal column damage
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Dorsal column damage
l Sensory ataxia
l Patient staggers; cannotperceive position ormovement of legs
l Visual clues help movement
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Central
Pathways
3. Spinocerebellar pathway
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3. Spinocerebellar pathway
l Carries unconscious
proprioception signalsl Receptors in muscles &
joints
l1st neuron: enters spinalcord through dorsal root
l 2nd neuron: ascends tocerebellum
l No 3rd neuron to cortex,hence unconscious
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Anterior Spinocerebellar Tract
l Originates in lower trunk and lower limbs.
l Consists of crossed fibers that recross in
pons and enter cerebellum through superiorcerebellar peduncles.
l Transmits ipsilateral proprioceptiveinformation to cerebellum.
Posterior Spinocerebellar
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Posterior Spinocerebellar
Tractl Originates in thoracic and upper lumbar
regions.
l Consists of uncrossed fibers that entercerebellum through inferior cerebellarpeduncles.
l Transmits ipsilateral proprioceptive
information to cerebellum.
Spinal Cord Ascending Tracts
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Spinocerebellar Tract
Modality: Unconscious Proprioception
Receptor: Muscle spindle, Golgi tendon organ
1st Neuron: Dorsal Root Ganglion
Posterior Root , [Posterior Column]2nd Neuron: 1. Clarkes column
Posterior Spinocerebellar Tract
2. Accessory Cuneate NucleusCuneocerebellar Tract
3. Posterior HornAnterior Spinocerebellar r Tract
Termination: Cerebellar Cortex
p g
Spinocerebellar Tract
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p
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Spinocerebellar Tract
Posterior SCbllT
Inferiorcerebellarpeduncle
Post. SCbllT andcuneocerebellartract
posteriorwhite column
posterior root
Anterior SCbllT
superiorcerebellarpeduncle
anteriorspinocerebellartract
anterior whitecommissure
posterior root
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Spino-Olivary Tracts
l Project to accessory olivary nuclei andcerebellum.
l Contribute to movement coordinationassociated primarily with balance.
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Spinotectal Tracts
l Project to superior colliculi of midbrain.
l Involved in reflexive turning of the head
and eyes toward a point of cutaneousstimulation.
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Spinoreticular Tracts
l Involved in arousing consciousness in thereticular activating system throughcutaneous stimulation.
Spinocerebellar tract damage
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Spinocerebellar tract damage
l Cerebellar ataxia
l Clumsy movements
l Incoordination of the limbs (intentiontremor)
l Wide-based, reeling gait (ataxia)
l Alcoholic intoxication produces similar
effects!
4.Somatosensory cortex
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Located in the postcentral gyrus of the
human cerebral cortex.
Spatial orientation of signals.1) Each side of
the cortex
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the cortex
receives
sensoryinformation
exclusively
from theopposite side of
the body
(the exception:the same side
of the face).
Spatial orientation of signals.2)The lips, faceand thumb are
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and thumb are
represented by
large areas in thesomatic cortex,
whereas the trunk
and lower part ofthe body,
relatively small
area.
3)The head in the most lateral portion, and the
lower body is presented medially
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Pain
Pain is an unpleasant sensory and emotional
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Pain is an unpleasant sensory and emotionalexperience associated with actual or potential
tissue damage or described in terms of suchdamage
International Association for the Study of Pain
Why feel pain?
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Why feel pain?
l Gives conscious awareness of tissuedamage
l Protection:
Remove body from danger Promote healing by preventing further damage
Avoid noxious stimuli
l Elicits behavioural and emotional responses
1.
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free nerve endings inskin respond tonoxious stimuli
Nociceptors
Nociceptorsi i l h d l
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l Nociceptors are special receptors that respond onlyto noxious stimuli and generate nerve impulses
which the brain interprets as "pain".
Nociopector
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Adequate Stimulation
Temperature Mechanical damage
Chemicals (released fromdamaged tissue)
Bradykinin, serotonin, histamine, K+,
acids, acetylcholine, and proteolyticenzymes can excite the chemical type ofpain.
Prostaglandins and substance Penhance the sensitivity of pain endingsbut do not directly excite them.
s
H l i
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Hyperalgesia:
The skin, joints, or muscles that have already beendamaged are unusually sensitive. A light touch to adamaged area may elicit excruciating pain;
Primary hyperalgesia occurs within the area ofdamaged tissue;
Secondary hyperalgesia occurs within the tissuessurrounding a damaged area.
2. Localization of PainS fi i l S ti P i i f ki
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Superficial Somatic Pain arises from skin areas
Deep Somatic Pain arises from muscle, joints, tendons& fascia
Visceral Pain arises from receptors in visceral organs localized damage (cutting) intestines causes no pain
diffuse visceral stimulation can be severe distension of a bile duct from a gallstone
distension of the ureter from a kidney stone
3. Fast and Slow Pain
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l Most pain sensation is a combination of the twotypes of message. If you prick your finger you first feel a sharp pain
which is conducted by the A fibres, and this is followed by a dull pain conveyed along C
fibres.
Comparison of Fast and Slow Pain------ Spinothalamic Tract
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Fast Pain Slow Pain
Sharp, pricking Dull, burning
Group III (A ) fiber Group IV (C) fiber
Short latency Slower onsetWell localized Diffuse
Short duration Long duration
Less emotional Emotional, autonomic response
Not blocked by morphine Blocked by morphine
Neospinothalamic Tract Paleospinothalamic Tract
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Comparison ofFast and Slow Pain
Neospinothalamic Tract
Paleospinothalamic tract - spinoreticular tract
- spinoreticulothalamic
tract
N STT P l STT
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Spinothalamic Tract& Spinoreticular Tract
Widespread
cortical region
CL (intralaminarthalamic nuclei)
reticulothalamicpathways
spinoreticulartract
Primary Motor
Area (M I)
VPLc (ventrobasalnuclear complex)
(spinal lemniscus)
spinothalamictract
thalamus
reticularformation
NeoSTT PaleoSTT
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Impulses transmitted to spinal cord by Myelinated A nerves: fast pain (80 m/s)
Unmyelinated C nerves: slow pain (0.4 m/s)
nociceptor
nociceptor
A nerve C nerve
spinothalamicpathway
to reticularformation
somato-sensory
cortex
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Impulses ascend to somatosensory cortex via: Spinothalamic pathway (fast pain)
Reticular formation (slow pain)
reticularformation
spinothalamicpathway
thalamus
4. Visceral pain
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Notable features of visceral pain:
Often accompanied by strong autonomicand/or somatic reflexes
Poorly localized;
may be referred
Mostly caused by distension of holloworgans or ischemia (localized mechanicaltrauma may be painless)
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Afferent innervation of the viscera.
Often anatomical separation nociceptiveinnervation (in sympathetic nerves) from non-nociceptive (predominantly in vagus).
Many visceral afferents are specialized
nociceptors, as in other tissues small (Ad and C)fibers involved.
Large numbers of silent/sleeping nociceptors,awakened by inflammation.
Nociceptor sensitization well developed in allvisceral nociceptors.
Referred pain
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p
l Pain originating fromorgans perceived ascoming from skin
l Site of pain may bedistant from organ
Convergence theory:This type of referred pain occurs becauseboth visceral and somatic afferents often
Referred
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both visceral and somatic afferents oftenconverge on the same interneurons in thepain pathways.
Excitation of the somatic afferent fibers isthe more usual source of afferentdischarge,
so we refer the location of visceralreceptor activation to the somatic source
even though in the case of visceral pain.
The perception is incorrect.
The convergence ofnociceptor input from theviscera and the skin.
pain
5.Control of Pain Transmission(Pain Modulation)
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1. Ascending controlGate control theoryof Wall and Melzak (1965)
Substantia gelatinosa Rolandi
2. Descending Control
Stimulation produced analgesia (SPA)
Raphe Magnus ------------ Raphespinal tract (Serotonin)
Periaqueductal Gray ----- fibers to raphe magnus
Lateral Tegmental Nucleus (Epinephrine)
Substantia gelatinosa Rolandi
enkephalin cf. endorphine, morphine
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Spinal and Brain Stem
Control of Pain
Descending pathways regulating thetransmission of pain information:i t it f i i i di id l d d d
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intensity of pain varies among individuals and depends oncircumstances (i.e. soldier wounded, athlete injured, duringstress).
Stimulation of PAG causes analgesia so profound thatsurgery can be performed.
PAG stimulation can ameliorate intractable pain. PAGreceives pain information via the spinomesencephalic tractand inputs from cortex and hypothalamus related tobehavioral states and to whether to activate the pain controlsystem. PAG acts on raphe & locus ceruleus to inhibit dorsalhorn neurons via interneurons and morphine receptors.
Application: Intrathecal morphine pumps
Analgesia -Morphine and Enkephalin
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opium
Endogenous body mechanisms forreducing pain
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reducing painl There are descending neurons that modulate the
perception of pain.l Many of these neurons originate in nuclei in the
brainstem and pass through the periaqueductal gray(PAG) area of the midbrain.
l The body possesses an additional mechanism tocontrol pain: the release of endogenous opioids,especially at the level of the PAG.
l There are neurons that release enkephalins,endorphins, and dynorphins at the PAG, and in thisway modulate its ability to modulate painperception.
l Other neurons can release their endogenous opioidsat the source of the pain, as well.
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l If this occurs, the transmission of pain informationfrom the nociceptors to the secondary neurons isblocked, and no pain is felt.
l Unfortunately, these endogenous mechanisms areoften damaged and unfunctional in people sufferingfrom allodynia, so the application of
pharmaceuticals is needed.
Allodynia
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l Allodynia, meaning "other pain", is apain due to a
stimulus which does not normally provoke pain andcan be either static or mechanical.
l Allodynia differs from referred pain, but can occurin areas other than the one stimulated.
l It is dysesthetic.
l Allodynia is different from hyperalgesia, an extremereaction to a stimulus which is normally painful.
Allodynia Typesl Th diff t ki d t f ll d i
http://en.wikipedia.org/wiki/Painhttp://en.wikipedia.org/wiki/Stimulus_(physiology)http://en.wikipedia.org/wiki/Referred_painhttp://en.wikipedia.org/wiki/Dysesthesiahttp://en.wikipedia.org/wiki/Hyperalgesiahttp://en.wikipedia.org/wiki/Hyperalgesiahttp://en.wikipedia.org/wiki/Dysesthesiahttp://en.wikipedia.org/wiki/Referred_painhttp://en.wikipedia.org/wiki/Stimulus_(physiology)http://en.wikipedia.org/wiki/Pain8/2/2019 1General Sensation
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l There are different kinds or types of allodynia:
l Mechanical allodynia (also known as tactileallodynia) Static mechanical allodynia pain in response to light
touch/pressure
Dynamic mechanical allodynia pain in response tobrushing
l Thermal (hot or cold) allodynia pain from
normally mild skin temperatures in the affected area
Pain Gate Theory
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Melzack & Wall (1965)
A gate, where pain impulses can be gated
The synaptic junctions between the peripheral nociceptorfiber and the dorsal horn cells in the spinal cord are the
sites of considerable plasticity.
A gate can stop pain signals arriving at the spinal cordfrom being passed to the brain
Reduced pain sensation
Natural pain relief (analgesia)
Gate Control Hypothesis:Wall & Melzack 1965
Hypothesized interneurons
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activated by A-beta fibers actas a gate, controlling primarily
the transmission of painstimuli conveyed by C fibers tohigher centers.
i.e. rubbing the skin near thesite of injury to feel better.
i.e. Transcutaneous electricalnerve stimulation (TENS).
i.e. dorsal column stim.
i.e. Acupuncture
How does pain gate work?
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How does pain gate work?
The gate = spinal cord interneurons thatrelease opioids.
The gate can be activated by:
Simultaneous activity in other sensory (touch)
neurons Descending nerve fibers from brain
Applications of pain gate
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Applications of pain gate
Stimulation of touch fibres for pain relief: TENS (transcutaneous electrical nerve stimulation)
Acupuncture
Massage
Release of natural opioids Hypnosis
Natural childbirth techniques
6 Pain Relief
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6. Pain Relief
l Aspirin and ibuprofen block formation ofprostaglandins that stimulate nociceptors
l Novocain blocks conduction of nerveimpulses along pain fibers
l Morphine lessen the perception of pain inthe brain.
Predominant Sensory Syndromes
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Herpes Zoster- inflammatory reactions of dorsal root ganglion
- severe pain on the dermatomes of affected ganglion
followed by rashes and vesicle
Tabes Dorsalis
- common variety of neurosyphilis- posterior colum and spinal posterior root lesion
- loss of discriminative touch sensation and
consciousproprioception below the level of lesion
- posterior column ataxia- lancinating pain
- loss of deep tendon reflex (DTR)
Herpes Zoster
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(Shingles) varicella-zoster virusreactivation fromthe dorsal root ganglia
unilateral vesiculareruption withina dermatome
T3 to L3 dermatome
lesions are frequent
zoster ophtahalmicus(ophthalmic division
of trigeminal n., V1)
Ramsey-Hunt syndrome(sensory br. of VII)
acyclovir, antiviral agent
Herpes Zoster(Shingles) Zoster ophthalmicus
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(Shingles) Zoster ophthalmicus
Tabes Dorsalis (Neurosyphilis)
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posterior column lesion - sensory symptoms, posterior column ataxia
Posterior Column
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Tabes Dorsalis
Subacute Combined
Degeneration
Ataxic Gait and Position:
- Watches the groundin walking
- Positive Romberg Test- Stick and Stamp Sign
Posterior Column
Ataxia
Syringomyelia
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initial symptom - bilateral loss of pain and temperature sensation
Syringomyelia
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Syringomyelia
initial symptom - bilateral loss of pain and temperature sensation
T i i l N l i
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Trigeminal Neuralgia(tic douloureux)
- excruciating episodic pain
in the area supplied by
trigeminal nerve, especially
second and third division
- trigger point- intense pain makes the
patient grimace (tic)
- antiepileptic drug (phenytoin,
carbamazepine) is effective
- surgical treatment
The effect of Painl Pain is a form of stress it increases sympathetic
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l Pain is a form of stress it increases sympatheticstimulation and the release of adrenaline which cancause:-- increased heart rate- increased cardiac output
- increased blood pressure- Increased breathing rate (hyperventilation, which can
lead to maternal alkalosis)
- Vasoconstriction causing decreased cerebral and uterine
blood flow- Nausea and vomiting due to delayed stomach emptying