1General Sensation

8/2/2019 1General Sensation

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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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1 stimulus inexcitatoryregion:

Feels like 1strong point


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2 stimuli inexcitatoryregions, farapart:

Feels like 2separate strongpoints


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2 stimuli inexcitatoryregions, closetogether (bothinhibiting andexciting eachother)

Feels like 1 point


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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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l Secondary fibers are joined in brainstem byfibers of the trigeminothalamic tract:

(Pain and temperature from face and teeth.)


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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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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



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p


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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


to reticularformation

somato-sensory

cortex


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Impulses ascend to somatosensory cortex via: Spinothalamic pathway (fast pain)

Reticular formation (slow pain)

reticularformation


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/Pain


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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

1General Sensation

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