cap II_curs 5 _6_ Somatosensory system_2014_2015_.pdf

Lect. univ. dr. Loredana - Cristina MEREUȚĂLaboratory of Biophysics & Med. Physics, Faculty of Physics,

'Alexandru Ioan Cuza' University of Iasi

I. General principles of sensory physiology

II. The somatosensory SystemIII. Chemical Senses: Taste and OlfactionIV. VisionV. Hearing and Equilibrium

SOMATIC SENSESThe somatic sensory system has two major

components:I. General somatic – include touch, pain, vibration,pressure, temperatureII. Proprioceptive – detect stretch in tendons andmuscle provide information on body position,orientation and movement of body in space.

Together, these two subsystems give humansand other animals the ability to identify the shapes andtextures of objects, to monitor the internal and externalforces acting on the body at any moment, and to detectpotentially harmful circumstances.

The somatosensory system is subdivided into:

1. Exteroreceptors - these are the familiar receptorsfound in the skin that mediate the sub-modalities oftouch, pain and temperature.

These types of sensory input can mediate bothrapid responses (e.g. reflexes) and reach the cerebralcortex and induce perception.

2. Interoreceptors - are found in internal organs andconvey signals that include distension of the stomach,carbon dioxide concentration in the blood etc. This typeof sensory input is obviously of great importance butdifficult to study experimentally (in the CNS).

The somatosensory system is subdivided into:

3. Proprioceptors.Proprioceptor afferents are found in muscles and

at joints; they mediate the detection of muscle stretchand the degree of extension at a joint.

For the most part, this type of sensory input isinvolved in motor control rather than perception.

The two major classes of proprioceptive inputcome from:(a) Golgi tendon organs(b) muscle spindles.

The components of thesomatic sensory systemprocess information conveyedby mechanical stimuli thatimpinge upon the body surfaceor that are generated within thebody itself (proprioception).

This processing isperformed by neuronsdistributed across several brainstructures that are connectedby both ascending anddescending pathways.

All Somatic Sensation Begins with Receptors and Ganglion Cells

First-order neurons arelocated in the dorsal rootand cranial nerve ganglia.Second-order neurons

are located in brainstemnuclei.Third-order neurons are

found in the thalamus,from where they projectto the cerebral cortex.


This activity is conveyed centrally via a chain ofneurons, referred to as the first-, second-, and third-order cells.


Transmission of afferent mechanosensoryinformation from the periphery to the brain begins witha variety of receptor types that initiate actionpotentials.

These pathways are topographically arrangedthroughout the system, the amount of cortical andsubcortical space allocated to various body parts beingproportional to the density of peripheral receptors.

The afferent fibers from the receptors for each ofthese sensations are projected to the dorsal horn of thespinal cord where they synapse with the dorsal rootganglion. From there the fibers split into two pathways:(a) The mediallemniscal tracttravels in the dorsalcolumn and carriestactile sensationand proprioceptiveinformation to thethalamus and thento thesomatosensorycortex.

Ascending Pathways

(b)The spinothalamic tract travels contralaterally up thespinal cord in the anterolateral column and carries painfuland thermal sensations to the thalamus and finally to thesomatosensory cortex.

Ascending Pathways

The sensory informationis kept somatotopicalyorganized throughout the entirepathway for each of the tracts.

Somatotopic map

(A) The human somatotopic map first defined in the1930s has remained generally valid.

(B) Diagram along theplane in (A) showing thesomatotopicrepresentation of bodyparts from medial tolateral.(C) Homunculus - theamount of somaticsensory cortex devotedto the hands and face ismuch larger than therelative amount of bodysurface in these regions.

In humans, thecutaneous area of eachdermatome has beendefined in patients inwhom specific dorsalroots were affected orafter surgicalinterruption (for relief ofpain or other reasons).Such studies show thatdermatomal maps varyamong individuals.

The innervation arising from a single dorsal rootganglion and its spinal nerve is called a dermatome.

Despite these limitations, knowledge ofdermatomes is essential in the clinical evaluation ofneurological patients, particularly in determining thelevel of a spinal lesion.

Dermatomal maps

Sagittal view of the spinal cord showing the origin of nerves corresponding to each of the dermatomes

Despite their variety, all somatic sensory receptorswork in fundamentally the same way:

Stimuli applied to the skin deform or otherwisechange the nerve endings, which in turn affects theionic permeability of the receptor cell membrane.

Changes in permeability generate a depolarizingcurrent in the nerve ending, thus producing a receptor(or generator) potential that triggers action potentials.

This overall process, in which the energy of astimulus is converted into an electrical signal in thesensory neuron, is called sensory transduction and isthe critical first step in all sensory processing.

How it works

The quality of a mechanosensory (or any other)stimulus (i.e., what it represents and where it is) isdetermined by the properties of the relevant receptorsand the location of their central targets.

The quantity or strength of the stimulus isconveyed by the rate of action potential dischargetriggered by the receptor potential (although thisrelationship is nonlinear and often quite complex).

Some receptors fire rapidly when a stimulus is firstpresented and then fall silent in the presence ofcontinued stimulation (which is to say they “adapt” tothe stimulus), whereas others generate a sustaineddischarge in the presence of an ongoing stimulus.

How it works

The usefulness of having some receptors thatadapt quickly and others that do not is to provideinformation about both the dynamic and staticqualities of a stimulus.

Receptors that initially fire in the presence of astimulus and then become quiescent are particularlyeffective in conveying information about changes in theinformation the receptor reports; conversely, receptorsthat continue to fire convey information about thepersistence of a stimulus.

How it works

Rapidly adapting, or phasicreceptors respond maximallybut briefly to stimuli; theirresponse decreases if thestimulus is maintained.Conversely, slowly adapting,or tonic receptors keep firingas long as the stimulus ispresent.

Accordingly, somatic sensory receptors and theneurons that give rise to them are usually classified intorapidly or slowly adapting types.

How it works

An applied pressure on the skin (e.g. pushing on akey on a computer keyboard, or sitting on a hardsurface) distorts the concentric layers. This results indeformation of the cell membrane of the sensoryneurone. Deformation opens the ion channels in themembrane (changes the shape of the protein slightly) soNa+ ions pass into the neurone.

How it works

This produces a depolarisation called a generatorpotential of about 1mV across the membrane. If thegenerator potential exceeds the threshold it triggers thegeneration of action potentials at the first node ofRanvier, nerve impulses are then transmitted along thelength of the sensory neurone to the CNS.

How it works

When the pressure stimulus is removed thecorpuscle resumes its normal shape producing anothertransitory deformation of the receptor membrane in theprocess and another brief generator potential, which willalso generate action potentials to indicate the pressure,has been removed.

The size of the generator potential isproportional to the amount of opening of the ionchannels, which is proportional to the amount ofdeformation, which is proportional to the intensity ofthe stimulus, so the size of the generator potential isproportional to the intensity of the stimulus (called agraded response).

How it works

The number of action potentials generated isproportional to the size of the generator potential, thestronger the stimulus the bigger the generator potentialthe more action potentials are generated (a frequencycoded response).

How it works

Sensory Information Is Transmitted Along Labeled Lines

In one particular population of somatosensoryneurons, activity is always interpreted by the CNS as apainful stimulus, no matter whether the stimulus isnatural (a sharp instrument) or artificial (electricalstimulation of the appropriate axons). An entirelyseparate population of neurons (colored blue) wouldsignal light pressure.Why this is so can be seenfrom the fact that receptorsare selective not only in whatdrives them, but also in thepostsynaptic targets withwhich they communicate.


Each ganglion cell transmits its activity into a well-defined region of the CNS, after which a strictlyorganized series of synaptic connections relaysinformation in a sequence that eventually leads to thethalamus and then to the cerebral cortex.

It is this orderly relayfrom receptor to ganglion cellto central neurons at each ofseveral stations that makes upa labeled line.


All sensory information arising from a single classof receptors is referred to as a modality (e.g., thesensations of pain and light pressure involve distinctmodalities).

Thus, theexistence of labeledlines means thatneurons in sensorysystems carryspecific modalities.

Somatic Receptors are divided into two groups:I. Free or Un-encapsulated nerve endings- are abundant in epithelia and underlying connectivetissue (nociceptors and thermoreceptors); Twospecialized types of free nerve endings:

Merkel discs –lie in theepidermis, slowlyadaptingreceptors for lighttouch

Hair folliclereceptors –Rapidly adaptingreceptors thatwrap around hairfollicles

Somatic Receptors are divided into two groups:II. Encapsulated nerve endings - consist of one ormore neural end fibers enclosed in connective tissue: Meissner’s corpuscles; Pacinian corpuscles; Ruffini’s

corpuscles Proprioceptors:

Muscle spindles – monitors thechanging length of a muscle,imbedded between musclefascicles

Golgi tendon organs – locatednear the muscle-tendon junction,monitor tension within tendons

Joint kinesthetic receptors -sensory nerve endings within thejoint capsules, sense pressure andposition.

A comprises thelargest and fastestaxons,

C the smallestand slowest.

Mechanoreceptoraxons generally fallinto category A.

In the 1920s and 1930s, there was a virtualindustry classifying axons according to theirconduction velocity. Three main categories werediscerned, called A, B, and C.

The A group is further broken down into threesubgroups designated:

alpha α (the fastest) betha β delta δ (the slowest)

Classifying axons according to their conduction velocity.

To make matters even moreconfusing, muscle afferentaxons are usually classifiedinto four additionalgroups: I (the fastest),II,III,and IV (the slowest)—withsubgroups designated bylowercase roman letters!

Somatic Receptors

There are many forms of exteroreceptors.

Based on function, this variety of somatic receptorscan be divided into three groups:

1. Mechanoreceptors – respond to mechanic stimuli

2. Nociceptors - respond to pain

3. Thermoceptors - respond to temperature

Somatic Receptors

The receptors that transduce light or sound arehighly specialized cells; in contrast, exteroreceptorsare merely nerve endings in the skin.

The most important distinction is between:

a. Touch or mechano-receptors - the nerveendings are usually associated with some specializedstructures such as hairs. The axons of touch fibers areheavily myelinated allowing them to conduct actionpotentials rapidly. The response of touch fibersdepends on their associated structures.

b. Pain and temperature receptors. The nerveendings of pain and temperature receptors are simpleand the axons are either unmyelinated (C type) orlightly myelinated.

Somatic Receptors

Simulated activity patterns in differentmechanosensory afferents as Braille is read.

The Skin Harbors Morphologically DistinctMechanoreceptors

Mechanoreceptors detect mechanical energy,typically consist of ion channels linked to external cellstructures (i.e. hairs) & internal structures (i.e.cytoskeleton). Bending/stretching plasma membranechanges permeability to sodium & potassium ions.

Four major types of mechanoreceptors arespecialized to provide information to the central nervoussystem about touch, pressure, vibration, andcutaneous tension:

Meissner’s corpuscles;

Pacinian corpuscles;

Merkel’s disks;

Ruffini’s corpuscles.

The Skin Harbors Morphologically DistinctMechanoreceptors

These receptors are referred to collectively aslow-threshold (or high-sensitivity) mechano-receptors because even weak mechanical stimulationof the skin induces them to produce action potentialsand they are innervated by relatively large myelinatedaxons (type Aβ), ensuring the rapid central transmissionof tactile information.

Mechanoreceptors

Touch ReceptorsFree nerve endings – Simplest mechanoreceptors

are in skin; – touch pressure & pain (nociceptors)More specialized tactile receptors

– Merkel Discs- touch and pressure– Meissner corpuscles- Light pressure– Ruffini corpuscles- Heavy pressure– Pacinian corpuscles- subcutaneous- vibration

Other Mechanoreceptors Proprioreceptors

– Muscle spindles- Stretch receptors– Golgi tendon organs- Force generated bymuscle

Blood Pressure – Measured at carotid sinus andaortic arch; Baroreceptors- Tension in walls ofvessels

There are differences in mechanosensorydiscrimination across the body surface.

The accuracy of our sense of touch is not the sameall over the body.

Fingers can distinguish things 2 mm apart, forearms40 mm apart.

Mechanosensory receptors are more numerous infinger tips and have smaller receptive fields.

Sensitivity of Tactile Discrimination Varies withLocation on the Body Surface

Tactile Discrimination

Sensitivity ofTactileDiscrimination Varieswith Locationon the BodySurface

Tactile Discrimination

Cutaneous Receptors1. Touch Receptors: fine touch

a. Meissner’s corpuscle – fine touch,discrimination; found concentrated in places where youneed to have a lot of responsiveness to a little input.

b. Merkel disks - found deep at the junction of theepidermis and dermis.

c. Root hair plexus - at the base of hair follicles.

Cutaneous Receptors2. Touch Receptors: pressure sensitive

a. Ruffini’s endings and Krause's end bulbs –encapsulated pressure sensors, dermis (andelsewhere), respond to continuous pressure

b. Pacinian corpuscles – deep pressure sensors,onion shaped capsule (layers of Schwann cells enclosedin a connective tissue membrane), respond to on-offpressure or vibration.

3. Temperature - Free nerve endings, someresponsive to heat and others responsive to cold

4. Pain - Free nerve endings, respond to chemicalsreleased from damaged tissues.

Lie between the dermal papillae just beneath theepidermis of the fingers, palms, and soles. Meissner’scorpuscles are the most common mechanoreceptors of“glabrous” (smooth, hairless) skin (the fingertips, forinstance), and their afferent fibers account for about40% of the sensory innervation of the human hand.

Encapsulated Nerve Endings-Meissner’s corpuscles

These corpuscles areparticularly efficient intransducing informationabout the relatively low-frequency vibrations (30–50 Hz) that occur whentextured objects aremoved across the skin.

Are elongated receptors formed by a connectivetissue capsule that comprises several lamellae ofSchwann cells. The center of the capsule contains oneor more afferent nerve fibers that generate rapidlyadapting action potentials following minimal skindepression.

Encapsulated Nerve Endings Meissner’s corpuscles

When a force isapplied to the dermalpapilla containing theMeissner corpuscle, thelaminar cells in thecorpuscle slide past oneanother.

How it works

This shearing force distorts the membranes of theaxon terminals located between the laminar cells, whichdepolarizes the axon terminals. If the force is sustained onthe dermal papilla, the laminar cells remain in their displacedpositions and no longer produce a shearing force on theaxon terminals. Consequently, a sustained force on thedermal papilla is transformed into a transient force on theaxon terminals of the Meissner corpuscle.

Encapsulated Nerve Endings Meissner’s corpuscles How it works

The 1° afferent axonresponse of a Meissnercorpuscle is rapidlyadapting and actionpotentials are only generatedwhen the force is first applied.

Example of a simple receptor - Paciniancorpuscles - are large encapsulated endings located inthe subcutaneous tissue (and more deeply ininterosseous membranes and mesenteries of the gut).

These receptors differ from Meissner’s corpuscles intheir morphology, distribution, and response threshold.

They makeup 10–15%of thecutaneousreceptors inthe hand.

The Pacinian corpuscle has an onion-likecapsule in which the inner core of membrane lamellae isseparated from an outer lamella by a fluid-filled space.One or more rapidly adapting afferent axons lie at thecenter of this structure.

The capsule acts as a filter, in this case allowingonly transient disturbances at high frequencies (250–350 Hz) to activate the nerve endings.

How it works

Because they arerapidly adapting, Paciniancorpuscles, likeMeissner’s corpuscles,provide informationprimarily about thedynamic qualities ofmechanical stimuli.

Pacinian corpuscles adapt more rapidly than Meissner’scorpuscles and have a lower response threshold. Theseattributes suggest that Pacinian corpuscles are involved inthe discrimination of fine surface textures or other movingstimuli that produce high-frequency vibration of the skin.In corroboration of this supposition, stimulation of Paciniancorpuscle afferent fibers in humans induces a sensation ofvibration or tickle.

How it works

Slowly adapting cutaneous mechanoreceptorsinclude Merkel’s disks and Ruffini’s corpuscles

Merkel’s disks are located in the epidermis,where they are precisely aligned with the papillae that liebeneath the dermal ridges.

They account forabout 25% of themechanoreceptorsof the hand and areparticularly dense inthe fingertips, lips,and externalgenitalia.

The slowly adapting nerve fiberassociated with each Merkel’s diskenlarges into a saucer-shaped endingthat is closely applied to anotherspecialized cell containing vesiclesthat apparently release peptides thatmodulate the nerve terminal. Selectivestimulation of these receptors inhumans produces a sensation of lightpressure. These several propertieshave led to the supposition thatMerkel’s disks play a major role in thestatic discrimination of shapes,edges, and rough textures.

How it works

The Merkel cell is coupled to the surrounding tissueand cannot shift its position relative to the surroundingtissue. Consequently, a force applied to the overlyingskin, distorts the Merkel cell for the duration of theapplied force. The distortion of the Merkel cell results inthe release of a stream of neuropeptides at itssynaptic junctions with the 1° Merkel disk.

As a result theaction potentialdischarges producedby the Merkelcomplex 1° afferentis slowly adapting.

How it works

Ruffini’s corpuscles, although structurally similar toother tactile receptors, are not well understood. Theseelongated, spindle-shaped capsular specializations arelocated deep in the skin, as well as in ligaments andtendons. The long axis of the corpuscle is usuallyoriented parallel to the stretch lines in skin.

Thus, Ruffini’scorpuscles areparticularly sensitiveto the cutaneousstretching producedby digit or limbmovements.

Ruffini’s corpuscles account for about 20% of thereceptors in the human hand and do not elicit any particulartactile sensation when stimulated electrically.

Stretching the Ruffini corpuscle produces a slowlyadapting (sustained) generator potential in the 1° afferentterminal that degrades slowly for the duration of the stretch.If the force applied to the 1° afferent terminal produces agenerator potential that is of sufficient amplitude at the axontrigger zone, a train of action potentials is generated thattravel along the axon to the terminals of the its centralprocess.

How it works

The action potentials in the central terminals initiate therelease of neurotransmitters on 2° somatosensory afferentneurons within the central nervous system, which results in adischarge of the 2° afferent.

The hair follicle receptoris anunencapsulatedcutaneousreceptor.

The 1° afferent terminal axons spiral around the hair follicle baseor run parallel to the hair shaft forming a lattice-like pattern. Mosthair follicle 1° afferents are the fast-adapting type; displacement ofthe hair produces a transient discharge of action potentials at theonset of the displacement and a maintained displacement of thehair often fails to produce a sustained discharge. The hair follicleafferents respond best to moving objects and signal the directionand velocity of the movement of a stimulus brushing against hairyskin. As Meissner corpuscles are absent from hairy skin, the hairfollicle endings are considered to be the discriminative touchsystem's movement sensitive receptors in hairy skin.

Response Properties of Fine Touch FibersThe structure of the accessory tissue of the nerve

ending determines its response.Meissner’s corpuscles -

fast adapting (FA) receptorsfor discriminative touch

Pacinian corpuscles -sensitive to deep pressure –fast (FA) adapting receptorsare good for detectingtexture.

Ruffini’s corpuscles -Monitor continuous pressureon the skin – adapt slowly(SA)- estimate the durationof contact.

Both the FA and SA afferent classes can besubdivided on the basis of other aspects of theirreceptive fields - defined as the region of skin fromwhich stimuli can evoke a response (i.e., change thefiring of the afferent axon).

Receptive field characteristics for type 1 andtype 2 sensory afferents. Plots in the top rowshow the threshold level of force needed toevoke a response as a function of the distanceacross the receptive field (shown on the hand)

Response Properties of Fine Touch Fibers

Type 1 units have smallreceptive fields with well-definedborders. Particularly for glabrousskin (i.e., hairless skin, such ason the palms of the hands andsoles of the feet), the receptivefield has a circular or ovoidshape, within which there isrelatively uniform and highsensitivity to stimuli thatdecreases sharply at the border.

Type 2 units have widerreceptive fields with poorlydefined borders and only a singlepoint of maximal sensitivity, fromwhich there is a gradual reductionin sensitivity with distance.

For comparison, a type 1unit's receptive field typically willcover approximately four papillaryridges in the fingertip, whereas atype 2 unit will have a receptivefield that covers most or all of afinger.


Receptive field characteristics for type 1 and type2 sensory afferents.

Type 1 units, particularly SA1 units, respond best to edges. Thatis, a larger response is elicited from them when the edge of astimulus cuts through their receptive field than when the entirereceptive field is indented by the stimulus.

modality,

location,

intensity and

timing,which are manifested insensation.

The four attributes of sensation are illustrated inthis figure for the somatosensory modality of touch.

The sensory systems encode four elementaryattributes of stimuli:


A. In the human hand the submodalities of touch aresensed by four types of mechanoreceptors. Specifictactile sensations occur when distinct types of receptorsare activated. Firing of all four receptors produces thesensation of contact with an object. Selective activationof Merkel cells and Ruffini endings producessensations of steady pressure on the skin above thereceptor. When the same patterns of firing occur only inMeissner's and Pacinian corpuscles, the tinglingsensation of vibration is perceived.


B. Location and other spatial properties of a stimulusare encoded by the spatial distribution of the populationof activated receptors. Each receptor fires actionpotentials only when the skin close to its sensoryterminals is touched, i.e., when a stimulus impinges onthe receptor's receptive field. The receptive fields ofmechanoreceptors - shown as red areas on the finger tip- differ in size and response to touch. Merkel cells andMeissner's corpuscles provide the most preciselocalization of touch, as they have the smallest receptivefields and are also more sensitive to pressure applied bya small probe.


C. The intensity of stimulation is signaled by the firingrates of individual receptors, and the duration ofstimulation is signaled by the time course of firing.

The spike trains below each finger indicate theaction potentials evoked by pressure from a small probeat the center of the receptive field.

Two of these receptors(Meissner's andPacinian corpuscles)adapt rapidly toconstant stimulation,while the other twoadapt slowly.


Mechanoreceptors in glabrous skin vary in the size andstructure of their receptive fields. Each colored area onthe hands indicates the receptive field of a differentsensory nerve fiber in the human median nerve.


A. The Merkel diskreceptor in the superficialskin and the subcutaneousRuffini ending are slowlyadapting receptors. TheMerkel disk receptor has asmall, highly localizedreceptive field, whereasthe Ruffini ending has alarge field (light purple)with a central zone ofmaximal sensitivity (darkpurple).

Depending on their location,individual Ruffini endings are excitedby stretch of the skin in specificdirections as indicated by arrows.

B. The Meissner's corpuscle in the superficial skin and the subcutaneousPacinian corpuscle are rapidly adapting receptors. Meissner's corpuscles onthe fingertips have receptive fields averaging 2-3 mm in diameter, whilereceptive fields on the palm average 10 mm in diameter. The receptive fields ofPacinian corpuscles cover larger continuous surfaces on the fingers or palm(light pink) but have a central zone of maximal sensitivity located directly abovethe receptor (red).


C. Expanded view of the receptivefields of mechanoreceptors in the skin.The relative sensitivity to pressure isshown as a contour map in which themost sensitive regions are indicated inred and the least sensitive areas in palepink. Receptive fields in the superficiallayers of the skin have many points ofhigh sensitivity, marking the positions ofthe Meissner's corpuscles or Merkeldisk receptors. Receptive fields in thedeep layers have a single point ofmaximal sensitivity overlying thePacinian or Ruffini receptor.

The distribution of receptor types in the human handvaries.


Meissner's corpuscles (RA) and Merkel disk receptors (SA I)are the most numerous receptors; they are distributedpreferentially on the distal half of the fingertip. Paciniancorpuscles (PC) and Ruffini endings (SA II) are much lesscommon; they are distributed more uniformly on the hand,showing little differentiation of the distal and proximal regions.

The number ofsensory nervefibers innervatingan area is indicatedby the stipplingdensity, with thehighest density ofreceptors shown bythe heavieststippling.

The number of mechanoreceptive fibers is reduced to120/cm2 in the proximal phalanges, and to 50/cm2 in the palm.


The fingertips are the most densely innervated region ofskin in the human body, receiving approximately 300mechanoreceptive nerve fibers per square centimeter.

A. The area of contact onthe skin determines thetotal number of stimulatedMerkel disk receptors. Thepink region on the fingertipshows the spread ofexcitation when probes ofdifferent diameters arepressed upon the skin withconstant force. The intensityof color is proportional to thefiring rates of the stimulatedreceptors.

The shape and size of objects touching the hand are encoded by populations of Merkel disk receptors.


1. A small-diameter, sharp probe activates a smallpopulation of Merkel receptors. However, the activereceptors fire intensely because all of the force isconcentrated at the small probe tip.


2. An intermediate-size probeexcites more receptors but the peakfiring rate in the population isreduced. The probe does not feel assharp as the small-diameter probe.3. A gently rounded, large-diameter probe stimulates a largepopulation of receptors spreadacross the width of the finger. Thesereceptors fire at low rates becausethe force is spread over a largerarea of skin.

The firing rate of individual Merkel disk receptorssignals the probe diameter.B. These recordings of action potentials fired by a Merkel diskreceptor illustrate the responses evoked when probes ofdecreasing size are pressed on the center of the receptive field.


All of the probes evoke a stronginitial response as contact ismade with the skin. The firingrate of the neuron during steadypressure is proportional to thecurvature of each probe. Theweakest responses are evokedby flat surfaces and gentlyrounded (large diameter)probes. The firing rate increasesas the probe diameter becomessmaller.

http://www.nature.com/nrn/journal/v12/n3/fig_tab/nrn2993_F1.html

Other Somatic Sensations - Warmth and Cold - Are Mediated by Thermal Receptors

Skin temperature is coded by warmth receptorsand cold receptors.A. Static temperatures. Cold receptors and warmthreceptors differ in the range of steady-statetemperatures to which they respond and in their peaktemperature sensitivities.

Cold receptors respond to steady-statetemperatures of 5-40°C. Warmth receptors aretonically active at steady temperatures of 29-45°C.

Cold receptors fire at highest rates at askin temperature of 25°C, while warmth receptorsare most active at 45°C.

At the normal skin temperature of 34°C,cold receptors are more active than warmthreceptors.

B. Dynamic temperatures. Both receptors are moresensitive to changes in skin temperature than toconstant temperatures.

Other Somatic Sensations - Warmth and Cold - Are Mediated by Thermal Receptors

Cooling the skin below the restinglevel evokes a sharp rise in the firing rate ofcold receptors and silences warmthreceptors. If the cold temperature ismaintained, the firing rates of the coldreceptors adapt. When the skin temperatureis rewarmed to the resting level, coldreceptors are briefly silenced, whereaswarmth receptors fire a burst of impulses.

Warming the skin produces theopposite firing patterns in warmth and coldreceptors.

Dynamic Aspects of Somatic Sensory Receptive Fields

When humans explore objects with their hands,multiple contacts between the skin and the objectsurface generate extraordinarily complex patterns oftactile stimuli.

As a consequence, the somatic sensory systemmust process signals that change continuously intime.

Nonetheless, we routinely discriminate the size,texture and shape of objects with great accuracy.


Psychophysical analysis of tactile performancesuggests that something more than the cutaneousperiphery is needed to explain variations in tactileperception.

For instance, even though we spend most of theday wearing clothes, we usually ignore the tactilestimulation that they produce. Some aspect of themechanosensory system allows us to filter out thisinformation and pay attention to it only whennecessary.


The fascinating phenomenon of “phantom limb”sensations after amputation provides further evidencethat tactile perception is not fully explained by theperipheral information that travels centrally.


The central nervous system clearly plays anactive role in determining the perception of themechanical forces that act on us.

Phantoms might simply be a curiosity - or aprovocative clue about higher order somatic sensoryprocessing - were it not for the fact that a substantialnumber of amputees also develop phantom pain.


This common problem is usually described as atingling or burning sensation in the missing part.

Sometimes, however, the sensation becomes amore serious pain that patients find increasinglydebilitating.


Phantom painis, in fact, one of themore common causesof chronic painsyndromes and isextraordinarily difficultto treat.

Mechanoreceptors Specialized for ProprioceptionWhereas cutaneous mechanoreceptors provide

information derived from external stimuli, another majorclass of receptors provides information aboutmechanical forces arising from the body itself, themusculoskeletal system in particular.These are called proprioceptors,roughly meaning “receptors for self.”

The purpose of proprioceptorsis primarily to give detailed andcontinuous information about theposition of the limbs and other bodyparts in space.

Mechanoreceptors Specialized for Proprioception

Whereas muscle spindles are specialized tosignal changes in muscle length (the degree to whichthey are being stretched), low-thresholdmechanoreceptors in tendons, called Golgi tendonorgans, inform the central nervous system aboutchanges in muscle tension.

Finally, joint receptors arerapidly adaptingmechanoreceptors in andaround joints which gatherdynamic information aboutlimb position and jointmovement.

Each spindle is 3 to10 mm long. Musclespindles consist of around3 to 12 tiny specializedintrafusal muscle fiberssurrounded by a capsuleof connective tissue.

Muscle SpindlesFound in all but a few striated (skeletal) muscles.Respond by causing muscle contraction

The intrafusal fibers are distributed among theordinary (extrafusal) fibers of skeletal muscle in aparallel arrangement.

Sensory Innervation of the Muscle Spindle - Centralregion of each of these fibers has few or no actin and myosinfilaments. Therefore, this central portion does not contractwhen the ends do. Instead, it functions as a sensoryreceptor.

Motor Innervation of Muscle Spindle -The end portionsthat do contract are excited by small gamma motor nervefibers (gamma efferent fibers) that originate from small typeA gamma motor neurons in the anterior horns of the spinalcord. The large alpha efferent fibers (type A alpha nervefibers) innervate the extrafusal skeletal muscle.

Sensory Innervation of the Muscle Spindle

Muscle spindle receptor can beexcited in two ways:

1) lengthening the whole musclestretches the midportion of thespindle and, therefore, excitesthe receptor.

2) contraction of the end portionsof the spindle's intrafusalfibers stretches the midportionof the spindle and thereforeexcites the receptor.

Two types of sensory endings are found in thiscentral receptor area of the muscle spindle:

Sensory Innervation of the Muscle Spindle

Primary Ending - this nerve fiber is a type Ia fiberaveraging 17 micrometers in diameter it transmits sensorysignals to the spinal cord at a velocity of 70 to 120 m/sec, asrapidly as any type of nerve fiber in the entire body.

Secondary Ending -usually one but sometimestwo smaller sensory nervefibers- type II fibers withan average diameter of 8micrometers-innervate thereceptor region on one orboth sides of the primaryending.

Sensing Blood Pressure

Blood pressure is monitored at two main sites inthe body: carotid sinus, an enlargement of the left andright internal carotid arteries, which supply blood to thebrain and the aortic arch, the portion of the aorta veryclose to its emergence from the heart.The walls of the bloodvessels at both sitescontain a highlybranched network ofafferent neuronscalled baroreceptors,which detect tensionin the walls.


When the blood pressure decreases, thefrequency of impulses produced by the baroreceptorsdecreases.

The CNS responds to this reduced input bystimulating the sympathetic division of the autonomicnervous system, causing an increase in heart rate andvasoconstriction.

Both effects help to raise the blood pressure, thusmaintaining homeostasis.

A rise in blood pressure, conversely, reducessympathetic activity and stimulates the parasympatheticdivision, slowing the heart and lowering the bloodpressure.

Mechanical distortion of theplasma membrane ofmechanoreceptorsproduces nerve impulsesthat serve to monitor musclelength from skeletal musclespindles and to monitorblood pressure frombaroreceptors withinarteries.


These mechanoreceptors, are encapsulatedsensory receptors innervated by branches of group Ibafferents and are distributed among the collagen fibersthat form the tendons.

Golgi Tendon Organ Helps Control Muscle Tension

About 10 to 15 musclefibers are usuallyconnected to each Golgitendon organ - isstimulated when this smallbundle of muscle fibers is"tensed" by contracting orstretching the muscle.

Thus, the major difference in excitation of theGolgi tendon organ versus the muscle spindle isthat the spindle detects muscle length and changes inmuscle length, whereas the tendon organ detectsmuscle tension as reflected by the tension in itself.


The tendon organ, like the primary receptor of themuscle spindle, has both a dynamic response and astatic response:

dynamic response: reacting intensely when themuscle tension suddenly increases

static response: settling down within a fraction of asecond to a lower level of steady-state firing that isalmost directly proportional to the muscle tension.

Thus, Golgi tendon organs provide the nervoussystem with instantaneous information on the degreeof tension in each small segment of each muscle.


Impulses from the tendon organ into the CNS aretransmitted through large, rapidly conducting type Ibnerve fibers that average 16 micrometers in diameter.


When the Golgi tendon organs ofa muscle tendon are stimulated byincreased tension in the connectingmuscle, signals are transmitted to thespinal cord to cause reflex effects in therespective muscle. This reflex isentirely inhibitory.

Thus, this reflex provides a negative feedbackmechanism that prevents the development of too muchtension on the muscle and a protective mechanism toprevent tearing of the muscle or avulsion of the tendonfrom its attachments to the bone.

Inhibitory Nature of the Tendon Reflex and Its Importance

Joint ReceptorsJoint receptors are found within the connective

tissue, capsule and ligaments of joints. The jointreceptors are free nerve endings and encapsulatedendings in the joint capsule and joint ligaments.

The encapsulated receptors in the joint capsuleresemble Pacinian and Ruffini endings whereas thosein the ligaments resemble Golgi tendon organs.

The joint 1° afferentsrespond to changes in theangle, direction, andvelocity of movement in ajoint. The responses arepredominantly rapidlyadapting with few joint 1°afferents signaling the resting(static) position of the joint.

Joint Receptors

It has been suggested that information from muscles,tendons, skin and joints are combined to provide estimatesof joint position and movement. For example, when the hipjoint is replaced — removing all joint receptors — the abilityto detect the position of the thigh relative to the pelvis is notlost.

PAIN nocireceptorsNOCICEPTION, THERMORECEPTION, AND ITCH

Most of the sensory and somatosensory modalitiesare primarily informative, whereas pain is a protectivemodality.

Although similar in some ways to the sensoryprocessing of ordinary mechanical stimulation, theperception of pain, called nociception, depends onspecifically dedicated receptors and pathways.

The relatively unspecialized nerve cell endingsthat initiate the sensation of pain are called nociceptors(noci is derived from the Latin nocere, “to hurt”).

Nociceptive means sensitive to noxious stimuli.Noxious stimuli are stimuli that elicit tissue damage andactivate nociceptors.

PAIN nocireceptors

Nociceptors are sensory receptors -free (bare)nerve endings found in the skin, muscle, joints, boneand viscera, that detect signals from damaged tissue orthe threat of damage and indirectly also respond tochemicals released from the damaged tissue.

Like other cutaneous and subcutaneous receptors,they transduce a variety of stimuli into receptorpotentials, which in turn trigger afferent action potentials.

PAIN nocireceptors

Moreover, nociceptors, like other somatic sensoryreceptors, arise from cell bodies in dorsal root ganglia (or inthe trigeminal ganglion) that send one axonal process to theperiphery and the other into the spinal cord or brainstem.

Different nociceptors/free nerve endings, and the fibers carrying painsensation from the nociceptors to the spinal cord.

Because peripheral nociceptive axons terminate inunspecialized “free endings,” it is conventional to categorizenociceptors according to the properties of the axonsassociated with them.

The axons associated with nociceptors, conductrelatively slowly, being only lightly myelinated or, morecommonly, unmyelinated.

PAIN nocireceptors

Accordingly, axons conveying information about painfall into:

Aδ group of myelinated axons, which conduct atabout 20 m/s,

C fiber group of unmyelinated axons, whichconduct at velocities generally less than 2 m/s.

aA - the largest and fastest axons: alpha (the fastest), betha, and delta (the slowest).C -the smallest and slowest.

Muscle afferent axons: group I (the fastest), II, III, and IV (the slowest) - with subgroups a, b.

Nociceptors are not uniformly sensitive. Theyfall into several categories, depending on theirresponses to mechanical, thermal, and/or chemicalstimulation liberated by the damage, tumor, and/orinflammation.

Skin nociceptors may be divided into four

categories based on function:

I. mechanonociceptors,

II. thermal nociceptors,

III. chemical nociceptors,

IV. polymodal nociceptors.

PAIN nocireceptors

The first type is termed high thresholdmechanonociceptors or specific nociceptors.

These faster-conducting Aδ nociceptors respondonly to intense mechanical stimulation such aspinching, cutting or stretching.

They are also excited by sharp objects thatpenetrate, squeeze, or pinch the skin and thereforemediate sensations of sharp or pricking pain.

Their firing rates increase with thedestructiveness of mechanical stimuli, from near-damaging to overtly destructive of the skin.

The afferent fibers for mechanical nociceptorshave bare nerve endings and, because they aremyelinated, are the fastest-conducting nociceptiveafferents.

PAIN nocireceptors

The bottom traces inparts A and B arethe output of a forcetransducer coupledto the stimulator.Pinching the skinwith serrated forceps(C), which is moretraumatic than a pinprick, produces thestrongest response.

Pressure on the cell's receptive field with a blunt-tipped probe elicits no response even if the skin isindented by 2 mm (A), but the tip of a needle thatpunctures the skin produces a clear response (B).

PAIN nocireceptors

The second type is the thermal nociceptors,are faster-conducting Aδ nociceptors which respondto the above stimuli as well as to thermal stimuli.

Thermal nociceptors are excited by extremes oftemperature as well as by strong mechanical stimuli.One group of thermal nociceptors is excited by noxiousheat (temperatures above 45°C). A second groupresponds to noxious cold (cooling the skin below 5°C).

PAIN nocireceptors

The third type is chemical nociceptors, whichrespond only to chemical substances.

A fourth type is known as polymodalnociceptors, specifically associated with C fibers, otherunmyelinated nociceptors, which respond to highintensity stimuli such as mechanical, thermal and tochemical substances like the previous three types.

Silent Nociceptors.In the skin and deep tissues there are additional

nociceptors called "silent" or "sleep" nociceptors.These receptors are normally unresponsive to noxious

mechanical stimulation, but become “awakened”(responsive) to mechanical stimulation duringinflammation and after tissue injury.One possible explanation of the "awakening"

phenomenon is that continuous stimulation from thedamaged tissue reduces the threshold of thesenociceptors and causes them to begin to respond.Many visceral nociceptors are silent nociceptors.

Joint Nociceptors.

The joint capsules and ligaments contain high-threshold mechanoreceptors, polymodal nociceptors,and "silent" nociceptors.

Many of the fibers innervating these endings inthe joint capsule contain neuropeptides, such assubstance P (SP) and calcitonin gene-related peptide(CGRP).

Liberation of such peptides is believed to play arole in the development of inflammatory arthritis.

Visceral Nociceptors.Visceral organs contain mechanical pressure,

temperature, chemical and silent nociceptors.The visceral nociceptors are scattered, with

several millimeters between them, and in some organs,there are several centimeters between each nociceptor.

Many of the visceralnociceptors are silent. Thenoxious information fromvisceral organs and skinare carried to the CNS indifferent pathways.

In general, two categories of pain perceptionhave been described: a sharp first pain and a moredelayed, diffuse, and longer-lasting sensation that isgenerally called second pain.

The receptive fields of all pain-sensitiveneurons are relatively large, particularly at the level ofthe thalamus and cortex, presumably because thedetection of pain is more important than its preciselocalization.

Pain perception

Aδ fibers are responsible for first painand C fibers are responsible for theduller, longer-lasting second pain.

(A) First and second pain, are carried by different axons,as can be shown by (B) the selective blockade of themore rapidly conducting myelinated axons that carry thesensation of first pain, or (C) blockade of the moreslowly conducting C fibers that carry the sensation ofsecond pain.

Pain perception

The number and frequencyof action potential dischargein the nociceptive axon,however, continues toincrease.

Pain perception

In the painful stimulus range, the axons ofthermoreceptors fire action potentials at the same rateas at lower temperatures.

(45°C is the approximatethreshold for pain).

Transduction of Nociceptive SignalsGiven the variety of stimuli (mechanical, thermal, andchemical) that can give rise to painful sensations, thetransduction of nociceptive signals is a complex task.The specific receptors associated with nociceptiveafferent endings are sensitive to both heat and tocapsaicin, the ingredient in chili peppers that isresponsible for the familiar tingling or burning sensationproduced by spicy foods. Since the same receptor isresponsive to heat as well as capsaicin, it is notsurprising that chili peppers seem “hot.”

(A) Some popular peppers that contain capsaicin.(B) The chemical structure of capsaicin.

Transduction of Nociceptive Signals

When applied to the mucus membranes of the oralcavity, capsaicin acts as an irritant, producing protectivereactions. When injected into skin, it produces a burningpain and elicits hyperalgesia to thermal and mechanicalstimuli. Repeated applications of capsaicin also desensitizepain fibers and prevent neuromodulators such as substanceP, VIP, and somatostatin from being released by peripheraland central nerve terminals.

Consequently, capsaicin is used clinically as ananalgesic and anti-inflammatory agent; it is usually appliedtopically in a cream (0.075%) to relieve the pain associatedwith arthritis, postherpetic neuralgia, mastectomy, andtrigeminal neuralgia. Thus, this remarkable chemicalirritant not only gives gustatory pleasure on anenormous scale, but is also a useful pain reliever!


Except for chemical sensitivity, nociception couldsimply be considered an extreme version of touch andtemperature sensation.

As is so clearly demonstrated in other sensorysystems (e.g., vision, olfaction, and taste), the cloningand functional characterization of sensory receptorsprovides the definitive molecular tools to elucidate thelogic of stimulus detection and perception (Julius &Nathans 2012).

Indeed, the studyof transient receptorpotential (TRP)channels validated theexistence of thenociceptor.


(TRP) channels consist of a large family of ion channels that playa wide diversity of physiological functions. In mammals, thesefamily consists of 28 different TRP members grouped in 7subfamilies. Expressed in a large number of tissues from nerve toepithelial cells, genetic studies have linked mutations in these ionchannels to human diseases.

Structurally, TRP channels resemblevoltage-gated potassium or cyclicnucleotide-gated channels, having sixtransmembrane domains with a pore betweendomains 5 and 6. The majority of TRP channelsare non-selective cation channels that permeateNa+ and K+, and most of them with significantCa2+ selectivity.

Under resting conditions the pore of the channel is closed.In the open, activated state, these receptors allow an influx ofsodium and calcium that initiates the generation of actionpotentials in the nociceptive fibers.

http://www.brauchilab.org/research.html


ThermoTRPshave become pivotaldrug targets, and thedevelopment oftherapeuticcompounds forpharmacologicalintervention isactively pursued bythe academy and theindustry.

Thermosensory channels, also named “thermoTRPs”,define a subfamily of the TRP channels that are activated bychanges in the environmental temperature, from noxiouscold (<15 °C) to injurious heat (>42 °C).

http://www.brauchilab.org/research.html

Pungent irritants from pepper, mint, and mustardplants have served as powerful pharmacological toolsfor identifying molecules and mechanisms underlyingthis initial step of pain sensation.

These natural products have revealed three members ofthe transient receptor potential (TRP) ion channelfamily—TRPV1, TRPM8, and TRPA1—as moleculardetectors of thermal and chemical stimuli that activatesensory neurons to produce acute or persistent pain.


Receptor Protein

Threshold or Temperature

Range for Activation (°C)

Other Characteristics

TRPV1 >42 Activated by capsaicinTRPV2 >52 -TRPV3 34-38 Activated by camphorTRPV4 27-34 -TRPM8 <25 Activated by mentholTRPA1 <18 Activated by mustard oil

The fourth letter in the nameidentifies the subfamily and waschosen because of the firstmember of the subfamilyidentified: V, vanilloid; M,melastatin; A, ankyrin-like.

Temperature dependence offiring rates in differentthermosensitive afferents.Below the firing curves areshown the ranges over whichthe different TRP channels areactivated.


TRP Family Proteins Involved in Thermal Transduction.


Schematic of the VR-1/capsaicin receptorchannel. TRPV1 domainsthat confer sensitivity tovarious stimuli: capsaicin(chili pepper) and relatedvanilloid ligands,extracellular protons,(lemon), or peptide toxinsfrom tarantula (spider).

The so-called vanilloid receptor (VR-1 or TRPV1) isfound in C and Aδ fibers and is activated by moderate heat(45°C—a temperature that is perceived as uncomfortable) aswell as by capsaicin.

Another type of receptor (vanilloid-like receptor, VRL-1or TRPV2) has a higher threshold response to heat (52°C), is notsensitive to capsaicin, and is found in Aδ fibers.

Following a painful stimulus associated with tissuedamage (e.g., cuts, scrapes, and bruises), stimuli in thearea of the injury and the surrounding region that wouldordinarily be perceived as slightly painful are perceivedas significantly more so, a phenomenon referred to ashyperalgesia. A good example of hyperalgesia is theincreased sensitivity to temperature that occurs after asunburn.

Sensitization

This effect is due to changesin neuronal sensitivity that occurat the level of peripheral receptorsas well as their central targets.

Peripheral sensitization results from the interaction ofnociceptors with the “inflammatory soup” of substancesreleased when tissue is damaged. These products of tissuedamage include extracellular protons, arachidonic acid andother lipid metabolites, bradykinin, histamine, serotonin,prostaglandins, nucleotides, and nerve growth factor (NGF),all of which can interact with receptors or ion channels ofnociceptive fibers, augmenting their respons.

Sensitization

Substances released by damagedtissues augment the response ofnociceptive fibers. In addition,electrical activation of nociceptorscauses the release of peptides andneurotransmitters that furthercontribute to the inflammatoryresponse.

The presumed purpose of the complex chemicalsignaling arising from local damage is not only to protect theinjured area (as a result of the painful perceptions producedby ordinary stimuli close to the site of damage), but also topromote healing and guard against infection by means oflocal effects such as increased blood flow and the migrationof white blood cells to the site.

Obviously the identification of the components of theinflammatory soup and their mechanisms of action is a fertilearea to explore for potential analgesics (i.e., compoundsthat reduce pain intensity). For example, so-callednonsteroidal anti-inflammatory drugs (NSAIDs), whichinclude aspirin and ibuprofen, act by inhibitingcyclooxygenase (COX), an enzyme important in thebiosynthesis of prostaglandins.

Sensitization

Pain has been classified into three major types:

1. Pricking pain. Pain caused by a needle, pin prick,skin cut, etc. - elicits a sharp, pricking quality, stingingpain sensation carried fast by the A delta fibers.- is precisely localized and of short duration.- is also called fast pain, first pain or sensory pain.- is present in all individuals and is a useful andnecessary component of our sensory repertoire; withoutthis type of protective pain sensation, everyday lifewould be difficult.- arises mainly from the skin, and carried mainly by Adelta fibers which permits discrimination (i.e., permitsthe subject to localize the pain).


2. Burning pain or soreness pain. Pain caused byinflammation, burned skin, etc., is carried by the Cfibers (slowly conducted pain nerve fibers).-is a more diffuse, slower to onset, and longer induration.- it is an annoying pain and intolerable pain, which is notdistinctly localized.-like pricking pain, burning pain arises mainly from theskin.-it is carried by the paleospinothalamic tract. (The oldprimitive transmission system for diffuse pain whichdoes not permit exact localization.)


3. Aching pain is a sore pain.-This pain arises mainly from the viscera and somaticdeep structures.-Aching pain is not distinctly localized and is anannoying and intolerable pain.-Aching pain is carried by the C fibers from the deepstructures to the spinal cord

The placebo effect is defined as a physiologicalresponse following the administration of apharmacologically inert “remedy.” The word placebomeans “I will please,” and the placebo effect has a longhistory of use (and abuse) in medicine.

Placebo effect

A common misunderstanding about the placebo effect isthe view that patients who respond to a therapeuticallymeaningless reagent are not suffering real pain, but only“imagining” it. Although the mechanisms by which the brain affectsthe perception of pain are only beginning to be understood, theeffect is neither magical nor a sign of a suggestible intellect. Inshort, the placebo effect is quite real.

Placebo effect

Understanding the central modulation of pain perception(on which the placebo effect is presumably based) was greatlyadvanced by the finding that electrical or pharmacologicalstimulation of certain regions of the midbrain produces relief ofpain. Precisely how pain is modulated is being explored in manylaboratories at present, motivated by the tremendous clinical (andeconomic) benefits that would accrue from still deeper knowledgeof the pain system and its molecular underpinnings.

We are always actively touching or passivelybeing touched by something - other people, furniture,clothes, spoons. Even if we are stark naked, our feetstill touch the ground, and the air touches our skin.

Therefore, the tactile sense, or sense of touch, isa huge sensory system that gives us informationneeded not only for visual perception, motor planning,and body awareness, but also for academic learning,emotional security, and social skills.

Two components make up the tactile sense:First is the protective (or defensive) system;Second is the discriminative system.

THE TACTILE SENSESENSORY INTEGRATION AND SENSORY INTEGRATION DYSFUNCTION

The Protective/Defensive SystemWhether we call it protective or defensive, it's the

same: the "uh, oh!" system. All creatures are bornwith a protective/defensive system. Its purpose is toalert us to potentially harmful stimuli. We need it tosurvive.

The receptors for the protective system are in theskin. Light touch is the stimulus that causes thereceptors to respond.

Sometimes light touch is alarming, such as amosquito alighting on our skin. We respond negative,for self-preservation. For example, when a strangergets too close, we shrink: when a lash gets in our eye,we blink.

SENSORY INTEGRATION AND SENSORY INTEGRATION DYSFUNCTION

The Discriminative System

The second component of the tactile sense is thediscriminative system: the "aha!" system. It tells us:

That we are touching something or that something istouching us.

Where on our body the touch occurs.

Whether the touch is light or deep.

How to perceive the attributes of the object, such asits size, shape, temperature, density and texture.


The Discriminative SystemIt develops as neurological maturation suppresses

the defensive system (the defensive system diminishesbut doesn't disappear. Indeed, messages between thetwo systems must continue to flow back and forth all ourlives so we can interpret tactile informationappropriately).

The receptors for this system are in the skin,especially on the hands and fingers, the soles of thefeet, and the mouth and tongue. Deep touch, or "touchpressure," is the stimulus that causes the receptors torespond.


CHARACTERISTICS OF TACTILE DYSFUNCTION

An individual with an inefficient tactile systemmay have one or more problems with the integration oftouch sensation. The person may:

1. Be defensive to touch (hypersensitive).

2. Be under-responsive to touch (hyposensitive).

3. Have poor tactile discrimination.


Tactile dysfunction is the inefficient processing inthe central nervous system of sensations perceivedthrough the skin.

The person who is hypersensitive to touch hastactile defensiveness, the tendency to react negativelyand emotionally to unexpected, light touch sensations.The person will react not only to actual touch but also tothe anticipation of being touched. Perceiving most touchsensations to be uncomfortable or scary, he overreactswith a fight-or-flight response. A person may typicallyavoid unexpected, light touch, but accept, firm touch(deep pressure), like a bear hug. This person needstouch information more than a person with a well-regulated tactile sense.


The person who is hyposensitive to touch tendsto under-react to tactile experiences.

Needing extra stimulation, he may constantlytouch objects and people. He may be under-responsiveto touch, whether the touch is soothing or painful.

Unlike the hypersensitive person, who over-reactsto protect himself', the hyposensitive person may notreact to touch effectively enough to do a good job of self-protection. In fact, he may he unaware of touchaltogether, unless the touch is very intense.

It is important to understand that the out-of-syncperson may be both hyposensitive and hypersensitive.For instance, he may jump when someone grazes hiselbow, yet be indifferent to a broken collarbone.


The person with hypersensitivity (tactiledefensiveness) may:- Overreact to physically painful experiences, making a"big deal" over a minor scrape or a splinter. He may bea hypochondriac.

- React similarly to dissimilar touch sensations. Araindrop on his skin may cause as adverse a reactionas a thorn.

- Avoid touching certain textures or surfaces, like somefabrics, blankets, rugs, or stuffed animals.

- Be unusually fastidious, hurrying to wash a tiny bit ofdirt off his hands.

- Avoid walking barefoot on grass or sand, or wading inwater, or walking on tiptoe to minimize contact with theground.


The person with hyposensitivity (under-responsiveness to touch) may:- Seem unaware of touch unless it is very intense.- Show little or no reaction to pain from scrapes,bruises, cuts, or shots.- Hurt other people or pets during play, seeminglywithout remorse, but actually not comprehending thepain that others feel.- Fail to realize he has dropped something.


The person with poor tactile discrimination may:- Seem out of touch with his hands, as if they wereunfamiliar appendages.- Be unable to identify which body parts have beentouched without looking.- Be fearful in the dark.- Be unable to perform certain motor tasks without visualcues, such as zipping, buttoning, and unbuttoningclothes.- Have difficulty holding and using tools, such as pens,scissors, and forks.


Proprioceptive dysfunction is the inefficientprocessing of sensations perceived through themuscles, joints, ligaments, tendons, and connectivetissue.

Proprioceptive dysfunction is usually accompaniedby problems with the vestibular and/or vestibularsystems.

Whereas it is common for an individual to haveonly tactile, or only vestibular, problems, it is less likelyfor an individual to have only proprioceptive problems.

CHARACTERISTICS OF PROPRIOCEPTIVE DYSFUNCTION

The person with proprioceptive dysfunction may:- Deliberately "bump and crash" into objects in theenvironment, e.g., jump from high places.- Stamp or slap his feet on the ground when walking.- Kick his heels against the floor or chair.- Bang a stick or other object on a wall or fence whilewalking.- Rub his hands on tables, bite or suck on his fingers, orcrack his knuckles.- Like to be tightly wrapped in a blanket or tucked in tightat bedtime.- Chew constantly on objects like shirt collars and cuffs,pencils, and gum.

CHARACTERISTICS OF PROPRIOCEPTIVE DYSFUNCTION

NEUROSCIENCE: Third Edition, Dale Purves et al., © 2004 Sinauer Associates, Inc. Fundamental neuroscience /by Larry Squire et al.—3rd ed. 2008, Elsevier Inc. Coding of Sensory Information, Esther P. Gardner John H. Martin;

http://homepage.psy.utexas.edu/homepage/class/psy394U/hayhoe/IntroSensoryMotorSystems/week3/Kandel%20Ch%2021,%2022,%2023.pdf

http://www.cogsci.ucsd.edu/~ajyu/Teaching/Cogs160_sp12/Lectures/lect1.pdf http://freedownloadb.net/ppt/sensory-and-motor-mechanisms-6026576.html www.austincc.edu/rfofi/BIO2304/2304LecPPT/2304Sensory.ppt www.mohsenparviz.ir/lesson/L5-%20Sensory%20Receptors.ppt www.med.uottawa.ca/Courses/NSC5104/.../NeuralSystemsSensory1.ppt www.med.muni.cz/biofyz/files/en/HEARING-finx.ppt www.jfmed.uniba.sk/.../Biofysics_of_sensory_p._receptors__vision.ppt faculty.weber.edu/nokazaki/.../PPT%20notes/Sensory%20System.ppt http://humanservices.alberta.ca/documents/pdd/pdd-central-sensory-integration-dysfunction.pdf TRP Channels and Pain, David Julius, Annu. Rev. Cell Dev. Biol. 2013. 29:355–84 Trafficking of ThermoTRP Channels, Clotilde Ferrandiz-Huertas et al., Membranes 2014, 4, 525-

564; doi:10.3390/membranes4030525 http://droualb.faculty.mjc.edu

References

http://www.cogsci.ucsd.edu/%7Eajyu/Teaching/Cogs160_sp12/Lectures/lect1.pdf

http://freedownloadb.net/ppt/sensory-and-motor-mechanisms-6026576.html

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