Sensory Physiology. Somatosensory Receptors

Transcription

1 Sensry Physilgy - Stimuli at the cnscius level f perceptin Special senses visin, hearing, taste, smell, equilibrium Smatic senses tuch, temperature, pain, itch and prpriceptin Prpriceptin = the awareness f bdy mvement and pstin in space Prprireceptrs in muscles/jints, either cnscius r uncnsius Sensry Pathways - Stimulus as physical energy sensry receptr Receptr acts as a transducer - Transducer cnverts stimulus int an intracellular signal Is usually a change in membrane ptential - Stimulus must be abve threshld Stimulus threshld actin ptential t CNS - Integratin in CNS cerebral crtex where sme stimuli reach are cnsciusly perceived and thers acted n subcnsciusly withut ur awareness Smatsensry Receptrs Simple receptrs are neurns with free nerve endings Cmplex neural receptrs have nerve endings enclsed in cnnective tissue capsules Pacinian Crpuscle which senses vibratin is a mechanreceptr Mst special senses receptrs are cells that release neurtransmitter nt sensry neurns, initiating an actin ptential Hair cell fund in Ear - Smell receptrs are neurns, ther 4 special senses are nn-nerual receptr cells When activated, the hair cell releases a neurtransmitter that initiates the actin ptential - Nn-neural accessry structures enhance inf-gathering capacity f receptrs E.g. crnea and lens help fcus light n phtreceptrs E.g. arm hairs help smatsensry receptrs sense mvement in the air abve skin surface Tuch Receptrs - Fund in the skin and in deeper regins n the bdy

2 Pacinian Crpuscle - The Lamellar crpuscle is val-shaped and apprximately 1 mm in length - The entire crpuscle is wrapped by a layer f cnnective tissue - It has cncentric lamellae cmpsed f fibrus cnnective tissue and fibrblasts, separated by gelatinus material - The lamellae are very thin, flat, mdified Schwann cells - In the centre f the crpuscle is the inner bulb, a fluid-filled cavity with a single afferent unmyelinated nerve ending - These receptrs detect vibratins and are rapidly adapting (phasic) receptrs - Pressure sensitive sdium channels lcated in crpuscle

3 Meissner's Crpuscles - Encapsulated unmyelinated endings - Cnsist f flattened supprtive cells arranged as hrizntal lamellae surrunded by a cnnective tissue capsule - Crpuscle is between μm in length and μm in diameter - A single nerve fibre meanders between the lamellae and thrughut the crpuscle - Any physical defrmatin in the crpuscle will cause an AP in the nerve - Since they are rapidly adapting r phasic, the APs generated quickly decrease and eventually cease (this is the reasn ne stps "feeling" ne's clthes) - Sense flutter and strking mvements Merkel Receptrs - Merkel nerve endings are mechanreceptrs fund in the skin and mucsa f that prvide tuch infrmatin t the brain. Each ending cnsists f a Merkel cell in clse appsitin with an enlarged nerve terminal. This is smetimes referred t as a Merkel cell neurite cmplex, r a Merkel disk receptr. A single afferent nerve fibre branches t innervate up t 90 such endings. They are classified as slwly adapting (tnic) mechanreceptrs - Sense steady pressure and texture

4 Ruffini r Bulbus Crpuscle - A slwly adapting (tnic) mechanreceptr fund in the subcutaneus tissue f humans - Ruffini crpuscles are highly branched nerve endings within a fluid filled capsule in the cnnective tissue - Small swellings at the end f each branch f the nerve ending and the receptr is said t respnd t displacement f cnnective tissue fibres that extend thrugh the fluid filled bulb Thught t sense stress r distensin in the deeper dermis - Respnd t skin stretch Types f Sensry Receptrs - Althugh receptrs are specific t ne frm f energy, they can respnd t mst ther frms if the intensity is high enugh Type f Receptr Example f Stimuli Chemreceptrs respnd t chemical Oxygen, ph, varius rganic mlecules such ligands that bind t the receptr as glucse Mechanreceptrs respnd t mechanical energy Phtreceptrs respnd t light Thermreceptrs respnd t temperature Pressure (barreceptrs), cell stretch (smreceptrs), vibratin, acceleratin, sund Phtns f light Varying degrees f heat Sensry Transductin - Transductin = the cnversin f stimulus energy int inf that can be prcessed by the CNS E.g. pening and clsing f in channels cnverts mechanical, light, chemical and thermal energy int a change in membrane ptential change - Adequate stimulus Preferred frm f stimulus by receptr e.g. thermreceptrs are mre sensitive t temperature whereas mechanreceptrs respnd t stimuli that defrm the cell membrane - Threshld Minimum stimulus required t activate a receptr - Receptr ptential = graded ptential, change in sensry receptr membrane ptential - Can initiate an AP OR influence the release f neurtransmitter secretin = alters electrical activity in an assciated neurn

5 Receptive Fields f Sensry Neurns - Receptive field specific physical area in which smatic sensry and visual neurns are activated by stimuli E.g. tuch receptive neurn in skin will respnd t pressure that falls within its receptive field - Primary sensry (first-rder) neurn 1RF assciated with 1 sensry neurn - Secndary sensry (secnd-rder) neurn primary synapses n 1 CNS neurn - Cnvergence multiple presynaptic neurns prvide input t a smaller number f pst synaptic neurns Allws multiple simultaneus sub-threshld stimuli t sum at the pstsynaptic neurn Individual receptive fields merge int a single large (secndary) receptive field Size f 2 nd ary receptive field determines an area s sensitivity t a stimulus Sensry Neurns: Tw-Pint Discriminatin - Less sensitive areas have larger receptive fields e.g. arms, legs Many primary neurns cnverge nt a single secndary neurn - Mre sensitive areas have smaller receptive fields e.g. finger tips

6 Cnvergence shws a 1:1 relatinship between primary neurns and secndary neurns Integratin by CNS - Sensry infrmatin either: Spinal crd t brain by ascending pathways Directly t brain stem via cranial nerves - Visceral reflexes integrated in brain stem r spinal crd usually d nt reach cnscius perceptin E.g. f uncnscius visceral reflex is cntrl f bld pressure by brain stem Mid-brain receives visual inf Medulla blngata receives input fr sund and taste Cerebellum prcesses inf abut balance and equilibrium These pathways prject t the thalamus relay befre passing inf t cerebrum Olfactry inf is nt ruted thrugh the thalamus Travels: lfactry bulb 1 st cranial nerve cerebrum - Perceptual threshld Level f stimulus intensity necessary fr yu t be aware f a particular sensatin Brain can turn ff sme stimuli t avid being verwhelmed - Nise can adequately stimulate sensry neurns (e.g. ear) but neurns higher in the brain can dampen the perceived signal s that it des nt reach the cnscius brain E.g. studying with the radi in the backgrund r zning ut during a lecture - Inhibitry Mdulatin decreased perceptin f a stimulus belw perceptual threshld Diminishes a suprathreshld stimulus until it is belw the perceptual threshld Often ccurs in the secndary and tertiary neurns Inhibitry mdulatin can be vercme

7 Sensry Pathways in the Brain - Mst pathways except the lfactry pass thrugh the thalamus n their way t the cerebral crtex Stimulus - CNS can distinguish fur prperties f a stimulus: Mdality/nature Lcatin Intensity Duratin Mdality r Nature - Indicated: By which sensry neurns are activated By where the pathway f activated neurns terminate in brain - Each receptr type is mst sensitive r specific t a particular mdality f stimulus E.g. sme neurns respnd mre t tuch, thers t changes in temp - Each sensry mdality can be subdivided E.g. clur visin detected by red, blue and green wavelengths - Labeled line cding The brain assciates a signal cming frm a specific grup f receptrs with a specific mdality 1:1 assciatin f receptr with sensatin e.g. stimulatin f a cld receptr will always be perceived as cld Lcatin - The lcatin f a stimulus is cded accrding t which receptive fields are activated - Sensry regins f cerebrum are highly rganised with respect t incming signals Input frm adjacent sensry receptrs is prcessed in adjacent regins f the crtex

8 E.g. tuch receptrs in the hand prject signals t a specific area f the cerebral crtex - Stimulatin f that part f the cerebral crtex during brain surgery is interpreted as a tuch t the hand, even if there was n cntact - Phantm pain ccurs when secndary neurns in spinal crd becme hyperactive resulting in the sensatin f pain in limb that is n lnger there - Auditry infrmatin is an exceptin Neurns in ear sensitive t different frequencies, have n receptive fields and prvide n infrmatin abut the lcatin f the sund Difference in time it takes fr sund stimuli t reach the tw sides f the auditry crtex is registered by brain and used t cmpute the sunds surce Sund frm directly in frnt will reach ears simultaneusly Sund placed ff t ne side will reach the clser ear several millisecnds befre - Brain uses timing differences rather than specific neurns t lcalise sund - Lateral inhibitin Increases cntrast between activated receptive fields and inactive neighburs and s islates the lcatin f the stimulus Used in visual system t sharpen edges - E.g. pressure stimulus t the skin: Pin pushing n skin activates 3 primary sensry neurns Each release neurtransmitters nt secndary neurn Nt all respnd in the same way Neurn B (clsest t stimulus) suppresses the respnse f the lateral neurns, where the stimulus is weaker allwing its wn pathway t prceed withut interference Inhibitin f lateral neurns = mre lcalised sensatin - Als demnstrates ppulatin cding multiple receptrs functining tgether t send the CNS mre inf

9 Intensity - Cded by: Number f receptrs activated (ppulatin cding) Frequency f actin ptentials cming frm thse receptrs (frequency cding) - Ppulatin cding fr intensity ccurs because nt all receptrs have the same threshld t their preferred stimulus E.g. nly mst sensitive receptrs respnd t lw threshld intensity - stimulus intensity = receptrs activated - CNS translates the number f active receptrs int a measure f stimulus intensity Prperties f Stimulus - Once stimulus reaches threshld actin ptentials fire - As stimulus intensity increases, the receptr ptential amplitude (strength) increases in prprtin and the frequency f actin ptentials in the primary sensry neurns increases up t a maximum rate - Receptr ptential is an initial respnse f a receptr cell t a stimulus, cnsisting f a change in vltage acrss the receptr membrane prprtinal t the stimulus strength - The intensity f the receptr ptential determines the frequency f actin ptentials travelling t the nervus system Duratin f Stimulus - Cded by duratin f actin ptentials Lnger stimulus = lnger series f actin ptentials in the primary neurn - Hwever, sme receptrs can adapt r cease t respnd Tnic and Phasic Receptrs - Receptrs fall int 2 classes depending n hw they adapt t cntinuus stimulatin In sme receptrs, K+ channels pen = replarisatin, in thers, Na+ channels inactivate

10 - Tnic receptrs: Slwly adapting fire rapidly when first activated, then slw and maintain their firing as lng as the stimulus is present E.g. barreceptrs, irritant receptrs, tactile receptrs, prpriceptrs (generally parameters under cnstant mnitring) - Phasic receptrs: Rapidly adapting fire when they first receive a stimulus but cease firing if the strength f the stimulus remains cnstant E.g. sense f smell. Can smell clgne in the mrning when applied but as day ges by lfactry receptrs adapt Allws the bdy t ignre inf that has been evaluated as nt being a threat t hmestasis Only way t create new signal increase intensity r remve stimulus and allw receptr t reset - Accessry structures may decrease the amunt f stimulus reaching a receptr E.g. tiny muscles in ear cntract + dampen vibratin f ssicles in respnse t lud sunds = sund signal befre reaching auditry receptrs Smatic Senses: Mdalities - Tuch

11 - Prpriceptin tells brain infrmatin abut lcatin f limbs in space. Muscle spindles and Glgi tendns infrm abut muscle length and tensin - Temperature - Nciceptin Pain Itch Smatic Senses: Pathways The Smatsensry Crtex part f the brain that recgnises where ascending sensry pathways riginate

12 Temperature Receptrs - Free nerve endings that terminate in subcutaneus layers f skin - Heat receptrs start t perceive heat abve 30 C and cntinue t perceive heat until the maximum receptr stimulatin which ccurs at 45 C Over 45 C, pain receptrs take ver t avid damaging the skin and bdy - Cld receptrs nly start t perceive cld belw 35 C Nrmal cre bdy temperature is 37 C S yu start t feel cld pretty quickly - There is bviusly a 5 crss-ver where bth ht and cld receptrs are perating - Temperature receptrs slwly adapt between C and their sustained respnse tells us abut the ambient temperature - Outside this range, where the likelihd f tissue damage is high, the receptrs d nt adapt TRPV1 (Vanillid Receptr) - Respnsible fr detectin and regulatin f bdy temperature - Prvides sensatin f scalding heat and pain (nciceptin)

13 - Capsaicin inhibits t varying degrees cmplex I and cmplex III f the electrn chain Cld Receptr - The transductin f temperature in cld receptrs is mediated in part by the TRPM8 channel

14 This channel passes a mixed inward catinic (predminantly carried by Na + ins althugh the channel and is als permeable t Ca 2+ ) current f a magnitude that is inversely prprtinal t temp The channel is sensitive ver a temperature range spanning abut C - TRPM8 can als be activated by the binding f an extracellular ligand Menthl can activate the TRPM8 channel in this way Since the TRPM8 is expressed in neurns whse physilgical rle is t signal cling, menthl applied t varius bdily surfaces evkes a sensatin f cling - The feeling f freshness assciated with the activatin f cld receptrs by menthl, particularly thse in facial areas with axns in the trigeminal (V) nerve, accunts fr its use in numerus tiletries incl. tthpaste, shaving ltins, facial creams etc. Nciceptrs (Pain Receptrs) - Free nerve ending f sensry neurn - Many stimuli have been fund t activate in channels present n nciceptr terminals that act as mlecular transducers t deplarise these neurns, thereby setting ff nciceptive impulses alng the pain pathways - Respnd t strng nxius stimulus that may damage tissue Chemical, thermal (heat) r mechanical - TRPV1 In channels are nn selective catin channels (mainly Na) Fund n nerve endings and mediate a pain respnse These respnd t heat e.g. stve and capsaicin in ht chilli peppers Nciceptr activatin is mdulated by lcal chemicals that are released upn injury E.g. histamine, prstaglandins, sertnin, platelets, substance P i.e. mediate inflammatry respnse These chemicals can activate the pain receptrs r sensitise them by lwering their activatin f threshld Inflammatry pain increased sensitivity t pain at sites f tissue damage - Nciceptrs activate 2 pathways Reflexive prtective respnse Integrated in spinal crd Withdrawal reflex Smatsensry Nerve Fibers Ascending pathway t cerebral crtex Becmes cnscius sensatin (pain r itch) Nciceptrs: Pain and Itch - Itch (puritis) Receptrs in skin Histamine activates C fibers causing itch

15 - Pain Subjective perceptin Fast pain Sharp and lcalised by Aδ fibers Slw pain Mre dull r diffuse by C fibers E.g. stubbed te quick stabbing sensatin (fast pain) fllwed shrtly by dull thrbbing pain (slw pain) The Gate-Cntrl Thery f Pain - In absence f input frm C fibers, tnically active inhibitry interneurn suppresses pain pathway - With strng pain, C fiber stps inhibitin f the pathway, allwing a strng signal t be sent t the brain - Pain can be mdulated by simultaneus smatsensry input This thery explains why rubbing a bumped elbw r shin lessens yur pain Referred Pain - Occurs because multiple primary sensry neurns cnverge n a single ascending tract

16 - Brain cannt distinguish visceral signals frm the mre cmmn signals arising frm smatic receptrs it interprets pain cming frm the smatic regins rather than the viscera