Understanding Somatosensory Systems: From Receptors to Perception

1. Somatosensory system • Perception • light touch: pressure, flutter, vibration • pain, temperature • Transduction • light touch • pain (damage from capcascin in hot peppers) • Central processing

2. General types of sensory receptors. • Idea of labeled line verses frequency code for sensation. • Idea of the adequate stimulus i.e. lowest threshold.

3. Types of Somatic sensory modalities. • Cutaneous sensation : Touch, vibration, tickle, itch, • deep pressure, heat, cold, pain. • Proprioceptors : Joint position, muscle stretch, • muscle tension. • Entroreceptors : Stomach stretch

7. Structural and Functional classification of receptors. • Structural classification - based on the specialized • morphology and the afferent innervation. • Physiological classification - based on the response • properties of receptive fields. • Correlate the structure with the function. In this way • you can use the number of receptors of a specific type • in each skin area determine the density. Here, however, • you would like to know the innervation ratio.

8. Glaborous skin. • Pacinian Corpuscles - have lamallae - many layers thick in the demis • Meissners - collagen fibers attaching a central spiral • nerve specialization -- epidermis • Merkels - NP shows the expanded endings flattened • against the epidermis • Ruffini’s - Nerve ending running transversely breaks into • fine mesh - dermis • Free nerve endings.

9. What’s wrong with this?? Fig. 8-3

10. Hairy skin. • Merkels, Ruffinis, Pacinians. • Hair follicles - • Guard hairs • down hairs • sinus hairs (eg. vibrissae). • Free nerve endings.

11. Mucocutaneous skin regions (eg mouth and lips) • Meissners, Merkels, Ruffinis, Pacinians • Increased number of free nerve endings.

12. Functional properties of cutaneous mechanoreceptors: • Lowenstein & Mendelson (1964) • Transduction. eg. Pacinian corpuscle has lamella with • ensheathing the nerve ending. • Adaptation RA eg. Pacinian - mechanical. But there is • additional filtering at the spike generator. So it is as if • nature has provided a double assurance that there is • complete high pass filtering. Only response to transient • vibrations.

13. What’s wrong with this?

16. Worm mechanoreceptor

17. Fly bristle receptor

18. Cochlear hair cells

19. Adaptation SA eg. in finger tip, fast component followed • by a sustained component. What does this remind you of? • Linear relation - stimulus strength and generator potential.

21. Functional classification of cutaneous mechanoreceptors. • Pacinian - PC. Obvious structure-function relation • Here the vibration response in the frequency domain • reinforces the responses seen as transients. • Rapidly adapting. Meissners, hair follicles • Slowly adapting - SA I (Merkels), SA II (Ruffini).

22. Frequency (Hz) Fig. 8-2

23. Contrast sensitivity function

25. Receptive field. Characteristics of cutaneous mechanoreceptors. • Pacinian - large receptive fields, • RA - small discrete rf’s. In finger - 10 to 12 hot spots • which matchesalmost exactly the number of Meissners • innervated by a single fiber. • SA I’s, small discrete rf’s - • SA II’s, larger rf’s, sometimes directional, • respond to stretch.

26. Light Touch Receptors

27. Fig. 8-6

28. Recording from fibers in the finger of humans Amplitude and Adaptation

29. Light Touch Receptors

31. Light Touch Receptors

33. Receptive field. Characteristics of cutaneous mechanoreceptors. • Pacinian - large receptive fields, • RA - small discrete rf’s. In finger - 10 to 12 hot spots • which matchesalmost exactly the number of Meissners • innervated by a single fiber. • SA I’s, small discrete rf’s - • SA II’s, larger rf’s, sometimes directional, • respond to stretch.

35. Thermoreceptors • :. • There are individual spots, ~ 1mm2 for • indvidual axons. • Cold receptors - 25 - 30 deg.C. myelinated A delta • fibers, dynamic response. Isolate a cold receptor spot • and stimulate > 45 deg.C get a sensation of • cold “paradoxical cold” - good labeled line evidence. • Warm receptors 40 - 43 deg. C, ‘C’ fibers, dynamic • response.

37. Nociceptors: • These are of two main types mechanical • nociceptors and mechanothermal. A delta and • C fibers, free nerve endings. • Proprioceptors: • There are a number of types of mechanorecptor • that signal the position of limbs and joints and are • important in the perception of movement and position.

38. . Peripheral representation of touch • Pressure, two point and point sensitivity on body surface • Magnitude estimation and SA fiber response • Psychometric and neurometric responses of human • Slide 17 and overhead. • Grating resolution & gap resolution need to draw the • grating on the Board.

42. PSYCHOPHYSICS & AFFERENTS • SPATIAL FORM - • Psychophysics gapThink of d’ as mean 2 - mean 1/sd1+sd2, • where the two sets of responses are drawn from Gaussian • distributions. Grating slope 4d’ units/mm gap size • Afferents and gratings • Here the experiment was withaperiodic gratings, shown • in the figure stepped across the rf center in 200 micron steps, • then indented into the skin. Shows the responses for SA’s • are modulated at the smallest size, while QA’s (RA’s ) show • no effect.

45. PSYCHOPHYSICS & AFFERENTS • SPATIAL FORM - • Psychophysics gapThink of d’ as mean 2 - mean 1/sd1+sd2, • where the two sets of responses are drawn from Gaussian • distributions. Grating slope 4d’ units/mm gap size • Afferents and gratings • Here the experiment was with aperiodic gratings, shown • in the figure stepped across the rf center in 200 micron steps, • then indented into the skin. Shows the responses for SA’s • are modulated at the smallest size, while QA’s (RA’s ) show • no effect.

50. For periodic square wave gratings there function relating • response to spatial period is linear, taking off at around • 1mm for SA fibers. • Modulation index Rmax - Rmin/Rmax+Rmin against period.