Somatosensory system

• Perception – light touch: pressure, flutter, vibration– pain, temperature

• Transduction– light touch – pain (damage from capcascin in hot peppers)

• Central processing

General types of sensory receptors.

Idea of labeled line verses frequency code for sensation.

Idea of the adequate stimulus i.e. lowest threshold.

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

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.

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.

Fig. 8-3

What’s wrong with this??

Hairy skin.Merkels, Ruffinis, Pacinians.

Hair follicles - Guard hairsdown hairs

sinus hairs (eg. vibrissae).

Free nerve endings.

Mucocutaneous skin regions (eg mouth and lips)

Meissners, Merkels, Ruffinis, Pacinians

Increased number of free nerve endings.

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.

What’s wrong with this?

Worm mechanoreceptor

Fly bristle receptor

Cochlear hair cells

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.

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).

Frequency (Hz) Fig. 8-2

Contrast sensitivity function

Receptive field. Characteristics of cutaneous mechanoreceptors.

Pacinian - large receptive fields,

RA - small discrete rf’s. In finger - 10 to 12 hot spots which matches almost 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.

Light Touch Receptors

Type Adaptation RF size

Meissner rapid small

Merkel slow small

Pacinian rapid large

Ruffini slow Large -directional

Fig. 8-6

Recording from fibers in the finger of humansAmplitude and Adaptation

Light Touch Receptors

Type Adaptation RF size

Meissner rapid small

Pacinian rapid large

Light Touch Receptors

Type Adaptation RF size

Merkel slow small

Ruffini slow Large - directional

Receptive field. Characteristics of cutaneous mechanoreceptors.

Pacinian - large receptive fields,

RA - small discrete rf’s. In finger - 10 to 12 hot spots which matches almost 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.

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.

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.

. 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.

PSYCHOPHYSICS & AFFERENTS

SPATIAL FORM -

Psychophysics gap Think 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.

PSYCHOPHYSICS & AFFERENTS

SPATIAL FORM -

Psychophysics gap Think 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.

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.

TEXTURE - Microtexture and macrotexture.

Macrotextured surfaces can be explored with metal and/or plastic gratings.

Microtextured surfaces have a very fine grain (2 -5 microns) i.e. sandpapersSA’s don’t respond to fine grain microtextures but RA’s do respond. So it must be RA’s and possibly PC’s that carry the microtexture texture code.

General characteristics for macrotexture detection.

A. movement is essential, but there’s no difference between active or passive exploration.B. relatively invariant to speed of movementC. lubricants have no effect - so it is independent of frictionD. indentation amplitude has little effectE. increasing the contact force increases perceived roughness

Roughness magnitude linearly proportional to increasing gap (“groove”) width .1 - 3 mm. The effect of ridge width is about 1/3 that of the groove width.

Fig. 2. From Blake et al J. Neurosci 17: 7480

Fig. 3. From Blake et al J. Neurosci 17: 7480

Fig. 4. From Blake et al J. Neurosci 17: 7480

Fig. 5. From Blake et al J. Neurosci 17: 7480

Fig. 6. From Blake et al J. Neurosci 17: 7480

Fig. 7. From Blake et al J. Neurosci 17: 7480

Fig. 8. From Blake et al J. Neurosci 17: 7480

From diCarlo(2000)

From diCarlo(2000)

Spatially selective

Mild orientation selectivity

If there is a big spatial offset between the delayedInhibitory component and the excitatory center—This could be the basis for direction selectivity.

Whiskers and Barrels

• Define the barrel by staining for the enzyme CO and counterstaining for thionine (Nissl stain).

• PW – principal whisker: many excitatory neurons respond to the principal whisker alone

• AW adjacent whisker

• Barreloids in the VPM supply have input mainly from one whisker and suuply feedforward input to a single barrel. But some cells in a barreloid have multiwhisker RF’s, so that they can be excited across

• Welker et al (1993) showed that thin spikes had short latencies, < 15ms

• In vitro studies showed that many fast spiking cells were GAD positive, smooth sellate cells likely to be inhibitory cells. From this they concluded that there was fast inhibition to the cortex. But who connects to the FSU’s??

• Intracellular recording suggests the 20-30% of barrel neuron spiny stellate pairs are reciprocally connected

• FSU’s multi whisker RF, tight coupling and higher probability of firing, short latency/

• RSU’s more single whisker RF’s higher thresholds, more temporal and spatial summation and longer latency.

• Studied the different classes of cells in layer 4 of barrel cortex:

• Density of cells in layer 4 112,000mm2

• A barrel is about 0.035 mm3, a barrel has about 4000 cells

• 10% are inhibitory, 90% excitaory each thamalocortical unit influences about 1300 regular spiking cells (3600 * .37) and 250 fast spiking units (400 * 0.63).

• Used paired recording and cross correlation analysis to determine if cells were connected,

• Figure 4 shows the proportion of cells of each class connected.

• Then they measured the efficacy of connections.

Bruno & Simons (2002) J Neurosci 22:10966

• Fig 1

Bruno & Simons (2002) J Neurosci 22:10966

• Fig 2

Bruno & Simons (2002) J Neurosci 22:10966

• Fig 3

Bruno & Simons (2002) J Neurosci 22:10966

• Fig 4

• Studied the different classes of cells in layer 4 of barrel cortex:

• Density of cells in layer 4 112,000mm2

• A barrel is about 0.035 mm3, a barrel has about 4000 cells

• 10% are inhibitory, 90% excitaory each thamalocortical unit influences about 1300 regular spiking cells (3600 * .37) and 250 fast spiking units (400 * 0.63).

• Used paired recording and cross correlation analysis to determine if cells were connected,

• Figure 4 shows the proportion of cells of each class connected.

• Then they measured the efficacy of connections.