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.
What’s wrong with this??
Fig. 8-3
Hairy skin.
Merkels, Ruffinis, Pacinians.
Hair follicles Guard hairs
down 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 humans
Amplitude 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. sandpapers
SA’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 movement
C. lubricants have no effect - so it is independent of
friction
D. indentation amplitude has little effect
E. 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 delayed
Inhibitory component and the excitatory center—
This could be the basis for direction selectivity.
Whiskers and Barrels
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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.
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Studied the different classes of cells in layer 4 of barrel cortex:
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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).
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Used paired recording and cross correlation analysis to determine if cells were connected,
Figure 4 shows the proportion of cells of each class connected.
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Then they measured the efficacy of connections.
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Fig 1
Bruno & Simons (2002) J Neurosci 22:10966
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Fig 2
Bruno & Simons (2002) J Neurosci 22:10966
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Fig 3
Bruno & Simons (2002) J Neurosci 22:10966
•
Fig 4
Bruno & Simons (2002) J Neurosci
22:10966
•
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.