Lec 5 : Somatic Sensory System (nearly word-for-word what was said in lecture)

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17 Feb 2012

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Lec 5 : The Somatic Sensory System
Slide 5:
Somatic sensation:
oActually multiple modalities (touch, proprioception, pain, temperature)
oTouch, pain, temperature; skip proprioception (another aspect of
somatosensation look at with motor control)
oEfference (not this week)
oHaptics: perception conveyed by touch, in combo w/ motor control that is
active exploration of objects (because of the way that you control your
fingers when you touch something, also gives you info about where
something is located); also gives much more vivid impression of shape /
size of objects; where touch plays a role in interaction w/ objects (such as
in recognizing them; touching something in darkness, reading Braille)
oTouch often goes together with motor control
oEnables body to feel itself & enviro, ache, chill
oSomatic sensory system = diff from other systems
Receptors = distributed throughout (esp on skin surface)
Responds to diff kinds of stimuli
Slide 6:
oWhat receptors are involved?
oPrimary afferent neurons
Relay info directly from skin to the brain (may have very long
axons if coming from foot)
oSpinal cord
oWhich pathways subserve touch?
oSomatosensory cortex
Slide 7:
Types of skin:
oHairy vs. glabrous (hairless)
Layers of skin:
oEpidermis (outer) vs. dermis (inner)
Functions of ksin:
oProtective fxn
oPrevents evaporation of body fluids
oProvides direct contact w/ world (give info about touch
oIn dermis
oMajority of somatosensory receptors are mechanoreceptors
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oNot just skin; also found in heart, blood vessels, organs, gum
oUnmyelinated axon branches w/ mechanosensitive ion channels
Slide 8:
Mechanoreceptor types:
Scientists Type Info
-near top of dermis, under epidermis
Merkel’s Disks -connects to a cell that isn’t a nerve
Italian Pacinian corpuscles
Ruffini’s endings
In skin covered with mucus, have other receptors like: Kraus end bulbs (similar to
Merkel’s disks but they have bulbs at the end)
Slide 9:
Mechanoreceptors look different and differ in what they do and in terms of spatial
resolution (receptive fields tend to differ)
oRecording from individual axons, in this case, the median nerve;
stimulated the hand to figure out where does this axon show
responsiveness (where can you get APs)
oFor Meissner’s corpuscles, you get very small receptive fields; Pacinian
corpuscles, much larger receptive fields can be due to the fact that
individual nerve endings are designed differently or due to it going to diff
Slide 10:
oDiffer in terms of receptive fields –
Meissner’s corpuscles and Merkel’s Disks have smaller receptive
fields could be b/c they’re closer to epidermis; the deeper you
go, the more diffuse the responsiveness will be (although there are
other reasons for why they have M&M have small receptive fields)
oDiffer in terms of time –
Meissner’s corpuscles and Pacinian corpuscles have higher degree
of adaptation
Curve: some kind of mechanical force as a function of time – only
see APs at the initial point but not when the mechanical force is
maintained and then it briefly responds again as the pressure is
released; this is even more true for Pacinian corpuscles = couple
APs at the onset of mounting pressure and then it discontinues
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Pacinian corpuscles are most responsive to higher freq; Meissner’s
corpuscles are responsive to frequencies around 50 Hz (skin slip
50 Hz = info about skin slip
oskin slip example = hold coffee cup in hand but don’t hold it strong
enough, you feel it slip thru your hand but you almost automatically
increase your force so it doesn’t fall
ograsping things
omeissner’s corpuscles
Pacinian corpuscles have even higher range of frequencies where they’re most
oOnly see response at onset of mounting pressure; as the mounting pressure
becomes continues, doesn’t respond
oFirst contact of pressure of low freq then suddenly to high freq = sensed
by Pacinian corpuscle
Merkel’s disk and Ruffini’s ending = slower adaptation = fire more continuously
oMerkel’s disk = show higher rate of APs as the pressure mounts and then a
bit less as it’s maintained; so they do adapt as well, but not at the same rate
oRuffini ending = adapt even more slowly, more sustained, but at some
point they do decrease a bit
Merkel’s disk:
oShow sustained activation and have smaller receptive fields therefore
good at pattern/texture recognition (Braille)
Ruffini ending:
oThings that are maintained longer, like lateral stretch; hand shape; for
example when you wrap your hand around something
So these mechanoreceptors show diff properties in terms of adaptation, which doesn’t
necessarily have to do with the nerve endings themselves, but rather with what they’re
wrapped in.
Slide 11:
What causes frequency tuning?
oPacinian corpuscles are wrapped in many layers (onion-like
encapsulation); these layers absorb the NRG from the external pressure; as
you push down with the device, the force increases transiently and works
on the plasticity of the corpuscle this is what creates these transient
responses (quick adaptation where the cell doesn’t respond anymore)
oWhen you strip layers off, this is the same response pattern of the same
nerve ending so, it’s the endings themselves and their physical
properties that create the differences btn the 4 mechanoreceptors
Gives rise to diff ranges of sensitivity to diff frequencies
oPacinian corpuscle = max around 300 Hz
oMeissner’s corpuscle = around 50 Hz
oBlue and red curves have diff minimums
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