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Flashcards in Post Midterm 3 Deck (80)
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why is the somatosensory system important

reach and grasp
how we interact by motor systems in the environment
implant to control robotic arm in paralysed man
-parietal cortex
= how robot arm controlled / where electrodes put
tongue used as a substitute for sight - grid on tongue, iphone camera used, projected onto tongue = allows to "see" `


skin is the ...

heaviest organ in the body


layers of the skin named and explained

epidermis - outer layer, made of flat cells (squamous cells) then round cells (basal) then melanocytes which produce melanin. for heat etc
dermis - below epidermis and contains four kinds of mechanoreceptors that respond to stimuli such as pressure, strecthing and vibration = this bit does transduction!


name the 4 mechanoreceptors

meissner corpuscles
ruffini cylinders in collgen (elastic bit)
pacian corpustules = deep down


over view / organisation types of mechanoreceptors

temporal response properties (how quikcly adaptation to pressure occurs)
SA (slowly adapting fibres) = merkel (SA1) and ruffini (SA2), fire continuously as long as pressure is applied
RA (rapidly adapting fibres) = meissner (RA1) and pacinian (RA2), fire at onset and offset of stimulation (but not during, rapidly adapt)
1s = superficial layers of dermis
2s = deeper, in collagen and subcutaneous fat


merkel cells

specialized epithelia cell - located in superficial layers
filled with vesicles, and a nerve fibre sitting in the cell
pressre causes the release of transmitters from vesicles
found in fingertips, supports high acuity light touch
slowly adapting responds continuosly to input
as long as stimulus is on = action potentials
just like in visual system, stronger driving of cell = more action potentials per unit time


how have mechanoreceptors been studied

barely at all compared to other sense
found a way to label/ highlight each type of cell
stimulus probe
when touch = displacement = ion channel open, clacium and sodium in (this is for merkel slow adapting cells)


meissner corpuscle

close to surface
stacked cells with afferent nerve fibre running through layers
deformation of layers in corpuscles triggers action potential (allows influx of positive ions)
rapidly adapting
work similarly to merkel
as cells displaced from pressure = ion channels open. layers so displaced and mishapen
removal signal too


ruffini cylinder

deep layer
register stretch of skin - useful for detecting slippage and adjusting grip apperture
as collagen stretches (connective tissue), will allow influx of positive ions to enter nerve fibre
slowly adapting - continuous firing as long as stimulus prestent


pacinian corpuscle

deep receptor
onion like shape - pushing causes rotation of disks with each other - this rotation allows influx of positive ions
requires more stimulation to activate, and is rapidly adapting


receptive fields of mechani receptros

surface receptors have smaller repceptive fields than deep receptors
so SA1 and RA1 = small
SA2 and RA1 = big


pathway from skin to cortex

nerve fibres travle in bundles (peripheral nerves) to the spinal cord
two major pathways in the spinal cord
-medical lemniscal pathway = touch
-spinothalamic pathway = pain
these cross over to the opposite of the bosy and synapse in the thalamus, and then on to the somatosensory cortex or S1
S1 = behind central sulcous


explain the medial lemniscal pathway

consists of large fibres that carry proprioceptive and touch information


explain the spinothalamic pathway

consists of smaller fibres that carry temperature and pain information


maps of the body on the cortex

signs travel from the thalamus to the somatosensory receiving area (S1) and the secondary receving are (S2) in the parietla lobe
body map (homonculus) on the cortes shows more cortical space allocated to parts of the body that are most sensitive to detail eg enormous hands, lips, tongue
exisits in somatosensory cortex and in motor cortex
large area = greater tactile sensistivity


plasitcity in S!

highly plastic
plasticity in neural functioning leads to flexible changes in how cortical cells are allocated to body parts
S1 is able to rapidly reorganise - even removal of body parts
eg train animal to use a particular part of its body = cortical part gets bigger
S1 doesn't grow, just takes up more space and other bits in S1 shrink
experience dependent


duct taping monkeys fingers together

S1 mapped before and after
once monkey learned to use two fingers as one unit the receptive fields crossed the boundaries betwee the two fingers = 3rd and fourth unit as one finger both physically and in the brain


phantom limb dis

example of how S1 can reorganise very quickly
the persistent sensation of an appendage, after removal by amputation or simple denervation
Ramachandran and colleagues have shown that touching the face of a phantom limb patient leads to sensations in the missing hand or arm
this leads to the hypothesis the brian is filling in for the missing stimulation in the hand and arm representation in the somatosensory system
connections in the cortex allow hand area to be stimulate by adjacent face region = how cortical representation can cross boundary
facial cortical space branches into space taken up by missing hand / arm


racoon experiment with phantom limb

cut off fourth finger
found cortical tissue immediately started responding to third and fifth digit = neighbours


how ramanchandran did it

Phantom limbs and plasticity: Ramachandran et al. (1992) tested a patient
whose right arm had been amputated some years earlier.
In this case, touching the face caused sensation of both the face and the
missing limb.
Note the proximity of the face and hand representation in cortex. The proposed
explanation is that connections within cortex allowed the hand areas (which now
did not receive any input from the hand) to be activated by stimulation of the
adjacent face regions. This stimulation was still experienced, however, as
stimulation of the hand.


graziano disagreement

drove cells in the brain until area physically moved (eg chewing area until monkey moved)
somatosensory cortex = map of actions / potential actinos
controversial hypothesis
still thought to be honculus (passive)


measuring tactile acuity

how to measure somatosensory function
2 ways
-two-point threshold, minimum separation needed between two points (2 needles) to perceive them as two units
-grating acuity, placing a grooved stimulus on the skin and asking the participant to indicate the orientation of the grating - change dimensions to get threshold
diff parts of the body = diff so fingertips dense in merkel so very sensitive


3 main factors that affect tactile acuity

1 density correlates with acuity - more dense = higher acuity. fixed receptive field size and density, but changing the separation of points (again excitatory centre and inhibitory surround)
2 size correlates with acuity - smaller = higher acuity. fixed separation of points, changing receptive field size
3 fingertips have special extra acuity = have diff brain representation in S1


special fingertips

there is a high density of merkel receptors / SA1 fibres in the fingertips
merkel receptors are densley packed on the fingertips - similar to cones in the fovea
both 2-point thresholds and grating show greater acuity for fingertips compared to base of finger compared to palm
but density of merkel cells is not enough to explain changes in acuity
thresholds increase from to the index finger to the pinky, but the density of merkel receptors is the same accross the fingers - something else is at play = cortical area in the brain (S1). index finger = lowest mean threshold so discriminate two needles quicker with smaller distance
central sulcus = greatest cortical representation for index over pinky finger . how experience effects cortical representation


object perception

somatosensory cortex shows cells that respond maximally to orientations and directions of movement
selectivity for different features, just like in the visual system
spaciotemporal receptive field
direction selective etc
have basic features but also more complicated


complex motor coding

specific reaching and tactile inputs
this selectivity is expressed in differetial response in somatosensory cortex to grasping different objects
so same graspoing movement but different objects = fifring for one object but not another from a particular single cell


similarities between tactile perception and vision

-Center-surround receptive fields
• Receptive fields of different sizes
• Acuity (as measured with the two-point threshold) varies with location on the body
• Sensitive to movement
• Selective with respect to edge orientation and direction of


star nosed mole

mole system for sensory cortex
does not see
has a bunch of fingers it uses to see the world around - nasal rays
taken notion of having notion of having lots of small and dense receptors
sensory substitution
thumps table alot = makes image of world around
blows bubbles to smell world around him


pain perception - nocioceptive

signals imdending damage to the skin
types of nocioceptors respond to heat, chemicals, severe pressure and cold
info carried back to nervous system carried by two routes
A-delta fibres


A-delta fibres (pathway)

– A-delta fibers –myelinated, fast conduction times (~20meters/s),
sharp initial response to mechanical or thermal input