somatosensory part 4

Question 1

where is the somatosensory cortex in the brain

Answer 1

post central gyrus (immediately behind the central sulcus)

Question 2

the primary somatosensory cortex consists of which brodmanns areas

Answer 2

3a, 3b, 1 and 2

Question 3

the posterior parietal cortex consists of which brodmanns areas

Answer 3

5 and 7

Question 4

what is the primary somatosensory cortex shortened to

Answer 4

S1 (or SI - roman numeral 1)

Question 5

magnification factor of somatotopic map =

Answer 5

area of cortical representation: area on body surface
hands and mouth have a larger area of cortex

Question 6

VPL (ventro-posterior lateral nucleus) and VPS (ventro-posterior superior nucleus) in the thalamus sends parallel connectiosn to 3a,3b,1,2. There are also connections between the different cortical areas themselves. the flow of information is from…

Answer 6

left to right (3a –> 2)

Question 7

if you lesion the hand area in 3b and 3a you silences the equivalent region in area 1. is this the same in the opposite direction

Answer 7

no

Question 8

possible mechanisms of plasticity for remapping

Answer 8

sleeping synapses awakened when normal input is lost
or
there’s formation of new lateral connections
(example: when hand was amputated and individual felt sensation of amputated hadn in the face - phantom limb syndrome)

Question 9

area 3a

Answer 9

mostly deep receptors (muscle spindle and joint receptors - proprioceptive info)

Question 10

area 3b

Answer 10

• cutaneous input from RA1 and SA1
• receptive fields are small and simple
• receptive fields show surround inhibition and excitatory centre (inhibition surrounding the excitation)
• convergence of many skin receptor inputs on to one cortical receptive field

Question 11

area 1

Answer 11

• cutaneous
• centre surround
• larger receptive fields than 3b
• some proprioceptive input

Question 12

area 2

Answer 12

• cutaneous, especially hand area
• many proprioceptive
• neurons that are sensitive to movement /direction/ orientation selectivity
• receptive fields may extend into other digits (biparitie)
• some neurones respond better to grasping particular objects shapes e.g ball vs bar

Question 13

powell and mountcastle (1959)
columnar organisation of the cortex

Answer 13

neurones are organised in columns which have similar properties associated with them
the columns run normal to the surface of the cortex
cells in the same column share the same receptor modality and represent the same skin locations
they also presented evidence for alternate input (SA/RA and cutaneous/deep receptors) for neighbouring digits

Question 14

when the electrode was put in normal to the cortex there was either only cutaneous or deep input

Answer 14

when the electrode is put across layers you can see both cutaneous and deep input

Question 15

neurons with similar receptor input were clustered together on the cortical surface

Answer 15

suggests function is the property of single receptor type
SA1 thought to be responsible for texture discrimination
and RA1 for detection of stimulus movement
(sub modality segregation hypothesis - specific functions are related to a specific receptor type and theyre segregated in that way)

Question 16

hypothesis for receptor input to cortical neurons
submodality segregation

Answer 16

individual receptor types identified at the skin level, their properties remain segregated that the cortical level

Question 17

submodality convergence

Answer 17

they don’t remain segregated at the cortical level
they get input from other receptors
if this is true then at the cortical level neurons cant be purely SA or RA type neurones there must be inputs from both receptor types

Question 18

submodality convergence hypothesis:
- responses assessed with limited stimulus set, not more complex natural stimuli, may miss what is really responds to
- evidence that one receptor may be insufficient in order toc code all aspects of a particular feature - e.g movement aid fine texture discrimination

Answer 18

for any particular task of recognition you benefit by having more than one type of receptor contributing to that process

Question 19

bensmaia found neurone that has an onset response, sustained component and an offset response

Answer 19

appear to get both SA and RA input
they also found neurons that have orientation selectivity (bar on the monkeys skin in different orientations caused a change in response)
orientation signal is minimal during onset and offset
this suggests the SA1 input is responsible for the orientation selectivity
the RA input is also present but not orientation selective
they found that the times when the firing rate was the highest (onset and offset), that this is where the orientation response was minimal
evidence that the cortical neurones are getting input from different types of receptors
neurone recorded in 3b

Question 20

sa1

Answer 20

shape and texture

Question 21

RA

Answer 21

motion and grip control

Question 22

PC

Answer 22

vibrations

Question 23

the responses of neurones become for complex in the higher areas (towards area 2)

Answer 23

curvature selective neurones in area 2
there is a preference for one angle over the others

Question 24

behavioural study (randolph and semmes 1974)
trained monkeys to do a set of different discriminations, some were texture and other were angle shape discriminations
they then made lesions in area 3b, 1 and 2 in different animals

Answer 24

the 3b area lesion caused the animals to be impaired in both the texture and angle/shape discriminations
lesion in area 1, the animals were impaired in the texture discrimination only
in they lesioned area 2 the animal was impaired in the angle/shape discrimination only
area 3b is necessary for both discriminations
area 1 is more involved in texture discrimination
area 2 is more involved in angle/shape discrimination

Question 25

astereognosis in patients with posterior S1 (~area 2) cortical lesions from stroke

Answer 25

• severe deficits in discriminating spheres from ellipses
• affects hand contralateral to lesion
• also deficits in passive position and movement of limbs
• drug block of cortical area 2 in monkey leads to deficits in complex hand movement

Question 26

behind the S1 is the posterior parietal cortex (area 5 and 7)

Answer 26

this is known as the association cortex
the input received by this cortex are from other cortical areas
area 5 main input is from area 1, 2 and 3, it also gets thalamic input
area 7 gets input from the prestriate visual cortex and the thalamus. the auditory cortex and the hippocampus (spatial memory)
- outputs of these areas go to premotor and motor cortex and to basal ganglia

Question 27

area 5

Answer 27

• complex
• fields may cover more than one limb
• activated by specific hand postures (grasping, reaching, manipulating)
• after damage to this area you cant recognise objects by manipulation
• somatosensory input to the region is intact bu causes a disruption of mental image
• animal shows deficit in reaching for objects that its going to pick up