Somatosensory System 2

Question 1

what is contained within grey and white matter of the spinal cord?

Answer 1

grey matter = cell bodies and sensory afferent terminals

white matter = ascending and descending tracts

Question 2

what are the divisions of grey matter in the spinal cord?

Answer 2

dorsal and ventral horns
10 distinct laminae of rexed
- laminae 1 and 2 = where nociceptors terminate
- laminae 3-6 = where LTMs terminate
- laminae 7-9 = where proprioceptors terminate

Question 3

what is the dorsal column/medial lemniscus pathway?

Answer 3

major route by which touch and proprioception ascends to the cerebral cortex (apart from anterior head)

Question 4

what are the 1st, 2nd and 3rd order neurones of the dorsal column pathway?

Answer 4

1st = enters dorsal horn and synapses on 2nd order neurones in deep dorsal horn/forms long ascending axon synapsing in gracile/cuneate nucleus in medulla
2nd = cross in great sensory decussation to other side of medulla and ascend in medial lemniscus to ventral posterior lateral (VPL) nucleus of thalamus
3rd = project to primary somatosensory cortex (S1) via posterior internal capsule

Question 5

what are the 5 abilities of the dorsal column pathway?

Answer 5

stereogenesis (ability to recognise objects by feeling them)
vibration detection (tested via tuning fork)
conscious proprioception
fine touch
weight discrimination

Question 6

what is lateral inhibition?

Answer 6

functions to remove the “noise” in sensory neuronal system (extra non-important sensory information which isn’t needed and gives false positives)
this sharpens up the sensation of the important information
collateral inhibitory interneurons branch from the neurone carrying the important information and synapse on adjacent/parallel nerves carrying “noise”, inhibiting these signals giving a simpler, sharper stimulus perception

Question 7

somatic information from the anterior head is mediated by what?

Answer 7

two trigeminothalamic pathways

Question 8

1st, 2nd and 3rd order neurones of sensory divisions of trigeminal nerve?

Answer 8

1st = area of sensation > trigeminal sensory ganglion in pons
2nd = pons > chief sensory nucleus/spinal nucleus > decussate and project via trigeminal lemniscus > VPM nucleus of thalamus
3rd = VPM of thalamus > thalamocortical neurones > cortex

Question 9

where on the thalamus do the dorsal column and trigeminal nerve synapse on the thalamus?

Answer 9

dorsal column = lateral

trigeminal = medial (VPM)

Question 10

what are the 2 parts of the somatosensory cortex?

Answer 10

both in post central gyrus

• S1 = posterior to central sulcus (primary somatosensory cortex)
• S2 = adjacent to posterior parietal cortex

Question 11

S1 somatosensory cortex is divided into what?

Answer 11

brodmann areas (BA)

• 1
• 2
• 3a
• 3b

Question 12

function of BA 1?

Answer 12

receives input from BA 3b and cutaneous mechanoreceptors

involved in texture discrimination

Question 13

function of BA 2 and what can damage here cause?

Answer 13

input from joint afferents, Golgi tendon organs, deep tissues, 3a and 3b
involved in pressure, joint position and object perception
damage causes impaired grasping and impaired size/shape discrimination (astereognosis)

Question 14

function of BA 3a?

Answer 14

receives information from proprioceptors (muscle spindles)

involved in body position sensation

Question 15

function of BA 3b?

Answer 15

receives input from 3rd order neurones which are associated with merkel cells and Meissner’s corpuscles and from 3a
involved in texture and shape perception

Question 16

how is the primary somatosensory cortex organised in terms of receptive fields?

Answer 16

receptive fields from S1 neurones form a somatotopic map (different parts of cortex relate to different parts of body)
= homunculus
(map isn’t proportional to size of body area or in order of body positions - i.e feet > face > hand > tongue etc)

Question 17

the somatosensory cortex is divided in which 2 ways?

Answer 17

divided into layers and columns

• 6 cell layers (I-VI)
• vertical columns extend across all 6 layers (each column = neurones with similar inputs and responses)

Question 18

thalamic inputs to S1 terminate mainly on neurones within which layer of the somatosensory cortex?

Answer 18

layer IV

cells in this layer then project to cell towards the surface of the cortex and also deeper layers

Question 19

there are multiple maps in the somatosensory system, where are these found?

Answer 19

adjacent somatotopic maps in BAs 1 and 3b
adjacent strips of cortex map the same parts of the body in parallel to each other
maps aren’t identical but are mirror images of each other

Question 20

what happens to the relevant area of S1 if a digit/limb etc is lost in an accident or is amputated?

Answer 20

cortical remapping - it becomes utilized by other sensory inputs

• after months the area now responds to stimulation from adjacent digits/limb
• involved in phantom limb pain

Question 21

what happens to the relevant area of S1 if the sensory input from a finger increases?

Answer 21

cortical representation of the digit in the cortex expands over time
common in string musicians etc

Question 22

what is the function of S2 (posterior parietal cortex area of somatosensory cortex)?

Answer 22

receives and integrates information from S1 and other cortical areas (visual/auditory etc) and sub-cortical areas (thalamus)
deciphers meaning from the information in S1 (e.g attaches meaning to visual information - attaching name to recognisable face)

Question 23

what can damage to S2 cause?

Answer 23

bizarre neurological symptoms with simple sensory skills in tact
e.g hemispatial neglect syndrome

Question 24

what is hemispatial neglect syndrome?

Answer 24

damage to the right parietal cortex causing patient to believe that the left side of the world does not exist and may even disclaim the existence of the left side of their body