Central Processing of Somatosensory Input

Question 1

what is the grey matter subdivided into?

Answer 1

dorsal (posterior) and ventral (anterior) horns and ten distinct laminae of rexed

Question 2

what is the fibre class of nociceptors and where do they terminate?

Answer 2

Aδ/C

terminate in laminae I and II

Question 3

what is the fibre class of LTMs and where do they terminate?

Answer 3

Aβ

terminate in laminae III to VI

Question 4

what is the fibre class of proprioceptors and where do they terminate?

Answer 4

Aα

terminate in laminae VII to IX

Question 5

describe the changes in grey and white matter as you move up the spinal cord and why is this change required?

Answer 5

more white matter and less grey matter as you move up from sacral to cervical

this is due to more afferent fibres joining further up and more white matter is needed to carry all these axons

Question 6

how is the grey matter in the ventral and dorsal horns of spinal cord divided?

Answer 6

into 10 laminae of rexed

Question 7

what laminae are found in the dorsal horn and what afferent signals terminate there?

Answer 7

I and II most posterior in dorsal horn - nociceptors found here

III to VI found in anterior of dorsal horn - low threshold mechanoreceptors found here

VII to IX found in ventral horn - proprioceptors found here

Question 8

using the dorsal column / medial leminiscus tract and the spinothalamic tract, explain what symptoms a patient with brown-sequard syndrome would experience if there was a hemisection of spinal cord?

Answer 8

dorsal column tract = touch, vibration
this would be lost on ipsilateral side of hemisection as sensory input does not cross until brainstem

spinothalamic tract = pain and temperature
this would be lost on the contralateral side to the hemisection as the sensory fibres cross segmentally in this tract

Question 9

what are the two sections of the dorsal column called and what levels do they extend to/from in spinal cord?

Answer 9

gracile = medial section (extends whole way up spinal cord)

cuneate = lateral section (extends from T6 upwards - deal with info from upper limbs)

Question 10

what can the dorsal column/medial leminscus pathway be used for?

Answer 10

stereognosis - recognise object by feeling it
vibration
fine touch - two point discrimination
conscious proprioception - awareness of body position and movement
weight discrimination

Question 11

what is lateral inhibition and why/where does it occur?

Answer 11

active neurone inhibits the activity of its neighbours via stimulating inhibitory interneurones lateral to it

this sharpens stimulus perception (eliminates background noise)

this happens in synapses of DCML pathway

Question 12

the DCML pathway does not exist for sensory information from anterior head - what picks up this information instead?

Answer 12

trigeminal divisions which feedback to the trigeminal ganglion and different nuclei in brainstem

Question 13

where is the primary somatosensory cortex located?

Answer 13

post central gyrus of the parietal cortex immediately posterior to central sulcus (SI)

adjacent to posterior parietal cortex (SII)

Question 14

what areas are found in the primary somatosensory cortex?

Answer 14

brodmann areas (BA) 1, 2, 3a and b

Question 15

what areas of the primary somatosensory cortex receive most information from thalamus?

Answer 15

areas 3a and b receive around 70%

Question 16

what sensations do each of the brodmann areas deal with?

Answer 16

3a = proprioception 
2b = touch (texture, shape, size)
1 = texture (as this receives input from 3b)
2 = pressure and joint position, distinguishing objects

Question 17

describe the organisation of the body parts in the primary somatosensory cortex?

Answer 17

the toes are at the top of the post central gyrus with tongue at lower end, but the hand separates the head from the face

Question 18

describe the organisation of the columns in the sensory cortex?

Answer 18

alternating columns of cells with rapidly adapting and slowly adapting sensory responses

Question 19

are each of the brodmann areas using the same somatotropic map?

Answer 19

yes - adjacent strips of cortex have the same parts of the body in parallel to each other

but maps are not identical, they are mirror images of each other

Question 20

what happens if the sensory input from a particular area of the body increases?

Answer 20

cortical space for that digit increases

relative to other inputs generating less activity

Question 21

describe a condition where cortical remapping can become an issue?

Answer 21

phantom limb pain

if a limb is amputated and other parts of the body take over that area of cortex, a pain stimulation in these areas can cause the brain to feel it as pain from non-existent limb

Question 22

what is the job of the posterior parietal cortex?

Answer 22

receives and integrates information from SI and other cortical areas (visual, auditory) and sub-cortical areas (thalamus)

Question 23

what condition can arise due to damage of posterior parietal cortex?

Answer 23

neglect syndrome

if damage is on RHS of posterior parietal cortex, then patients fail to acknowledge the left side of world exists
no vision problems, patients just disclaim existence of things, including own left leg