the somatosensory system- organisation of sensory information in the cortex

Question 1

what type of matter does the spinal cord consist of?

Answer 1

-grey matter and white matter

the proportion of white and grey matter varies along the length of the spinal cord

Question 2

what does grey matter consist of?

Answer 2

-cell bodies and sensory afferent terminals

Question 3

what does white matter consist of?

Answer 3

-fibre tracts

Question 4

what can grey matter be subdivided into ?

Answer 4

-dorsal (posterior) horns
- ventral (anterior) horns
-ten distinct laminae of Rexed

Question 5

what does the dorsal column consist of?

Answer 5

Medial gracile tract and the more lateral cuneate tract

Question 6

where does sensory input from T6 and below travel compared to above T6 in the dorsal column?

Answer 6

T6 and below (including legs and lower trunk):
-gracile tract

> T6 (including arms and upper trunk):
-cuneate tract

Question 7

describe the dorsal column medial lemniscal pathway (part of the ascending tract)?

Answer 7

-Major route by which touch and conscious proprioceptive information ascends to the cerebral cortex (apart from anterior head)!!

• 1st order neurone enters dorsal horn and branches forming
  
  • Synapses deep in the dorsal horn upon 2nd order neurones (important in spinal reflexes)
  • Long ascending axon (via the dorsal column gracile, or cuneate tracts) synapsing in either the dorsal column gracile nucleus or cuneate nucleus
• Axons of 2nd order neurones of the dorsal column nuclei cross collectively in the great sensory decussation and ascend in the medial leminscus to the posterior lateral nucleus of the thalamus
• 3rd order thalamic neurones project to the primary somatosensory cortex via the posterior internal capsule

Question 8

what type of sensory information does the DCML pathway conduct?

Answer 8

• Sterognosis (ability to recognise an object by feeling it)
• Vibration detection
• Fine touch (two point discrimination)
• Conscious proprioception (awareness of body position and awareness of body movements)

Question 9

what is lateral inhibition?

Answer 9

-a neuron’s response to a stimulus is inhibited by the excitation of a neighbouring neuron

-this means only the neurons stimulated/ least inhibited will be excited

-it is used to sharpen stimulus perception and is one mechanism by which information is transformed at the synapses of the DCML pathway

Question 10

what is contrast enhancement?

Answer 10

as information is conveyed from one neurone to another in the sensory pathway, differences in the activity of adjacent neurones are amplified

Question 11

explain the trigeminal system

Answer 11

-General somatic information from the anterior head (including oral and nasal cavities, paranasal air sinuses, teeth, intracranial dura and cerebral arteries) is mediated by two trigeminothalamic pathways

-Impulses originate at the terminals of the trigeminal nerves

-soma (cell body) of the sensory neurones are located in the trigeminal sensory ganglion

-central terminals of the trigeminal nerve synapse upon second order neurones in the chief sensory nucleus (general tactile stimuli) or spinal nucleus (pain, temperature information) which in turn decussate and project via the trigeminal lemniscus to the ventroposteriomedial (VPM) nucleus of the thalamus

-third order neurones relay information to the cortex via the thalamocortical neurones

Question 12

where is the somatosensory cortex located?

Answer 12

-in the post central gyrus of the parietal cortex
-immediately posterior to the central sulcus and adjacent to the posterior parietal cortex

Question 13

what are Brodmann areas?

Answer 13

-the somatosensory cortex is described as consisting of Brodmann areas 1, 2, 3a and 3b

Question 14

what is the homunculus and why is it important in somatosensory?

Answer 14

-there are 3 homonculi in the brain (2 motor and 1 somatosensory)

• The homunculus reveals the relative area of cortex devoted to each body part and is correlated with the density of sensory input from that part (not its size) and hence sensory acuity and importance of sensory input
• The homunculus is discontinuous - the toes are at the top of the post central gyrus with the tongue at the lower end, but the had separates the head from the face

Question 15

how many layers are there in the somatosensory cortex?

Answer 15

6 layers (I-VI)

Question 16

what extends across the 6 layers of the somatosensory cortex?

Answer 16

vertical columns of neurones extend across the 6 layers
-each column consists of neurones with similar inputs and responses

Question 17

what is corticol plasticity/ neuroplasticity?

Answer 17

-the brain has the ability to reorganize itself by forming new neural connections based on individual experiences, lifestyle and environment

e.g.
1. If the sensory input from a finger increases, the cortical representation of that digit, over months, increases, relative that of inputs generating less activity

2. If a finger (say D3) is e.g. lost, the area of the cortex formerly representing that digit after several months will now respond to stimulation of the adjacent digits (D2 and D4)

Question 18

role of the posterior parietal cortex?

Answer 18

-it receives and integrates information from the somatosensory and other corticol areas (visual, auditory) and sub corticol areas (thalamus)