Central somatosensory pathways

Question 1

What is the purpose of somatosensory afferents?

Answer 1

To convey information from the skin surface to the central circuits

Question 2

Describe the main tactile mechanoreceptive pathway?

Answer 2

3 neuron sequence to reach the somatosensory cortex. Third neuron is in the ventral posterior nucleus of thalamus.

Decussation occurs in the caudal medulla.

Question 3

For example, describe the pathway of how being pinched on your toe would be perceived in your brain?

Answer 3

Mechanoreceptor axons from lower body would travel up the lumbar spinal cord, through the cervical spinal cord, decussate in the caudal medulla gracile nucleus (for all pathways from the lower body).

The fibre would then pass through the medial lemniscus of the pons and midbrain before synapsing in the ventral posterior nucleus of the thalamus.

From there, nerve fibres would travel up to the primary somatic sensory cortex.

Question 4

If you were pinched in the toe the corresponding afferent nerve axon would decussate in the gracile nucleus of the caudal medulla. Had you been pinched in the arm where would the nerve fibre decussate?

Answer 4

Also the caudal medulla, but in the cuneate nucleus (all pathways of upper body).

Question 5

Describe the primary somatosensory cortex?

Answer 5

Also called SI, its located in the parietal lobe (postcentral gyrus), comprises four distinct regions known as Brodmann’s areas 3a, 3b, 1, and 2.

Each of the 4 areas contain a complete and seperate representation of the body.

Question 6

How is each region within the primary somatosensory cortex divided in terms of response to stimuli?

Answer 6

1 & 3b respond to cutaneous stimuli.
2 responds to tactile and proprioceptive stimuli.
3a responds to proprioceptive stimuli mainly.

Question 7

how does the functional hierarchy of the somatosensory cortex work?

Answer 7

ventral thalamus projects to particular regions of primary somatosensory cortex (1,2,3a,3b).

Then primary projects to secondary somatosensory cortex (cognitive touch) OR to Parietal areas 5,7.

Secondary cortex projects to amygdala and hippocampus. While the Parietal areas project to motor and premotor cortical areas.