Lecture 7: Somatosensory Pathways

Question 1

The dorsal column is mainly involved in?

Answer 1

Conscious Proprioception.

Question 2

Which are the three main long tracts of the nervous system?

Answer 2

(1) Lateral corticospinal tract, (2) posterior column - medial lemniscal pathway and (3) anterolateral pathways.

Question 3

For the posterior column - medial lemniscal pathway, name the function as well as the name and level of decussation.

Answer 3

Function: sensory, including vibration, joint position sense and fine touch.

Name and level of decussation: internal arcuate fibers (lower medulla).

Question 4

For the anterolateral pathways, name the function as well as the name and level of decussation.

Answer 4

Function: sensory, including pain, temperature and crude touch.

Name and level of decussation: anterior commissure (spinal cord).

Question 5

Which subdivisions of the anterolateral pathways can be differentiated? Which essential brainstem areas form part of these subdivisions and what is the related function?

Answer 5

(1) Spinothalamic tract and (2) spinomesencephalic tract:
project to periaqueductal grey (PAG) and is involved in central modulation of pain.

(3) spinoreticular tract: projects to locus caeruleus (LC) and is involved emotion / arousal aspect of pain.

Question 6

Name the three types of pain and explain each.

Answer 6

(1) Nociceptive pain: nociceptor gets stimulated by chemical, thermal or mechanical event that could cause tissue damage.
(2) Neuropathic pain: disease or trauma damages the nervous system itself (in PNS: neurogenic).
(3) Central pain: type of neuropathic pain, due to damage the brain, brainstem or spinal cord (allodynia).

Question 7

Mainly three types of fibers are involved in sensory information transmission. Name and describe their main features. Which are involved in nociception?

Answer 7

(1) A-Beta fibers: thick, myelinated fibers, quick in information processing.
(2) A-Delta fibers: thin, myelinated fibers.
(3) C fibers: unmyelinated fibers.

A Delta and C fibers are mainly involved in nocicpetion.

Question 8

Give the name of a pain mediating neurotransmitter.

Answer 8

Substance P.

Question 9

Name the two main pathways involved in pain transmission. Where do they project mainly and what is the specific function?

Answer 9

(1) Paleospinothalamic (medial) tract: projects to prefrontal cortex and is involved in the emotional aspect of pain.
(2) Neospinothalamic (lateral) tract: projects to the somatosensory cortex and is involved in the perceptual aspect of pain.

Question 10

While ascending from the spinal cord to the cortex, the pain pathways project to at least three supraspinal pain suppressing systems. Name all of them.

Answer 10

(1) Locus caeruleus, (2) periaqueductal grey and (3) the prefrontal cortex.

Question 11

Explain the so-called ‘gate-control theory of pain’.

Answer 11

When experiencing pain, C fibers are dominant in signal transmission to laminar 5 of the spinal cord (open gate). However, as soon as one is rubbing over the painful spot (light touch), A-Beta fibers will be activated, being much faster in projecting to laminar 5 where they activate an inhibitory interneuron, which closes the ‘gate’ for nociceptive information (note: this is only a matter of degree; there is still pain, just less of it).