Peripheral Somatosensory Mechanisms

Question 1

List the 4 types of mechanoreceptors in glabrous skin, and whether each is superficially or deeply located

Answer 1

Meissner corpuscles (superficial)
Merkel complexes (superficial)
Ruffini organs (deep)
Pacinian corpuscles (deep)

Question 2

What innervates the mechanoreceptors in the skin?

Answer 2

Afferent AB neurons: large myelinated axons with cell bodies in the DRG

Question 3

What is the difference between slowly and rapidly adapting mechanoreceptors?

Answer 3

Slowly adapting receptors continuously encode the size of a stimulus over time until is removed
Rapidly adapting receptors only care about the change in a stimulus

Question 4

What is the difference in terms of stimulation for tacticle and nociceptive primary afferents?

Answer 4

Tactile primary afferents are low threshold
Nociceptive primary afferens have a high threshold; will only fire when tissue damage begins to occur (i.e. more informative about the internal than external environment)

Question 5

Where are Merkel complexes located? What do they respond to? Are they slowly or rapidly adapting? Do they have a high density and a small receptor field, or a low density and a large receptor field?

Answer 5

At the tips of epidermal ridges (superficial)
Indentation
Slowly adapting
High density

Question 6

Where are Ruffini organs located? What do they respond to? Are they slowly or rapidly adapting? Do they have a high density and a small receptor field, or a low density and a large receptor field?

Answer 6

In the upper dermis (deep)
Skin movement
Slowly adapting
Low density

Question 7

Where are Meissner corpuscles located? What do they respond to? Are they slowly or rapidly adapting? Do they have a high density and a small receptor field, or a low density and a large receptor field?

Answer 7

Near the skin surface (superficial)
Skin movement
Rapidly adapting
High density

Question 8

Where are Pacinian corpuscles located? What do they respond to? Are they slowly or rapidly adapting? Do they have a high density and a small receptor field, or a low density and a large receptor field?

Answer 8

Deep in the dermis and hypodermis (deep)
Vibration
Rapidly adapting
Low density

Question 9

What receptors detect pain and temperature? What sensory afferents carry this information to the CNS?

Answer 9

Free nerve endings

Thin unmyelinated C (or lightly myelinated AD)

Question 10

How are tracts of the dorsal column organised topographically?

Answer 10

Caudally to rostrally medial to lateral (lower limb closest to midline)

Question 11

Which dorsal column fasciculus and nucleus carry information relating to the lower limb?

Answer 11

Gracile

Question 12

Which dorsal column fasciculus and nucleus carry information relating to the upper limb?

Answer 12

Cuneate

Question 13

Describe the dorsal column medical lemniscus tract

Answer 13

1st-order neurons travel via the dorsal column tracts to the medulla oblongata where they synapse in the dorsal column nuclei
2nd-order neurons decussate at the caudal medulla as internal arcuate fibres to form the medial lemniscus, travel up through the midbrain and synapse in the ventral posterolateral nucleus in the thalamus
3rd-order neurons project from the thalamus to the primary somatosensory cortex

Question 14

Describe the location of the medial lemniscus in the brainstem

Answer 14

At the rostral medulla, the medial lemniscus is joined by fibres from the trigeminal nerve, and sits in line with the midline, with information about the neck, trunk and extremities carried more ventrally and information about the head carried more dorsally
By the mid-pons, the medial lemiscus has rotated so that head and upper limb information is carried in tracts close to the midline, and lower limb information is carried in more lateral tracts

Question 15

What is the role of the spinocerebellar pathway?

Answer 15

Important for coordination of movement

Question 16

What Brodmann’s area(s) make(s) up the primary somatosensory cortex?

Answer 16

1, 2, 3a, 3b

Question 17

Describe the somatotopic organisation of the primary somatosensory cortex

Answer 17

Distal structures are located most medially

Proximal structures are located most laterally

Question 18

Describe cortical plasticity in the representation of different skin regions

Answer 18

Increased stimulation can cause the cortical representation to increase in size, and vica versa (but only at the expense of surrounding regions)

Question 19

What are some possible problems encountered in peripheral nerve regeneration?

Answer 19

Peripheral nerves don’t always go to the right place and don’t change identity (so will signal for their originallocation); the central plasticity (i.e. the cortical representation) does not change (cortical representations can increase in size when exposed to increased stimulation, but cannot encode for a change in location or pattern of information)