NEURO: Somatosensation I

Question 1

What does the somatosensory system convey?

Answer 1

The somatosensory system conveys sensations from the body such as:

• touch
• proprioception
• heat, cold
• pain, itch

Question 2

Describe the structure of a peripheral nerve.

Answer 2

A nerve is essentially a bundle of axons.

The nerve itself is covered in epineurium, a connective sheath covering. Inside it are several fascicles, each of which are covered in perineurium. Each fascicle contains multiple axons, each of which are covered in endoneurium.

Question 3

Broadly, there are two anatomically and functionally distinct systems in the somatosensory system.
Explain.

Answer 3

One has large fibres (have a large diameter, myelinated, fast conduction): tactile and proprioceptive.
It’s called the dorsal column medial lemniscal system (DCML), and it mediates discriminative touch, vibration and proprioception.

The other has small fibres (small diameter, thinly-myelinated or non-myelinated, medium or slow conducting): temperature, pain, itch, crude touch.
It’s called the spinothalamic tract (STT, also known as anterolateral system), and it mediates touch, temperature and pain.

Question 4

List the receptors of the somatosensory system.

Answer 4

PROPRIOCEPTION:
A-α afferents: large diameter, myelinated, fastest conducting (≤100 m/s)
- Muscle spindles

TACTILE AFFERENTS (discriminative touch):
A-β afferents: large diameter, myelinated, 2nd fastest conducting (30-70 m/s).
Cutaneous:
- Meissner’s corpuscles (rapidly adapting)
- Merkel’s discs (slowly adapting)

Deep:

• Ruffinni corpuscles (slowly adapting)
• Pacinian corpuscles (rapidly adapting)

FREE NERVE ENDINGS (low-resolution tactile, temperature, pain):

• A delta fibres - small diameter, thinly myelinated, moderate conduction velocity (≤30 m/s)
• C fibres - small diameter, unmyelinated, slow conducting

Question 5

Describe the regional variation in the cortical cytoarchitecture.

Answer 5

Different areas of cortex have the same basic cell types organised in layers, with the same basic organisation.
Regional differences can be identified on the basis of relative thickness of the different layers, cell size and density.

Brodmann’s areas are cytoarchitectonic areas.

Question 6

Describe the columnar organisation of the cortex.

Answer 6

Cells with similar receptive fields are organised in vertical columns.

Question 7

What information does the dorsal root convey?

Answer 7

sensory

Question 8

What information does the ventral root convey?

Answer 8

motor

Question 9

Where are cell bodies located?

Answer 9

dorsal root ganglion - these are the sensory receptors of the somatosensory system

Question 10

What class is used for braille?

Answer 10

A-beta

Question 11

What does the ability to localise depend on?

Answer 11

depends on sensory receptive fields

Question 12

What are the two major central pathways of the somatosensory system?

Answer 12

Dorsal column – medial lemniscal system (DCML)
mediates discriminative touch, vibration, proprioception
Inputs from A-β and A-α afferent fibres

Spinothalamic tract (STT, also known as anterolateral system)
coarse touch, temperature, pain
Inputs from A-δ and C fibres

Question 13

Explain the DCML pathway for the body

Answer 13

DCML is the ascending pathway.
Info relayed from one side of the body will be processed in the opposite side of the brain.

Lower limb - thoracic SC - just has gracile nucleus
Upper limb - cervical SC - has gracile and cuneate nucleus

For the body:
A-beta may send out information into the spinal cord (cervical or thoracic) where the main axon ascends the white matter (dorsal columns) after passing through dorsal root ganglion where cell bodies are found

Neurons then project to the dorsal column nuclei in the medulla where they synapse to 2nd order neurons

These then cross the medial leminiscus (midline) of the medulla to the midbrain

They synapse again in the ventral posterior nucleus of the thalamus into 3rd order neurons

These 3rd order neurons project to the primary somatosensory cortex in the cerebral cortex

Question 14

Explain the DCML pathway for the face

Answer 14

This is mediated by the cranial nerves instead of the spinal nerves

Mechanosensory receptors from the face send information via axons through the trigeminal ganglion (where cell bodies are found to the principal nucleus of trigeminal complex in the medulla where they snyapse

The 2nd order neurons then pass the medial leminiscus and project to the ventral posterior medial nucleus (synpase again) of the thalamus via the trigeminal leminiscus

The 3rd order neurons then project to the primary somatosensory cortex