The Somatosensory System

Question 1

How can you classify sensation?

Answer 1

General sensation (body wall and viscera) - then into somatic (conscious) and visceral (unconscious)
Special sensation

Question 2

What are the modalities of somatic sensation?

Answer 2

Temperature
Pain
Pressure / crude touch
Vibration
Proprioception
Fine touch
Two point discrimination

Question 3

What modalities of of somatic sensation are controlled by the spinothalamic system?

Answer 3

Temperature (thermoreceptors)

Pain (nociceptors)

Pressure / crude touch (mechanoreceptors)

Question 4

What modalities of of somatic sensation are controlled by the dorsal column-medial leminiscus system?

Answer 4

Vibration (mechanoreceptors)

Proprioception

Fine touch (mechanoreceptors)

Two point discrimination (mechanoreceptors)

Question 5

What is another term for ‘primary sensory neurones’?

Answer 5

Dorsal root ganglion neurones

Primary afferent neurones

First order sensory neurones

pseudo unipolar neurones

Question 6

What are rapidly adapting receptors?

Answer 6

Mechanoreceptors are an example.

These respond best to change in strength of stimulation. However, frequency of firing diminishes rapidly after initial stimulus.

These are the response you are not aware of the clothes on your skin.

Question 7

What are slowly adapting receptors?

Answer 7

Nociceptors are an example.

They change their frequency of firing very little after the initial stimulus.

This explains why pain is persistent.

Question 8

What are receptive fields?

Answer 8

The area of skin that a single primary sensory neurone supplies.

Question 9

How do the receptive fields influence acuity?

Answer 9

If an area of skin is supplied by sensory neurones with a relatively large receptive field, it will have low sensory acuity (poor two-point discrimination). E.g. the back.

If an area of skin is supplied by sensory neurones with relatively small receptive fields, it will have gift sensory acuity (good two-point discrimination). E.g. the finger tips and lips.

Question 10

Why can dermatomes have “fuzzy” boundaries?

Answer 10

Because the receptive fields of adjacent sensory neurones overlap.

Question 11

How many neurones is the somatosensory system made from?

Answer 11

A chain of three neurones.

1st order, 2nd order and 3rd order.

Question 12

Describe the characteristics of the first order sensory neurones in the somatosensory system.

Answer 12

Have their cell bodies in the dorsal root ganglion.

Communicate with a receptor.

Their central axon projects ipsilateral to the cell body.

Projects onto second order neurones.

Question 13

Describe the characteristics of the second order sensory neurones in the somatosensory system.

Answer 13

Have their cell bodies in the spinal cord dorsal horn or medulla.

Decussate.

Project onto third order neurones.

Question 14

Describe the characteristics of the third order sensory neurones in the somatosensory system.

Answer 14

Have their cell bodies in the thalamus.

Project to the primary sensory cortex.

Question 15

What is somatotopy / topographical representation?

Answer 15

The idea that every point on the surface of the body has an equivalent point the can be identified along the sensory pathway.

(Mostly), adjacent body regions map to adjacent regions of the sensory system.

Question 16

Why is somatotopy good?

Answer 16

It minimises the amount of ‘wiring’ required to transmit sensory information.

Question 17

Where do the 1st order neurones from the lower body (T7 and below) of the dorsal column-medial leminiscus system travel through?

Answer 17

Ascend through the gracile fasciculus to the gracile nucleus in the medulla.

Question 18

Where do the 1st order neurones from the upper body (T6 and above) of the dorsal column-medial leminiscus system travel through?

Answer 18

Ascend through the cuneate fascicilus to the cuneate nucleus in the medulla.

Question 19

Describe the path of the 2nd order neurones of the DCML system

Answer 19

Neurones in both the gracile and the cuneate nucleus project to the contralateral thalamus in the medial leminiscus.

Question 20

Describe the path of the 3rd order neurones of the DCML system

Answer 20

Thalamic neurones form the lower half of the body (via gracile nucleus) project to the medial part of the primary sensory cortex.

Thalamic neurones form the upper half of the body (via cuneate nucleus) project to the lateral part of the primary sensory cortex.

Question 21

How are the dorsal columns organised?

Answer 21

Axons from the lower parts of the body run most medially.

Axons from progressively superior body segments are added laterally to the dorsal columns.

Question 22

Describe the path of the first order neurones in the spinothalamic tract

Answer 22

They project onto second order neurones in the ipsilateral (same) spinal cord dorsal horn in the segment at which they enter the cord through the dorsal root (generally)

Question 23

Describe the path of the second order neurones in the spinothalamic tract

Answer 23

Their cell bodies are in the dorsal horn.

Their axons decussate in the ventral white commissure of the cord and then go on to form the spinothalamic tract.

The spinothalamictract projects to the thalamus.

Question 24

Describe the path of the third order neurones in the spinothalamic tract

Answer 24

Thalamic neurones receiving information ultimately from more inferior parts of the body project to the medical part of the primary sensory cortex.

Thalamic neurones receiving information ultimately from more superior parts of the body project to the lateral part of the primary sensory cortex.

Question 25

Describe the topographical organisation of the spinothalamic tract.

Answer 25

Axons from the lower parts of the body run most laterally / superficially.

Axons from progressively superior body segments are added medially / deeper onto the spinothalamic tract.

This is the opposite the situation for the dorsal columns, and is due to the decussation of the SST second order neurones at the level of entry of the first order neurones.

Question 26

What structures will be completely destroyed in Brown-Sequard syndrome?

Answer 26

Unilaterally:

The dorsal horn
The ventral horn
All other cord grey matter
All white matter pathways
Dorsal and central routes

Question 27

What are the signs of Brown-Sequard syndrome?

Answer 27

Ipsilateral (same side) segmental anaesthesia affecting a single dermatome (due to the destruction of the dorsal root and dorsal horn).

Ipsilateral loss of dorsal column modalities below the destroyed segment.

Contralateral loss of spinothalamic modalities at and below the destroyed segment (although level can be up to a couple of segments lower due to asset of some of the primary afferents in Lissauer’s tract)

Question 28

Why does rubbing a sore area relieve pain?

Answer 28

Because it stimulates the mechanoreceptors which activate the inhibitory interneurones (containing encephalin) so stop the impulses being sent down the second order neurones and therefore stop you feeling pain.

These neurones are also activated in hypnosis, hence why it can be used for pain relief.