Session 3: Somatosensory System

Question 1

Types of sensation

Answer 1

General

Special

Question 2

How can general sensation further be subdivided?

Answer 2

Somatic (conscious)

Visceral (unconscious)

Question 3

Which two systems are most clinically relevant for somatic sensation?

Answer 3

Spinothalamic

Dorsal column-medial lemniscus system

Question 4

Give the modalities of the spinothalamic system and their associated receptors.

Answer 4

Temperature - thermoreceptors

Pain - nocireceptors

Crude touch - mechanoreceptors

Question 5

Give the modalities of the dorsal column-medial lemniscus system and their associated receptors.

Answer 5

Vibration - mechano

Proprioception - muscle spindles and golgi tendon organs

Fine touch - mechano

Two point discrimination - mechano

Question 6

What are analogue and digital receptor signals?

Answer 6

Analogue is the ion flux during generator potential

Digital is the frequency of action potential in primary sensory neuron.

Question 7

What type of signal is the somatosensory system associated with?

Answer 7

Digital

Question 8

Explain where the analogue signal can be found and where it goes into digital signal.

Answer 8

Found at receptor and signal is converted from analogue to digital at the synapse between receptor and 1st order neuron.

Question 9

What does a strong receptor activation consist of?

Answer 9

High frequency of APs in PSN

Question 10

What does a weak receptor activaiton consist of?

Answer 10

Low frequency of APs in PSN.

Question 11

What is a rapidly adapting receptors?

Answer 11

A receptor where the frequency of firing diminishes rapidly after initial stimulus.

This means that the receptor gets used to the stimulus and adapts.

Question 12

Give an example of a rapidly adapting receptor.

Answer 12

Mechanoreceptors in our buttocks e.g.

You realise that you sit down initially but get used to it and don’t consciously think about it afterwards.

Question 13

What are slowly adapting receptors?

Answer 13

Frequency of firing diminished slowly after initial stimulus.

Question 14

Give an example of a slowly adapting receptor.

Answer 14

Nocireceptors which explains why pain can be persisent and you never really get used to it.

Question 15

What is a receptive field?

Answer 15

A single primary sensory neuron supplying a given area of skin.

Question 16

What is the difference between dermatome and receptive field?

Answer 16

A dermatome is made up of thousands of receptive fields.

Question 17

How does the size of the receptive field relate to acuity?

Answer 17

A large receptive field means that there is a low sensory acuity. An example being there is poor two-point discrimination.

A small receptive field means that there is a high sensory acuity.

Question 18

Give examples of large and small receptive fields.

Answer 18

Large = back

Small = fingertips

Question 19

Explain why dermatomes can have fuzzy boundaries.

Answer 19

Because receptive fields can overlap between spinal nerves.

This means that it is important to check sensory of a dermatome in the middle of that dermatome.

Question 20

Explain the general path of the somatosensory system.

(1st order etc…)

Answer 20

1st order sensory with its receptor has a central axon that projects ipsilateral. It will synapse with the cell body of the 2nd order neuron.

The 2nd order neuron will decussate. The 2nd order neuron is always the one to decussate. The 2nd order neuron will then synapse with the cell body of the 3rd order neuron.

The 3rd order neuron will then synapse with the primary sensory cortex.

Question 21

Where can the cell body of the 1st order neuron be found?

Answer 21

In the dorsal root ganglion

Question 22

Where can the cell body of the 2nd order neuron be found?

Answer 22

In the spinal cord dorsal horn or in the medulla.

Question 23

Where can the 3rd order cell body be found?

Answer 23

In the thalamus

Question 24

Sensory of the dorsal column-medial lemniscus.

Answer 24

Proprioception

Fine touch

Two-point discrimination

Vibration

Question 25

Explain the route of L3 in dorsal column-medial lemniscus system.

Answer 25

1st order neuron will enter the spinal canal and ipsilaterally ascend the spinal canal via the gracile fasciculus (ground) to synapse with the 2nd order cell body in the gracile nucleus of the medulla.

The 2nd order neuron in the medulla will then decussate and synapse with the contralateral thalamus in medial lemniscus.

The 3rd order neuron will then travel via internal capsule to synapse with the medial part of the primary sensory cortex.

Question 26

Explain the route of C5 in dorsal column-medial pathway.

Answer 26

1st order neuron will enter the spinal canal ipsilaterally and also laterally to the neurons coming from inferiorly. It will then ascend the spinal canal via cuneate fasciculus to the cuneate nucleus of the medulla and synapse with the 2nd order neuron there.

2nd order neuron will then decussate and synapse with the 3rd order neuron in the contralateral thalamus in the medial lemniscus.

The 3rd order neuron will then travel via internal capsule and synapse with the lateral part of the primary sensory cortex.

Question 27

Where is the cut off point of which axons travel via gracile fasciculus and which travel via cuneate fasciculus?

Answer 27

T7 and below travel via gracile

T6 and above travel via cuneate

Question 28

Sensory of spinothalamic system.

Answer 28

Pain

Temperature

Crude touch

Question 29

Explain the route of L3 in spinothalamic pathway.

Answer 29

1st order neuron will enter the spinal canal and synapse with then synapse with the 2nd order neuron at the level of the spinal nerve and also in ipsilateral spinal cord dorsal horn.

The 2nd order neuron will then decussate in the ventral white commisure and travel laterally on the contralateral side to synapse with the 3rd order neuron in the thalamus.

The 3rd order neuron will then travel to synapse with the medial part of the primary sensory cortex.

Question 30

Explain the route of C5 in spinothalamic pathway.

Answer 30

The 1st order neuron will enter the spinal canal and synapse with 2nd order neuron at the level of the spinal nerve and also at the ipsilateral spinal cord dorsal horn.

The 2nd order neuron will then decussate at the ventral white commissure and travel medially to the lower ascending axons. It will then ascend to synapse with the 3rd order neuron in the thalamus.

3rd order neuron will then travel to synapse with the lateral part of the primary sensory cortex.

Question 31

What is the Lissauer’s tract?

Answer 31

A tract where the spinothalamic 1st order neuron might ascend the spinal cord ipsilaterally a little bit first before synapsing with the 2nd order neuron and then decussate.

Question 32

What is the relevance of Lissauer’s tract?

Answer 32

If there is an injury the spinothalamic pathway modalities may be both contralateral and ipsilateral where as they are most commonly only contralateral below the level of injury.

Question 33

What will the signs be of destruction of one lateral half of a single cord segment?

Answer 33

Ipsilateral complete anaesthesia of one single dermatome

Ipsilateral loss of dorsal column modalities below destroyed segmet

Contralateral loss of spinothalamic modalities

Ipsilateral loss of motor function

Question 34

Explain why rubbing a sore area will alleviate the pain.

Answer 34

A fibres carry impulse from mechanoreceptors in skin. Activation of the mechanoreceptors will alleviate pain by exciting inhibitory enkephalinergic interneurons in the cord and inhibit the C fibres that carry pain.