3: Somatosensory System

Question 1

What type of receptors detect pain?

Answer 1

Nociceptors

Question 2

What is meant by receptors being either tonic or phasic?

Answer 2

Tonic receptors will continue to fire action potentials for as long as the stimulus is occurring. They are slow adapting. E.g. nociceptors.

Phasic receptors are fast adapting and will decrease the frequency of APs even if the stimulus remains, eg the feeling of our own clothes on our body, spraying perfume.

Question 3

What is the difference between the 1st, 2nd, and 3rd order neurone?

Answer 3

1st will collect info from a given receptor and go to the spinal cord (PNS)

2nd will cross the midline (dessucate) and pass up to the thalamus (CNS)

3rd will project from the thalamus to the sensory cortex via corona radiata (CNS)

Question 4

What is the function of the thalamus?

Answer 4

To relay motor and sensory signals to the cerebral cortex

Question 5

What is lateral inhibition?

Answer 5

When adjacent second order neurones to the neurone central to the stimulus are inhibited in order to localise a signal to its exact location on the body wall.

Question 6

What does the dorsal column medial lemnsicus pathway transfer sensations of?

Answer 6

Fine touch
Vibration
Two point discrimination
Proprioception

Question 7

Where are the first order neurones located?

Answer 7

In the dorsal root ganglion (except in head and neck)

Question 8

In the DCML pathway, are upper limb fibres more medial or lateral?

Answer 8

Lateral

Fibres from high levels are added to the outside of the dorsal column

Question 9

In the DCML pathway, where do 1st order neurones synapse?

Answer 9

At the junction between the spinal cord and medulla oblongata…

If upper limb, synapse with cuneate nucleus
If lower limb, synapse with gracile nucleus

Question 10

What do second order neurones pass up through in the DCML pathway?

Answer 10

Medial lemniscus (white matter pathway in the brainstem), before synapsing in the thalamus

Question 11

At what level do the 2nd order neurones dessucate in the DCML pathway?

Answer 11

At the level of the medulla

Different to other pathways which dessucate at the level of the spinal cord

Question 12

What sensations does the spinothalamic tract carry?

Answer 12

Pain
Temperature
Crude touch
Pressure

Question 13

Where do 1st order neurones synapse in the spinothalamic tract?

Answer 13

Tip of the dorsal horn (substantia gelatinosa)

Question 14

What sensations does the lateral spinothalamic tract convey?

Answer 14

Pain and temperature

Question 15

What sensations does the anterior spinothalamic tract convey?

Answer 15

Crude touch and pressure

Question 16

What sensation does the spinocerebellar tracts carry?

Answer 16

Unconscious proprioceptive information from muscles to the cerebellum to help our brain coordinate motor movements

Question 17

Do spinocerebellar tracts go to the ipsilateral or contralateral cerebellum?

Answer 17

Ipsilateral
(As posterior spinocerebellar tracts do not dessucate, and anterior spinocerebellar tracts dessucate twice so are still ipsilateral)

Question 18

Which tracts carry unconscious proprioceptive information?

Answer 18

Spinocerebellar tracts

Question 19

Where do 3rd order neurones of the spinothalamic tract pass?

Answer 19

From the thalamus, through the internal capsule and terminate at the primary sensory cortex of the brain

Question 20

If a lesion to the spinal cord occurred, would the sensory loss of fine touch and proprioception be contralateral or ipsilateral?

Answer 20

Ipsilateral

As the second order neurones of the DCML pathway do not dessucate until the medulla oblongata

Question 21

What does Brown Sequard syndrome refer to?

Answer 21

One sided lesion of the spinal cord, often due to traumatic injury

Question 22

In Brown Sequard syndrome, would the loss of pain and temperature sensation be contralateral or ipsilateral?

Answer 22

Contralateral

As the lateral spinothalamic tract which carries pain and temperature sensation will dessucate in the spinal cord

Question 23

Which fibres carry pain?

Answer 23

C fibres

Question 24

What are enkephalins?

Answer 24

A more line substance that can be agonised by analgesics

Question 25

Why can rubbing the skin relive pain?

Answer 25

By activating mechanoreceptors which excites enkephalinergic interneurones in the cord

Question 26

How can we test a patient’s proprioception?

Answer 26

Rombergs test
(Px standing and asked to close their eyes and increased loss of balance is a positive rombergs test)
Requires healthy functioning of dorsal columns
If a patient is ataxic but has a -ve rombergs test, this suggests the ataxia is cerebellar in nature

Question 27

How can ataxia of a cerebellar cause and a dorsal column cause be distinguished between?

Answer 27

If rombergs test is +ve, this suggests the ataxia is sensory in nature (ie DCML)
If rombergs test is -ve it suggests the ataxia is cerebellar in nature