4+5) Somatic Sensation and Ascending Tracts

Question 1

What is the function of the somatosensory system?

Answer 1

Carries conscious sensation from body wall

Question 2

Give examples of some modalities in the somatosensory system:

Answer 2

Temperature, pain, vibration, pressure, stretch, 2-point discrimination, proprioception

Question 3

Why can we experience different modalities in the somatosensory system?

Answer 3

Travel through different pathways in CNS and have different receptors

Question 4

Describe phasic receptors and give an example:

Answer 4

Adapt to original stimulus by decreasing rate of firing so eventually become unaware of original stimulus e.g. mechanoreceptors in the feel of clothes

Question 5

Describe tonic receptors and give an example:

Answer 5

Don’t adapt to stimulus to frequency of AP remains the same e.g. nociceptor in pain

Question 6

How is sensory acuity related to receptive field?

Answer 6

Acuity is inversely proportional to size of receptive field

Question 7

Describe how lateral inhibition takes place:

Answer 7

Max stimulation at one neurone and inhibitory interneurones from this neurone laterally inhibit nearby neurones

Question 8

What is the purpose of lateral inhibition?

Answer 8

Maximise contrast of sensation

Question 9

What is topographical representation?

Answer 9

For every point of the body surface there is a corresponding point in the neural cortex

Question 10

What is the stereognosis and what is it used to test?

Answer 10

Test for dorsal columns

Ask someone to close their eyes, place an object in their hand and ask them what it is

Question 11

What are the functions of the dorsal columns?

Answer 11

Sophisticated aspects of touch: light tough, joint position sense, vibration, two-point discrimination

Question 12

What are the parts of the dorsal columns?

Answer 12

Fasciculus gracilis

Fasciculus cuneatus

Question 13

What are ascending tracts?

Answer 13

White matter connection between two areas of grey matter

Question 14

Describe the general path of a neurones up the DCML pathway:

Answer 14

Primary afferent from body wall and travel up to medulla where they synapse. Second order neurones then travel to thalamus and synapse here, then third order neurones project onto sensory cortex

Question 15

Where do the different orders of neurones run in the DCML pathway?

Answer 15

1st order in dorsal columns
2nd order in medial lemniscus
3rd order between thalamus and primary sensory cortex

Question 16

Describe the difference between lower limb and upper limb neurones in the DCML pathway:

Answer 16

Fibres from lower levels travel closer to midline in dorsal columns
Lower limb neurones project medially onto sensory cortex

Question 17

Where do 1st order neurones in the upper half of the body synapse in dorsal column pathway?

Answer 17

Cuneate nucleus

Question 18

Where do 1st order neurones in the lower half of the body synapse in dorsal column pathway?

Answer 18

Gracile nucleus

Question 19

What are the functions of the spinothalamic tracts?

Answer 19

Pain, temperature and crude touch

Question 20

Describe the general path of a neurones up the spinothalamic pathway:

Answer 20

1st order neurones from body wall synapse at same level in the spinal cord (apart from in leg). 2nd order neurones cross midline and ascends to thalamus. 3rd order from thalamus to primary sensory cortex

Question 21

Describe the difference between lower limb and upper limb neurones in the spinothalamic pathway:

Answer 21

Lower limb neurones are more lateral in the cord

Lower limb neurones project medially onto sensory cortex

Question 22

Where are the dorsal columns located?

Answer 22

Posterior spinal cord

Question 23

Where is the spinothalamic tract located?

Answer 23

Ventral spinal cord

Question 24

What is the difference between A and C fibres?

Answer 24

A fibres carry impulses from mechanoreceptors in the skin

C fibres carry pain

Question 25

What is the mechanism behind the rubbing it better phenomenon?

Answer 25

Activation of mechanoreceptors alleviates pain by exciting inhibitory enkepahlinergic interneurones in the cord

Question 26

Describe the descending pathway controlling pain transmission:

Answer 26

Control from cerebral cortex and peri-aqueductal gray, which send descending fibres to nucleus raphe magnus in medulla causing activation of enkephalinergic interneurones to switch off pain transmission