Somatosensory pathways

Question 1

What is a modality?

Answer 1

Type of stimulus

Question 2

What modalities are sensed by a mechanoreceptor?

Answer 2

Touch, pressure, vibration, proprioception

Question 3

Give an example of a type of mechanoreceptor?

Answer 3

Meissner’s corpuscle

Question 4

Describe the function of TRP channels

Answer 4

= free nerve endings with thermal sensitivity, activated at different temperatures

Question 5

Recall the 3 main classifications of sensory nerves

Answer 5

A-beta
A-delta
C

Question 6

Recall what each class of sensory fibre is responsible for sensing

Answer 6

A-beta = innocuous mechanical stimuli
A-delta = pain, temperature
C = slower, achey pain; temperature

Question 7

Compare the structures of each class of sensory fibres

Answer 7

A-beta = large, myeinated
A-delta = medium, myelinated
C = narrow, unmyelinayed

Question 8

Define “stimulus threshold”

Answer 8

Level of tsimulus that produces a response 50% of the time

Question 9

Define “adaptation”

Answer 9

change over time in the responsiveness of the sensory system to a constant stimulus

Question 10

Recall the 2 different classes of receptors with respect to adaptation, and explain the difference

Answer 10

Tonic: detect continuous signal strength
Phasic: detect a change in signal strength

Question 11

Give an example of a receptor that demonstrates tonic activity

Answer 11

Merkel cells

Question 12

Describe the function of tonic receptors

Answer 12

Continue to transmit to the brain as long as the stimulus is present

Question 13

Describe the function of phasic receptors

Answer 13

Transmit a signal at the beginning and end of a stimulus

Question 14

Define “receptive field”

Answer 14

The region of skin which causes activation of a single sensory neuron when activated

Question 15

Explain the significance of receptive field size

Answer 15

The smaller the receptive field, the more precise and fine the sensitivity due to higher neuron density

Question 16

Explain the function of lateral inhibition

Answer 16

Since receptive fields may overlap, stimuli may seem more perfuse than they really are. LI aids localisation of the stimulus via interbeurons in the dorsal horn

Question 17

Recall the 2 types of nocioceptor

Answer 17

1 - A-mechanoheat receptors

2 - A-mechanoreceptors

Question 18

How are fibres entering the dorsal horn organised?

Answer 18

Rexed laminae - layers of grey matter

Question 19

What sort of sensory fibres are capable of sensing pain and temperature, and in which rexed laminae do they terminate?

Answer 19

A-delta and C: I and II

Question 20

What sort of sensory fibres are capable of innocuous mechanical stimuli, and in which rexed laminae do they terminate?

Answer 20

A-beta and A-alpha: III-VI

Question 21

Describe the 2 roles of interneurons in the spinal cord

Answer 21

1. Connect between laminae

2. Connect adjacent peripheral inputs (for lateral inhibition)

Question 22

Where do A-beta fibres enter the spinal cord?

Answer 22

Dorsal horn

Question 23

What are the dorsal columns involved in sensing?

Answer 23

Fine discriminative touch and vibration

Question 24

At which vertebral level is the upper and lower-limb spinal nerve division?

Answer 24

T6

Question 25

Describe the 2 tracts in the dorsal columns

Answer 25

1. Gracile tract: information from lower limbs; medial

2. Cuneate tract: information from upper limbs; lateral

Question 26

What is a fasciculus?

Answer 26

Bundle of axons

Question 27

Compare the termination of 1st, 2nd and 3rd order neurons between the dorsal columns and the spinothalamic tract

Answer 27

1st Order:
Dorsal columns = medulla
Spinothalamic tract = immediately in spinal cord
Both: 
2nd - VPL nucleus in thalamus
3rd - SI

Question 28

What is the medial lemniscus?

Answer 28

Section of dorsal column that runs between medulla and VPL

Question 29

Compare the point at which the ascending pathway changes side between the dorsal columns and the spinothalamic tract

Answer 29

Dorsal column: medulla

Spinothalamic tract: immediate in spinal cord

Question 30

What is the spinothalamic tract involved in sensing?

Answer 30

Light touch, vibration, 2-point discrimination

Question 31

Describe the ascending path of the trigeminal nerve

Answer 31

1. Synapses with 2nd order neuron at trigeminal cranial nucleus
2. Relays through the VPM nucleus to cross midline
3. Joins medial end of medial lemniscus
4. –> SI

Question 32

Via what tract do pain pathways interact with the limbic system

Answer 32

Spinoreticular tract

Question 33

Recall the functions of SI, SII and the posterior parietal cortex

Answer 33

Information is first processed in SI before entering SII/ PPC