L4 Somatosensory System

Question 1

Types of sensation

Answer 1

General sensation - pain and touch etc.

Special sensory - sight, smell, hearing, taste, balance

Question 2

2 types of general sensation

Answer 2

Somatic:

• conscious
• from body surface

Visceral

• unconscious
• most sensation does not reach consciousness

Question 3

Spinothalamic modalities

Answer 3

Temperature
Pain
Pressure - crude touch

Question 4

Spinothalamic pathway

Answer 4

• Responsible for modalities of sensation that are crucial for survival
• Located anterolaterally

Question 5

Dorsal column modalities

Answer 5

• vibration
• fine touch
• proprioception
• 2 point discrimination

Question 6

Dorsal column

Answer 6

• Responsible for more sophisticated modalities

- located posteriorly

Question 7

Modalities

Answer 7

• Units of sensation that cannot be subdivided
• mediated by a single type of receptor
• stickiness for example is made up of multiple modalities

Question 8

Intensity of a stimulus

Answer 8

Analogue signal - can vary continuously

Analogue signal converted to digital signal (0 and 1s) into frequency of action potentials

Question 9

Rapidly adapting receptors

Answer 9

• initial high frequency of action potential firing but then becomes less frequent
• in e.g. cutaneous mechanoreceptors

E.g when sitting down, you are initially are of it but then you can’t consciously fell it anymore

Question 10

Slowly adapting receptors

Answer 10

E.g nociceptors - pain

• action potential frequency remains high until the stimulus is removed
• frequency doesn’t change with time

Question 11

Receptive field

Answer 11

• Region of skin that a sensory neurone supplies

- can overlap between sensory neurones therefore loss of sensation is less than expected

Question 12

Size of receptive field

Answer 12

Large receptive field - decreased sensory acuity as receptors are more spread out

Small receptive field - high sensory acuity as receptors are more concentrated

Question 13

Sensory acuity

Answer 13

Acuity is inversely proportional to the size of the receptive field

Acuity is directly proportional to the number of sensory neurones present e.g high in tongue

Question 14

Primary sensory neurone

Answer 14

• communicates with receptors of the same type
• has multiple dendrites
• cell body is in the dorsal root ganglion
• projects ipsilaterally into spinal cord (same side as cell body)
• synapses on to the second order sensory neurone

Question 15

Secondary sensory neurone

Answer 15

• cell body in the dorsal horn or the medulla
• DECUSSATES - crosses the midline
• synapses on to the third order sensory neurone in the thalamus

Question 16

Tertiary sensory neurone

Answer 16

• cell body in the thalamus

- projects to the primary sensory cortex in the post central gyrus via the internal capsule

Question 17

Somatotopy

Answer 17

1 to 1 correspondence between the surface of the body (dermatome) and an area of the primary motor cortex (homunculus)

Conversion and convergence occurs in the thalamus in an organised system

Question 18

Dorsal column medial lemniscus pathway

Answer 18

1. Primary neurone - projects into cord on the ipsilateral side
2. Ascends in the gracililis fasciculus CLOSE TO THE MIDLINE
3. Synapses in the gracile nucleus (lower limb) in the medulla onto the 2nd order neurones
4. The second order neurones decussate and ascends via the medial lemniscus tract
5. In the thalamus it synapses onto the third order neurones
6. Third order neurones project to the associated sensory cortex via the internal capsule

Question 19

Upper limb

Answer 19

• Joins ascending neurones laterally
• the first order sensory neurone synapses in the cuneate nucleus in the medulla
• the first order sensory neurones ascend via the cuneate fasciculus

Question 20

Gracile fasciculus

Answer 20

Region of the dorsal column
Below T6
Contains the gracile nucleus

Question 21

Cuneate fasciculus

Answer 21

Region of the dorsal column
Above T6
Contains the cuneate nucleus

Question 22

Injury to medial dorsal column

Answer 22

Affects lower limbs

Question 23

Injury to lateral dorsal column

Answer 23

Affects upper body

Question 24

Spinothalamic system

Answer 24

1. Primary sensory neurone enters the cord ipsilaterally
2. Synapses onto the secondary sensory neurones in the dorsal horn and DECUSSATES EARLY IN THE DORSAL HORN
3. Secondary sensory neurones ascend via the spinothalamic tract LATERALLY to the thalamus
4. Synapses on to the third order sensory neurone in thalamus and projects on to the primary sensory cortex

Question 25

Where do second order sensory neurones in the spinothalamic tract decussate? The

Answer 25

Ventral white commissure

Question 26

Lissauer’s tract

Answer 26

The first order sensory neurone can ascend a few segments and synapse to a higher secondary neurone via the Lissauer’s tract

E.g the C7 primary sensory neurone can ascend and synapse on to the C5 secondary neurone

Therefore the spinothalamic loss may be less than the dorsal column loss

Question 27

A fibres

Answer 27

Carry impulses from mechanoreceptors in the skin

Question 28

C fibres

Answer 28

carry pain impulses

Question 29

Rubbing wound

Answer 29

1. Activation of mechanoreceptors stimulates A fibres.
2. Stimulates inhibitory enkephalinergic interneurones which releases enkephalins (endorphins)
3. Inhibits secondary spinothalamic sensory neurones therefore pain decreases

Question 30

Huge trauma and pain

Answer 30

Higher relay station in the periaqueductal region of grey matter inhibits secondary spinothalamic sensory neurones therefore cannot feel pain

Question 31

Opiates

Answer 31

E.g morphine

Inhibit secondary spinothalamic sensory neurones in the dorsal horn

Question 32

Why is the hand particularly vulnerable to damage with brain tumours

Answer 32

• Represents a large area of the homunculus - very tactile and large sensory innovation
• hand is represented laterally

Question 33

Which modalities are affected in a brain tumour of the primary sensory cortex

Answer 33

All modalities as both the dorsal column and spinothalamic tracts are affected

Question 34

Glove and stocking paraesthesia

Answer 34

Only affects hands and feet

• Due to peripheral neuropathy e.g diabetes or vitamin B12 deficiency
• 2 lesions in the primary sensory cortex

Question 35

Affect of destroying then right half of the C5 segment

Answer 35

Spinothalamic tract:

• sensory loss of both left and right side at C5
• contralateral sensory loss from C5 below (left)
• affects lower spinal levels as lower spinal level primary sensory neurones can ascend via Lissauer’s tracts
• loss of pain, temperature and crude touch sensations

Dorsal column:

• ipsilateral sensory loss from C5 below
• loss of vibration, fine touch and proprioception