L04 - The Somatosensory System

Question 1

What are two features of primary sensory neurones?

Answer 1

• All excitatory (majority use glutamate NT)

- Can generate APs at peripheral nerve ending

Question 2

What are the 4 fibre types and what are their functions?

Answer 2

• A⍺ - carry information from proprioceptors
• Aβ - carry tactile information from mechanoreceptors of the skin
• Aδ - carry pain and temperature information
• C - carry pain, temperature and itch information

Question 3

What is the difference between a receptive field and a dermatome?

Answer 3

• A receptive field applies to a single sensory neurone and its terminal branches
• A dermatome is an area of skin supplied by a single spinal nerve, which contains many sensory neurones, and so the dermatome is made up of the combined areas of all the receptive fields of the neurones in that spinal nerve and which have their cell bodies in the dorsal root ganglion

Question 4

What is an adequate stimulus?

Answer 4

The type of energy to which a sensory receptor will respond with the initiation of an action potential

Question 5

Which part of a neurone determines the adequate stimulus and the signal threshold?

Answer 5

Nerve ending

Question 6

What is the function of slow adapting and non-adapting receptors?

How do slow adapting and non-adapting receptors respond to unchanging stimuli?

Why is this useful?

Answer 6

• Slow adapting and non-adapting receptors function to detect the strength of a stimulus
• They continue to respond to a stimulus for as long as it is sustained within some reasonable time frame (constant AP frequency)
• This is important for stimuli which an individual needs to stay aware of, for example pain or stretch (of skin)

Question 7

What is the function of fast adapting receptors?

How do fast adapting receptors respond to unchanging stimuli?

Why is this useful?

Answer 7

• Fast adaptation functions to detect the rate of change of a stimulus
• They don’t respond to sustained stimuli, only to changes in the strength of stimuli
• This is important for stimuli which an individual can stop paying attention to when the stimulus is no longer important, for example tactile (touch) receptors

Question 8

What are the 3 types of cutaneous sensory receptor and what stimuli do they respond to?

Answer 8

1 - Mechanoreceptors (touch, pressure and vibration)

2 - Thermoreceptors (temperature)

3 - Nociceptors (noxious stimuli)

Question 9

In which layer of the skin are Meissner’s corpuscles found?

Answer 9

In the papillary (superficial) dermis of glabrous skin

Question 10

What stimuli do Meissner’s corpuscles respond to?

Answer 10

1 - Light touch

2 - Vibration

Question 11

What type of adaptation do Meissner’s corpuscles exhibit?

What is the implication of this?

Answer 11

• Fast adaptation

- They require constantly changing stimuli

Question 12

Where are Merkel’s receptors found?

Answer 12

• In high densities in the epidermis of the digits and around the mouth
• In low densities in other glabrous skin

Question 13

What stimulus do Merkel’s receptors respond to?

Answer 13

Sustained light touch

Question 14

What type of adaptation do Merkel’s receptors exhibit?

Answer 14

• Slow adaptation

- They can sustain a constant light touch stimulus

Question 15

What apparatus forms Merkel’s receptors?

Answer 15

Specialised keratocytes

Question 16

Where are Ruffini’s corpuscles found?

Answer 16

In the dermis of glabrous skin

Question 17

Which stimuli do Ruffini’s corpuscles respond to?

Answer 17

1 - Lateral movement (stretching) of the skin

2 - Deep touch

Question 18

What type of adaptation do Ruffini’s corpuscles exhibit?

Answer 18

Slow adaptation

Question 19

What apparatus forms Ruffini’s corpuscles?

Answer 19

Networks of collagen fibres

Question 20

Where are Pacinian corpuscles found?

Answer 20

In the reticular (deep) dermis of both glabrous and hairy skin

Question 21

What stimuli do Pacinian corpuscles respond to?

Answer 21

Strong stimuli:

1 - Strong poking

2 - Deep touch

3 - High-frequency vibration (this is the primary stimulus)

Question 22

What type of adaptation do Pacinian corpuscles exhibit?

Answer 22

Fast adaptation

Question 23

What is the structure of Pacinian corpuscles?

Answer 23

Fully encapsulated nerve endings

Question 24

Where are hair follicle receptors found?

Answer 24

Dermis

Question 25

Which stimulus do hair follicle receptors respond to?

Answer 25

1 - Hair deflection

2 - Light touch

Question 26

What type of adaptation do hair follicle receptors exhibit?

Answer 26

Fast adaptation -> require constantly changing stimulus

Question 27

What type of adaptation do cutaneous thermoreceptors exhibit?

Answer 27

Slow adaptation

Question 28

What are the two subcategories of cutaneous thermoreceptors?

Answer 28

Cold receptors found on C & Aδ fibres & warm receptors on C fibres

Question 29

How is the sense of temperature determined?

Answer 29

• Cutaneous thermoreceptors are poor indicators of absolute temp, but are very sensitive to temp changes
• Sense of temp comes from the comparison of signals from warmth & cooling receptors

Question 30

To which family of ion channels do thermoreceptor ion channels belong?

Answer 30

• Transient receptor potential (TRP) family

- They are non-specific cation channels that open in response to particular temp changes

Question 31

What is the name of the ion channels that belong to cold cutaneous thermoreceptors?

When do these channels open?

Answer 31

• TRPM8 channels = cold receptors -> open between 10-38℃ (max opening at 25℃) -> also opened by menthol
• TRPA1 channels (ANKTM1) = cold receptors -> open at <17℃

Question 32

What is the name of the ion channels that belong to hot cutaneous thermoreceptors?

When do these channels open?

Answer 32

TRPV3/4 channels = warm receptors -> open between 29-45℃ (max opening at 45℃)

Question 33

Why do mints feel cold in the mouth?

Answer 33

• They contain menthol, which opens TRPM8 channels

- These are cold receptors

Question 34

Which stimuli do cutaneous nociceptors respond to?

Answer 34

• Pain
• Damaging heat
• Noxious chemicals

Question 35

What type of adaptation do cutaneous nociceptors exhibit?

Answer 35

Non-adaptation

Question 36

What can be said about the threshold of cutaneous nociceptors?

Answer 36

• The threshold is very high & an adequate stimulus must be capable of damaging tissue
• This prevents everything feeling painful

Question 37

What are the two types of cutaneous nociceptors?

Answer 37

• Mechanical nociceptors (Aδ fibre) – activated by strong shearing force in skin and give sensation of sharp pain; Aδ fibres myelinated -> faster transmission
• Polymodal nociceptors (C fibre) – respond to many stimuli and give sensation of dull, burning pain; C fibres non-myelinated -> slower transmission

Question 38

Which substance opens TRPV1 channels?

Answer 38

Capsaicin

Question 39

What 4 properties of the musculoskeletal system are conveyed in proprioception?

Answer 39

1 - Joint position

2 - Muscle length & movement

3 - Acceleration

4 - Tension/force

Question 40

What are muscle spindles?

Answer 40

• Specialised intrafusal muscle fibres found within fibrous capsule -> detect muscle length & acceleration -> contribute to tone
• Innervated by 1a sensory afferents & stretch sensitive -> ↑ firing
• Positioned in parallel w/ muscle fibres

Question 41

What are Golgi tendon organs?

Answer 41

• Junction of muscle & tendon -> detect muscle tension
• Innervated by 1b sensory afferents & sensitive to tension generated by muscle contraction
• Positioned in series w/ muscle fibres

Question 42

What stimuli excite fibres in the lemniscal pathway?

Answer 42

• Touch
• Vibration
• Two-point discrimination
• Proprioception

Question 43

What stimuli excite fibres in the spinothalamic pathway?

Answer 43

• Pain

- Temperature

Question 44

What fibres are involved in the lemniscal pathway?

Answer 44

Large sensory Aβ fibres

Question 45

What fibres are involved in the spinothalamic pathway?

Answer 45

Small sensory Aδ & C fibres