L04: Somatosensory Receptors

Question 1

What is the receptive field?

Answer 1

Area of skin innervated by a single axon

Question 2

What type of neurone is a sensory neurone? Where does the AP start?

Answer 2

Pseudounipolar

AP starts at nerve ending NOT at axon hillock of cell body (e.g. like in motor neurones)

Question 3

Types of primary afferent fibres

Answer 3

Aa: carries info from skeletal proprioceptors
Ab: info from mechanoreceptors
Ad: pain & temp
C: pain, temp, itch

Question 4

Which are the thickest and fastest conducting fibres?

Answer 4

Aa

Question 5

Which type of afferent fibre isn’t myelinated?

Answer 5

C

Question 6

What does size of receptive field determine?

Answer 6

Precision of stimulus + 2 point discrimination

Large receptive field —> poor 2 point discrimination

Question 7

What is an adequate stimulus?

Answer 7

Strength of stimulus that causes AP to be fired in an axon (threshold)
Changes depending on axon + receptor

Question 8

How does rate of AP propagation relate to the strength of the stimulus?

Answer 8

High energy stimulus = high strength —> faster rate of propagation (remember strength of AP can’t change)

Question 9

What are slow/non adapting sensory receptors?

Answer 9

AP is maintained if stimulus present - sustained, tonic signalling e.g. pain, stretch
Usually to let body know there is damage occurring etc.
Responds to strength of stimulus

Question 10

What are adapting sensory receptors?

Answer 10

AP stops if there is sustained stimulus - phasing signalling e.g. touch, feeling of clothes on skin
Usually for non dangerous stimuli
Responds to changes in stimuli

Question 11

Types of cutaneous sensory receptors?

Answer 11

Mechanoreceptors
Thermoreceptors
Nociceptors

Question 12

What are mechanoreceptors?

Answer 12

Tactile receptors responding to touch, pressure + vibration

Question 13

What fibres are mechanoreceptors associated with + what are the consequences of this?

Answer 13

Ab fibres - large and myelinated for fast conduction

Question 14

Types of mechanoreceptors

Answer 14

Merkel’s disc (specialised keratinocyte) - sustained light touch
Meissners corpuscles - light touch + vibration
Pancinian corpuscles - deep touch + high freq vibration
Ruffini endings - deep touch & stretch
Hair follicle receptor - v light touch

Question 15

Describe how Merkel’s discs work

Answer 15

Respond to sustained light touch, found mainly in epidermis of digits + around mouth
Slowly adapting
Allows perception of texture e.g. reading braille
Replaces hair receptors in glabrous skin?

Question 16

Describe how Meissners corpuscles work

Answer 16

Respond to light touch & vibration, found deep in papillary dermis
Rapidly adapting so detects changing stimuli
Senses loss of grip on something

Question 17

Describe how pancinian corpuscles work

Answer 17

Respond to deep touch & high freq vibration as it’s found deeper in dermis
Stronger stimulus required compared to meissner’s
Rapidly adapting
Onion like structure that deforms due to pressure

Question 18

Describe how ruffini endings work

Answer 18

Respond to lateral movements/stretching of skin, found in dermis beneath meissner’s (but above pancinian)
Network of collagen fibres that move across eachother when skin is stretched causing opening of channels

Question 19

Describe how hair follicle receptors work

Answer 19

Nerve ending wrapped aound follicle so deflection of hair causins AP
Responds to light touch
Rapidly adapting so responds to changes in stimuli

Question 20

What are thermoreceptors?

Answer 20

Slowly adapting, bare nerve endings either responding to heat or cold
Detects changes in temp from normal by comparing signals from hot/cold receptors (no indication of absolute temp)

Question 21

What type of channels do thermoreceptors contain?

Answer 21

TRIP family of receptors - contain non-specific cation channels that determine sensitivity

Question 22

why does menthol feel cold?

Answer 22

Opens TRPM8 channel on thermoreceptor that usually responds to cold (8-38 degrees)

Question 23

What is paradoxical cold perception

Answer 23

TRPA1 channel usually opens when cold (<17 degrees) BUT can also open by high temp >50 –> feeling cold

Question 24

What type of fibres do hold & cold receptors have?

Answer 24

Cold - Ad & C

Hot - C fibres

Question 25

What are nociceptors?

Answer 25

Bare nerve endings that detect noxious stimuli

Non-adapting w high threshold of activation (adequate stimulus must be capable of damaging tissue)

Question 26

What are the types of nociceptors?

Answer 26

High threshold mechanoreceptors (Ad fibres) 
Polymodal nociceptors (C fibres)

Question 27

What are high threshold mechanoreceptors?

Answer 27

Type of nociceptor containing Ad fibres, activated by strong shearing force, detects localised pricking pain e.g. papercut

Question 28

What are polymodal nociceptors?

Answer 28

Contan C fibres, dectects range ofdamaging stimului e.g. temp>46, noxious chemicals, sharp blow
–> poorly localised burning pain

Question 29

Why does capsaicin cause pain?

Answer 29

Opens TRPV1 channel on polymodal nociceptor - usually opened due to mechanical damage

Question 30

What type of fibres do proprioceptors connect to?

Answer 30

thick, v myelinated Aa fibres

Question 31

What are muscle spindles?

Answer 31

Specialised muscle fibres innervated by sensory axons (intrafusal fibres)
Constantly firing & contributes to muscle tone
Detects STRETCH as it’s paralell to muscle

Question 32

What are golgi tendon organs?

Answer 32

Detects tension generated by contraction
In series w muscle so detects length
Found in the junction between muscle & tendon