L04: Somatosensory Receptors Flashcards
What is the receptive field?
Area of skin innervated by a single axon
What type of neurone is a sensory neurone? Where does the AP start?
Pseudounipolar
AP starts at nerve ending NOT at axon hillock of cell body (e.g. like in motor neurones)
Types of primary afferent fibres
Aa: carries info from skeletal proprioceptors
Ab: info from mechanoreceptors
Ad: pain & temp
C: pain, temp, itch
Which are the thickest and fastest conducting fibres?
Aa
Which type of afferent fibre isn’t myelinated?
C
What does size of receptive field determine?
Precision of stimulus + 2 point discrimination
Large receptive field —> poor 2 point discrimination
What is an adequate stimulus?
Strength of stimulus that causes AP to be fired in an axon (threshold)
Changes depending on axon + receptor
How does rate of AP propagation relate to the strength of the stimulus?
High energy stimulus = high strength —> faster rate of propagation (remember strength of AP can’t change)
What are slow/non adapting sensory receptors?
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
What are adapting sensory receptors?
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
Types of cutaneous sensory receptors?
Mechanoreceptors
Thermoreceptors
Nociceptors
What are mechanoreceptors?
Tactile receptors responding to touch, pressure + vibration
What fibres are mechanoreceptors associated with + what are the consequences of this?
Ab fibres - large and myelinated for fast conduction
Types of mechanoreceptors
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
Describe how Merkel’s discs work
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?
Describe how Meissners corpuscles work
Respond to light touch & vibration, found deep in papillary dermis
Rapidly adapting so detects changing stimuli
Senses loss of grip on something
Describe how pancinian corpuscles work
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
Describe how ruffini endings work
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
Describe how hair follicle receptors work
Nerve ending wrapped aound follicle so deflection of hair causins AP
Responds to light touch
Rapidly adapting so responds to changes in stimuli
What are thermoreceptors?
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)
What type of channels do thermoreceptors contain?
TRIP family of receptors - contain non-specific cation channels that determine sensitivity
why does menthol feel cold?
Opens TRPM8 channel on thermoreceptor that usually responds to cold (8-38 degrees)
What is paradoxical cold perception
TRPA1 channel usually opens when cold (<17 degrees) BUT can also open by high temp >50 –> feeling cold
What type of fibres do hold & cold receptors have?
Cold - Ad & C
Hot - C fibres
What are nociceptors?
Bare nerve endings that detect noxious stimuli
Non-adapting w high threshold of activation (adequate stimulus must be capable of damaging tissue)
What are the types of nociceptors?
High threshold mechanoreceptors (Ad fibres) Polymodal nociceptors (C fibres)
What are high threshold mechanoreceptors?
Type of nociceptor containing Ad fibres, activated by strong shearing force, detects localised pricking pain e.g. papercut
What are polymodal nociceptors?
Contan C fibres, dectects range ofdamaging stimului e.g. temp>46, noxious chemicals, sharp blow
–> poorly localised burning pain
Why does capsaicin cause pain?
Opens TRPV1 channel on polymodal nociceptor - usually opened due to mechanical damage
What type of fibres do proprioceptors connect to?
thick, v myelinated Aa fibres
What are muscle spindles?
Specialised muscle fibres innervated by sensory axons (intrafusal fibres)
Constantly firing & contributes to muscle tone
Detects STRETCH as it’s paralell to muscle
What are golgi tendon organs?
Detects tension generated by contraction
In series w muscle so detects length
Found in the junction between muscle & tendon
