L04 - The Somatosensory System Flashcards
What are two features of primary sensory neurones?
- All excitatory (majority use glutamate NT)
- Can generate APs at peripheral nerve ending
What are the 4 fibre types and what are their functions?
- 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
What is the difference between a receptive field and a dermatome?
- 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
What is an adequate stimulus?
The type of energy to which a sensory receptor will respond with the initiation of an action potential
Which part of a neurone determines the adequate stimulus and the signal threshold?
Nerve ending
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?
- 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)
What is the function of fast adapting receptors?
How do fast adapting receptors respond to unchanging stimuli?
Why is this useful?
- 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
What are the 3 types of cutaneous sensory receptor and what stimuli do they respond to?
1 - Mechanoreceptors (touch, pressure and vibration)
2 - Thermoreceptors (temperature)
3 - Nociceptors (noxious stimuli)
In which layer of the skin are Meissner’s corpuscles found?
In the papillary (superficial) dermis of glabrous skin
What stimuli do Meissner’s corpuscles respond to?
1 - Light touch
2 - Vibration
What type of adaptation do Meissner’s corpuscles exhibit?
What is the implication of this?
- Fast adaptation
- They require constantly changing stimuli
Where are Merkel’s receptors found?
- In high densities in the epidermis of the digits and around the mouth
- In low densities in other glabrous skin
What stimulus do Merkel’s receptors respond to?
Sustained light touch
What type of adaptation do Merkel’s receptors exhibit?
- Slow adaptation
- They can sustain a constant light touch stimulus
What apparatus forms Merkel’s receptors?
Specialised keratocytes
Where are Ruffini’s corpuscles found?
In the dermis of glabrous skin
Which stimuli do Ruffini’s corpuscles respond to?
1 - Lateral movement (stretching) of the skin
2 - Deep touch
What type of adaptation do Ruffini’s corpuscles exhibit?
Slow adaptation
What apparatus forms Ruffini’s corpuscles?
Networks of collagen fibres
Where are Pacinian corpuscles found?
In the reticular (deep) dermis of both glabrous and hairy skin
What stimuli do Pacinian corpuscles respond to?
Strong stimuli:
1 - Strong poking
2 - Deep touch
3 - High-frequency vibration (this is the primary stimulus)
What type of adaptation do Pacinian corpuscles exhibit?
Fast adaptation
What is the structure of Pacinian corpuscles?
Fully encapsulated nerve endings
Where are hair follicle receptors found?
Dermis
Which stimulus do hair follicle receptors respond to?
1 - Hair deflection
2 - Light touch
What type of adaptation do hair follicle receptors exhibit?
Fast adaptation -> require constantly changing stimulus
What type of adaptation do cutaneous thermoreceptors exhibit?
Slow adaptation
What are the two subcategories of cutaneous thermoreceptors?
Cold receptors found on C & Aδ fibres & warm receptors on C fibres
How is the sense of temperature determined?
- 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
To which family of ion channels do thermoreceptor ion channels belong?
- Transient receptor potential (TRP) family
- They are non-specific cation channels that open in response to particular temp changes
What is the name of the ion channels that belong to cold cutaneous thermoreceptors?
When do these channels open?
- TRPM8 channels = cold receptors -> open between 10-38℃ (max opening at 25℃) -> also opened by menthol
- TRPA1 channels (ANKTM1) = cold receptors -> open at <17℃
What is the name of the ion channels that belong to hot cutaneous thermoreceptors?
When do these channels open?
TRPV3/4 channels = warm receptors -> open between 29-45℃ (max opening at 45℃)
Why do mints feel cold in the mouth?
- They contain menthol, which opens TRPM8 channels
- These are cold receptors
Which stimuli do cutaneous nociceptors respond to?
- Pain
- Damaging heat
- Noxious chemicals
What type of adaptation do cutaneous nociceptors exhibit?
Non-adaptation
What can be said about the threshold of cutaneous nociceptors?
- The threshold is very high & an adequate stimulus must be capable of damaging tissue
- This prevents everything feeling painful
What are the two types of cutaneous nociceptors?
- 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
Which substance opens TRPV1 channels?
Capsaicin
What 4 properties of the musculoskeletal system are conveyed in proprioception?
1 - Joint position
2 - Muscle length & movement
3 - Acceleration
4 - Tension/force
What are muscle spindles?
- 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
What are Golgi tendon organs?
- 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
What stimuli excite fibres in the lemniscal pathway?
- Touch
- Vibration
- Two-point discrimination
- Proprioception
What stimuli excite fibres in the spinothalamic pathway?
- Pain
- Temperature
What fibres are involved in the lemniscal pathway?
Large sensory Aβ fibres
What fibres are involved in the spinothalamic pathway?
Small sensory Aδ & C fibres
