The Somatosensory System Flashcards
What is the receptive field?
the area of skin innervated by a single axon
What are the two potential routes of primary sensory neurones?
arise at the sensory receptor
cell body in DRG
then synapses in dorsal horn or ascends up to brain
What are the 4 axon classifications of the primary afferents?
Aalpha = largest diameter --> proprioceptors of skeletal muscle Abeta = mechanoreceptors of skin Adelta = pain, temperature C = smallest diameter, no myelin --> temperature, pain, itch
Receptive filed sizes vary, determining precision of ………
What are receptive field size defined by?
localisation
- 2 point discrimination
- related to the area of cerebral cortex devoted to each region
What is a dermatome?
an area of skin that is innervated by afferent axon fibres, signalling all its sensation via a single nerve from a single spinal root
What is the adequate stimulus?
What is the threshold a signal?
What do these both depend on?
adequate stimulus = stimulus that produces maximal response
threshold = the threshold that once reached will fire an action potential
- depend on the nerve ending
Firing rate is proportional to what?
stimulus strength
What is meant by a graded response?
receptor potential can vary in size, unlike an action potential (these are all or nothing)
What is the difference between phasic and tonic receptors?
Phasic = fast-adapting, detects how fast it changes, constantly changing stimulus required - useful where it is important to signal a change in stimulus, also to stop paying attention to stimuli that are no longer important e.g. tactile receptors Tonic = slow adapting or non-adapting, detects strength, important when maintaining information about a stimulus is valuable e.g. amount of stretch or pain
What is line code?
identifying the incoming sensory information by knowing which the particular axon is carrying it
What are the 5 major types of mechanoreceptors?
Meissener's corpuscle Merkel's receptor (disk) Ruffini's corpuscle (ending) Pacinian corpuscle Hair follicle receptor
At the end of which axons are cutaneous mechanoreceptors found?
What is the apparatus comprised of?
- Abeta fibres
- a specialised cell
Meissner’s Corpuscle:
- where are they found?
- are they rapidly or slowly adapting?
- what do they detect?
- what kind of stimulus is required?
- gives 2 examples of when they are used
- papillary dermis
- rapidly adapting
- light touch / vibration
- a constantly changing stimulus is required
e. g. putting on clothes but not wearing
e. g. adjustment of grip force when an object is slipping out of your hand
Merkel’s receptor (disk):
- where are they found in high density?
- are they rapidly or slowly adapting?
- what do they detect?
- what is the apparatus comprised of?
- epidermis of digits and around mouth
- slowly adapting
- sustained light touch, perception of form and texture, sustained pressure up to several seconds in duration
- specialised keratocyte
Ruffini’s Corpuscle:
- what do they respond to?
- what is their apparatus comprised of?
- deep touch and stretch
- apparatus is a network of collagen fibre
Pacinian Corpuscle:
- where are they found?
- are they rapidly or slowly adapting?
- what do they respond to?
- describe their structure
- found in deeper layers of dermis
- rapidly adapting
- respond to deep poke, high frequency vibration
- fully encapsulated nerve ending, layers
Hair Follicle Receptor:
- what do they respond to?
- what is their structure?
- what kind of stimulus is required?
- light touch, hair deflection detected
- nerve fibre wrapped around hair follicle
- constantly changing stimulus is required
Describe cutaneous thermoreceptors and how they detect temperature
- bare nerve endings
- slowly adapting sensory receptor
- poor indicators of absolute temperature
- but very sensitive to changes in temperature
- sense of temperature comes from the comparison of the signals for warm and cold receptors
What family are thermoreceptors channels from?
Describer them
Transient Receptor Potential Family
- non-specific cation channels
- nerve ending sensitivity dependant on which transducer channels are expressed
What are the 3 thermoreceptor channels called?
What are they opened by?
TRPV3/4 = warm channels, open 29-45 TRPM8 = cold channels, open 8-38 (opened by menthol) TRPA1 = cold >17
What fibres are cold thermoreceptors found on?
What fibres are warm thermoreceptors found on?
cold = C and Adelta warm = C fibres
What is paradoxical cold perception?
cold receptors also excited by high temperatures
Describe cutaneous nociceptors
bare nerve endings
non adapting sensory receptor, high threshold
adequate stimulus must be capable of damaging tissue
What are the two types of cutaneous nociceptors?
High threshold mechanoreceptors (Adelta)
- well localised pricking pain
Polymodal nociceptor (C fibre)
- sensitive to mechanical stimulus, damaging heat and noxious chemicals
- poorly localised burning pain
- TRPV1 an identified transducer channel (opened by capsaicin, chillies)
What are the receptors for proprioception?
What do they detect?
What do they provide information about?
- muscle spindle, golgi tendon organ
- detects the mechanical stress of the musculo-skeletal system
- joint position, muscle length, muscle movement, acceleration, tension/force
Describe how the muscle spindle detects length and acceleration
- specialised muscle fibres in a fibrous capsule
- termed intrafusal fibres (cf extrafusal, power generating fibres
- Aalpha afferents wrap around central (sensory) portion
- firing contributes to muscle tone
- stretch sensitive –> increase firing
Describe the stretch reflex (proprioception)
muscle spindle –> Aalpha afferents –> spinal cord –> alpha motor neurone –> effector muscle
How do golgi tendon organs detect muscle tension?
- located at the junction of muscle and tendon
- innervate by Abeta sensory afferents
- position in series with muscle
- sensitive to tension generated by contraction (cf spindle in parallel, sensitive to length)
