Somatosensory system Flashcards
Describe a primary afferent neurone
Sensory receptor in skin Axon fibre Cell body on stalk Cell body in dorsal root ganglia Terminates in dorsal horn of spinal cord
What is the name of the area that a sensory neurone receives information from?
The receptive field
Where is an action potential generated in a primary afferent sensory neurone?
At the peripheral end of the axon
What are the four classifications of primary afferent axons?
Aalpha
Abeta
Adelta
C
Which is the thickest myelinated axon?
Aalpha
What is the classification of an unmyelinated axon?
C
Where will you find Aalpha axons?
Proprioceptors of skeletal muscle
Where will you find Abeta axons?
Mechanoreceptors of skin
Where will you find Adelta axons?
Pain, temperature
Where will you find C axons?
Temperature, pain, itch
Explain the relationship between nerves and receptive fields
Each nerve axon innervates a specific receptive field
Describe receptive fields
Can vary greatly in size
This determines precision of localisation
What defines the size of a receptive field?
2 point discrimination
What is 2 point discrimination?
The smallest distance between two points where you can still distinguish two stimuli
How does sensory localisation vary over the body?
Greatly
Fingers are very sensitive
Area like the back isn’t
What in the brain relates to the sensory localisation?
The size of the cerebral cortex for that region of the body
What is a dermatome?
An area of skin that is innervated by afferent axon fibres
Signalling all its sensation via a signal nerve from a single spinal nerve root
What is an adequate stimulus?
The right size and type of stimulus for that nerve ending that will stimulate a response
Is threshold dependent on nerve endings?
Yes
What is the line code?
The CNS interprets incoming action potentials by the specific axon they arrived via
What is generated in the nerve ending of a sensory neurone?
A receptor potential
Stimulus strength determines what?
The size of the graded receptor potential
What creates the graded receptor potential?
Channels opening
Membrane depolarisation
Firing rate is proportional to…
…stimulus strength
What is a slow or non-adapting stimulus?
A stimulus that is always present
Important when maintaining information about a stimulus is valuable
e.g. amount of stretch or pain
What is a fast adapting stimulus?
Constantly changing stimulus
Useful where it is important to signal a change in stimulus
Also to stop paying attention to stimulus
e.g. touch
Name the three kinds of cutaneous sensory receptors
Mechanoreceptors
Thermoreceptors
Nociceptors
What do mechanoreceptors sense?
Touch
Pressure
Vibration
What do thermoreceptors sense?
Temperature
What do nociceptors sense?
Noxious stimulation
Pain
Name sensory organs (mechanoreceptors) found in human skin
Ruffini’s corpuscle
Meissner’s corpuscle
Pacinian corpuscle
Merkel’s receptor
What classification of axon does a cutaneous mechanoreceptor have?
Abeta
The structure of sensory apparatus determines what?
Structure determines function
Function determines location
Group the mechanoreceptors of the skin by location
Superficial
Meissner’s corpuscle
Merkel’s receptor
Deep
Ruffini’s corpuscle
Pacinian corpuscle
Describe Meissner’s corpuscles
Found in papillary dermis Rapidly adapting Constantly changing stimulus required Light touch Vibration
Why does the need for constantly changing stimulus make a Meissner’s corpuscle effective
Example:
Detect putting clothes on
Do not detect wearing the clothes
Describe Merkel’s receptors
High density in epidermis of digits and lips Lower density elsewhere (glabrous) Very low density (hairy) Slowly adapting Light touch
What actually is a Merkel cell?
An adapted keratocyte
What stimuli do Merkel cells respond to?
Initial indentation of skin
Sustained pressure of up to a few seconds
What do Merkel’s receptors allow us to perceive?
Form and texture
Describe Ruffini’s corpuscle
Responds to lateral movement or stretching of skin
Deep touch
Describe the structure of a Ruffini’s corpuscle
Network of collagen fibres
Describe a Pacinian corpuscle
Found in deeper layers of dermis
Rapidly adapting
Stronger stimulus
High frequency vibration
What is the structure of a Pacinian corpuscle
A fully encapsulated nerve ending
Onion structure
Relieves pressure on nerve ending
What does the activation of a Pacinian corpuscle feel like?
Vibration
Describe a hair follicle receptor
Light touch
Rapidly adapting
Constantly changing stimulus required
Describe a cutaneous thermoreceptor
Bare nerve endings
Slowly adapting sensory receptor
What are the two types of thermoreceptor?
Respond to cold
Respond to warm
What is important to remember about thermoreceptors?
They do not indicate absolute temperature
They are sensitive to change in temperature
How do we sense temperature?
Comparing signals from warm and cold receptors
What are thermoreceptor channels?
Non-specific cation channels
Nerve ending sensitivity dependant on which transducer channels are expressed
Describe TRPV3/4
Warm thermoreceptor
Channels open 29-45
Maximum 45
Describe TRPM8
Cold thermoreceptor
Channels open 8-38
Maximum 25
Also opened by menthol
Describe TRPA1
Cold thermoreceptor
Channels open <17
Cold receptors have what kind of axons?
C and Adelta
Warm receptors have what kind of axon?
C
What is paradoxical cold perception?
Cold receptors activated at very high temperatures
Feeling of cool at high temperatures
Describe cutaneous nociceptors
Bare nerve endings
Non-adapting sensory receptors
High threshold
Adequate stimulus must be capable of damaging tissue
Name the 2 types of nociceptors
High threshold mechanoreceptors
Polymodal nociceptor
Describe a high threshold mechanoreceptor
Pricking pain
Adelta fibre
Describe a polymodal nociceptor
Poorly localised burning pain
C fibre
Sensitive to mechanical stimulus, heat and chemicals
What is proprioception?
Detecting the mechanical status of the muscle-skeletal system
What do proprioceptors provide information about?
Joint position Muscle length Muscle movement Acceleration Tension/force
Describe a muscle spindle
Specialised muscle fibres in a fibrous capsule Termed intrafusal fibres 1a afferents wrap around central portion Firing contributes to muscle tone Stretch sensitive = increased firing
Describe a golgi tendon organ
Located at the junction of the muscle and tendon
Made of collagen fibrils
Innervated by 1b sensory afferent neurone
Sensitive to tension generated by contraction
Its position is in series with the muscle
