somatosensation I Flashcards
what does the somatosensory system do?
connects the body to the CNS via peripheral nerves
what sensations does the somatosensory system convert from the body?
- heat, cold
- pain, itch
- proprioception
- touch
what is proprioception?
being aware of the position of the body (like sixth sense)
what types of peripheral nerves exist?
spinal and cranial
structure of peripheral nerves?
a nerve is a bundle of axons, covered in a connective sheath called an epineurium
- the nerve consists of several fascicles, all separate by connective tissue called perineurium
- the fascicles contain axons, and individually myelinated axons are wrapped in endoneurium
what does the dorsal root contain?
cell bodies of incoming sensory neurones
what does the ventral root contain?
cell bodies of outgoing motor neurones
what us the dorsal root ganglion?
occurs just as the dorsal and ventral roots fuse, located just outside of the spinal cord
-contain cell bodies of the sensory neurones that are part of the somatosensory system
dorsal and ventral root reunite to form what?
spinal nerve
white matter
ascending and descending axons tracks
grey matter
cell bodies, dendrites, synapses, glial cells
what is the spinal cord divided into
incoming sensory info and outgoing motor info
what are dorsal root ganglion cells?
sensory receptors of the somatosensory system
explain the 2 functionally distinct systems of the somatosensory system
2 different white matter tracts
- (DCML)
- mechanosensory afferent fibres
- a-beta and a-alpha afferent fibres
- large fibres, large diameter, myelinated, fast conduction
- touch, vibration, tactile, proprioception
- enter the spinal cord and enter the medulla and then cross - (STT)
- thin fibres
- a-delta and C fibres
- small diameter, thinly myelinated/unmyelinated, medium/slow conducting
- pain, itch, crude touch coarse, temperature
- enter the spinal cord, make contact with second order neurones in the spinal cord - these cross to the other side, then ascend up to the brain
- the receptor endings/axon terminals are in areas of the body eg. the fingers
- the axon travels centrally via the dorsal root
- axon can then terminate or they may ascend the spinal cord all the way to the brainstem
crude touch
crude touch is touch without knowing the location
what does the quality of sensation depend upon?
the afferent fibre type
if the afferent fibre is a member of the small class of fibre what will the sensation be?
temperature related, pain or crude touch
if the afferent fibre is a member of the large class of fibre what will the sensation be?
proprioception
cutaneous receptors of the somatosensory system?
larger diameter afferents - 4 major classes in the skin:
2 are cutaneous, receptor endings between the dermis and epidermis - tactile afferents, sensitive to light touch - Meissners Corpuscle and Merkels discs
2 are deeper in the dermis, or between dermis and hypodermic - Ruffini’s and Pacinian corpuscles
also there are free nerve endings
proprioceptors of the somatosensory system?
group 1 and 3 afferent axons detect changes in length
- muscle spindles are stretched and increase their firing rate
- muscle spindle itself can be made to contract by the gamma motor neurone - recalibration of muscle spindle length
what are muscle spindles?
specialised muscle fibres, detect changes in muscle length
receptors of the somatosensory system?
- proprioception
- muscle spindles, A-alpha afferent fibres, large diameter, fastest conduction - tactile afferents - A-beta afferent fibres, large diameter, second fastest conduction
a) superficial
Meissners Corpuscle and Merkels discs
b) deep
Ruffini’s and Pacinian corpuscles - free nerve endings
- end in the dermis/epidermis
- A delta fibres - thin, myelinated, moderate conduction
- C fibres - thin, unmyelinated, slow conduction
what do Pacinian Corpuscles detect?
vibration
what do Ruffini Corpuscles detect?
stretch
what are small receptive field needed for?
precise detail
having a large receptive field mean?
the stimulus cannot be localised apart from the fact that its within that large receptive field
larger areas of the cortex are devoted to?
areas of the body where receptive fields are small
