the somatosensory system Flashcards
types of sensory receptors
encapsulated nerve endings:
- pacinium corpuscle
- Meissner corpuscles
- ruffini corpuscles
unencapsulated nerve endings:
- merkel discs
- free nerve endings
what are the different classes of mechanoreceptor response?
- rapidly adapting (phasic) = give information about changes in the stimulus
- slowly adapting (tonic) = continue to respond as long as stimulus is present
how are axons classified?
classified according to conduction velocity, which broadly reflects diameter
axons coming from skin designated by letters A=fastest, C=slowest
axons coming from muscles designated by roman numerals I=fastest IV=slowest
how is sensory information initially organised?
starting with layers in the spinal cord dorsal horn
cells grouped in DRG and projection organised to different layers of dorsal horn
what main routes get sensory information from spinal cord to brain?
medial lemniscal tracts - carry mechanoreceptive and proprioceptive signals to the thalamus
spinothalamic tract - carries pain and temp signals to thalamus
what neurons carry sensory information to higher centres?
first-order neurons = detect stimulus and transmit to spinal cord
second-order neurons = relay signal to thalamus
third-order neurons = carry signal from thalamus to cortex
what do first-order neurons synapse onto in medial lemniscal tract?
axons are topologically organised
first-order axons from upper body follow lateral pathways and synapse on 2nd order neurons in cuneate nucleus
first-order axons from lower body follow medial pathway and synapse on neurons in gracile nucleus
together the nuclei are known as dorsal column neclei
what is topological organization?
the spatial arrangement of the objects relative to one another
what is the role of second- and third-order neurons in the medial lemniscal tract?
axons are topologically organised
2nd order axons cross midline and ascend in the medial lemniscus, topology reversed so lower body axons are more lateral etc
3rd order axons again reverse topology so lower body axons are medial etc
what is the result of the topological projection of the medial lemniscal tract?
a map of the body in the cortex
reflects the fact that each dorsal root ganglion innervates a specific dermatome
what is a dermatome?
the specific region of skin innervated by a dorsal root ganglion
they arise because dermis of each reason is derived from a somite (each one associated with a DRG)
how is the cortex a topographic map?
represents physical distribution of parts on the surface
how to determine size of receptive field?
two point discrimination - assessing the ability to discriminate two sharp points set apart at different distances
if the subject feels two pin points then the distance between the points is larger than the receptive field
why are the fingers a larger area on the cortex?
more cortex is dedicated to regions where receptive fields are small, high number of endings from different neurons
