somato-sensory system Flashcards
sensory receptors with encapsulated nerve endings
meissner (or tactile) corpuscles
pacinian (or lamellated)
corpuscles
ruffini corpuscles
sensory receptors with unencapsulated nerve endings
merkel (or tackle) discs
free nerve endings
‘encapulsated nerve endings’
means they can filter out frequencies
to give specific frequencies
meissner corpuscles location
dermal papillae of skin esp hands, palms, eyelids, lips, tongue ec
modality of messier corpuscles
light touch, texture (movement)
sensitive to 30-50 Hz
rapidly adapting
pancinian corpuscles location
dermis
joint capsules
viscera
pancinian corpuscles modality
deep pressure stretch tickle vibration sensity to 150-350 Hz rapidly adapting
ruffini corpuscles location
dermis
subcutaneous tissue
joint capsules
ruffini corpuslces modality
heaving touch, pressure, skin stretch
joint movements
slowing adapting
merkel discs location
superficial skin (epidermis)
merkel discs modality
light touch, texture , edges shapes
slowing adapting
free nerve ending locations
widespread in epithelia and connective tissue
free nerve ending modality
pain, heat and cold
hair follicles location
widespread in epithelia
hair follicles modality
varied according to type
both rapid and slowly adapting subtypes
different classes of machine receptor response
phasic and tonic
tonic
slowing adapting
continue to response as long as the stimulus is present (info about persistence of stimulus)
e.g. ruffini corp
phasic
rapidly adapting five info about changes in the stimulus
e.g. pacinian corp
primary afferent axon subtypes are classified according to….
conduction velocity
which broadly reflects diameter (faster=longer diameter)
axons coming from the skin are designated by
letters ( A, B, C)
a= fastest/largest
c=slowest/smallest
A group further broken down by greek character
alpha, beta delta
a= fastest
d=slowest
axons coming from muscles designated by …
roman numerals
I II III IV
I = largest IV= smallest
pain fibres are ….. than proprioceptors
slower
evolutionarily important- ability to stand, feedback has to be very fast
how is sensory info organised
The cell bodies of sensory neurons are grouped in the Dorsal root ganglion and their projections are organised to different layers of the dorsal horn
two main routes that send info from all sensory subtypes to brain
1) medial lemniscal tracts
2) spinothalamic tracts
medial lemniscal tract
carry mechanoreceptive and proropceptive signals to the thalamus
spinothalamic tract
carries pain and temp signals to the thalamus
Typically, sensory information travels through 3 neurons to reach higher centres:
First-order neurons detect the stimulus and transmit to spinal cord via Dorsal Root Ganglion
Second-order neurons relay the signal from spinal cord to the thalamus, the “gateway” to the cortex
Third-order carry the signal from the thalamus to the cortex
commissural
cross the midline to the contralateral side
in both medial lemniscal tracts and spin thalamic tracts 2nd order axons do what?
cross the midline i.e. are commissural
in the medial lemiscal system, 1st order neurones from the upper body…..
follow lateral pathway and synapse on 2nd order neurones in the cuneate nucleus (in medulla)
in the medial lemiscal system, 1st order neurones from the lower body…. (below vertebra T6)
follow the more medial pathway and synapse on neurons in the gracile nucleus
the doors column nuclei
the cuneate and the gracile nucleus
2nd order axons in the medial lemiscal system
cross the midline
and the topology is reversed relative to the midline so that lower body axons are more lateral on reaching the thalamus
3rd order axons in the medial lemiscal system
again reverse the topoglofy so that lower body axons synapse on the more medial cortical neurones
medial ness and laternal ness of axons
1st ordeR: upper= lateral
lower = medial
2nd order: upper= medial
lower= lateral
3rd order: upper=lateral
lower = medialsomatopoic order in the brain is not simply lower vs upper body axons, the maps is very fine which reflects…..
the fact that each dorsal root ganglion innervate a specific domain of the body called a dermatone
dermatome
area of the skin that is mainly supplied by a single spinal nerve
dermatomes arise from
embryonic somite (which are repeating structures that give rise to the underlying musculature and skeleton)
each DRG is associated with a specific somite and subsequently innervates the tissues surrounding that somite
receptive fields
an axon splits into multiple endings and extend over a n area
fingers receptive fields
small (high discrimination)
cortex more dedicated to these areas
receptive fields in legs
large (low discrimination)
receptive field size
In these regions, although the number of receptors (nerve endings) may be similar, the number of endings from different neurons per unit area is higher, therefore the amount of derived information is also higher
largest receptive fields
upper arm, thigh, calf
what factor effects the velocity of propagation of the action potential
AXONAL DIAMETER
The influx of sodium that occurs during an action potential causes depolarisation of the axon membrane that lies just ahead of it. As this region reaches threshold, voltage-gated sodium channels open and the action potential is propagated along the axon. The influx of positive charge associated with the inward sodium current has two paths that it can take, it can flow down the inside of the axon or it can cross the axonal membrane. The more charge that flows down the inside of the axon the faster the propagation of the action potential. Therefore, axons with a wider diameter provide more opportunity for the current to flow down the centre of the axon, rather than across the membrane, giving large diameter axons a faster conduction velocity.
