Somatic Sensory System Flashcards
somatic sesnory system =
representation of body in the brain
2 major inputs
mechanical stimuli
painful stimuli and tempertautre
input and interpretation enables us to…..
identify shape and texture
monitor internal and external forces acting on body
detect potentially harful circumstances
sensory receptors
golgi tendon organ
muscle spindles
sensory receptors
meissner (tactile) corpuscles
location - dermal papillae of skin, eyes, palms etc
modality - light, touch, texture (movement e.g friction)
sensitive to 30-50Hz
rapidly adapting
sensory receptors
pacinian (lamellated) corpuscles
location - dermis, joint capsules, viscera
modality - deep pressure, stretch, tickle, vibration
sensitive to 250 - 350 Hz
rapidly adapting
encapsulated nerve endings
meissener
pacinian
Ruffini
sensory receptors
ruffini corpuscles
location - dermis, subcutaneous tissue, joint capsules
modality - heavy touch, pressure, skin stretch
slowly adapting
sensory receptors
merkel (tactile) discs
location - superficial skin - epidermis
modality - light touch, texture, edges, shapes
slowly adapting
sensory receptors
free nerve endings
location - widespread in epithelia and connective tissues
modality - pain, heat, cold
unencapsulated nerve endings
merkel
free nerve endings
sensory receptors
hair follices
location - widespread in eptihelia
modality - varied according to type
both rapid and slow adapting
cellular vs molecular receptors
molecular = molecule on surface to detect other molecules cellular = group of cells organised to form receptor - detect pain, temp etc
sensory receptors = not always neurons
in above examples, cells are doing the sensing = they are the sensory neurons themselves
but, photoreceptors, auditory and vestibular hair cells are specialist cell types that synapse with sensory afferent to relay to CNS
classes of mechanoreceptor response
phasic
tonic
phasic response
rapidly adapting
give info about changes in the stimulus e.g. pacinian corpuscles
tonic
slow adapting
continue to respond as long as stimulus is present - gives info about presence of stimulus e.g. ruffini corpuscles
primary afferent sub types
according to conduction velocity which broadly reflects diameter (faster = larger diameter)
axons coming from skin
labelled with letters
A = fastest/largest
can be further broken down by greek letters, alpha = fastest
axons coming from muscles
Roman numerals
I = largest/fastest
Ia = faster than Ib
organisation of sensory info
in layers in spinal cord dorsal horn
cell bodies of different classes of sensory neurons are grouped in DRG and their projections organised to different layers of dorsal horn
somatosensory projections
2 routes to brain
medial lemniscal tract
spinothalamic tract
medial lemniscal tract
cary mechanoreceptive and proprioceptive signals to thalamus
spinothalamic tract
carries pain and temperature signals to thalamus
3 types of neurons
first order - detect stimulus and transmit to spinal cord - from periphery to spinal cord
second order - relay signal to thalamus, gateway to cortex
third order - carry signal from thalamus to cortex
medial lemniscal system
1st orders
topologically organised = spatial arrangement of objects relative to one another
1st order axons from upper body follow lateral pathway and synapse on 2nd order in cuneate nucleus
1st order from lower body follow medial pathway and synapse on neurons in gracile nucleus
both nuclei = dorsal column nuclei
medial lemniscal system
2nd orders
2nd order acons cross midline and ascend in medial lemniscus
topology is reversed, relative to midline, so lower body axons are lateral
medial lemniscal system
3rd orders
again reverse topology so lower body axons are medial again and upper are lateral
somatotropic order in brain
results of topology projection = map of body in cortex
system is posterier to central sulcus in brain
very fine map as each DRG innervates specific domains of body = dermatone
dermatones
specific region of skin innvernated by each sensory ganglion
due to dermis of each region being derived from a specific somite
somites give rise to underlying musculature and skeleton
in embryo, each DRG is associated with specific somite
topographic map
larger areas in cortex = reflects behavioural significance
receptive fields
each sensory neuron has a receptive field
size of field depends on where it is in body
large field = low discrimination e.g arm
small field = high discrimination e.g. fingers
higher number of ending from different neurons
sensory modality in cortex
somatotopic map - preserved in coronal plane in postcentral gyrus
different sensory modalities are localised along saggital axis
e.g. texture discrimination, finger co ordination = Brodmann areas
cortical map plasticity
cut off monkeys finger
= area in cortex disappeared and other areas grew larger
