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Flashcards in The Somatosensory System Deck (14)

Conscious sensation types

Exteroception (external state)
-> somatic receptors
-> feel receptors


Unconscious sensation types

->position sense
-> kinesthetic sense
Enteroception (internal state)


Sensory pathways

Dorsal column-medial lemniscus -> two point touch discrimination/touch/vibration and Proprioception
Spinothalamic-> noiception, temperature, light touch


Somatosensory receptors

Skin-> pancian corpuscles, merkels disks, meisseners corpuscles, ruffini ending
Muscle-> muscle spindles
Tendons-> Golgi tendons
Joint capsules-> ruffini endings
Gut-> stretch receptors
Free nerve endings-> chemical pain


Sensory tract order

1st order-> cell bodies in dorsal route ganglion-> primary afferent sensory fibres
2nd order-> cell bodies lie in spinal cord
3rd order-> cell bodies lie in thalamus


Primary afferent sensory fibres Aa and Ab

Aa and Ab-> large diameter, myelinated, no thermal threshold
Aa-> skin/group 1 muscle-> proprioceptive
Ab-> skin/group 2 muscle-> mechanoreceptive, slower
-> Proprioception and light touch, first realisation of pain


A gamma fibres

Lightly myelinated
Small diameter
Noiception-> mechanical,thermal, chemical
Thermal threshold 53 degrees


C fibres

Small diameter-> slow transmission-> second pain
Innocuous temp
Nociception-> mechanical, thermal, chemical, thermal threshold 43 degrees


Termination of sensory fibres in the spinal cord

Arrangement of lamina varies with spinal cord level depending on how much sensory input there is
VII only present in limb enlargements
Substantial gelatinosa-> layer II
Mechanoreceptors synapse in layers II and IV and also IX for reflex pathways
Noiception-> layer I to V


Medial lemniscus pathway

Aa and b
Two point discrimination
No intervening synapses-> fast -> up dorsal colum
Synapse in nuclei then decussates at medulla-> contralateral-> enter medial lemniscus -> arm fibres medial
Gracile nuclus-> feet up till arms
Cunneate nucleus-> arms up till face
Synthase on ventral posterior nucleus of thalamus


Fine touch

Discriminative touch
Dorsal-column medial lemniscus


Tow point discrimination

Ability to differentiate between two points
Large receptive field-> two stimuli perceived as a single point-> many primary sensory neurones converge on a single secondary neurone
Small receptive field-> fewer neurones converge-> two stimuli are in different receptive fields-> perceived as distinct stimuli


Somatosensory cortex

Located in the post central gyrus
3b-> somatosensory cortex
3a-> somatosensory association cortex, receives Proprioception info from the thalamus
1 and 2-> receive input from 3b
1-> texture
2-> mainly size and shape


Spinothalamic tract

Lateral-> pain and temperature
Anterior-> light touch
Synapse in dorsal horn
Second order neurones immediately Decussate
Some fibres ascend or descend a few levels first in the dorsal tract of lissuer before decussating
Form the spinal lemniscus tract at the level of the pons
Posterior limb of internal capsule from thalamus
Leg, trunk, arms, neck lateral to medial