The Somatosensory System

Question 0

Unconscious sensation types

Answer 0

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

Question 1

Conscious sensation types

Answer 1

Exteroception (external state)

• > somatic receptors
• > feel receptors

Question 2

Sensory pathways

Answer 2

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

Question 3

Somatosensory receptors

Answer 3

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

Question 4

Sensory tract order

Answer 4

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

Question 5

Primary afferent sensory fibres Aa and Ab

Answer 5

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

Question 6

A gamma fibres

Answer 6

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

Question 7

C fibres

Answer 7

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

Question 8

Termination of sensory fibres in the spinal cord

Answer 8

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
Noiception

Question 9

Medial lemniscus pathway

Answer 9

Ipsilateral
Aa and b
Two point discrimination
Touch/vibration
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

Question 10

Fine touch

Answer 10

Discriminative touch

Dorsal-column medial lemniscus

Question 11

Tow point discrimination

Answer 11

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

Question 12

Somatosensory cortex

Answer 12

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

Question 13

Spinothalamic tract

Answer 13

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