somatosensory pathways Flashcards
Somatic sensation
from the body
- carried by peripheral nerve to spinal cord
- Passed rostrally (t/w head) thru sp cord, brainstem, thalamus, to cerebral cortex
Chain of 3 neurons rule:
1. sensory receptor at peripheral extremity
- cell body in dorsal root ganglion
- Synapses upon entry to CNS
2. crosses to opposite side
- ascends to thalamus
- in thalamus, synapses w 3rd neuron
3. thalamocortical neuron
- Projects to somatosensory cortex in parietal lobe
Touch and proprioception
carried from periphery by sensory I and II (myelinated and fast)
- Reach sp cord, ascend in white matter on same side w/o synapsing, via dorsal columns
- Synapse in caudal medulla in dorsal column nuclei
- axon of 2nd order neuron crosses immediately
- ascends through medulla medially (medial lemniscus)
Pain and temp
Carried from periphery by sensory III and IV (slow)
- 1st order neurons synapse in dorsal horn
- 2nd order cross right away, ascend spinothalamic tract (ventrolateral white matter)
In spinal cord:
Touch and proprioception same side (cross in brainstem)
Pain and temp opposite side (cross right away)
Pain + temp combine w touch + proprioception
In medulla and pons: medial lemniscus (touch+proprio) is medial, spinothalamic (pain+temp) is lateral
At midbrain: combine into sensory lemniscus
Sensory lemniscus fiber organization
lateral to medial: leg, torso, arm and neck, head
Sensory info from head?
- Carried by trigeminal nerve
- Enters at pons
- 2nd order cross immediately and join sensory lemniscus
Sensory lemniscus continues to…
- Thalamus (large group of nuclei)
- Named according to position in thalamus
- Somatosensory nuclei: Ventroposterior nuclei
- Lateral division: legs
- Medial division: upper body fibers, head most medial
- 3rd order axons go thru internal capsule to postcentral gyrus of parietal lobe
Corticospinal system
Voluntary movement
- one neuron from cortex to sp cord
- cell bodies in precentral gyrus of frontal lobe
Corticospinal tract
- Axons run thru internal capsule
- Thru cerebral peduncle on ventral surf of midbrain
- Thru pons (clumps)
- Thru medulla in pyramids (@ medulla-sp cord junction)
- Cross to lateral column of white matter (lateral corticospinal tract)
- Axons leave tract at appropriate level
- Enter ventral horn of grey matter and synapse w motor neurons
Corticobulbar system
Face muscles (bulbar=brainstem)
- Same as corticospinal except it terminates on cranial nerve nuclei in brainstem
- Axons run thru cerebral peduncle in midbrain
- Thru pons (clumps)
- Thru medulla in pyramids
- Cranial nerve nuclei in brainstem
Reticulospinal tract
Reticular formation forms central core of grey matter, runs whole length of brainstem
- Gives rise to reticulospinal fibers (project bilaterally)
- Influence gamma-MNs
- Mingled w other systems in ventral columns of spinal cord
Vestibulospinal tract
From vestibular nuclei (in medulla and pons)
- Medial vestibulospinal tract: from medial vestibular nucleus -> MNs for neck and trunk
- Lateral vestibulospinal tract: from lateral vestibular nucleus -> MNs for limbs
- Ipsilateral and do not branch
- Mainly extensor motor neurons
alpha-gamma coactivation
Corticospinal system activates alpha and gamma MNs simultaneously
- Spindle sensitivity maintained even during contraction
