Motor System Part 2 Flashcards
(12 cards)
What are Upper Motor Neurons (UMNs)
Cell Bodies:
- Motor Cortex
- Brainstem centers
Axons:
- Primarily synapse with local circuit neurons
- Responsible for initiation of voluntary movement and complex spatio-temporal skilled activities
Explain the motor cortex
Primary Motor Cortex: Precentral gyrus
- Contralateral control of movement
- Encodes for movement in personal space
Premotor Cortex: frontal lobe (initiating movement)
- Intention of movement from external cues (ex: see a pot hole, plan to go around it)
- Receives input from the cerebellum (coordinated movement)
Supplementary Motor Cortex
- Prepares for movement
- Intention of movement from internal cues (ex: I’m hungry, im going to the fridge)
- Receives input from put amen of the basal ganglia
Topographical Organization (HAL)
Corticobulbar tract (bulbar=brainstem)
- Face
Corticospinal tract
- Upper Extremity
- Trunk
- Lower Extremity
What are Betz cells (pyramidal cells)?
The celebrity of UMNs
- In medulla -> travel through medullary pyramids
- Found in cortical layer 5
- UMN’s of the primary motor cortex
- LARGEST neurons in the CNS (by soma size)
-> Account for 5% of the axons that project from the motor cortex
-> Betz and non-Betz cell axons descend via the corticospinal and corticobulbar pathways
What pathway controls fractionated and distal limb movements ?
Lateral Corticospinal Tract
Fractionation: Ability to activate individual muscles independent of other muscles
- Tracts descend through the lateral white matter of the spinal cord
- Motor neuron pools located in the ventral horn of the spinal cord
What pathway terminate bilaterally via collateral branches that cross midline?
Ventral Corticospinal Tract
- Synapse with motor neurons that control the neck, shoulder, and trunk muscles
- Stays ipsilateral, then terminates and provides bilateral projections
- Since it’s bilateral, there’s backup !!
Explain the Corticobulbar Tract Cranial Nerve Nuclei
- Majority of axons terminate bilaterally on local circuit neurons in the reticular formation rather than directly onto the LMN’s motor nuclei (synapse w/ LCN in reticular formation THEN connects to LMNs)
- Local circuits coordinate output to the cranial nerve motor nuclei
- Result -> Minimal impairment with unilateral trauma - Motor neurons innervates the muscles of the face, tongue, pharynx, larynx, traps, and SCM
- CN nuclei for III, IV, V, VI, VII, (IX, X), XI, and XII
What are the EXCEPTIONS to bilateral termination?
Corticobulbar input arises from BOTH cerebral hemispheres BUT favors the CONTRALATERAL cortex in the following…
- Hypoglossal nucleus
- Trigeminal motor nucleus
- Facial motor nucleus -> Lower face innervation (raising/lowering one corner of the mouth)
What is the difference between UMN and LMN lesions in terms of facial paralysis/paresis?
UMN lesion
- Damage to the unilateral motor areas
- Resultant CONTRALATERAL weakness of INFERIOR facial muscles
LMN Lesion
- Damage to the facial motor nucleus (in the pons) or its nerve
- Resultant ipsilateral weakness of the SUPERIOR and INFERIOR facial muscles
What do Brainstem Motor Pathways control?
- Control axial musculature of the trunk and proximal musculature of the extremities
-> Maintain balance, regulation of posture, initiation/regulation of locomotion, and orientation of visual gaze - Tracts reside in the MEDIAL white matter of the spinal cord
- Information relayed to medial motor neuron pools in the spinal cord
Explain the Medial Vestibulospinal Tract
- Originate in the medial vestibular nucleus (pons)
-> Terminates BILATERALLY in the medial ventral horn of the cervical spinal cord - Projects to the muscles of the neck and upper back (makes sense why they terminate in the cervical sc)
-> Regulates head position by reflex activation of neck muscles in response to inner ear - Provides feedback response to postural instability via vestibular labyrinth
-> Ex: semicircular canals
Explain the Lateral Vestibulospinal Tract
- Originate in lateral vestibular nucleus
-> Terminates IPSILATERALLY - Project to proximal muscles of the limbs
-> Activate limb EXTENSORS in response to inner ear
-> Maintains one’s center of gravity over their base of support - Provide feedback response to postural instability via vestibular labyrinth
-> Ex: otolith organs
Explain the Lateral Corticospinal Tract
Controls fractionated and distal limb movements
(Ability to use muscles independently without others tagging along, like playing the piano)
- Originates in the primary motor cortex (precentral gyrus) and travels through internal capsule
- Travel ipsilaterally down the brainstem
- Decussate at the caudal medulla (medullary pyramids) only lateral decussates
- Travel down lateral white matter tract
- Synapse with Local Circuit Neurons (fine tune movement) before synapsing with Lower motor neuron in ventral horn
- Go to skeletal muscle
