Motor Tracts Flashcards
Motor tracts big picture:
Direct (cortex to SC) and indirect pathways (passing through something)
2 tract system (UMN, LMN)
Upper motor neurons vs Lower motor neurons
Located in the cortex or brainstem and axons(descend down to their target LMNs)
Located in the periphery (spinal cord or brain stem) and axons go and innervate the muscles).
CNs with motor component are LMNs
Gamma motor neuron vs. alpha motor neuron
Types of LMNs. GMN are medium sized and myelinated, go to the intrafusal muscle fibers while AMNs are large and myelinated, go to the extrafusal muscle fibers.
Direct vs indirect somatic motor pathways
Direct refers to cortex to spinal cord to muscles (some direct fibers send collaterals that modulate indirect pathways)
Indirect refers to pathways that synapse in other parts of the brain before getting to the muscles
Corticospinal tract is what type of somatic motor pathway?
Direct
Medial vs lateral fibers of the CST
Medial: innervate the postural muscles
Lateral: innervate the limb muscles, and fractionation (manipulating fingers for precise movements)
Primary motor cortex
Area 4 of the precentral gyrus. Right side controls the contralateral side and conversely.
Neurons with cell bodies here are called the Upper motor neurons
Motor innervation on the primary motor cortex (motor “homunculus”
Similar to sensory, muscles are unequally innervated (some are more innervated than others). More motor units = more innervation = more cortical tissue
(distribution also similar to sensory - leg, hand, face)
What is the lateral corticospinal tract?
When CST axons arise on the lateral aspect of the spinal cord. The continue descending down the spinal cord to synapse on their respective LMNs on the ventral horn of the spinal cord
What is the medial corticospinal tract? How is it different from the lateral CST?
Controls postural and proximal movements (muscles of the neck, shoulder and trunk)
Fibers do not cross at the medulla
Not very clinically significant
What is the corticobulbar/corticonuclear tract?
Originates from the ventral area of the primary motor cortex down the brainstem and innervates the nuclei of the cranial nerves (thereby innervating the muscles they innervate).
Which cranial nerves are controlled by the corticobulbar tract?
V, VII, IX, X, XI, XII
Axons cross and control the contralateral muscles
Differences between the corticobulbar and corticospinal tract?
Enters via the genu of the internal capsule. Passes through the cerebral peduncle, anterior pons and pyramids and terminate at its specific motor nucleus
At pons level, describe the distribution of the CN V terminal fibers
Terminate bilaterally = equal contribution from each hemisphere
At the medulla level, describe the distribution of the CN IX, X and XII terminal fibers
Mostly bilateral input but palates receive contralateral input (uvula deviates away from lesion)
Tongue muscles get contralateral input from XII (deviates towards side of lesion)
At spinal cord level, describe the distribution of the CN XI terminal fibers
Ipsilateral input
LMN anatomical organization
Found on the anterior horn.
Medial LMNs go to axial muscles while Lateral LMNs go to limb muscles
Extensor axons tend to be ventral while flexor axons tend to be dorsal
What do indirect pathways innervate?
Antigravity and axial LMNs (for tone when sitting or standing/staying upright)
What are the indirect medial UMN tracts?
Tectospinal, medial reticulospinal, lateral and medial vestibulospinal. Medial LMNs receive info from these. (axial/postural muscles). These are generally located in the anterior funiculus
Ipsilateral
What are the indirect lateral UMN tracts?
Rubrospinal and lateral reticulospinal tracts. Lateral LMN receive info from these tracts, along with lateral corticospinal tract (limbs)
Lateral vestibulospinal tract:
Indirect medial UMN tract from lateral vestibular nuclei to spinal cord.
Synapse to LMNs that innervate the postural muscles and limb extensors (keeping you upright, gravity info)
Medial vestibulospinal tract
Indirect medial UMN tract from medial vestibular nuclei to spinal cord.
Synapse to LMNs that innervate neck and shoulder muscles (coordinates head movements)
Medial (pontine) reticulospinal tract:
Indirect medial UMN tract from pontine reticular formation to spinal cord.
Synapse to LMNs innervating postural muscles and limb extensors (postural reflexes)
Lateral (medullary) reticulospinal tract:
Indirect lateral UMN tract from medullary reticular formation to spinal cord.
Stimulates flexor motor neurons and inhibits extensor motor neurons (inhibits spinal segmental reflexes)
Rubrospinal tract:
Indirect lateral UMN tract from the red nucleus to the spinal cord
Synapse to LMNs innervating upper limb flexors
Tectospinal tract:
Indirect medial UMN tract from superior colliculus to upper spinal cord
Synapse to LMNs that innervate neck muscles (coordination of head with eye movements)
Lower motor neuron lesions usually presents with ….
Flaccid paralysis, wasting, hyporeflexia, hypotonia and fasciculations (muscle twitches)
Upper motor neuron signs
Loss of the direct corticospinal tract and loss of regulation from the indirect brainstem motor control pathways
Symptoms of upper motor neuron signs:
CST problems:
Indirect pathways modulation problems:
Loss of distal extremity strength Babinski sign (inverted plantar reflex
Hypertonia (spastic or rigid)
Hyperreflexia (clonus)
Clasp knife phenomenon and spasticity
Pronator drift)
Clues in LMN lesions
Clinical signs ipsilateral to lesion
Decorticate posture:
bent arms, clenched fists, and legs held out straight, feet extended and inverted
Lesion above the level of the red nucleus
Decerebrate posture:
Upper extremity in pronation and extension and lower extremity in extension
Lesion below the level of the red nucleus
Complete transection of spinal cord
All sensation 1 or 2 levels below lesion lost due to overlap of dermatomes Incontinence Spinal shock (loss of tendon reflexes) UMN signs at levels below the lesion LMN signs at the level of lesion
Hemisection of spinal cord (Brown Sequard)
Pain and temp loss from contralateral side of body and loss of discriminative touch and proprioception on the ipsilateral side
LMN signs at level of lesion (flaccid paralysis)
UMN signs on ipsilateral side of lesion
Syringomyelia
Cysts in spinal cord. Affects pain and temp sensation first due to impingement of the anterior white commissure. Cape pattern sensation loss.
Motor also lost (LMN signs if ventral horn is affected or UMN signs if lateral corticospinal tract is affected)
Anterior cord syndrome
Damage to anterior spinal cord (usually due to spinal cord infarction, herniation or myelopathy
Central cord syndrome
Central spinal cord damage usually due to cervical hyperextension
Central Seven Palsy
Corticobulbar tract lesion involving CN VII. Upper face controlled equally by both hemispheres. Lower face is controlled by contralateral hemisphere.
Lesions rostral to CN VII nucleus results in muscle drooping on corner of mouth (contralateral side of lesion)
Bell’s palsy
Ipsilateral flaccid paralysis of upper and lower face
Spastic cerebral palsy
Spastic movement dysfunction.
Paresis, abnormal reflexes, abnormal postural coordination
Amytrophic lateral sclerosis (ALS)
UMNs and LMNs destruction. UMN and LMN signs
CN involvement results in difficulty breathing, swallowing or speaking
Polyneuropathy
Progress distal to proximal due to impaired axonal transport or demyelization. Affects sensory, motor and autonomic functions
