Descending Pathways

Upper motor neuron (UMN) vs lower motor neuron (LMN)

UMN axons descend from the cerebral cortex; LMNs innervate striated skeletal muscle with cell bodies in the brainstem or spinal cord

What type of neuron are motor neurons based on structure?

Multipolar neurons

What is the functional modality of descending motor tracts?

Somatic efferent

What is an upper motor neuron (UMN)?

A motor neuron with its cell body in the cerebral cortex whose axon descends to influence lower motor neurons

What is a lower motor neuron (LMN)?

The last neuron in the motor pathway that directly innervates skeletal muscle with its cell body in the ventral horn of the spinal cord or brainstem cranial nerve nuclei

What are the clinical signs of an UMN lesion?

Spastic paralysis; hypertonia; hyperreflexia; mild disuse atrophy; clonus; pathological reflexes including Babinski sign

What is the Babinski sign?

Dorsiflexion of the big toe with fanning of the other toes when the side of the heel is stroked

What are the clinical signs of an LMN lesion?

Flaccid paralysis; atonia; areflexia; fasciculations and fibrillations; severe atrophy

Where are LMN cell bodies located?

Ventral horn of the spinal cord or brainstem cranial nerve nuclei

What is a motor unit?

"One motor neuron and all the myofibers it innervates"

How does motor unit size relate to muscle control?

Smaller motor units allow finer control; larger motor units produce stronger contractions

How many myofibers per motor unit are in extraocular muscles?

Approximately 10 myofibers per motor unit

How many myofibers per motor unit are in large antigravity muscles like gastrocnemius?

Hundreds to up to 1000 myofibers per motor unit

How are motor units recruited?

In order of size

Do motor units contain mixed fiber types?

No; motor units consist of one fiber type only (Type I or Type II)

How is motor control organized hierarchically?

The cortex decides what movement should occur and directs lower motor neurons

How is motor control organized in parallel?

Premotor cortex can directly communicate with lower motor neurons

What is the role of basal ganglia and cerebellum in motor control?

They influence cerebral cortical output but have very few outputs to the spinal cord

Do basal ganglia and cerebellum directly affect LMNs?

No direct effect on LMNs

What motor deficits result from basal ganglia or cerebellar damage?

Involuntary movements; uncoordinated movements; difficulty initiating movement without weakness

What are the two main descending motor pathways?

Corticospinal tract and corticobulbar tract

What is another name for the corticospinal tract?

Pyramidal tract

What is another name for the corticobulbar tract?

Corticonuclear tract

What other descending motor pathways are discussed?

Rubrospinal; reticulospinal; vestibulospinal; tectospinal tracts

What is the function of the corticospinal tract?

Control of voluntary movement

What is the function of the corticobulbar tract?

Control of motor nuclei in the brainstem

What are the cortical origins of the corticospinal tract?

Primary motor cortex; premotor cortex; supplementary motor cortex; somatosensory cortex; superior parietal lobule

What percentage of corticospinal fibers arise from the primary motor cortex?

Approximately 40%

What percentage of corticospinal fibers arise from the premotor cortex?

Approximately 20%

What percentage of corticospinal fibers arise from the supplementary motor cortex?

Approximately 10%

What percentage of corticospinal fibers arise from the somatosensory cortex?

Approximately 25%

What percentage of corticospinal fibers arise from the superior parietal lobule?

Approximately 5%

What is the function of the primary motor cortex?

Execution of contralateral voluntary movements and fine digital control

Where is the primary motor cortex located?

Precentral gyrus

What is the effect of a lesion in the primary motor cortex?

Paralysis of contralateral muscles

What is the homunculus?

Somatotopic organization of body representation in the motor cortex

What is the function of the premotor cortex?

Plans movements in response to external cues and controls proximal and axial musculature

Where is the premotor cortex located?

Lateral aspect of the cerebrum anterior to the precentral gyrus

What is the function of the supplementary motor cortex?

Plans internally paced movements and assembles learned and new motor sequences

Where is the supplementary motor cortex located?

Medial aspect of the cerebrum anterior to the precentral gyrus

What parietal lobe areas project to motor cortex?

Somatosensory cortex (areas 3

What is a key characteristic of the corticospinal tract?

It is a complex tract with multiple origins and destinations

Do all voluntary movements depend on the corticospinal tract?

No

What movement deficit remains permanently after corticospinal tract section in monkeys?

Loss of fine finger movements

Trace the course of the corticospinal tract from cortex to muscle

"Cortex → internal capsule → cerebral peduncle → pons → medullary pyramids → spinal cord → synapse on LMN in ventral horn → skeletal muscle"

Where does the lateral corticospinal tract decussate?

At the pyramidal decussation in the medulla

What does the lateral corticospinal tract primarily innervate?

Distal limb musculature

What does the anterior corticospinal tract primarily innervate?

Axial and proximal limb musculature

Where does the anterior corticospinal tract decussate?

At the level of the spinal cord via the anterior commissure

What percentage of corticospinal fibers form the lateral corticospinal tract?

Approximately 80%

What percentage of corticospinal fibers form the anterior corticospinal tract?

Approximately 10%

What percentage of corticospinal fibers remain uncrossed as lateral CST?

Approximately 10%

Where is the corticospinal tract located in the spinal cord?

Anterior and lateral funiculi

What are anterior horn motor neurons called?

Alpha motor neurons

How are motor neurons organized somatotopically in the anterior horn?

Proximal muscles are medial; distal muscles are lateral; flexors are posterior to extensors

What is the function of the corticobulbar pathway?

Controls muscles innervated by cranial nerve motor nuclei

Where does the corticobulbar pathway originate?

Face and mouth portion of the motor cortex and nearby areas

Do corticobulbar fibers decussate as a single tract?

No; they split off at the level of the target nucleus

Which cranial nerve motor nuclei receive bilateral corticobulbar input?

V; VII; nucleus ambiguus; XI; XII

Which cranial nerve nuclei receive no direct corticobulbar input?

III; IV; VI

What is unique about facial motor nucleus innervation?

Upper facial muscles receive bilateral input; lower facial muscles receive mainly contralateral input

What is the effect of unilateral corticobulbar damage on facial movement?

Inability to smile symmetrically with preserved forehead wrinkling

What is the function of the reticulospinal tract?

Supports rhythmic motor actions such as walking

Where does the reticulospinal tract originate?

Reticular formation in the pons and medulla

How does the medial reticulospinal tract descend?

Ipsilaterally near the MLF in the anterior funiculus

How does the lateral reticulospinal tract descend?

Bilaterally in the lateral funiculus

What is the function of the vestibulospinal tracts?

Mediates postural adjustments and head movements

Where do vestibulospinal tracts originate?

Vestibular nuclei in the pons

What is the function of the lateral vestibulospinal tract?

Facilitates antigravity muscles

How does the lateral vestibulospinal tract descend?

Ipsilaterally via the lateral funiculus

What is the function of the medial vestibulospinal tract?

Controls head movements in response to gravity

How does the medial vestibulospinal tract descend?

Bilaterally via the anterior funiculus to cervical levels