Lecture #30: Introduction to Supraspinal Motor Systems Part I: Cerebral Cortex and Brainstem

What are supraspinal motor systems?

Supraspinal motor systems are motor control pathways originating above the spinal cord, primarily in the cerebral cortex and brainstem, that regulate voluntary movement, posture, and muscle tone by influencing lower motor neurons indirectly.

What is the key difference between upper and lower motor neurons?

Upper motor neurons originate in the cerebral cortex or brainstem and project to lower motor neurons, while lower motor neurons originate in the spinal cord or cranial nerve nuclei and directly innervate skeletal muscle.

Where are lower motor neuron cell bodies located?

Lower motor neuron cell bodies are located in the ventral horn of the spinal cord or in cranial nerve motor nuclei in the brainstem.

What defines the pyramidal motor system?

The pyramidal system consists of cortical motor projections to the spinal cord and brainstem, primarily through the corticospinal and corticobulbar tracts, responsible for voluntary movement.

Which cortical areas contribute to the pyramidal system?

The primary motor cortex, premotor cortex, and supplemental motor area all contribute fibers to the pyramidal system, with the primary motor cortex contributing the largest proportion.

What is the functional role of the primary motor cortex?

The primary motor cortex is directly involved in executing voluntary movements and controlling small groups of muscles with precise force and timing.

What roles do the premotor cortex and supplemental motor area play?

The premotor cortex is involved in motor planning and preparation based on external cues, while the supplemental motor area is important for initiating movements and coordinating action sequences.

How does the corticospinal tract descend through the brain?

Corticospinal fibers pass through the internal capsule, crus cerebri, ventral pons, medullary pyramids, then decussate in the lower medulla before descending in the spinal cord.

What percentage of corticospinal fibers decussate in the medulla?

Approximately 80–85% of corticospinal fibers decussate in the lower medulla to form the lateral corticospinal tract.

What is the function of the ventral corticospinal tract?

The ventral corticospinal tract contains uncrossed fibers that descend ipsilaterally and primarily control axial and proximal muscles, with many fibers crossing at the spinal level.

How does cortical control relate to body laterality?

Because most corticospinal fibers decussate, each cerebral hemisphere primarily controls voluntary movement on the contralateral side of the body.

What are typical clinical signs of an upper motor neuron lesion?

Upper motor neuron lesions cause weakness, spasticity, hyperreflexia, abnormal synergies, and the Babinski sign due to loss of inhibitory cortical control.

Why does spasticity develop after an upper motor neuron lesion?

Spasticity develops due to the release of brainstem and spinal reflex circuits that are normally regulated by descending cortical inhibition.

What is spinal shock?

Spinal shock is the initial phase following acute spinal cord injury characterized by flaccid paralysis and absent reflexes before spasticity develops.

What characterizes a lower motor neuron lesion?

Lower motor neuron lesions cause flaccid paralysis, muscle atrophy, fasciculations, reduced reflexes, and decreased muscle tone.

What is the corticobulbar tract?

The corticobulbar tract consists of cortical motor projections to cranial nerve motor nuclei that control muscles of the face, jaw, tongue, pharynx, and larynx.

How is facial muscle innervation unique in the corticobulbar system?

Upper facial muscles receive bilateral cortical input, while lower facial muscles receive primarily contralateral cortical input, making lower face weakness more prominent in unilateral cortical lesions.

What facial deficit results from a corticobulbar tract lesion?

A corticobulbar lesion causes weakness of the contralateral lower face while sparing the forehead and eye closure.

How does a hypoglossal corticobulbar lesion affect tongue movement?

A unilateral corticobulbar lesion causes the tongue to deviate away from the side of the cortical lesion due to contralateral genioglossus weakness.

What distinguishes Bell’s palsy from a corticobulbar lesion?

Bell’s palsy is a lower motor neuron lesion of the facial nerve causing complete ipsilateral facial paralysis, including the forehead.

What is the functional role of Broca’s area?

Broca’s area is involved in motor planning for speech production; damage results in expressive aphasia with preserved comprehension but impaired speech output.

What are the frontal eye fields responsible for?

The frontal eye fields control voluntary eye movements toward the contralateral visual field.

What brainstem nuclei contribute to axial and proximal limb control?

The red nucleus, reticular formation, and vestibular nuclei are key brainstem centers involved in controlling posture and proximal musculature.

What is the rubrospinal tract?

The rubrospinal tract originates from the red nucleus, decussates, and primarily influences upper limb flexor muscles, though it is relatively small in humans.

What is the function of the reticulospinal tract?

The reticulospinal tract regulates posture, muscle tone, locomotion, and reflexes, often acting bilaterally to coordinate axial and proximal muscles.

What role does the vestibulospinal tract play?

The vestibulospinal tract maintains balance and posture, with strong activation of extensor muscles to support antigravity functions.

What is decorticate posture?

Decorticate posture is characterized by flexed upper limbs and extended lower limbs, indicating damage above the red nucleus.

What is decerebrate posture?

Decerebrate posture involves extension of both upper and lower limbs, reflecting more severe damage below the red nucleus.

What causes locked-in syndrome?

Locked-in syndrome results from pontine lesions interrupting corticospinal and corticobulbar tracts, leaving the patient conscious but unable to move except for limited eye movements.

How do extrapyramidal systems differ from pyramidal systems?

Extrapyramidal systems, including the basal ganglia and cerebellum, modulate motor activity by influencing upper motor neurons rather than directly innervating lower motor neurons.

Why are basal ganglia and cerebellar disorders called extrapyramidal?

They are termed extrapyramidal because they do not project directly through the pyramidal tracts but instead regulate motor output indirectly.

What is the overall function of supraspinal motor control?

Supraspinal motor control integrates voluntary movement, posture, reflexes, and coordination through complex hierarchical and parallel pathways connecting cortex, brainstem, and spinal cord.