Neuro Week 9: The Motor System

What is the motor system described as?

A hierarchical system that becomes less sophisticated as it descends and more sophisticated as it ascends.

What does the premotor cortex (PMC) respond to?

External cues such as visual, auditory, and tactile information.

Which areas provide input to the premotor cortex?

The prefrontal and parietal cortices.

What is the role of the premotor cortex?

It selects appropriate motor programs in response to sensory cues or learned associations.

What cues does the supplementary motor area (SMA) respond to?

Internal cues, such as memory.

What is the role of the supplementary motor area?

It plans and organizes complex movement sequences and stores and retrieves learned movement sequences (motor memory).

What is a tract?

A group of nerve fibers sharing a common origin, destination, and function.

Which two neurons are needed for voluntary movement?

An upper motor neuron and a lower motor neuron.

Where are upper motor neuron cell bodies located?

In higher motor centers in the brain and brainstem.

What do upper motor neuron axons form?

The descending motor pathways.

Where are lower motor neuron cell bodies located?

In the spinal ventral horns or the corresponding cranial motor nuclei.

What do lower motor neuron axons do?

Travel through peripheral somatic nerves to innervate skeletal muscles.

What is the traditional basis for classifying descending motor pathways as pyramidal and extrapyramidal?

Whether the pathway passes through the medullary pyramids.

Why is the corticospinal tract called a pyramidal tract?

It originates in the cerebral cortex and passes through the medullary pyramids on its way to the spinal cord.

What is the pyramidal system's main role?

It is the major voluntary pathway for movement.

What are the three pathways listed in the pyramidal system?

Corticospinal, corticonuclear (corticobulbar), and corticopontine tracts.

What is the lateral corticospinal tract made of?

Corticospinal fibers that cross in the medulla.

What does the lateral corticospinal tract supply?

All levels of the spinal cord.

What is the anterior corticospinal tract made of?

Uncrossed corticospinal fibers that cross near the level where they synapse with lower motor neurons.

Which areas does the anterior corticospinal tract supply according to the slides?

The neck and upper limbs.

Where do corticospinal fibers descend after leaving the cerebral cortex?

Through the posterior limb of the internal capsule.

Through which midbrain structure do corticospinal fibers pass?

The middle portion of the cerebral peduncles.

What happens to corticospinal fibers in the pons?

They are separated by transverse pontine fibers.

Where do corticospinal fibers form the pyramids?

In the upper medulla oblongata.

What is the medullary decussation?

The crossing of most corticospinal fibers in the lower medulla to the opposite side.

Approximately what percentage of corticospinal fibers cross in the medulla?

About 80–90% according to the course slide.

What happens to the corticospinal fibers that do not cross in the medulla?

About 10% descend on the same side as the anterior corticospinal tract and cross near their spinal level of termination.

What are two functions of the corticospinal tract?

Adding speed and agility to conscious movement and providing fine motor control, especially of the fingers.

What is the corticobulbar tract's primary target region?

The head.

Where do most corticobulbar fibers terminate?

In the reticular formation near cranial nerve nuclei.

What do association neurons do in the corticobulbar pathway?

They leave the reticular formation and synapse in cranial nerve nuclei with lower motor neurons.

Which cranial nerve nuclei receive corticobulbar input in the midbrain?

Cranial nerves III and IV.

Which cranial nerve nuclei receive corticobulbar input in the pons?

Cranial nerves V, VI, and VII.

Which cranial nerve nuclei receive corticobulbar input in the medulla?

Cranial nerves IX, X, XI, and XII.

Do corticobulbar fibers generally innervate cranial motor nuclei on one side or both sides?

Mostly both sides of the brainstem.

Which corticobulbar targets are noted as exceptions that receive only contralateral cortical innervation?

The lower part of the facial nerve nucleus and the hypoglossal nerve nucleus.

What are extrapyramidal tracts?

Descending motor tracts other than the corticospinal tracts, as defined in the slides.

What are the main components of the extrapyramidal system?

The basal ganglia and brainstem.

Which four brainstem tracts are listed as part of the extrapyramidal system?

Rubrospinal, vestibulospinal, reticulospinal, and tectospinal tracts.

Where does the rubrospinal tract originate?

In the red nucleus of the midbrain.

What inputs does the red nucleus receive?

Input from the cerebellum and cerebral cortical motor areas.

Where does the rubrospinal tract send output?

To contralateral spinal motor neurons.

What movements and functions are associated with the rubrospinal tract?

Distal limb movements, regulation of tone and posture; it excites flexors and inhibits extensors.

Where are the vestibular nuclei located?

In the pons and medulla.

What inputs do the vestibular nuclei receive?

Input from the vestibular section of the inner ear and the cerebellum.

Does the vestibulospinal tract remain ipsilateral or cross to the opposite side?

It remains ipsilateral.

What is the function of the vestibulospinal tract?

It helps maintain balance in response to head movement by regulating muscle tone; it excites ipsilateral extensors and inhibits flexors.

Where does the reticulospinal tract originate?

In the reticular formation in the central gray matter of the brainstem.

What inputs are listed for the reticular formation?

Sensory input from the spinal cord, vestibular nuclei, cerebellum, sensorimotor cortex, globus pallidus, and hypothalamus.

Where does the pontine reticulospinal tract run?

Ipsilaterally from nuclei in the pons.

What is the stated function of the pontine reticulospinal tract?

It excites axial extensor muscles.

Where does the medullary reticulospinal tract run?

Ipsilaterally, with some fibers crossing, from nuclei in the medulla.

What movements does the reticulospinal tract help mediate?

Larger trunk and limb movements that do not require balance or fine upper-limb movements.

Where does the tectospinal tract originate?

In the superior colliculus of the midbrain.

What kind of input is associated with the tectospinal tract?

Visual stimuli.

What is the function of the tectospinal tract?

It helps control neck muscles and moves the head and eyes in response to visual stimuli.

What does the extrapyramidal or multineuronal system regulate?

Posture and balance, coordination and control of voluntary movement, and muscle tone.

How does the pyramidal system differ from the extrapyramidal system in motor control?

The pyramidal system provides more direct control of spinal motor neurons through the primary motor cortex; the extrapyramidal system uses multiple synapses and brain regions.

How can pyramidal and extrapyramidal systems work together?

Fine work such as needlework uses the pyramidal system, while the extrapyramidal system helps maintain the posture needed to perform the task.

Where are cell bodies contributing to the pyramidal tracts located, according to the slides?

In the precentral gyrus (primary, premotor, and supplementary motor cortex) and somatosensory area.

Where do extrapyramidal tracts originate?

In midbrain and brainstem nuclei influenced by the cerebral cortex, basal ganglia, and cerebellum.

What is the lateral corticospinal tract especially important for?

Fine finger movements, such as writing and needlework.

What is the ventral/anterior corticospinal tract associated with?

Axial or postural movement.

What is the final common pathway in the speech motor system?

Lower motor neurons, including cranial and spinal nerves.

What is the direct activation pathway in the speech motor system?

Upper motor neurons in the pyramidal system, including corticobulbar and corticospinal tracts.

What is the indirect activation pathway in the speech motor system?

Upper motor neurons in the extrapyramidal system, including rubrospinal and reticulospinal tracts.

What control circuits are listed in the speech motor system?

The basal ganglia and cerebellum.

What does the alpha motor neuron innervate?

Skeletal muscle.

What inputs does the alpha motor neuron receive?

Input from higher centers and sensory input from muscle stretch and tension receptors.

Which structures commonly have dysfunction in movement disorders and lesions, according to the slides?

The basal ganglia and cerebellum.

How do the basal ganglia and cerebellum influence movement areas of the cerebral cortex?

Through the thalamus.

What are the two basal ganglia pathways named in the slides?

The direct pathway and the indirect pathway.

What is the overall effect of the basal ganglia direct pathway on motor activity?

It increases motor activity.

What is the overall effect of the basal ganglia indirect pathway on motor activity?

It decreases motor activity.

What structures are listed for the direct basal ganglia pathway?

Caudate, putamen, globus pallidus, and substantia nigra.

What structures are listed for the indirect basal ganglia pathway?

Caudate, putamen, globus pallidus, subthalamic nucleus, and substantia nigra.

What is the sequence described for the direct pathway?

The striatum projects to the internal globus pallidus and substantia nigra, which then project to the thalamus.

How does the direct pathway promote movement?

The thalamus is released from tonic inhibition, increasing excitatory thalamic drive to the supplementary motor area and turning up motor activity.

What is the key pathology described for Parkinson's disease?

Severe loss of dopamine-producing neurons in the substantia nigra.

How much dopamine-producing cell loss may have occurred by the time Parkinson's symptoms appear?

About 60–80% or more of substantia nigra dopamine-producing cells.

What happens to the thalamus in the Parkinson's disease circuitry described in the slides?

It receives more inhibition and therefore provides less excitation to the motor cortex, resulting in decreased movement.

What are the four cardinal symptoms of Parkinson's disease listed in the slides?

Resting tremor, bradykinesia, rigidity, and postural instability.

What is bradykinesia?

Slowness of movement.

What is the primary neuronal loss described in Huntington's disease?

Neuronal death in the striatum, especially the caudate and putamen.

Which neurotransmitters are reduced in Huntington's disease according to the slides?

GABA, acetylcholine (ACh), and metenkephalin.

How does the slide describe abnormal basal ganglia circuitry in Huntington's disease?

Relatively higher dopamine and norepinephrine lead to excessive excitation of thalamocortical pathways and choreiform movements.

What happens to the thalamus in the Huntington's disease circuitry described in the slides?

It is not inhibited enough, so it sends more excitation to the cortex and movement increases.

What are the primary characteristics of Huntington's chorea listed in the slides?

Choreic movements, personality and psychiatric disturbances, and dementia.

What other signs and symptoms are listed for Huntington's chorea?

Dysarthria, dysphagia, muscle wasting, sleep disorders, depression, and apraxia.

How do the direct and indirect basal ganglia pathways normally work together?

They counterbalance each other to maintain normal movement function.

What are common signs of an upper motor neuron (UMN) lesion?

Increased tone (spasticity), increased reflexes, clonus, and a positive Babinski sign.

What is clonus?

Repetitive muscle contraction and relaxation in an oscillating pattern.

What is a positive Babinski sign in an adult?

Stimulation along the outer border of the sole causes the big toe to extend upward and the other toes to fan.

What does a positive Babinski sign indicate according to the slides?

An upper motor neuron lesion and damage to the lateral corticospinal tract.

At what age can the Babinski reflex be normal according to the slides?

Below one year of age.

What are common signs of a lower motor neuron (LMN) lesion?

Decreased tone (hypotonia or flaccidity), decreased muscle power, decreased reflexes, and muscle wasting.

How does muscle wasting typically differ between UMN and LMN lesions?

UMN lesions usually cause slight wasting from disuse, while LMN lesions cause pronounced wasting.

How do tendon reflexes differ between UMN and LMN lesions?

They are increased in UMN lesions and diminished or absent in LMN lesions.

What does hemiplegia mean?

Complete paralysis of one half of the body.

What does hemiparesis mean?

Partial weakness of one half of the body.