4. Motor Control & Reflexes

Comprehensive flashcards covering motor control pathways, cortical areas, brainstem nuclei, cerebellum, basal ganglia, motor units, muscle receptors, spinal cord reflexes, and autonomic pupillary reflexes.

What systems are required for smooth voluntary movement?

Voluntary movement requires complex interaction of the corticospinal (pyramidal) tracts, basal ganglia, and cerebellum to ensure smooth, purposeful movement without extraneous muscular contractions.

What is the skeletal motor axis?

The projection from the motor cortex to the motor neuron.

What structures provide input to the skeletal motor axis?

The thalamus, brainstem, and cerebellum.

What are the three subdivisions of the motor cortex?

Primary motor cortex, premotor area, and supplemental motor area.

What is the primary motor cortex?

A motor cortex region with unequal topographic representation where stimulation elicits fine motor movement.

What does unequal topographic representation in the primary motor cortex mean?

Different body regions occupy different amounts of cortical area depending on the degree of fine motor control required.

What happens when the primary motor cortex is stimulated?

Fine motor movement is elicited.

What is the premotor area?

A motor cortex region involved in movement patterns required to perform specific tasks.

What happens when the premotor area is stimulated?

Movement of groups of muscles organized into patterns for specific tasks occurs.

How does the premotor area function?

It works in concert with other motor areas.

What is the supplemental motor area?

A motor cortex region that works with the premotor area to provide attitudinal, fixation, and positional movements of the body.

What role does the supplemental motor area play in fine motor control?

It provides the background support needed for fine motor control of the arms and hands by the premotor and primary motor cortices.

What are the two major pathways of cortical motor signal transmission?

The direct (corticospinal/pyramidal) pathway and the indirect (extrapyramidal) pathway.

What is the corticospinal tract?

The direct pyramidal pathway that directs discrete, detailed movements and fine-skilled motor behavior.

What structures are involved in the extrapyramidal pathway?

The basal ganglia, cerebellum, and brainstem nuclei.

What functions are associated with the extrapyramidal pathway?

Control of trunk musculature, upright posture, locomotion, and orienting responses.

What are Betz cells?

Giant pyramidal cells located in the primary motor cortex that give rise to large, rapidly conducting corticospinal fibers.

Where do corticospinal tract fibers cross?

In the pyramids of the medulla.

Where do corticospinal tract fibers terminate?

They descend the spinal cord and synapse with interneurons or directly with motor neurons.

What is the primary function of extrapyramidal tracts?

Modulation of corticospinal signals reaching motor neurons.

What sensory information do extrapyramidal tracts integrate?

Sensory and posture-related information.

Why are extrapyramidal tracts important?

They are important for trunk musculature, upright posture, locomotion, and smooth muscle movements.

What are extrapyramidal symptoms?

Symptoms resulting from dysfunction of extrapyramidal motor systems, leading to impaired movement control.

Which muscles support the body against gravity?

The muscles of the spinal column and the extensor muscles of the legs.

Which brain structures influence antigravity muscles?

Brainstem nuclei.

What is the function of the pontine reticular nuclei?

They excite antigravity muscles.

What is the function of the medullary reticular nuclei?

They inhibit antigravity muscles.

What is the cerebellum?

The "little brain" responsible for coordinating and sequencing muscle activity.

What afferent inputs does the cerebellum receive from the cortex?

Information about intended movements from motor and cortical areas.

What afferent inputs does the cerebellum receive from muscle receptors?

Proprioceptive information from muscle spindles and Golgi tendon organs regarding contraction, tension, and limb position.

What efferent functions does the cerebellum have?

Equilibrium control and coordination of opposing influences on motor neurons.

How does the cerebellum regulate movement?

It compares intended movements with actual movements and makes corrective changes.

What are the basal ganglia?

Four nuclei involved in controlling complex patterns of motor activity.

What cortical regions provide input to the basal ganglia?

Primary motor, premotor, supplemental motor, and prefrontal cortex.

What type of output do the basal ganglia provide to the cortex?

Inhibitory output.

Why do the basal ganglia inhibit the cortex?

To suppress conflicting motor activity.

What role do the basal ganglia play in posture?

They maintain postural stability.

What postural changes occur with basal ganglia damage?

The head falls over the chin and the person walks bent forward at the waist.

What role do the basal ganglia play in motor programs?

They convert overall goals into specific movement programs.

How is Parkinson disease related to basal ganglia function?

Parkinson disease causes inability to initiate movements.

What symptoms may occur in a patient with extrapyramidal dysfunction?

Loss of movement smoothness, tremor, difficulty initiating movement, and difficulty maintaining upright posture.

What neurological disease commonly produces extrapyramidal symptoms?

Parkinson disease.

What is the function of the cortical level in motor control?

It issues commands, controls intensity, and modifies timing of movement patterns.

What is the function of the brainstem level in motor control?

Maintaining equilibrium by adjusting axial tone.

What is the role of the cerebellum in integrated motor control?

Adjusting spinal motor activity, equilibrium, and planning of motor activity.

What is the role of the basal ganglia in integrated motor control?

Assisting the cortex in executing learned subconscious movement patterns and planning sequential movements.

What is the role of the spinal cord in integrated motor control?

Preprogramming movement patterns such as withdrawal reflexes and walking movements.

Why is the spinal cord more than a conduit for nerve fibers?

It contains neuronal circuits for walking and various reflexes.

What is the role of higher brain centers in spinal cord circuits?

They activate and command circuits responsible for walking and equilibrium.

What inputs do motor neurons receive?

Excitatory and inhibitory inputs from afferent neurons, spinal interneurons, and descending cortical pathways.

What is always true about motor neuron output to skeletal muscle?

It is always excitatory.

Where are motor neurons located in the spinal cord?

In the anterior portion of the spinal cord.

How do motor neurons compare in size to other neurons?

They are approximately $50\text{--}100\%$ larger.

What are interneurons?

Small highly excitable neurons that make up reflex circuitry and receive most descending brain signals.

How abundant are interneurons compared with anterior motor neurons?

About $30$ times more abundant.

What are propriospinal fibers?

Fibers that travel up and down the spinal cord for $1\text{--}2$ segments.

What is the function of propriospinal fibers?

They provide pathways for multisegmental reflexes and contribute to proprioception.

What is proprioception?

Sensing the relative position of body parts and the strength of effort used in movement.

What are alpha motor neurons?

Large Aα fibers approximately $14\,\mu\text{m}$ in diameter that innervate extrafusal muscle fibers.

What is a motor unit?

A single alpha motor neuron and all the muscle fibers it innervates.

How many muscle fibers can one alpha motor neuron excite?

About $3$ to more than $100$ extrafusal muscle fibers.

What are gamma motor neurons?

Smaller Aγ fibers approximately $5\,\mu\text{m}$ in diameter that innervate intrafusal muscle fibers in muscle spindles.

What is the function of gamma motor neurons?

Maintenance of muscle tone through stimulation of intrafusal fibers.

What occurs during normal muscle contraction involving alpha and gamma motor neurons?

They are co-activated.

What are extrafusal muscle fibers?

The main force-generating muscle fibers that make up most of skeletal muscle.

Which neurons innervate extrafusal fibers?

Alpha motor neurons.

What are intrafusal muscle fibers?

Small specialized fibers within muscle spindles that function as sensory receptors.

How do intrafusal fibers compare with extrafusal fibers?

They are smaller, encapsulated, and much shorter, about $10\,\text{mm}$ in length.

What is a muscle spindle?

A sensory receptor in the muscle belly that senses muscle length and rate of change in length.

Where is a muscle spindle located?

In the muscle belly.

What sensory information does a muscle spindle detect?

Muscle length and the rate of change in muscle length.

What is a Golgi tendon organ?

A sensory receptor in tendons that senses tension and rate of change in tension.

Where is a Golgi tendon organ located?

In the tendon.

What sensory information does a Golgi tendon organ detect?

Tendon tension and rate of change in tension.

Do signals from muscle sensory receptors reach the brain?

Yes, they are transmitted to the cerebrum and cerebellum as well as the spinal cord.

What is the primary role of muscle spindle and Golgi tendon organ signals?

Intrinsic subconscious control of muscle function.

How many intrafusal fibers are found in a muscle spindle?

Three to twelve intrafusal fibers.

Do intrafusal fibers contribute to muscle tension?

No, they function only as sensory receptors.

What neurons innervate intrafusal fibers?

Gamma motor neurons.

What are bag fibers?

Intrafusal fibers that detect rapid, dynamic changes in muscle length.

What are chain fibers?

Intrafusal fibers that detect static changes in muscle length.

What is the stretch reflex (myotatic reflex)?

A reflex in which sudden muscle stretch causes contraction of the stretched muscle and inhibition of the opposing muscle.

What stimulates the stretch reflex?

Sudden muscle stretch.

How many synapses are in the stretch reflex arc?

One synapse; it is monosynaptic.

What is the response of the stretch reflex?

Contraction of the extensor muscle and opposition to further stretching.

What is the function of Golgi tendon organs?

Equalization of force among muscle fibers.

What type of receptor is a Golgi tendon organ?

A low-threshold mechanoreceptor.

How does increased muscle tension activate Golgi tendon organs?

Compression of nerve endings opens stretch-sensitive ion channels.

What is the autogenic inhibition reflex?

Sudden relaxation of a muscle at very high tensions to prevent muscle tearing.

Do Golgi tendon organs function only at high tensions?

No, they signal muscle force throughout the physiological range.

What is the inverse stretch reflex (Golgi tendon reflex)?

A protective reflex that relaxes a muscle when excessive tension develops.

What stimulus activates the Golgi tendon reflex?

Excessive muscle contraction and tension.

How many synapses are in the Golgi tendon reflex?

Two synapses; it is disynaptic.

How does the Golgi tendon reflex work?

The sensory afferent activates an inhibitory interneuron that hyperpolarizes the alpha motor neuron, causing muscle relaxation.

What is transmitted to higher brain centers from muscle spindles and Golgi tendon organs?

Instantaneous information about muscle length and tension.

How fast is muscle length and tension information transmitted to the brain?

Approximately $120\,\text{m/s}$.

Why is transmission of muscle length and tension information important?

It provides feedback control of motor activity.

What is the flexor withdrawal reflex?

A reflex that withdraws a limb from a painful stimulus.

What receptors activate the flexor withdrawal reflex?

Nociceptors.

Is the flexor withdrawal reflex monosynaptic or polysynaptic?

Polysynaptic.