MOTOR PHYSIOLOGY I

What are the major levels involved in generating and controlling movement?

Movement involves several interacting levels:

Cerebral cortex → descending motor pathways → spinal cord → lower motor neurons → skeletal muscle

Movement is also modulated by:

• Basal ganglia → movement initiation, selection, suppression of unwanted movements

• Cerebellum → coordination, timing, accuracy, error correction

• Sensory systems → provide feedback about body position and movement

• Spinal reflex circuits → rapid automatic responses

What is the basic distinction between voluntary movement and reflex movement?

• Voluntary movement: initiated by higher brain centers and consciously planned.

• Reflex movement: automatic response to a sensory stimulus, usually involving a relatively short neural circuit.

What is the general pathway for voluntary motor control?

Motor planning areas → primary motor cortex → descending motor pathway → spinal cord → lower motor neuron → skeletal muscle

The motor cortex generates the command, while spinal circuits and sensory feedback help refine the movement.

What is the primary motor cortex?

The primary motor cortex, located in the precentral gyrus, is the major cortical region responsible for initiating voluntary skeletal muscle movement.

It corresponds primarily to Brodmann area 4.

What does the premotor cortex do?

The premotor cortex helps plan and organize movements, particularly movements based on sensory information and learned motor patterns.

Where are the cell bodies of lower motor neurons located in the spinal cord?

In the ventral (anterior) horn of the spinal cord gray matter.

Their axons leave through the ventral roots and ultimately innervate skeletal muscle.

What is a lower motor neuron (LMN)?

A motor neuron that directly innervates skeletal muscle.

The LMN is the final common pathway through which the CNS produces skeletal muscle contraction.

How are motor neurons organized according to muscle function?

Motor neuron pools are organized according to the muscles they control.

Generally:

• Medial motor neurons → axial/proximal muscles

• Lateral motor neurons → distal limb muscles

• More dorsal/ventral organization also relates to flexor/extensor functions.

What are the basic components of a reflex arc?

Stimulus → sensory receptor → afferent neuron → CNS synapse/interneuron → efferent neuron → effector

For a skeletal muscle reflex, the effector is a muscle.

What is the myotatic reflex?

The myotatic reflex, or stretch reflex, is a reflex in which stretching a muscle causes that same muscle to contract.

Its major purpose is to:

• Maintain muscle length

• Maintain posture

• Resist sudden changes in muscle length

What receptor detects muscle stretch?

The muscle spindle.

It is a proprioceptor located within skeletal muscle.

What happens when a muscle is suddenly stretched?

Muscle stretch → muscle spindle activated → Ia sensory afferent fires → spinal cord → α-motor neuron activated → same muscle contracts

This helps oppose the original stretch.

Why is the stretch reflex considered a monosynaptic reflex?

Because the primary sensory neuron can synapse directly on the α-motor neuron in the spinal cord.

Therefore:

Ia afferent → α-motor neuron → muscle

There is only one synapse in the basic pathway.

What is the classic clinical example of the stretch reflex?

The patellar/knee-jerk reflex.

Tapping the patellar tendon stretches the quadriceps → muscle spindle activation → Ia afferent → spinal cord → quadriceps α-motor neurons → quadriceps contraction → lower leg extends.

What is a muscle spindle?

A sensory receptor that detects:

• Muscle length

• Changes in muscle length

• Especially rapid changes in length

It provides proprioceptive information to the CNS.

What are intrafusal fibers?

Specialized muscle fibers located inside the muscle spindle.

They are different from ordinary force-producing extrafusal muscle fibers.

What are extrafusal muscle fibers?

The ordinary skeletal muscle fibers that generate the actual force of muscle contraction.

They are innervated by α-motor neurons.

What is the difference between intrafusal and extrafusal fibers?

Intrafusal	Extrafusal
Inside muscle spindle	Main skeletal muscle
Sensory function	Force production
Detect muscle length	Produce contraction
Associated with γ-motor neurons	α-motor neurons

What are Ia afferents?

Large, rapidly conducting sensory fibers associated with muscle spindles.

They provide information about muscle stretch, especially rapid changes in muscle length.

What are γ-motor neurons responsible for?

They innervate the intrafusal fibers of muscle spindles.

Their activity keeps the muscle spindle sensitive during muscle contraction.

Why is γ-motor activity important during voluntary muscle contraction?

If only extrafusal fibers contracted, the spindle could become slack and stop detecting changes in muscle length.

γ-motor activation contracts the ends of intrafusal fibers and keeps the sensory portion of the spindle stretched.

This is called α-γ coactivation.

What does the Golgi tendon organ detect?

It detects muscle tension/force.

Where is the Golgi tendon organ located?

At the muscle-tendon junction, where it is arranged in series with the muscle fibers.

What sensory fiber carries information from the Golgi tendon organ?

The Ib afferent fiber.

What is the inverse myotatic reflex?

The inverse myotatic reflex is a reflex in which increased muscle tension causes relaxation of that same muscle.

Basic pathway:

High tension → GTO → Ib afferent → inhibitory interneuron → ↓ α-motor neuron activity → muscle relaxation

Why is the inverse myotatic reflex protective?

It helps prevent excessive muscle tension from damaging the muscle or tendon.

What happens during the flexion reflex?

A painful stimulus causes the affected limb to flex and withdraw.

Example:

Step on something painful → nociceptors activated → spinal cord → flexor muscles activated → limb withdraws

Why is the flexion reflex polysynaptic?

Because sensory neurons communicate with motor neurons through one or more interneurons.

What is the crossed-extension reflex?

While one limb flexes away from a painful stimulus, the opposite limb extends to help support body weight.

Example:

Right leg painful → right leg flexes + left leg extends

What is the major difference between the stretch reflex and withdrawal reflex?

Stretch reflex:

• Muscle stretch → same muscle contracts

• Fast

• Primarily monosynaptic

Withdrawal reflex:

• Painful stimulus → limb flexes away

• Polysynaptic

• Involves interneurons