14.3 Motor Responses

Motor System Processing Stream

The motor system translates your intentions into physical movement. It connects brain regions that plan and initiate actions with the spinal cord neurons that actually make muscles contract. Understanding this processing stream is central to grasping how voluntary movement works and what goes wrong in motor disorders.

Components of Motor System Processing

Several structures contribute to motor control, each with a distinct role:

• Premotor cortex receives input from sensory areas and plans movements before they happen.
• Primary motor cortex (M1) executes voluntary movements by sending signals down to lower motor neurons. M1 sits in the precentral gyrus of the frontal lobe.
• Basal ganglia modulate motor activity and help select and initiate appropriate movements. They receive input from the cerebral cortex and send output to the thalamus and brainstem. Key structures include the caudate nucleus, putamen, and globus pallidus.
• Cerebellum fine-tunes and coordinates movements by comparing intended actions with actual sensory feedback. It receives input from sensory systems and sends corrective output to the brainstem and thalamus.
• Brainstem contains motor nuclei that control cranial nerves and relay descending signals to the spinal cord. Its three regions (midbrain, pons, medulla oblongata) each house different motor nuclei.
• Spinal cord contains the lower motor neurons that directly innervate skeletal muscles.

Pathway of Motor Commands

Here's how a voluntary motor signal travels from brain to muscle:

1. The premotor cortex and primary motor cortex (M1) in the frontal lobe generate the motor command.

2. The corticospinal tract (also called the pyramidal tract) carries that signal from the cortex to the spinal cord:

   • Axons from M1 descend through the internal capsule.
   • At the pyramids of the medulla oblongata, about 85-90% of these axons decussate (cross to the opposite side).
   • The lateral corticospinal tract consists of these crossed fibers, which descend contralaterally in the lateral white matter of the spinal cord. This tract controls skilled, voluntary movements of the limbs.
   • The anterior corticospinal tract consists of the uncrossed fibers, which descend ipsilaterally in the anterior white matter. These fibers typically cross at the spinal level where they synapse.

3. Lower motor neurons (alpha motor neurons) in the anterior horn of the spinal cord receive the descending signal.

   • Their axons exit the spinal cord via ventral roots and travel through peripheral nerves to reach skeletal muscles.

4. At the neuromuscular junction, the axon terminal of the lower motor neuron synapses with a skeletal muscle fiber. The neurotransmitter acetylcholine (ACh) is released here, triggering muscle contraction.

Because most corticospinal fibers cross at the medulla, the left motor cortex controls the right side of the body, and vice versa.

Descending Motor Pathways

Descending pathways fall into two major categories based on where they originate and what type of movement they control.

Pyramidal vs. Extrapyramidal Tracts

The pyramidal (corticospinal) tract originates from M1 and controls fine, precise, voluntary movements, especially of the distal limbs (hands, fingers). Think of tasks like writing or buttoning a shirt.

The extrapyramidal tracts originate from brainstem motor nuclei rather than the cortex. They control posture, balance, muscle tone, and more automatic movements. There are four main extrapyramidal tracts:

• Rubrospinal tract originates from the red nucleus in the midbrain. It facilitates flexor muscle tone, particularly in the upper limbs.
• Tectospinal tract originates from the superior colliculus. It coordinates head and neck movements in response to visual stimuli (turning your head toward a sudden flash of light, for example).
• Reticulospinal tracts originate from the reticular formation. They control axial and proximal limb muscles involved in posture and balance during activities like sitting and standing.
• Vestibulospinal tracts originate from the vestibular nuclei. They adjust posture and balance in response to vestibular input from the inner ear.

Neurological Connections and Movement Initiation

Voluntary movement doesn't start with a single signal. The cortical motor areas (premotor cortex and M1) integrate information from multiple sources before generating a motor command:

• Sensory cortices (somatosensory, visual, auditory) provide information about the body's current state and the surrounding environment.
• Basal ganglia help select the right movement and suppress competing ones.
• Cerebellum provides timing and coordination data based on prior movement experience.

Once this information is integrated, M1 generates a motor command that travels via the corticospinal tract to lower motor neurons in the spinal cord. Those lower motor neurons then directly innervate skeletal muscles, producing the movement.

Motor Control and Coordination

Hierarchical Organization of Motor Control

Motor control is organized in a hierarchy:

• Upper motor neurons reside in the cerebral cortex and brainstem. They initiate and modulate voluntary movements but do not directly contact muscles.
• Lower motor neurons reside in the anterior horn of the spinal cord (or brainstem motor nuclei for cranial nerves). They are the final common pathway because every motor command must pass through them to reach skeletal muscle.
• A motor unit consists of a single lower motor neuron plus all the muscle fibers it innervates. Small motor units (few muscle fibers per neuron) allow fine control, like in the muscles of the eye. Large motor units (many fibers per neuron) generate powerful but less precise movements, like in the quadriceps.

Sensory Feedback and Motor Control

Movement doesn't happen in a vacuum. The nervous system constantly adjusts motor output based on sensory feedback.

• Proprioception provides real-time information about body position, joint angle, and movement. Proprioceptors include muscle spindles, Golgi tendon organs, and joint receptors. Without proprioception, even simple tasks like reaching for a cup become extremely difficult.
• Central pattern generators (CPGs) are networks of interneurons in the spinal cord that produce rhythmic, repetitive motor patterns. Walking and breathing are classic examples. CPGs can operate without continuous input from the brain, though the brain can modify their output.

Reflex Arcs

Reflexes are rapid, involuntary motor responses to specific stimuli. They occur at the spinal cord level, which means they don't require input from the brain (though the brain is informed after the fact). Three key reflexes show up repeatedly in this course.

Stretch Reflex (Myotatic Reflex)

This is a monosynaptic reflex, meaning only one synapse separates the sensory neuron from the motor neuron. Its purpose is to maintain muscle tone and resist sudden, unwanted changes in muscle length.

1. A muscle spindle detects a sudden stretch of the muscle.
2. A Ia sensory fiber (the afferent neuron) carries the signal to the spinal cord.
3. The Ia fiber synapses directly on an alpha motor neuron in the anterior horn.
4. The alpha motor neuron (the efferent neuron) fires back to the same muscle, causing it to contract and resist the stretch.

The classic example is the patellar tendon reflex (knee-jerk reflex). Tapping the patellar tendon stretches the quadriceps, triggering a quick contraction that extends the knee.

Golgi Tendon Reflex (Inverse Myotatic Reflex)

This is a polysynaptic reflex that protects muscles and tendons from excessive force.

1. A Golgi tendon organ at the muscle-tendon junction detects increasing tension.
2. A Ib sensory fiber carries the signal to the spinal cord.
3. An inhibitory interneuron in the spinal cord receives the signal and inhibits the alpha motor neuron of the same muscle.
4. The alpha motor neuron reduces its firing, causing the muscle to relax and reducing tension.

Notice this reflex does the opposite of the stretch reflex: instead of contracting the muscle, it causes relaxation. That's why it's sometimes called the inverse myotatic reflex.

Withdrawal Reflex (Flexor Reflex)

This is a polysynaptic reflex that protects the body from harmful stimuli by pulling a limb away.

1. Nociceptors (pain receptors) or mechanoreceptors in the skin detect a harmful stimulus.

2. A sensory fiber carries the signal to the spinal cord.

3. Multiple interneurons in the spinal cord process the signal:

   • Excitatory interneurons activate alpha motor neurons to flexor muscles, pulling the limb away.
   • Inhibitory interneurons suppress alpha motor neurons to extensor muscles, allowing the limb to flex freely.

4. The limb withdraws from the stimulus.

A common example: touching a hot stove causes your hand to pull back before you even consciously feel the pain. The crossed extensor reflex often accompanies the withdrawal reflex on the opposite limb, extending it to maintain balance (if you step on a tack, the opposite leg straightens to support your weight).