5.2 Motor cortex and voluntary movement

Motor Cortex Organization

The motor cortex is the brain's command center for voluntary movement. It doesn't just fire off random signals to muscles. Instead, it's organized into distinct areas that plan, prepare, and execute movements in a coordinated sequence. Understanding this organization is key to understanding how damage to specific brain regions produces specific movement deficits.

Primary Motor Cortex and Somatotopic Organization

The primary motor cortex (M1) sits in the precentral gyrus of the frontal lobe, just in front of the central sulcus. It's somatotopically organized, meaning different regions of M1 map onto different body parts. This map is called the motor homunculus.

The homunculus layout, from medial to lateral:

• Feet and legs are represented medially (near the top/midline of the brain)
• Trunk and arms occupy the middle portion
• Hands and face are represented laterally (along the side)

A critical detail: the amount of cortex devoted to each body part reflects how much fine motor control that part requires, not its physical size. Your hands and face take up a huge portion of M1 compared to your trunk, because tasks like writing, speaking, and facial expressions demand extremely precise muscle control.

Neurons in M1 send their axons down through the corticospinal tract to directly activate motor neurons in the spinal cord, which in turn contract skeletal muscles.

Premotor and Supplementary Motor Areas in Movement Planning

Before M1 fires, movements need to be planned. Two regions anterior to M1 handle this:

• The premotor cortex sits just in front of M1 on the lateral surface. It plans and prepares movements by integrating sensory information and cognitive goals. It receives input from the prefrontal cortex (decision-making) and the basal ganglia (action selection).
• The supplementary motor area (SMA) sits on the medial surface of the frontal lobe. It specializes in planning complex movement sequences and coordinating movements that involve both hands (bimanual coordination). The SMA is particularly active when you mentally rehearse a movement before performing it.

Both regions send projections to M1 and directly to the spinal cord, giving them influence over how planned movements are actually carried out.

Motor Programs and Execution

Concept of Motor Programs

A motor program is a pre-planned sequence of muscle contractions that can be executed as a unit, often with minimal conscious effort. Think of typing your name or reaching for a cup of coffee: you don't consciously plan each individual muscle contraction.

Motor programs are thought to be stored across the premotor cortex, SMA, and cerebellum. When executed, they activate M1, which sends signals down the corticospinal tract to the appropriate muscles.

These programs aren't rigid. They can be modified on the fly based on sensory feedback. If you reach for a cup and it's heavier than expected, your brain adjusts grip force mid-movement. This flexibility is what makes motor programs adaptive rather than robotic.

Role of the Cerebellum

The cerebellum acts as a timing and coordination center for motor programs. It doesn't initiate movements on its own, but it fine-tunes them by comparing what you intended to do with what's actually happening (using sensory feedback from muscles, joints, and the vestibular system).

The cerebellum receives input from the motor cortex, spinal cord, and sensory systems, then sends corrective signals back to ensure movements are smooth and accurate.

Damage to the cerebellum produces ataxia, a condition marked by clumsy, uncoordinated movements, difficulty with balance, and problems performing smooth, accurate actions like touching your finger to your nose.

Descending Motor Pathways

Voluntary movement signals travel from the brain to the spinal cord through several descending pathways. These are broadly divided into the corticospinal tract (for fine voluntary control) and several brainstem pathways (for posture, balance, and more automatic movements).

Corticospinal Tract

The corticospinal tract is the primary pathway for voluntary movement. It originates in M1, descends through the brainstem, and terminates in the spinal cord. Most of its fibers cross (decussate) at the medullary pyramids, which is why the left motor cortex controls the right side of the body and vice versa.

This tract is especially important for fine, precise movements of the distal muscles, particularly the hands and fingers. Damage to it can cause weakness, paralysis, or loss of fine motor control on the opposite side of the body.

Other Descending Motor Pathways

These brainstem pathways handle more automatic aspects of motor control:

• Rubrospinal tract: Originates in the red nucleus of the midbrain. Controls proximal limb muscles and helps maintain muscle tone. In humans, its role is relatively minor compared to the corticospinal tract.
• Reticulospinal tract: Originates in the reticular formation of the brainstem. Controls posture, locomotion, and automatic movements like walking adjustments.
• Vestibulospinal tract: Originates in the vestibular nuclei. Maintains balance and upright posture by adjusting trunk and limb muscles in response to vestibular (inner ear) input.
• Tectospinal tract: Originates in the superior colliculus of the midbrain. Coordinates head and neck movements in response to visual and auditory stimuli, like turning your head toward a sudden sound.