5 Motor Systems

Distributed control of movement

Movement is not produced by a single command center. Cortical and subcortical circuits select and shape actions, descending pathways carry signals toward the spinal cord and brainstem, and local circuits combine those signals with sensory feedback. Skeletal muscles are activated by lower motor neurons, the final link in this chain.

Voluntary action involves several interacting levels of control:

• Association and premotor areas help choose an action and prepare its sequence, often using sensory information and the current context.

• The primary motor cortex (M1) contributes to execution and fine control of voluntary movement. Its activity is distributed across populations of neurons rather than organized as a simple one-neuron-to-one-muscle map.

• The basal ganglia help select and initiate appropriate actions while reducing competing actions.

• Cerebellar circuits compare intended and actual movement, help adjust timing and coordination, and support error-based motor learning.

• Brainstem and spinal circuits organize posture, reflexes, locomotion, and the muscle-level details of movement.

These structures form interacting loops. The basal ganglia and cerebellum influence movement largely through circuits that return to the cortex or act through brainstem motor centers; they do not directly activate most limb muscles.

Descending pathways

Descending pathways carry motor signals from the brain toward spinal circuits and brainstem motor nuclei.

Cortical pathways

The corticospinal tract carries signals from the cerebral cortex toward spinal circuits. Many of its fibers pass through the internal capsule and brainstem. Most cross at the lower medulla and descend as the lateral corticospinal tract, which is important for skilled movements, especially those involving the distal limbs. Other fibers descend in the anterior cord and influence trunk and proximal muscles, often through spinal interneurons.

The corticobulbar pathway carries cortical motor signals to brainstem cranial-nerve motor nuclei. It helps control muscles of the face, jaw, tongue, and neck. Many corticobulbar connections are bilateral, although some muscles receive stronger input from one hemisphere.

Brainstem pathways

Brainstem pathways help regulate posture, balance, and orienting movements. Vestibulospinal pathways use vestibular information to help stabilize posture. Reticulospinal pathways contribute to postural adjustments and coordinated movement. Tectospinal pathways help orient the head and neck toward salient sights or sounds. These pathways work alongside cortical commands rather than forming a separate, isolated motor system.

Spinal circuits and muscle output

Descending commands reach interneurons and lower motor neurons in the spinal cord. Interneurons link sensory input, motor-neuron activity, and muscle groups, helping coordinate actions such as withdrawing a limb or alternating the legs during walking. Local circuits can organize basic movement patterns, but normal voluntary action depends on their interaction with descending control and sensory input.

A motor unit consists of one lower motor neuron and all the muscle fibers it activates. Small motor units allow fine control. Recruiting additional motor units and increasing their firing rates can increase muscle force. Lower motor neurons are the final common pathway because their activity directly drives skeletal-muscle contraction.

Feedback guides movement

The nervous system continually combines motor commands with information about the body and its surroundings. Proprioceptors report muscle stretch, tension, and joint position, while vision and touch provide additional information. Spinal reflexes can rapidly adjust muscle activity, and the brain uses sensory feedback to correct actions and update future movements.

When reaching for a cup, the brain estimates the cup’s location, selects and plans the reach, and sends descending signals to coordinate the shoulder, arm, and hand. As the arm moves, visual and proprioceptive feedback help correct its path. The cerebellum contributes to detecting and reducing movement error, while the basal ganglia contribute to selecting the action and suppressing competing ones.

Effects of disrupted motor circuits

Different circuit disruptions produce different movement problems. Damage to corticospinal pathways can impair voluntary strength and skilled movement. Cerebellar damage can cause poorly timed, inaccurate, or unsteady movement. Basal ganglia dysfunction can interfere with initiating or scaling movement and can produce unwanted movements.

These differences show that coordinated action depends on cooperation among multiple systems, not only on the muscles or motor cortex.