Descending motor pathways are the nervous system’s communication lines, carrying movement instructions from the brain down to the spinal cord. When the brain formulates a plan for movement, these descending tracts transmit that plan, ensuring muscles receive the information needed to perform actions. This system orchestrates everything from large-scale movements to the finest motor tasks.
The Corticospinal Tracts
The primary route for voluntary movement is the corticospinal tract, a collection of nerve fibers that begins in the brain’s cerebral cortex. These fibers originate from several areas, including the primary motor cortex, the premotor cortex, and the supplementary motor area. From the cortex, these fibers converge and travel down through a dense white matter structure called the internal capsule.
As the corticospinal tract continues its descent, it passes through the brainstem. In the lowest part of the brainstem, in a region called the medulla, decussation occurs. Here, the majority of the fibers—between 75% and 90%—cross over to the opposite side of the central nervous system. This crossing explains why the right hemisphere of the brain controls the left side of the body, and the left hemisphere controls the right.
This pathway is responsible for precise and skilled voluntary movements, especially those involving the hands and feet, such as writing, typing, or playing a piano. The tract is divided into two main parts: the lateral corticospinal tract and the anterior corticospinal tract. The lateral tract is formed from the fibers that decussate in the medulla and controls the muscles of the limbs. The smaller anterior tract consists of the fibers that do not cross and manages the muscles of the trunk.
Brainstem Pathways for Posture and Balance
Beyond voluntary actions, the brainstem houses pathways that manage automatic functions like posture and balance. These are often referred to as extrapyramidal tracts and operate without conscious thought. They provide the stable foundation upon which voluntary movements can be executed, ensuring the body remains upright against gravity.
Two of the main pathways in this group are the vestibulospinal and reticulospinal tracts. The vestibulospinal tracts originate from the vestibular nuclei in the brainstem, which process balance information from the inner ear. These tracts send signals to the spinal cord to adjust posture and maintain balance, such as when you automatically correct your position to avoid falling. The vestibulospinal tracts primarily influence the flexor muscles of the arms and the extensor muscles of the legs.
The reticulospinal tracts arise from a network of neurons in the brainstem called the reticular formation. These pathways are instrumental in regulating muscle tone to support the body and control posture. For example, the constant muscle tension that keeps a person standing upright is managed by the reticulospinal tracts. They also help maintain muscle tone during movements like lifting a heavy object.
Brainstem Pathways for Head and Limb Orientation
Other brainstem pathways specialize in orienting the body in response to external cues by coordinating movements of the head, eyes, and limbs. This function is distinct from the general postural control provided by the vestibulospinal and reticulospinal tracts.
One such pathway is the tectospinal tract, which originates in a midbrain structure called the superior colliculus. The superior colliculus receives visual and auditory information, and the tectospinal tract uses this input to coordinate reflex movements of the head and eyes. For instance, when a sudden loud noise occurs to your side, the tectospinal tract directs your head to turn toward the source of the sound.
The rubrospinal tract, originating from the red nucleus in the midbrain, also contributes to motor control. While its exact role in humans is debated, it is understood to be involved in the coordination of upper limb movements. It is thought to work with the corticospinal tract, acting as a supplementary pathway to facilitate precise movements of the hands and arms.
Consequences of Motor Pathway Disruption
Damage to these descending motor pathways can lead to motor deficits. Such injuries are known as upper motor neuron (UMN) lesions, as they affect the neurons that originate in the brain or brainstem. Conditions that can cause UMN lesions include strokes, spinal cord injuries, multiple sclerosis, and traumatic brain injuries. The specific symptoms depend on the location and extent of the damage.
UMN lesions produce a characteristic set of symptoms known as upper motor neuron syndrome. These signs include:
- Muscle weakness
- Spasticity (an increase in muscle tone)
- Clonus (rhythmic, involuntary muscle contractions)
- Hyperreflexia (exaggerated reflexes)
For example, a brisk stretch of a muscle may result in an abrupt increase in tone, a phenomenon called clasp-knife rigidity.
A classic indicator of UMN damage is the Babinski sign. In a healthy adult, stroking the sole of the foot causes the toes to curl downward. Following a UMN lesion, the same stimulus may cause the big toe to extend upward while the other toes fan out.