Movement Disorders Health Article

Definition

Movement disorders are a group of neurological diseases and syndromes that involve the motor and movement systems' ability to produce and control movement.

Description

Though it seems simple and effortless, normal movement actually requires an astonishingly complex system of control. Disruption of any portion of this system can cause a person to produce movements that are too weak, too forceful, too uncoordinated, or too poorly controlled for the task at hand. Unwanted movements may occur at rest. Intentional movement may become impossible. These conditions are examples of movement disorders.

Abnormal movements themselves are symptoms of underlying disorders. In some cases, the abnormal movements are the only symptoms. The more common diseases causing motor disorders include:

• spinal cord injury (SCI)
• stroke
• multiple sclerosis (MS)
• muscular dystrophy (MD)
• huntington's chorea (HC)
• cerebral palsy (CP)
• dystonias
• tremor
• myasthenia gravis (MG)
• parkinsonism (PD)
• Tourette syndrome

Other causes of motor disorders are Wilson's disease (WD), inherited ataxias (Friedreich's ataxia, Machado-Joseph disease, and spinocerebellar ataxias), and encephalopathies.

Causes

Movement is produced and coordinated by several interacting brain centers, including the motor cortex, the cerebellum, and a group of structures in the inner potions of the brain called the basal ganglia. Sensory information provides critical input on the current position and velocity of body parts, and spinal nerve cells (neurons) help prevent opposing muscle groups from contracting simultaneously.

To understand how movement disorders occur, it is helpful to consider a normal volunteer movements, such as reaching to touch a nearby object with the right index finger. To accomplish the desired movement, the arm must be lifted and extended. The hand must be held out to align with the forearm, and the forefinger must be extended while the other fingers remain flexed.

THE MOTOR CORTEX. Voluntary motor commands begin in the motor cortex located on the outer, wrinkled surface of the brain. Movement of the right arm is begun by the left motor cortex, which generates a large volley of signals to the involved muscles. These electrical signals pass along upper motor neurons, through the midbrain, to the spinal cord (SC). Within the SC, these signals connect to lower motor neurons, which convey the signals from the SC to the surface of the muscles involved. Neural activation of the muscles causes contraction, and the force of contraction pulling on the skeleton causes movement of the arm, hand, and fingers.

Damage to, or death of any of the neurons along this path, can cause weakness or paralysis of the affected muscles.

THE CEREBELLUM. Once the movement of the arm is initiated, sensory information is needed to guide the finger to its precise destination. In addition to sight, the most important source of information comes from the "position sense," provided by the many sensory receptors located within the limbs (proprioception). Proprioception allows a person to touch his or her nose with a finger even with the eyes closed. The balance organs in the ears provide important information about posture. Both postural and proprioceptive information are processed by a structure at the rear of the brain, called the cerebellum. The cerebellum sends out electrical signals to modify movements as they progress, "sculpting" the barrage of voluntary commands into a tightly controlled, constantly evolving pattern. Cerebellar disorders cause inability to control the force, fine positioning, and speed of movements (ataxia). Disorders of the cerebellum may also impair the ability to judge distance, so that a person under- or overreaches the target (dysmetria). Tremor during voluntary movements can also result from cerebellar damage.

THE BASAL GANGLIA. Both the cerebellum and the motor cortex send information to a set of structures deep within the brain that helps control involuntary components of movement (basal ganglia). The basal ganglia send output messages to the motor cortex, helping to initiate movements, regulate repetitive or patterned movements, and control muscle tone.

Circuits within the basal ganglia are complex. Within this structure, some groups of cells begin the action of other basal ganglia components, and some groups of cells block the action. These complicated feedback circuits are not entirely understood. Disruptions of these circuits are known to cause several distinct movement disorders. A portion of the basal ganglia, called the substantia nigra, sends electrical signals that block output from another structure, the subthalamic nucleus. The subthalamic nucleus sends signals to the globus pallidus, which in turn blocks the thalamic nuclei. Finally, the thalamic nuclei send signals to the motor cortex. The substantia nigra, then begins movement, and the globus pallidus blocks it.

This complicated circuit can be disrupted at several points. Loss of substantia nigra, cells increases blocking of the thalamic nuclei and prevents them from sending signals to the motor cortex. Degeneration of these nerve cells, as in PD, results in lower production of dopamine and fewer connections with other nerve cells and muscles, leading to a loss of movement (motor activity).

In contrast, cell loss in early HD decreases the blocking of signals from the thalamic nuclei, causing more cortex stimulation and stronger, but uncontrolled, movements.

Disruptions in other portions of the basal ganglia are thought to cause tics, tremors, dystonia, and a variety of other movement disorders, although the exact mechanisms are not well understood.

Some movement disorders, including HD, are caused by inherited genetic defects and inherited ataxias. Some diseases that cause sustained muscle contraction limited to a particular muscle group (focal dystonia) are inherited, but others are caused by trauma. The cause of most cases of PD is unknown, although genes have been identified for some familial forms.

ANTAGONISTIC MUSCLE PAIRS. This picture of movement, however, is too simple. One important refinement to it comes from considering the role of opposing, or antagonistic, muscle pairs. Contraction of the bicep muscle, located on the top of the upper arm, pulls on the forearm to flex the elbow and bend the arm. Contraction of the triceps, located on the opposite side, extends the elbow and straightens the arm. Within the spine, these muscles are normally wired so that willed (voluntary) contraction of one is automatically accompanied by blocking of the other. In other words, the command to contract the biceps provokes another command within the spine to prevent contraction of the triceps. In this way, these antagonist muscles are kept from resisting one another. Spinal cord or brain injury, can damage this control system and cause involuntary simultaneous contraction and spasticity, an increase in resistance to movement during motion.

While the peripheral mechanism, antagonistic muscle pairs, is certainly important, it is not the only one of concern with regard to movement disorders. Central pattern generators (CPGs) in the spinal cord are especially relevant because of their role in sensory processing. Filtration and processing of sensory input is accomplished locally, where the response of spinal pattern generator circuitry fits into continual movement, as necessary. Thus, although the brain receives much of the sensory input, the responses to spinal inputs are first the responsibility of the local spinal circuitry. Multi-segmental reflexes and anticipatory postural adjustments are as critical in the etiology of these syndromes.