The previous motor system chapters have deconstructed the motor system into its component parts, in an effort to portray how the brain’s “divide and conquer” strategy assigns different motor control tasks to different brain regions. This chapter describes the types of disorders that result from damage or disease to different parts of the motor system. In the process, the different components of the motor system are reviewed to see how they work together to produce the fluid, effortless body movements that we take for granted. An emphasis is placed on trying to explain the causes and symptoms of motor system disorders in terms of the basic principles of neuroanatomy and neuronal function that you learned in the earlier chapters.
Lower Motor Neuron Syndrome
The first level of the motor system hierarchy is the spinal cord, the location of the alpha motor neurons that constitute the “final common pathway” of all motor commands. Alpha motor neurons directly innervate skeletal muscle, causing the contractions that produce all movements. Reflex circuits and other circuitry within the spinal cord underlie the automatic processing of many of the direct commands to the muscles (the “nuts and bolts” processing), thereby freeing higher-order areas to concentrate on more global, task-related processing.
Motor system dysfunction can result from damage or disease at any level of the motor system hierarchy and side-loops. Differences in the symptoms that result from damage at different levels allow the clinician to localize where in the hierarchy the damage is likely to be. Damage to alpha motor neurons results in a characteristic set of symptoms called the lower motor neuron syndrome (lower motor neurons refer to alpha motor neurons in the spinal cord and brain stem; all motor system neurons higher in the hierarchy are referred to as upper motor neurons). This damage usually arises from certain diseases that selectively affect alpha motor neurons (such as polio) or from localized lesions near the spinal cord. Lower motor neuron syndrome is characterized by the following symptoms:
- The effects can be limited to small groups of muscles. Recall that a motor neuron pool is a nucleus of alpha motor neurons that innervate a single muscle (link to Motor Unit Figure 2). Furthermore, nearby motor neuron pools control nearby muscles. Thus, restricted damage to lower motor neurons, either within the spinal cord or at the ventral roots, will affect only a restricted group of muscles.
- Muscle atrophy. When alpha motor neurons die, the muscle fibers that they innervate become deprived of necessary trophic factors and eventually the muscle itself atrophies.
- Weakness. Because of the damage to alpha motor neurons and the atrophy of muscles, weakness is profound in lower motor neuron disorders.
- Fasciculation. Damaged alpha motor neurons can produce spontaneous action potentials. These spikes cause the muscle fibers that are part of that neuron’s motor unit to fire, resulting in a visible twitch (called a fasciculation) of the affected muscle (Figure 6.1).
Figure 6.1
Fasciculations and fibrillations. Click on buttons to see demonstration.
- Fibrillation. With further degeneration of the alpha motor neuron, only remnants of the axons near the muscle fibers remain. These individual axon fibers can also generate spontaneous action potentials; however, these action potentials will only cause individual muscle fibers to contract. This spontaneous twitching of individual muscle fibers is called a fibrillation (Fig. 1). Fibrillations are too small to be seen as a visible muscle contraction. They can only be detected with electrophysiological recordings of the muscle activity (an electromyogram).
- Hypotonia. Because alpha motor neurons are the only way to stimulate extrafusal muscle fibers, the loss of these neurons causes a decrease in muscle tone.
- Hyporeflexia. The myotatic (stretch) reflex is weak or absent with lower motor neuron disorders, because the alpha motor neurons that cause muscle contraction are damaged.
Upper Motor Neuron Syndrome
Damage to any part of the motor system hierarchy above the level of alpha motor neurons (not including the side loops) results in a set of symptoms termed the upper motor neuron syndrome. Some of these symptoms are opposite of those of lower motor neuron disorders. Thus, one of the critical determinations a clinician must make is whether a patient presenting with motor problems has an upper motor neuron disorder or a lower motor neuron disorder.
Upper motor neuron disorders typically arise from such causes as stroke, tumors, and blunt trauma. For example, strokes to the middle cerebral artery, lateral striate artery, or the medial striate artery can cause damage to the lateral surface of cortex or to the internal capsule, where the descending axons of the corticospinal tract collect. The symptoms of upper motor neuron syndrome are:
- The effects extend to large groups of muscles. Recall from the Motor Cortex chapter that muscles from different body parts are activated by stimulation of parts of motor cortex, consistent with the notion that motor cortex represents movements that are controlled by many joints, rather than individual muscles. Thus, a stroke in a particular part of motor cortex will affect the activation of many muscles in the body. Likewise, a stroke that affects the internal capsule or crus cerebri could affect muscles on the entire contralateral side of the body.
- Atrophy is rare. Because alpha motor neurons are present, muscles will continue to receive trophic agents necessary for their survival. A mild amount of atrophy may result from disuse, but it will not be as pronounced as that resulting from a lower motor neuron disorder.
- Weakness. Upper motor neuron disorders produce a graded weakness of movement (paresis), which differs from the complete loss of muscle activity caused by paralysis (plegia).
- Absence of fasciculations. Because alpha motor neurons themselves are spared, fasciculations do not occur.
- Absence of fibrillations. Likewise, fibrillations do not occur.
- Hypertonia. Upper motor neuron disorders result in an increase in muscle tone. Recall that descending motor pathways can modulate the intrinsic circuitry that is present in the spinal cord. This modulatory input can be either inhibitory or excitatory. Through mechanisms that are not well understood, the loss of descending inputs tends to result in an increased firing rate of alpha and/or gamma motor neurons. The higher firing rate causes an increase in the resting level of muscle activity, resulting in hypertonia.
- Hyperreflexia. Because of the loss of inhibitory modulation from descending pathways, the myotatic (stretch) reflex is exaggerated in upper motor neuron disorders. The stretch reflex is a major clinical diagnostic test of whether a motor disorder is caused by damage to upper or lower motor neurons.
- Clonus. Sometimes the stretch reflex is so strong that the muscle contracts a number of times in a 5-7 Hz oscillation when the muscle is rapidly stretched and then held at a constant length. This abnormal oscillation, called clonus, can be felt by the clinician.
- Initial contralateral flaccid paralysis. In the initial stages after damage to motor cortex, the contralateral side of the body shows a flaccid paralysis. Gradually, over the course of a few weeks, motor function returns to the contralateral side of the body. This gradual recovery of function results from the ability of other motor pathways to take over some of the lost functions. Recall that there are multiple descending motor pathways by which high-order information can reach the spinal cord. Thus, descending pathways such as the rubrospinal and the reticulospinal tracts, which receive direct or indirect cortical input, can take over the function lost by the damage to the corticospinal tract. Moreover, primary motor cortex itself is capable of reorganizing itself to recover some lost function. Thus, if the part of motor cortex that controls a certain body movement is damaged, neighboring parts of the motor cortex that are undamaged can, to some extent, alter their function to help compensate for the damaged areas. The one major exception to the recovery of function is that fine control of the distal musculature will not be regained after a lesion to the corticospinal tract. Recall that there are direct connections from primary motor cortex neurons to alpha motor neurons controlling the fingers. These connections presumably underlie our abilities to manipulate objects with great precision and to do such tasks as playing a piano and performing microsurgery. None of the other descending pathways have direct connections onto spinal motor neurons, and none of them can compensate for the loss of fine motor control of the hands and fingers after damage to the corticospinal tract.
- Spasticity. A clinical sign of upper motor neuron disorder is a velocity dependent resistance to passive movement of the limb. If the clinician moves a patient’s limb slowly, there may be little resistance to the movement. As the passive movement becomes quicker, however, at a certain point the muscle will sharply resist the movement. This is referred to as a “spastic catch.” The mechanism for this spasticity is not entirely known, but altered firing rate of gamma motor neurons and their regulating interneurons may be involved, as well as an increase in alpha motor neuron activity, causing an inappropriately powerful stretch reflex to a fast stretch of the muscle. Sometimes, the resistance becomes so great that the autogenic inhibition reflex is initiated, causing a sudden drop in the resistance; this is referred to as the clasp-knife reflex.
Figure 6.2
Babinski sign.
- Babinski sign. A classic neurological test for corticospinal tract damage is the Babinski test. In this test, the clinician strokes the sole of the foot firmly with an instrument. This elicits a normal plantar response in normal individuals, as the toes curl inward. In patients with an upper motor neuron disorder, however, an abnormal extensor plantar response is elicited, as the big toe extends upward and the remaining toes fan out. This is called a positive Babinski sign (Figure 6.2). Interestingly, the positive Babinski sign is normal in infants for the first 2 years of life. During development, however, the reflex changes to the normal adult pattern, presumably as corticospinal circuits mature.
In addition to the above symptoms, damage to the motor cortex and association cortex can result in impairments in motor planning and strategies and in an inability to perform complex motor tasks. Performance of simple tasks is intact, but patients are unable to perform complex, practiced tasks. This symptom is known as apraxia. For example, patients may be unable to arrange a set of blocks to match an example block-structure in front of them. They can move the blocks individually, but cannot come up with a motor plan to arrange them properly. This disorder is known as constructional apraxia. Other apraxias include dressing apraxia (inability to dress oneself) and verbal apraxia (inability to coordinate mouth movements to produce speech).
Paralysis
A section or crush of the spinal cord will result in paralysis of all parts of the body below the damaged region. Even though such an injury occurs in the spinal cord, it is not considered a lower motor neuron disorder, as the alpha motor neurons themselves are not directly damaged. If the damage occurs at the cervical level, then all four limbs will be paralyzed (quadriplegia). If the damage occurs below the cervical enlargement, then only the legs are paralyzed (paraplegia). Other terms used to describe patterns of paralysis are hemiplegia (paralysis to one side of the body) and monoplegia (paralysis of a single limb).
