Question

1. Describe why syndromes associated with damage to the lateral aspect of the lower brainstem differ from those associated with the medial aspect of the lower brainstem.

Answer 1

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:

1. 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.
2. 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.
3. Weakness. Because of the damage to alpha motor neurons and the atrophy of muscles, weakness is profound in lower motor neuron disorders.
4. 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
5. 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).
6. 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.
7. 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.