Damaged alpha motor neurons can produce spontaneous action potentials. Because of the damage to alpha motor neurons and the atrophy of muscles, weakness is profound in lower motor neuron disorders. When alpha motor neurons die, the muscle fibers that they innervate become deprived of necessary trophic factors and eventually the muscle itself atrophies. 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.

Furthermore, nearby motor neuron pools control nearby 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). The effects can be limited to small groups of muscles.Lower motor neuron syndrome is characterized by the following symptoms: This damage usually arises from certain diseases that selectively affect alpha motor neurons (such as polio) or from localized lesions near the spinal cord. 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). 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. Motor system dysfunction can result from damage or disease at any level of the motor system hierarchy and side-loops. Reflex circuits and other circuitry within the spinal cord underlie the automatic processing of many of the direct commands to the muscles (the вЂњnuts and boltsвЂќ processing), thereby freeing higher-order areas to concentrate on more global, task-related processing. Alpha motor neurons directly innervate skeletal muscle, causing the contractions that produce all movements. The first level of the motor system hierarchy is the spinal cord, the location of the alpha motor neurons that constitute the вЂњfinal common pathwayвЂќ of all motor commands.

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. 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. This chapter describes the types of disorders that result from damage or disease to different parts of the motor system. The previous motor system chapters have deconstructed the motor system into its component parts, in an effort to portray how the brainвЂ™s вЂњdivide and conquerвЂќ strategy assigns different motor control tasks to different brain regions.

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