Key Points

• Lower motor neurons represent the final common pathway that links the nervous system to the appropriate muscle group. Lower motor neurons for muscles of the head and neck are located within cranial nerve nuclei in the brain stem.

• Excitatory and inhibitory interneurons make reciprocal connections among groups of lower motor neurons innervating antagonistic and synergistic muscles.

• Upper motor neurons reside at higher levels of the neuronal axis and provide input to interneurons and lower motor neurons, thus creating a hierarchical arrangement of the motor system.

• Lower motor neurons for muscles of the body are arranged somatotopically at different levels within the central gray matter of the spinal cord.

• The appropriate pool of motor neurons and the individual muscle fibers they each innervate work synergistically to cause contraction of an entire muscle.

• Intrinsic properties of individual motor neurons and muscle fibers have an important influence on the response characteristics of the motor unit.

There is but one final common pathway between the nervous system and the somatic musculature. The lower motor neuron forms the essential neuronal link with the individual muscle cell, and all motor commands must be delivered through it. With loss of the lower motor neuron, muscles cannot respond to any neuronal drive and flaccid paralysis results. Lower motor neurons for muscles of the head and neck are located in motor nuclei of the brain stem, with peripheral fibers projecting through cranial nerves. Lower motor neurons for muscles of the body have cell bodies in the ventral horn of the spinal cord and processes that exit in the ventral roots of the spinal nerves. Details of the synaptic interactions at the neuromuscular junction between lower motor neurons and innervated muscle are discussed in Chapter 6.

Lower motor neurons do not act in isolation; rather, they are themselves under continuous control of a number of different inputs generally organized in hierarchical fashion. The lowest control level is represented by direct sensory inputs to the lower motor neuron that can drive simple reflex activity in appropriate muscle groups. The next highest control level is represented by groups of upper motor neurons organized within various motor nuclei of the brain stem or within areas of the gray matter of the spinal cord. Intermediate levels of motor processing can also occur in other motor areas such as the basal ganglia or cerebellum. These motor neurons coordinate more complex muscle reflexes. The highest level of control is represented by upper motor neurons in the motor cortex that elicit conscious movements and learned motor behaviors. Chapters 55 through 59 are devoted to discussions of the different levels within the hierarchical organization of the motor system and how they function in producing the complex repertoire of human movements.

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