Most of the synaptic input to motor neurons from the descending pathways and afferent neurons does not go directly to motor neurons but rather to interneurons that synapse with the motor neurons. These interneu-rons are of several types. Some are confined to the general region of the motor neuron upon which they synapse and thus are called local interneurons. Others have processes that extend up or down short distances in the spinal cord and brainstem, or even throughout much of the length of the central nervous system. The interneurons with longer processes are important in movements that involve the coordinated interaction of, for example, a shoulder and arm or an arm and a leg.
The local interneurons are important elements of the lowest level of the motor control hierarchy, integrating inputs not only from higher centers and peripheral receptors but from other interneurons as well (Figure 12-3). They are crucial in determining which muscles are activated and when. Moreover, interneurons can act as "switches" that enable a movement to be turned on or off under the command of higher motor centers. For example, if we pick up a hot plate, a local reflex arc will be initiated by pain receptors in the skin of the hands, normally resulting in our dropping the plate. But if it contained our dinner, descending commands could inhibit the local activity, and we would hold onto the plate until we could put it down safely.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.