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The Neuromuscular Junction and Muscle Excitation Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 30, 2011

Higher Motor Neurons Within The Cns

We have explained how a lower motor neuron in the gray matter of the spinal cord can excite skeletal muscle fibers to contract. The next question to consider is, “What is it that excites a lower motor neuron, so that it can then excite skeletal muscle?”

The answer is that a series of one or more higher motor neurons within a particular motor pathway eventually stimulates a lower motor neuron. The cell bodies of such higher motor neurons generally lie within the Central Nervous System, which is abbreviated as CNS. The Central Nervous System (CNS) is the portion of the nervous system that is centrally located within the skull and the vertebral column. Specifically,

The Neuromuscular (Nerve-Muscle) Connection Motor Pathways Higher Motor Neurons Within The Cns

The brain is technically the upper portion of the CNS that lies within the skull. The spinal cord, in contrast, is the narrow, “cord”-like lower portion of the CNS that is housed within the vertebral column.

Figure 14.10 provides an overview of the CNS, along with the relative positions of the brain and spinal cord. One especially important location for higher motor neurons in the brain is an area called the precentral gyrus ( JEYE -rus). The name gyrus means “ring” or “fold.” The precentral gyrus, therefore, is a raised ring or fold of brain tissue located just “before” or “in front of” (pre-) a groove called the central sulcus ( SUL -kus).

The precentral gyrus lies at the superior end of the cerebrum (seh- REE -brum) or “main brain mass.” Since this gyrus is at the surface, it is part of the cerebral cortex ( KOR -tex) or gray matter “bark” covering the “main brain mass.” The upper motor neurons situated within the cerebral cortex of the precentral gyrus often direct voluntary movements of the body to occur.

The Neuromuscular (Nerve-Muscle) Connection Motor Pathways Higher Motor Neurons Within The Cns

Fig. 14.10 Higher motor neurons and the hidden puppeteer.

The precentral gyrus is sometimes called the primary motor area . It is here that upper motor neurons “decide” to initiate the movements of various skeletal muscles (usually on the opposite side of the body). From the primary motor area (precentral gyrus), motor nerve fibers descend, cross over the body midline , and then stimulate skeletal muscles on the opposite side of the body to contract. To move your right hand, for instance, upper motor neurons within the left precentral gyrus would send down a message, which would eventually cross to the right side of the spinal cord, then out to the muscles in the right hand.

Study suggestion: Imagine a mischievous little puppeteer hiding within your left precentral gyrus. The strings manipulating his puppet hang down (descend) for some distance. These “strings” are actually nerve fibers in a motor pathway. After the strings are pulled (excited by a muscle action potential), they activate the puppet (skeletal muscle) to contract.

 

Practice problems for these concepts can be found at:  The Neuromuscular (Nerve-Muscle) Connection Test

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