The Neuromuscular Junction and Muscle Excitation Help (page 2)
Introduction to The Neuromuscular Junction and Muscle Excitation
A critical element in the neuromuscular connection is the motor neuron. As its name suggests, a motor neuron is a nerve cell that excites a muscle to contract (or a gland to secrete), thereby causing movement. Thus, this type of neuron results in the moving of a vertebrate body, in somewhat the same sense that a motor ultimately causes the body of a car to move.
Figure 14.8 illustrates the main parts of a typical motor neuron. There are a number of dendrites ( DEN -dryts) at one end of the nerve cell. These dendrites are actually slender branches of cytoplasm that carry excitation toward the cell body (major central portion) of the neuron.
Study suggestion: Why do you think this word derives from the Greek for “of a tree”?
When the cell body is sufficiently excited, it fires off an action potential or nerve impulse. The action potential (nerve impulse) can most simply be considered a wave. It is a traveling wave of chemical excitation that passes from the neuron cell body, and then onto the neuron axon ( AX -ahn).
The axon is like a long slender “axle” holding two car wheels. Although the axon doesn’t move, it is a long, slender branch of cytoplasm that carries excitation (an action potential) away from the cell body of a neuron. The action potential or nerve impulse travels down the axon to its branching tips, called the axon terminals.
Within the axon terminals are tiny vesicles ( VES -ih-kls). Each vesicle is literally a “tiny bladder” that consists of a membrane surrounding thousands of neurotransmitter ( NUR -oh- trans -mit-er) molecules. Each time an action potential travels down into the axon terminals, the membranes surrounding some of the vesicles rupture.
Hundreds of neurotransmitter molecules are then released into the neuromuscular junction, which is also called the motor end plate. The neuromuscular junction (motor end plate) is the flat, plate-like area where the axon terminals of a motor neuron almost (but not quite) touch the cell membrane of a muscle fiber.
The Process Of Muscle Fiber Excitation
The released neurotransmitter molecules diffuse across the narrow, saltwater-filled gap of the neuromuscular junction, then attach to transmitter binding sites on the muscle fiber. What happens to the muscle fiber, then, is largely determined by the nature of the neurotransmitter molecules that are released and attached to the binding sites.
Excitatory neurotransmitter molecules stimulate or excite the skeletal muscle fiber to contract. The main excitatory neurotransmitter in the human neuromuscular junction is called acetylcholine (uh- see -tul- KOH -leen), abbreviated as ACh.
The neuromuscular junction (motor end plate) can be considered the functional end of a particular motor pathway . A motor pathway is a sequence of one or more motor neurons that carries an action potential to a neuromuscular junction, thereby resulting in muscle contraction.
From this definition, you can see that a lower motor neuron is the one whose axon and axon terminals actually makes a neuromuscular junction with one or more skeletal muscle fibers. “Where is the cell body of this lower motor neuron located?” the inquiring mind needs to know. Quite often, the cell body of the lower motor neuron is located in the gray matter of the spinal cord (Figure 14.9). The axon then extends outward into the white matter , entering a spinal nerve .
The spinal nerve travels some distance away from the spinal cord. The motor neuron axons within the spinal nerve carry the action potential away from the central region of the whole body, and hence to the “outer portion” or periphery ( per - IF -eh-ree). As it reaches the body periphery, the spinal nerve branches into one or more peripheral ( per - IF -er-al) nerves . It is the peripheral nerve, then, that forms the last major linkage in the motor pathway. From here, the axon terminals of the lower motor neuron branch out and make a neuromuscular junction with one or more skeletal muscle fibers. Excitatory neurotransmitter (such as acetylcholine, ACh) is released, the skeletal muscle fibers are stimulated, and the fibers contract.
The spinal nerves and individual peripheral nerves leading to various skeletal muscles are considered parts of the Peripheral Nervous System or PNS. This name is due to the fact that they supply the body periphery (outer portion).
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 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 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.
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