Somatic motor neurons form synapses with skeletal muscle cells (muscle fibers). At these synapses, or neuromuscular junctions, the postsynaptic membrane of the muscle fiber is known as a motor end plate. Therefore, the EPSPs produced by ACh in skeletal muscle fibers are often called end-plate potentials. This depolarization opens voltage-regulated channels that are adjacent to the end plate. Voltage-regulated channels produce action potentials in the muscle fiber, and these are reproduced by other voltage-regulated channels along the muscle plasma membrane. This conduction is analogous to conduction of action potentials by axons; it is significant because action potentials produced by muscle fibers stimulate muscle contraction (as described in chapter 12).
Clinical Investigation Clue
Remember that Sandra had flaccid paralysis and difficulty breathing after eating mussels and clams gathered from the local shore.
What caused her difficulty in breathing?
If any stage in the process of neuromuscular transmission is blocked, muscle weakness—sometimes leading to paralysis and death—may result. The drug curare, for example, competes with ACh for attachment to the nicotinic ACh receptors and thus reduces the size of the end-plate potentials (see table 7.6). This drug was first used on blow-gun darts by South American Indians because it produced flaccid paralysis in their victims. Clinically, curare is used in surgery
The Nervous System: Neurons and Synapses as a muscle relaxant and in electroconvulsive shock therapy to prevent muscle damage.
Autonomic motor neurons innervate cardiac muscle, smooth muscles in blood vessels and visceral organs, and glands. As previously mentioned, there are two classifications of autonomic nerves: sympathetic and parasympathetic. Most of the parasympathetic axons that innervate the effector organs use ACh as their neurotransmitter. In some cases, these axons have an inhibitory effect on the organs they innervate through the binding of ACh to muscarinic ACh receptors. The action of the vagus nerve in slowing the heart rate is an example of this inhibitory effect. In other cases, ACh released by autonomic neurons produces stimulatory effects as previously described. The structures and functions of the autonomic system are described in chapter 9.
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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.