When acetylcholine (ACh) binds to its receptor, it directly or indirectly causes the opening of chemically regulated gates. In many cases, this produces a depolarization called an excitatory postsynaptic potential, or EPSP. In some cases, however, ACh causes a hyperpolarization known as an inhibitory postsynaptic potential, or IPSP
Acetylcholine (ACh) is used as an excitatory neurotransmit-ter by some neurons in the CNS and by somatic motor neurons at the neuromuscular junction. At autonomic nerve endings, ACh may be either excitatory or inhibitory, depending on the organ involved.
The varying responses of postsynaptic cells to the same chemical can be explained, in part, by the fact that different post-synaptic cells have different subtypes of ACh receptors. These receptor subtypes can be specifically stimulated by particular toxins,
The Nervous System: Neurons and Synapses and they are named for these toxins. The stimulatory effect of ACh on skeletal muscle cells is produced by the binding of ACh to nico-tinic ACh receptors, so named because they can also be activated by nicotine. Effects of ACh on other cells occur when ACh binds to muscarinic ACh receptors; these effects can also be produced by muscarine (a drug derived from certain poisonous mushrooms).
An overview of the distribution of the two types of ACh receptors demonstrates that this terminology and its associated concepts will be important in understanding the physiology of different body systems.
1. Nicotinic ACh receptors. These are found in specific regions of the brain (chapter 8), in autonomic ganglia (chapter 9), and in skeletal muscle fibers (chapter 12). The release of ACh from somatic motor neurons and its subsequent binding to nicotinic receptors, for example, stimulates muscle contraction.
2. Muscarinic ACh receptors. These are found in the plasma membrane of smooth muscle cells, cardiac muscle cells, and the cells of particular glands (chapter 9). Thus, the activation of muscarinic ACh receptors by ACh released from autonomic axons is required for the regulation of the cardiovascular system (chapter 14), digestive system (chapter 18), and others.
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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.