Edward G. Moczydlowski
The ionic gradients that cells maintain across their membranes provide a form of stored electrochemical energy that cells can use for electrical signaling. The combination of a resting membrane potential of −60 to −90 mV and a diverse array of voltage-gated ion channels allows excitable cells to generate action potentials that propagate over long distances along the surface membrane of a single nerve axon or muscle fiber. However, another class of mechanisms is necessary to transmit such electrical information from cell to cell throughout the myriad of neuronal networks that link the brain with sensory and effector organs. Electrical signals must pass across the specialized gap region between two apposing cell membranes that is called a synapse. The process underlying this cell-to-cell transfer of electrical signals is termed synaptic transmission. Communication between cells at a synapse can be either electrical or chemical. Electrical synapses provide direct electrical continuity between cells by means of gap junctions, whereas chemical synapses link two cells together by a chemical neurotransmitter that is released from one cell and diffuses to another.
In this chapter we introduce the general principles of synaptic transmission and then focus mainly on synaptic transmission between a motor neuron and a skeletal muscle fiber. This interface between the motor neuron and the muscle cell is called the neuromuscular junction. In Chapter 13, we expand upon this topic with a focus on synaptic transmission between neurons in the central nervous system (CNS).