8 Synaptic Transmission between Neurons
In contrast to the way in which information travels within individual neurons as electrical signals, information is usually transmitted between neurons through the release of neurotransmitters at specialized junctions called synapses. And in contrast to unvarying, always depolarizing action potentials, a wide variety of slow graded potentials may be produced at the synapses on an individual neuron—some depolarizing, some hyperpolarizing, some milliseconds in duration, others seconds, minutes, or even hours.
There Are Five Steps in Conventional Chemical Synaptic Transmission
The fundamental elements of a chemical synapse (Fig. 8-1) are a presynaptic ending from which neurotransmitter is released, a synaptic cleft across which it diffuses, and a postsynaptic element containing receptor molecules to which the neurotransmitter binds. Although the presynaptic ending is usually an axon terminal and the postsynaptic ending usually a dendrite, any part of a neuron can be presynaptic to any part of another neuron. The essential processes at chemical synapses are presynaptic synthesis, packaging, and release of neurotransmitter; binding to postsynaptic receptors; and termination of neurotransmitter action.
Presynaptic Endings Release Neurotransmitters into the Synaptic Cleft
Neurotransmitter release is a secretory process triggered by an increase in presynaptic Ca2+ concentration. The membranes of presynaptic terminals contain voltage-gated Ca2+ channels that open when an action potential spreads into the terminal (Fig. 8-2). Ca2+ influx causes one or more vesicles to fuse with the presynaptic membrane and dump its neurotransmitter content into the synaptic cleft. Because small vesicles are close to the synaptic cleft, they are the first to release their contents. Because the large vesicles are farther away, release of their contents requires more Ca2+ entry (hence, more presynaptic action potentials) and more time.
