18.17: Synaptic Signaling
Neurons communicate at synapses, or junctions, to excite or inhibit the activity of other neurons or target cells, such as muscles. Synapses may be chemical or electrical.
Most synapses are chemical, meaning an electrical impulse or action potential spurs the release of chemical messengers called neurotransmitters. The neuron sending the signal is called the presynaptic neuron, and the neuron receiving the signal is the postsynaptic neuron.
The presynaptic neuron fires an action potential that travels through its axon. The end of the axon, or the axon terminal, contains neurotransmitter-filled vesicles. The action potential opens voltage-gated calcium ion channels in the axon terminal membrane. Ca2+ rapidly enters the presynaptic cell (due to the higher external Ca2+ concentration), enabling the vesicles to fuse with the terminal membrane and release neurotransmitters.
The space between presynaptic and postsynaptic cells is called the synaptic cleft. Neurotransmitters released from the presynaptic cell rapidly populate the synaptic cleft and bind to receptors on the postsynaptic neuron. The binding of neurotransmitters instigates chemical changes in the postsynaptic neuron, such as opening or closing of ion channels. This, in turn, alters the membrane potential of the postsynaptic cell, enabling it to fire an action potential.
To end the synaptic signaling, neurotransmitters in the synapse are degraded by enzymes, reabsorbed by the presynaptic cell, diffused away, or cleared by glial cells.
Electrical synapses are present in the nervous system of both invertebrates and vertebrates. They are narrower than their chemical counterparts and transfer ions directly between neurons, allowing faster signal transmission. However, unlike chemical synapses, electrical synapses cannot amplify or transform presynaptic signals. Electrical synapses synchronize neuronal activity, which is favorable for controlling rapid, invariable signals, such as the danger escape in squids.
Neurons can send signals to, and receive them from, many other neurons. The integration of numerous inputs received by postsynaptic cells ultimately determines their action potential firing patterns.
