6.7:
Synaptic Signaling
Neurons communicate to each other and to other cells mainly through chemical signaling at synapses. These specialized regions are where the axon terminal of the presynaptic cell, the neuron sending the message, meets the postsynaptic cell receiving the message.
The signal consists of neurotransmitter molecules which are stored in the axon terminal within membrane-bound organelles called synaptic vesicles.
When an electrical signal, known as an action potential, occurs in the presynaptic neuron, it triggers these vesicles to fuse to the cell membrane. When the vesicles fuse, they release their neurotransmitter into the synaptic cleft, the narrow space between cells.
The neurotransmitter then diffuses across and binds to its postsynaptic receptors. This binding elicits a response in the postsynaptic cell, which, in this case, is a neuron, and an action potential may be produced. Ultimately, synaptic signaling allows neurons to transmit information to other cells, near and far.
6.7:
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. That means that an electrical impulse—or action potential—spurs the release of chemical messengers. These chemical messengers are also called neurotransmitters. The neuron sending the signal is called the presynaptic neuron. 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 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 ion channels. This, in turn, alters the membrane potential of the postsynaptic cell, making it more or less likely to fire an action potential.
To end 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 transmission of the signal. However, unlike chemical synapses, electrical synapses cannot amplify or transform presynaptic signals. Electrical synapses syncronize neuron 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.
Suggested Reading
Kennedy, Mary B. “Synaptic Signaling in Learning and Memory.” Cold Spring Harbor Perspectives in Biology 8, no. 2 (February 2016). [Source]