18.6: The Synapse
Neurons communicate with one another by passing on their electrical signals to other neurons. A synapse is the location where two neurons meet to exchange signals. At the synapse, the neuron that sends the signal is called the presynaptic cell, while the neuron that receives the message is called the postsynaptic cell. Note that most neurons can be both presynaptic and postsynaptic, as they both transmit and receive information.
An electrical synapse is one type of synapse in which the pre- and postsynaptic cells are physically coupled by proteins called gap junctions. This allows electrical signals to be directly transmitted to the postsynaptic cell. One feature of these synapses is that they can transmit electrical signals extremely quickly—sometimes at a fraction of a millisecond—and do not require any energy input. This is often useful in circuits that are part of escape behaviors, such as that found in the crayfish that couples the sensation of a predator with the activation of the motor response.
In contrast, transmission at chemical synapses is a stepwise process. When an action potential reaches the end of the axonal terminal, voltage-gated calcium channels open and allows calcium ions to enter. These ions trigger fusion of neurotransmitter-containing vesicles with the cellular membrane, releasing neurotransmitters into the small space between the two neurons, called the synaptic cleft. These neurotransmitters—including glutamate, GABA, dopamine, and serotonin—are then available to bind to specific receptors on the postsynaptic cell membrane. After binding to the receptors, neurotransmitters can be recycled, degraded, or diffuse away from the synaptic cleft.
Chemical synapses predominate the human brain and, due to the delay associated with neurotransmitter release, have advantages over electrical synapses. First, a few or many vesicles may be released, resulting in a variety of postsynaptic responses. Second, binding to different receptors may cause an increase or decrease membrane potential in the postsynaptic cell. Additionally, the availability of neurotransmitters in the synaptic cleft is regulated by recycling and diffusion. In this way, chemical synapses achieve neuronal signaling that can be highly regulated and fine-tuned.