18 Synapse

Synapses

A synapse is the site of communication between a neuron and another cell. There are two types of synapses: chemical synapses and electrical synapses. In a chemical synapse, a chemical signal, which is called a neurotransmitter, is released from the neuron and binds to a receptor on the other cell. In an electrical synapse, the membranes of two cells directly connect through a gap junction so that ions can pass directly from one cell to the next, transmitting a signal. Both types of synapses occur in the nervous system, though chemical synapses are more common.

An example of a chemical synapse is the neuromuscular junction (NMJ), which will be described in the chapter on muscle tissue. In the nervous system, there are many additional synapses that utilize the same mechanisms as the NMJ. All chemical synapses have common characteristics, which can be summarized in Table 18.1:

Neurotransmitter Release & the Synaptic Cleft

When an action potential reaches the axon terminals, voltage-gated Ca²⁺ channels in the membrane of the synaptic end bulb (axon terminal) open. Ca²⁺ diffuses down its concentration gradient and enters into the presynaptic neuron axon terminal (end bulb). Once Ca²⁺ is inside the presynaptic end bulb, it binds to proteins that trigger exocytosis—the process of vesicles fusing with the cell membrane to release neurotransmitters into the synaptic cleft. The released neurotransmitter moves into the small gap between the cells, called the synaptic cleft.

Once in the synaptic cleft, the neurotransmitter diffuses the short distance to the postsynaptic membrane and can bind to neurotransmitter receptors. Receptors are specific for the neurotransmitter, and the two fit together like a lock and key, and so a neurotransmitter will not bind to receptors for other neurotransmitters (Figure 18.1). This process converts an electrical signal in the presynaptic neuron into a chemical signal across the synapse, and then back into an electrical signal in the postsynaptic cell.

The neurotransmitter is cleared from the synapse either by enzymatic degradation, neuronal reuptake, or glial reuptake. In some cases, surrounding glial cells (like astrocytes) remove neurotransmitters from the synaptic cleft. This ensures that the signal ends in a timely manner and prevents continuous stimulation of the postsynaptic cell. Together, these steps ensure that neurons can communicate efficiently, precisely, and briefly—allowing the nervous system to coordinate rapid and complex responses.

Agonists and Antagonists

Some drugs and toxins affect synaptic transmission by mimicking or blocking neurotransmitters. A substance that activates a receptor by mimicking a neurotransmitter is called an agonist. A substance that blocks a receptor and prevents the neurotransmitter from binding is called an antagonist. For example, nicotine is an agonist at certain acetylcholine receptors, while curare is an antagonist that blocks those same receptors at the neuromuscular junction.

Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 12.