Synapse

In the nervous system, a synapse^[1] is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending on the mechanism of signal transmission between neurons. In the case of electrical synapses, neurons are coupled bidirectionally with each other through gap junctions and have a connected cytoplasmic milieu.^[2][3][4] These types of synapses are known to produce synchronous network activity in the brain,^[5] but can also result in complicated, chaotic network level dynamics.^[6][7] Therefore, signal directionality cannot always be defined across electrical synapses.^[8]

Chemical synapses, on the other hand, communicate through neurotransmitters released from the presynaptic neuron into the synaptic cleft. Upon release, these neurotransmitters bind to specific receptors on the postsynaptic membrane, inducing an electrical or chemical response in the target neuron. This mechanism allows for more complex modulation of neuronal activity compared to electrical synapses, contributing significantly to the plasticity and adaptable nature of neural circuits.^[9]

Synapses are essential for the transmission of neuronal impulses from one neuron to the next,^[10] playing a key role in enabling rapid and direct communication by creating circuits. In addition, a synapse serves as a junction where both the transmission and processing of information occur, making it a vital means of communication between neurons.^[11]

At the synapse, the plasma membrane of the signal-passing neuron (the presynaptic neuron) comes into close apposition with the membrane of the target (postsynaptic) cell. Both the presynaptic and postsynaptic sites contain extensive arrays of molecular machinery that link the two membranes together and carry out the signaling process. In many synapses, the presynaptic part is located on the terminals of axons and the postsynaptic part is located on a dendrite or soma. Astrocytes also exchange information with the synaptic neurons, responding to synaptic activity and, in turn, regulating neurotransmission.^[10] Synapses (at least chemical synapses) are stabilized in position by synaptic adhesion molecules (SAMs)[1] projecting from both the pre- and post-synaptic neuron and sticking together where they overlap; SAMs may also assist in the generation and functioning of synapses.^[12] Moreover, SAMs coordinate the formation of synapses, with various types working together to achieve the remarkable specificity of synapses.^[11][13] In essence, SAMs function in both excitatory and inhibitory synapses, likely serving as the mediator for signal transmission.^[11]