A synapse is a specialized junction through which a neuron communicates with another neuron or a target cell, such as a muscle cell. Synapses connect individual cells into functional circuits in the brain and other parts of the nervous system. They transmit signals, influence whether receiving cells become active, and permit changes in the effectiveness of communication. Two principal forms are recognized: chemical synapses, which use neurotransmitters, and electrical synapses, which conduct current directly between cells. (ncbi.nlm.nih.gov)
Structure and organization
At a chemical synapse, the presynaptic component releases a transmitter, while the postsynaptic component contains receptors that respond to it. Their cell membranes are separated by a narrow extracellular space, the synaptic cleft, typically about 20 nanometers wide in mammalian brain synapses. The presynaptic terminal contains transmitter-filled synaptic vesicles and specialized release sites called active zones. On the receiving side, receptors and associated proteins are organized into molecular assemblies; excitatory synapses commonly have a prominent postsynaptic density visible by electron microscopy. (pmc.ncbi.nlm.nih.gov)
Many neuronal synapses join an axon terminal to a dendrite, often on a small protrusion called a dendritic spine. Others contact the cell body or another axon. These arrangements allow signals to influence different aspects of neuronal processing, including the integration of incoming activity and the release of transmitter from another terminal. Synaptic structure varies considerably across cell types and brain regions. (pmc.ncbi.nlm.nih.gov)
Chemical transmission
Chemical transmission commonly begins when an action potential reaches a presynaptic terminal. The resulting membrane depolarization opens voltage-gated calcium channels. Calcium ions enter the terminal and trigger vesicles to fuse with its membrane, releasing transmitter into the cleft. The transmitter travels by diffusion and binds to postsynaptic receptors. Because release machinery and receiving receptors occupy different sides of the junction, this process normally transmits information in one direction. (ncbi.nlm.nih.gov)
Transmitter release is organized into discrete packets associated with individual vesicles. A presynaptic impulse therefore does not necessarily produce an identical response every time: transmission depends on vesicle availability, release probability, and the receiving cell’s responsiveness. Vesicle membrane is subsequently retrieved and recycled, allowing terminals to sustain repeated signaling. (ncbi.nlm.nih.gov)
Two broad receptor classes produce different responses. Ionotropic receptors are transmitter-gated ion channels and generally produce rapid changes in membrane conductance. Metabotropic receptors activate intracellular signaling pathways that can alter channels and other cellular processes, usually with slower and longer-lasting effects. A transmitter’s action depends on its receptor and the receiving cell’s ionic conditions, not simply on the transmitter’s identity. (ncbi.nlm.nih.gov)
Signaling ends as transmitter leaves the receptor region through diffusion, transport back into neurons or surrounding glial cells, or breakdown by enzymes. The relative importance of these mechanisms differs among transmitters. Clearance limits the duration and spread of signaling and can permit transmitter components to be reused. (ncbi.nlm.nih.gov)
Electrical transmission
Electrical synapses connect cells through gap junctions, whose channels link their interiors. Current passes through these channels without an intervening transmitter-release step, allowing transmission with very little delay. Many electrical synapses conduct in both directions, although some favor one direction. They can convey both depolarizing and hyperpolarizing changes and help coordinate activity among connected neurons. (pmc.ncbi.nlm.nih.gov)
Electrical and chemical transmission are not mutually exclusive. Some neuronal contacts contain both kinds of junction, forming mixed synapses. Electrical coupling can also be modified by cellular signaling and activity; it is not merely a fixed connection. Interactions between electrical and chemical synapses contribute to the organization and operation of neural circuits. (pmc.ncbi.nlm.nih.gov)
Excitation, inhibition, and integration
An excitatory synaptic response increases the likelihood that a receiving neuron will fire an action potential; an inhibitory response reduces it. These effects arise from changes in membrane conductance and depend on which ions can cross the membrane and their electrochemical gradients. Inhibition may hyperpolarize the cell or reduce the effectiveness of simultaneous excitatory inputs without producing a large voltage change. (ncbi.nlm.nih.gov)
Most individual synaptic responses are graded local signals rather than action potentials. Neurons combine activity arriving at different locations through spatial summation and activity arriving in rapid succession through temporal summation. Synapse location, response timing, and membrane properties affect how strongly each input influences firing. A neuron’s output consequently reflects the integration of multiple inputs rather than the simple relay of a single incoming impulse. (ncbi.nlm.nih.gov)
Plasticity
Synaptic effectiveness can change with previous activity, a property called synaptic plasticity and an important component of neuroplasticity. Changes may involve transmitter release, receptor abundance or function, and synaptic structure. Long-term potentiation produces a persistent increase in synaptic effectiveness, whereas long-term depression produces a persistent decrease. These processes are studied as cellular mechanisms contributing to learning and memory, rather than as complete explanations of either. (ncbi.nlm.nih.gov)
At many excitatory synapses in the hippocampus, NMDA receptors admit calcium under appropriate conditions and help initiate lasting modifications. Plasticity mechanisms differ among synapses, however, and changes in synaptic strength are influenced by previous activity and broader cellular regulation. (ncbi.nlm.nih.gov)
Neuromuscular synapses and disease
The neuromuscular junction is a chemical synapse between a motor neuron and a skeletal muscle fiber. Its transmitter, acetylcholine, activates receptors on the muscle membrane and initiates excitation leading to contraction. In myasthenia gravis, antibodies interfere with acetylcholine receptors or other junctional proteins, impairing nerve-to-muscle transmission. This illustrates how disruption of a particular synaptic component can alter the function of an otherwise connected nerve and target cell. (ninds.nih.gov)