Neuronal synapses

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A synapse is a junction between two cells, serving as the primary site of communication. Neurons can form synapses with other neurons or with effector cells, such as muscles and glands. These neuronal synapses are categorized into two types: electrical and chemical synapses.

Electrical synapses facilitate direct communication between cells through protein channels known as connexons, allowing ions to pass rapidly between them. As a result, electrical synapses are fast and bidirectional.

Chemical synapses, in contrast, are slower and unidirectional. They involve the release of neurotransmitters from vesicles in the presynaptic neuron. When an action potential reaches the presynaptic neuron terminal, sodium ions enter the cell via voltage-gated sodium channels. This results in a change in the cell's membrane potential, which triggers the opening of voltage-gated calcium channels. The influx of calcium causes vesicles to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft, the gap between the presynaptic and postsynaptic cells.

The neurotransmitters bind to neuroreceptors on the postsynaptic membrane. There are two kinds of neuroreceptors: ionotropic and metabotropic receptors. Ionotropic receptors are ligand-gated ion channels that open directly in response to the binding of a neurotransmitter, resulting in a fast and short-lived response. On the other hand, metabotropic receptors are G-protein coupled receptors which use second messengers to indirectly open ion channels, resulting in a slow and long-lasting response.

Neurotransmitters can have either excitatory or inhibitory effects on the postsynaptic neuron. Excitatory postsynaptic potentials (EPSPs) result from depolarization, increasing the likelihood of action potential generation in the neuron. In contrast, inhibitory postsynaptic potentials (IPSPs) are produced by hyperpolarization, decreasing the likelihood of action potential initiation.

Postsynaptic potentials are graded potentials, capable of summation. Temporal summation occurs when multiple signals from a single presynaptic neuron arrive in rapid succession, amplifying the overall effect. Spatial summation happens when signals from multiple presynaptic neurons converge on a single postsynaptic neuron, leading to a cumulative effect. Summation can involve both excitatory and inhibitory signals, determining the overall response of the postsynaptic neuron.

Watch the following video to understand how neurons communicate using synapses.

Explore concepts

Electrical vs chemical synapses

Functionally, synapses could be electrical or chemical. Learn about their differences with these images.

Structure of a chemical synapse

Synaptic transmission

Neurons communicate across chemical synapses using neurotransmitters. Learn the events that take place during that process step-by-step.

Postsynaptic potentials

Depending on the receptor, neurotransmitters could excite or inhibit the postsynaptic neurons generating postsynaptic potentials.

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Summary