The Synapse

The cells of your nervous system are constantly receiving and transmitting information, from basic bodily function to sensory stimulus. Neurons communicate with electrical signals called action potentials. These action potentials originate in the cell body and travel along the axon to the axon terminal where they are passed on to the next cell.

The point at which two neurons meet is called the synapse. Electrical synapses allow direct communication between cells using gap junctions, and are often involved in coordination of rapid activity. However, most synapses are chemical synapses that contain a synaptic cleft, the physical space that exists between the neuron sending the signal, known as the presynaptic cell, and the neuron receiving it, called the postsynaptic cell.

Action potentials cannot travel across the synaptic cleft, so neurons convert the electrical signal into a chemical signal at the synapse. This is accomplished by the release of molecules known as neurotransmitters. There are many neurotransmitters, each with different effects on the postsynaptic neurons, including the excitatory glutamate and the inhibitory GABA, among others.

When the action potential reaches the presynaptic terminal, voltage-gated calcium channels on the presynaptic membrane open. Calcium rushes into the cell, which triggers the fusion of vesicles with the membrane and the release of neurotransmitters into the synaptic cleft. These are then able to bind to receptors on the postsynaptic cell.

The binding of neurotransmitters to receptors may result in and increased or decreased postsynaptic membrane potential, changing the likelihood of an action potential initiating in the postsynaptic cell. Neurons can have thousands of synapses and receive information from many cells. These signals are combined in the soma of the postsynaptic neuron where the cell determines whether or not to pass the message forward.

After briefly binding to postsynaptic receptors, neurotransmitters may diffuse away, be degraded, or recycled. Reuptake proteins on the presynaptic cell are often responsible for recycling neurotransmitters. The release and binding of neurotransmitters across synapses allow the electrical signals of action potentials to be communicated to adjacent neurons. This multi-step process is critical to neuron function.