In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another cell (neural or otherwise). Synapses are essential to neuronal function, The key to neural function is the synaptic signaling process. Synaptic signals to other neurons are transmitted by the axon; signals from other neurons are received by the soma and dendrites. Synapses can be excitatory or inhibitory and will either increase or decrease activity in the target neuron. The human brain contains about 100 billion neurons and 100-500 trillion synapses; each neuron may have thousands of input synaptic connections. It is the complex integration of these synaptic signals that controls all of the body functions including learning, memory, sensory integration, motor coordination, and emotional responses.
In an electrical synapse, the presynaptic and postsynaptic cell membranes are connected by special channels called gap junctions that are capable of passing electric current, causing voltage changes in the presynaptic cell to induce voltage changes in the postsynaptic cell. The main advantage of an electrical synapse is the rapid transfer of signals from one cell to the next. In a chemical synapse, the presynaptic neuron releases a chemical called a neurotransmitter that binds to receptors located in the postsynaptic cell, usually embedded in the plasma membrane. The neurotransmitter may initiate an electrical response or a secondary messenger pathway that may either excite or inhibit the postsynaptic neuron. Because of the complexity of receptor signal transduction, chemical synapses can have complex effects on the postsynaptic cell.