A synapse is the space between two neurons, the presynaptic and the postsynaptic neuron, and it is the channel through which neurotransmitters and electric activity are released. There are two kinds of synapses in the brain: electrical and chemical synapse. This article will look at the electrical synapse.
Electrical synapses are relatively simple compared to the chemical synapse. They occur at specialized sites called gap junctions, which are spanned by proteins called connexins. Six connexins come together to form a connexon, and two connexons form a gap junction channel. This channel allows ions to pass from one cell to the next. Most gap junctions allow ionic currents to pass in either direction, towards either cells, unlike chemical synapses which allow neurotransmitters to be released in one direction only (which is why chemical synapses are labeled as being unidirectional). Because electrical currents, in the form of ions, can pass through these channels, cells connected by gap junctions are known as being electrically coupled.
When two neurons are electrically coupled, an action potential in the presynaptic neuron causes a current to flow across the gap junction into another neuron. This current causes a postsynaptic potential (PSP), which then can induce a PSP in the presynaptic neuron. By itself, a PSP is not large enough to induce an action potential in the postsynaptic neuron, but coupled, several PSPs can induce an action potential (nerve impulse) in a neuron. This is considered to be synaptic integration, where the neurons work together to induce an action potential.
Electrical synapses vary in role according to where in the brain they are located. They are often found in regions where the activity of neighboring neurons are highly synchronized (going back to the idea of synaptic integration). Gap junctions are especially common between neurons during the embryonic stage of neural development.
Most synaptic transmission in the human adult brain is chemical in nature, as opposed to electrical. The presynaptic and postsynaptic chemical synapse membranes are separated by a synaptic cleft that is filled with fibrous protein that help to make the pre- and postsynaptic membranes adhere to each other. This adherence allows the neurotransmitters, which are the chemical messengers of the brain, to pass freely and target certain areas on the postsynaptic membrane more easily. The presynaptic side is filled with synaptic vesicles which are filled with the neurotransmitters.
In the central nervous system (CNS), which is composed of the brain and spinal cord, different synapse types may be described by where they are on a neuron. If the postsynaptic membrane is on a dendrite, the synapse is called axodendritic. If the postsynaptic membrane is on the cell body, the synapse is called axosomatic. In some cases, the postsynaptic membrane is on another axon, and the synapse then is called axoaxonic. Sometimes, dendrites form synapses with one another and so these synapses are called dendrodritic.
Synapses also exist outside the CNS, such as on muscle and cardiac cells. Chemical synapses between axons of motor neurons of the spinal cord and skeleton muscles are called neuromuscular junctions. The postsynaptic membrane is then called a motor-end plate. Neuromuscular synaptic transmission is fast, as can be evidenced by the quick movements your muscles can make, such as during a reflex. The neurosmuscular junction has clinical significance: disease, drugs, toxins, etc. can interfere with these chemical synapses and can have a significant effect on bodily function, such as the heart and respiration.