Synapse

A synapse or synaptic junction is a small gap at the end of a neuron and the region of contact between two adjacent neurons. There are billions of neurons or nerve cells in the human brain acting as an information-processing system. Each neuron comprises an axon or output end, a cell body, and dendrites or input end. To communicate, nerve impulses must send signals across the synaptic gap but cannot pass directly from one to another. With the aid of a neurotransmitter, messages travel from the axon of one neuron to the nearby dendrites or receptor area of the next neuron. Similar to a lock and key or jigsaw puzzle, however, each neurotransmitter fits only into a tailor-made receptor area on the surface of the target neuron. Derived from the Greek words sun and haptein—together and join, and coined by physiologist Michael Foster in his 1897 Textbook of Physiology—synapse is the space across which nerve cells join together to communicate.

Background

Nerve cells called neurons are the primary functional units or building blocks of the brain and nervous system. While neurons are similar to other cells in the human body, the unique structure of neurons allows them to receive and send information to other neurons through chemical and electric signals. All thoughts, feelings, sensations, and movements result from signals passing between neurons or from neurons to cells of the glands or muscles.

The nucleus, where most of the neuron's molecules are manufactured, is housed in the cell body. Axons take information away from the cell body while special projections called dendrites, which extend out from cells like tree branches, bring information to the cell body. Axons can be short or long, carrying signals to adjacent cells or extending, for example, from the brain down the spinal cord. The body's longest axon, measuring approximately three feet, runs from the bottom of the spine down to the big toe. Dendrites can also be long or short and sparsely or highly branched. Some neurons have only one dendrite while others have many dendrites. Central nervous system dendrites are particularly long and complex, capable of receiving signals from thousands of other neurons.

For information to flow, it must cross the gap between nerve cells, the synapse. There are three parts to the synapse: a pre-synaptic ending that houses neurotransmitters; a post-synaptic ending with receptor sites for neurotransmitters; a synaptic cleft or the space between one nerve cell's pre-synaptic ending and the next nerve cell's post-synaptic ending.

When a brain signal reaches the end of an axon, it triggers the release of a small sac of chemicals—the neurotransmitters—stored in synaptic vesicles at the end of the axon. These neurotransmitters cross the synapse and attach themselves to the receptors of a neighboring cell. Neuroreceptors have specific binding sites for specific neurotransmitters. Once a certain threshold is reached, an electric impulse called action potential is fired. If the receiving cell is also a neuron, transmission of a signal continues to the next cell in a chain-ball fashion. In what is known as the all-or-none law, neurons never fire partially; action potentials either occur or do not occur. This ensures that neurons always fire at full strength, and that signals are carried at their full intensity and do not weaken or get lost as they travel from source to destination.

Overview

There are two primary types of synapses: chemical ones and electrical ones. In a chemical synapse process, electrical activity in a presynaptic neuron triggers the release of neurotransmitters that bind to special receptors of the postsynaptic cell. Neurotransmitters can either excite or inhibit the postsynaptic cell, leading to a firing of action potential or to the prevention of signal transmission.

In an electrical synapse process, two neurons are connected by special channels called gap junctions; gap junctions allow positive current from the presynaptic neuron to directly reach the postsynaptic cell. As a result, electrical signals between pre- and postsynaptic cells travel much faster than chemical synapses, rapidly speeding up the transfer of signals. Transmission speeds in chemical synapses can take several milliseconds; electrical synapse transmissions are almost instantaneous. The gap between electrical synapses averages about 3.5 nm; the gap between chemical synapses averages about 20 nm. Electrical synapses are solely excitatory while chemical synapses can be excitatory or inhibitory.

Despite their speed advantage, the strength of electrical synapse signals diminishes as they travel from one cell to the next, so that it can take a large presynaptic neuron to influence a smaller postsynaptic cell. Chemical synapse signals are slower but stronger; even a small presynaptic neuron can influence large postsynaptic cells.

Neurotransmitters, also known as chemical messengers, are responsible for much of human daily functioning and experiences, affecting processes such as learning, muscle contraction, emotions, pain perception, and other functions. Scientists have identified more than one hundred neurotransmitters, the primary ones being acetylcholine, GABA, endorphins, dopamine, and serotonin.

Acetylcholine, also known as the excitatory neurotransmitter, governs muscle contraction and affects hormone secretion, learning, and memory function. A lack or shortage of acetylcholine in the brain is associated with Alzheimer’s disease.

GABA, short for gamma-aminobutyric acid, is an inhibitory neurotransmitter that can make cells less excitable. Helping control muscle activity, drugs that increase GABA levels are used to treat individuals with Huntington's disease or epileptic seizures. GABA levels are also an important part of the visual system.

Endorphins, chemical messengers released by the human body in response to fear or trauma, are associated with emotions and pain perception. They act in a similar manner as opiate drugs such as morphine, but they are stronger than the opiate drugs.

Dopamine is another inhibitory neurotransmitter and is associated with mood, pleasurable feelings, thought, and control of complex movements. Too much or too little dopamine manifests in disorders such as schizophrenia (excess dopamine) and Parkinson's disease (dopamine deficit). Medications used to treat these conditions modify the action of dopamine in the brain.

Serotonin is involved in temperature regulation and stimulation of smooth muscles; it affects sleep and the production of blood vessels.

In a healthy brain, synapse functions operate quickly and automatically. However, problems can occur when drugs and chemicals such as morphine, cocaine, LSD, alcohol, or strychnine interfere, leading to over-excitation or inhibition of synapse function. Neurotransmitters are also susceptible to the effects of fatigue, oxygen deficiency, anesthetics, toxic chemicals, certain therapeutic drugs, and conditions such as Alzheimer's disease. Since the brain learns by adding and deleting connections between neurons and by altering the strength of neural connections, human experience is significantly altered by synapse and neurotransmitter actions.

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