From EvoWiki

A synapse is a subcellular structure specialized for communication between adjacent animal cells. Most synapses use chemicals as messenger molecules that pass between cells at the synapse. Some synapses form cytoplasmic bridges between the adjacent cells and ions pass between the two cells.

The chemicals that pass between cells at synapses are usually called neurotransmitters. There are several classes of neurotransmitters that can be distinguished by their chemical structures. Several of the most important neurotransmitters are amino acids. The functional properties of neurotransmitters generally depend on receptor proteins that physically bind the neurotransmitters in a lock-and-key manner. The two major categories of neurotransmitter receptors are the inhibitory and excitatory receptors. When excitatory receptors bind a neurotransmitter, the cell is more likely to produce new signals that will pass to other cells. When inhibitory receptors are activated, the cell becomes less likely to produce new signals that will pass to other cells.

Many neurotransmitters are stored inside vesicles that are often visible in electron micrographs of synapses. In such synapses, release of neurotransmitter into the extracellular space between the two adjacent cells depends on exocytosis. Electrical activity in the cell containing the neurotransmitter vesicles causes the vesicles to fuse with the cell surface membrane, allowing the neurotransmitter to escape from the cell. Once outside the cell, the neurotransmitter molecules diffuse through the extracellular space to reach receptor proteins.

An interesting evolutionary difference between invertebrates and vertebrates is that some invertebrates use both excitatory and inhibitory synapses to control skeletal muscle contraction. Vertebrates only use excitatory synapses to control skeletal muscle contraction.

Changes in the structure and function of synapses between neurons are particularly important for learning and memory formation. Animals with a high capacity to learn depend heavily on changes in synapse structure and function that make it possible for behaviors to change in response to the demands of the environment. An important aspect of human evolution has been changes in the brain that allow for extensive synaptic plasticity and social learning. It is common for young social mammals to have a juvenile period of greater synaptic plasticity followed by an adult phase of life with reduced capacity for learning. The human juvenile period is long and adult human brains retain many of the characteristics of a juvenile brain, allowing life-long learning.

External Links

• Wikipedia article on synapses [1].