A synapse is a connection which allows for the transmission of nerve impulses. Synapses can be found at the points where nerve cells meet other nerve cells, and where nerve cells interface with glandular and muscular cells. In all cases, this connection allows for the one-way movement of data. The human body contains trillions of these connections, and at any given time, huge numbers of these connections are active.
Understanding the anatomy of a nerve cell can be helpful when learning about synapses. Each nerve cell consists of a cell body, with attached axons and dendrites. These branching structures carry information to and from the cell body. Dendrites bring information in, with the use of receptors on their tips which collect information from other cells. Axons bring information away by sending signals across the synapse between the nerve cell and another cell.
In most cases, the connection is chemical, which means that neurotransmitters are passed along the connection, in contrast with an electrical synapse, in which electrical signals are transmitted. In the case of a chemical synapse, the chemicals are released by the vesicles on the ends of the axons of a nerve cell. These vesicles open in response to signals which enter the nerve cell through its dendrites, and the opening results in the release of neurotransmitters which connect with receptors on the destination cell.
Chemical synapses take the form of a structure called the synaptic cleft, which is actually a tiny gap between the cells, while electrical synapses involve cell to cell contact for the purpose of transmitting data. The speed at which impulses travel across the connections can vary, depending on the cell and the type of signals. Once the neurotransmitter has moved across the connection, it is broken down by enzymes to ensure that the signal is only transmitted once, and to clear the way for the next neurotransmitter.
Synapses can carry a wide assortment of types of information, ranging from a message from the brain to do something to a signal from an extremity to alert the brain to an ongoing situation. Neurons are very responsive to sensory input, and they can send back information about sensory input to the brain at remarkable speeds. The neurons within the brain can also rapidly process and shuffle information so that the brain can respond in the fraction of a second, sometimes before the brain's owner is even consciously aware of an event.