The synapse is the junction at which neurons trade information. It is not a physical component of a cell but rather a name for the connection between two cells: the presynaptic cell (giving the signal) and the postsynaptic cell (receiving the signal). There are two types of possible reactions at the synapse: a chemical reaction and an electrical reaction. During a chemical reaction, a chemical called a neurotransmitter is released from one cell into another . In an electrical reaction, the electrical charge of one cell is influenced by another .
All synapses have a few common characteristics:
- Presynaptic cell: a specialized area within the axon of the giving cell that transmits information to the dendrite of the receiving cell
- Synaptic cleft: the small space at the synapse that receives neurostransmitters
- G-protein coupled receptors: receptors that sense molecules outside the cell and thereby activate signals within it
- Ligand-gated ion channels: receptors that are opened or closed in response to the binding of a chemical messenger
- Postsynaptic cell: a specialized area within the dendrite of the receiving cell that contains receptors designed to process neurotransmitters
The Chemical Synapse
Chemical reactions require chemical messengers such as neurotransmitters to operate, but they also involve electrical modifications at the postsynaptic membrane.
A basic chemical reaction at the synapse undergoes a few steps:
- The process begins with a wave of electrochemical excitation called an action potential, which travels along the membrane of the presynaptic cell until it reaches the synapse.
- The electrical depolarization of the membrane at the synapse causes channels to open that are permeable to certain ions.
- The ions flow through the presynaptic membrane, rapidly increasing their concentration in the interior.
- The high concentration activates a set of ion-sensitive proteins attached to vesicles that contain a neurotransmitter chemical.
- These proteins change shape, causing the membranes of some "docked" vesicles to fuse with the membrane of the presynaptic cell, thereby opening the vesicles and dumping their neurotransmitter contents into the synaptic cleft, the narrow space between the membranes of the pre- and post-synaptic cells.
- The neurotransmitter diffuses within the cleft. Some of it escapes, but some of it binds to chemical receptor molecules located on the membrane of the post-synaptic cell.
- The binding of neurotransmitter causes the receptor molecule to be activated in some way. Several types of activation are possible, described in more detail below. In any case, this is the key step by which the synaptic process affects the behavior of the postsynaptic cell.
- Due to thermal shaking, neurotransmitter molecules eventually break loose from the receptors and drift away.
- The neurotransmitter is either reabsorbed by the presynaptic cell and then repackaged for future release, or else it is broken down metabolically.
The Electrical Synapse
The process of an electrical reaction at the synapse is similar to a chemical reaction, but with important differences. For example, although their simplicity results in a faster reaction, electrical synapses can only produce simple behaviors compared to the more complex chemical synapses.
Notable effects of electrical synapses include the following:
- Electrical synapses are faster than chemical synapses because receptors do not need to recognize chemical messengers. The synaptic delay for a chemical synapse is typically about 2 milliseconds, while the synaptic delay for an electrical synapse may be about 0.2 milliseconds.
- Because electrical synapses do not involve neurotransmitters, electrical neurotransmission is less modifiable than chemical neurotransmission.
- The response is always the same sign as the source. For example, depolarization of the pre-synaptic membrane will always induce a depolarization in the post-synaptic membrane, and vice versa for hyperpolarization.
- The response in the postsynaptic neuron is generally smaller in amplitude than the source. The amount of attenuation of the signal is due to the membrane resistance of the presynaptic and postsynaptic neurons.
- Long-term changes can be seen in electrical synapses. For example, changes in electrical synapses in the retina are seen during light and dark adaptations of the retina.