The Synapse

The synapse is the functional connection between one neuron and another. It is the site at which information transfer takes place. There are essentially 3 parts to a synapse: the presynaptic neuron, the synaptic cleft, and the postsynaptic neuron. There are two kinds of synapses in the nervous system: electrical and chemical. Chemical synapses are the most common in mammals, as they allow for greater flexibility. While a synapse can occur between different neuronal structures, the most common one is between axon and dendrite, called axodendritic.

The presynaptic neuron is the cell that sends information (i.e., transmits chemical messages).
The postsynaptic neuron is the cell that receives information (i.e., receives chemical messages).
The synaptic cleft is the small space separating the presynaptic membrane and postsynaptic membrane (usually the dendritic spine).
Neurotransmitters are chemicals that are released by neurons and bind to receptor sites of membrane proteins.
A synaptic vesicle is an organelle that houses chemical neurotransmitters.
The cell membrane contains two layers of phospholipids that surround the cell, separating its contents from the extracellular fluid.
Microtubules are small tubes that transport molecules and help give the cell its shape.
Mitochondria are cellular structures that perform metabolic activities that provide energy.
Ions are electrically charged molecules. The movement of ions across neural membranes creates electrical activity.
A receptor is an area on the surface of a protein to which chemicals can bind. When neurotransmitters are released from the presynaptic neuron, they bind to receptors and alter the activity of the postsynaptic neuron.

Click the action potential button to see the basic processes underlying synaptic communication. When the action potential arrives at the axon terminal, vesicles are mobilized to release neurotransmitter into the synaptic cleft. The transmitter then subsequently binds to receptors on the postsynaptic target.

Knowledge of chemical synaptic transmission is central to understanding brain and behaviour, and is fundamental to neuroscience research. The plasticity of synapses is crucial to memory and other higher order brain functions, and can be altered in response to an organism's experiences. Many medical conditions affect behaviour by altering neurotransmission. Also, drugs that have psychoactive effects do so by influencing chemical reactions at synapses. Consequently, in order to understand how drugs work, it is crucial to explore the ways in which they influence synaptic action.

Upon closer inspection of the synapse, we can see the important stages of neurotransmission.

Step by Step

Synthesis: Neurotransmitters are synthesized in the neuron, and packaged into vesicles.
Release: In response to an action potential, calcium mobilizes the neurotransmitters to be released into the synaptic cleft via exocytosis.
Receptor action: The neurotransmitter crosses the synaptic cleft and binds to a receptor on the postsynaptic neuron, altering its activity.
Inactivation: The neurotransmitter is separated from the receptors and is either diffused away, converted into inactive chemicals, taken back into the presynaptic neuron, or taken up by glial cells.

See Tutorial 11 for additional information.

Types of Receptors

On the postsynaptic neuron, chemical messengers may bind to either ionotropic or metabotropic receptors. Ionotropic receptors have 2 main parts, a binding site for the neurotransmitter and a channel. When the transmitter binds to the site, the receptor changes and directly affects the flow of ions, which in turn results in rapid changes in the voltage across the membrane. Metabotropic receptors have a binding site but lack the ion channel. Consequently, binding to these types of receptors bring about indirect changes in adjacent ion channels or activate other cellular processes.

See Tutorial 12 for additional information.