synapse

The synapse is the connection between nerve cells (neurons) in animals including humans. The synapse joins the axons in one neuron to the dendrites in another. Here is a diagram showing how the synapse connects axons to dendrites:

The synapse consists of:

• The presynaptic terminal at the end of an axon. This contains tiny vesicles which contain neurotransmitters - the small molecules which carry the nerve impulse from the sending neuron to the receiving neuron.

• The synaptic cleft - a gap between the two neurons across which the neurotransmitters migrate.

• The postsynaptic terminal usually in the dendrites of receiving neurons. This contains receiving sites for the neurotransmitters.

Nerve impulses are transmitted down to the presynaptic terminal in the axon of one neuron and across the synaptic cleft to the postsynaptic terminal in the dendrite of another neuron.

Synapses do not only join axons to dendrites (axodendritic synapses) - they can also joins axons to other axons (axoaxonic synapses) or to the soma - the neuronal cell body - (axosomatic synapses).

A typical neuron in the central nervous system (CNS) has around 100,000 synapses.

The effect of the release of neurotransmitters across the synapse may either be excitory - making the triggering of a nerve impulse (action potential) in the receiving neuron more likely - or inhibitory - making it less likely.

An action potential is an on-off event and is always -55 millivolts - there are no grades of nerve impulse. Neurotransmitters, on the other hand, are additive - if the net effect of all the excitory neurotransmitters minus all the inhibitory ones achieves this threshold then an action potential will be initiated.

Synapse links:

Neuroscience for Kids - The Synapse

Neuron Dendrite Spine & Synapse

Neuron Forming a Chemical Synapse

Transmission of Signals through a Neuron and Across a Synapse

Neural Synapses