What is a synapse and how is information transmitted across it?

Synapse = the junction between two neurones (nerve cells) OR the junction between a neuron and an effector cell such as a muscle cell

A synapse between two neurones consists of:

• a presynaptic neuron
  
• a post synaptic neuron

The presynaptic neuron has a synaptic terminal which is in close contact with the dendrites of the post synaptic neuron. The two neurons don't actually touch. The space between them is called the synaptic cleft.

When an impulse is sent down a neuron an action potential is generated and propagated.

Action potential = a change in the electrical potential (charge) of a neuron as an impulse is transmitted along it.

REMEMBER: at rest the inside of a neurone is negatively charged with respect to the outside.

The membrane of the synaptic terminal contains voltage gated calcium channels (this means they open when there is a change in voltage). When the action potential arrives at the synaptic cleft the charge inside the neurone becomes positive causing the calcium channels to open and calcium to enter the synaptic terminal.

Vesicles containing acetylcholine (a type of neurotransmitter) are present in the synaptic terminal and the influx of calcium causes these to release neurotransmitter into the synaptic cleft.

These neurotransmitter molecules are able to bind to ligand gated channels on the postsynaptic neuron causing them to open, allowing an influx of positive ions (usually calcium in the case of neurons). If this was a muscle cell (an effector cell) rather than a post synaptic neurone the positive ion would likely be sodium.