Synapses

A synapse is where one neurone makes functional contact with another neurone or with an effector. Most synapses are between the axon terminals of the stimulating neurone and the dendrites of the stimulated neurone. Typically a gap, the synaptic cleft, of about 15 nm is found between the two cells. Most synapses use neurotransmitters such as acetylcholine (ACh) to cross this gap, but some synapses are purely electrical. Synapses in which the neurotransmitter is ACh are said to be cholinergic. 

Transmission Across a Cholinergic Synapse

• 1. An action potential arrives at the axon terminal. 
• 2. Calcium ion channels open and calcium ions (Ca2+) diffuse into the axon terminal (synaptic knob). 
• 3. This causes the synaptic vesicles , which contain ACh made from choline and acetylcoenzyme A, to move to the presynaptic membrane. This is an active process and needs ATP from the many mitochondria. 
• 4. The…