Cholinergic synapses

• A synapse is a junction that transfers an action potential between two neurones or a neurone and a muscle fibre
• A synapse is made up of a presynaptic neurone and a postsynaptic neurone, separated by a synaptic cleft
• The end of the presynaptic neurone is called the synaptic knob
• The synaptic knob is filled with mitochondria and synaptic vesicles that store neurotransmitter molecules
• The presynaptic membrane contains voltage gate sodium ion channels and voltage gated calcium ion channels
• The postsynaptic membrane contains sodium channels with receptor sites, complementary to the neurotransmitter
• Synapses are unidirectional
• Summation can be temporal or spatial.
• When an action potential arrives at the presynaptic neurone, the voltage gated sodium ion channels open, and sodium ions enter the neurone, depolarising the synaptic knob and triggering an action potential
• The action potential in the synaptic knob causes the voltage gated calcium ion channels to open, and calcium ions travel down their electrochemical gradient
• Vesicles in the presynaptic neurone are filled with neurotransmitter molecules. Next, when the synapse is a cholinergic synapse, these molecules are acetylcholine molecules
• An influx of calcium ions cause synaptic vesicles to move down the synaptic knob and fuse with the presynaptic membrane, releasing acetylcholine molecules into the synaptic cleft
• Synapses that use acetylcholine as a neurotransmitter are called cholinergic

  synapses
• Acetylcholine diffuse through the synaptic cleft until they reach the sodium ion channels on the postsynaptic membrane
• Sodium ion channels on the postsynaptic membrane have two receptor sites, specific for acetylcholine
• The binding of two acetylcholine molecules changes the sodium ion channel's tertiary structure, opening it and allowing sodium ions into the postsynaptic neurone
• The diffusion of sodium ions down their electrochemical gradient depolarises the postsynaptic membrane, until the threshold value is reached, where an action potential occurs
• The synaptic cleft is short, meaning there is a short diffusion pathway for acetylcholine
• Once an action potential is triggered, the acetylcholine molecules are removed by the enzyme acetylcholinesterase. This enzyme hydrolyses them, producing choline and ethanoic acid
• Choline and ethanoic acid diffuse back across the synaptic cleft to be recycled into acetylcholine by an enzyme
• Choline is charged so requires facilitated diffusion via a transport protein
• Ethanoic acid is not charged so can pass into the presynaptic membrane by simple diffusion
• ATP is used to actively transport acetylcholine into a vesicle
• Ethanoic acid is also known as acetic acid