Synapses

A synapse is the junction between two neurones, or between a neurone and an effector cell such as a muscle fibre (a neuromuscular junction). An action potential cannot cross the gap, so the signal is carried across by a chemical, a neurotransmitter.A synapse has three parts:• the presynaptic neurone ends in a swelling called the synaptic knob, which contains many mitochondria and synaptic vesicles full of neurotransmitter
• the synaptic cleft, a gap about 20 nm wide between the two cells
• the postsynaptic membrane, which has receptor proteins that bind the neurotransmitterSynapses that use acetylcholine are called cholinergic synapses.Synapses have several important functions:• One-way transmission. Only the presynaptic knob contains vesicles and only the postsynaptic membrane has receptors, so impulses pass in one direction.
• Summation. One impulse may release too little neurotransmitter to trigger an action potential. In spatial summation, several presynaptic neurones release neurotransmitter onto one postsynaptic neurone at the same time. In temporal summation, one presynaptic neurone fires several impulses in quick succession. Either way, enough neurotransmitter builds up to reach threshold.
• Integration. Some synapses are excitatory and others inhibitory, so a neurone can combine many inputs and 'decide' whether to fire.
• Filtering. Weak background stimuli are ignored if they do not release enough neurotransmitter.Because transmission at a synapse involves several separate steps, synapses are where many drugs act.