To prevent continuous stimulation of the postsynaptic neuron, neurotransmitters must be rapidly removed from the cleft. In cholinergic synapses, the enzyme acetylcholinesterase hydrolyzes acetylcholine into choline and ethanoic acid (acetate).
The breakdown products are reabsorbed into the presynaptic knob via active transport. Once inside, they are recombined into acetylcholine using ATP from mitochondria and stored back into vesicles for future use.
This recycling process is essential for maintaining the sensitivity of the synapse and ensuring that signals remain discrete rather than merging into a single, constant stimulus.
Synapses ensure unidirectionality because the structural components are asymmetrical. Neurotransmitter vesicles are only found in the presynaptic knob, while the specific receptors are only located on the postsynaptic membrane.
Divergence occurs when a single presynaptic neuron connects to multiple postsynaptic neurons, allowing one signal to be broadcast to various parts of the nervous system simultaneously.
Convergence is the opposite, where multiple presynaptic neurons terminate on a single postsynaptic neuron. This allows the postsynaptic cell to integrate information from multiple sources before 'deciding' whether to fire an action potential.
Summation is the process by which multiple sub-threshold stimuli are added together to reach the threshold potential required for an action potential. This acts as a filtering mechanism to ignore low-level 'background noise' in the nervous system.
Temporal summation occurs when a single presynaptic neuron fires multiple impulses in rapid succession. The neurotransmitter builds up in the cleft faster than it can be broken down, eventually opening enough sodium channels to reach the threshold.
Spatial summation involves multiple different presynaptic neurons releasing neurotransmitters simultaneously at different locations on the same postsynaptic neuron. Their combined effect triggers the action potential.
Identify the Ion: Always distinguish between (presynaptic, triggers vesicle fusion) and (postsynaptic, triggers depolarization). Confusing these is a very common error.
Sequence Matters: Be prepared to describe the step-by-step pathway from the arrival of the action potential to the generation of the new one. Use terms like 'diffusion', 'exocytosis', and 'binding to receptors'.
Explain Unidirectionality: If asked why signals only travel one way, mention the specific location of vesicles vs. receptors. A simple 'because of the gap' is insufficient for full marks.
Summation Logic: Remember that summation is about reaching the threshold potential. If the total depolarization does not reach this specific voltage, no action potential will occur regardless of how many synapses are active.
