synapse

Meaning: A synapse is the site of communication between a presynaptic neuron and a postsynaptic cell. At a chemical synapse, an action potential depolarizes the axon terminal and opens voltage-gated Ca²⁺ channels. Ca²⁺ entry makes neurotransmitter-filled vesicles fuse with the membrane (exocytosis). The neurotransmitter diffuses across the synaptic cleft and binds receptors on the postsynaptic membrane. These are often ligand-gated ion channels, but they can also be receptors that act through second messengers. The signal ends when the neurotransmitter is broken down by enzymes, taken back up by the presynaptic cell, or diffuses away. Electrical synapses use gap junctions, pass ions directly, and are faster.

On the MCAT: Passages describe a toxin, drug, or mutation and ask what happens downstream. Blocking presynaptic Ca²⁺ channels stops release. Blocking reuptake or the degrading enzyme prolongs the signal. Excitatory input causes a depolarizing EPSP (for example, Na⁺ entering). Inhibitory input causes a hyperpolarizing IPSP (for example, Cl⁻ entering or K⁺ leaving). The postsynaptic neuron sums these inputs over time and space, and it fires if the membrane at the axon hillock reaches threshold. The neuromuscular junction, which uses acetylcholine, is the usual example.

Common confusion: The action potential does not cross the cleft. A chemical signal does, which is why chemical synapses have a short delay and conduct in one direction only. Also, a neurotransmitter is not excitatory or inhibitory by itself. The postsynaptic receptor and the ion it admits decide the effect.