synaptic transmission

Meaning: At a chemical synapse, an action potential in the presynaptic terminal opens voltage-gated Ca²⁺ channels. Ca²⁺ entry makes synaptic vesicles fuse with the membrane and release neurotransmitter into the cleft. It binds postsynaptic receptors, opening ion channels. Na⁺ entry depolarizes (EPSP); Cl⁻ entry or K⁺ exit hyperpolarizes (IPSP). The signal ends by enzymatic breakdown, reuptake, or diffusion. Electrical synapses pass current through gap junctions without neurotransmitter.

On the MCAT: Expect questions on what happens when Ca²⁺ is removed (release falls), or when a drug blocks reuptake or the degrading enzyme (neurotransmitter lingers, so the response is larger or longer). Postsynaptic potentials are graded and summate, spatially and temporally, at the axon hillock; threshold decides whether an action potential fires. Chemical synapses usually conduct one way, since vesicles are presynaptic and receptors postsynaptic.

Common confusion: The action potential does not cross the cleft at a chemical synapse; the signal is converted to a chemical one and back. A neurotransmitter is also not excitatory or inhibitory by itself: the receptor and the ion channel it controls set the effect. Acetylcholine, for example, excites skeletal muscle but slows the heart.