| Comments |
"Synaptic transmission is initiated when an action potential triggers neurotransmitter
release from a presynaptic nerve terminal (Katz 1969). An action potential
induces the opening of Ca2+ channels, and the resulting Ca2+ transient stimulates
synaptic vesicle exocytosis (Figure 1). After exocytosis, synaptic vesicles
undergo endocytosis, recycle, and refill with neurotransmitters for a new round of
exocytosis. Nerve terminals are secretory machines dedicated to repeated rounds
of release. Most neurons form >500 presynaptic nerve terminals [BNID 111276] that are often
widely separated from the neuronal cell bodies. Action potentials, initiated in the
neuronal cell body, travel to all of the cell body’s nerve terminals to be transformed
into synaptic secretory signals. Nerve terminals do not convert reliably
every action potential into a secretory signal but are “reliably unreliable” (primary source). In most terminals, only 10%–20% of action potentials trigger
release. The relationship between action potentials and release in a nerve terminal is regulated by intracellular messengers and extracellular modulators and
is dramatically altered by repeated use of a synapse. Thus in addition to secretory
machines, nerve terminals are computational units where the relation of input
(action potential) to output (neurotransmitter release) continuously changes in response
to extra- and intracellular signals." |