The most common form of synapse between neurons is the
chemical synapses. At chemical synapses, a narrow (= 20 nm), water-filled space
called the synaptic cleft separates the presynaptic neuron from the postsynaptic
neuron. Neurotransmitter molecules are used by the presynaptic neuron to send
a message across the cleft to the postsynaptic neuron. The neurotransmitter molecules
are stored in synaptic vesicles located in the expanded tips (= synaptic knobs)
of the presynaptic neuron.
(A) An action potential at the synaptic knob signals the synaptic vesicles to
fuse with the presynatpic membrane and release their contents into the synaptic
cleft. Some of the neurotransmitters diffuse across the synaptic cleft and bind
with receptors in the postsynaptic membrane. (B) In response, membrane channels
open and ions (Na+, K+, & Cl-) enter and exit the postsynatpic neuron. The exchange
of ions excites (depolarizes) or inhibits (hyperpolarizes) the postsynatpic neuron.
Transmission at chemical synapses is slower than the rate at which action potentials move down axons. The reduced rate of current flow at the synapse is known as synaptic delay.
