Question

describe how the neurotransmitter is synthesized in the presynaptic neuron, how it is released (mechanism of vesicular release), how it affects the postsynaptic cell (excitation or inhibition) and how it is degraded or recycled.

Answer 1

There are two types of neurotransmitters small-molecule, rapidly acting transmitters and the neuropeptides which are of much larger molecular size that usually acts much slowly. The small-molecule, rapidly acting transmitters cause most acute responses of the nervous system, such as transmission of sensory signals to the brain and of motor signals back to the muscles. The neuropeptides, usually cause more prolonged actions. In most cases, the small-molecule types of transmitters are synthesized in the cytosol of the presynaptic terminal. The neuropeptides are synthesized as integral parts of large-protein molecules by ribosomes in the neuronal cell body.

The membrane of the presynaptic terminal is called the presynaptic membrane. It contains large numbers of voltage-gated calcium channels. When an action potential depolarizes the presynaptic membrane, these calcium channels open and allow large numbers of calcium ions to flow into the terminal. When the calcium ions enter the presynaptic terminal, they bind with special protein molecules on the inside surface of the presynaptic membrane, called release sites. This binding in turn causes the release sites to open through the membrane, allowing a few transmitter vesicles to release their transmitter into the cleft after each single action potential.

The transmitter vesicles contain the neurotransmitter that, when released into the synaptic cleft, either excites or inhibits the postsynaptic neuron. It excites the postsynaptic neuron if the neuronal membrane contains excitatory receptors, and it inhibits the neuron if the membrane contains inhibitory receptors.

Vesicles that store and release small-molecule transmitters are continually recycled and used over and over again. After they fuse with the synaptic membrane and open to release their transmitter substance, the vesicle membrane at first simply becomes part of the synaptic membrane. The vesicle portion of the membrane invaginates back to the inside of the presynaptic terminal and pinches off to form a new vesicle. The new vesicular membrane still contains appropriate enzyme proteins or transport proteins required for synthesizing new transmitter substance inside the vesicle. Acetylcholine is a typical small-molecule transmitter that obeys the principles of synthesis and release.