Overview of Synaptic Transmission and Neurotransmitters in Neurons

Nerve impulses travel from neuron to neuron along complex pathways that involve synapses. The synapse is the junction between two neurons, where a gap called a synaptic cleft exists. The process of transmitting the impulse across the synapse involves the release of a chemical messenger called a neurotransmitter. This message is transmitted from the presynaptic neuron to the postsynaptic neuron, triggering a response that leads to the formation of new action potentials.

Neurons Communicate At Synapses

Neurotransmitter Substances

Neurotransmitters include acetylcholine, which stimulates skeletal muscle contractions, and a group of compounds called monoamines (such as epinephrine, norepinephrine, dopamine, and serotonin), which are formed from modified amino acid molecules. Acetylcholine is the main transmitter for synapses outside the CNS, while others are mostly involved in synaptic transmission within the brain and spinal cord.

Figure Showing Synaptic Transmission

The synaptic transmission is illustrated in the following numbered sequence:

An action potential (red arrow) arrives at the synaptic knob.

The action potential triggers chemical changes that make neurotransmitter vesicles fuse with the plasma membrane of the transmitting cell.

The fused vesicles release their neurotransmitter molecules (green) into the synaptic cleft.

The released neurotransmitter molecules diffuse across the cleft and bind to receptor molecules on the receiving cell's plasma membrane.

The binding of neurotransmitter to receptor opens chemical-sensitive ion channels in the receiving cell's membrane. When the channels open, ions can diffuse into the receiving cell and trigger new action potentials.

After being released, some neurotransmitters are decomposed by enzymes present in the synaptic cleft by being transported back into the synaptic knob that released them or into nearby neurons or neuroglial cells.