INTRODUCTION

The ability of our nervous system to orchestrate complex behaviors, deal with complex concepts, and learn and remember depends upon communication between vast numbers of neurons. Communication between neurons occurs at specialized junctions called synapses. The most common type of synapse in the brain is the chemical synapse—one in which chemical messages released by a presynaptic cell induce changes in a postsynaptic cell. Neurons also communicate with muscle cells through chemical synapses. The classic synapse that has been extensively studied is the neuromuscular junction—the synapse through which a motor neuron causes a muscle cell to contract.

This animation depicts the events that are involved in transmitting the signal from the nerve ending of a motor neuron to a muscle cell at a chemical synapse using the neurotransmitter acetylcholine (ACh).

ANIMATION SCRIPT

Depolarization of the axon terminal membrane causes voltage-gated Ca²⁺ channels to open, and Ca²⁺ ions rush into the axon terminal.

The Ca²⁺ ions trigger the release of neurotransmitter by causing the synaptic vesicles closest to the active zone of the synapse to fuse with the presynaptic membrane.

When the vesicles fuse with the membrane, they release their content of neurotransmitter into the synaptic cleft. The neurotransmitter moves across the cleft and binds to receptors on the postsynaptic membrane.

The physiological response of the postsynaptic cell depends upon the particular neurotransmitter and receptor combination. In this example, acetylcholine binds to an ion channel, the channel opens, and sodium ions enter the postsynaptic cell. The signal is thus propagated to the postsynaptic cell.

After a neurotransmitter has achieved its effect, it must be inactivated. For acetylcholine, an enzyme in the synaptic cleft—acetylcholinesterase—breaks down the acetylcholine into acetyl CoA and choline. The release of transmitter from the receptors causes the channels to close. The choline is then taken back up into the synaptic terminal for resynthesis into acetylcholine.

The synaptic vesicles are also recycled, via endocytosis of the presynaptic membrane. The recycled vesicles are later re-filled with neurotransmitter molecules and are ready for another round of synaptic transmission.

CONCLUSION

We have just seen how the electrical signal—the action potential—is passed from a presynaptic cell (a neuron) to a postsynaptic cell (a muscle cell) via the chemical synapse using the neurotransmitter acetylcholine.

In the human brain, more than 25 neurotransmitters are now recognized, and which neurotransmitter is used by a particular neuron will determine whether a synapse is excititory or inhibitory. In addition, each neurotransmitter may bind to several receptor subtypes. Ultimately, the action of a particular neurotransmitter will depend on the receptor to which it binds.

Note that in this animation we have depicted only a single synaptic vesicle within the terminal. An actual synapse contains a multitude of synaptic vesicles. In addition, the arrival of the action potential triggers the release of neurotransmitter only from those vesicles closest to the synaptic membrane. Vesicles farther away from the membrane then move into position in preparation for another round of transmitter release.