The nervous system relies on communication to coordinate body functions. This communication occurs through specialized chemical messengers known as neurotransmitters. These chemicals transmit signals throughout the body, allowing neurons to relay information.
Understanding the Synaptic Terminal
Neural communication takes place at a synapse, where signals transmit from one neuron to the next. The “sending end” is the synaptic terminal, also known as the axon terminal or presynaptic terminal.
This terminal is part of the presynaptic neuron. It is separated from the postsynaptic neuron by a synaptic cleft. Within the presynaptic terminal, neurotransmitters are stored in small, membrane-bound sacs called synaptic vesicles.
The Sequence of Neurotransmitter Release
Neurotransmitter release begins when an action potential travels down the presynaptic neuron’s axon to its terminal, converting the electrical signal into a chemical one for synaptic communication. When the action potential arrives, voltage-gated calcium channels located in the presynaptic membrane open. This opening allows calcium ions, which are present in higher concentrations outside the neuron, to rapidly flow into the presynaptic terminal. This sudden influx of calcium ions is a trigger for the subsequent steps of neurotransmitter release.
The increase in calcium concentration inside the terminal prompts synaptic vesicles, which contain the neurotransmitters, to move towards the presynaptic membrane. These vesicles then “dock” or bind to specific sites on the presynaptic membrane, positioning them for release. Proteins such as synapsin help mobilize vesicles from a reserve pool to these active zones.
Following docking, the vesicle membrane fuses with the presynaptic membrane. This fusion process, known as exocytosis, creates an opening that allows the neurotransmitters to be released into the synaptic cleft. This rapid expulsion of chemical messengers ensures efficient and timely signal transmission to the postsynaptic neuron.
The Fate of Neurotransmitters
Once neurotransmitters are released into the synaptic cleft, they quickly diffuse. They then bind to specific receptor proteins on the postsynaptic neuron, much like a key fitting into a lock. This binding initiates a response in the postsynaptic cell, which can be either excitatory, making the neuron more likely to fire its own electrical signal, or inhibitory, making it less likely.
To ensure precise and controlled communication, neurotransmitters must be removed from the synaptic cleft shortly after they have delivered their message. One common mechanism is reuptake, where specialized transporter proteins on the presynaptic neuron reabsorb the neurotransmitters back into the terminal. Once reabsorbed, they can either be repackaged into vesicles for future use or broken down.
Another mechanism for clearing neurotransmitters is enzymatic degradation. Enzymes present in the synaptic cleft break down the neurotransmitter molecules into inactive components. For example, acetylcholine is broken down by acetylcholinesterase directly in the cleft. Some neurotransmitters also simply diffuse away from the synaptic cleft, moving into the surrounding fluid where they are no longer able to bind to receptors.