An action potential represents a rapid, temporary shift in the electrical charge across a neuron’s membrane. This electrical signal travels along the neuron’s axon until it reaches a specialized structure known as the terminal button, also referred to as the axon terminal or synaptic knob. The terminal button serves as the point where a neuron transmits signals to another neuron or a target cell. This conversion of an electrical signal into a chemical one at the terminal button is a fundamental process in the communication network of the nervous system.
The Arrival of the Signal
Upon reaching the terminal button, the action potential causes a significant change in the electrical potential of the presynaptic membrane. This depolarization, or change in voltage, triggers the opening of specialized voltage-gated calcium channels embedded within this membrane. These channels allow calcium ions (Ca2+) to enter the terminal button from the extracellular space. The influx of calcium ions into the presynaptic terminal occurs within milliseconds of the action potential’s arrival. This surge of intracellular calcium triggers the subsequent steps in transmitting the neuronal message across the synapse.
Unleashing Chemical Messengers
The sudden increase in calcium concentration inside the terminal button initiates molecular events. Calcium ions bind to specific proteins associated with synaptic vesicles. These membrane-bound sacs store neurotransmitters, the chemical messengers of the nervous system.
The binding of calcium prompts the movement of synaptic vesicles towards the active zones of the presynaptic membrane, where they dock for release. This process culminates in exocytosis, where the synaptic vesicles fuse with the presynaptic membrane and release their contents, the neurotransmitters, into the synaptic cleft. The synaptic cleft is the microscopic gap separating the transmitting neuron from the receiving neuron or cell.
Receiving the Message
Once released into the synaptic cleft, neurotransmitters quickly diffuse across this narrow space to reach the postsynaptic membrane, which is part of the receiving neuron or cell. Here, they encounter and bind to specific receptor proteins located on the postsynaptic membrane. This binding event causes a conformational change in the receptor proteins, leading to the opening of associated ion channels, often referred to as ligand-gated ion channels. The opening of these channels allows ions to flow across the postsynaptic membrane, which alters the electrical potential of the postsynaptic neuron. This change can be either excitatory, making the postsynaptic neuron more likely to generate its own action potential by causing depolarization, or inhibitory, making it less likely to fire by causing hyperpolarization.
Ending the Signal
For precise and efficient communication, the action of neurotransmitters in the synaptic cleft must be brief and tightly regulated. Several mechanisms work to terminate the signal, preventing continuous stimulation of the postsynaptic neuron.
One mechanism is reuptake, where specific transporter proteins on the presynaptic terminal membrane reabsorb neurotransmitters back into the transmitting neuron. Once reabsorbed, these neurotransmitters can be repackaged into vesicles for future use or broken down.
Another method is enzymatic degradation, where enzymes located within the synaptic cleft break down neurotransmitters into inactive metabolites. For instance, acetylcholinesterase rapidly breaks down acetylcholine.
Finally, neurotransmitters can simply diffuse away from the synaptic cleft, moving out of the immediate vicinity of the receptors and into the surrounding extracellular fluid. These processes ensure that the synapse is quickly ready to receive new signals, maintaining the dynamic and responsive nature of neuronal communication.