Action Potential

Neuronal Synapses and Action Potentials

At an inner neuronal synapse, neurotransmitters can be excitatory or inhibitory, which affects the postsynaptic terminal's membrane potential. Excitatory synapses lead to membrane depolarization, while inhibitory synapses cause hyperpolarization. A neuron receives inputs from multiple axons, which may be excitatory or inhibitory, leading to a complex decision-making process about whether to generate an action potential.

This decision-making is summative; if the overall excitatory signals outweigh the inhibitory signals, an action potential is likely to be generated. The key factor in this process is the threshold potential, which is the minimum level of depolarization needed to trigger an action potential. A neuron doesn’t need to completely depolarize to zero but must reach a range of approximately -55 to -42 millivolts.

The influx of specific ions, such as sodium, calcium, or chloride, is critical in determining the membrane potential. These ions diffuse to the axon's base (the axon hillock), where the action potential is evaluated. If sufficient sodium ions enter and the threshold is met, voltage-gated sodium channels open, leading to a rapid depolarization of the membrane. This depolarization spreads along the axon, creating a chain reaction as more voltage-gated sodium channels open in sequence.

Following depolarization, potassium ions exit the cell, leading to the repolarization of the membrane. There's a brief moment of hyperpolarization, but the membrane then returns to resting potential. This sequence occurs very quickly, allowing neurons to send signals rapidly. It is critical to understand the difference between hyperpolarization and repolarization: hyperpolarization occurs when the starting point is resting potential, while repolarization occurs from a depolarized state.

The action potential's propagation varies between myelinated and unmyelinated axons. In unmyelinated axons, the action potential requires successive openings of voltage-gated sodium and potassium channels, which slows down signal transmission. In contrast, myelinated axons conduct action potentials significantly faster due to the insulation provided by myelin, allowing the action potential to jump between nodes of Ranvier, a process known as saltatory conduction. This jumping motion enables quicker propagation of the action potential along the axon, effectively speeding up neural communication. Additionally, the presence of myelin not only increases the speed of conduction but also conserves energy, as fewer ions need to be exchanged across the membrane during the process.