Action Potentials

Action potential refers to the electrical signal produced by neurons as they transmit information from one part of the body to another. It involves rapid changes in voltage across a cell membrane that can be seen on an electrode recording or using other techniques. This process occurs when a neuron becomes excited and develops into a nerve impulse, which then travels along the axon to the next neuron. Let us delve deeper into understanding this phenomenon through its various subtopics.

Subthreshold Potentials

Before discussing action potentials, it is essential to understand subthreshold potentials. Neurons have a resting membrane potential due to the presence of ions inside and outside the cell. When an excitatory neurotransmitter binds to a receptor on the dendrites, the ion channels open, causing sodium ions to rush into the cell. This influx of positive ions causes a depolarization of the membrane potential from approximately -70mV to -55mV. If this depolarization is not enough to reach the threshold value for generating an action potential (-45mV), it is considered a subthreshold event.

Action Potentials

Now let us move on to action potentials. When the membrane potential reaches its threshold value (-50mV to -45mV), voltage-gated channels open, allowing the flow of ions across the membrane. The primary flow occurs via sodium and potassium ions.

Depolarization Phase

The opening of these channels begins the depolarization phase, where sodium ions rush into the cell. As more ions enter, the membrane potential moves towards 0mV. The depolarization phase is rapid and lasts only a few milliseconds.

Repolarization Phase

After the peak of the action potential has been reached, the voltage-gated channels close, and potassium ions start to flow out of the cell. This process pushes the membrane potential back towards its resting state. The repolarization phase is slower than the depolarization phase and can take tens of milliseconds to complete.

Refractory Period

Once an action potential has occurred in a neuron, it enters a period known as the refractory period. During this time, any subsequent stimuli applied would not be able to generate another action potential until the refractory period ends.

Action Potentials in Muscle Cells

While we have mainly discussed action potentials in neurons, they also occur in muscle cells. These electrical signals are responsible for contraction in muscles during movement. In skeletal muscle fibers, when a motor neuron generates an action potential at its axon terminal, calcium channels open, allowing calcium ions to enter. This influx in calcium leads to the release of acetylcholine from the synaptic vesicles into the synapse gap between the terminals and the muscle fiber's sarcolemma. Acetylcholine then binds to receptors on the muscle fiber surface, causing voltage-gated sodium channels to open, initiating an action potential within the muscle cell.

In summary, action potentials are crucial in transmitting information throughout the body via electrical impulses. Understanding subthreshold potentials, the generation of action potentials, and their phases (depolarization, repolarization, and refractory period) provides insight into their role in both neurons and muscle cells.