Definition and Importance of Action Potentials
– Action potentials are rapid changes in the membrane potential of excitable cells.
– They play a crucial role in cell-to-cell communication and signaling.
– Action potentials occur in neurons, muscle cells, and certain endocrine cells.
– They are essential for the propagation of signals along neurons and the release of neurotransmitters.
– Action potentials are also known as nerve impulses or spikes.
Process and Characteristics of Action Potentials
– Action potentials are generated by voltage-gated ion channels in the cell’s plasma membrane.
– When the membrane potential reaches a threshold, sodium channels open, causing depolarization.
– Repolarization occurs when potassium channels open and potassium ions move out of the cell.
– The impulse travels in one direction only, from the axon to the axon terminal.
– Action potentials are followed by a transient negative shift called the afterhyperpolarization.
– The membrane potential of a cell is typically maintained at a negative voltage relative to the exterior.
– Action potentials in animal cells can last for milliseconds to seconds.
– Sodium-based action potentials are shorter in duration than calcium-based action potentials.
– Slow calcium spikes can trigger a burst of rapidly emitted sodium spikes in some neurons.
– In cardiac muscle cells, a fast sodium spike primes the rapid onset of a calcium spike for muscle contraction.
Membrane Potential and Voltage-Gated Ion Channels
– Cell membranes maintain a voltage difference called the membrane potential.
– The interior of the cell has a negative voltage relative to the exterior.
– Voltage-gated ion channels in the membrane switch between closed and open states based on the voltage difference.
– These channels allow the flow of ions across the membrane, driving action potentials.
– Action potentials are driven by voltage-gated sodium and calcium channels in different types of cells.
Polarization and Excitability in Neurons
– All cells in animal body tissues are electrically polarized.
– Neurons have different electrical properties in different parts of the cell.
– The axonal initial segment is the most excitable part of a neuron.
– Neurons have resting potential and threshold potential levels of membrane potential.
– Action potentials are triggered when the membrane potential reaches the threshold level.
Biophysical Basis and Ion Movement during Action Potentials
– Action potentials result from the presence of voltage-gated ion channels in a cell’s membrane.
– Voltage-gated ion channels have three key properties: they can assume multiple conformations, one of which creates a channel permeable to specific ions, and their conformation is influenced by the membrane potential.
– Different types of voltage-gated ion channels exist, such as sodium channels for fast action potentials and calcium channels for slower action potentials.
– Sodium (Na+) and potassium (K+) ions play a crucial role in action potentials.
– The principal ions involved in action potentials are sodium and potassium cations.
– Sodium ions enter the cell during depolarization, while potassium ions leave during repolarization.
– The exchange of ions during an action potential has a negligible impact on overall ionic concentrations. Source: https://en.wikipedia.org/wiki/Action_potential
An action potential occurs when the membrane potential of a specific cell rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and in some plant cells. Certain endocrine cells such as pancreatic beta cells, and certain cells of the anterior pituitary gland are also excitable cells.
In neurons, action potentials play a central role in cell–cell communication by providing for—or with regard to saltatory conduction, assisting—the propagation of signals along the neuron's axon toward synaptic boutons situated at the ends of an axon; these signals can then connect with other neurons at synapses, or to motor cells or glands. In other types of cells, their main function is to activate intracellular processes. In muscle cells, for example, an action potential is the first step in the chain of events leading to contraction. In beta cells of the pancreas, they provoke release of insulin. Action potentials in neurons are also known as "nerve impulses" or "spikes", and the temporal sequence of action potentials generated by a neuron is called its "spike train". A neuron that emits an action potential, or nerve impulse, is often said to "fire".
Action potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane. These channels are shut when the membrane potential is near the (negative) resting potential of the cell, but they rapidly begin to open if the membrane potential increases to a precisely defined threshold voltage, depolarising the transmembrane potential. When the channels open, they allow an inward flow of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential towards zero. This then causes more channels to open, producing a greater electric current across the cell membrane and so on. The process proceeds explosively until all of the available ion channels are open, resulting in a large upswing in the membrane potential. The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate. As the sodium channels close, sodium ions can no longer enter the neuron, and they are then actively transported back out of the plasma membrane. Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state. After an action potential has occurred, there is a transient negative shift, called the afterhyperpolarization.
In animal cells, there are two primary types of action potentials. One type is generated by voltage-gated sodium channels, the other by voltage-gated calcium channels. Sodium-based action potentials usually last for under one millisecond, but calcium-based action potentials may last for 100 milliseconds or longer.[citation needed] In some types of neurons, slow calcium spikes provide the driving force for a long burst of rapidly emitted sodium spikes. In cardiac muscle cells, on the other hand, an initial fast sodium spike provides a "primer" to provoke the rapid onset of a calcium spike, which then produces muscle contraction.