Action potential is an event in which the membrane potential of a cell first increases and then decreases. It is a characteristic property of a number of animal cells namely, neurons, muscle cells and endocrine cells. Some plant cells also produce them. Neurons play a very crucial role in cell-to-cell communication. In beta cells of pancreas action potential is associated with the release of insulin. Plasma membrane has special type of voltage-gated ion channels for producing action potential. These channels remain closed when the membrane potential is close to the resting potential of the cell but if the membrane potential increases then the channels become active. When the channels open they allow an inward flow of sodium ions which increases the membrane potential. The rapid inward flow of sodium ions results in changing the polarity of the membrane and as a result they become inactive. After that potassium channels become activated and there is an outward flow of the potassium ions due to which the membrane returns to its resting state. A transient negative shift known as refractory period prevents back travelling of the action potential. In animal cells generally two types of these potentials are known for example, one which is produced due to the voltage-gated sodium channels and the other which is produced due to voltage-gated calcium channels.
Nearly all cells maintain a voltage difference between their interior and exterior environment. The voltage of the cell is measured in milli volts (mV). The voltage of a typical animal cell is -70 mV. In majority of cells very little voltage fluctuations are known to Synapse xt occur but in some cells this is a usual feature. Some cells also show up and down cycles as the voltage increases and declines. The duration of these potentials vary over a wide range. In brain cells, the up and down cycle of action potential are completed in less than thousandth of a second while in other types of cells the cycles may take several seconds. The electrical property of an animal cell is dependent upon the membrane structure. Cell membrane is made up of a layer of lipids and proteins. The lipid layer acts as an insulator.
Generation of action potential within a neuron
All animal cells are electrically polarized as they maintain a voltage difference known as membrane potential. In case of neurons, axons, dendrites and cell body have different electrical properties. The most excitable part of a neuron is the axon hillock but axon and cell body also become excited. At the axon hillock the resting potential is -70 mV and the threshold potential is -55 mV. Synaptic inputs to the neuron result in depolarization causing the membrane potential to rise or fall. Action potential is produced when enough depolarization accumulates and the membrane potential reaches a threshold value.
Mechanism involved in action potential
Action potential is the result of activity of voltage-gated ion channels present in the plasma membrane of a cell. A voltage-gated ion channel is actually a cluster of proteins that remain embedded in plasma membrane and produces action potential because it can give rise to a positive feedback loop. Membrane potential is responsible for controlling the state of ion channels. Action potential is generated when the positive feedback cycle proceeds with full intensity. The time and amplitude are determined by the bio-physical properties of the voltage-gated ion channels. Several types of ion channels are known that produce positive feedback cycles and ultimately result in the production of these potentials. Voltage-gated sodium channels are involved in generation of faster potentials like those of nerve impulse. Slower ones like those produced in the muscle cells are mediated by the calcium ion channels.The most intensively studied voltage-gated ion channels are the sodium ion channels participating in faster nerve impulse conduction.