What is an Action Potential?
How Does this Process Work?
To initiate an action potential, a neuron must be excited by some kind of stimulus or impulse. A sensory neuron receives stimulus from external sources (such as light) while interneurons and motor neurons receive impulses from other neurons. In its normal state, a neuron will bring in 2 potassium ions for every 3 Na+ ions that it sends out. The cell maintains this ratio of ions through a process called active transport. This is where the cell expends energy to pump ions in and out of the cell with the use of ATP. By keeping a higher potassium concentration inside the cell, the cell holds a charge of -70mV at its normal resting state. When excited, some sodium ion channels open and allow sodium to diffuse. If the impulse is strong enough to cause the cell's internal charge to rise to -55mV, neighboring sodium ion channels open causing more Na+ ions to diffuse through at a higher rate depolarizing the cell. The sequence of sodium ions channels opening, depolarizing the nearby area, and causing nearby sodium ion channels to open up occurs down the axon.
Individual areas of the cell go from an internally negative to positive state due to all the sodium ions until the reach the charge of +30-40mV. This point is known as the overshoot, after which, the sodium ion channels close, potassium ion channels open hyperpolarizing the cell to a negative environment. This transition to a negative state is known as the absolute refractory period and subsequent action potentials cannot be fired. This is because the rate at which potassium ions enter the cell makes it impossible for depolarization to occur.
The absolute refractory period is followed by a relative refractory period where potassium ions cause the cell to undershoot its resting potential and, instead, reach an internal state of -90mV. At this time, an action potential can still be fired, however, a stronger stimulus would be required to reach the -55mV threshold.
Simply, action potentials are the way in which nerve cells communicate using signals generated through both electrical and chemical means. Upon receiving an external stimulus, neurons generate and fire action potentials to relay a message to the next targeted neuron, muscle fiber, organ, or gland.
Before diving into this concept, it is important to understand the properties of ions and ion channels. Ions are atoms that carry an electric charge due to the loss or gain of electrons. The presence of ions can cause either a net positive or negative charge. The main ions responsible for action potentials are Sodium and Potassium Ions. A relatively larger concentration of potassium is present inside the cell as opposed to a larger concentration of sodium present outside the cell. Although both ions are positive in nature, the resting potential of potassium is negative 80 millivolts (mV) as opposed to sodium whose resting potential is positive 62 mV.
How Does this Process Work?
To initiate an action potential, a neuron must be excited by some kind of stimulus or impulse. A sensory neuron receives stimulus from external sources (such as light) while interneurons and motor neurons receive impulses from other neurons. In its normal state, a neuron will bring in 2 potassium ions for every 3 Na+ ions that it sends out. The cell maintains this ratio of ions through a process called active transport. This is where the cell expends energy to pump ions in and out of the cell with the use of ATP. By keeping a higher potassium concentration inside the cell, the cell holds a charge of -70mV at its normal resting state. When excited, some sodium ion channels open and allow sodium to diffuse. If the impulse is strong enough to cause the cell's internal charge to rise to -55mV, neighboring sodium ion channels open causing more Na+ ions to diffuse through at a higher rate depolarizing the cell. The sequence of sodium ions channels opening, depolarizing the nearby area, and causing nearby sodium ion channels to open up occurs down the axon.
Individual areas of the cell go from an internally negative to positive state due to all the sodium ions until the reach the charge of +30-40mV. This point is known as the overshoot, after which, the sodium ion channels close, potassium ion channels open hyperpolarizing the cell to a negative environment. This transition to a negative state is known as the absolute refractory period and subsequent action potentials cannot be fired. This is because the rate at which potassium ions enter the cell makes it impossible for depolarization to occur.
The absolute refractory period is followed by a relative refractory period where potassium ions cause the cell to undershoot its resting potential and, instead, reach an internal state of -90mV. At this time, an action potential can still be fired, however, a stronger stimulus would be required to reach the -55mV threshold.
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