Action Potential Basic Features Underlying Mechanism

As the traveling signals of nerves and as the localized changes that contract muscle cells, action potentials are an essential feature of animal life. They set the pace of thought and action, constrain the sizes of evolving anatomies and enable centralized control and coordination of organs and tissues.

1 Basic features

When a biological cell or patch of membrane undergoes an action potential—or electrical excitation—the polarity of the transmembrane voltage swings rapidly from negative to positive and back. Within any one excitable cell, consecutive action potentials typically are indistinguishable. Also between different cells the amplitudes of the voltage swings tend to be roughly the same. But the speed and simplicity of action potentials vary significantly between cells, in particular between different cell types.

Minimally, an action potential involves a depolarization, a repolarization and finally a hyperpolarization (or "undershoot"). In specialized muscle cells of the heart, such as the pacemaker cells, a plateau phase of intermediate voltage may precede repolarization.

2 Underlying mechanism

Schematic three dimensional cross section of a cell membrane. As the cell membrane is hydrophobic, charged molecules or ions are not able to easily diffuse through it, this property permits a potential difference to exist across the membrane. In order for this potential to be changed, as occurs during the propagation of an action potential, charged entities, principally K⁺ and Na⁺Sodium is the chemical element in the periodic table that has the symbol Na Natrium in Latin) and atom number 11. Sodium is a soft, waxy, silvery reactive metal belonging to the alkali metals that is abundant in natural compounds (especially halite). must be translocated from one side to the other via transmembrane proteins such as ion channelsIon channels are present in the membranes that surround all biological cells. By conducting and controlling the flow of ions, these pore-forming enzymes help establish the small negative voltage that all cells possess at rest (see cell potential). Basic f, pumps, and transportersNaKATPase (also known as the Na+/K+ pump or Na+/K+ exchanger is an enzyme ( EC ) located in the plasma membrane (specifically an electrogenic transmembrane ATPase). It is found in the plasma membrane of virtually every human cell and is common to all cell.

The transmembrane voltage changes that take place during an action potential result from changes in the permeability of the membraneA component of every biological cell, the cell membrane (or plasma membrane is a thin and structured bilayer of phospholipid and protein molecules that encapsulate the cell. It separates a cell's interior from its surroundings and controls what moves in a to specific ions, the internal and external concentrations of which cells maintain in an imbalance. In the axon fibers of nerves, depolarization results from the inward rush of sodium ions, while repolarization and hyperpolarization arise from an outward rush of potassium ions. Calcium ions make up most or all of the depolarizing currents at an axon's presynaptic terminus, in muscle cells (including the heart's) and in some dendrites.

Changes in membrane permeability and the onset and cessation of ionic currents reflect the opening and closing of voltage-gated ion channels, which provide portals through the membrane for ions. Residing in and spanning the membrane, these proteins sense and respond to changes in transmembrane potential.

3 Initiation

Action potentials are triggered by an initial depolarization to the point of threshold. This threshold potential varies but generally is about 15 millivolts above the resting potential of the cell. Typically action potential initiation occurs at a synapse, but may occur anywhere along the axon. In his discovery of "animal electricity," Luigi Galvani elicited an action potential through contact of his scalpel with the sciatic motor nerve of a frog he was dissecting, causing one of its legs to kick as in life.

4 Wave propagation

In the fine fibers of simple (or unmyelinated) axons, action potentials propagate as waves, which travel at speeds up to 120 meters per second.

The propagation speed of these impulses is faster in fatter fibers than in thin ones, other things being equal. In their Nobel prize-winning work uncovering the wave nature and ionic mechanism of action potentials, Alan Hodgkin and Andrew Huxley performed experiments on the giant fiber of Atlantic squid . Responsible for initiating flight, this axon is fat enough to be seen without a microscope (100 to 1000 times larger than is typical). This is assumed to reflect an adaptation for speed. Indeed, the velocity of nerve impulses in these fibers is among the fastest in nature.

5 Saltatory propagation

Many neurons have insulating sheaths of myelin surrounding their axons, which enable action potentials to travel faster than in unmyelinated axons of the same diameter. The myelin sheathing normally runs along the axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier.

Because the salty cytoplasm of the axon is electrically conductive, and because the myelin inhibits charge leakage through the membrane, depolarization at one node is sufficient to elevate the voltage at a neighboring node to the threshold for action potential initiation. Thus in myelinated axons, action potentials do not propagate as waves, but recur at successive nodes and in effect hop along the axon. This mode of propagation is known as saltatory conduction.

The disease multiple sclerosis (MS) is due to a breakdown of myelin sheathing, and degrades muscle control by destroying axons' ability to conduct action potentials.

6 Detection and observation

Action potentials (APs) are measured with the recording techniques of electrophysiology. In the case of an archetypal nerve action potential on an oscilloscope, the relatively large swing to a more positive value, followed by the repolarization recovery and undershoot together trace an arc that could be described as a distorted sine wave—or like the blips on hospital EKG machines that can be seen on TV (these EKG waves are a smear of all the action potentials in one heartbeat, so they enact more slowly than any individual AP and have a somewhat more complicated shape). In an unmyelinated axon that is firing an action potential, the transmembrane potential at any instant will vary from point to point along the fiber, with its amplitude depending on whether the AP wave has reached that point or passed it, and how long ago. A recording from a single point will show the various stages of the action potential enacted—depolarization, repolarization, hyperpolarization—as the wave passes.

7 Detailed features

Prototypically, depolarization and repolarization together are complete in about two milliseconds, while undershoots can last hundreds of milliseconds, depending on the cell. In neurons, the exact length of the roughly two-millisecond delay in repolarization can have a strong effect on the amount of neurotransmitter released at a synapse. The duration of the hyperpolarization determines a nerve's refractory period (how long until it may conduct another action potential) and hence the frequency at which it will fire under continuous stimulation. Both of these properties are subject to biological regulation, primarily (among the mechanisms discovered so far) acting on ion channels selective for potassium.

In pacemaker and other cardiac muscle cells, inward calcium currents determine shape and duration of the plateau phase, which in turn controls the strength and duration of contraction. See cardiac action potential, ventricular action potential, atrial action potential , and pacemaker action potential for more details.

8 References

Bear, M.F., B.W. Conners, and M.A. Paradisa. 2001. Neuroscience: Exploring the Brain. Baltimore: Lippincott.[1]
Kandel, Eric, James Schwartz, and Thomas Jessel. 2000. Principles of Neural Science. 4th ed. McGraw-Hill, New York.[2]

Nervous system

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