Action Potential: Definition, Propagation, Steps, And Functions in Various Cells.

Action Potential Definition

Action potentials happen while the voltage on a cell membrane quickly rises and falls. Evey’s action potential (impulse) is the same size. Action potentials could have a big effect on how a nerve or muscle cell responds to them. This could be because of the frequency and length of the action potentials. To induce a particular change in voltage, to get an action potential going, it needs some positive ions (threshold value). It happens after a certain amount of internal cell membrane depolarization, which is a rise in electrical charge inside the cell.

What is an Action Potential?

An action potential is then what you need to do something. Numerous definitions are rather confusing, especially while you are studying action potentials for the first time. To fully solve the riddle, we need to first examine the term’s definition.

The term “potential” does not relate to the possibility of attaining anything; rather, an electric potential is what it’s all about. This is a constant energy field most often associated with physics. In biology, potentials may be found on the inner and outer borders of cell membranes. Potential energy is constant since it is stored energy. A ball possesses potential energy while it stays motionless. When a neuron does not fire, it is said to be dormant. It has stored energy in the form of potential energy. Rather than referring to a cell—or more precisely, its membrane—as having potential energy, we refer to it as having a resting potential.

When a ball is subjected to force, it is going to move. Kinetic energy, often known as the energy of motion, is converted from potential energy. This force comes from somewhere other than the ball, and it transfers potential energy to kinetic energy. The ball comes to a stop when its kinetic energy is spent (and no additional external force is applied). It regains its potential energy after that.

Ions are charged atoms that cause action in the cell membrane in the same way that motion does. When a neuron does not fire and the cell membrane prevents substantial quantities of particular products from entering or leaving the cell (we shall discuss them later), the cell has resting potential. When electrical activity is induced, the potential ceases to rest due to the generation of electrical movement (an action potential).

We need a deeper knowledge of the electrical charge – the energy field – associated with the resting and active phases to fully appreciate this process. Atoms produce an electrical charge. If you know how an atom is made up of a proton and a neutron nucleus surrounded by an electron cloud, you know that protons are positively charged and electrons are negatively charged. This is a fact; there is no reason to question it.

An atom usually contains an equal number of positive and negative protons and electrons. If it doesn’t, it tries to join with other atoms in order to balance the charges and stabilise the atom. An atom is nothing more than an atom in its neutral condition. When an atom gains a negative charge due to an overabundance of electrons, it is referred to as a negatively charged ion. The positive charge on the proton is raised when an atom has insufficient electrons (but the number of protons stays constant), and the atom is referred to as a positively charged ion. When it comes to electrical signalling through action potentials, these ions are crucial. Action potentials are used by neurons and muscle cells, for example.

Electrical Cell Membranes

Electrical current flows across cell membranes. They use ions on both sides of the cell—extracellular ions and intracellular ions—to create a charge that runs the length of the cell membrane. When nothing is occurring, it is stated that the cell membrane has a resting potential. Numerous compounds may diffuse into the cell via pores or the bilayer phospholipid membrane. Every cell’s membrane has a resting potential. However, not all cells are capable of generating an action potential.

Numerous particles, including charged particles and ions, need assistance to enter or exit a cell. They need the use of specialised channels that seal and open. The various mechanisms by which ions enter and exit a cell can be studied in the passive and active transport articles; all you need to know about the action potential is that it is the process by which a cell switches from a resting to an action state via changes in electrical charge caused by changes in (resting) internal and external ion concentrations.

A membrane potential is a mathematical expression that defines how an electrical charge is dispersed over a membrane. It is expressed as millivolts (mV). This is most often determined by comparing the charge on the exterior of the cell (the side that contains extracellular fluid) to the charge on the interior of the cell (the cytosol or intracellular fluid). To simplify computations, it is assumed that the outer side has a value of zero mV.

Generally, inside the cell and in the surrounding fluid, the ratio of negative and positive ions is equal. and so the cell is neutral. However, a difference may be noticed in an area that is particularly near to the membrane’s inner and outer surfaces.

When a cell is in its resting state, or resting potential, the channels that enable charged particles to enter and exit the cell are mostly closed. There are extremely particular ion concentrations at the inner and outer membranes that are close to the surfaces. Inside the cell, there are more positive potassium ions (K+) than outside; outside the cell, there are more positive sodium ions (N+). Within the cell, negative charges are mostly constituted of bigger proteins termed anions that cannot pass through the membrane’s ion channels. Thus, electrical signalling is the outcome of positive ion mobility.

Ion Channels

As previously stated, ion channels are required. This university’s website contains a song to help you recall the various channel kinds. Ions are hydrophilic and cannot pass through a membrane’s lipids. They need specialised, structured proteins that form tunnels or channels to travel within and outside the membrane. Ion channels are, unsurprisingly, channels that transport ions. When an ion binds to an ion channel, the protein channel’s shape changes and the associated ion may pass through.Alternatively, these channels may be induced to open by stretching the cell membrane (stretch-gated channels) or by changes in cell membrane voltage (voltage-gated channels) (voltage-gated channels). A protein is considered to be ligand-gated when it requires the presence of a ligand to open a channel. Each kind of channel is further broken into subtypes. This means that a single neuron membrane might include up to ten different ion channel types.

Each second, a single ion channel may carry up to a hundred million ions. This would ordinarily take a lot of energy, but the body saves it by allowing the ions to pass over a concentration gradient while they are at rest. If there are too many Na+ ions outside the cell membrane, they will leak into the cell via non-gated leak channels, which operate as passive transport since they do not need an external source of energy. The quicker the ion movement, the greater the concentration difference on either side of the membrane. As concentrations return to normal, the rate slows.

Another approach is the sodium-potassium pump, which shunts three sodium ions out and two potassium ions in during both the resting and action potential phases. This is an ATP-dependent active transport mechanism.

How can electrical activity be created when both sodium and potassium ions have positive charges? Numerous anions (negative atoms) may be found within the cell, and bigger molecules (proteins) that typically remain put because the larger molecules are too huge to escape. Allowing positive ions from outside the cell to enter reverses the negative charge within the cell membrane. Allowing positive ions to leave the cell allows it to revert to a negatively charged resting potential. Inside the cell, the negative charges stay stationary.

As we all know, negative and positive charges have the ability to be captured and rejected. This has implications for the action potential since we must now account for the mobility of the change in electrical charge. The attraction between negative and positive charges then acts as a pulling force, causing voltage fluctuations to flow in a particular direction—an impulse.

Action Potentials in Neurons

Neurons are capable of performing a wide variety of actions, including sensory and motor messages. We cannot operate without action potential. The nervous system takes data from our senses, analyses it, and transmits additional data that causes us to react. Each message is sent by an action potential that passes through the neuron’s membrane. An action potential is a kind of electrical stimulation that may cause another cell to generate a hormone, liberate a neurotransmitter, or contract a muscle. Action potentials are analogous to the anatomical telephone lines and Ethernet cables that existed before the advent of wireless communications.

Neurons may be rather lengthy. The human body’s longest neuron axon stretches from the spine’s base until the big toe. A blue whale’s dorsal root ganglion is more than sixteen metres long and connects the spine to the brain. The recurrent laryngeal nerve traverses the length of a giraffe’s neck—about two and a half metres. The shift in membrane potential must be maintained in some way. This is performed by gradually transitioning from resting to action potential and maintaining the electrical message until it hits the nerve’s endpoint (synapse) and transmits the message to the next cell.

Electrochemical communication is how neurons communicate. They need chemical messengers (neurotransmitters) to begin, accelerate, decelerate, and terminate an action potential. Additionally, they require ions. Electrochemical energy is the source of all electricity created in a living body.

Sodium and potassium (both with a single positive charge), calcium (Ca2+) with two positive charges, and chloride (Cl–) with one negative charge are the primary ions in the human body and most animals’ neurological systems. Chloride is an anion that is used to delay the firing rate of action potentials in some neuronal circuits.

Resting Potential vs Action Potential

The difference between resting potentials and action potentials is mostly determined by the inner membrane voltage differential. A nerve cell’s resting potential is roughly -70 mV. Keeping in mind that the outside of the membrane is zero, we can see that negatively charged ions within the cell outnumber positively charged ions. The membrane of a resting neuron or muscle cell is always negative. Furthermore, there are more Na+ ions on the outside than inside a neuron or muscle cell, and more K+ ions on the inside than in the extracellular fluid when the cell is at rest.

What, therefore, transforms a resting potential into an acting potential? Your sense of smell is an excellent illustration of the action potential. Are you smelling anything at the moment?

Most likely, you will not. That is not to say that your nose is not operating; rather, it’s possible that you’ve been analysing action potentials in the same place for some time. Perhaps you noticed the aroma of rain flowing through the air via an open window or yesterday’s pizza when you initially sat at your desk or laptop. The receptor cells in your nose identify smell particles, which are the initial step in the production of an olfactory action potential. They either create an action potential or release chemicals that cause a closely connected neuron to fire. Sensory transduction is the term for this process.

Therefore, what ‘triggers’ an action potential? Consider that when you entered your research location for the first time, old pepperoni pizza gaseous bubbles were connected to the receptor proteins on the surface of your olfactory cilia cells. The activation and release of a protein known as G-protein is elicited by these receptors. A G-protein boosts the quantities of cyclic adenosine monophosphate, another protein, via a chemical reaction (cAMP). The sodium ion channels in cilia cells open when cyclic AMP is present.

Positively charged sodium ions pour into the cilia cell. The internal potential of the cell membrane has risen from -70 mV to +70 mV. Depolarization is the process by which the inside of the cell becomes more positively charged. This is an important term to remember. Depolarization is described as a change in electrical charge on the cell’s interior, especially near the membrane, where the new charge is more positive than the ‘at rest’ charge.

An action potential may be initiated when the charge reaches a threshold of -55 mV. If the charge falls below-65 mV when the receptor cells stop responding, no action potential is generated.

To have an electrical impact, there must be a significant quantity of positively charged ions within the cell—in biology terms, the cytoplasm on the inner side of the cell membrane must surpass a threshold of roughly-55 mV to-50 mV before an action potential can occur. The depolarization has now reached a level sufficient to have an impact.

When the resting potential begins to rise (let us say from-69 mV), when the membrane potential reaches a high of +30 to +40 mV, the process is complete. Depolarization is not an action potential, contrary to common assumption. It’s a basic word that refers to a positive charge rising.

Sodium ions keep flowing in after the threshold is passed; depolarization persists throughout the firing of an action potential. Additionally, an impulse, a spike, or a message are all terms for the same thing. An action potential’s size is always the same. It is not more potent or larger when sodium ions are influxed into the cell membrane from the outside. The message’s significance will not be increased if the threshold is surpassed. The size of the action potential is fixed. At work, the “all or none” concept states that either the threshold is passed and an impulse is launched, or it is not.

The only reason humans react more strongly to some stimuli is because of their rate of firing, not their size. In our scenario, when entering your study location, due to the rapid firing rate of the cilia action potentials, you may perceive a stale pizza odour. The odour identified was novel. After a period of time, the cilia receptors become temporarily sensitised. They cease noticing the previous odour and begin focusing on the new one. In terms of flavour, nothing compares to the old pepperoni pizza. You will be completely unaware of any stench. The magnitude of the action potentials has not decreased; they are either present or absent. Cilia simply ceases to react to a single odour; depolarization comes to an end. If your nose detects the aroma of burning, it will do so for a much longer period of time. A fire is dangerous, but a pepperoni pizza is not.

Action Potential Propagation

Propagation of an action potential is the process by which an impulse travels through a cell membrane, most commonly the axon of a nerve cell. We already know that a large number of neurons are very lengthy. To guarantee that an action potential does not decay or the quantity of depolarization falls below the threshold (some ions will continue to exit the cell through leaky channels), the action potential must continue down the axon. To maximise efficiency in neurons without an insulating myelin coating, parts of the cell membrane depolarize sequentially, pushing the action potential in a single direction toward a target cell. This section-by-section movement is action potential propagation. Initiation comes first, followed by dissemination.

Consider a single ion channel inside a membrane that has been activated by cAMP, allowing sodium ions to enter the cell. How can the nerve cell in question, which is located in the giraffe’s neck, sustain adequate sodium flow and stay over the threshold levels while moving? How can the action potential move in one direction all the way to the nerve cell synapse if cAMP impacts numerous ion channels? The idea of negative and positive attraction enters the picture at this point.

The part of the membrane nearest to the ion channel gets more positive when sodium ions enter the cell. Atoms take time to diffuse completely into the cytoplasm, but neurons move at breakneck speed. During the brief time period to which it has been allocated, the threshold charge resides immediately to the left and right of the ion channel. Consider a single plus sign surrounded by negative signs underneath an ion channel.

Due to the attraction of positive and negative charges, that plus is drawn toward the adjacent ion channel; since it has a positive charge, it partially neutralises the negative charge that draws it. Due to the passage of the first ion channel, the region immediately underneath it has lost its positive charge and has returned to a negative charge. When the positive charge gets close to the subsequent point, it is dragged closer by the adjoining negative charge.

This voltage shift activates additional sodium channels that are not cAMP-dependent but are activated by a charge change—voltage-gated sodium channels. Any sodium ion leakage is corrected in this way. As a result, an action potential is similar to a string of light bulbs that are all turned on in the same direction. The illuminated bulb shows the presence of a positive charge.

Following depolarization, repolarization occurs. Given that depolarization results in a rising (positive) charge differential across the inner membrane of the cell, repolarization, we may assume, is the inverse: the inner membrane returns to its resting potential by returning to its original negative charge. Repolarization is the inverse of depolarization, which is an upward-pointing line. Only potassium ions make this possible. When a positive electrical charge is present within the cell, voltage-dependent potassium ion channels open, assisting in the ejection of K+ ions. Simultaneously, sodium ion channels collapse. When all of these positive ions have left or are unable to enter the cell, the resting potential is reached.

In fact, the resting potential is completely bypassed. Initially, the electrical charge of the inner cell membrane is lower than the resting potential. When the charge is smaller than-70 mV, hyperpolarization occurs. This is due to the fact that when the sodium and potassium channels approach 70 mV, they are unable to close quickly.

Action potential propagation occurs in peripheral neurons between individual Schwann cells at the Ranvier nodes. Schwann cells surround the neuron’s axon with myelin, a kind of insulation that stops sodium ions from entering voltage-dependent sodium channels. While this insulating layer reduces leakage, when sodium ions diffuse away from the immediate region, depolarization decreases. Nerve impulses would begin, but they would fade away over time.

To avoid this, unmyelinated nodes between Schwann cells enable sodium ions to pass via ion channels. In this illustration, each Ranvier node depicts a positively charged light bulb that conveys the message through an axon. Furthermore, the inner positive to negative pull ensures that the action potential progresses.

However, how can action potential travel in a straight line? How is it that the following negative charge’s pull moves the action potential forward instead of backward?

The refractory time of the ion channel results in one-way action potentials. When an ion channel opens, it closes for a short period of time, usually one to two milliseconds. This is enough to ensure that the next voltage-dependent ion channel in the chain detects the charge shift and allows the next batch of Na+ ions into the cell, raising the charge on the inner membrane surrounding that channel.

Skeletal Muscle Action Potentials

Muscle fibres contract as a result of action potentials. Muscles are made up of groups of hundreds or thousands of skeletal muscular fibres, each of which is made up of a single cylindrical muscle cell (cells). At the conclusion of this part, you may observe how a muscle is produced. Skeletal muscle action potentials need a greater sodium inflow due to the muscle cell’s inner voltage being about -90 mV.

Action potentials pass from the brain to skeletal muscle through a motor neuron. At the neuromuscular junction, the neuron and muscle are practically fused together, with just a small gap (synaptic cleft) separating them. The action potential induces the release of acetylcholine, a neurotransmitter, at the motor nerve terminal. This neurotransmitter is recognised by receptors on the muscle membrane after it crosses the synaptic cleft.

When muscle receptors detect acetylcholine, ion channels in the muscular membrane open. Sodium ions enter the cell when the threshold is reached, and the muscle cell subsequently releases its calcium ion stores (Ca2+). Muscle fibres must have calcium in order to contract. The sodium ion channels close after the action potential is finished, enabling the muscle cell to relax.

Action potentials do not influence a single muscle cell in skeletal muscle. Rather than that, a motor neuron is coupled to many muscle fibres or cells. When a single neuron transmits action potentials to the muscle cells, they all contract. Due to the fact that a muscle is composed of several groups of muscle cells, each group enclosed in a membrane, more than one motor neuron is needed to enable it to contract. These motor neurons do not activate simultaneously to maintain consistent contraction—otherwise, we would have highly jerky motions as the muscles alternated between contract and relax mode.

Physiologists analyse muscle movement by tracking the electrical activity of all muscle fibres’ action potentials at the same time. The sum of many action potentials from several motor neurons in a single muscle or group of muscles is referred to as a compound action potential (CAP).

Cardiac Action Potentials

Heart cells are involved in cardiac action potentials. They begin in pacemaker cells located on the inner surface of the heart muscle, such as the SA node or sinoatrial node. Our sympathetic and parasympathetic nervous systems regulate the rate at which our hearts beat. This is not the time to relax, since our bodies will not get enough oxygen if the heart stops beating.

Numerous cardiovascular medicines exert their impact on the cardiac action potential as well as the sites of action of several heart drugs. The cardiac action potential graph is divided into four distinct phases: 

  • Phase Zero: Depolarization is the first phase (sodium and calcium ion influx)
  • Phase One: gradual repolarization — a brief period during which sodium ion gates close and potassium ion gates open
  • Phase Two: Gradual repolarization-calcium ion influx to help in muscle contraction
  • PhaseThree: Increased repolarization
  • Phase Four: Diastole and pacemaker potential in Phase Four

Additionally, you may record the location of these action potentials with respect to an ECG.

The pacemaker potential is an action potential that occurs in pacemaker cells and replaces the resting potential phase in neurons and skeletal muscle cells. Because the heart is never at rest, this is the case. Because the inner cell membrane’s resting voltage is -90 mV, greater positive ion influx is required, just as it is for skeletal muscle action potentials.

Action Potential Steps

The processes of an action potential may now be summarised since the previous material has provided you with a better understanding of nerve impulses and cell membrane potentials, their significance and mechanism.

The many stages or steps of the action potential are as follows:

Possibility of rest. Around -70 mV or -90 mV. There are no voltage-gated ion channels open. The sodium-potassium pump and leakage-gated ion channels contribute to the stability of the resting potential.

A chemical stimulus, like cAMP or a neurotransmitter, activates the sodium ion channels that are most closely associated with the stimulus.

Depolarization The ensuing inflow of positively charged sodium ions into the cell results in a quick increase in voltage on the cell’s inner side.

The possibility of crossing the threshold The negative charge within the cell membrane grows to a threshold voltage of roughly-55 mV as it increases. Voltage-gated ion channels are activated at the threshold. The following phase may occur only if the inner voltage hits this level.

The commencement of an action potential. When the threshold for initiating an action potential is met, the action potential commences. If the threshold value is not reached, the action potential will not be initiated.

Action potential propagation

in neurons that are not myelinated. The action potential goes along the membrane’s inner side. Sodium ions continue to flow in through voltage-gated ion channels, raising the internal potential of each cell membrane segment to around +40 mV.

In myelinated peripheral neurons, Sodium ion channels can contribute exclusively at the nodes of Ranvier because the myelin layer prevents ion transport across the cell membrane.

Repolarization. Following the action potential, the sodium ion channels shut down and the potassium ion channels responsible for excreting this cation open. As these positive ions are ejected from the cell, the charge within the cell becomes negative.

Hyperpolarization Positively charged ions continue to escape the cell via unresponsive ion channels. At the threshold potential value, the inner side of the cell membrane accumulates a greater negative charge than the outer side. During hyperpolarization, the voltage will be between -71 and -75 mV (undershoot).

Possibility of rest. To maintain the -70 mV inner voltage during hyperpolarization, all voltage-gated channels shut and sodium and potassium ions permeate through the cell membrane through concentration gradients (leakage channels) and pumps. We’ve returned to the starting point.

Action Potential Graph

A graph of action potentials should now make perfect sense. These plots depict the voltage within the cell membrane (in millivolts) on the vertical axis and the time in milliseconds on the horizontal axis.

The graph below illustrates the voltage shift that occurs at each action potential step. At 70 mV, the cell membrane is resting-the flat line indicates the resting potential. This voltage stays steady when potassium and sodium ions move in a concentration gradient across the cell membrane.

When sodium ion channels open in response to stimulation, the charge within the cell quickly rises to a positive charge in a phase known as depolarization-the sudden upward spike. An action potential will occur only when the charge reaches a threshold value of around-55 mV.

Negatively charged potassium ions depart the cell, and sodium ion channels close when an action potential is started and transmitted via the axon or cell membrane. The sharp descending gradient indicates repolarization.

Due to the slow response of the ion channels, hyperpolarization occurs when the inner side of the cell membrane becomes more negative than its resting value for a short period of time. The voltage returns to -70 mV when the ion channels react, restoring the resting state.


  • Rhoads R A, Bell D R. (2009) Medical Physiology: Principles for Clinical Medicine. Baltimore, Lippincott Williams & Wilkins.
  • Brown A G. (2001) Nerve Cells and Nervous Systems: An Introduction to Neuroscience. London, Springer-Verlag.
  • Levitan I B, Kaczmarek L K. (2002) The Neuron: Cell and Molecular Biology. Oxford, Oxford University Press.
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