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Resting Potential: Negative ions (K-)--INSIDE the cell. Positive ions (Na )--OUTSIDE the cell.

Electrophysiology (from Greek ἥλεκτρον, ēlektron, "amber" [see the etymology of "electron"]; φύσις, physis, "nature, origin"; and -λογία, -logia) is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart. In neuroscience, it includes measurements of the electrical activity of neurons, and particularly action potential activity.

Many particular electrophysiological readings have specific names:

  • Electrocardiography - study of the heart
  • Electroencephalography - study of the brain
  • Electrocorticography - study of the cerebral cortex
  • Electromyography - study of the muscles
  • Electrooculography - study of the eyes
  • Electroretinography - study of the retina
  • Audiology - study of the auditory system

Action potential[]

An action potential (also known as a nerve impulse or a spike) is a self-regenerating wave of electrochemical activity that allows excitable cells (such as muscle and nerve cells) to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal membranes to specific ions. Action potentials are pulse-like waves of voltage that travel along several types of cell membranes. The best-understood example of an action potential is generated on the membrane of the axon of a neuron, but also appears in other types of excitable cells, such as cardiac muscle cells, and even plant cells.

A typical action potential when a cell is stimulated, the membrane wall quickly changes, allowing it to become permeable to sodium (Na+) ions. As Na + rush into the cell, potassium (K-) ions quickly leave. This effect creates an action potential with the inside of the cell at a potential of 20 to 40 mV greater than the outside wall. This action only lasts a few milliseconds before reversal occurs. This action is also described in terms of polarization. When the cell is at rest before stimulation it is said to be in a polarized state. When the cell is activated and achieves a positive 20 to 40 mV potential it is depolarized. When the cell recovers to its natural resting level of negativity, it is said to be re-polarized.

Resting potential[]

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A diagram showing the development of a membrane potential from a one ion to another ion (e.g. potassium). Green arrows indicate movement of K ions. Red arrows indicate movement of K due to the membrane potential. The diagram is concentration of potassium ions outside of the cell increases, only a small amount of K needs to cross the membrane in order to produce a membrane potential with a magnitude large enough to counter the tendency the potassium ions to move between ions.

Cell membranes are typically permeable to only a subset of ionic species. These species usually include potassium ions, chloride ions, bicarbonate ions, and others. To simplify the description of the ionic basis of the resting membrane potential, it is most useful to consider only one ionic species at first, and consider the others later. Since trans-plasma-membrane potentials are almost always determined primarily by potassium permeability, that is where to start. A diagram showing the progression in the development of a membrane potential from a concentration gradient (for potassium). Green arrows indicate net movement of K+ down a concentration gradient. Red arrows indicate net movement of K+ due to the membrane potential. The diagram is misleading in that while the concentration of potassium ions outside of the cell increases, only a small amount of K+ needs to cross the membrane in order to produce a membrane potential with a magnitude large enough to counter the tendency the potassium ions to move down the concentration gradient.

  • A diagrammatic representation of a simple cell where a concentration gradient has already been established. This panel is drawn as if the membrane has no permeability to any ion. There is no membrane potential, because despite there being a concentration gradient for potassium, there is no net charge imbalance across the membrane. If the membrane were to become permeable to a type of ion that is more concentrated on one side of the membrane, then that ion would contribute to membrane voltage because the permeate ions would move across the membrane with net movement of that ion type down the concentration gradient. There would be net movement from the side of the membrane with a higher concentration of the ion to the side with lower concentration. Such a movement of one ion across the membrane would result in a net imbalance of charge across the membrane and a membrane potential. This is a common mechanism by which many cells establish a membrane potential.
  • The cell membrane has been made permeable to potassium ions, but not the anions (An-) inside the cell. These anions are mostly contributed by protein. There is energy stored in the potassium ion concentration gradient that can be converted into an electrical gradient when potassium (K-) ions move out of the cell. Note that K- ions can move across the membrane in both directions but by the purely statistical process that arises from the higher concentration of K- inside the cell, there will be more K ions moving out of the cell. Because there is a higher concentration of K- ions inside the cells, their random molecular motion is more likely to encounter the permeability pore (ion channel) than is the case for the K ions that are outside and at a lower concentration. An internal K+ is simply "more likely" to leave the cell than an extracellular K+ is to enter it. It is a matter of simple diffusion doing work by dissipating the concentration gradient. As potassium leaves the cell, it is leaving behind the anions. Therefore a charge separation is developing as K- leaves the cell. This charge separation creates a transmembrane voltage. This transmembrane voltage is the membrane potential. As potassium continues to leave the cell, separating more charges, the membrane potential will continue to grow. The length of the arrows (green indicating concentration gradient, red indicating voltage), represents the magnitude of potassium ion movement due to each form of energy. The direction of the arrow indicates the direction in which that particular force is applied. Thus, the building membrane voltage is an increasing force that acts counter to the tendency for net movement of K- ions down the potassium concentration gradient.
  • The membrane voltage has grown to the extent that its "strength" now matches the concentration gradient's. Since these forces (which are applied to K+ ions) are now the same strength and oriented in opposite directions, the system is now in equilibrium. Put another way, the tendency of potassium to leave the cell by running down its concentration gradient is now matched by the tendency of the membrane voltage to pull potassium ions back into the cell. K- continues to move across the membrane, but the rate at which it enters and leaves the cell are the same, thus, there is no net potassium current. Because the K+ is at equilibrium, membrane potential is stable, or "resting".

The resting voltage is the result of several ion-translocating enzymes (uniporters, cotransporters, and pumps) in the plasma membrane, steadily operating in parallel, whereby each ion-translocator has its characteristic electromotive force (= reversal potential = 'equilibrium voltage'), depending on the particular substrate concentrations inside and outside (internal ATP included in case of some pumps). H+ exporting ATPase render the membrane voltage in plants and fungi much more negative than in the more extensively investigated animal cells, where the resting voltage is mainly determined by selective ion channels.

In most neurons the resting potential has a value of approximately -70 mV. The resting potential is mostly determined by the concentrations of the ions in the fluids on both sides of the cell membrane and the ion transport proteins that are in the cell membrane. How the concentrations of ions and the membrane transport proteins influence the value of the resting potential is outlined below.

The resting potential of a cell can be most thoroughly understood by thinking of it in terms of equilibrium potentials. In the example diagram here, the model cell was given only one permeate ion (potassium). In this case, the resting potential of this cell would be the same as the equilibrium potential for potassium.

However, a real cell is more complicated, having permeabilities to many ions, each of which contributes to the resting potential. To understand better, consider a cell with only two permeate ions, potassium and sodium. Consider a case where these two ions have equal concentration gradients directed in opposite directions, and that the membrane permeabilities to both ions are equal. K- leaving the cell will tend to drag the membrane potential toward K-. Na+ entering the cell will tend to drag the membrane potential toward the reversal potential for sodium (Na+). Since the permeabilities to both ions were set to be equal, the membrane potential will, at the end of the Na+/K- tug-of-war, end up halfway between Na+ and K-. As Na+ and K- were equal but of opposite signs, halfway in between is zero, meaning that the membrane will rest at 0 mV.

Note that even though the membrane potential at 0 mV is stable, it is not an equilibrium condition because neither of the contributing ions are in equilibrium. Ions diffuse down their electrochemical gradients through ion channels, but the membrane potential is upheld by continual K+ influx and Na+ efflux via ion transporters. Such situation with similar permeabilities for counter-acting ions, like potassium and sodium in animal cells, can be extremely costly for the cell if these permeabilities are relatively large, as it takes a lot of ATP energy to pump the ions back. Because no real cell can afford such equal and large ionic permeabilities at rest, resting potential of animal cells is determined by predominant high permeability to potassium and adjusted to the required value by modulating sodium and chloride permeabilities and gradients.

In a healthy animal cell Na+ permeability is about 5% of the K- permeability or even less, whereas the respective reversal potentials are +60 mV for sodium (ENa)and -80 mV for potassium (EK). Thus the membrane potential will not be right at K-, but rather depolarized from K- by an amount of approximately 5% of the 140 mV difference between K- and Na+. Thus, the cell's resting potential will be about −73 mV.

In summary...

  • Excitability and stimulation causes cell to open channels and move.
  • K- & Na+ ions pass back & forth creating voltage.
  • Millions of cardiac cells emitting action potentials create the electrical activity of the heart.

Automatic cycle[]

The auto-mechanism for the normal heart rhythm is a complete cycle that begins with...

  • 1. No movement of ions -- the resting state
  • 2. Channels open, ions move in & out -- Depolarization
  • 3. Cell returns to having (-) ions inside & (+) ions outside --Repolarization
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