Cell Membrane Potential

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Chapter: Anatomy and Physiology for Health Professionals: Control and Coordination: Neural Tissue

1. Explain the various phases of action potentials. 2. D efine action potential and threshold potential. 3. Explain the effects of myelination on an axons ability to conduct action potentials. 4. What is the all-or-none phenomenon? 5. Describe “resting potential.”

Cell Membrane Potential

Cell Membrane Potential

A cell membrane’s surface is usually electrically charged (polarized) compared with its inner contents. This is due to unequal amounts of positive and negative ions and is important for conduction of nerve and muscle impulses. Adequate stimulation of a neuron causes gen-eration of an electrical impulse in response. An action potential is a change in neuron membrane polarization and a return to its resting state. An action potential forms a nerve impulse propagated along an axon. The human body is electrically neutral, with the same num-ber of positive and negative electrical charges. Some regions, however, are positively or negatively charged. Opposite charges attract each other, requiring energy to separate them. Therefore, when opposite charges come together, energy is liberated to perform cellular work.


Action Potential

The threshold potential causes permeability to sud-denly change. Sodium channels open and sodium diffuses freely inward. The membrane becomes depo-larized. Nearly simultaneously, potassium channels open to allow potassium to diffuse freely outward. The inside of the membrane becomes briefly hyperpolar-ized and then repolarized to its resting potential. For example, if the resting membrane potential is ‒70 mV and the threshold is ‒60 mV, a membrane potential of ‒62 mV will not produce an action potential. The action potential is defined as this rapid sequence of ­depolarization and repolarization, taking only about one-thousandth of a second. Only cells with excitable membranes can generate action potentials. These cells include muscle cells and neurons. Synaptic activity produces graded potentials in postsynaptic cell plasma membranes.

The terms depolarization and hyperpolarization describe membrane potential changes that are related to resting membrane potential. In depolarization, the inside of the membrane becomes less negative (e.g., it moves from ‒70 to ‒60 mV). In hyperpolarization, the inside of the membrane becomes more negative. Many action potentials can occur, and resting potentials can be reestablished before the concentrations of sodium and potassium significantly change. Active transport inside the membranes maintains the original concen-trations on either side. Action potentials do not decay with greater distance traveled.

In skeletal muscle cells and neurons, action poten-tial generation and transmission occur in the same way. Action potentials are usually generated only in axons, with neurons generating them only when enough stimulation occurs. Stimuli change the neuron membrane’s permeability via the opening of certain voltage-gated channels on the axon. Changes in the membrane potential cause these channels to open and close (FIGURE 11-8A). Graded potentials (local currents) activate them, with these currents spreading toward axons along the dendrite and cell body membranes. The method in which an action potential is generated is shown in FIGURE 11-8B.


Depolarization Phase

Beginning with a neuron in its polarized (resting) state, all gated sodium and potassium ion channels are closed. Leakage channels are the only ones open, and they maintain resting membrane potential. Depolariza-tion opens sodium channels and then inactivates them. The axon membrane is first depolarized, followed by sodium channels opening and the rushing in of sodium ions into the cell. These positively charged ions depo-larize the local membrane area to a greater degree, opening more sodium channels. The cell interior, there-fore, becomes less negative on a continual basis. When threshold is reached at the stimulation site, depolar-ization becomes self-generating. Threshold is usually between ‒55 mV and ‒50 mV. Positive feedback assists the process, with depolarization being driven by ionic currents that were created by the sodium ion influx.

Repolarization Phase

In repolarization, sodium channels are inactivating, with potassium channels open. The intense rise of action potential is only about 1 msec in length, being self-limited because of the slow inactivation gates of the sodium channels closing at this time. The membrane permeability to sodium, therefore, reduces to resting levels. The net influx of sodium ions completely stops, meaning the action potential spike stops its rise. The slow potassium channels open and ­potassium quickly leaves the cell along its electrochemical gradient. The internal negativity of the resting neuron is, therefore, restored (repolarization). The fast decline in sodium permeability and the increased potassium permeability aid in repolarization.

Hyperpolarization

In hyperpolarization, some potassium ion channels remain open and sodium ion channels are reset. Usu-ally, the time in which increased potassium permea-bility occurs lasts longer than is actually required to restore the resting state. A hyperpolarization is seen on the action potential curve, appearing as a small dip after the spike; this is the result of excessive potassium ion efflux before the closure of the potassium channels.

All-or-None Phenomenon

Action potentials are not always produced by depo-larization events. For depolarization to produce an action potential, it must reach threshold values to make an axon “fire.” Threshold may be the membrane potential at which potassium movement’s outward current is identical to sodium movement’s inward current. When the membrane has been depolarized by 15 mV to 20 mV from its resting value, threshold is usually reached. An action potential is, therefore, an all-or-none phenomenon, a principle occurring completely or not at all.

Refractory Periods

A neuron cannot respond to any amount of stimulus when an area of neuron membrane is generating an action potential and its voltage-gated sodium chan-nels are opened. The absolute refractory period is the period from which sodium channels open until they begin to reset to their resting state. This period results in each action potential being a unique all-or-one occurrence. It also enforces the one- way ­transmission of the action potential. The interval that follows­ the absolute refractory period is the relative­ refractory period. At this time, most sodium ­channels have resumed their resting state, but some potassium channels are still open, and repolarization is occurring.

Propagation of Action Potentials

Propagation of action potentials may occur in several­ different ways. In unmyelinated axons, con-tinuous propagation is the method by which action ­potentials move. At the first segment of the axon, there is a brief change of the transmembrane poten-tial when it becomes positive instead of negative.

As sodium ions begin to move, a local current devel-ops, spreads, and depolarizes nearby areas of the membrane. A chain reaction occurs, with the process repeating and moving the action potential forward but not backward. Backward movement is prevented by the previous axonal segment still being in its abso-lute refractory period. The furthest portions of the plasma membrane are eventually affected. The action potential that reaches the synaptic terminal is identi-cal to the one that was initially generated. Continuous propagation occurs at a rate of approximately 1 meter per second, which is about 2 miles per hour. A ­second stimulus is needed for a second action potential to occur at the same site.

In the CNS and PNS, saltatory propagation moves action potentials much more quickly along axons. An action potential appearing at the first segment­ of a myelinated axon avoids the internodes and depolarizes the nearest node to threshold. The action potential jumps the 1–2 mm distance between the nodes in a large, myelinated axon. This requires less energy than continuous propagation, because there is less surface area involved and not as many sodium ions are needed to be pumped from the ­cytoplasm.


Ion Distribution

Because of active transport, body cells have more sodium ions outside of them and more potassium ions inside them. These cells’ cytoplasm contains many negatively charged phosphate ions, sulfate ions, and proteins. Distribution of ions is determined partly by selectively permeable channels located in the cell mem-branes. Some are open, whereas others can open or close. Certain channels can allow specific ions to pass through, whereas others cannot. Potassium ions pass through cell membranes more easily than sodium ions. Therefore, potassium ions contribute greatly to membrane polarization. Membrane channels are large proteins­ with subunits. Their amino acid chains are distributed in a back and forth pattern across the membrane. Those known as leakage (nongated) channels remain open constantly. These are also called passive channels . Gated channels open and close based on certain signals. These are also called active channels . Every gated channel can either be closed but able to open, open (also called activated), or closed and unable to open (also called inactivated).

Chemically gated channels bind to specific chem-icals to open or close. Examples include the ­receptors that bind acetylcholine (ACh) at neuromuscular junctions. These channels are most common on dendrites and cell bodies of neurons. Voltage­-gated channels respond to changes in the transmembrane potential by opening or closing, and are common on excitable membranes, which are those that can generate or conduct action potentials. These channels are common on the sarcolemma of skeletal or cardiac muscle fibers and the axons of unipolar and multipolar neurons. Mechanically gated channels respond to physical distortions of membrane surfaces by opening or closing. They are important in sensory receptors involving pressure, touch, or vibration.


Resting Potential

Sodium and potassium ions move from areas of high concentration to areas of low concentration, based on permeability. Because resting cell membranes are more permeable to potassium ions than sodium ions, the potassium ions diffuse out of cells more rapidly than sodium ions diffuse. More positive charges leave the cell by diffusion than enter it, making the outside of the cell membrane positive and the inside negative. This difference in charges is called a potential differ-ence. In a resting nerve cell, this potential difference is called a resting potential. If undisturbed, the cell membrane remains polarized. Sodium and potas-sium continue to be actively transported, maintaining the concentrations needed for diffusion. Membrane voltage can be measured by using a voltmeter, with one microelectrode inserted into a neuron and the other into its extracellular fluid. Membrane voltage is approximately ‒70 mV. The minus sign means that the cytoplasmic (inside) portion of the membrane is neg-atively charged compared with the outside portion. In different types of neurons, the value of the resting membrane potential ranges from ‒40 to ‒90 mV. The resting potential only exists across the membrane, with solutions inside and outside the cell being elec-trically neutral. The resting potential is generated by either differences in permeability of the plasma mem-brane to the intracellular and extracellular fluids or differences in the ionic composition of these fluids.

Ionic Composition Differences

A lower concentration of sodium ions and a higher concentration of potassium ions exist in the cell cyto-sol in comparison with the extracellular fluid. Anionic (negatively charged) proteins aid in balancing the pos-itive charges of intracellular cations, which are mostly potassium ions. The positive charges of sodium and other cations are mostly balanced by chloride ions in the extracellular fluid. There are many other solutes in both fluids, including glucose and urea. Even so, potassium plays the chief role in generating the mem-brane potential.

Plasma Membrane Permeability Differences

When resting, the plasma membrane is not perme-able to large anionic cytoplasmic proteins. It is slightly permeable to sodium, but nearly 25 times more per-meable to potassium. Also, it is highly permeable to chloride ions. These permeabilities are related to the leakage ion channel properties. Along their concen-tration gradient, potassium ions diffuse out of the cell very easily in comparison with how easily sodium ions can enter. The cell becomes more negatively charged inside as potassium ions flow outward. The slowly entering sodium ions cause the cell to become just slightly more positive than it would be if only potas-sium was flowing. Therefore, at resting potential, the cell’s negative interior is based much more on the abil-ity of potassium to diffuse outward than it is on the entrance of sodium. Via the adenosine triphosphate (ATP)-controlled sodium-potassium pump, three sodium ions are ejected from the cell followed by two potassium ions being transported back in. This pump stabilizes the resting potential via maintenance of con-centration gradients for both sodium and potassium.


Changes in Potential

Nerve cells respond with excitability to changes in the surroundings. Some can detect specific changes such as temperature, pressure, or light from outside the body. Many neurons respond to neurotransmit-ters from other neurons, usually affecting the resting potential in a certain part of the nerve cell’s mem-brane. If the resting potential decreases, it is described as being depolarized. The amount of change in resting potential is proportional to the amount of the stim-ulus. The greater the stimulus, the greater the depo-larization. Sufficiently depolarized neurons cause the membrane potential to reach its threshold potential. When this happens, an action potential is reached, which is the basis for the nerve impulse. The primary method in which neurons­ send signals over long dis-tances throughout the body is the generation and transmission of action potentials.


1. Explain the various phases of action potentials.

2. D efine action potential and threshold potential.

3. Explain the effects of myelination on an axons ability to conduct action potentials.

4. What is the all-or-none phenomenon?

5. Describe “resting potential.”

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