Got Potential A. Membrane Potentials
1. Cell Membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside enviroment. The cell membrane is selective permeable to ions and organic molecules and controls the movement of substances in and out of cells.The basic function of the cell membrane is to protect the cell from its surroundings. It consists of the lipid bilayer with embedded proteins. Cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signaling and serve as the attachment surface for several extracellular structures, including the cell wall, glycocalyx, and intracellular cytoskeleton. Cell membranes can be artificially reassembled.
The membranes of all nerve cells have a potential difference across them, with the cell interior negative with respect to the exterior (a). In neurons, stimuli can alter this potential difference by opening sodium channels in the membrane. For example, neurotransmitters interact specifically with sodium channels (or gates). So sodium ions flow into the cell, reducing the voltage across the membrane.
Once the potential difference reaches a threshold voltage, the reduced voltage causes hundreds of sodium gates in that region of the membrane to open briefly. Sodium ions flood into the cell, completely depolarizing the membrane. This opens more voltage-gated ion channels in the adjacent membrane, and so a wave of depolarization courses along the cell — the action potential.
As the action potential nears its peak, the sodium gates close, and potassium gates open, allowing ions to flow out of the cell to restore the normal potential of the membrane
Membranes are polarized or, in other words, exhibit a resting membrane potential. This means that there is an unequal distribution of ions (atoms with a positive or negative charge) on the two sides of the nerve cell membrane. This potential generally measures about 70 millivolts (with the inside of the membrane negative with respect to the outside). So, the resting membrane potential is expressed as -70 mV, and the minus means that the inside is negative relative to (or compared to) the outside. It is called a resting potential because it occurs when a membrane is not being stimulated or conducting impulses (in other words, it's resting)
Bibliography:
Budin, Itay; Devaraj, Neal K. (December 29, 2011). "Membrane Assembly Driven by a Biomimetic Coupling Reaction" Journal of the American Chemical Society134 (2): 751–753. doi:10.1021/ja2076873. http://pubs.acs.org/doi/abs/10.1021/ja2076873. Retrieved February 18, 2012.
Staff (January 25, 2012). "Chemists Synthesize Artificial Cell Membrane". ScienceDaily. http://www.sciencedaily.com/releases/2012/01/120125132822.htm Retrieved February 18, 2012
Staff (January 26, 2012). "Chemists create artificial cell membrane" kurzweilai.net. http://www.kurzweilai.net/chemists-create-artificial-cell-membrane. Retrieved February 18, 2012
(Gutkin and Ermentrout 2006
2. Polarity
Cell polarity refers to spatial differences in the shape, structure, and function of cells. Almost all cell types exhibit some sort of polarity, which enables them to carry out specialized functions. Classical examples of polarized cells are described below, including epithelial cells with apical-basal polarity, neurons in which signals propagate in one direction from dendrites to axons, and migrating cells.
Most neurons can be anatomically characterized as:
The conduction of nerve impulses is an example of an all-or-none response. In other words, if a neuron responds at all, then it must respond completely. Greater intensity of stimulation does not produce a stronger signal but can produce a higher frequency of firing. There are different types of receptor response to stimulus, slowly adapting or tonic receptors respond to steady stimulus and produce a steady rate of firing. These tonic receptors most often respond to increased intensity of stimulus by increasing their firing frequency, usually as a power function of stimulus plotted against impulses per second. This can be likened to an intrinsic property of light where to get greater intensity of a specific frequency (color) there have to be more photons, as the photons can't become "stronger" for a specific frequency.
4. Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential. Hyperpolarization is the opposite of depolarization, and inhibits the rise of an action potential.
If, for example, a cell has a resting potential of -70mV, once the membrane potential changes to -50mV, then the cell has been depolarized. Depolarization is often caused by influx of cations, e.g. Na+ through Na+ channels, or Ca2+ through Ca2+ channels. On the other hand, efflux of K+ through K+ channels inhibits depolarization, as does influx of Cl– (an anion) through Cl– channels. If a cell has K+ or Cl– currents at rest, then inhibition of those currents will also result in a depolarization.
Because depolarization is a change in membrane voltage, electrophysiologists measure it using current clamp techniques. In voltage clamp, the membrane currents giving rise to depolarization are either an increase in inward current, or a decrease in outward current.
5. Repolarization
After the peak of action potential, called spike potential, the permeability of the membrane to Na+ decreases, while it becomes more permeable for K+ which rapidly diffuses out from the cytoplasm to extracellular fluid due to electrochemical gradient. Soon, this part of membrane regains its original polarity and becomes electropositive on outside and electronegative on inside. This is known as repolarization and the nerve fibre is called repolarized nerve fibre. A repolarized nerve fibre has same polarity as that of a polarized nerve fibre but has different ionic distribution. It has more K+ outside and more Na+ inside. The repolarized nerve fibre undergoes a refractory period of a few milliseconds during which the original ionic distribution is restored by a sodium potassium exchange pump which actively transport sodium ions out and potassium ion in.
6. Hyperpolarization
Hyperpolarization is a change in a cell's membrane potential that makes it more negative. It is the opposite of a depolarization. It inhibits action potentials by increasing the stimulus required to move the membrane potential to the action potential threshold.
Hyperpolarization is often caused by efflux of K+ (a cation) through K+ channels, or influx of Cl– (an anion) through Cl– channels. On the other hand, influx of cations. e.g. Na+ through Na+ channelsor Ca2+ through Ca2+ channels, inhibits hyperpolarization. If a cell has Na+ or Ca2+ currents at rest, then inhibition of those currents will also result in a hyperpolarization.
Because hyperpolarization is a change in membrane voltag,, electrophysiologists measure it using current clamp techniques. In voltage clamp, the membrane currents giving rise to hyperpolarization are either an increase in outward current, or a decrease in inward current.
In neurons, the cell enters a state of hyperpolarization immediately following the generation of an action potential. While hyperpolarized, the neuron is in a refractory period that lasts roughly 2 milliseconds, during which the neuron is unable to generate subsequent action potentials. Sodium-potassium ATPases redistribute K+ and Na+ ions until the membrane potential is back to its resting potential of around –70 millivolts, at which point the neuron is once again ready to transmit another action potential.
Pack, Phillip E. "Cliffs AP Biology 3rd Edition
7. Refractory Period
In physiology, a refractory period is a period of time during which an organ or cell is incapable of repeating a particular action, or (more precisely) the amount of time it takes for an excitable membrane to be ready for a second stimulus once it returns to its resting state following an excitation. It most commonly refers to electrically excitable muscle cells or neurons. Absolute refractory period corresponds to depolarisation and repolarisation, whereas relative refractory period corresponds to hyperpolarisation.
After initiation of an action potential, the refractory period is defined two ways:
1. Cell Membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside enviroment. The cell membrane is selective permeable to ions and organic molecules and controls the movement of substances in and out of cells.The basic function of the cell membrane is to protect the cell from its surroundings. It consists of the lipid bilayer with embedded proteins. Cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signaling and serve as the attachment surface for several extracellular structures, including the cell wall, glycocalyx, and intracellular cytoskeleton. Cell membranes can be artificially reassembled.
The membranes of all nerve cells have a potential difference across them, with the cell interior negative with respect to the exterior (a). In neurons, stimuli can alter this potential difference by opening sodium channels in the membrane. For example, neurotransmitters interact specifically with sodium channels (or gates). So sodium ions flow into the cell, reducing the voltage across the membrane.
Once the potential difference reaches a threshold voltage, the reduced voltage causes hundreds of sodium gates in that region of the membrane to open briefly. Sodium ions flood into the cell, completely depolarizing the membrane. This opens more voltage-gated ion channels in the adjacent membrane, and so a wave of depolarization courses along the cell — the action potential.
As the action potential nears its peak, the sodium gates close, and potassium gates open, allowing ions to flow out of the cell to restore the normal potential of the membrane
Membranes are polarized or, in other words, exhibit a resting membrane potential. This means that there is an unequal distribution of ions (atoms with a positive or negative charge) on the two sides of the nerve cell membrane. This potential generally measures about 70 millivolts (with the inside of the membrane negative with respect to the outside). So, the resting membrane potential is expressed as -70 mV, and the minus means that the inside is negative relative to (or compared to) the outside. It is called a resting potential because it occurs when a membrane is not being stimulated or conducting impulses (in other words, it's resting)
Bibliography:
Budin, Itay; Devaraj, Neal K. (December 29, 2011). "Membrane Assembly Driven by a Biomimetic Coupling Reaction" Journal of the American Chemical Society134 (2): 751–753. doi:10.1021/ja2076873. http://pubs.acs.org/doi/abs/10.1021/ja2076873. Retrieved February 18, 2012.
Staff (January 25, 2012). "Chemists Synthesize Artificial Cell Membrane". ScienceDaily. http://www.sciencedaily.com/releases/2012/01/120125132822.htm Retrieved February 18, 2012
Staff (January 26, 2012). "Chemists create artificial cell membrane" kurzweilai.net. http://www.kurzweilai.net/chemists-create-artificial-cell-membrane. Retrieved February 18, 2012
(Gutkin and Ermentrout 2006
2. Polarity
Cell polarity refers to spatial differences in the shape, structure, and function of cells. Almost all cell types exhibit some sort of polarity, which enables them to carry out specialized functions. Classical examples of polarized cells are described below, including epithelial cells with apical-basal polarity, neurons in which signals propagate in one direction from dendrites to axons, and migrating cells.
Most neurons can be anatomically characterized as:
- Unipolar or pseudounipolar; dendrite and axon emerging from same process.
- Bipolar; axon and single dendrite on opposite ends of the soma.
- Multipolar;: more than two dendrites:
- Golgi I; neurons with long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells.
- Golgi II; neurons whose axonal process projects locally; the best example is the granule cell.
The conduction of nerve impulses is an example of an all-or-none response. In other words, if a neuron responds at all, then it must respond completely. Greater intensity of stimulation does not produce a stronger signal but can produce a higher frequency of firing. There are different types of receptor response to stimulus, slowly adapting or tonic receptors respond to steady stimulus and produce a steady rate of firing. These tonic receptors most often respond to increased intensity of stimulus by increasing their firing frequency, usually as a power function of stimulus plotted against impulses per second. This can be likened to an intrinsic property of light where to get greater intensity of a specific frequency (color) there have to be more photons, as the photons can't become "stronger" for a specific frequency.
4. Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential. Hyperpolarization is the opposite of depolarization, and inhibits the rise of an action potential.
If, for example, a cell has a resting potential of -70mV, once the membrane potential changes to -50mV, then the cell has been depolarized. Depolarization is often caused by influx of cations, e.g. Na+ through Na+ channels, or Ca2+ through Ca2+ channels. On the other hand, efflux of K+ through K+ channels inhibits depolarization, as does influx of Cl– (an anion) through Cl– channels. If a cell has K+ or Cl– currents at rest, then inhibition of those currents will also result in a depolarization.
Because depolarization is a change in membrane voltage, electrophysiologists measure it using current clamp techniques. In voltage clamp, the membrane currents giving rise to depolarization are either an increase in inward current, or a decrease in outward current.
5. Repolarization
After the peak of action potential, called spike potential, the permeability of the membrane to Na+ decreases, while it becomes more permeable for K+ which rapidly diffuses out from the cytoplasm to extracellular fluid due to electrochemical gradient. Soon, this part of membrane regains its original polarity and becomes electropositive on outside and electronegative on inside. This is known as repolarization and the nerve fibre is called repolarized nerve fibre. A repolarized nerve fibre has same polarity as that of a polarized nerve fibre but has different ionic distribution. It has more K+ outside and more Na+ inside. The repolarized nerve fibre undergoes a refractory period of a few milliseconds during which the original ionic distribution is restored by a sodium potassium exchange pump which actively transport sodium ions out and potassium ion in.
6. Hyperpolarization
Hyperpolarization is a change in a cell's membrane potential that makes it more negative. It is the opposite of a depolarization. It inhibits action potentials by increasing the stimulus required to move the membrane potential to the action potential threshold.
Hyperpolarization is often caused by efflux of K+ (a cation) through K+ channels, or influx of Cl– (an anion) through Cl– channels. On the other hand, influx of cations. e.g. Na+ through Na+ channelsor Ca2+ through Ca2+ channels, inhibits hyperpolarization. If a cell has Na+ or Ca2+ currents at rest, then inhibition of those currents will also result in a hyperpolarization.
Because hyperpolarization is a change in membrane voltag,, electrophysiologists measure it using current clamp techniques. In voltage clamp, the membrane currents giving rise to hyperpolarization are either an increase in outward current, or a decrease in inward current.
In neurons, the cell enters a state of hyperpolarization immediately following the generation of an action potential. While hyperpolarized, the neuron is in a refractory period that lasts roughly 2 milliseconds, during which the neuron is unable to generate subsequent action potentials. Sodium-potassium ATPases redistribute K+ and Na+ ions until the membrane potential is back to its resting potential of around –70 millivolts, at which point the neuron is once again ready to transmit another action potential.
Pack, Phillip E. "Cliffs AP Biology 3rd Edition
7. Refractory Period
In physiology, a refractory period is a period of time during which an organ or cell is incapable of repeating a particular action, or (more precisely) the amount of time it takes for an excitable membrane to be ready for a second stimulus once it returns to its resting state following an excitation. It most commonly refers to electrically excitable muscle cells or neurons. Absolute refractory period corresponds to depolarisation and repolarisation, whereas relative refractory period corresponds to hyperpolarisation.
After initiation of an action potential, the refractory period is defined two ways:
- The absolute refractory period is the interval during which a second action potential absolutely cannot be initiated, no matter how large a stimulus is applied.
- The relative refractory period is the interval immediately following during which initiation of a second action potential is inhibited but not impossible.
