Depolarization in the context of biology refers to the sudden change within a
cell during which the cell undergoes a dramatic electrical change. Most cells,
especially those that compose the tissues of highly organized animals,
typically maintain an internal environment that is negatively charged
compared to the cell's surrounding environment. This difference in charge
is known as the membrane potential of the cell. In the process of
depolarization, the negative internal charge of the cell becomes positive for
a very brief period of time. This shift from a negative to a positive internal
cellular environment allows for the transmission of electrical impulses both
within a cell and, in certain instances, between cells. This communicative
function of depolarization is essential to the function of many cells,
communication between cells, and the overall function of an organism.
Physiology The process of depolarization is
entirely dependent upon the intrinsic electrical nature of most cells. When a
cell is at rest, the cell maintains what is known as a resting potential. The
resting potential generated by nearly all cells results in the interior of the
cell having a negative charge compared to the exterior of the cell. In order to
maintain this electrical imbalance, microscopic positively and negatively
charged particles called ions are transported across the cell's plasma
membrane. The transport of the ions across the plasma membrane is
accomplished through several different types of transmembrane proteins embedded
in the cell's plasma membrane that function as pathways for ions both into
and out of the cell, such as ion channels, sodium potassium pumps, and
voltage gated ion channels. = Resting potential=
The resting potential must be established within a cell before the
cell can be depolarized. There are many mechanisms by which a cell can establish
a resting potential, however there is a typical pattern of generating this
resting potential that many cells follow. The cell utilizes ion channels,
ion pumps, and voltage gated ion channels in order to generate a negative
resting potential within the cell. However, the process of generating the
resting potential within the cell also creates an environment outside of the
cell that favors depolarization. The sodium potassium pump is largely
responsible for the optimization of conditions on both the interior and the
exterior of the cell for depolarization. By pumping three positively charged
sodium ions out of the cell for every two positively charged potassium ions
pumped into the cell, not only is the resting potential of the cell
established, but an unfavorable concentration gradient is created by
increasing the concentration of sodium outside of the cell and increasing the
concentration of potassium within the cell. Although there is an excessive
amount of potassium in the cell and sodium outside of the cell, the
generated resting potential keeps the voltage gated ion channels in the plasma
membrane closed, preventing the ions that have been pumped across the plasma
membrane from diffusing to an area of lower concentration. Additionally, note
that despite the high concentration of positively-charged Potassium, most cells
contain internal components which accumulate to establish a negative
inner-charge. = Depolarization=
After a cell has established a resting potential, that cell has the capacity to
undergo depolarization. During depolarization, the charge within the
cell rapidly shifts from negative to positive. In order for this rapid change
to take place within the interior of the cell, there are several events that must
occur along the plasma membrane of the cell as well. While the sodium potassium
pump continues to work, the voltage gated ion channels that had been closed
while the cell was at resting potential have been opened by an electrical
stimulus. As the sodium rushes back into the cell the positive sodium ions raise
the charge inside of the cell from negative to positive. Once the interior
of the cell becomes positively charged, depolarization of the cell is complete.
= Repolarization= After a cell has been depolarized, it
undergoes one final change in internal charge. Following depolarization, the
voltage gated sodium ion channels that had been open while the cell was
undergoing depolarization close again. The increased positive charge within the
cell now causes the potassium channels to open. Potassium ions begin to move
down the electrochemical gradient. As potassium moves out of the cell the
potential within the cell plummets and approaches its resting potential once
more. The sodium potassium pump works continuously throughout this process.
Hyperpolarization The process of repolarization causes an
overshoot in the potential of the cell. Potassium ions continue to move out of
the axon so much so that the resting potential is exceeded and the new cell
potential becomes more negative than the resting potential. The resting potential
is ultimately re-established by the closing of all voltage-gated ion
channels and the activity of the sodium potassium ion pump.
Neurons Depolarization is essential to the
functions of many cells in the human body, which is exemplified by the
transmission of stimuli both within a neuron and between two neurons. The
reception of stimuli, neural integration of that stimuli, and the neuron's
response to stimuli all rely upon the ability of neurons to utilize
depolarization to transmit stimuli either within a neuron or between
neurons. = Response to stimulus=
Stimuli to neurons can be a physical, electrical, chemical stimulus, which can
either inhibit or excite the neuron being stimulated. An inhibitory stimulus
is transmitted to the dendrite of a neuron, causing hyperpolarization of the
neuron. The hyperpolarization following an inhibitory stimulus causes a further
decrease in voltage within the neuron below the resting potential. By
hyperpolarizing a neuron, an inhibitory stimulus results in a greater negative
charge that must be overcome in order for depolarization to occur. Excitation
stimuli, on the other hand, will increase the voltage in the neuron,
which will lead to a neuron that is easier to depolarize than the same
neuron in the resting state. Regardless of excitatory or inhibitory, the stimuli
travel down the dendrites of a neuron to the cell body for integration.
= Integration of stimuli= Once the stimuli have reached the cell
body, the nerve must integrate the various stimuli before the nerve can
respond. The stimuli that have traveled down the dendrites converge at the axon
hillock, where they are summed to determine the neuronal response. If the
sum of the stimuli reaches a certain voltage, known as the threshold
potential, depolarization will continue from the axon hillock down the axon.
= Response= The surge of depolarization traveling
from the axon hillock to the axon terminal is known as an action
potential. Action potentials reach the axon terminal, where the action
potential triggers the release of neurotransmitters from the neuron. The
neurotransmitters that are released from the axon continue on to stimulate other
cells such as other neurons or muscle cells. After an action potential travels
down the axon of a neuron, the resting membrane potential of the axon must be
restored before another action potential can travel the axon. This is known as
the recovery period of the neuron, during which the neuron cannot transmit
another action potential. = Rod cells of the eye=
The importance and versatility of depolarization within cells can be seen
in the relationship between rod cells in the eye and their associated neurons.
When rod cells are in the dark, they are depolarized. In the rod cells, this
depolarization is maintained by ion channels that remain open due to the
higher voltage of the rod cell in the depolarized state. The ion channels
allow calcium and sodium to pass freely into the cell, maintaining the
depolarized state. Rod cells in the depolarized state constantly release
neurotransmitters which in turn stimulate the nerves associated with rod
cells. This cycle is broken when rod cells are exposed to light; the
absorption of light by the rod cell causes the channels that had facilitated
the entry of sodium and calcium into the rod cell to close. When these channels
close, the rod cell produces less neurotransmitter, which is perceived by
the brain as light. In the case of rod cells and neurons, depolarization
actually prevents a signal from reaching the brain as opposed to stimulating the
transmission of the signal. Vascular endothelium
Endothelium is a thin layer of simple squamous epithelial cells that line the
interior of both blood and lymph vessels. The endothelium that lines
blood vessels is known as vascular endothelium, which is subject to and
must withstand the forces of blood flow and blood pressure from the
cardiovascular system. In order to withstand these cardiovascular forces,
endothelial cells must simultaneously have a structure capable of withstanding
the forces of circulation while also maintaining a certain level of
plasticity in the strength of their structure. This plasticity in the
structural strength of the vascular endothelium is essential to the overall
function of the cardiovascular system; endothelial cells within blood vessels
can alter the strength of their structure in order to maintain the
vascular tone of the blood vessel they line, prevent vascular rigidity, and
even help to regulate blood pressure within the cardiovascular system.
Endothelial cells are able to accomplish these feats by utilizing depolarization
in order to alter their structural strength. When an endothelial cell
undergoes depolarization, the result is a marked decrease in the rigidity and
structural strength of the cell by altering the network of fibers that
provide these cells with their structural support. Depolarization in
vascular endothelium is essential not only to the structural integrity of
endothelial cells, but also to the ability of the vascular endothelium to
aid in the regulation of vascular tone, prevention of vascular rigidity, and the
regulation of blood pressure. Heart
Depolarization occurs in the four chambers of the heart: both atria first,
and then both ventricles. The sinoatrial node on the wall of the
right atrium initiates depolarization in the right and left atria, causing
contraction, which is symbolized by the P wave on an electrocardiogram.
The SA node sends the depolarization wave to the atrioventricular node which
–- with a delay of about 100 milliseconds to allow the atria to
finish contracting –- then causes contraction in both ventricles, seen in
the QRS wave. At the same time, the atria are repolarized and relaxed.
The ventricles are repolarized and relaxed at the T wave.
This process continues regularly, unless there is a problem in the heart.
Depolarization blockers There are drugs, called depolarization
blocking agents, that cause prolonged depolarization by opening channels
responsible for depolarization and not allowing them to close, preventing
repolarization. Examples include the nicotinic agonists suxamethonium and
decamethonium. References
Further reading Purves D, Augustine GJ, Fitzpatrick D;
et al., eds.. Neuroscience. Sunderland, Mass: Sinauer Assoc. ISBN 0-87893-742-0.
External links "Depolarization". Psychology Department,
Hanover College. Retrieved 18 May 2013.