Editing Depolarization (section)

==Neurons==

[[File:1206 The Neuron.jpg|thumb|left|Structure of a neuron]]

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 those 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 physical, electrical, or chemical, and can either inhibit or excite the neuron being stimulated. An inhibitory stimulus is transmitted to the dendrite of a neuron, causing [[Hyperpolarization (biology)|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 for depolarization to occur.

Excitation stimuli, on the other hand, increase the voltage in the neuron, which leads to a neuron that is easier to depolarize than the same neuron in the resting state. Regardless of it being excitatory or inhibitory, the stimulus travels down the dendrites of a neuron to the cell body for integration.

===Integration of stimuli===

[[File:1224 Post Synaptic Potential Summation.jpg|thumb|right|[[Summation (neurophysiology)|Summation]] of stimuli at an [[axon hillock]]]]

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 [[Summation (neurophysiology)|summed]] to determine the neuronal response. If the sum of the stimuli reaches a certain voltage, known as the [[threshold potential]], depolarization continues 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 [[neurotransmitter]]s 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 cell]]s 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 cells produce fewer neurotransmitters, which is perceived by the brain as an increase in light. Therefore, in the case of rod cells and their associated neurons, depolarization actually prevents a signal from reaching the brain as opposed to stimulating the transmission of the signal.<ref>{{cite book|last1=Lodish|first1=H|last2=Berk|first2=A|last3=Kaiser|first3=C|last4=Krieger|first4=M|last5=Bretscher|first5=A|last6=Ploegh|first6=H|last7=Amon|first7=A|title=Molecular Cell Biology|url=https://archive.org/details/molecularcellbio00lodi|url-access=registration|date=2000|publisher=W. H. Freeman and Company|location=New York, NY|pages=[https://archive.org/details/molecularcellbio00lodi/page/695 695]|edition=7th|isbn=978-0-7167-3136-8}}</ref>{{page needed|date=February 2015}}