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Электрические сигналы и химические сигналы. Мозг

A Brief Introduction to the Brain http://www.ifisiol.unam.mx/Brain/segunda.htm

Электрические сигналы и химические сигналы http://www.ifisiol.unam.mx/Brain/mempot.htm Хорошие картинки, но лень сейчас копировать

MEMBRANE POTENTIAL

Information transmission can be understood in terms of two major components: Electrical signals and chemical signals. Transient electrical signals are important for transferring information over long distances rapidly within the neuron. Chemical signals, on the other hand, are mainly involved in the transmission of information between neurons.


Electrical signals (receptor potential, synaptic potential and action potential) are all caused by transient changes in the current flow into and out of the neuron, that drives the electrical potential across the plasma membrane away of its resting condition.


Every neuron has a separation of electrical charge across its cell membrane. The membrane potential results from a separation of positive and negative charges across the cell membrane. The relative excess of positive charges outside and negative charges inside the membrane of a nerve cell at rest is maintained because the lipid bilayer acts as a barrier to the diffusion of ions, and give rise to an electrical potential difference, which ranges from about 60 to 70 mV.


The potential across the membrane when the cell is at rest (i.e. when there is no signaling activity) is known as the resting potential. Since , by convention, the potential outside the cell is arbitrarily defined as zero, and given the relative excess of negative charges inside the membrane; the potential difference across the membrane is expressed as a negative value:


Vr = -60 to -70 mV.

Being Vr, the resting potential.

The charge separation across the membrane, and therefore the resting membrane potential, is disturbed whenever there is a net flux of ions into or out of the cell. A reduction of the charge separation is called depolarization; an increase in charge separation is called hyperpolarization. Transient current flow and therefore rapid changes in potential are made possible by ion channel, a class of integral proteins that traverse the cell membrane. There are two types of ion channels in the membrane: gated and nongated. Nongated channels are always open and are not influenced significantly by extrinsic factors. They are primarily important in maintaining the resting membrane potential. Gated channels, in contrast, open and close in response to specific electrical, mechanical, or chemical signals. Since ion channels recognize and select among specific ions, the actual distribution of ionic species across the membrane depends on the particular distribution of ion channels in the cell membrane.

Ionic species are not distributed equally on the two sides of a nerve membrane. Na and Cl are more concentrated outside the cell while K and organic anions (organic acids and proteins) are more concentrated inside. The overall effect of this ionic distribution is the resting potential. However, what prevents the ionic gradients from being dissipated by passive diffusion of ions across the membrane through the passive nongated channels?.

There are two forces acting on a given ionic species. The driving force of the chemical concentration gradient tends to move ions down this gradient (chemical potential). On the other hand the electrostatic force due to the charge separation across the membrane tends to move ions in a direction determined by its particular charge. Thus, for instance, chloride ions which are concentrated outside the cell tend to move inward down its concentration gradient through nongated chloride channels. However the relative excess of negative charge inside the membrane tend to push chloride ions back out of the cell. Eventually an equilibrium can be reached so that the actual ratio of intracellular and extracellular concentration ultimately depends on the existing membrane potential.




The same argument applies to the potassium ions. However these two forces act together on each Na ion to drive it into the cell. First, Na is more concentrated outside than inside and therefore tends to flow into the cell down its concentration gradient. Second, Na is driven into the cell by the electrical potential difference across the membrane. Therefore, if the cell is to have a steady resting membrane potential, the movement of Na ions into the cell must be balanced by the efflux of K ions. Although these steady ionic interchange prevents can prevent irreversible depolarization, this process cannot be allowed to continue unopposed. Otherwise, the K pool would be depleted, intracellular Na would increase, and the ionic gradients would gradually run down, reducing the resting membrane potential.


Dissipation of ionic gradients is ultimately prevented by Na-K pumps, which extrudes Na from the cell while taking in K. Because the pump moves Na and K against their net electrochemical gradients, energy is required to drive these actively transported fluxes. The energy necessary for this process is obtained from the hydrolysis of ATP (an energy carrying molecule). In addition, some cells also have chloride pumps that actively transport chloride ions toward the outside so that the ratio of extracellular to intracellular concentration of Cl is greater than the ratio that would result from passive diffusion alone.

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Tags: Неврология, Статьи
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