Silvia Helena Cardoso, PhD

Eletricity is a natural process in our organism and is involved in special cells in the brain and the body. Every pattern of light, sound, hot, pain, every twitch of your finger, every thought, gets translated into a sequence of electrical pulses. The electrical mechanisms that promote communication between cells take place in solution, with substances dissolved in water.

When a substance like salt (NaCl) se dissolve em água, ela deixa de existir como átomos e moléculas. Os átomos perdem ou ganham elétrons tornando-se partículas flutuando livremente, chamadas de íons (no caso do sal, ele se tornará cloreto de sódio ou seja, Na+ Cl-). Os íons possuem cargas positivas e negativas, e o movimento desses íons carregados dentro e fora da célula viva, possuem eletricidade.

The organelles of a neuron are embedded internally in a cytoplasm that is made up mostly of water, proteins and inorganic salts. Externaly, it is also bathed by substances which feed the cell, creating conditions in which it can works.

The ability of nerve cells to process electrical information depends on the special properties of the cell membrane, which controls the flux of nutritive substances to the internal side of the cell. Canais sobre a membrana permitem que uma substância passe do meio interno para o meio externo da célula e vice-versa. Ionic movements through channels are influenced by the process of difusion, osmose and electricity.

Veja o processo de dissolução

Ions and molecules dissolved in solution move randomly. This temperature dependent, constant movement will tend to distribute the ions throughout the solution. In this manner, there will be movement of ions from regions of high concentration to regions of low concentration. This movement is called diffusion. As an example, add a spoon of milk to a glass of water. The milks spreads through the water solution.

Below, dissolution of a crystal of salt (one of the substances of the cell) in water.

It has numerous projecting parts such as long wires ou branches of a tree. The neuron has one long process called an axon (from greek axoon, axis) and one or more short, with many branches called dendrites (from greek dendron, árvore).

Neurons differ from other cells in that they are able to produce a nerve impulseor action potential. A nerve impulse is the transmission of a coded signal from a given stimulus, along the membrane of the neuron from the point that it was stimulated.
Substances dissolved in water transform in ions charged of electricity -

Em soluções salinas muito diluídas, a maior parte das moléculas se decompõem em íons, isto é, em partes moleculares carregadas de eletricidade, positiva ou negativamente. Os átomos carregados de eletricidade positiva são os cátions e os de eletricidade negativa são os ânions.

Se, por exemplo, for dissolvido em água, o sal de cozinha (NaCl) decompõe-se no cátion Na+ e no ânion Cl-. Nas soluções aquosas, são os íons os únicos portadores de cargas

Se, ----------------- --

A. H2O (Water)------B. Crystal of Salt (NaCl)--C. Sodium Chlroride


A.The two hydrogen atoms and the oxigen atom of the water molecule (H2O) are 
bounded together covalently, sharing electrons.

B.In crystals of salt, (NaCl), the sodium and chlorine atoms are bonded by sharing one electron.

C. In solution, crystals of salt separate and no longer exists as the usual atoms. Salt dissolves in water because the charged portions of the water molecule have stronger attraction for the ions than they have for it other. Sodium loses the electron to forma positive ion, Na+; chlorine gains one to make a negative ion, Cl-. These two, along with the potassion ion K+, are the main ones involved in nerve impulses. 

ions- particles charged of electricity, 
anions - atoms which gain electrons
cations - atoms which lose electrons).

Neurons are are able to produce a nerve impulse or action potential. A nerve impulse is the transmission of a coded signal from a given stimulus, along the membrane of the neuron from the point that it was stimulated. A neuron is activated by other in a continuous fashion, and transmitts information to other neurons or muscles. Two types of phenomena are involved in processing of nerve impulse: electrical and chemical. Electrical events propagate a signal within a neuron, and chemical processes transmit the signal from one neuron to another neuron or to a muscle cell. On its journey to the brain, a nerve impulse must pass from one neuron to the next by crossing a contact point known as a synapse.

In this section, you are going to understand how stimuli arising outside the cell are converted into electrical signals.

The Resting Cell

At rest, that is, when not stimulated, a nerve cell has an excess of negative charges (anions) on the inside of the membrane and an excess of positive charges (cations) on the outside.

But ions tend to drift around so that they become evenly spread out and balanced - they will diffuse until their concentration is equal. Keeping ions separated against their tendency to equalize charge and concentration uses up energy, which the cell has to expend.
The negative charge inside the cell is maintained by two features:

1. Selective permeability of the cell membrane, which is more permeable to potassium than sodium.

2. Sodium pumps within the cell membrane that restrains ions from passing freely via channels into or out of the cell (it actively pump sodium out of the cell). (page134 kandel). 

This separation of particles with opposite charges produces an electrical difference or potential difference is known as the resting potential an can be measured with sensitive scientific equipment around -70 milivolts, with the inside the cell negatively charged between the inside and the outside of the membrane.

The Stimulated Cell

When stimuli coming from the environment such as pain or any other kind of stimulating current received by the neuron, the membrane permeability is changed, allowing a sudden influx of sodium ions into the cell.

The high concentration of sodium changes the overall charge within the cell from negative to positive. The local change in ion concentration triggers similar reactions along the membrane, propagating the nerve impulse. In these cases, ions sodium penetrate inside the cell while potassium ions go outside. This makes the inside momentarily positive by about 50 millivolts. This rise in voltage is an action potential. Logo em seguida, o sódio volta para fora e o potássio retorna ao interior da célula. After a brief period called the refractory period, during which the ionic concentration returns to resting potential, the neuron can repeat this process.

Nerve impulses travel at different speeds, depending on the cellular composition of a neuron. Where speed of impulse is important, as in the nervous system, axons are insulated with a membranous substance called myelin. The insulation provided by myelin maintains the ionic charge over long distances. Nerve impulses are propagated at specific points along the myelin sheath; these points are called the nodes of Ranvier. Examples of myelinated axons are those in sensory nerve fibers and nerves connected to skeletal muscles. In non-myelinated cells, the nerve impulse is propagated more diffusely.
Pumps. The cell membrane allows the cell to generate a resting potential.
This is done using energy-consuming pumps in the membrane which make 
sure that ions stay segregated against their natural tendency to spread out 
and equalize their electrical charges and chemical concentartions. Sodium
ions are pumped out of the cell, and potassium ions pumped back in.

The separation of particles with opposite charges produces a potential difference, an electrical difference between the inside and outside of the membrane. That is, the membrane nomally is electrically polarized, with the inside 50 to 90 millivolts negative to the outside. This is known as the resting potential. The resting potential occurs when the neuron is not stimulated.

How stimuli arising outside the cell are converted into electrical signals ?

Every each fraction of a second, each neuron decides whether or not to send a signal.

Action Potential
A single action potential is an all-or-nothing event of a fixed amplitude. It comes about when the tens of thousands of special ion channels clustered at a neuron's axon hillock open in concert. These are voltage-gated ion channels, and they stay closed until the local potential (voltage) at the hillock reaches the threshold. Once triggered, the action potential proceeds at this strenght along the lenght of the axon.

Links on Neurophysiology