contd.: Communication Between Neurons

The electrical mechanisms that promote communication between cells take place in solution, with substances dissolved in water.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.

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An atom is made up of a positively charged central nucleus and particles called
electrons which spin around it an which have an equivalent negative charge, so the
whole atom is electrically neutral. In solution, an atom
gains or loses one or more electrons. The result is a particle, or ion, that has charge.

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

Neurons comunicate with each other at ending 
nerve called sinapse. Without touching the dendrite of other cell, the axon 
release chemical substances called neurotransmitters, which se 
encaixam in receptors of the next neuron. In this phase, the electrical 
transmission takes place to a chemical reaction. 

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 nerve cell has similar properties of any other cell in many aspects: it feed, breath, sofre variations of electrical charge, e está vinculada to processes of difusion (movement of molecules from regions of high concentration to low concentration), osmosis (difusion of a fluid through a semipermeable membrane until there is an equal concentration of fluid on either side of the membrane), However, they differ in one important aspect: they process information.

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. 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).

Os neurônios ficam todos "engatados" uns aos outros, como os vagões de um trem, formando as chamadas cadeias neuronais. Por essas cadeias os impulsos nervosos caminham e são retransmitidos. O engate entre um neurônio e outro, denominado sinapse, é feito entre a terminação do axônio de uma célula e os dendritos e corpo celular de outra. A direção do impulso é do corpo celular para o axônio. A sinapse é o "interruptor" encarregado de ligar ou desligar uma célula nervosa de outra.

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.

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.

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.

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.

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

Links on Neurophysiology