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Neuron Conduction

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Introduction

During a thunderstorm in 1786, Luigi Galvani touched a frogâs leg with a metal instrument and noticed the muscles twitching. He concluded that the storm had generated electricity, which conducted through the frogâs nerves and caused the muscles to contract. Nerves do transmit impulses from one part of the body to another, but in a different way than in an ordinary conductor. The electrical properties are different in neural conduction because it is slower and does not very in strength (it is a all-or-nothing conduction).

A nerve cell (neuron) is the basic building block of the nervous system and is specialized to transmit information. It consists of a cell body and two types of branchlike fibers, dendrites and axons (top of Figure 1). Dendrites, along the cell body, receive information in the form of stimuli from sensory receptors or from other nerve cells. The axon is a long, thin cellular extension responsible for transmitting information to other nerve cells, and is filled with a viscous intracellular fluid called the axoplasm. If stimuli received by the dendrites or the cell body is above the cellâs intensity threshold, a nerve impulse is initiated which propagates along the axon. It flows along the axon away from the cell body toward the terminal branches. Once a nerve impulse reaches the terminal branches, neurotransmitter substances release, conveying the impulse to receptors on the next cell.

The Resting Potential of the Nerve Cell

Critical to the function of the nerve cell, the cell membrane maintains intracellular conditions that differ from those of the extracellular environment. There is an excess of negative ions inside the cell membrane and an excess of positive ions outside (middle of Figure 1). The electrochemical gradient across the membrane is the means of nerve impulse transmission. The concentration of potassium (K+) is 30 times greater in the fluid inside the cell than outside and the concentration of sodium ions (Na+) is nearly 10 times greater in the fluid outside the cell than inside (See Table 1). Anions, particularly chloride (Cl--), are also unevenly distributed. Nerve cells use both passive diffusion and active transport to maintain these differentials across their cell membranes. The unequal distribution of Na+ and K+ is established by an energy-dependant Na+-K+ ãpumpä, moving Na+ out of the cell and K+ into the cell. Specialized proteins embedded in the nerve cell membrane function as voltage-dependant channels, passing through Na+ and K+ during nerve impulse transmission.
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