The Central Nervous System

The Central Nervous System (CNS) is composed of neurons or nerve cells. The brain, the spinal cord and the peripheral ganglia are intricately well-connected through these neurons. Each nerve cell has two principal parts: an axon and dendrites. Between these two parts lies the cell body or soma of the nerve cell. Electric and chemical signals are the two main methods of signal propagation. Chemical signals occur at the junction of neurons called synapses. The entire neuron is wrapped in a layer of lipid and protein called the ‘myelin sheath.

’ Some nerve cells are myelinated and the others are non-myelinated. Though a single nerve cell can have multiple branching dendrites or dendritic filaments that by their biological definition emerge from the cell body, a soma or cell body never has more than one axon, the upper part of the nerve cell that links to the soma. This paper examines the function of the myelin sheath and describes the events of membrane polarization. In order of fastest to slowest conduction rate would be Axon Y has a diameter of 10 micrometers, is 25 cm long and is myelinated followed by Axon Z is 2.

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5 micrometers in diameter, is 2. 5 cm long and is myelinated and the slowest would be Axon X has a diameter of 1. 0 micrometers, is 10 cm long and is unmyelinated. The first important reason is myelinated nerve cells are faster in conduction when compared with unmyelinated nerve cells due to the phenomenon of salutatory conduction between the nodes of Ranvier. The second important reason is between two myelinated nerve cells, the thicker nerve or the nerve cell with the larger diameter will have a thicker axon, which in turn, will have a thicker myelin sheath.

This layer covers only the axon part of the nerve cell and it helps in insulation and faster conduction of signals. Depolarization is the observed difference in the membrane potential of the nerve cell. The potential tends to become less negative or more positive numerically. Nerve stimulation results in the creation of an electric impulse that travels through the length of the nerve cell. The resting (original, prior to deploarization) potential of the nerve cell membrane changes from -60 mV to +45 mV(post depolarization).

The influx of sodium and calcium ions through sodium and calcium ion channels respectively causes membrane depolarization. In a polarized state the electrical charges outside the membrane are positive while in the depolarized state the membranes become increasingly positive in their charge. The sodium/potassium pumps help selective open and close the sodium and potassium channels. The sodium channels open in the membrane of the axon in response to a stimulus that generates an electric impulse by changing the membrane potential using positive Na+ and Ca+ ions. Reference Siegfried, D. R. , Understanding the transmission of nerve impulses.

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