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Neurons

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Introduction

The human body is made up of different body systems that work independently yet interact to support the very basic and essence of their existence which is life. These systems include the circulatory system that ensures that essential element needed by the body are transported to they point of need, the respiratory system, the excretory system, the endocrinal system among others (Robert, p.62). However of interest to this piece of work is the nervous system in which the neurons are to be found. This essay first defines what neurons are, secondly looks at their structure and how they are related to specific functions and roles that they perform. Lastly, we have examined how important these roles are important to the nervous system and the body as a whole.

The ability to respond to stimuli is a characteristic of all living cells. The specific cells that are specialized to conduct the impulses and react to the stimuli are known as the neurons. Three types of neurons do exist. There classification is based on their specific functions; sensory or otherwise referred to as afferent neurons conduct impulses from the sensory organs to the spinal cord and the brain also known as central nervous system. The motor or the efferent neurons on the other hand transmits impulses from the central nervous systems to the muscles and gland which are also called the effector organs (Davies, p.124). The third and indeed the last neuron derives its name to its function and is thus known as the interneuron, association or the connector neurons these join the sensory neurons to the motor neurons.

The structure

The motor neuron is made up of cytoplasmic extensions the dendrites that join to a large cell body grey in color at one end. On the other end axon extends to join either the dendrites of the adjacent neuron or to make a motor endplate in a muscle. While the axons are long non-branched processes the dendrite on the other hand are short and heavily divided. The impulses are conducted through the motor neuron only in one direction into the cell body and away from it by the dendrites and the axons respectively (Chudler, para.5). The cell membrane encloses the cell nucleus as well as the other cell organelles. Within the cytoplasm granulated Nissl body occur, other organelles to be found in the cell body include the neurofibrils which stretch from the dendrite into the axon. The myelin sheath surrounds the axon and function to provide insulation together with the Schwann cells; they are both referred to medullary sheath.  In addiction to this the myelin sheath plays an important role in speeding up the impulse transmission. At regular Intervals the medullary sheath are interrupted by nodes of Ranvier (López-Muñoz, Boya, Alamo, p.393).

The sensory neurons are more or less similar to the motor neurons structurally except that they have a single dendrite and axon. As such they are unipolar while the motor and interneuron are multipolar (Siegfried, para.6). It is imperative noting at this point that although the entire nerve cell is capable of transmitting the impulse the axon is structurally designed for this. Axons are enormously elongated extension of the cell, specifically adapted for this purpose.

Transmission of the impulse

Nerve impulses have a contingence effect. Upon receiving an impulse the neuron must pass it to the next one, ensuring the continuity of the correct impulse on the same path. The dendrites through a chain of chemical happenings, receives the impulse and pass it to the axon and onto the next neuron(Robert  p.273).This takes places in a number of steps.

The first step is the polarization of the nerve cell membrane. In a normal setting the sodium ions are usually on the outer side of the membrane and the potassium on the inner side.  The belief over time has been that the nerve impulse is an electrical phenomenon in which the axon and dendrites act as the electric cables. As such when the neuron is non stimulated, then its membrane is said to be polarized (Peters, Palay, Webster, p.75). This in essence means that the outer side of the membrane is positively charged or rather has a positive electric charge. There are excess of sodium ions on the outer side of the cell and excess potassium on the inner side of the cell membrane.

But the presence of negatively charged nucleic acid and some protein molecules inside the cell is responsible for this partial negative charge. It is worth noting that the equilibrium of maintaining the sodium ions and the potassium ions on the outer and the inner side of the cell respectively is made possible by the presences of Na+/K+ pump that pumps back the sodium ions to the outside and vice versa. If the neuron is polarized and inactive then it is said to be a resting potential (Williams, & Herrup, p.483).

On a stimulus reaching a resting neuron gated ion channels immediately open to allow Na+ into the cell. The neuron thus gets depolarized. The inside becomes positively charged, a threshold is thus reached. If the stimulus goes beyond this threshold the more gated channels open allowing the flow of even more Na+ ions and complete depolarization occurs (Robert p.264). Action potential is created. The stimulus is then transmitted through the axon.

The inside is now flood with sodium ions, the inside gates open to allow the potassium ions to move out of the cell. As this happens the sodium gates simultaneous closes and the initial state of the cell is restore with outer side positively charged and the inner side negatively charged only that there is an interchange of ions. The neuron is said to be repolarised.

Hyperpolarisation occurs as more potassium ions and fewer sodium ions on the outside and inside respectively. The cell is lower than the resting potential, this stage is short lived and after the impulse has been transmitted through the nerve cell then action potential is over and the resting potential takes over (Guillery, p.1283). After this the K+ and Na+ return to their normal state i.e. Na+ on the outside and the K+ on the inside. This period is known as the refractory period and sees the neuron get polarized once again.

Transmission between nerve cells

There exists a gap known as the synapse that separate an axon of one neuron to the dendrite of the adjacent neuron. Only in the brain that an electric charge transmits and through synapses, in all other parts of the body this happens through a chemical reaction or rather a chemical change (Robert p.267).

The membrane depolarizes at the ending of the axon from which the impulse emanates, the Ca2+ get into the cell as the gated ion channels are opened. Immediately a neurotransmitter called acetycholine is released into the synapse. Acetycholine moves across the synapse to bind proteins of the neuron membrane that is receiving the impulse. The impulse is thus transmitted to the next neuron. Whether that impulse is transmitted through the next neuron will depends on neurotransmitters ability to make the Na+ gated channel to open.

In conclusion the importance of the neurons to the nervous system cannot be over emphasized. It is for this reason that neuron of human do neither divides nor multiplies. Thus they cannot mutate nor convert into cancerous cells.

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