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Neurons are the cells that transmit nerve messages to all parts of our
body. We have about 100 billion neurons in our brain! Neurons vary in
size and shape but all have dendrites, a cell body and an axon. There
are two types of dendrites, apical and basal dendrites. The dendrites
pass information to the cell body. The cell body contains organelles
that maintain its life, and the axon passes information away from the
cell body. To visualise a neuron, imagine a pyramid. You might want to take a piece of paper to draw on. At the top of the pyramid are the apical dendrites, which branch outwards of the top of the pyramid like a fan. Next draw a line which links the apical dendrites to the apex of the cell body, which is in the middle of the pyramid and which is shaped like a triangle. Then from the two other edges of the triangular cell body are two lines that link to the basal dendrites that branch outwards of the two opposite sides of the pyramid. From the middle of the base of the cell body is the axon, a long line linking to the synaptic terminals that branch outwards of the bottom of the pyramid. Now remove the pyramid and leave the neuron there. Ta da! There are three types of neuron, sensory neurons, motor neurons and interneurons. Sensory neurons have long dendrites and a short axon, motor neurons have the opposite and interneurons connect neurons. Sensory neurons receive information from sensory receptors, such as our tongue, skin, nose and so on, and pass the information to the central nervous system. Motor neurons pass information from the central nervous system to the muscles and glands of the body where the appropriate action is carried out after receiving the information. Many axons are covered in a myelin sheath, a layer of fatty substances, which insulate the axon. It is also surrounded by the neurilemma, a thin membrane, which provides nourishment for the axon. There are gaps at intervals in the sheath, the nodes of Ranvier, where the neurilemma sinks down coming into contact with the axon. These are places serving as generators for nerve signals. Nerve signals jump from node to node, travelling much faster than they would if they travelled along the surface of the axon. Hence our brain is able to send information all the way down to our toes in thousandths of a second! Neurons are grouped together to form nerve bundles that have blood vessels and connective tissue to provide food and bunch the neurons together. |
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[İMartin and Ambrose 2001] |
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