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The genius and spinal cord argon made up of many carrells, including neurons and glial cells. Neurons ar cells that send and receive electro-chemical signals to and from the brain and nervous system. There are about 100 billion neurons in the brain. There are many more glial cells they provide support functions for the neurons, and are far more numerous than neurons. There are many type of neurons. They vary in size from 4 microns (. 004 mm) to 100 microns (. 1 mm) in diameter. Their continuance varies from a fraction of an inch to several feet.Neuronsare center cells that transmit nerve signals to and from the brain at up to 200 mph. The neuron consists of acell body (or soma)with branchingdendrites(signal receivers) and a projection called anaxon, which conduct the nerve signal. At the other end of the axon, theaxon terminalstransmit the electro-chemical signal across asynapse(the go between the axon terminal and the receiving cell). The word neuron was coined by the German s cientist Heinrich Wilhelm Gottfried von Waldeyer-Hartz in 1891 (he also coined the term chromosome).Theaxon, a long extension of a nerve cell, and take information away from the cell body. Bundles of axons are known as nerves or, within the CNS (central nervous system), as nerve tracts or pathways. Dendrites fetch information to the cell body. Myelincoats and insulates the axon (except for periodic breaks called nodes of Ranvier), increasing transmission speed along the axon. Myelin is manufactured by Schwanns cells, and consists of 70-80% lipids (fat) and 20-30% protein.Thecell body (soma)contains the neurons nitty-gritty (with DNA and typical nuclear organelles). Dendrites branch from the cell body and receive messages. A typical neuron has about 1,000 to 10,000 synapses (that is, it communicates with 1,000-10,000 other neurons, muscle cells, glands, etc. ). distinguishable TYPES OF NEURONS There are different types of neurons. They all carry electro-chemical nerve signals, but differ in structure (the number of processes, or axons, emanating from the cell body) and are name in different parts of the body. Sensory neurons or Bipolar neuronscarry messages from the bodys sense receptors (eyes, ears, etc. ) to the CNS. These neurons have two processes. Sensory neuron account for 0. 9% of all neurons. (Examples are retinal cells, olfactory epithelium cells. ) * Motoneurons or Multipolar neuronscarry signals from the CNS to the muscles and glands. These neurons have many processes originating from the cell body. Motoneurons account for 9% of all neurons. (Examples are spinal motor neurons, pyramidal neurons, Purkinje cells. * Interneurons or Pseudopolare (Spelling) cellsform all the neural wiring within the CNS. These have two axons (instead of an axon and a dendrite). One axon communicates with the spinal cord one with either the skin or muscle. These neurons have two processes. (Examples are dorsal root ganglia cells. ) LIFE SPAN OF NEURONS Unlike almost o ther cells, neurons cannot regrow after damage (except neurons from the hippocampus). Fortunately, there are about 100 billion neurons in the brain. GLIAL CELLS Glial cells make up 90 percent of the brains cells.Glial cells are nerve cells that dont carry nerve impulses. The various glial (meaning glue) cells perform many important functions, including digestion of parts of dead neurons, manufacturing myelin for neurons, providing personal and nutritional support for neurons, and more. Types of glial cells include Schwanns Cells, Satellite Cells,Microglia, Oligodendroglia, and Astroglia. Neuroglia (meaning nerve glue) are the another type of brain cell. These cells guide neurons during fetal development.