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Neurons Short Version Dr. Ron Pohala Nervous System Central Nervous System consists of the brain and the spinal cord. Peripheral Nervous System consists of any of the nerve tissue outside of the central nervous system such as the thorax, abdomen or the arms and legs. Type of nerve cells Neuroglia The supporting structure of nervous tissue. It consists of a fine web of tissue made up of modified ectodermal elements, in which are enclosed peculiar branched cells known as neuroglial cells or glial cells. The neuroglial cells are of three types: astrocytes and oligodendrocytes which appear to play a role in myelin formation, transport of material to neurons, and maintenance of the ionic environment of neurons; and microcytes (microglia), which phagocytize waste products of nerve tissue. Neuroglia (glia) Function to physically support or assist with metabolism. 90 percent of the nerve cells in the brain are neuroglia Four types of neuroglia exist, astrocytes, oligodendrocytes, ependyma cells. Sometime microglia come under this listing. Neuroglia A special type of connective tissue cell holding functioning neurons together Astrocytes, star shaped neuroglia attach neurons to small blood vessels Microglia, smaller than astrocytes, generally are stationary but can move about as microbe eating scavengers (phagocytes) The oligodendroglia ( oligodendrocytes) hold nerve fibers together and produce myelin in the CNS ASTROCYTES Play a very important role in the blood- brain barrier. Since neurons are so vitally important and can’t carry out mitosis for replacement the body particularly tries to prevent dangerous substances from reaching them. Play a role in supplying the neurons with glucose. Neurons It is important that you know the anatomy of the neuron and how it functions and to learn the basics of the neurons and how they communicate with one another and other type of cells. Nerve cells that transmit impulses are known as neurons. Neuron A neuron consists of three main parts the cell body or perikaryon or soma, dendrites and axons. The cell body is the central region which is the most important part of the neuron containing the nucleus of the cell. Dendrites are extensions that carry impulses toward the cell body and are referred to as being afferent fibers. Neurons Axons carry impulses away from the cell body and are known as efferent fibers. The axon is the section of the neuron that communicates with another neuron by releasing neurotransmitters. Point of Reference for nerve fibers The point of reference for the fibers carrying the impulse is the cell body, perikaryon or soma. Dendrites carry the impulse toward the cell body.(afferent fibers) Axons carry impulses away from the cell body. (efferent fibers) Neuron (cont.) The region between the two neurons is known as the synaptic gap or snaptic cleft or neural junction. Personally I prefer to use the word synapse as an action verb and refer to the actual transmission as the synapse not the space. Neurons May be classified by how many fibers are extending from the cell body. Unipolar neurons has one extension from the cell body. Bipolar neurons have two extensions from the cell body. Multipolar neurons ( which are the most common) and usually the one referred to has many dendrites and usually one axon. Classification of Neurons More commonly neurons are based on the location and the direction the neuron conducts the impulse. A sensory neuron carries and impulse toward the CNS. AKA an afferent neuron. Point of Reference The point of reference for differentiating neurons is the Central Nervous System. So the brain and the spinal cord have neurons carrying impulses toward or away from them. Motor neurons Carry impulses away from Central Nervous System. Classifying neurons according to number of fibers . Unipolar Bipolar Multipolar Interneuron Also know as internuncial neuron, connecting neuron, association neuron. Addionally known also called relay neuron, association neuron or local circuit neuron) A human brain contains about 100 billion interneurons. Sheaths The neuron may have either a neurilemma sheath, myelin sheath or both or none at all. Neurilemma sometimes spelled neurolemma is one that is cytoplasmic in its makeup and allows for the repair of damaged fibers. It is found only in the peripheral neurons none are found in the central nervous system neurons. Neurilemma or Neurolemma Neurolemma (also known as neurilemma or sheath of Schwann) is the outermost nucleated cytoplasmic layer of Schwann cells that surrounds the axon of the neuron. It forms the outermost layer of the nerve fiber in the peripheral nervous system.[1] It is important to note that in CNS, axons are myelinated by oligodendrocytes, thus lack neurolemma. White and gray matter in CNS White matter –myelinated axons. Gray matter- unmyelinated axons, cell bodies, dendrites. White verses Gray Matter Most of the surface and outer few millimeters is gray matter, while most of the inner tracts are composed of white matter (myelinated neurons). Remember in the brain the outer layers are gray matter and the inner is white matter while in the spinal cord the outer layer is white matter and the inner layer is gray matter.