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7 A-The Nervous system, Neurons and synapses

7 A-The Nervous system, Neurons and synapses - The Nervous...

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The Nervous System Neurons and Synapses-A Lecturer: Dr. R. Ahangari University of Central Florida, Orlando Human Physiology by S.I. Fox and human anatomy by Marieb & Mallat
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Cells of the Nervous tissue: Cells are densely packed and intertwined. Two main cell types -Neurons: transmit electrical signals, found in grey matter of CNS and ganglia. -Neuroglial cells (support cells): nonexcitable, surround and wrap neurons. Neurons are basic structural units of the nervous system and possess a cell body and processes called neurites. Human body contains billions of neurons. Characteristics of Neurons: Conduct electrical impulses along the plasma membrane Produce nerve impulse Produce action potential Longevity: can live and function for a lifetime Do not divide: fetal neurons lose their ability to undergo mitosis High metabolic rate: require abundant oxygen and glucose
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Structure of a neuron: 1- cell body (perikaryon) Size varies from 5–140µm Has the normal cell organelles *Nissl bodies (rER), Neurofibrils and Lipofuscin 2- axon (long) Transmits impulses away from Neuron. Neuron has only one axon No protein synthesis in axon No Nissl bodies in axoplasm. Initial segment: after Axon hillock; most excitable site, origination site of action potential. Neurofilaments, microtubules and actin microfilaments are present. Is smooth, without many synapses. Axon terminal (boutons). 3- dendrite (short) Increases neuron’s receptive area. Transmits impulses towards the Neuron. Nissl bodies in its basal parts.
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Type of neurons:
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Supporting cells -Provide supportive functions for neurons. -Cover nonsynaptic regions of the neurons. There are two types of supporting cells in the peripheral nervous system: a. Schwann cells, which form myelin sheaths around peripheral axons. b. Satellite cell, or ganglionic gliocytes, which support neuron cells bodies within the ganglia of the PNS. There are four types of supporting cells, called neuroglial cells, in the central nervous system. a. Oligodendrocytes, which form myelin sheaths around axons of the CNS. b. Microglia, which migrate through the CNS and phagocytose foreign and degenerated material. c. astrocytes, which help to regulate external environment of neurons in the CNS d. Ependymal cells, which line the ventricles of the brain and the central canal of the spinal cord.
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Supporting cells Oligodendrocytes (Oligs): small cell bodies, no filaments in their cytoplasm, one oligodendrocyte can myelinate many fibers. Oligodendrocytes are not surrounded by a basement membrane, unlike the Schwann cells in PNS. Functions of Oligs: 1- Myelination in the CNS Oligodendrocytes that surround nerve cell bodies, (Stellite Oligs) may influence the biochemical environment of neurons.
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