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Introduction The Nervous System: Neural Tissue Chapter 13 • Nervous system = control center & communications network • Functions – Stimulates movements – Maintains homeostasis (with endocrine system) Human Anatomy, 3rd edition Prentice Hall, © 2001 Organization of the Nervous System Functional Classification of the Peripheral Nervous System • Sensory (afferent) division • Nerve fibers that carry information to the central nervous system • Motor (efferent) • Nerve fibers that carry information from the central nervous system Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 Human Anatomy, 3rd edition Prentice Hall, © 2001 Histology of Nervous Tissue • 2 types of cells – Neurons • Structural & functional part of nervous system • Specialized functions – Neuroglia (glial cells) Neuroglia • Neuroglia of CNS – Astrocytes – Oligodendrocytes – Microglia – Ependymal cells • Gli = glue • Support & protection of nervous system 1 Neuroglia of CNS • Astrocytes Neuroglia of CNS • Oligodendrocytes • Form the bloodbrain barrier • Produce myelin sheath around nerve fibers in the central nervous system • Structural framework for CNS • Repair damaged neural tissue • Control the interstitial environment of the brain Marieb, Essentials of Anatomy and Physiology, 8th edition, 2006 Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 Neuroglia of CNS Neuroglia of CNS • Ependymal cells • Microglia • Line ventricles of the brain and spinal cord • Spider-like phagocytes • Secrete cerebrospinal fluid • Dispose of debris Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 Neuroglia of CNS Neuroglia of PNS • Schwann cells – Form myelin sheaths of PNS • Satellite cells Marieb, Essentials of Anatomy and Physiology, 8th edition, 2006 Human Anatomy, 3rd edition Prentice Hall, © 2001 2 Neurons • Function • Long, specialized Axon Structure – Collaterals = branches – Telodendria = termination of axons & collaterals – Conduct electrical impulses • Cytoplasm = axoplasm • Plasma membrane = axolemma • Structure – Cell body • Nucleus with nucleolus • Cytoplasm (perikaryon) – Cytoplasmic processes • Dendrites • Axon Human Anatomy, 3rd edition Prentice Hall, © 2001 Anatomy of a Neuron Human Anatomy, 3rd edition Prentice Hall, © 2001 Nerve Fibers of the PNS • An axon and its sheaths – Myelinated axon • Axon is surrounded by a myelin sheath – Unmyelinated axon • Axon has no myelin sheath Human Anatomy, 3rd edition Prentice Hall, © 2001 Myelin • White matter of nerves, brain, spinal cord • Composed primarily of phospholipids • Production • Function of myelin – Increases speed of impulse conduction – Insulation and maintenance of axon – Developing Schwann cells wind around axon • Nodes of Ranvier – Unmyelinated gaps between segments of myelin – Impulses “jump” from node to node • Neurilemma – Peripheral cytoplasmic layer of the Schwann cell enclosing the myelin sheath A Myelinated Axon Marieb, Essentials of Anatomy and Physiology, 8th edition, 2006 Human Anatomy, 3rd edition Prentice Hall, © 2001 3 Nerve Fibers of the CNS Structural Classification of Neurons • Based on the number of cytoplasmic processes • Unmyelinated • Myelinated – Production of myelin is from oligodendrocytes – Nodes of Ranvier are less numerous Human Anatomy, 3rd edition Prentice Hall, © 2001 Human Anatomy, 3rd edition Prentice Hall, © 2001 Functional Classification of Neurons Nerve Impulse • Based on the direction of impulse transmission – Sensory neurons – Motor neurons – Interneurons (association) • A change in charge that travels as a wave along the membrane of a neuron • Called an action potential • Depends on the movement of sodium ions (Na+) and potassium ions (K+) between the interstitial fluid and the inside of the neuron. Human Anatomy, 3rd edition Prentice Hall, © 2001 Resting Potential • Sodium ions are in large concentration along the outside of the cell membrane • Potassium ions are in large concentration along the inside of the cell membrane Beginning of a Nerve Impulse • Requires a stimulus of adequate strength • Membrane is irritable – Neuron may respond to a stimulus and convert it to an impulse. • When? – If above threshold Human Anatomy, 3rd edition Prentice Hall, © 2001 4 Starting a Nerve Impulse The Action Potential • If the action potential starts, it is propogated over the entire axon • Potassium ions rush out of the neuron after sodium ions rush in • Depolarization – a stimulus depolarizes the neuron s membrane • A depolarized membrane allows sodium (Na+) to flow inside the membrane • The exchange of ions initiates an action potential in the neuron – Repolarizes the membrane Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20102/Bio%20102%20lectures/nervous%20system/neuron6.gif Return to Resting Potential • Sodium-potassium pump restores original configuration Nerve Impulse Propagation • The impulse continues to move away from the cell body – Requires ATP • Impulses travel faster when fibers have a myelin sheath Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20102/Bio%20102%20lectures/nervous%20system/neuron6.gif Continuation of the Nerve Impulse Between Neurons Synaptic Cleft • Impulses are able to cross a synapse to another nerve – Neurotransmitter is released from the axon terminal (synaptic knob) into synaptic cleft – The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 5 Postsynaptic Membrane Receptor Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 Neural Regeneration After Injury Human Anatomy, 3rd edition Prentice Hall, © 2001 Neural Regeneration Human Anatomy, 3rd edition Prentice Hall, © 2001 Synapse Human Anatomy, 3rd edition Prentice Hall, © 2001 Neural Regeneration Human Anatomy, 3rd edition Prentice Hall, © 2001 Neural Regeneration Human Anatomy, 3rd edition Prentice Hall, © 2001 6 Nerves • Neurons are bundled into fasciculi which are bundled into nerves. – Endoneurium surrounds each nerve fiber (axon) – Groups of fibers are bound into fascicles • Surrounded by the perineurium – Fascicles are bound together into a nerve • Surrounded by the epineurium Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 7