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The Nervous System: Neural Tissue Chapter 9a Introduction – Nervous system = control center & communications network – Functions • Stimulates movements • Maintains homeostasis (with endocrine system) Organization of the Nervous System – Central nervous system (CNS) • Brain & spinal cord – Peripheral Nervous System (PNS) • Afferent (Sensory) System • Efferent (Motor) System – Somatic Nervous System – Autonomic Nervous System » Sympathetic Nervous System » Parasympathetic Human Anatomy, 3rd edition Nervous System Prentice Hall, © 2001 Organization of the Nervous System 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) – Gli = glue » Support & protection of nervous system Neurons – Function • Conduct electrical impulses – Structure • Cell body – Nucleus with nucleolus – Cytoplasm • 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 http://www.britannica.com/EBchecked/topic-art/409665/66781/Conduction-of-the-action-potential-In-a-myelinated-axon-the Myelin – White matter of nerves, brain, spinal cord – Composed primarily of phospholipids – Production • Developing Schwann cells wind around axon – Function • Increases speed of impulse conduction • Insulation and maintenance of axon Human Anatomy, 3rd edition Prentice Hall, © 2001 Myelin – Nodes of Ranvier • Unmyelinated gaps between segments of myelin • Impulses “jump” from node to node Human Anatomy, 3rd edition Prentice Hall, © 2001 Nerve Fibers of the CNS – Umyelinated – Myelinated • Production of myelin is from oligodendrocytes • Nodes of Ranvier are less numerous Human Anatomy, 3rd edition Prentice Hall, © 2001 Classification of Neurons – Functional – based on the direction of impulse transmission • Sensory neurons • Motor neurons • Interneurons (association) Functional Classification of Neurons Human Anatomy, 3rd edition Prentice Hall, © 2001 Nerve Impulse – A change in charge that travels as a wave along the membrane of a neuron – Depends on the movement of K+ and Na+ across the nerve cell membrane. – Called an action potential Resting Neuron – 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 – Membrane is polarized 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 = a critical level – Minimum stimulus required to elicit a response Starting a Nerve Impulse • Depolarization – a stimulus begins the change in charge on 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 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The Action Potential • If the action potential (nerve impulse) starts, it is propagated over the entire axon • Potassium ions rush out of the neuron after sodium ions rush in • Sodium and potassium are actively transported back to their original positions = repolarization • Membrane is at rest again Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Nerve Impulse Propagation • The impulse continues to move away from the cell body • Impulses travel faster when fibers have a myelin sheath Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Continuation of the Nerve Impulse between Neurons • Impulses are able to cross the synapse to another nerve • Neurotransmitter is released from a nerve’s axon terminal • The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter How Neurons Communicate at Synapses Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Synapses 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 Neural Regeneration Human Anatomy, 3rd edition Prentice Hall, © 2001 Neural Regeneration Human Anatomy, 3rd edition Prentice Hall, © 2001 Structure of a Nerve • Endoneurium surrounds each fiber • Groups of fibers are bound into fascicles by the perineurium • Fascicles are bound together by epineurium Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Classification of Nerves • Mixed nerves • Both sensory and motor fibers • Afferent (sensory) nerves • Carry impulses toward the CNS • Efferent (motor) nerves • Carry impulses away from the CNS