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The Nervous System: Neural Tissue Chapter 13 Introduction • 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 Human Anatomy, 3rd edition Prentice Hall, © 2001 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 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 Neuroglia • Neuroglia of CNS – – – – Astrocytes Oligodendrocytes Microglia Ependymal cells Neuroglia of CNS • Astrocytes • Form the bloodbrain barrier • Structural framework for CNS • Repair damaged neural tissue • Control the interstitial environment of the brain Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 Neuroglia of CNS • Oligodendrocytes • Produce myelin sheath around nerve fibers in the central nervous system Marieb, Essentials of Anatomy and Physiology, 8th edition, 2006 Neuroglia of CNS • Microglia • Spider-like phagocytes • Dispose of debris Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 Neuroglia of CNS • Ependymal cells • Line ventricles of the brain and spinal cord • Secrete cerebrospinal fluid Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 Neuroglia of CNS Human Anatomy, 3rd edition Prentice Hall, © 2001 Neuroglia of PNS • Schwann cells – Form myelin sheaths of PNS • Satellite cells Marieb, Essentials of Anatomy and Physiology, 8th edition, 2006 Neurons • Function – Conduct electrical impulses • Structure – Cell body • Nucleus with nucleolus • Cytoplasm (perikaryon) – Cytoplasmic processes • Dendrites • Axon Human Anatomy, 3rd edition Prentice Hall, © 2001 • Long, specialized Axon Structure – Collaterals = branches – Telodendria = termination of axons & collaterals • Cytoplasm = axoplasm • Plasma membrane = axolemma 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 Myelin • White matter of nerves, brain, spinal cord • Composed primarily of phospholipids • Production – Developing Schwann cells wind around axon • Neurilemma – Peripheral cytoplasmic layer of the Schwann cell enclosing the myelin sheath Marieb, Essentials of Anatomy and Physiology, 8th edition, 2006 A Myelinated Axon • Function of myelin – Increases speed of impulse conduction – Insulation and maintenance of axon • 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 • Unmyelinated • Myelinated – Production of myelin is from oligodendrocytes – Nodes of Ranvier are less numerous Human Anatomy, 3rd edition Prentice Hall, © 2001 Structural Classification of Neurons • Based on the number of cytoplasmic processes Human Anatomy, 3rd edition Prentice Hall, © 2001 Functional Classification of Neurons • Based on the direction of impulse transmission – Sensory neurons – Motor neurons – Interneurons (association) Human Anatomy, 3rd edition Prentice Hall, © 2001 Nerve Impulse • 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. 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 Human Anatomy, 3rd edition Prentice Hall, © 2001 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 Starting a Nerve Impulse • 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 Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 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 – Repolarizes the membrane 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 – Requires ATP http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20102/Bio%20102%20lectures/nervous%20system/neuron6.gif Nerve Impulse Propagation • The impulse continues to move away from the cell body • Impulses travel faster when fibers have a myelin sheath Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 Continuation of the Nerve Impulse Between Neurons • 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 Synaptic Cleft Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 Postsynaptic Membrane Receptor Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006 Synapse Human Anatomy, 3rd edition Prentice Hall, © 2001 Neural Regeneration After Injury 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 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