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The Nervous System: Neural Tissue

Chapter 13

Human Anatomy, 3rd edition Prentice Hall, © 2001

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 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

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Neuroglia of CNS •  Astrocytes

•  Form the blood-brain 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

Human Anatomy, 3rd edition Prentice Hall, © 2001

Neuroglia of CNS Neuroglia of PNS •  Schwann cells

– Form myelin sheaths of PNS •  Satellite cells

Marieb, Essentials of Anatomy and Physiology, 8th edition, 2006

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Human Anatomy, 3rd edition Prentice Hall, © 2001

Neurons •  Function

–  Conduct electrical impulses

•  Structure –  Cell body

•  Nucleus with nucleolus

•  Cytoplasm (perikaryon)

–  Cytoplasmic processes

•  Dendrites •  Axon Human Anatomy, 3rd edition

Prentice Hall, © 2001

Axon Structure •  Long, specialized

–  Collaterals = branches –  Telodendria = termination of axons & collaterals

•  Cytoplasm = axoplasm •  Plasma membrane = axolemma

Human Anatomy, 3rd edition Prentice Hall, © 2001

Anatomy of a Neuron 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

Human Anatomy, 3rd edition Prentice Hall, © 2001

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

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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)

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.

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

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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

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Postsynaptic Membrane Receptor

Marieb, Essentials of Human Anatomy and Physiology, 8th edition, 2006

Human Anatomy, 3rd edition Prentice Hall, © 2001

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

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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