The Nervous System: Neural Tissue

Chapter 9a

Introduction

– Nervous system = control center & communications network

– Functions• Stimulates movements• Maintains homeostasis (with endocrine

system)

Human Anatomy, 3rd editionPrentice Hall, © 2001

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

Nervous System

Human Anatomy, 3rd editionPrentice Hall, © 2001

Organization of the Nervous System

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

Human Anatomy, 3rd editionPrentice Hall, © 2001

Neurons– Function

• Conduct electrical impulses

– Structure• Cell body

– Nucleus with nucleolus

– Cytoplasm

• Cytoplasmic processes

– Dendrites

– Axon

Human Anatomy, 3rd editionPrentice 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

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Human Anatomy, 3rd editionPrentice Hall, © 2001

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 editionPrentice Hall, © 2001

Myelin

– Nodes of Ranvier• Unmyelinated gaps

between segments of myelin

• Impulses “jump” from node to node

Human Anatomy, 3rd editionPrentice Hall, © 2001

Nerve Fibers of the CNS

– Umyelinated– Myelinated

• Production of myelin is from oligodendrocytes

• Nodes of Ranvier are less numerous

Classification of Neurons

– Functional – based on the direction of impulse transmission

• Sensory neurons

• Motor neurons

• Interneurons (association)

Human Anatomy, 3rd editionPrentice Hall, © 2001

Functional Classification of Neurons

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 ImpulseStarting a Nerve Impulse

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• 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

The Action PotentialThe Action Potential

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• 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

Nerve Impulse PropagationNerve Impulse Propagation

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• The impulse continues to move away from the cell body

• Impulses travel faster when fibers have a myelin sheath

Continuation of the Nerve Impulse between Continuation of the Nerve Impulse between NeuronsNeurons

• 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 SynapsesHow Neurons Communicate at Synapses

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Human Anatomy, 3rd editionPrentice Hall, © 2001

Synapses

Human Anatomy, 3rd editionPrentice Hall, © 2001

Neural Regeneration

Human Anatomy, 3rd editionPrentice Hall, © 2001

Neural Regeneration

Human Anatomy, 3rd editionPrentice Hall, © 2001

Neural Regeneration

Human Anatomy, 3rd editionPrentice Hall, © 2001

Neural Regeneration

Structure of a NerveStructure of a Nerve

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Endoneurium surrounds each fiber

• Groups of fibers are bound into fascicles by the perineurium

• Fascicles are bound together by epineurium

Classification of NervesClassification 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