Nerve Cells & Tissues
Structure & Function
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
Human Anatomy, 3rd editionPrentice 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
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
http://www.britannica.com/EBchecked/topic-art/409665/66781/Conduction-of-the-action-potential-In-a-myelinated-axon-the
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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