Transcript Chapter 9

Chapter 9
Nervous Tissue
Copyright 2010, John Wiley & Sons, Inc.
End of Chapter 9

Copyright 2010 John Wiley & Sons, Inc.
All rights reserved. Reproduction or translation of this
work beyond that permitted in section 117 of the 1976
United States Copyright Act without express permission
of the copyright owner is unlawful. Request for further
information should be addressed to the Permission
Department, John Wiley & Sons, Inc. The purchaser may
make back-up copies for his/her own use only and not
for distribution or resale. The Publishers assumes no
responsibility for errors, omissions, or damages caused
by the use of theses programs or from the use of the
information herein.
Copyright 2010, John Wiley & Sons, Inc.
Structures of the Nervous System
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Brain: neurons enclosed within skull
Spinal cord: connects to brain and enclosed
within spinal cavity
Nerves: bundles of many axons of neurons
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Cranial nerves (12 pairs) emerge from brain
Spinal nerves (31 pairs) emerge from spinal cord
Ganglia: groups of neuron cell bodies located
outside of brain and spinal cord
Enteric plexuses: networks in digestive tract
Sensory receptors: monitor changes in internal
or external environments
Copyright 2010, John Wiley & Sons, Inc.
Structures of the Nervous System
Copyright 2010, John Wiley & Sons, Inc.
Functions of the Nervous System
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Sensory receptors and sensory nerves
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Integration: information processing
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Carry information into brain and spinal cord
Perception = awareness of sensory input
Analyzing and storing information to help lead to
appropriate responses
Motor activity: efferent nerves
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Signals to muscles and glands (effectors)
Copyright 2010, John Wiley & Sons, Inc.
Organization of the Nervous System
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Central Nervous System (CNS)
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Brain and spinal cord
Peripheral Nervous System (PNS)
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All nervous system structures outside of the CNS
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Histology of the Nervous System
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Neurons
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Can respond to stimuli and convert stimuli to
electrical signals (nerve impulses) that travel along
neurons
Neuroglia cells: support, nourish and protect
neurons
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Neuroglia critical for homeostasis of interstitial fluid
around neurons
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Neuronal Structure
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Cell body: nucleus, cytoplasm with typical
organelles
Dendrites: highly branched structures that
carry impulses to the cell body
Axon: conducts away from cell body toward
another neuron, muscle or gland
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Emerges at cone-shaped axon hillock
Axon terminals: contain synaptic vesicles
that can release neurotransmitters
Copyright 2010, John Wiley & Sons, Inc.
Neuronal
Structure
Copyright 2010, John Wiley & Sons, Inc.
Structural Classes of Neurons
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Multipolar
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Bipolar
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Have several or many dendrites and one axon
Most common type in brain and spinal cord
Have one dendrite and one axon
Example: in retina of eye and inner ear
Unipolar
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Have fused dendrite and axon
Sensory neurons of spinal nerves
Copyright 2010, John Wiley & Sons, Inc.
Functional Classes of Neurons
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Sensory (afferent)
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Motor (efferent)
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Convey impulses into CAN (brain or spinal cord)
Convey impulses from brain or spinal cord out
through the PNS to effectors (muscles or glands)
Interneurons (association neurons)
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Most are within the CNS
Transmit impulses between neurons, such as
between sensory and motor neurons
Copyright 2010, John Wiley & Sons, Inc.
Neuroglia
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Cells smaller but much more numerous than
neurons
Can multiply and divide and fill in brain areas
Gliomas: brain tumors derived from neuroglia
Functions
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Do not conduct nerve impulses
Do support, nourish and protect neurons
Copyright 2010, John Wiley & Sons, Inc.
Neuroglia
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Astrocytes: help form blood brain barrier
Oligodendrocytes: produce myelin in CNS
Microglia: protect CNS cells from disease
Ependymal cells: form CSF in ventricles
Schwann: produce myelin around PNS
neurons; help to regenerate PNS axons
Satellite cells: support neurons in PNS ganglia
Copyright 2010, John Wiley & Sons, Inc.
Myelination
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Axons covered with a myelin sheath
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Nodes of Ranvier: gaps in the myelin
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Many layers of lipid and protein: insulates neurons
Increases speed of nerve conduction
Appears white (in white matter)
Nodes are important for rapid signal conduction
Some diseases destroy myelin:
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Multiple sclerosis
Tay-Sachs
Copyright 2010, John Wiley & Sons, Inc.
Collections of Nervous Tissue
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Clusters of neuron cell bodies
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Ganglion: cluster of cell bodies in PNS
Nucleus: cluster of cell bodies in CNS
Bundles of axons
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Nerve: bundle of axons in PNS
Tract: bundle to axons in CNS
Copyright 2010, John Wiley & Sons, Inc.
Gray and White Matter
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White matter: primarily myelinated axons
Gray matter: cell bodies, dendrites,
unmyelinated axons, axon terminals, neuroglia
Locations of gray and white matter
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Spinal cord: white matter (tracts) surround centrally
located gray matter “H” of “butterfly”
Brain: gray matter in thin cortex surrounds white
matter (tracts)
Copyright 2010, John Wiley & Sons, Inc.
Neuron Regeneration
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Regeneration of PNS neurons
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Axons and dendrite in the PNS can be repaired if
cell body is intact and Schwann cells functional.
These form a regeneration tube and grow axons
or dendrites if scar tissue does not fill the tube
Regeneration of CNS neurons
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Very limited even if cell body is intact
Inhibited by neuroglia and by lack of fetal growthstimulators
Copyright 2010, John Wiley & Sons, Inc.
Organization of the Nervous System
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Central nervous system (CNS) structures:
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Brain
Spinal cord
Peripheral nervous system (PNS) structures:
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Cranial nerves and branches
Spinal nerves and branches
Ganglia
Sensory receptors
Copyright 2010, John Wiley & Sons, Inc.
Organization of the Nervous System
Copyright 2010, John Wiley & Sons, Inc.
Organization of the Nervous System
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Peripheral nervous system (PNS) divisions
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Somatic (SNS)
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Sensory neurons from head, body wall, limbs, special
sense organs
Motor neurons to skeletal muscle: voluntary
Autonomic (ANS) nervous systems
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Sensory neurons from viscera
Motor neurons to viscera (cardiac muscle, smooth
muscle, glands): involuntary
Sympathetic: “fight-or-flight”
 Parasympathetic: “rest-and-digest”
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Enteric nervous system (ENS): “brain of the gut”
Copyright 2010, John Wiley & Sons, Inc.
Organization of the Nervous System
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Peripheral nervous system (PNS),
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Enteric nervous system (ENS): “brain of the gut”
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Sensory neurons monitor chemical changes and
stretching of GI wall
Motor neurons regulate contractions, secretions and
endocrine secretions (involuntary)
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Structure and Function of the Nervous System
Interactions Animation
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Introduction to Structure and Function of the
Nervous System
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Copyright 2010, John Wiley & Sons, Inc.
Action Potentials
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Action potentials = nerve impulses
Require
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A membrane potential: a charge difference across
cell membrane (polarization)
Ion channels: allow ions to move by diffusion from
high to low concentration
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Leakage channels: allow ions to leak through membrane;
there are more for K+ than for Na+
Gated channels
Open and close on command
 Respond to changes in membrane so can generate and
conduct action potentials
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Copyright 2010, John Wiley & Sons, Inc.
Resting Membrane Potential
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Typically –70 mV
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Inside of membrane more negative than outside
Caused by presence of ions:
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Inside (more negative) because cytosol has:
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Many negative ions (too large to leak out): amino acids
(in cellular proteins) and phosphates (as in ATP)
K+ that easily leaks out through many K+ channels
Outside (more positive) because interstitial fluid
has:
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Few negative ions
Na+ that does not leak out of cell: few Na+ channels
Membrane “pumps” that quickly pump out Na+ that
does leak (diffuse) into cell
Copyright 2010, John Wiley & Sons, Inc.
Resting Membrane Potential
Copyright 2010, John Wiley & Sons, Inc.
Action Potential
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Series of events that activate cell membrane
in neuron or muscle fiber
An initial event (stimulus) is required
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Triggers resting membrane to become more
permeable to Na+
Causes enough Na+ to enter cell so that cell
membrane reaches threshold (~ –55 mv)
If so, the following events occur: action potential
which spreads along neuron or muscle fiber
Copyright 2010, John Wiley & Sons, Inc.
Action Potential
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Depolarizing phase
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Na+ channels open  as more Na+ enters cell,
membrane potential rises and becomes positive
(–70  0  + 30 mv)
Repolarizing phase
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K+ channels open  as more K+ leave cell,
membrane potential is returned to resting value
(+ 30  0  –70 mv)
May overshoot: hyperpolarizing phase
Typically depolarization and repolarization take
place in about 1 millisecond (1/1000 sec)
Copyright 2010, John Wiley & Sons, Inc.
Action Potential
Copyright 2010, John Wiley & Sons, Inc.
Action Potential
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Recovery
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Levels of ions back to normal by action of Na+/K+
pump
Refractory period (brief): even with adequate
stimulus, cell cannot be activated
All-or-none principle
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If a stimulus is strong enough to cause
depolarization to threshold level, the impulse will
travel the entire length of the neuron at a constant
and maximum strength.
Copyright 2010, John Wiley & Sons, Inc.
Membrane Potentials
Interactions Animations
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Membrane Potentials
You must be connected to the internet to run this animation.
Copyright 2010, John Wiley & Sons, Inc.
Conduction of Nerve Impulses
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Nerve impulse conduction (propagation)
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Each section triggers the next locally as even more
Na+ channels are opened (like row of dominos)
Types of conduction
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Continuous conduction
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Saltatory conduction
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In unmyelinated fibers; slower form of conduction
In myelinated fibers; faster as impulses “leap” between
nodes of Ranvier
Factors that increase rate of conduction
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Myelin, large diameter and warm nerve fibers
Copyright 2010, John Wiley & Sons, Inc.
Conduction
of Nerve
Impulses
Copyright 2010, John Wiley & Sons, Inc.
Conduction
of Nerve
Impulses
Copyright 2010, John Wiley & Sons, Inc.
Synaptic Transmission
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Similar sequence of events occurs at
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Synapse (neuron-neuron)
Neuromuscular junction (neuron-muscle fiber:
chapter 8)
Neuroglandular junction (neuron-gland)
Triggered by action potential (nerve impulse)
Components of synapse:
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Sending neuron: presynaptic neuron (releases
neurotransmitter)
Space between neurons: synaptic cleft
Receiving neuron: postsynaptic neuron
Copyright 2010, John Wiley & Sons, Inc.
Synaptic Transmission
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Action potential arrives at presynaptic
neuron’s end bulb
Opens voltage gated Ca2+ channels  Ca2+
flows into presynaptic cytosol
Increased Ca2+ concentration  exocytosis
of synaptic vesicles
Neurotransmitter (NT) released into cleft
NT diffuses across cleft and binds to
receptors in postsynaptic cell membrane
Copyright 2010, John Wiley & Sons, Inc.
Synaptic Transmission
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NT serves as chemical trigger (stimulus) of
ion channels
Postsynaptic cell membrane may be
depolarized or hyperpolarized
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Depends on type of NT and type of postsynaptic
cell
1000+ neurons converge on synapse; the sum of
all of their NTs determines effect
If threshold reached, then postsynaptic cell
action potential results
Copyright 2010, John Wiley & Sons, Inc.
Synaptic Transmission
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One-way transmission only because
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Only presynaptic cells release NT
Only postsynaptic cells have receptors for NT
binding
Finally, NT must be removed from the cleft.
Three possible mechanisms
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Diffusion out of cleft
Destruction by enzymes (such as ACh-ase) in cleft
Transport back (recycling) into presynaptic cell
Copyright 2010, John Wiley & Sons, Inc.
Signal Transmission at the Chemical
Synapse
Copyright
Copyright 2010,
2009 John
John Wiley
Wiley &
& Sons,
Sons, Inc.
Inc.
38
Neurotransmitters
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Acetylcholine (ACh): common in PNS
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Amino acids
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Glutamate, aspartate, gamma aminobutyric acid
(GABA), glycine
Modified amino acids
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Stimulatory (on skeletal muscles)
Inhibitory (on cardiac muscle)
Norepinephrine (NE), dopamine (DA), serotonin
Neuropeptides such as endorphins
Nitric oxide (NO)
Copyright 2010, John Wiley & Sons, Inc.
End of Chapter 9

Copyright 2010 John Wiley & Sons, Inc.
All rights reserved. Reproduction or translation of this
work beyond that permitted in section 117 of the 1976
United States Copyright Act without express permission
of the copyright owner is unlawful. Request for further
information should be addressed to the Permission
Department, John Wiley & Sons, Inc. The purchaser may
make back-up copies for his/her own use only and not
for distribution or resale. The Publishers assumes no
responsibility for errors, omissions, or damages caused
by the use of theses programs or from the use of the
information herein.
Copyright 2010, John Wiley & Sons, Inc.