Copyright 2010, John Wiley & Sons, Inc. Chapter 9 Nervous Tissue.

Copyright 2010, John Wiley & Sons, Inc.

Chapter 9

Nervous Tissue


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.


Structures of the Nervous System Brain: neurons enclosed within skull Spinal cord: connects to brain and enclosed

within spinal cavity Nerves: bundles of many axons of neurons

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


Structures of the Nervous System


Functions of the Nervous System Sensory receptors and sensory nerves

Carry information into brain and spinal cord Integration: information processing

Perception = awareness of sensory input Analyzing and storing information to help lead to

appropriate responses Motor activity: efferent nerves

Signals to muscles and glands (effectors)


Organization of the Nervous System Central Nervous System (CNS)

Brain and spinal cord Peripheral Nervous System (PNS)

All nervous system structures outside of the CNS


Histology of the Nervous System Neurons

Can respond to stimuli and convert stimuli to electrical signals (nerve impulses) that travel along neurons

Neuroglia cells: support, nourish and protect neurons Neuroglia critical for homeostasis of interstitial fluid

around neurons


Neuronal Structure 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 Emerges at cone-shaped axon hillock

Axon terminals: contain synaptic vesicles that can release neurotransmitters


Neuronal Structure


Structural Classes of Neurons Multipolar

Have several or many dendrites and one axon Most common type in brain and spinal cord

Bipolar Have one dendrite and one axon Example: in retina of eye and inner ear

Unipolar Have fused dendrite and axon Sensory neurons of spinal nerves


Functional Classes of Neurons Sensory (afferent)

Convey impulses into CAN (brain or spinal cord) Motor (efferent)

Convey impulses from brain or spinal cord out through the PNS to effectors (muscles or glands)

Interneurons (association neurons) Most are within the CNS Transmit impulses between neurons, such as

between sensory and motor neurons


Neuroglia Cells smaller but much more numerous than

neurons Can multiply and divide and fill in brain areas Gliomas: brain tumors derived from neuroglia Functions

Do not conduct nerve impulses Do support, nourish and protect neurons


Neuroglia 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


Myelination Axons covered with a myelin sheath

Many layers of lipid and protein: insulates neurons Increases speed of nerve conduction Appears white (in white matter)

Nodes of Ranvier: gaps in the myelin Nodes are important for rapid signal conduction

Some diseases destroy myelin: Multiple sclerosis Tay-Sachs


Collections of Nervous Tissue Clusters of neuron cell bodies

Ganglion: cluster of cell bodies in PNS Nucleus: cluster of cell bodies in CNS

Bundles of axons Nerve: bundle of axons in PNS Tract: bundle to axons in CNS


Gray and White Matter White matter: primarily myelinated axons Gray matter: cell bodies, dendrites,

unmyelinated axons, axon terminals, neuroglia Locations of gray and white matter

Spinal cord: white matter (tracts) surround centrally located gray matter “H” of “butterfly”

Brain: gray matter in thin cortex surrounds white matter (tracts)


Neuron Regeneration Regeneration of PNS neurons

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 Very limited even if cell body is intact Inhibited by neuroglia and by lack of fetal growth-

stimulators


Organization of the Nervous System Central nervous system (CNS) structures:

Brain Spinal cord

Peripheral nervous system (PNS) structures: Cranial nerves and branches Spinal nerves and branches Ganglia Sensory receptors


Organization of the Nervous System


Organization of the Nervous System Peripheral nervous system (PNS) divisions

Somatic (SNS) Sensory neurons from head, body wall, limbs, special

sense organs Motor neurons to skeletal muscle: voluntary

Autonomic (ANS) nervous systems Sensory neurons from viscera Motor neurons to viscera (cardiac muscle, smooth

muscle, glands): involuntary Sympathetic: “fight-or-flight” Parasympathetic: “rest-and-digest”

Enteric nervous system (ENS): “brain of the gut”


Organization of the Nervous System Peripheral nervous system (PNS),

Enteric nervous system (ENS): “brain of the gut” Sensory neurons monitor chemical changes and

stretching of GI wall Motor neurons regulate contractions, secretions and

endocrine secretions (involuntary)


Structure and Function of the Nervous SystemInteractions Animation Introduction to Structure and Function of the

Nervous System

You must be connected to the internet to run this animation.


Action Potentials Action potentials = nerve impulses Require

A membrane potential: a charge difference across cell membrane (polarization)

Ion channels: allow ions to move by diffusion from high to low concentration

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


Resting Membrane Potential Typically –70 mV

Inside of membrane more negative than outside Caused by presence of ions:

Inside (more negative) because cytosol has: 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:

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


Resting Membrane Potential


Action Potential Series of events that activate cell membrane

in neuron or muscle fiber An initial event (stimulus) is required

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


Action Potential Depolarizing phase

Na+ channels open as more Na+ enters cell, membrane potential rises and becomes positive (–70 0 + 30 mv)

Repolarizing phase 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)


Action Potential


Action Potential Recovery

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 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.


Membrane PotentialsInteractions Animations

Membrane Potentials

You must be connected to the internet to run this animation.


Conduction of Nerve Impulses Nerve impulse conduction (propagation)

Each section triggers the next locally as even more Na+ channels are opened (like row of dominos)

Types of conduction Continuous conduction

In unmyelinated fibers; slower form of conduction Saltatory conduction

In myelinated fibers; faster as impulses “leap” between nodes of Ranvier

Factors that increase rate of conduction Myelin, large diameter and warm nerve fibers


Conduction of Nerve Impulses


Synaptic Transmission Similar sequence of events occurs at

Synapse (neuron-neuron) Neuromuscular junction (neuron-muscle fiber:

chapter 8) Neuroglandular junction (neuron-gland)

Triggered by action potential (nerve impulse) Components of synapse:

Sending neuron: presynaptic neuron (releases neurotransmitter)

Space between neurons: synaptic cleft Receiving neuron: postsynaptic neuron


Synaptic Transmission 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


Synaptic Transmission NT serves as chemical trigger (stimulus) of

ion channels Postsynaptic cell membrane may be

depolarized or hyperpolarized 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


Synaptic Transmission One-way transmission only because

Only presynaptic cells release NT Only postsynaptic cells have receptors for NT

binding Finally, NT must be removed from the cleft.

Three possible mechanisms Diffusion out of cleft Destruction by enzymes (such as ACh-ase) in cleft Transport back (recycling) into presynaptic cell


Copyright 2009 John Wiley & Sons, Inc. 38

Signal Transmission at the Chemical Synapse


Neurotransmitters Acetylcholine (ACh): common in PNS

Stimulatory (on skeletal muscles) Inhibitory (on cardiac muscle)

Amino acids Glutamate, aspartate, gamma aminobutyric acid

(GABA), glycine Modified amino acids

Norepinephrine (NE), dopamine (DA), serotonin Neuropeptides such as endorphins Nitric oxide (NO)


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. Chapter 9 Nervous Tissue.

Documents

Transcript of Copyright 2010, John Wiley & Sons, Inc. Chapter 9 Nervous Tissue.