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Chapter 12Nervous Tissue
The Nervous System Objectives
Describe the two anatomical divisions of the nervous system.
List the general functions of both divisions.
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Copyright 2009 John Wiley & Sons, Inc.
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Major Structures of the Nervous System
The Nervous System The nervous system functions to:
monitor the internal and external environment
integrate sensory information
coordinate voluntary and involuntary responses of other organ systems
Functions are performed by neurons, the functional unit of the nervous system.
Neuroglia support and protect the neurons.
Two major subdivisions of the nervous system are:
central nervous system (CNS)
peripheral nervous system (PNS)
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Central Nervous System
consists of the brain and the spinal cord.
integrates and coordinates sensory data and motor commands.
is the seat of higher functions (intelligence, memory, and emotion)
Peripheral Nervous System
provides communication between the CNS and the rest of the body.
includes all the neural tissue outside the CNS.
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afferent division - brings sensory information to CNS
Ex. From touch receptor to brain
efferent division - brings motor commands to muscles and glands
Ex. From brain to triceps brachii
Peripheral Nervous System
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Subdivisions of the PNS Somatic (voluntary) nervous system (SNS)
motor neurons to skeletal muscle tissue – controls skeletal muscle contractions
Autonomic (involuntary) nervous systems (ANS) motor neurons to smooth & cardiac muscle and glands –
involuntary control
sympathetic division (speeds up heart rate)
parasympathetic division (slow down heart rate)
Enteric nervous system (ENS) involuntary sensory & motor neurons control GI tract neurons function independently of ANS & CNS
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Copyright 2009 John Wiley & Sons, Inc.
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Organization of the Nervous System
Cellular Organization
Objective: Distinguish between neurons and
neuroglia on the basis of their structure and
function.
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Cellular Organization All neural tissue consists of two kinds of cells. Neurons
functional unit of the nervous system all neural functions involve the communication of
neurons with each other and with other cells Cannot divide
Neuroglia regulate the environment around the neurons provide a support framework for neural tissue act as phagocytes retain the ability to divide much smaller and much more numerous than
neurons
Dendrites
Cell body
Axon
Nuclei ofneuroglia
cell body
several branching sensitive dendrites receive incoming signals
elongated axon carry outgoing signals
one or more synaptic terminals communicates with other
cells
Neurons - Structure
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Neurons - Structure
Cell body large nucleus with large nucleolus
mitochondria
free and fixed ribosomes
nissl bodies - gray clusters of RER and free ribosomes
usually no centrioles
Neurons
Membrane of the cell body and dendrites are sensitive to chemical, mechanical, and electrical stimulation.
Stimulation results in an action potential from the axon hillock.
Axon may branch into collaterals each with a synaptic terminal.
Synapse is the site where communication occurs.
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Structure of a Multipolar Neuron
Structural & Functional Classification of Neurons Structural
multipolar neuron
unipolar neuron
bipolar neuron
Functional
sensory neurons
motor neurons
interneurons
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Multipolar Neuron
Multiple processes extending away from the cell body.
Very common in the CNS
Unipolar Neuron
Dendrites and axon are continuous and the cell body lies to one side.
Action potential begins at the base of the dendrites and the rest of the process is considered an axon.
Most sensory neurons of the PNS are unipolar.
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Bipolar Neurons
Cell body lies between the one dendrite and one axon.
Rare, occur in special sense organs such as the eye and ear.
Sensory Neurons (afferent)
There are about 10 million neurons in the afferent division. These are all sensory neurons
Receptors are categorized based on the information that they carry somatic sensory receptors
external receptors proprioceptors
visceral receptors
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Motor Neurons (efferent)
There are about .5 million motor neurons of the efferent division.
There are two efferent divisions of the PNS:
Somatic motor neurons of the somatic nervous system innervate the skeletal muscles.
Visceral motor neurons of the autonomic nervous system innervate smooth muscle, cardiac muscle, and glands
Interneurons
There are about 20 billion interneurons.
Located within the brain and spinal cord.
Interconnect other neurons.
Responsible for the distribution of sensory information and the coordination of motor activity.
A stimulus that requires a more complex response involves a greater number of interneurons.
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Neuroglia Objectives
Describe the locations of neuroglia.
Explain the functions of each type of neuroglia.
Neuroglia
found in the CNS and PNS.
The CNS has the greatest variety of glial cells.
There are four types of glial cells in the CNS.
astrocytes
oligodendrocytes
microglia
ependymal
Schwann cells are the major glial cells in the PNS.
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Copyright 2009 John Wiley & Sons, Inc.
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Neuroglia of the CNS
Ependymal Cells
Epithelial cells that line the central canal of the spinal cord and the ventricles of the brain.
Produce cerebrospinal fluid (CSF) in certain regions of the brain.
Cilia help to circulate the CSF in brain ventricles.
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Astrocytes
Largest and most numerous glial cells.
Secrete chemicals that maintain blood brain barrier.
Create a structural framework for the CNS.
Perform repairs in damaged neural tissue.
Direct growth and interconnection for developing neurons in the embryonic brain.
Adjust composition of ECF.
Oligodendrocytes
Cytoplasmic extensions create a myelin sheath around axons.
Myelin increases the speed of an action potential.
nodes of Ranvier (nodes): gaps between adjacent oligodendrocytes
internodes: areas covered in myelin.
Lipid rich myelin causes axons to appear glossy white.
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Microglia
Smallest and rarest of the neuroglia in the CNS.
Phagocytic cells.
Schwann Cells
Most important glial cells in the PNS.
Cover every axon outside the CNS whether myelinated or not.
The Schwann cell covering of the axon is called the neurilemma.
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Neuroglia of the PNS
Demyelination
The progressive destruction of myelin sheaths.
inflammation
axon damage
scarring of neural tissue
Results in gradual loss of sensation and motor control.
numbness / weakness
paralysis
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Anatomical Organization
Neurons and their axons form bundles with distinct anatomical boundaries.
PNS neuron cell bodies are located in ganglia axons are bundled in nerves
spinal nerves connected to spinal cord cranial nerves connected to brain
Anatomical Organization CNS
group of neuron cell bodies with a common function is a center (or nucleus if it has a distinct boundary).
brain surface is covered by a thick layer of gray matter called neural cortex
the most complex integration centers, nuclei, and cortical areas in the brain are called higher centers.
white matter containing bundles of axons sharing common origins, destinations, and functions are tracts
tracts in the spinal cord form larger groups called columns. Pathways link the centers of the brain with the rest of the
body sensory (ascending) pathways take info to the brain motor (descending) pathways result in motor control at
the skeletal muscles
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?????
1. What would damage to the afferent division of the nervous system affect?
2. Examination of a tissue sample shows unipolar neurons. Are these more likely to be sensory neurons or motor neurons?
3. Which type of glial cell would you expect to be present in large numbers in brain tissue from a person suffering from an infection of the central nervous system?
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1. Afferent division of the nervous system is composed of nerves that carry sensory information to the brain and spinal cord. Damage would interfere with a persons ability to experience a variety of sensory stimuli.
2. Sensory neurons of the peripheral nervous system are usually unipolar. This tissue is most likely associated with a sensory organ.
3. Microglial cells are small phagocytic cells found in increased number in damaged and diseased areas of the CNS.