Transcript Human Anatomy, First Edition McKinley&O'Loughlin

Human Anatomy,
First Edition
McKinley & O'Loughlin
Chapter 14 Lecture Outline:
Nervous Tissue
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The Nervous System
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The nervous system is the body’s
primary communication and control
system.
The nervous system can be divided
according to structural and functional
categories.
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Nervous System: Structural
Organization
Structural subdivisions of the nervous system:
 Central nervous system (CNS)
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brain and spinal cord
Peripheral nervous system (PNS)
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cranial nerves (nerves that extend from the brain)
spinal nerves (nerves that extend from the spinal cord)
ganglia (clusters of neuron cell bodies located outside
the CNS)
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Nervous System: Functional Organization
Functional divisions of the nervous system:
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Sensory division — receives sensory information
(input) from receptors and transmits this
information to the CNS.
Motor (or efferent) division — transmits motor
impulses (output) from the CNS to muscles or
glands.
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Sensory Division
The sensory division is subdivided into two
components:
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Somatic sensory components are the general
somatic senses—touch, pain, pressure, vibration,
temperature, and proprioception.
Visceral sensory components transmit nerve
impulses from blood vessels and viscera to the
CNS. The visceral senses primarily include
temperature and stretch (of the organ wall).
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Motor Division
The motor division is subdivided into two components:
 The somatic motor component (somatic nervous system;
SNS) conducts nerve impulses from the CNS to skeletal
muscles.
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also known as the voluntary nervous system
The autonomic motor component (autonomic nervous
system; ANS) innervates internal organs, regulates
smooth muscle, cardiac muscle, and glands.
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also known as the visceral motor system or involuntary nervous
system
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Nerve Cells
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Two distinct cell types form nervous tissue.
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Neurons, which are excitable cells that initiate and transmit
nerve impulses
Glial cells, which are nonexcitable cells that support and
protect the neurons
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Characteristics of Neurons
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Neurons have a high metabolic rate.
Neurons have extreme longevity.
Neurons typically are non-mitotic.
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Neuron Structure
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Neurons come in all shapes and sizes, but all neurons
share certain basic structural features.
A typical neuron has a cell body, dendrites, and
axons.
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Neuron Structure – Cell Body
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The cell body serves as the neuron’s control
center and is responsible for receiving,
integrating, and sending nerve impulses.
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Neuron Structure – Dendrites
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Dendrites tend to be shorter, smaller processes that
branch off the cell body.
Some neurons have only one dendrite, while others
have many.
Dendrites conduct nerve impulses toward the cell
body; they receive input and then transfer it to the
cell body for processing.
The more dendrites a neuron has, the more nerve
impulses that neuron can receive from other cells.
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Neuron Structure – Axon
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The larger, typically longer nerve cell process
emanating from the cell body is the axon,
sometimes called a nerve fiber.
Most neurons have only one axon.
The axon transmits a nerve impulse away
from the cell body toward another cell.
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Classifications of Neurons
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Neurons vary widely in morphology and location.
They can be classified according to either their
structure or their function.
Neurons can be classified according to the number of
processes extending from the cell body.
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unipolar neuron has a single process
bipolar neurons have two processes
multipolar neurons have three or more processes
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Interneurons
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Interneurons, or association neurons, lie entirely
within the CNS and are multipolar.
They receive nerve impulses from many other
neurons and carry out the integrative function of the
nervous system.
Thus, interneurons facilitate communication between
sensory and motor neurons.
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Glial Cells
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Sometimes referred to as neuroglia, occur within
both the CNS and the PNS.
Glial cells are smaller and capable of mitosis.
Glial cells do not transmit nerve impulses.
Glial cells physically protect and help nourish
neurons, and provide an organized, supporting
framework for all the nervous tissue.
Glial cells far outnumber neurons.
Glial cells account for roughly half the volume of the
nervous system.
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Glial Cells of the CNS
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Astrocytes exhibit a starlike shape due to projections
from their surface.
Astrocytes are the most abundant glial cells in the
CNS, and they constitute over 90% of the tissue in
some areas of the brain.
Help form the blood-brain barrier (BBB) that strictly
controls substances entering the nervous tissue in
the brain from the bloodstream.
Regulate tissue fluid composition.
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Functions of Glial Cells
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Forming a structural network.
Replacing damaged neurons.
Assisting neuronal development.
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Myelination
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Neurolemmocytes also called Schwann cells,
are associated with PNS axons and are
responsible for myelinating PNS axons.
Myelination is the process by which part of an
axon is wrapped with a myelin sheath, a
protective fatty coating that gives it glossywhite appearance.
The myelin sheath supports, protects, and
insulates an axon.
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Myelination
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No change in voltage can occur across the membrane
in the insulated portion of an axon.
In the PNS, myelin sheaths form from
neurolemmocytes.
In the CNS, they form from oligodendrocytes.
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Mylenated vs. Unmylenated
Axons
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In a myelinated axon, the nerve impulse “jumps”
from neurofibril node to neurofibril node and is
known as saltatory conduction.
In an unmyelinated axon, the nerve impulse must
travel the entire length of the axon, a process called
continuous conduction.
A myelinated axon produces a faster nerve impulse.
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Mylenated vs. Unmylenated
Axons
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In an unmyelinated axon, a nerve impulse
takes longer to reach the end of the axon.
A myelinated axon also requires less energy
(ATP) than does an unmyelinated axon.
Using continuous conduction, unmyelinated
axons conduct nerve impulses from pain
stimuli.
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Regeneration of PNS Axons
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PNS axons are vulnerable to cuts, crushing injuries,
and other trauma.
A damaged axon can regenerate, however, if at least
some neurilemma remains.
PNS axon regeneration depends upon three factors.
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the amount of damage
neurolemmocyte secretion of nerve growth factors to
stimulate outgrowth of severed axons
the distance between the site of the damaged axon and the
effector organ
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Structure of a Nerve
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A nerve is a cable-like bundle of parallel axons.
Like a muscle, a nerve has three successive
connective tissue wrappings.
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endoneurium - a delicate layer of loose connective tissue
perineurium - a cellular and fibrous connective tissue layer
that wraps groups of axons into bundles called fascicles
epineurium - a superficial connective tissue covering
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This thick layer of dense irregular fibrous connective tissue encloses the
entire nerve, providing both support and protection
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Nerves
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Nerves are a component of the peripheral nervous
system.
Sensory (afferent) nerves convey sensory information
to the CNS.
Motor (efferent) nerves convey motor impulses from
the CNS to the muscles and glands.
Axons terminate as they contact other neurons,
muscle cells, or gland cells.
An axon transmits a nerve impulse at a specialized
junction with another neuron called synapse.
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Synapses
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Presynaptic neurons transmit nerve impulses
along their axonal membranes toward a
synapse.
Postsynaptic neurons conduct nerve impulses
through their dendritic and cell body
membranes away from the synapse.
Axons may establish synaptic contacts with
any portion of the surface of another neuron,
except those regions that are myelinated.
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Electrical Synapses
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Electrical synapses are not very common in
mammals.
In humans, these synapses occur primarily between
smooth muscle cells where quick, uniform innervation
is essential.
Electrical synapses are also located in cardiac muscle.
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Chemical Synapses
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The most numerous type of synapse is the chemical
synapse.
It facilitates most of the interactions between
neurons and all communications between neurons
and effectors.
At these junctions, the presynaptic membrane
releases a signaling molecule called a
neurotransmitter, such as acetylcholine (ACh).
Other types of neurons use other neurotransmitters.
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Neurotransmitters
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Are released only from the plasma membrane of the
presynaptic cell.
It then binds to receptor proteins found only on the
plasma membrane of the postsynaptic cell.
A unidirectional flow of information and
communication takes place.
Two factors influence the rate of conduction of the
impulse: the axon’s diameter and the presence (or
absence) of a myelin sheath.
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Neuronal Pools (or Neuronal
Circuits or Pathways)
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Billions of interneurons within the CNS are grouped in
complex patterns called neuronal pools (or neuronal
circuits or pathways).
Neuronal pools are defined based upon function, not
anatomy, into four types of circuits:
converging
 diverging
 reverberating
 parallel-after-discharge
A pool may be localized, or its neurons may be distributed in
several different regions of the CNS.
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