Chapter 17: Nervous System
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Nervous Tissue
The nervous system is divided into a
central nervous system (CNS),
consisting of the brain and spinal cord,
and a peripheral nervous system (PNS),
consisting of nerves carrying sensory
and motor information between the
CNS and muscles and glands.
Both systems have two types of cells:
neurons that transmit impulses and
neuroglial cells that service neurons.
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Organization of the nervous
system
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Neuron Structure
Neurons are composed of dendrites that
receive signals, a cell body with a
nucleus, and an axon that conducts a
nerve impulse away.
Sensory neurons take information from
sensory receptors to the CNS.
Interneurons occur within the CNS and
integrate input.
Motor neurons take information from the
CNS to muscles or glands.
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Types of neurons
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Myelin Sheath
Long axons are covered by a protective
myelin sheath formed by neuroglial
cells called Schwann cells.
The sheath contains lipid myelin which
gives nerve fibers their white,
glistening appearance.
The sheath is interrupted by gaps called
nodes of Ranvier.
Multiple sclerosis is a disease of the
myelin sheath.
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Myelin sheath
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The Nerve Impulse
The nervous system uses the nerve
impulse to convey information.
The nature of a nerve impulse has been
studied by using excised axons and a
voltmeter called an oscilloscope.
Voltage (in millivolts, mV) measures the
electrical potential difference between
the inside and outside of the axon.
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Resting Potential
When an axon is not conducting a nerve
impulse, the inside of an axon is negative
(-65mV) compared to the outside; this is
the resting potential.
A sodium-potassium pump in the
membrane actively transports Na+ out of
the axon and K+ into the axon to establish
resting potential.
The membrane is more permeable to K+
and much of the resting potential is due
to the excess of K+ outside of the neuron.
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Resting potential
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Action Potential
An action potential is a rapid change in
polarity as the nerve impulse occurs.
The action potential occurs if a stimulus
causes the membrane to depolarize past
threshold.
An intense stimulus causes many firings
(reaching action potential) in an axon; a
weak stimulus may cause only a few.
The action potential requires two types of
gated channel proteins: one each for Na+
and K+.
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Sodium Gates Open
The gates of sodium channels open first
and Na+ flows into the axon.
The membrane potential depolarizes to
+40 MV.
Potassium Gates Open
The gates of potassium channels open
next and K+ flows to the outside of the
axon.
The membrane potential repolarizes to
–65 MV.
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Action potential
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Propagation of an Action
Potential
The action potential travels the length of an
axon, with each portion of the axon
undergoing depolarization then
repolarization.
A refractory period ensures that the action
potential will not move backwards.
In myelinated fibers, the action potential
only occurs at the nodes of Ranvier.
This “jumping” from node-to-node is called
saltatory conduction.
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Transmission Across a Synapse
The tip of an axon forms an axon bulb that
is close to a dendrite or cell body of
another neuron; this region of close
proximity is called the synapse.
Transmission of a nerve impulse takes
place when a neurotransmitter molecule
stored in synaptic vesicles in the axon
bulb is released into a synaptic cleft
between the axon and the receiving
neuron.
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When a nerve impulse reaches an axon
bulb, gated channels for calcium open
and Ca2+ flow into the bulb.
This sudden rise in Ca2+ causes synaptic
vesicles to move and merge with the
presynaptic membrane, releasing their
neurotransmitter molecules into the
cleft.
The binding of the neurotransmitter to
receptors in the postsynaptic
membrane causes either excitation or
inhibition.
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Synapse structure and function
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Synaptic Integration
Many synapses per single neuron is not
uncommon.
Excitatory signals have a depolarizing
effect, and inhibitory signals have a
hyperpolarizing effect on the postsynaptic membrane.
Integration is the summing up of these
excitatory and inhibitory signals.
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Integration
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Neurotransmitter Molecules
Out of 25, two well-known
neurotransmitters are acetylcholine
(ACh) and norepinephrine (NE).
Neurotranmitters that have done their job
are removed from the cleft; the enzyme
acetylcholinesterase (AChE) breaks
down acetylcholine.
Neurotransmitter molecules are removed
from the cleft by enzymatic breakdown
or by reabsorption, thus preventing
continuous stimulation or inhibition.
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The Central Nervous System
The central nervous system (CNS)
consists of the spinal cord and brain.
Both are protected by bone, wrapped in
protective membranes called
meninges, and surrounded and
cushioned with cerebrospinal fluid that
is produced in the ventricles of the
brain.
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The ventricles are interconnecting
cavities that produce and serve as a
reservoir for cerebrospinal fluid.
The CNS receives and integrates sensory
input and formulates motor output.
Gray matter contains cell bodies and
short, nonmyelinated fibers; white
matter contains myelinated axons that
run in tracts.
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Organization of the nervous
system
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The Spinal Cord
The spinal cord extends from the base of
the brain through the vertebral canal.
Structure of the Spinal Cord
A central canal holds cerebrospinal fluid.
Gray matter of the spinal cord forms an
“H” and contains interneurons and
portions of sensory and motor neurons.
White matter consists of ascending tracts
taking sensory information to the brain
and descending tracts carrying motor
information from the brain.
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Spinal cord
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Functions of the Spinal Cord
The spinal cord is the center for many
reflex arcs.
It also sends sensory information to the
brain and receives motor output from
the brain, extending communication
from the brain to the peripheral nerves
for both control of voluntary skeletal
muscles and involuntary internal
organs.
Severing the spinal cord produces
paralysis.
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The Brain
The brain has four cavities called
ventricles.
The cerebrum has two lateral ventricles,
the diencephalon has the third
ventricle, and the brain stem and
cerebellum have the fourth ventricle.
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The human brain
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The Cerebrum
The cerebrum or telencephalon has two
cerebral hemispheres connected by the
corpus callosum.
Learning, memory, language and speech
take place in the cerebrum.
Sulci divide each hemisphere into lobes
including the frontal, parietal, occipital,
and temporal lobes.
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Cerebral hemispheres
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The Cerebral Cortex
The cerebral cortex is a thin, highly
convoluted outer layer of gray matter
covering both hemispheres.
The primary motor area is in the frontal
lobe; this commands skeletal muscle.
The primary somatosensory area is dorsal
to the central sulcus or groove.
The primary visual area is at the back
occipital lobe.
The temporal lobe has the primary auditory
area.
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The parietal lobe provides taste sensation.
All have adjacent association areas that
integrate signals; the prefrontal area is
an important association area for
appropriate behavior.
White matter consists mostly of long
myelinated axons forming tracts; these
cross over so the left side of the brain
handles right side information.
Basal nuclei are masses of gray matter
deep within the white matter integrate
motor commands.
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The lobes of a cerebral hemisphere
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The Diencephalon
The hypothalamus and thalamus are in the
diencephalon that encircles the third
ventricle.
The hypothalamus controls homeostasis
and the pituitary gland, and the thalamus
receives all sensory input except smell
and integrates it and sends it to the
cerebrum.
The pineal gland is also located here and
secretes melatonin that may regulate our
daily rhythms.
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The Cerebellum
The cerebellum receives sensory input
from eyes, ears, joints and muscles and
receives motor input from the cerebral
cortex.
It integrates this information to maintain
posture and balance.
The cerebellum is involved in learning of
new motor skills, such as playing the
piano.
A thin layer of gray matter covers the
white matter.
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The Brain Stem
The brain stem contains the medulla
oblongata, pons, and midbrain.
The medulla oblongata and pons have
centers for vital functions such as
breathing, heartbeat, and
vasoconstriction.
The medulla also coordinates swallowing
and some other automatic reactions.
The midbrain acts as a relay station
between the cerebrum and spinal cord
or cerebellum.
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The Reticular Formation
The reticular formation is a complex
network of nuclei and fibers that extend
the length of the brain stem.
One portion of the reticular formation,
called the reticular activating system,
arouses the cerebrum via the thalamus
causing alertness.
An inactive reticular activating system
results in sleep.
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The reticular activating system
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The Limbic System and Higher
Mental Functions
Limbic System
The limbic system is involved in our
emotions and higher mental functions.
The limbic system is a complex network
of tracts and nuclei involving cerebral
lobes, basal nuclei and the
diencephalon.
Two structures, the hippocampus and
amygdala are essential for learning and
memory.
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The limbic system
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Higher Mental Functions
Animal research, MRI, and PET scans
allow researchers to study the
functioning of the brain.
Memory and Learning
Memory is the ability to hold a thought in
mind or recall events from the past.
Learning takes place when we retain and
utilize past memories.
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Short-term memory involves activity in
the prefrontal area.
Long-term memory includes semantic
memory (numbers, words, etc.) and
episodic memory (persons, events,
etc.).
Skill memory involves ability to ride a
bike, for example, and involves all
motor areas of the cerebrum below the
level of consciousness.
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Long-term Memory Storage and
Retrieval
Our long-term memories are stored in bits
and pieces throughout the sensory
association areas of the cerebral cortex.
The hippocampus is a bridge between
sensory association areas and the
prefrontal area where memories are
utilized.
The amygdala associates danger with
sensory stimuli.
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Long-term memory circuits
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Long-Term Potentiation
Long-term potentiation is increased
response at synapses within the
hippocampus and is essential to longterm memory.
However, a postsynaptic neuron in the
hippocampus can become too excited
and then die.
Excitotoxicity, a form of cell death, is due
to the neurotransmitter glutamate
rushing in too quickly.
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Language and Speech
Language and speech are dependent
upon Broca’s area (a motor speech
area) and Wernicke’s area (a sensory
speech area) that are involved in
communication.
These two areas are located only in the
left hemisphere; the left hemisphere
functions in language in general and
not just in speech.
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Language and speech
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The Peripheral Nervous System
The peripheral nervous system (PNS)
contains nerves (bundles of axons) and
ganglia (cell bodies).
Sensory nerves carry information to the
CNS, motor nerves carry information
away, and mixed nerves have both
types of fibers.
Humans have 12 pairs of cranial nerves
and 31 pairs of spinal nerves.
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Nerve structure
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Cranial nerves
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The dorsal root of a spinal nerve contains
sensory fibers that conduct sensory
impulses from sensory receptors
toward the spinal cord.
Dorsal root ganglia near the spinal cord
contain the cell bodies of sensory
neurons.
The ventral root of a spinal nerve
contains motor fibers that conduct
impulses away from the spinal cord to
effectors.
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Spinal nerves
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Somatic System
The somatic system serves the skin,
skeletal muscles, and tendons.
The brain is always involved in voluntary
muscle actions but somatic system
reflexes are automatic and may not
require involvement of the brain.
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The Reflex Arc
Involuntary reflexes allow us to respond
rapidly to external stimuli.
In reflexes, sensory receptors generate
nerve impulses carried to interneurons
in the spinal cord.
Next, interneurons signal motor neurons
which conduct nerve impulses to a
skeletal muscle that contracts, giving
the response to the stimulus.
Pain is not felt until the brain receives
nerve impulses.
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A spinal nerve reflex arc
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Autonomic System
The autonomic system of the PNS
regulates the activity of cardiac and
smooth muscle and glands.
The system is divided into sympathetic
and parasympathetic divisions that:
1) Function automatically and
involuntarily;
2) Innervate all internal organs; and
3) Use two neurons and one ganglion.
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Sympathetic Division
The sympathetic division is associated
with responses that occur during times
of stress, including “fight or flight”
reactions.
The postganglionic axon releases mainly
norepinephrine which acts similar to
adrenaline, the hormone from the
adrenal medulla.
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Parasympathetic Division
The parasympathetic system is
associated with responses that occur
during times of relaxation and
promotes “housekeeper” activities.
The postganglionic neurotransmitter
used by the parasympathetic division is
acetylcholine.
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Autonomic nervous system
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Drug Abuse
Stimulants increase excitation, and
depressants decrease excitation; either
can lead to physical dependence.
Each type of drug has been found to
either promote or prevent the action of
a particular neurotransmitter.
Medications that counter drug effects
work by affecting the release,
reception, or breakdown of dopamine, a
neurotransmitter responsible for mood.
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Drug actions at a synapse
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Drug use
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Alcohol
Alcohol may affect the inhibiting
transmitter GABA or glutamate, an
excitatory neurotransmitter.
Alcohol is primarily metabolized in liver
and heavy doses can cause liver scar
tissue and cirrhosis.
Alcohol is an energy source but it lacks
nutrients needed for health.
Cirrhosis of the liver and fetal alcohol
syndrome are serious conditions
associated with alcohol intake.
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Nicotine
Nicotine is an alkaloid derived from
tobacco.
In the CNS, nicotine causes neurons to
release dopamine; in the PNS, nicotine
mimics the activity of acetylcholine and
increases heart rate, blood pressure,
and digestive tract mobility.
Nicotine induces both physiological and
psychological dependence.
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Cocaine
Cocaine is an alkaloid derived from the
shrub Erythroxylum cocoa, often sold
as potent extract termed “crack.”
Cocaine prevents uptake of dopamine by
the presynaptic membrane, is highly
likely to cause physical dependence,
and requires higher doses to overcome
tolerance.
This makes overdosing is a real
possibility; overdosing can cause
seizures and cardiac arrest.
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Heroin
Derived from morphine, heroin is an
alkaloid of opium.
Use of heroin causes euphoria.
Heroin alleviates pain by binding to
receptors meant for the body’s own
pain killers which are the endorphins.
Tolerance rapidly develops and
withdrawal symptoms are severe.
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Marijuana
Marijuana is obtained from the plant
Cannabis sativa that contains a resin
rich in THC (tetrahydrocannabinol).
Effects include psychosis and delirium
and regular use can lead to
dependence.
Long-term marijuana use may lead to
brain impairment, and a fetal cannabis
syndrome has been reported.
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Chapter Summary
The nervous system consists of two
types of cells: neurons and mesoglia.
Neurons are specialized to carry nerve
impulses.
A nerve impulse is an electrochemical
change that travels along the length of
a neuron fiber.
Transmission of signals between
neurons is dependent on
neurotransmitter molecules.
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The central nervous system is made up
of the spinal cord and the brain.
The parts of the brain are specialized for
particular functions.
The cerebral cortex contains motor
areas, sensory areas, and association
areas that are in communication with
each other.
The cerebellum is responsible for
maintaining posture; the brainstem
houses reflexes for homeostasis.
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The reticular formation contains fibers
that arouse the brain when active and
account for sleep when they are
inactive.
The limbic system contains specialized
areas that are involved in higher mental
functions and emotional responses.
Long-term memory depends upon
association areas that are in contact
with the limbic system.
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There are particular areas in the left
hemisphere that are involved in
language and speech.
The peripheral nervous system contains
nerves that conduct nerve impulses
toward and away from the central
nervous system.
The autonomic nervous system has
sympathetic and parasympathetic
divisions with counteracting activities.
Use of psychoactive drugs such as
alcohol, nicotine, marijuana, cocaine,
and heroin is detrimental to the body.
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