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Chapter 11: Nervous System
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Function of the Nervous System
 To coordinate the actions of your body
 To ensure effective behaviour
 To maintain the internal environment within safe
limits (homeostasis)
Messages are relayed throughout the body via
electrochemical messages from the brain or through
chemical messengers – hormones (hormones require
more time than nervous transmission but are long
lasting)
There are more nerve cells in the body than there are
visible stars in the Milky Way!
1 cm3 of brain tissue houses several million neurons
with each connecting with several thousand others
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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 support
neurons.
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Organization of the nervous system
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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
(nonmyelinated).
Motor neurons take information from the CNS to
muscles or glands.
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Types of neurons
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dendrites – receive information (either from
receptor cells or other nerve cells), conducting
towards the cell body (~200 dendrites/cell body)
cell body – location of the nucleus, high metabolic
rate (so contains mitochondria)
axon– may be 1m long, very thin, conducts the
impulse towards other neurons or effectors, starts
at axon hillock, the smaller the neuronal diameter,
the faster the neuronal transmission
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nodes of Ranvier– the unmyelinated sections of a
myelinated neuron, impulses “jump” between the
nodes of Ranvier
neurilemma– a thin layer encompassing neurons in the
peripheral nervous system, promoting their
regeneration
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Schwann cell – responsible for the myelin synthesis, type
of glial cell (supporting and nourishing cell found in
the nervous system)
Axon Bulb – either at a synaptic bulb or end plate to
muscle, contains neurotransmitter
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Myelin Sheath
Myelination covers long axons with a protective myelin
sheath (made 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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FYI
Nerves are generally comprised of many neurons
together (like fibre optic cable)
Myelinated neurons in the brain are termed white
matter (the myelin makes them look white)
White matter may regenerate after injury, whereas grey
matter (unprotected) will not
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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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Membrane Polarization (Resting Potential)
When an axon is not conducting a nerve impulse, the inside
of an axon is negative (-70mV) compared to the
outside(+40mV); this is the resting potential.
To establish the –70mV potential in the cell:
 Na+ is actively pumped out of the cell
 K+ is actively pumped into the cell
Sodium pump
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Membrane Polarization (Resting Potential)
 Na+ and K+ diffuse down the concentration gradient,
but K+ diffuses faster due to an increased number of
ion channels (gates) open to K+ ions
 Since there is a net loss of positive ions to the outside
of the cell, -70 mV is established inside the neuron
 There are also large negative proteins inside the
neuron that contribute to the negative charge
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Resting potential
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Membrane Depolarization
When the nerve cell is excited, the membrane
DEPOLARIZES (Action Potential)
The membrane’s polarity changes:
– Na+ channels open, Na+ rushes in, K+ gates close
The positive ions flowing in causes a charge reversal to
+40 mV inside the neuron
(gated channel proteins)
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Action potential
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Membrane Repolarization
Once the charge becomes positive, the Na+ gates close,
K+ gates open, eventually restoring the charge inside
the neuron to –70 mV (but the Na+ excess is inside
and K+ excess is outside!)
The Na/K Pump restores the ion concentrations inside
and outside the cell
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Membrane Repolarization
During the repolarization, the nerve cannot be
reactivated – this is called the refractory period (1 to 10
ms) and is a recovery time for the neuron
The pump requires ATP in order to operate
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The Na/K Pump
To be ready for another action potential, the membrane
re-establishes the proper concentration gradient for
sodium and potassium
Three sodium ions are actively transported across the
membrane and to the ECM
Two potassium ions are then carried across to the
cytoplasm
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Movement of the Action Potential
The action in the neuron adjacent to an area of resting
membrane causes that area to depolarize, moving the
action potential along (due to attraction of opposite
charges)
Since the area from which the action potential came is
still in recovery, the action potential will only move in
one direction
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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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Schwann cell
Depolarized region
(node of Ranvier)
Cell body
Myelin
sheath
Axon
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The All-or-None Response (Threshold Potential)
All neurons provide an all-or-none response:
- in response to a stimulus, they either activate (fire)
and provide a certain level of response, or don’t fire
at all
A neuron will only fire if it is stimulated with an
intensity of at least threshold level
Every action potential for a neuron is identical in
strength and duration (regardless of how much
beyond threshold the stimulus is)
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Threshold Potential
All neurons differ in their threshold level
To inform the brain of the intensity of a stimulus:
- the frequency of firing is increased (not speed, which
is constant for each neuron)
- the number of neurons that respond to that level of
stimulus can increase (neurons may have different
threshold)
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Transmission Across a Synapse
 The junction between neurons or neurons & effectors 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, calcium
channels 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 synapse
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 Summation
Many synapses per single neuron is not uncommon.
Excitatory signals have a depolarizing effect, and
inhibitory signals have a hyperpolarizing effect on the
post- synaptic membrane.
Summation is the summing up of these excitatory and
inhibitory signals.
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Summation
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Summation
Neurotransmitter Molecules
Out of 25, two well-known neurotransmitters are
acetylcholine (ACh) and norepinephrine (NE).
Neurotransmitters 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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FYI
 most synapses involve more than just 2 neurons
(or neuron/effectors)
 neurotransmitters move only by diffusion, so
synaptic transmission is MUCH slower than
axonal transmission.
 insecticides interfere with enzymes that break
down neurotransmitters causing their hearts to
remain contracted,
 whereas LSD and other hallucinogens are believed
to bind to the receptor sites for neurotransmitters
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 Lidocaine, an anesthetic works by stabilizing the
neuronal membrane so it can’t depolarize
 Endorphins and enkephalins are “natural”
painkillers produced in the CNS, blocking the pain
transmitter that usually attaches to the injured
organ allowing the perception of pain
 opiates (heroin, codeine, morphine) block the
production of the pain transmitter. Since they act
to decrease the production of natural painkillers,
the amount of opiate taken must be increased or at
least maintained to maintain the effect
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 Valium and other depressants are believed to enhance
the action of inhibitory synapses
 Alcohol acts to increase the polarization of the
membrane, increasing the threshold
 Since many neurons will connect to a postsynaptic
neuron, it is the summation of the effects of the
presynaptic neurons that determine whether or not
the postsynaptic neuron or effector will depolarize
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Neural Circuits – includes neuronal and synaptic
transmission
There are two types of neural circuits
 complicated neural circuits, involving conscious
thought
 reflex arcs – without brain coordination

often unconscious, involuntary and faster than when thought
is required (why are these useful?)
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Nervous Control (in general)
StimulusReceptorSensoryNeuronInterneuron
BrainInterneuronMotorNeuronEffectorResponse
Reflex Arc (see diagram – the reflex arc)
StimulusReceptorSensoryNeuronInterneuron
(spinal cord)MotorNeuronEffectorResponse
When the response is made at the spinal cord level
(information does not have to go to the brain to be
processed), the response is quick (and always correct
given the circumstances)
Reflexes protect the body from injury
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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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The Brain
 consumes more oxygen and glucose than any other
part of the body
 meninges – outer layers (protection) – dura mater,
arachnoid and pia mater
 cerebrospinal fluid –between the inner, middle
meninges & central canal of s.cord, carries nutrients,
acts as a shock absorber, relays waste by diffusion &
fac. diffusion, flows within ventricles – four “spaces” in
the brain
The human brain
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Fig. 49-15
Frontal lobe
Parietal lobe
Speech
Frontal
association
area
Somatosensory
association
area
Taste
Reading
Speech
Hearing
Smell
Auditory
association
area
Visual
association
area
Vision
Temporal lobe
Occipital lobe
Fig. 49-17
Max
Hearing
words
Seeing
words
Min
Speaking
words
Generating
words
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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.
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Forebrain (cerebrum)
 contains two hemispheres for coordinating sensory
and motor information – speech,
reasoning, memory, personality, which may be located
on one side only
 the outer layer is called the cerebral cortex (only 1 mm
thick), deeply folded into fissures(to increase surface
area)
Cerebral hemispheres
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Forebrain Continued
- the two hemispheres are connected by the corpus
callosum allowing info to be shared between the
hemispheres (a collection of nerve fibres) which are
sometimes severed to control epilepsy leading to
interesting results
- the cerebrum can be subdivided into 4 lobes
1. Frontal (walking, speech, intellect, personality),
2. temporal (hearing,vision, memory, interpretation),
3. parietal (interpreting sensory info receptors, long term
memory) and
4. occipital (vision) lobes
Broca’s area - a part of the left hemisphere usually where
speech centre is located
The lobes of a cerebral hemisphere
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Forebrain Continued
 thalamus- below cerebrum, coordinates and interprets




sensory info
hypothalamus – below the thalamus, related to
pituitary,
connects endocrine to the nervous system, receives
sensory info, instincts, temperature control (ANS)
pituitary gland – influenced by the hypthalamus, part of
the endocrine system (master gland)
pineal gland – part of the endocrine system – melatonin
production
 midbrain - less developed in humans than the
forebrain, 4 spheres – relay centre for some eye and
ear reflexes
 Hindbrain - located behind the midbrain,
connects brain to spinal cord
 contains cerebellum (coordinates movement, balance,
muscle tone), The cerebellum is involved in learning of
new motor skills, such as playing the piano.
 pons (relay station between cerebellum areas, and
cerebellum & medulla)
 medulla oblongata (connection between peripheral and
CNS, involuntary movements – heart rate, breathing
(ANS), crossover of control)
 FYI
 much brain research takes place during brain
surgery & after people have strokes
 epileptics also provide insight into brain
differentiation when they undergo severing of the
corpus callosum to relieve extremely serious
seizures
 although the brain must control the entire body,
the volume of brain allocated to each part of the
body is not proportional to that body part’s size –
the face and hands account for the majority of the
motor cortex’s attention
Fig. 49-16
Parietal lobe
Frontal lobe
Leg
Genitals
Toes
Jaw
Primary
motor cortex
Abdominal
organs
Primary
somatosensory cortex
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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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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 ventral root (towards front of body) carries motor
neuron messages to muscles
 dorsal root (towards back) carries sensory neuron
messages from the body
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 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
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.
 nerves running to skeletal muscle system (under
voluntary control)
 motor neurons  voluntary effectors (skeletal
muscle)
 control exists in the cerebrum & cerebellum
(coordination)
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Homeostasis and the Autonomic Nervous
System
 All autonomic nerves are motor nerves that
regulate the organs of the body without conscious
control; involuntary
 Control exists in the medulla
 Effectors are smooth muscle (digestive system),
cardiac muscle (heart) and glands (exocrine &
endocrine)
 Responsible for maintaining homeostasis during
times of rest and during emergencies
Consists of two parts:
Sympathetic
 prepares the body for stress, including “fight or flight”
response
 short preganglionic nerve (Ach), long postganglionic nerve
(NEp)
 originate in the thoracic vertebrae (ribs) or lumbar
vertebrae (small of back)
 Parasympathetic
 restores normal balance; times of relaxation
 long preganglionic nerve (Ach), short postganglionic nerve
(ACh)
 originate in the brain (cranial nerves) or the spinal cord
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Fig. 49-8
Sympathetic division
Parasympathetic division
Action on target organs:
Action on target organs:
Constricts pupil
of eye
Dilates pupil
of eye
Stimulates salivary
gland secretion
Inhibits salivary
gland secretion
Constricts
bronchi in lungs
Cervical
Sympathetic
ganglia
Relaxes bronchi
in lungs
Slows heart
Accelerates heart
Stimulates activity
of stomach and
intestines
Inhibits activity
of stomach and
intestines
Thoracic
Stimulates activity
of pancreas
Inhibits activity
of pancreas
Stimulates
gallbladder
Stimulates glucose
release from liver;
inhibits gallbladder
Lumbar
Stimulates
adrenal medulla
Promotes emptying
of bladder
Promotes erection
of genitals
Inhibits emptying
of bladder
Sacral
Synapse
Promotes ejaculation and
vaginal contractions
Autonomic nervous system
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Disorders Associated With the
Nervous System
 Parkinson’s Disease:
inadequate production of
dopamine in the brain causes involuntary muscle
contractions and tremors; can be partially alleviated
with L-dopa (synthetic dopamine)
 Alzheimer’s Disease: decrease in CNS levels of
acetylcholine
 Multiple Sclerosis: degeneration of the Myelin sheath;
Many symptoms, partial paralysis, double
vision,speech problems
 Amyotrophic lateral sclerosis (Lou Gehrig's disease
(ALS) : genetic disease causing motor neurons to die;
muscle control is lost, increased salivation, cramping,
twitching
 Epilepsy: brain injury or lack of oxygen to the brain;
Seizures – grand mal or petit mal – transient loss of
muscle control
 Spinal Cord Injuries: through injury or disease, the
spinal neurons are damaged, Results in loss of motor
control -degree of which depends on where the
damage occurred
 Hydrocephalus: “water on the brain” – excess
cerebrospinal fluid in the brain Increased pressure
may lead to brain damage
 Cerebral Palsy: Usually caused by oxygen deficiency
before/during birth, reduced muscle coordination
(cerebral damage)
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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
A drug can affect a neurotransmitter in these
ways:
(a) cause leakage out of a synaptic vesicle into the
axon bulb;
(b) prevent release of the neurotransmitter into the
synaptic cleft;
(c) promote release of the neurotransmitter into the
synaptic cleft;
(d) prevent reuptake by the presynaptic membrane;
(e) block the enzyme that causes breakdown of the
neurotransmitter; or
(f) bind to a receptor, mimicking the action or
preventing the uptake of a neurotransmitter.
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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