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Chapter 11: Nervous System
17-1
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. 17-3
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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 (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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Fig. 48-13
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 postsynaptic 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
Fig. 49-1
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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
17-59
Organization of the nervous
system
17-60
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
17-67
Cranial nerves
17-68
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
17-70
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
17-83
•
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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