Download Chapter 33 Nervous System

Chapter 33 Nervous System
Spinal cord and nerves
Nerves passing through a vertebra
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Neurons
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Start-Up Activities
LAUNCH Lab
How does information travel in the nervous system?
Your body is bombarded by sounds, odors, sights, tastes, and physical contact almost
constantly. The nervous system makes sense of these stimuli, and reacts in ways that
promote your survival. In this lab, you will model that communication process.
Procedure
Form groups of four and assign one student to each of the following roles: a sensor, a relayer,
an interpreter, and an actor.
Brainstorm situations, such as touching a hot object, in which your senses receive informa–tion
and you respond.
Model one situation. The sensor should describe what he or she senses to the relayer, who
passes the information to the interpreter, who decides on a body response. The relayer then
passes the response to the actor to act out the response.
Repeat Step 3 using different situations.
Analysis
Explain What factors could cause the situations you modeled to vary in speed?
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Section 33.1 Structure of the Nervous System
Neurons conduct electrical impulses that allow cells, tissues, and organs to detect and
respond to stimuli.
Real-World Reading Link Imagine that you wake up in the middle of the night and get out of
bed. On your way to the kitchen to get something to eat, you stub your toe on the corner of
the bed. You know right away what happened. Was it one, two, or three seconds before
you said “ouch??? Or was it less than that? How did your brain get the message so quickly
that something hurt?
Neurons
Electricity and chemistry were both involved as your brain received the message that you
stubbed your toe. Neurons are specialized cells that help you gather information about
your environment, interpret the information, and react to it. Neurons make up an enormous
communi–cation network in your body called the nervous system. You will learn more
about how this communication works electrically and chemically later in the chapter.
Figure 33.1 shows that a neuron consists of three main regions: the dendrites, a cell body, and
an axon. Dendrites receive signals called impulses from other neurons and conduct the
impulses to the cell body. A single neuron might have many dendrites. The nucleus of the
neuron and many of the cell organelles are found in the cell body. Lastly, an axon carries
the nerve impulse from the cell body to other neurons and muscles.
Figure 33.1 There are three main parts of a neuron: the dendrites, a cell body, and an axon.
Neurons are highly specialized cells that are organized to form complex networks.
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Figure 33.2 A simple reflex involves a sensory neuron, an interneuron, and a motor neuron.
Interneurons can also carry impulses to the brain.
Explain How might a reflex be completed before the brain interprets the event?
There are three kinds of neurons: sensory neurons, interneurons, and motor neurons. Sensory
neurons send impulses from receptors in the skin and sense organs to the brain and spinal
cord. Sensory neurons signal interneurons, which are found in the spinal cord and brain.
Interneurons carry the impulse to motor neurons, which carry impulses away from the
brain and spinal cord to a gland or muscle, which results in a response. Refer to Figure
33.2 to follow the path of an impulse for a simple, invol–untary reflex. The nerve impulse
completes what is called a reflex arc. A reflex arc is a nerve pathway that consists of a
sensory neuron, an inter-neuron, and a motor neuron. Notice that the brain is not involved.
A reflex arc is a basic structure of the nervous system.
A Nerve Impulse
A nerve impulse is an electrical charge travel–ing the length of a neuron. An impulse results
from a stimulus, such as a touch or perhaps a loud bang that causes you to jump.
A neuron at rest
Figure 33.3 shows a neuron at rest—it is not con–ducting an impulse. Notice that there are
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more sodium ions (Na ) out–side the cell than inside the cell. The reverse is true for
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potassium ions (K )—there are more potassium ions inside the cell than outside the cell.
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Figure 33.3 The distribution of Na and K ions, and the presence of negatively charged
protein molecules in the cytoplasm, keep the inside of the cell more negatively charged
than the outside when a neuron is at rest.
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Recall from Chapter 7 that ions tend to diffuse across the plasma membrane from an area of
high concentration of ions to an area of low concentration of ions. Proteins found in the
plasma membrane work to counteract the diffusion of the sodium ions and potassium ions.
These proteins, called the sodium-potassium pump, actively transport sodium ions out of
the cell and potassium ions into the cell.
For every two potassium ions pumped into a neuron, three sodium ions are pumped out. This
maintains an unequal distribution of posi–tively charged ions, resulting in a positive charge
outside the neuron and a negatively-charged cytoplasm inside the neuron.
An action potential
Another name for a nerve impulse is an action potential. The minimum stimulus to cause an
action potential to be produced is a threshold. However, a stronger stimulus does not
generate a stronger action potential. Action potentials are described as being “all or
nothing,?? meaning a nerve impulse is either strong enough to travel along the neuron or it
is not strong enough.
When a stimulus reaches threshold, channels in the plasma mem–brane open. Sodium ions
rapidly move into the cytoplasm of the neuron through these channels, causing a temporary
reversal in elec trical charges. The inside of the cell now has a positive charge, which
causes other channels to open. Potassium ions leave the cell through these chan–nels,
restoring a positive charge outside the cell. Figure 33.4 shows that this change in charge
moves like a wave along the length of the axon.
Figure 33.4 Follow as an action potential moves along an axon from left to right. Notice what
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happens to the Na and K and how this changes the relative electrical charges inside and
outside the neuron.
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Figure 33.5 A nerve impulse moves from node to node along myelinated axons.
Explain What happens at a node when an impulse moves along a myelinated axon?
Speed of an action potential
The speed of an action potential varies. Many axons have a covering of a lipid called myelin,
which forms an insulating layer called a sheath around the axon. The myelin sheath has
many gaps, called nodes, along the length of the axon, as shown in Figure 33.5. Sodium
ions and potassium ions cannot diffuse through myelin, but they can reach the plasma
membrane at these nodes. This allows the action potential to jump from node to node,
greatly increasing the speed of the impulse as it travels the length of the axon.
In the human body, there are neurons that have myelin, and neurons that do not have myelin.
Neurons with myelin carry impulses that are associated with sharp pain; neurons that lack
myelin carry impulses associated with dull, throbbing pain. The action potentials in these
neu–rons travel much more slowly than they do in neurons with myelin. When you stubbed
your toe, which kind of neurons were involved?
Reading Check Explain the relationship of a threshold to an action potential.
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Visualizing Action Potential
Figure 33.6 To cause the voluntary contraction of a muscle, a signal from the brain creates an
action potential in a motor neuron. This action potential travels along the motor neuron,
which leads to the release of a neurotransmitter that signals the fibers of the muscle to
contract.
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Figure 33.7 A single neuron can have multiple connections with other neurons.
The synapse
A small gap exists between the axon of one neuron and the dendrite of another neuron. This
gap is called a synapse (SIH naps). When an action potential reaches the end of an axon,
small sacs called vesicles carrying neuro transmitters fuse with the plasma membrane and
release a neuro transmitter by exocytosis. When a motor neuron synapses with a muscle
cell, as illustrated in Figure 33.6, the released neurotrans–mitter crosses the synapse and
causes a muscle to contract.
A neurotransmitter is a chemical that dif–fuses across a synapse and binds to receptors on the
dendrite of a neighboring neuron. This causes channels to open on the neighboring cell and
creates a new action potential.
There are over 25 known neurotransmitters. Once a neuro–transmitter has been released into a
synapse, it does not remain there for long. Depending on the neurotransmitter, it might
simply diffuse away from the synapse, or enzymes might break it down. Some
neurotransmitters are recycled and used again. Figure 33.7 shows that a single neuron can
communicate with many other neurons.
Section 33.1 Assessment
Section Summary
? There are three major parts of a neuron.
? There are three basic types of neurons.
? A nerve impulse is an electrical charge and is called an action potential.
? Neurons use chemicals and electricity to relay impulses.
Understand Main Ideas
Compare How is the nervous system similar to the Inter–net as a communication network?
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Infer why energy is necessary to counteract the diffusion of Na and K
plasma mem–brane of a neuron.
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ions across the
Explain Suppose the sensory nerves in a person's right foot are completely nonfunctional. If
this person's right foot was severely burned, would the person feel the burn?
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Section 33.2 Organization of the Nervous System
The central nervous system and the peripheral nervous system are the two major
divisions of the nervous system.
Real-World Reading Link Imagine you have studied for a test and are con–fident that you
will do well. When you look at the first question, you are not sure how to answer it. You
concentrate. You picture a page in your textbook. Your memory clicks and you answer the
question. How does this happen?
The Central Nervous System
The nervous system consists of two major divisions: the central ner–vous system (CNS) and the
peripheral nervous system (PNS). The brain and the spinal cord make up the central
nervous system. The peripheral nervous system consists of the sensory neurons and
motor neurons that carry information to and from the CNS.
The CNS is made up mostly of interneurons. Its function is to coor–dinate all of the body's
activities. The CNS relays messages, processes information, and analyzes responses.
Sensory neurons carry informa–tion about the environment to the spinal cord. Interneurons
in the spi–nal cord might respond via a reflex arc, or they might relay this information to
the brain, where it is processed in at least two ways. Some brain neurons send a message
by way of the spinal cord to motor neurons, and the body responds appropriately. Other
neurons in the brain might store the information to be recalled later.
Figure 33.8 Brainstorm
For thousands of years, scientists have stud–ied the brain and investigated ways to treat
neurological disease.
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Figure 33.9 Left: A photograph of a human brain shows distinct sections.
Right: The major sections of the brain are the cerebrum, the cerebellum, and the brain stem.
The brain
Over 100 billion neurons are found in the brain. Because the brain maintains homeostasis and is
involved with almost all of the body's activities, it is sometimes called the control center of
the body. Refer to Figure 33.8 to learn about important events that have led to
understanding of the functions of the brain.
Refer to Figure 33.9. The cerebrum (suh REE brum) is the largest part of the brain and is
divided into two halves called hemispheres. The two hemispheres are not independent of
each other—they are connected by a bundle of nerves. The cerebrum carries out thought
processes involved with learning, memory, language, speech, voluntary body movements,
and sensory perception. Most of these higher thought processes occur near the surface of
the brain. The folds and grooves on the surface of the cerebrum, as shown in Figure 33.9,
increase the surface area and allow more compli–cated thought processes.
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The cerebellum controls balance, posture, and coordination, and is located at the back of the
brain. The cerebellum controls the smooth and coordinated movement of skeletal muscles
and also is involved with motor skills, such as playing the piano or riding a bike.
The brain stem connects the brain to the spinal cord and is made up of two regions called the
medulla oblongata and the pons. The medulla oblongata relays signals between the brain
and the spinal cord. It also helps control breathing rate, heart rate, and blood pressure. The
pons relays signals between the cerebrum and the cerebellum. The pons also helps control
the rate of breathing. Have you ever felt a gagging sensation when your doctor put a
tongue depressor in your mouth? The interneurons that form a reflex center for
swallowing, vomiting, coughing, and sneezing are located in the medulla oblongata.
Located between the brain stem and the cerebrum, the hypothala–mus is essential for
maintaining homeostasis. The hypothalamus (hi poh THA luh mus) regulates body
temperature, thirst, appetite, and water balance. It also partially regulates blood pressure,
sleep, aggres–sion, fear, and sexual behavior. It is about the size of a fingernail and
performs more functions than any other brain structure of its size.
The spinal cord
The spinal cord is a nerve column that extends from the brain to the lower back. It is protected
by the vertebrae. Spinal nerves extend from the spinal cord to parts of the body and
connect them to the central nervous system. Reflexes are processed in the spinal cord.
Reading Check Review the functions of the CNS.
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The Peripheral Nervous System
When you hear the word nerve, you might initially think of a neuron. However, a nerve is
actually a bundle of axons. Many nerves contain both sensory and motor neurons. For
example, there are 12 cra–nial nerves that lead to and from the brain and 31 spinal nerves
(and their branches), as shown in Figure 33.10, that lead to and from the spinal cord. You
could think of these as two-way streets. Information travels to and from the brain through
these sensory and motor neurons.
Refer to Figure 33.11 as you read about the periph–eral nervous system. This system includes
all neurons that are not part of the central nervous system, includ–ing sensory neurons and
motor neurons. Neurons in the peripheral nervous system can be classified further as being
either part of the somatic nervous system or part of the autonomic nervous system.
The somatic nervous system
Nerves in the somatic nervous system relay information from external sensory receptors to the
central nervous system, and motor nerves relay information from the central nervous
system to skeletal muscles. Usu–ally, this is voluntary. However, not all reactions of the
central nervous system are voluntary. Some responses are the result of a reflex, which is a
fast response to a change in the environment. Reflexes do not require conscious thought
and are involun–tary. Most signals in reflexes go only to the spinal cord, and not to the
brain. Remember the example of stubbing your toe? Refer back to Figure 33.2 and note
that the illustrated reflex is part of the somatic nervous system.
The autonomic nervous system
Remember the last time you had a scary dream? You might have awakened and realized that
your heart was pounding. This type of reaction is the result of the action of the autonomic
nervous system. The autonomic nervous system carries impulses from the central nervous
sys–tem to the heart and other internal organs. The body responds involuntarily, not under
conscious control. The autonomic nervous system is important in two different kinds of
situations. When you have a bad nightmare or perhaps find yourself in a scary situation,
your body responds with what is known as a fight-or-flight response. When everything is
calm, your body rests and digests.
Reading Check Compare and contrast voluntary responses and involuntary responses.
Figure 33.10 Thirty-one pairs of spinal nerves extend from the spinal cord.
Differentiate How is a neuron related to a nerve?
Figure 33.11 Each division of the nervous system functions in the control of the body and the
communication within the body.
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Table 33.1 The Autonomic Nervous System
Structure
Sympathetic Stimulation
Parasympathetic Stimulation
Iris (eye muscle)
Pupil dilation
Pupil constriction
Salivary Glands
Saliva production reduced
Saliva production increased
Oral/Nasal Mucosa
Mucus production reduced
Mucus production increased
Heart
Heart rate and force increased Heart rate and force decreased
Lung
Bronchial muscle relaxed
Bronchial muscle contracted
Stomach
Muscle contractions reduced
Gastric juice secreted; motility
increased
Small Intestine
Muscle contractions reduced
Digestion increased
Large Intestine
Muscle contractions reduced
Secretions and motility
increased
There are two branches of the autonomic nervous system and they act together. The
sympathetic nervous system is most active in times of emergency or stress when the heart
rate and breathing rate increase. The parasympathetic nervous system is most active
when the body is relaxed. It counterbalances the effects of the sympathetic system and
restores the body to a resting state after a stress–ful experience. Table 33.1 compares and
contrasts the two systems. Both the sympathetic and parasympathetic systems relay
impulses to the same organs, but the overall response depends on the intensities of the
opposing signals.
Section 33.2 Assessment
Section Summary
? The nervous system has two major divisions—the central nervous system and the peripheral
nervous system.
? The brain and spinal cord make up the central nervous system.
? The somatic nervous system and the autonomic nervous system make up the peripheral
nervous system.
? The sympathetic nervous system and the parasympathetic nervous system are branches of the
autonomic ner–vous system.
Understand Main Ideas
Compare the struc–tures of the central nervous system with the structures of the peripheral
nervous system and explain their relationships.
Assess the similarities and differ–ences between the somatic nervous system and the autonomic
nervous system.
Explain Which part of the nervous system is involved in a fight-or-flight response? Why is
such a response important?
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Section 33.3 The Senses
Sensory receptors allow you to detect the world around you.
Real-World Reading Link Who can resist the smell of chocolate-chip cookies baking in the
oven? When the aroma travels from the kitchen, you actually are responding to chemicals
in the air. Senses allow you to be aware of changes in your environment. You are
interpreting the environment around you every second. You even were reacting to
environmental stimuli before you were born.
Taste and Smell
Specialized neurons in your body called sensory receptors enable you to taste, smell, hear, see,
and touch, and to detect motion and temperature.
The senses of taste and smell are stimulated by chemicals and often function together.
Specialized receptors located high in the nose respond to chemicals in the air and send the
information to the olfac–tory bulb in the brain. Taste buds are areas of specialized
chemical receptors on the tongue that detect the tastes of sweet, sour, salty, and bitter.
These receptors detect the different combinations of chemicals in food and send this
information to another part of the brain.
The receptors associated with taste and smell are shown in Figure 33.12. Signals from these
receptors work together to create a combined effect in the brain. Try eating while holding
your nose. You will find that your food loses much of its flavor.
Figure 33.12 The receptors of taste and smell function together and are stimulated in similar
ways. Food often is smelled as it is tasted.
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Figure 33.13 Light travels through the cornea and the pupil to the lens, which focuses the
image on the retina. Rods and cones in the retina send information to the brain through the
optic nerve.
Sight
Figure 33.13 shows the path of light as it travels through the eye. Light first enters the eye
through a transparent, yet durable, layer of cells called the cornea. The cornea helps to
focus the light through an opening called the pupil. The size of the pupil is regulated by
muscles in the iris—the colored part of the eye. Behind the iris is the lens, which inverts
the image and projects it onto the retina. The image travels through the vit–reous humor,
which is a colorless, gelatinlike liquid between the lens and the retina. The retina contains
numerous receptor cells called rods and cones. Rods are light-sensitive cells that are
excited by low levels of light. Cones function in bright light and provide information about
color to the brain. These receptors send action potentials to the brain via the neurons in the
optic nerve. The brain then interprets the specific com–bination of signals received from
the retina and forms a visual image.
Hearing and Balance
Hearing and balance are the two major functions of the ear. From a soft sound, like whispering,
to a loud sound, such as a crowd cheering at a sporting event, specialized receptors in the
ear can detect both the volume and the highness and lowness of sounds. Canals in the inner
ear are responsible for your sense of balance, or equilibrium.
Hearing
Vibrations called sound waves cause particles in the air to vibrate. Figure 33.14 illustrates the
path of sound waves as they travel through the ear.
Sound waves enter the auditory, or ear, canal and cause a membrane, called the eardrum or
tympanum, at the end of the ear canal to vibrate. These vibrations travel through three
bones in the middle ear—the malleus (also called the hammer), the incus (anvil), and
stapes (stirrup). As the stapes vibrates, it causes the oval window—a membrane that
separates the middle ear from the inner ear—to move back and forth. In the inner ear, a
snail-shaped structure called the cochlea (KOH klee uh) is filled with fluid and lined with
tiny hair cells. Vibrations cause the fluid inside the cochlea to move like a wave against the
hair cells. The hairs cells respond by generating nerve impulses in the auditory nerve and
transmitting them to the brain.
Reading Check Summarize how each sense organ detects changes in the environment.
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Figure 33.14 Sound waves cause the tympanum to vibrate, and the vibrations travel through
the bones of the middle ear to the cochlea. Hair cells in the cochlea generate nerve
impulses, which are sent to the brain through the auditory nerve.
Balance
The inner ear also contains organs for balance, including three semicircular canals.
Semicircular canals transmit information about body position and balance to the brain.
The three canals are posi–tioned at right angles to one another and, like the cochlea, they
are fluid-filled and lined with hair cells. When the position of your head changes, fluid
moves through the canals. This causes the hair cells to bend, which in turn sends nerve
impulses to the brain. The brain then is able to deter–mine your position and whether your
body is still or in motion.
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Figure 33.15 Many types of receptors are found in the skin. A person can tell if an object is hot
or cold, sharp or smooth.
Touch
Many types of sensory receptors that respond to temperature, pressure, and pain are found in
the epidermis and dermis layers of the skin. Figure 33.15 illustrates the different types of
receptors —some that respond to light touches, and others that respond to heavy pressure.
Distribution of receptors is not uniform in all areas of the body. The tips of the fingers have
many receptors that detect light touch. The soles of the feet have many receptors that
respond to heavy pressure. Pain receptors are very simple, consisting of free nerve endings
that are found in all tissues of the body except the brain. The brain constantly receives
signals from these receptors and responds appropriately.
Section 33.3 Assessment
Section Summary
? The senses of taste and smell work together.
? The eye has two types of receptors.
? The ear is involved in both hearing and balance.
? The skin has many types of sensory receptors.
? Some sensory receptors are more complex than others.
Understand Main Ideas
Diagram the route of a sound wave from the auditory canal until it causes a nerve impulse to
be generated.
Predict what might be the result if the cornea was damaged.
Analyze the importance of the kind of receptors found in the fingers.
Explain why it might be difficult to taste when you have a cold and your nasal passages are
clogged.
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Section 33.4 Effects of Drugs
Some drugs alter the function of the nervous system.
Real-World Reading Link Some drugs help to restore health; some help to maintain health.
The use of drugs such as caffeine or tobacco can become a habit that is difficult to change.
People who abuse drugs say they use them either to feel good or to escape temporarily
from problems in their lives. What are some ways to make yourself feel good or to relieve
stress that do not involve the use of drugs?
How Drugs Work
A drug is a substance, natural or artificial, that alters the function of the body. Some types of
drugs are shown in Table 33.2. Antibiotics are pre–scription drugs that fight bacterial
infections. Cocaine and marijuana are illegal drugs. There are over-the-counter drugs for
pain relief. There are other common substances that people often do not think of as drugs,
such as caffeine, nicotine, and alcohol. However, they are classified as drugs.
Drugs can affect a person's body in many different ways. Not all drugs affect the nervous
system. However, those that cause changes in the nervous system work in one or more of
the following ways:
• a drug can cause an increase in the amount of a neurotransmitter that is released into a
synapse
• a drug can block a receptor site on a dendrite, preventing a neurotransmitter from binding
• a drug can prevent a neurotransmitter from leaving a synapse
• a drug can imitate a neurotransmitter
Table 33.2 Some Common Drugs
Alcohol
Caffeine
Prescription
Drugs
beer, wine
coffee, tea, soda, antibiotics, pain
chocolate
medications
Over-theCounter
Drugs
Tobacco
aspirin, cold
medications
cigarettes, cigars
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Figure 33.16 Dopamine crosses the gap from one neuron and binds to receiver sites, or
receptors, on the membrane of another neuron. This occurs at a synapse.
Many drugs that affect the nervous system influence the level of a neurotransmitter called
dopamine. Dopamine (DOH puh meen) is a neurotransmitter found in the brain that is
involved with the control of body movements and other functions. Dopamine also is
strongly involved with feelings of pleasure or reward. Dopamine normally is removed
from a synapse by being re absorbed by the neuron that released it, as illustrated in Figure
33.16.
Classes of Commonly Abused Drugs
Drug abuse does not necessarily involve the use of illegal drugs. Any use of a drug for reasons
other than legitimate medical purposes, whether deliberate or unintentional, can be
considered abuse of that drug.
Stimulants
Drugs that increase alertness and physical activity are stimulants. Figure 33.17 indicates some
common stimulants.
Nicotine Nicotine in cigarette and cigar smoke increases the amount of dopamine released into
a synapse. Nicotine also constricts blood vessels, raising blood pressure and causing the
heart to work harder than normal. Cigarette smoking has been linked to about 90 percent of
all lung cancer cases.
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Caffeine The most commonly used, and often abused, stimulant probably is caffeine. Caffeine
is found in coffee, tea, some soft drinks, and even some foods like chocolate. Caffeine
works by binding to ade-nosine receptors on neurons in the brain. Adenosine slows down
neural activity, causing drowsiness. When caffeine binds to these receptors, it has the
opposite effect. It makes users feel awake and alert. Caffeine also temporarily raises
epinephrine (adrenaline) levels in the body, giving a quick burst of energy that soon wears
off.
Depressants
Drugs that tend to slow down the central nervous sys–tem are depressants. These drugs can
lower blood pressure, interrupt breathing, and slow the heart rate. Depressants can relieve
anxiety, but they also can cause the noticeable effect of sedation.
Alcohol Alcohol is a depressant. It affects the central nervous system and is one of the most
widely abused drugs in the world today. It is pro–duced by the fermentation of grains and
fruits. Alcohol is known to affect at least four different neurotransmitters, resulting in a
feeling of relaxation and sluggishness. Short-term alcohol use impairs judgment,
coordination, and reaction time. Long-term effects of alcohol abuse include a reduction in
brain mass, liver damage, stomach and intestinal ulcers, and high blood pressure.
Consumption of alcohol during preg–nancy is the number-one cause of fetal alcohol
syndrome, which can result in damage to a baby's brain and nervous system.
Inhalants Inhalants are chemical fumes that have an influence on the nervous system.
Exposure to inhalants might be accidental due to poor ventilation. Inhalants generally work
by acting as a depressant on the central nervous system. Inhalants might produce a shortterm effect of intoxication, as well as nausea and vomiting. Death can occur. Long-term
exposure to inhalants can cause memory loss, hearing loss, vision problems, peripheral
nerve damage, and brain damage.
Figure 33.17 There are many common stimulant drugs, such as coffee, tea, cocoa, and
chocolate.
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Illegal drugs Amphetamines and cocaine both increase dopamine levels and both prevent
dopamine from being reabsorbed, so it remains in the synapse. This ultimately increases
the levels of dopamine in the brain, which results in a feeling of pleasure and well-being.
The use of cocaine and amphetamines has short-term and long-term effects. Cocaine abuse
might result in disturbances in heart rhythm, heart attacks, chest pain, respiratory failure,
strokes, seizures, head–aches, abdominal pain, and nausea. Abuse of amphetamines might
result in rapid heart rate, irregular heartbeat, increased blood pressure, and irreversible,
stroke-producing damage to small blood vessels in the brain. Elevated body temperature,
called hyperthermia, and convulsions can result from an amphetamine or cocaine overdose,
and if not treated immediately, can result in death. Abusers also can experience episodes of
violent behavior, paranoia, anxiety, confusion, and insomnia. It can take a year or longer
for users of methamphetamine—the strongest type of amphetamine—to recover after
quitting the drug.
Marijuana is the most-used illegal drug in the United States. The active chemical in marijuana
is tetrahydrocannabinol, or THC. Smok–ing marijuana quickly gets THC into the
bloodstream where it is car–ried to the brain. THC binds to receptors on neurons in the
brain, which produces the effect of intense pleasure. These receptors are found on neurons
associated with many body activities. Short-term effects of marijuana use include problems
with memory and learning, loss of coordination, increased heart rate, anxiety, paranoia,
and panic attacks. Long-term smoking of marijuana might also cause lung cancer.
Reading Check Explain the function of a neurotransmitter.
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Tolerance and Addiction
Drug tolerance can lead to addiction. Tol er a nce occurs when a person needs more and more
of the same drug to get the same effect. The dos–age increases because the body becomes
less responsive to the drug.
Addiction
The psychological and/or physiological dependence on a drug is addiction. Current research
suggests that the neurotransmitter dopamine is involved with most types of physiological
addiction. Recall that dopamine normally is removed from a synapse as it is reabsorbed by
the neuron that released it. However, certain drugs prevent that reabsorption, which results
in an increase of dopamine in the brain. A person addicted to drugs derives pleasure from
increased levels of dopamine and builds up a tolerance to the drug. As a result, the person
takes more of the drug. When people who are addicted try to quit, the levels of dopamine
decrease, making it difficult to resist going back to the drug.
Addictions can also be psychological. An individual with a psycho–logical dependence on a
drug such as marijuana has a strong desire to use the drug for emotional reasons. Both
physiological and psychologi–cal dependence can affect emotional and physical health.
Both types are strong, making it difficult to quit a drug.
Treatment
People who are either psychologically or physiologically dependent on a drug experience
serious withdrawal symptoms without it. It is very difficult for dependent users to quit on
their own. They might be able to quit for short periods of time, but they are likely to use
the drug again. Medical supervision is necessary when people who are psychologically and
physiologically dependent on a drug try to quit.
The best way to avoid an addiction is never to use drugs in the first place, even when pressured
to use them. Encourage people who abuse drugs to seek treatment for drug dependency.
Physicians, nurses, coun–selors, clergy, and social workers are trained to direct people to
the resources they need to get help, as illustrated in Figure 33.18.
Figure 33.18 Counseling often is necessary to break an addiction.
Section 33.4 Assessment
Section Summary
? Drugs affect the nervous system in four different ways.
? Common substances such as caffeine and alcohol are considered drugs.
? Many addictive drugs increase levels of dopamine.
? Drug abuse has many negative consequences.
? A person can become psychologi–cally and/or physiologically addicted to drugs.
Understand Main Ideas
Describe four ways that drugs can influence the nervous system.
Compare the actions of cocaine, amphetamines, and nicotine.
Explain why the effects of stimu–lants and depressants do not neces–sarily counteract each
other.
Evaluate why students who abuse amphetamines are likely to experi–ence failing grades.
982
CUTTING-EDGE BILOGY
BRAIN-CONTROLLED LIMBS: NO LONGER SCIENCE FICTION
For centuries, the only recourse for people who lose an arm or leg to accident or dis–ease
has been a prosthetic limb. These limbs help people regain some of the func–tions of a
real arm or leg. However, their effectiveness is limited because the limbs are not
controlled by the brain. Current sci–entific research is about to change all that.
What are brain-controlled prostheses?
Scientists currently are developing thought-controlled robotic arms with fully mobile shoulders
and elbows. The hand is in the shape of a gripper that functions much like a real hand.
Used primarily with monkeys in research, these arms are connected to the brain using
implants.
How do the implants work? The implants are in the form of hundreds of electrodes that are as
thin as a human hair. The electrodes are placed in the motor cortex of a monkey's brain 3
mm beneath the skull to pick up nerve signals in the brain. The implant transmits these
sig–nals to a computer. A mathematical procedure translates them into instructions for the
arm. Within 30 milliseconds of the command, the arm can, for example, pick up food and
bring it to the monkey's mouth. The arm is equipped with several motors, and moves in
three dimensions just like a human arm. The arm responds and brings food to the monkey
when the monkey thinks about the food.
During these experiments, the monkey used its own arms to experiment with a joystick to get
used to work–ing with the robotic arm. Once the monkey had prac–ticed with the joystick,
the scientists removed it and gently restrained the monkey's own arm. To their amazement,
the robotic arm began to move as a result of the monkey's thoughts.
Scientists want to refine the technology so the system is completely wireless. One concern is
that the current electrodes last only about six months. Due to tissue growth, there appears
to be interference with message transmission.
How might these brain-controlled devices help society? Scientists plan to begin researching
and using these devices with humans in the next few years. The hope is that these braincomputer interfaces (BCI) will help people who are parapalegic regain some movement or
ability to communicate with others. Brain implants also could allow hand-free control of
small robots that could perform everyday tasks. BCIs also might benefit people who are
not paralyzed or who have not lost a limb. BCIs could be used to perform tasks in
dangerous environments or war zones, for example.
983
BIOLAB
HOW DO NEURAL PATHWAYS DEVELOP AND BECOME MORE EFFICIENT?
Background: Imagine forging a narrow path through a wooded area. As the path is
traveled over time, it becomes more defined and easier to follow. In a similar manner,
neural pathways are developed in the brain when you learn something new. As you
practice what you learned, connec–tions between neurons strengthen, causing nerve
impulses to pass more quickly and efficiently along the circuit.
Question: What effect do learning strate–gies have on the efficiency of a neural circuit?
Materials
graph paper
pencil
paper
calculator
Procedure
Read and complete the lab safety form.
Work with one student in your group to write a list of 20 concrete words that describe specific
physical objects. Assign a number, 1 to 20, to each word.
Read the list aloud to three other mem–bers of your group—the test subjects. Immediately, and
without discussion, have them write down as many words as they can remember from the
list.
Calculate and record the percent recall for each word: divide the number of subjects who
recalled each word by the total num–ber of subjects. Multiply by 100.
Graph the percent recall for each word. Note patterns in the data.
Calculate the average percent recall: add the percent recall for each word, divide by 20, and
multiply by 100.
Brainstorm techniques to increase the average percent recall. Choose one tech–nique. Predict
how it will affect the aver–age percent recall. Design an experiment to test the prediction.
Once your teacher approves the plan, implement it with the same test subjects, using another
list of 20 concrete words that describe specific physical objects.
Repeat Steps 4-6 to evaluate changes in the average percent recall.
Analyze and Conclude
Identify patterns in the percent recall data after the list was read the first time. Which words
were most likely to be remembered?
Interpret Data Describe the technique you used to increase the average percent recall.
Compare the average percent recall before and after the technique was used.
Analyze Did the technique strengthen the neural circuits responsible for remem–bering the list
of words as well as you predicted? Explain.
Error Analysis Identify factors, other than the technique you used, that might have affected the
average percent recall.
984
Chapter 33 Study Guide
Activity Pretend that you must develop a new drug. Explain how your drug works on the
nervous system. How could you determine what side effects the drug might have?
Vocabulary
Key Concepts
Section 33.1 Structure of the Nervous System
Neurons conduct electrical impulses that
allow cells, tissues, and organs to
detect and respond to stimuli.
• action potential (p. 964)
• There are three major parts of a neuron.
• axon (p. 962)
• There are three basic types of neurons.
• cell body (p. 962)
• dendrite (p. 962)
• neuron (p. 962)
• neurotransmitter (p. 967)
• node (p. 965)
• reflex arc (p. 963)
• synapse (p. 967)
• threshold (p. 964)
• A nerve impulse is an electrical charge and
is called an action potential.
• Neurons use chemicals and electricity to
relay impulses.
Section 33.2
The central nervous system and the peripheral
nervous system are the two major
divisions of the nervous system.
• autonomic nervous system (p. 971)
• central nervous system (p. 968)
• cerebrum (p. 969)
• The nervous system has two major
divisions—the central nervous system
and the peripheral nervous system.
• The brain and spinal cord make up the
central nervous system.
• hypothalamus (p. 970)
• medulla oblongata (p. 970)
• parasympathetic nervous system (p. 972)
• peripheral nervous system (p. 968)
• pons (p. 970)
• somatic nervous system (p. 971)
• sympathetic nervous system (p. 972)
• The somatic nervous system and the
autonomic nervous system make up the
peripheral nervous system.
• The sympathetic nervous system and the
parasympathetic nervous system are
branches of the autonomic nervous
system.
Section 33.3 Senses
Sensory receptors allow you to detect the
world around you.
• cochlea (p. 974)
• The senses of taste and smell work together.
• cone (p. 974)
• The eye has two types of receptors.
• lens (p. 974)
• The ear is involved in both hearing and
balance.
• retina (p. 974)
• rod (p. 974)
• semicircular canal (p. 975)
• The skin has many types of sensory
receptors.
• Some sensory receptors are more complex
than others.
• taste bud (p. 973)
Section 33.4
Some drugs alter the function of the nervous
system.
• addiction (p. 981)
• Drugs affect the nervous system in four
different ways.
• depressant (p. 978)
• dopamine (p. 978)
• drug (p. 977)
• stimulant (p. 978)
• Common substances such as caffeine and
alcohol are considered drugs.
• Many addictive drugs increase levels of
dopamine.
• Drug abuse has many negative
consequences.
• tolerance (p. 981)
• A person can become psychologically
and/or physiologically addicted to a drug.
985
Chapter 33 Assessment
Section 33.1 Vocabulary Review
For each set of terms below, choose the one term that does not belong and explain why it does
not belong.
axon—dendrite—reflex arc
cell body—synapse—neurotransmitter
myelin—node—threshold
Understand Key Concepts
Use the diagram below to answer question 4.
What is occurring in the diagram above?
+
K ions are entering the neuron.
Negatively charged proteins are leaving the neuron.
+
Na ions are entering the neuron.
The myelin coat has broken down, allowing ions to freely cross the plasma membrane.
Which is the correct path a nerve impulse will follow in a reflex arc?
motor neuron ? interneuron ? sensory neuron
interneuron ? motor neuron ? sensory neuron
motor neuron ? sensory neuron ? interneuron
sensory neuron ? interneuron ? motor neuron
Constructed Response
Short Answer Hypothesize why it takes more energy for a nerve impulse to travel an axon that
lacks myelin as opposed to an axon that has myelin.
Short Answer Explain the following analogy: A neuron is like a one-way street, while a nerve
is like a two-way street.
Think Critically
Infer In most animals, an action potential will travel only in one direction along a neuron. Infer
what the result might be in humans if nerve impulses could travel in both directions on a
single neuron.
Section 33. Vocabulary Review
For each set of terms below, choose the one term that does not belong and explain why it does
not belong.
somatic system—parasympathetic system —sympathetic system
cerebrum—pons—medulla oblongata
autonomic nervous system—somatic nervous system—central nervous system
Understand Key Concepts
Which is characteristic of the sympathetic division of the autonomic system?
stimulates digestion
dilates the bronchi
slows the heart rate
converts glucose to glycogen
Use the diagram below to answer question 13.
If the portion indicated by the arrow was damaged due to trauma, what effects would this
person most likely experience?
partial or complete memory loss
body temperature fluctuations
trouble maintaining balance
rapid breathing
Which nervous system is the hypothalamus most involved in regulating?
voluntary
peripheral
sensory
autonomic
986
Constructed Response
Open Ended Suppose you are on the debate team at school. Yo u must support the following
statement: The autonomic nervous system is more involved with homeostasis than the
somatic nervous system. Build your case.
Think Critically
Critique You might have heard the statement “humans use only ten percent of their brains.??
Use the Internet or other sources to compile evidence that either supports or refutes this
idea.
Analyze The human cerebrum is disproportion–ately large compared to the cerebrum of other
ani–mals. What advantage does this give to humans?
Section 33.3 Vocabulary Review
Distinguish between the terms in each of the following sets:
rods —cones
cochlea—semicircular canals
retina—taste buds
Understand Key Concepts
If there were a power outage in a movie theater and only a few dim emergency lights were lit,
which cells of the retina would be most important for seeing your way to the exit?
rods
cones
Rods and cones are equally important.
Which represents the correct sequence as sound waves travel in the ear to trigger an impulse?
cochlea, incus, stape, eardrum
tympanum, bones in the middle ear, cochlea, hair cells
auditory canal, tympanum, hair cells, cochlea
hair cells, auditory canal, cochlea, malleus
With which sense are free nerve endings associated?
taste
hearing
touch
sight
Use the diagram below to answer question 24.
Some rides at amusement parks cause a person to become dizzy when the ride stops. Which
structure in the diagram is most likely involved with the dizzy feeling?
A
B
C
D
Constructed Response
Open Ended A rare condition exists in which a person cannot feel pain. Is this desirable or
unde–sirable? Explain your response.
Think Critically
Explain You have receptors for light (soft) touch all over your body. In terms of what you
know about the nervous system, why are you not always con–scious of things like wearing
clothes or a wristwatch?
Categorize Rate the senses from 1 to 5 in order of importance (with 1 representing the most
impor–tant.) Be prepared to debate this issue with other students in the class.
Section 33.4 Vocabulary Review
Explain the difference between the terms in each set. Then explain how the terms are related.
stimulants—depressants
tolerance—addiction
dopamine—drug
987
Understand Key Concepts
Which of the following decreases brain activity?
nicotine
amphetamines
cocaine
alcohol
What is the most likely function of amphetamines?
to stimulate the sympathetic nervous system
to stimulate the parasympathetic nervous system
to stimulate the sympathetic and para -sympathetic systems equally
do not affect either the sympathetic or para-sympathetic nervous system
Use the diagram below to answer question 33.
If a person is suffering from depression, which drug is one recommended treatment of the pre–
synaptic neuron?
one that increases the re-uptake of dopamine.
one that increases the production of dopamine
one that decreases the receptors for dopamine
one that decreases the re-uptake of dopamine
Constructed Response
Short Answer What does it mean when some–one is addicted to a drug?
Open Ended Discuss what consequences might arise if a person's gene for the production of
dopamine was defective.
Think Critically
Defend Form a conclusion about the following statement: “It is more difficult for someone to
get addicted to drugs than it is to stop using drugs.?? Defend your position.
988
Standardized Test Practice
Cumulative
Multiple Choice
Which characteristic is unique to mammals?
hair
endothermy
four-chambered heart
internal fertilization
Use the diagram below to answer questions 2 and 3.
In which part of the diagram above would you expect to find myelin?
1
2
3
4
In which part of the diagram above would you expect to find neurotransmitters when an action
potential reaches the end of the neuron?
1
2
3
4
What is the purpose of the epithelial tissue in the integumentary system?
cover the body surface and protect its tissues
move joints and bones
provide a structural framework for the body
transmit nerve signals
Which animal is a placental mammal?
hummingbird
kangaroo
duck-billed platypus
whale
Use the diagram below to answer questions 6 and 7.
Which part of the eye is made of muscles that respond to stimuli?
1
2
3
4
If a person cannot see certain colors, what part of the eye might be damaged?
1
2
3
4
Use the graph below to answer question 8.
The graph above shows the circadian pattern of body temperature in humans. When does the
body tempera–ture of humans seem to be the lowest?
after eating
in the afternoon
just before dawn
late at night
989
Short Answer
Use the diagram below to answer questions 9 and 10.
The figure above shows the teeth of two different types of mammals. From these teeth, what
can you infer about the diets of these mammals?
Which animal's teeth most closely resemble those of humans? Explain your answer.
Explain how spiders predigest their food and com–pare this process to the digestion process of
another animal with which you are familiar.
Suppose that a person who used to drink one cup of coffee to stay awake at night finds she
needs to drink two cups. What is the name of this phenom–enon and what causes it?
What is the role of the gametophyte generation in seed plants?
Extended Response
Two abandoned whooping crane chicks are found several days after they had hatched. A
scientist wants to raise the chicks. To make the chicks feel comfort–able, the scientist uses
a hand puppet that looks like a whooping crane. The scientist offers the chicks mealworms
but they will not take them. Formulate a hypothesis that gives a possible explanation of the
actions of the chicks.
How are the actions of myosin and actin fibers related to the contraction of a muscle?
What is the main difference between segmented worms and other worms? What is the
importance of this difference?
Essay Question
Each year doctors perform more than 450,000 joint repair and replacement surgeries. This
surgery reduces pain and increases movement in the joints. Joint repair surgery involves
removing any debris or excess bone growth from around the joint. This restores the
functioning of the joint. Joint replace–ment surgery involves replacing the joint with a
synthetic joint. The synthetic joint is made of polyethylene, ceramic, or metal. Joint
replacement enables the joint to function in the same way as a natural joint. Joint
replacements usually are per–formed on the knee, hip, or shoulder.
Using the information in the paragraph above, answer the following question in essay format.
Doctors usually only replace knee or hip joints on older patients who are less active than
younger patients. Explain why doctors recommend this.
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