Download The Sensory Nervous System (p. 623)

28 THE NERVOUS SYSTEM
EXTENDED LECTURE OUTLINE
Neurons and How They Work (p. 608)
28.1
28.2
28.3
Evolution of the Animal Nervous System (p. 608; Figs. 28.1, 28.2, 28.3)
A. Animals sense changes in the environment using sensory neurons, and react to those changes
using motor neurons; these two types of neurons are linked by a nervous system.
B. Neurons in the brain and spinal cord are part of the central nervous system.
C. The peripheral nervous system consists of neurons leading to and from the central nervous
system.
D. Invertebrate Nervous Systems
1. The simplest nervous systems that provide reflexes are found in the Cnidaria.
2. More complex nervous systems, such as those in the flatworms, provide associative
activities.
3. The evolutionary path to vertebrates shows more complex sensory mechanisms,
differentiation into central and peripheral nervous systems, differentiation of sensory and
motor neurons, increased complexity of association, and elaboration of the brain.
Neurons Generate Nerve Impulses (p. 610; Fig. 28.4, 28.5)
A. Neurons
1. Neurons are the basic structural units of the nervous system, and each is made up of
three parts.
a. Dendrites are branching cellular extensions that detect impulses and bring them into
the cell.
b. The cell body is the expanded, central portion of the cell that houses the nucleus.
c. Leading away from the cell body is a long, slender cellular extension called an axon
that conveys information away from the cell to the next neuron.
2. Neuroglia are the helper cells that protect and nourish neurons.
3. One type of neuroglial cell, the Schwann cell, wraps axons in insulating material called a
myelin sheath.
4. Each Schwann cell wraps a portion of a neuron, and there are gaps between adjacent
Schwann cells, called nodes of Ranvier.
5. The nervous impulse jumps rapidly from one node to the next as it travels down the
neuron.
B. The Nerve Impulse
1. In its resting state, when not transmitting an impulse, a neuron is said to be polarized and
in its resting state.
2. In this state, it has an excess of positively charged ions on the outside of the cell
membrane and an excess of negative ions on the inside.
3. When something interrupts the resting potential and there is enough pressure or other
stimulus to disturb the cell membrane of a neuron, voltage-gated channels open up,
allowing a rush of sodium to the inside of the cell; it is now depolarized.
4. Sodium channels close rapidly, and sodium is pumped back out of the cell.
5. If the stimulus is great enough, it will set up a chain reaction of sodium channels opening
down the length of the neuron and generating a nerve impulse.
6. This chain reaction is referred to as reaching action potential, and a nerve
Impulse travels the length of the nerve.
The Synapse (p. 611; Figs. 28.6, 28.7, 28.8)
A. At the junctions between two neurons lies a small space called the synapse.
B. A nervous impulse is conveyed from the presynaptic neuron to the postsynaptic neuron.
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28.4
C. Neurotransmitters
1. Since the nervous impulse cannot travel across a synapse, the presynaptic neuron releases
a chemical called a neurotransmitter into the synapse.
2. The postsynaptic neuron has specific receptors in its cell membrane that can detect the
neurotransmitter.
3. Once received, the sodium gates of the postsynaptic neuron open up, action potential is
reached, and the nervous impulse is on its way.
D. Kinds of Synapses
1. Synapses are considered the control switches of the nervous system.
2. Some synapses are excitatory, meaning that the receipt of a given neurotransmitter
generates action potential in the postsynaptic neuron.
3. Other synapses are inhibitory, and receipt of a different neurotransmitter inhibits the
impulse from traveling further.
4. A process called integration results when excitatory and inhibitory synapses
counterbalance one another.
E. Neurotransmitters and Their Function
1. Acetylcholine is the neurotransmitter released at the neuromuscular junction.
a. It forms excitatory synapses with skeletal muscle but has the opposite effect on
cardiac muscle.
b. Glycine and GABA are inhibitory neurotransmitters.
c. Biogenic amines including norepinephrine, dopamine, seratonin, and epinephrine
have various effects on the body.
Addictive Drugs Act on Chemical Synapses (p. 614; Figs. 28.9)
A. Neuromodulators
1. At times, additional neurotransmitters are released that modify the transmission of an
impulse at a synapse, either prolonging or inhibiting it.
2. These additional chemicals are called neuromodulators, and are responsible for mood
and emotion in the brain.
B. Drug Addiction
1. Addictive drugs, such as cocaine, often act as neuromodulators.
2. The brain adjusts its synapses to accommodate such drugs, and the result is
physiological dependence, or addiction.
C. Is Addiction to Smoking Cigarettes Drug Addiction?
1. Cigarette smokers find it difficult to quit smoking because they have become addicted to
nicotine, a powerful neuromodulator.
2. Addiction to nicotine occurs because the brain compensates for the many changes
nicotine induces by making other changes; eventually the newly coordinated system
requires nicotine to achieve an appropriate balance of nerve pathway activities.
The Central Nervous System (p. 616)
28.5
Evolution of the Vertebrate Brain (p. 616; Figs. 28.11, 28.12, 28.13)
A. Despite extensive study, scientists are still not certain how the brain performs many of its
functions.
B. Fossil agnanthans from 500 million years ago showed evidence of the hindbrain
(rhombencephalon), the midbrain (mesencephalon), and the forebrain (prosencephalon).
1. The hindbrain is an extension of the spinal cord devoted mostly to coordinating motor
reflexes.
2. In more advanced vertebrates, the cerebellum, an extension of the hindbrain, plays an
important role as a coordinating center.
3. The midbrain is devoted to the reception and processing of sensory information and
includes the optic lobes.
4. Brains of fishes continue to grow throughout life, unlike the brains of other vertebrates
that stop growing in infancy.
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C.
28.6
The Dominant Forebrain
1. Beginning with the amphibians, the forebrain becomes more developed and plays an
increasing role in sorting out sensory input.
2. In reptiles, amphibians, birds, and mammals, the forebrain is composed of the
diencephalon (consisting of thalamus and hypothalamus), and the telencephalon
(cerebrum in mammals).
D. The Expansion of the Cerebrum
1. The increase in brain size in mammals largely reflects the great enlargement of the
cerebrum.
How the Brain Works (p. 618; Figs. 28.14, 28.15, 28.16, 28.17)
A. The Cerebrum Is the Control Center of the Brain
1. The cerebrum is the center for thought and association, and makes up about 85% of the
weight of the brain.
2. It is divided into right and left halves, called cerebral hemispheres.
3. The neuron cell bodies lie in the thin cerebral cortex that covers the cerebrum; it is here
that much thinking occurs.
4. Deeper into the cerebrum are the myelinated nerve fibers that rapidly shuttle information
from one portion of the brain to another.
5. A bundle of neurons, called a tract, connects the two hemispheres.
6. The left side of the brain controls the right side of the body, and vice versa.
7. Different areas of the cerebral cortex control different body activities.
8. When brain blood vessels are clogged, it can lead to a stroke, and a portion of the brain
dies.
B. The Thalamus and Hypothalamus Process Information
1. The second portion of the brain contains the thalamus and hypothalamus, which are deep
in the brain and serve to process information.
2. The thalamus processes sensory information, including sound and balance, and sends
messages to the proper portion of the cerebrum.
3. The hypothalamus controls all of the body's internal functions: it regulates heart rate,
blood pressure, the secretions of the pituitary, and many other organs.
4. An extensive network of neurons in the cerebrum and hypothalamus, called the limbic
system, controls hunger, thirst, pleasure, pain, and emotional drives.
C. The Cerebellum Coordinates Muscle Movements
1. The cerebellum, located at the back of the brain, is where the control of muscular
coordination, balance, and posture are located.
2. It is even better developed in birds because of the nature of their aerial acrobatics.
D. The Brain Stem Controls Vital Body Processes
1. The brain stem, sometimes called the medulla oblongata, connects the brain with the
spinal cord. It controls breathing rate, heartbeat, and blood vessel diameter.
2. The reticular formation, a network of nerves, runs through the brain stem to other
portions of the brain.
3. The reticular formation is responsible for shutting the brain down for sleep and for its
arousal when waking is needed.
E. Language and Other Higher Functions
1. The left hemisphere is the dominant one for language in 90% of right-handed people and
two-thirds of left-handers.
2. The dominant hemisphere is adept at sequential reasoning while the nondominant
hemisphere is adept at spatial reasoning, such as that which is needed for artwork and
music.
3. While the process of memory is not fully understood, there appear to be fundamental
differences between short-term and long-term memory.
4. Short-term memories are transient and are stored electrically in the form of a transient
neural excitation.
5. Long-term memories, by contrast, involve structural changes within certain neural
connections in the brain.
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The Mechanism of Altzheimer’s Disease is Still a Mystery
1. Two hypotheses have been proposed.
a. The nerve cells in the brain are killed from the outside in by abnormal production of
beta-amyloid protein (an external protein) causing plaque formation.
b. The nerve cells are killed from the inside out by an internal protein (tau) that causes
tangles inside of the nerve cell.
The Spinal Cord (p. 621; Figs. 28.19)
A. The spinal cord begins at the brain stem and extends down through the vertebral column.
B. Sensory and motor nerve tracts run the length of the spinal cord, conveying messages at great
speed to the brain and back.
C. Spinal Cord Regeneration
1. In the past scientists have been unable to rejoin severed sections of the spinal cord.
2. With the discovery of fibroblast growth factor, neurobiologists have been at least partly
successful in gluing on rat nerves with fibrin mixed with the growth factor.
3. This technique holds promise for future spinal cord repair.
F.
28.7
The Peripheral Nervous System (p. 622)
28.8
Voluntary and Autonomic Nervous Systems (p. 622; Fig. 28.20, 28.21, 28.22)
A. Motor nerve pathways can be divided into the voluntary nervous system and the autonomic
nervous system.
B. Voluntary Nervous System
1. The voluntary nervous system controls skeletal muscles; it is under conscious control.
2. Reflexes enable quick action.
3. Reflexes are automatic reactions that protect the body from danger.
4. These are usually wired through an interneuron housed within the spinal cord and do not
travel to the brain.
C. Autonomic Nervous System
1. The autonomic nervous system has two divisions.
2. The sympathetic nervous system is involuntary, and prepares the body for danger.
a. Blood pressure and heart rate increase, and the body is ready for “fight-or-flight.”
3. The parasympathetic nervous system slows down reactions and conserves energy when it
is not needed.
4. Most internal organs and smooth muscles are innervated with fibers from both the
sympathetic and parasympathetic nervous systems.
5. This arrangement allows the brain to fully control the activities of the body to ensure
that homeostasis is maintained.
The Sensory Nervous System (p. 623)
28.9
Sensory Perception (p. 623; Fig. 28.23, 28.24, 28.25)
A. The sensory nervous system detects information at the exterior, or in the interior, of the body
and carries it to the CNS.
B. Sensory Receptors
1. Special neurons called sensory receptors detect changes in the environment.
2. Often sensory receptors are housed in specialized organs called sensory organs.
C. The Path of Sensory Information
1. The path of sensory information to the brain is composed of stimulation, transduction
(initiating the impulse), and transmission of the impulse.
2. All sensory receptors are able to initiate nerve impulses by opening or closing stimulusgated channels.
3. Sensory receptors located inside the body are known as interoceptors.
4. We are not consciously aware of the input from interoceptors.
5. Exteroceptors detect environmental changes at the surface of the body.
6. The senses of touch, pain, and temperature involve exteroceptors.
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28.10
28.11
28.12
28.13
D. Sensing the Internal Environment
1. Examples of sensory receptors in the internal environment include receptors capable of
detecting changes in blood chemistry, pain and tissue damage, muscle contraction and
posture, blood pressure, and touch below the skin’s surface.
Sensing Gravity and Motion (p. 626; Fig. 28.26)
A. Balance and motion are detected by sensory organs housed in the inner ear.
B. Balance is detected using little rocks called otoliths located on top of sensory receptors.
1. Whenever the otoliths shift, a nervous impulse is generated, sending information about
the body's position to the brain.
C. Motion is detected by sensory cells housed in semicircular canals.
1. The sensory cells have cilia that extend into a gelatinous material called a cupula.
2. When the cupula shifts in one or more of the semicircular canals, a change in motion is
perceived.
Sensing Chemicals: Taste and Smell (p. 627; Figs. 28.27, 28.28)
A. The senses of smell and taste both involve sensory receptors capable of detecting chemical
changes.
B. The tongue houses the taste receptors.
1. Four classes of chemicals can be detected: sour, sweet, bitter, and salty.
2. Impulses are set up and sent to the brain, which interprets the taste of a food.
C. Smell receptors are located in the upper nasal passages.
1. When odors are detected, nervous impulses are sent to the brain, which then determines
the nature of the odors, based on experience.
Sensing Sounds: Hearing (p. 628; Fig. 28.29, 28.30, 28.31)
A. Sounds are detected when air pressure waves press against the eardrum, transferring
vibrations to three little bones of the middle ear.
B. The three little bones amplify the sound waves as they vibrate against a membrane of the
cochlea of the inner ear.
C. The cochlear membrane vibrates against the fluid inside the cochlea, which in turn vibrates
membranes holding hair cells that are hearing receptors.
D. The hair cells shear against a membrane overlying them, generating nervous impulses that
are sent to the brain.
E. Sounds of different pitches stimulate different parts of the membrane holding the hair cells,
so pitch can be detected.
F. Volume is determined by how often the neurons send impulses.
G. The Lateral Line System
1. Fish use the lateral line system in water to sense pressure waves.
2. It consists of sensory structures within a longitudinal canal in the fish’s skin and within
several canals in the head.
3. The sensory structures are hair cells extending into a cupula; vibrations in the water
produce movement of the cupula.
H. Sonar
1. Some animals, like bats, whales, and dolphins, use sonar to bounce sound waves off
objects and generate a three-dimensional image of what lies ahead; this process is called
echolocation.
Sensing Light: Vision (p. 630; Figs. 28.33, 28.34, 28.35, 28.36, 28.37, 28.38, 28.39, 28.40)
A. Vision (the perception of light) is carried out by a special sensory apparatus called the eye.
B. Evolution of the Eye
1. Many invertebrates have simple visual systems with photoreceptors clustered in an
eyespot that detect the presence and direction of light only.
2. The members of the annelids, mollusks, arthropods, and vertebrates have independently
evolved image-forming eyes.
C. Structure of the Vertebrate Eye
1. The eye is the organ of sight, capable of detecting light.
2. Light first travels through a transparent cornea, which bends light waves toward the
back of the eye.
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3.
28.14
A lens completes the focusing, with the aid of ciliary muscles that contract or relax
involuntarily.
4. How much light enters the eye through the pupil is adjusted by the rings of muscle
known as the iris.
5. At the back of the eye, the retina houses the special sight receptors, the rods and cones.
6. Rods detect black and white and help us see in dim light; cones are responsible for color
vision.
7. The lens focuses light on a portion of the retina called the fovea, which has the greatest
concentration of sight receptors.
D. How Rods and Cones Work
1. Sight receptors work because of special pigments inside them.
2. A light-detecting complex called rhodopsin is inside the rods.
3. When light strikes the rods, the rhodopsin undergoes a change in shape that initiates a
series of reactions to generate a nervous impulse.
E. Color Vision
1. Cones function in a manner similar to that of the rods, but have three different pigments,
each detecting a different set of wavelengths of light; thus, different colors can be
detected.
F. Conveying Light Information to the Brain
1. When rods and cones detect light, impulses are sent to bipolar cells and then to ganglion
cells which relay impulses to the occipital lobe of the cerebral cortex.
2. The pattern of activity among the ganglion cells across the retina encodes a map of the
receptive field, allowing the retina and brain to image objects in visual space.
G. Binocular Vision
1. Because humans have two eyes positioned on the front of the head, they have binocular
vision that enables them to judge distances.
2. The fields of vision of the two eyes overlap, so the brain can estimate the distance to an
object.
3. The ability to judge distance is a learned one, not one we are born with.
Other Types of Sensory Reception (p. 634; Fig. 28.41)
A. Heat
1. Pit vipers have heat-detecting pit organs that help them locate warm prey by infrared
radiation.
B. Electricity
1. Many aquatic invertebrates can locate prey and ascertain the contours of their
environment by means of electroreceptors.
C. Magnetism
1. Eels, sharks, and many birds appear to orient themselves with respect to the earth’s
magnetic field by a poorly understood mechanism.
LEARNING OBJECTIVES
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List the evolutionary trends that occurred as nervous systems became more complex.
Describe the structure of a neuron.
Explain how nervous impulses are generated and carried down neurons.
Understand how impulses travel across synapses, and how neurotransmitters operate.
Explain the relationship between neuromodulators and drug addiction.
Discuss the evolution of the vertebrate brain.
Describe the structure of the cerebrum, and know its functions.
Discuss the location of the thalamus and hypothalamus and their functions.
List the jobs of the cerebellum and brain stem.
Explain how hemisphere dominance occurs and list the capabilities of each hemisphere.
Know the function of the spinal cord.
Describe the divisions of the motor nervous system and their functions.
Explain the purposes of reflexes.
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Understand the differences between the sympathetic and parasympathetic nervous systems.
Describe the purposes of exteroceptors and interoceptors.
Know where and how sensory receptors for balance and motion operate.
Understand how the senses of taste and smell work.
Discuss how sound waves stimulate hearing receptors.
List ways animals sense body position.
Discuss the evolution of the eye and describe the structure of the vertebrate eye.
Understand the role of visual pigments in detecting light.
Know the importance of binocular vision.
List other types of sensory reception.
KEY TERMS
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central nervous system (CNS) (p. 608) consisting of the brain and spinal cord in more complex
animals
motor neuron (p. 608) carries impulses from CNS to target organ or muscle
sensory neuron (p. 608) carries impulses from periphery to CNS
neuroglial cells (p. 610) helper cells that provide nutrition to neurons, among other functions
resting potential (p. 611) charge separation that occurs on either side of the neuron membrane when
not conducting an impulse
voltage-gated channels (p. 611) protein channels in the neuron membrane that open and close in
response to electrical voltage
action potential (p. 611) a local reversal in voltage that initiates a nervous impulse
synapse (p. 612) junction between an axon and another cell
neurotransmitter (p. 612) chemical messenger secreted by an axon
integration (p. 613) process in which various excitatory and inhibitory electrical effects tend to cancel
or reinforce one another
neuromodulator (p. 612) long-lasting chemicals secreted into the synapse that modify the release of
neurotransmitters
cerebral cortex (p. 618) thin outer gray layer of cerebrum consisting of neuron cell bodies
hypothalamus (p. 619) area of the brain that integrates all internal activities
limbic system (p. 619) network of neurons throughout the brain that drives emotions
spinal cord (p. 621) nerve highway inside vertebral column
voluntary nervous system (p. 620) relays commands to skeletal muscles
autonomic nervous system (p. 620) stimulates glands and smooth muscles of the body
reflex (p. 620) an automatic response enabled by a circuit of sensory neuron, interneuron, and motor
neuron
sympathetic nervous system (p. 623) a division of the autonomic nervous system; responsible for
“fight-or-flight” responses
parasympathetic nervous system (p. 623) a division of the autonomic nervous system; responsible for
normal activity levels
stimulus-gated channel (p. 624) All sensory receptors are able to initiate nerve impulses by opening
or closing stimulus-gated channels within sensory neuron membranes.
exteroceptor (p. 624) sensory neuron that senses stimuli that arise from the external environment
interoceptor (p. 624) sensory neuron that senses stimuli that arise from within the body
cochlea (p. 628) coiled bony portion of inner ear that houses hearing receptors
retina (p. 631) the light-sensing portion located at the back of the eye
rod (p. 631) abundant type of visual receptor that perceives objects in dim light; black and white
cone (p. 631) visual receptor that detects colors, responsible for sharp vision in good light
rhodopsin (p. 631) light-detecting visual pigment system of rods
binocular vision (p. 632) slight displacement of two images from two eyes allows perception of depth
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LECTURE SUGGESTIONS AND ENRICHMENT TIPS
1.
2.
3.
Test Your Senses of Smell. Have your students work in small groups to demonstrate the ability of their
senses of smell. Fill small jars (baby food jars work well) or test tubes with a variety of common foods
or flavorings (for example, onion, peanut butter, cinnamon, pepper, ginger, lemon, and vanilla).
Instruct one student in each group to close his or her eyes, and the other student(s) to place a jar under
the first student's nose. Students should readily identify many of the odors. Have students switch places
and try again.
Test Your Touch Receptors. Again, have your students work in small groups. Pass out toothpicks to
each group. Use the toothpicks to determine two-point discrimination on several areas of the body.
Have one student operate the toothpicks on a second student whose eyes are closed. A third student can
estimate or measure distances between toothpicks. Start with the points of the toothpicks placed on the
skin of the back of the hand about one inch apart, then lift the toothpicks and position them slightly
closer together. Each time, ask the student who is feeling the toothpicks whether he feels two or one
toothpick. At some point, the toothpicks will be close enough together that the student will feel only
one toothpick, even though two are there. Repeat the procedure on the palm, the forearm, and the back
of the neck. This exercise demonstrates that the number of sensory receptors in the skin varies,
depending on the functions of different body parts.
Detached Retinas. Detachment of the retina can occur because of a fall or blow to the head, and can
also occur in those who are severely nearsighted. The inner layer of the retina containing the sensory
receptors actually becomes separated from the dark-colored vascular layer that nourishes the sensory
receptors. When this happens, the sensory receptors are deprived of oxygen and eventually die. To the
person whose retina is becoming detached, it appears as though a black sheet were coming down over a
portion or all of the field of vision in one eye. When this happens, the person needs to seek medical
attention immediately. The retina can be reattached using laser or other surgical means.
CHANGES TO THE NEW EDITION
Refer to the Johnson instructor web site at http://www.mhhe.com/biosci/genbio/tlw4 for a complete list of
changes to this edition.
CRITICAL THINKING QUESTIONS
1.
2.
3.
Why do people sometimes become tolerant of certain levels of pain medication and require higher
dosages?
Why do you think hemisphere dominance evolved?
Few sensations, other than those for pain, originate in the viscera of the body. Why is this an important
arrangement?
FILMS/MEDIA SUGGESTIONS
(Telephone and fax numbers and/or web sites for the sources of the following materials are listed in the
Appendix.)
The Brain. This video begins in the emergency room of a large hospital where head injuries are
commonplace. Showing how the brain is scanned using a variety of methods, this video explores the
importance of the brain, what is known about it, and how it operates. 1995. 20 minutes.
Films for the Humanities and Sciences, #BVL5988; also Carolina Biological Supply, CE-49-3707
The Senses. How the senses gather information, and why that is important, are the topics highlighted in this
video. The structures and functions of eyesight and balance are introduced. 1995. 20 minutes.
Films for the Humanities and Sciences, #BVL5989; also Carolina Biological Supply, CE-49-3708
167
Journey to the Center of the Brain. This five-part video series explores many aspects of the brain, from
behavior and the effects of drugs to development and personality.
1. The Electric Ape, shows the structures of the brain, how it generates electricity, and uses chemical
communication. 58 minutes. #BVL5693
2. Through a Glass Darkly explores how scientists examine the brain, evolution of the brain, and a
comparison of animal brains. 58 minutes. #BVL5694
3. Bubble, Bubble, Toil and Trouble is a view of the chemical circuitry of the brain, how
communication within the brain is influenced by drugs, and how drugs effect behavior. 58 minutes.
#BVL5695
4. The Seven Ages of the Brain focuses on the development of the brain from embryo to adult, and how
the pattern of development can be influenced by environmental factors. 58 minutes. #BVL5696
5. The Mind's I is a look at how the brain is unique in each of us. How regions of the brain function, and
how the regions work together to make a thinking individual is examined. 58 minutes. #BVL5697
Films for the Humanities and Sciences, five-part series, #BVL5636
The Hijacked Brain. How addiction affects the brain, and how heredity determines alcoholism are two of
many topics covered here. 1998. 57 minutes.
Carolina Biological Supply, CE-49-3907
The Development of the Human Brain. This video follows brain development from conception to age 8.
40 minutes.
Carolina Biological Supply, CE-49-3750
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