Download Biology and Behavior

CHAPTER 2
Biology and Behavior
CHAPTER OUTLINE
Biological psychology studies the cells, genes, and organs of the body, and the physical and chemical
changes involved in behavior and mental processes.
I.
CELLS OF THE NERVOUS SYSTEM
What are neurons and what do they do?
A.
B.
C.
Neurons
1. There are two main cell types in the nervous system.
a) Neurons are specialized to respond rapidly to signals and send signals of their
own.
b) Glial cells hold neurons together, guide their growth, maintain a stable chemical
environment, provide energy, help restore damage, and respond to signals from
neurons.
2. All cells have some features in common.
a) An outer membrane selectively allows only some substances to pass in and out.
b) The cell body contains the nucleus.
c) Mitochondria turn oxygen and glucose into energy.
3. Neurons have special features that permit effective signal communication.
a) An axon is a cell fiber that carries signals away from the cell body. Most neurons
have just one axon.
b) A dendrite is a cell fiber that receives signals from other neurons and carries
information toward the neuron’s cell body. Most neurons have many dendrites.
Action Potentials
1. Action potentials are electrochemical pulses that shoot down the neuron’s axon. They
are “all-or-none”: A neuron either fires an action potential at full strength or does not
fire at all.
2. After an action potential, there is a brief recovery time called a refractory period,
during which a neuron cannot fire another action potential.
3. The speed of an action potential depends on the thickness of the axon and on the
presence of myelin, a white, fatty substance that speeds up action potentials.
Synapses and Communication Between Neurons
1. At the axon end the action potential causes bag-like vesicles to release stored chemicals
called neurotransmitters into a space between the two neurons.
2. This space is called a synapse, a connection that is a narrow gap separating the axon of
one neuron from the dendrites of another. It is the means by which two neurons
communicate.
3. Released neurotransmitters “float” across the synapse to “bind” with receptors,
proteins on a dendrite of a receiving neuron.
4. The interaction between neurotransmitters and receptors is very specific, like a lock
and key. A specific receptor (a “lock”) can only be stimulated by a specific
neurotransmitter (a “key”).
Copyright © Houghton Mifflin Company. All rights reserved.
Chapter 2: Biology and Behavior
27
5.
D.
This interaction creates a signal called a postsynaptic potential (PSP) that might make
action potentials in the receiving, or postsynaptic, neuron either more or less likely. A
number of PSPs sum together at the junction of the cell body and the axon. Whether or
not an action potential “fires” depends on the kind of signals that are most numerous.
Organization of the Nervous System
The nervous system is organized into two main parts:
1.
II.
The central nervous system (CNS), encased in bone, consists of the brain and spinal
cord. The CNS is the nervous system’s central executive.
2. The peripheral nervous system, extends throughout the body and relays information
to and from the brain.
THE PERIPHERAL NERVOUS SYSTEM: KEEPING IN TOUCH WITH THE WORLD
How do sights and sounds reach my brain?
The peripheral nervous system has two subsystems:
A.
The Somatic Nervous System
The somatic nervous system carries signals between the senses and CNS and between the
CNS and skeletal muscles. Sensory neurons bring information to the brain, and motor
neurons send information from the brain to the muscles.
B.
The Autonomic Nervous System
The autonomic nervous system (ANS) carries messages between the CNS and the heart,
lungs, and other organs and glands. The ANS has two divisions:
1.
The sympathetic system directs the body to spend energy (e.g., increased heart rate,
faster breathing, sweating, sometimes called the “fight-or-flight” response) to react to
stress.
2. The parasympathetic system directs the body’s functions to conserve energy (e.g.,
slower heart rate, increased digestive activity). Parasympathetic activity helps “calm” a
person after increased sympathetic arousal.
3. Both systems may act on the same body areas, with their relative “balance” regulating
the state of the targeted organs.
III. THE CENTRAL NERVOUS SYSTEM: MAKING SENSE OF THE WORLD
How is my brain "wired"?
In the CNS different functions are performed by different networks of neurons. Clusters of
neurons are called a nuclei, and pathways that connect the networks are bundles of axons called
fiber tracts.
A.
The Spinal Cord
The spinal cord receives and sends signals to and from the brain.
1.
B.
Reflexes are simple, involuntary behaviors controlled by spinal cord neurons, without
requiring instructions from the brain.
2. Reflexes are controlled by a feedback system. Information about the consequences of
an action goes back to the source of the action for further adjustment, if necessary.
The Brain
A number of tools have been developed for monitoring the brain’s structure and activity:
1.
2.
The earliest technique is called an electroencephalogram (EEG), which measures
general electrical activity through electrodes on the scalp.
A newer technique is a positron emission tomography (PET) scan, which records the
location of radioactive substances that were injected into the bloodstream. These show
the location of brain activity during specific tasks.
Copyright © Houghton Mifflin Company. All rights reserved.
28
Chapter 2: Biology and Behavior
3.
C.
D.
Magnetic resonance imaging (MRI) records radio frequency waves after exposure to a
magnetic field providing clear pictures of the anatomical structure of the brain.
4. Functional MRI ( fMRI) detects changes in blood flow to provide a “moving picture”
of neuronal activities.
5. The newest techniques provide even more information about brain activity, structure,
and functioning. These include diffusion tensor imaging (DTI) and transcranial
magnetic stimulation (TMS).
Thinking Critically: What Can fMRI Tell Us about Behavior and Mental Processes?
fMRI scans show where brain activity occurs as people think and experience emotion. Like
phrenology in the nineteenth century, which claimed that personality traits and other mental
features could be determined from bumps on people’s skulls, some feel that people will
uncritically accept the claim that fMRI scans also indicate how the mind works.
1.
What am I being asked to believe or accept?
fMRI scans cannot indicate the anatomical locations—in other words, the biological
causes—of particular thoughts and emotions.
2.
Is there evidence available to support the claim?
Although brain areas do “light up” when a person thinks or feels something, fMRI
scans of these areas are not precise. First, they do not directly measure brain cell
activities but just reflect blood flow and oxygen in the brain that are related in some
unknown way to neuron firings. Second, an fMRI scan may miss brain cell activities
that do not create simple increases in blood flow. Third, coordinated changes in
millions of neurons are necessary before a detectable fMRI signal occurs. Fourth,
many of the results of fMRI research depend on how the researchers decide to interpret
them—on judgments. And, finally, no one knows what it really means when certain
brain areas appear to be activated during certain experiences.
3.
Can that evidence be interpreted another way?
Supporters point to important fMRI research on brain mechanisms involved with
experiencing empathy and learning by watching others. Mirror neuron mechanisms
were found in parts of the brain including Broca’s area. Neurons in these areas become
activated not only when a person actually experiences something, but also when he/she
watches someone else do or feel the same thing. Some fMRI studies have found
malfunctioning mirror mechanisms in people diagnosed with autism, a disorder that
includes problems with language development, imitative skills, and empathy.
4.
What evidence would help to evaluate the alternatives?
As fMRI technology continues to improve, knowledge about correlation and causation
in fMRI must also grow. Sharing information from fMRI experiments will help to
accomplish this. An fMRI data center has been established to store research data for
review.
5.
What conclusions are most reasonable?
fMRI is an exciting tool that offers images of the structure and functioning of the brain.
However, by itself, fMRI probably will not be able to explain exactly how the brain
creates behavior and mental processes. Critical thinking must always underlie analysis
of results of fMRI research.
The Hindbrain
The hindbrain is found just above the spinal cord and is composed of the following
structures:
1.
The medulla controls vital life functions (e.g., blood pressure, heart rate, breathing).
Copyright © Houghton Mifflin Company. All rights reserved.
Chapter 2: Biology and Behavior
29
2.
3.
E.
F.
The reticular formation is a web of neurons is involved in arousal and attention.
The cerebellum coordinates fine motor movements, stores a memory code for wellrehearsed behaviors, and participates in cognitive tasks such as reading.
The Midbrain
The tiny midbrain relays information from the eyes, ears, and skin and controls certain
types of automatic behaviors. The midbrain and its connections to the forebrain permit the
smooth initiation of movement.
The Forebrain
The forebrain, the largest part of the brain, regulates many complex aspects of behavior and
mental phenomena. Interior structures include the following:
1.
G.
H.
The thalamus processes inputs from sense organs (except for smell) and then relays
sensory information to appropriate “higher” forebrain areas. It is the primary sensory
relay into the rest of the brain.
2. The hypothalamus regulates many physiological feedback systems, coordinating
hunger, thirst, temperature regulation, and sexual behavior. It directly influences both
the autonomic and the endocrine systems. It contains the suprachiasmatic nuclei, the
brain’s “clock” that sets biological rhythms for the body.
3. The limbic system includes the amygdale and the hippocampus.
a) The amygdala is involved in memory and emotion. It links different kinds of
sensory information together in memory. The amygdala also plays a role in fear
and other emotions, linking emotions to sensations.
b) The hippocampus is critical to the ability to form new memories.
The Cerebral Cortex
1. The forebrain’s outer surface, the cerebral cortex, is a thin sheet of neurons. In
humans, the sheet folds in on itself, giving the brain its characteristic wrinkled
appearance.
2. The cerebral cortex is divided down the middle, creating two halves called the left and
right cerebral hemispheres. The corpus callosum connects the two halves.
3. The folds of cortex produce gyri (ridges) and sulci, or fissures (valleys or wrinkles), on
the brain’s outer surface. Several deep sulci make convenient markers for dividing the
cortex of each hemisphere into four anatomical areas: the frontal, parietal, occipital,
and temporal lobes.
Sensory and Motor Cortex
The sensory cortex and the motor cortex are two of the functional areas of the cortex.
1.
I.
Each region of the sensory cortex receives and processes input from a single sensory
organ.
a) Inputs from the eyes are sent to the visual cortex in the occipital lobe.
b) Inputs from the ears are sent to the auditory cortex in the temporal lobe.
c) Inputs from the skin sensory organs connect to the somatosensory cortex in the
parietal lobe. Neighboring body areas send somatosensory inputs to neighboring
parts of the somatosensory cortex. This pattern of brain organization is called a
homunculus.
2. Neurons in the motor cortex, in the frontal lobe, initiate voluntary movements of
specific body parts. These neurons are organized so that the combined activity of
neighboring groups of neurons controls movements of neighboring body regions.
Focus on Research: The Case of the Disembodied Woman
1. What was the researcher’s question?
Why was an apparently healthy woman falling and dropping things?
2.
How did the researcher answer the question?
Copyright © Houghton Mifflin Company. All rights reserved.
30
Chapter 2: Biology and Behavior
Dr. Sacks conducted a case study of Christina to check her sensory feedback from her
joints and muscles.
J.
K.
L.
3.
What did the researcher find?
Christina’s sensory neurons that usually supply kinesthetic information had stopped
working.
4.
What do the results mean?
Our sense of our bodies comes not just from seeing them, but also from
proprioception.
5.
What do we still need to know?
Dr. Sack’s research provided detailed information on what neurological problem
Christina experienced, but it did not show what caused her condition. Did megadoses
of vitamin B6 contribute to Christina’s problem and, if so, why? Are there other causes
of this kinesthetic disorder?
Association Cortex
1. Most of the cortex in each lobe is association cortex, with no specific sensory inputs
or direct motor outputs. Rather, the association cortex combines inputs from various
senses and is involved in many different mental processes.
a) Some regions of the association cortex are specifically involved in language
processing.
(1) Broca’s area is a region of association cortex, usually in the left frontal
lobe. Damage to this region causes difficulty speaking smoothly and
grammatically, a condition called Broca’s aphasia.
(2) Wernicke’s area is a region of the association cortex, usually in the left
temporal lobe. Damage to this region leaves fluency intact but makes it
difficult to understand the meaning of words or to speak understandably.
b) Other association areas in the front of the brain called the prefrontal cortex are
involved in complex, higher-level thought processes.
The Divided Brain: Lateralization
1. The physically separate left and right hemispheres perform different functions.
2. Most sensory and motor pathways cross as they enter or leave the brain. As a result,
the left hemisphere receives information from and controls movements of the right side
of the body, and the right hemisphere does the same for the left side of the body.
3. Studies of split-brain patients highlight the different functions of the two hemispheres.
a) The left and right hemispheres communicate through the corpus callosum, a
bundle of over a million fibers. To relieve seizures in some epilepsy patients, a
“split-brain” operation cuts the corpus callosum. In such patients, the two
hemispheres operate somewhat independently of each other.
b) Special techniques were used to present information to only the left or right
hemisphere of split brains. Patients could verbally name only those objects shown
to the left hemisphere; they could use their hands to recognize objects shown to
either hemisphere. This suggested that the left hemisphere, more than the right, is
specialized for language.
c) The right hemisphere appears to be specialized for tasks involving spatial
relationships and recognizing human faces.
d) Because of the corpus callosum connection, the two hemispheres work closely
together.
Plasticity in the Central Nervous System
1. Brains show synaptic plasticity, adding or changing synapses due to one’s experiences.
This may be a physical basis for forming memories and learning new things.
Copyright © Houghton Mifflin Company. All rights reserved.
Chapter 2: Biology and Behavior
31
2.
Brain damage is hard to repair because the adult nervous system does not automatically
replace damaged cells and restore lost functions. A number of surgical techniques have
tried to help damaged central nervous systems.
a) Fetal brain tissue grafts have not been successful in humans over the long term.
b) Transplants of brain tissue from other species have been rejected by humans.
c) Scientists are currently concentrating on coaxing neural stem cells that exist in
adult brains to form new neurons. Current work tries to solve the problem that
glial cells and proteins block new neurons from making replacement synaptic
connections for those that have been lost or damaged.
(1) When a protein called Nogo is blocked in rats, neurons were able to make
new connections.
(2) Brain grafts can be made more effective by adding naturally occurring
proteins called growth factors, which promote the survival of neurons and
survival of neurons.
(3) To promote neural plasticity, special mental and physical exercise programs
seem to help “rewire” damaged brains.
IV. LINKAGES: HUMAN DEVELOPMENT AND THE CHANGING BRAIN
A. PET and fMRI scans, which measure neuronal activity, have shown that brain functioning
changes with age.
1. Newborns’ brain activity is high in the thalamus and low in the part of the forebrain
related to smooth movement. This pattern of brain activity and motor function
resembles that seen after the brain damage in Huntington’s disease patients.
2. In the second and third months, brain activity increases in regions of the cortex. This is
paralleled by a loss of reflexes not under cortical control.
3. In the eighth and ninth months, brain activity increases in the frontal cortex, paralleled
with the apparent blossoming of cognitive activity.
4. The brain matures through adolescence, creating more efficient communication in
major fiber tracts.
B. Synaptic plasticity, not the growth of new cells, is associated with development. In the first
years of life the number of synapses and dendrites increases greatly, then drops in early
adolescence. The brain overproduces neural connections, then “prunes” unneeded ones.
C. In studies on rats the richness of the environment, in other words experience, determines the
number of synapses that are developed and retained throughout life.
V. THE CHEMISTRY OF BEHAVIOR: NEUROTRANSMITTERS
How do biochemicals affect my mood?
Different sets of neurons use different neurotransmitters. About 100 neurotransmitters have been
identified. A group of neurons that communicate using the same neurotransmitter is called a
neurotransmitter system.
A.
Three Classes of Neurotransmitters
1. Small molecules
a) Acetylcholine is used by sets of neurons involved in controlling movement of the
body, in making memories, and in slowing the heartbeat and activating the
digestive system. Alzheimer’s disease may result from disruptions of this
system.
b) Norepinephrine affects arousal, wakefulness, learning, and mood. Disruptions of
this system have been linked to depression.
c) Serotonin affects sleep, mood, aggression, and impulsive behaviors. Serotonin
levels can be affected by what is eaten.
(1) Malfunctions in serotonin systems can result in mood and appetite problems
seen in some types of obesity, premenstrual tension, and depression.
Copyright © Houghton Mifflin Company. All rights reserved.
32
Chapter 2: Biology and Behavior
(2)
Antidepressant medications such as Prozac, Zoloft, and Paxil are thought to
act on serotonin systems to relieve some of the symptoms of depression.
d) Dopamine is used by sets of neurons involved in controlling movement, and
damage to these systems contributes to shakiness experienced by people with
Parkinson’s disease. Other dopamine systems are involved in the experiencing of
reward, or pleasure, which is vital in shaping and motivating behavior. Certain
other dopamine systems are suspected to be responsible for the perceptual,
emotional, and thought disturbances associated with schizophrenia.
e) GABA (gamma-amino butyric acid) is the main inhibitory neurotransmitter in the
brain—it slows down the brain’s neural activity.
(1) Some drugs amplify the inhibitory action of GABA. One example is
alcohol, which results in impairments of thinking, judgment, and motor
skills. Drugs that interfere with GABA’s inhibitory effects produce intense
repetitive electrical discharges, known as seizures.
(2) Impaired GABA systems are thought to contribute to severe anxiety,
Huntington’s disease, and epilepsy.
f)
Glutamate is the main excitatory neurotransmitter in the brain. Its release is
associated with the ability of a synapse to “strengthen” its connection between
two neurons, perhaps as part of the physical basis of memory formation.
(1) Overactivity of glutamate synapses can cause neurons to die by “exciting
them to death.” Blocking glutamate receptors immediately after brain
trauma can prevent permanent brain damage.
(2) Glutamate may contribute to the loss of brain cells in Alzheimer’s disease.
2. Peptides: Hundreds of chemicals called peptides have been found to act as
neurotransmitters. Examples of these are endorphins, which are used in brain systems
involved in pain perception. Opiate drugs (e.g., morphine) relieve pain by binding to
endorphin receptors.
3. Gases: Two toxic gases that contribute to air pollution have been recently discovered
to act as neurotransmitters: nitric oxide and carbon monoxide. Rather than bind to
receptors, these gases affect the chemical reactions inside nearby neurons. Nitric oxide
is not stored in vesicles and can be released from any part of the neuron. Nitric oxide
appears to be one of the neurotransmitters responsible for penile erection and the
formation of memories.
VI. THE ENDOCRINE SYSTEM: COORDINATING THE INTERNAL WORLD
How can my hormones help me in a crisis?
A.
Like the nervous system, the endocrine system is specialized for cell-to-cell
communication. Cells of endocrine glands release chemicals called hormones into the
bloodstream. Then, cells of target organs use specific receptors to detect specific hormones,
causing specific cell responses.
1. The hypothalamus in the brain controls the pituitary gland, which controls endocrine
organs in the body. An endocrine organ’s hormone product affects cells of a specific
target organ of the body.
2. Each part of the system uses hormones to signal the next or to provide feedback for
subsequent hormonal regulation.
a) When threat is perceived, the hypothalamus directs the pituitary to release the
hormone ACTH into the bloodstream.
b) ACTH causes the adrenal gland to release the hormone cortisol.
(1) Adrenal hormones and sympathetic arousal together result in the fight-orflight response (e.g., faster heart rate, increased energy use) to help the
body respond to danger.
Copyright © Houghton Mifflin Company. All rights reserved.