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CHAPTER 3
Biological Aspects of Psychology
OUTLINE
I.
THE NERVOUS SYSTEM
A. Cells of the Nervous System
1. Cells of the nervous system share many characteristics with other cells in the body.
2. Many cells communicate with one another in a process whereby the internal activity of
one cell changes in response to external stimuli (such as chemicals) released by another
cell.
3. Nervous system cells, like other body cells, each have an outer membrane, a cell body,
and a nucleus. Neurons respond to environmental changes by means of three special
features: structures called axons and dendrites, “excitable” surface membranes, and
synapses. Axons carry signals away from the neuron to points where communication
occurs with other neurons, whereas dendrites detect and carry information from other
nerve cells to the cell body. Other nervous system cells, called glial cells, hold neurons
in place, direct their growth and repair, and keep their chemical environment stable.
B. Action Potentials
1. The selective permeability of the neuronal membrane keeps positively charged sodium
and calcium ions from freely entering the axon through the gates or channels. As a
result, the membrane becomes electrically polarized, such that the inside of the cell is
more negatively charged than the outside. When an action potential occurs, some part
of the axon membrane becomes depolarized, causing a sodium or calcium gate to open.
Sodium rushes into the axon, causing the neighboring sodium gates to open as well,
and more sodium rushes in. This chain of events occurs along the entire length of the
axon.
2. The neuron either fires or does not fire, in an all-or-none type of signal. The speed of
the action potential is constant as it travels down the axon. If a neuron is larger or is
coated with myelin, action potentials will be faster. The length of the pause between
action potentials is known as the refractory period and determines the rate or number
of action potentials that occurs within a given time unit. Messages in the nervous
system are coded by the speed and rate of action potentials.
C. Synapses and Communication Between Neurons
Communication between neurons occurs at the synapse.
1.
2.
Neurotransmitters. Communication at the synapse is chemical in nature.
Neurotransmitters are chemicals that carry the signal across the synapse to the
postsynaptic cell (usually a dendrite). When neurotransmitters bind to receptors in the
postsynaptic cell, a membrane potential is created.
Excitatory and Inhibitory Signals. When a postsynaptic cell is reached by a
neurotransmitter, the postsynaptic membrane becomes depolarized or hyperpolarized,
creating a postsynaptic potential, and the signal once again becomes electric in nature.
A depolarized membrane will cause an excitatory postsynaptic potential (EPSP), and a
hyperpolarized membrane will cause an inhibitory postsynaptic potential (IPSP). The
combined impact of the many EPSPs and IPSPs will determine whether or not an
action potential will occur.
D.
II.
Organization and Functions of the Nervous System
1. Neurons in the brain and spinal cord are organized into groups called neural networks.
The sensory system provides input about the environment via the five senses. The
motor system directs the response to the environment by influencing the muscles and
other organs.
2. The peripheral nervous system is made up of the sensory and motor systems. The
central nervous system (CNS) processes information and is encased in bone. The CNS
is made up of the brain and spinal cord.
THE PERIPHERAL NERVOUS SYSTEM: KEEPING IN TOUCH WITH THE WORLD
The peripheral nervous system has two components: the somatic and autonomic systems.
A.
The Somatic Nervous System
The somatic system carries information from the senses to the CNS and sends movement
instructions back to the muscles.
B.
The Autonomic Nervous System
The autonomic system, via the parasympathetic and sympathetic branches, transmits
messages between the CNS and the body’s organs and glands. The sympathetic branch
prepares the body for action through the “fight-or-flight” response. The parasympathetic
branch does the opposite: It slows organ and gland activity to conserve the body’s energy.
III. CENTRAL NERVOUS SYSTEM: MAKING SENSE OF THE WORLD
The central nervous system (CNS) is made up of collections of neuronal cell bodies called nuclei
and the fiber tracts or pathways that connect them.
A.
B.
C.
The Spinal Cord
1. The spinal cord carries messages to and from the brain. Reflexes—quick, involuntary
muscular responses (through efferent neurons) that are initiated on the basis of
incoming sensory information (through afferent neurons)—occur in the spinal cord
without instruction from the brain. The brain is informed of each reflex after it occurs.
2. The spinal cord is an example of a feedback system, a process in which information
about an action’s results are conveyed back to the source of the action so that further
adjustments to the activity can be made.
The Brain
1. The Hindbrain. Hindbrain structures such as the medulla control vital functions (for
example, blood pressure, heart rate, and breathing). The reticular formation is a
network of cells running throughout the hindbrain that alters the activity of other brain
structures. For example, the locus coeruleus, an area thought to be involved in the state
of vigilance, is activated by the reticular formation. The cerebellum controls finely
coordinated movements, including speech.
2. The Midbrain. Located between the hindbrain and forebrain, the midbrain controls
certain automatic behaviors. The substantia nigra is a midbrain structure that, together
with the striatum, is involved in initiating smooth movement.
3. The Forebrain. The forebrain is the most highly developed part of the human brain.
The thalamus lies deep within the brain and relays sensory signals. Below it lies the
hypothalamus, which regulates basic drives. The suprachiasmatic nuclei, part of the
hypothalamus, determines our biological rhythms. The amygdala and hippocampus are
part of the limbic system, which plays an important role in regulating emotion and is
involved in memory and other thought processes.
Thinking Critically: What Can fMRI Tell Us About Behavior and Mental Processes.
What am I being asked to believe or accept?
Scientists using fMRI can determine what parts of the brain cause various behaviors and
mental processes
What evidence is available to support the assertion?
When a participant in an fMRI experiment thinks or feels something, you can see the colors
in the brain scan change.
Are there alternative ways of interpreting the evidence?
Sometimes the brain scan looks similar regardless of whether the person is actually doing
the behavior in question or watching someone else do it (mirror-image mechanisms).
What additional evidence would help to evaluate the alternatives?
As the quality of fMRI improves, better images will result, but a greater understanding of
correlation and causation in fMRI is needed.
What conclusions are most reasonable?
Although fMRI is a useful tool that offers detailed images of the brain structure and
function, it will not likely explain how the brain creates behavior and mental processes on its
own.
D.
Focus on Research Methods: Manipulating Genes in Animal Models of Human Disease
To test hypotheses about the cause of Alzheimer’s disease, experimenters implanted a gene
in mice and found that it created brain damage similar to Alzheimer’s. If the damage creates
similar memory impairments, scientists may have succeeded in making an animal model of
the disease that will allow more specific research on Alzheimer’s.
E.
The Cerebral Cortex
The cerebral cortex is the outer surface of the cerebrum or cerebral hemispheres. The frontal,
parietal, occipital, and temporal lobes are used as physical landmarks for describing the
cortex. The functional areas of the cortex include the sensory, motor, and association cortex.
1.
2.
3.
F.
G.
Sensory Cortex. The sensory cortex receives sensory information.
Motor Cortex. The motor cortex neurons control the onset of voluntary movement.
Association Cortex. The association cortex receives information from more than one
sense and combines sensory and motor information. Aphasia, a deficit in
understanding and producing language, is caused by damage to Broca’s area or
Wernicke’s area. Many areas of the brain are related to language; several are
associated with specific semantic abilities.
The Divided Brain in a Unified Self
1. Split-Brain Studies. Split-brain (severed corpus callosum) data demonstrate that each
hemisphere is superior in certain abilities. The left hemisphere controls spoken
language, and the right controls recognition of faces and tasks dealing with spatial
relations, such as drawing three-dimensional shapes. In addition, the left hemisphere
controls the right side of the body, and the right hemisphere controls the left side.
2. Lateralization of Normal Brains. Data collected from people with intact brains
demonstrate the superiority of the left hemisphere in logical thinking and language
abilities. The right hemisphere exhibits better spatial, artistic, and musical abilities. But
keep in mind that, although the two halves of the brain may have lateralized abilities,
the cerebral hemispheres work closely together.
Plasticity in the Central Nervous System
1. Plasticity is the brain’s ability to strengthen neural connections and establish new
connections. Unfortunately, synaptic plasticity is somewhat limited. New neurons
cannot be generated, and exact replication of the many synaptic connections prior to
H.
brain damage is almost impossible. However, old neurons do produce new axons and
dendrites, which make new connections.
2. Scientists are continually trying new methods to reproduce and grow nerve cells.
Several methods for enhancing brain-damage recovery are under study, such as tissue
transplants into the damaged area from another brain, nerve growth factor for
stimulation, and guidance of axon growth.
Linkages: Human Development and the Changing Brain
Patterns of behavioral development in infants are correlated with plastic changes in activity
and structure in the developing brain. During development, the brain overproduces neural
connections and, based on experience, establishes which connections are needed and then
eliminates the extras. Even in adulthood, the number of connections is affected by
experience: stimulating environments produce greater numbers of connections.
IV. THE CHEMISTRY OF PSYCHOLOGY
A group of neurons that communicates with the same neurotransmitter is called a neurotransmitter
system. Some neurotransmitter systems are responsible for certain behaviors or problems.
A.
V.
Three Classes of Neurotransmitters
1. Small molecules are found in both the CNS and PNS. Below are six examples of small
molecules that function as neurotransmitters.
a) Acetylcholine controls the contraction of muscles and is used by neurons in the
parasympathetic nervous system. In the brain, cholinergic neurons are involved in
movement and memory. A loss of cholinergic neurons is linked to Alzheimer’s
disease.
b) Arousal is the main task of norepinephrine, which is also known as
noradrenaline. Noradrenaline is used by neurons in the sympathetic nervous
system and in the locus coeruleus.
c) Sleep, moods, and appetite are influenced by serotonin.
d) Problems with the dopamine system in the substantia nigra and striatum
contribute to Parkinson’s disease, which leads to difficulty in the initiation of
movement. Dopamine is also involved in the experience of pleasure.
Malfunctioning dopaminergic neurons may be partly responsible for
schizophrenia, a psychological disorder characterized by distorted perception,
emotion, and thought.
e) Gamma-amino butyric acid, or GABA, is the major inhibitory transmitter of the
central nervous system. Huntington’s disease causes a loss of GABA-using
neurons and results in uncontrollable movement of the arms and legs.
f)
The major excitatory neurotransmitter in the CNS is glutamate. Glutamate helps
strengthen synaptic connections, which may be the origin of learning and
memory.
2. Peptides were discovered as scientists investigated opiates, substances derived from
the poppy flower.
a) Endorphins, natural opiate-like compounds, can reduce pain and cause sleep.
3. Gases were most recently discovered to act as neurotransmitters.
a) Nitric oxide and carbon monoxide are gases that function as neurotransmitters.
B. Thinking Critically: Are There Drugs That Can Make You Smarter?
THE ENDOCRINE SYSTEM: COORDINATING THE INTERNAL WORLD
The endocrine system, like the nervous system, influences a wide variety of behaviors. Glands
secrete hormones, which travel via the bloodstream and affect coordinated systems of target
tissues and organs, producing such responses as the fight-or-flight syndrome. A negative feedback
system involving the brain regulates the amount of hormone released.
VI. THE IMMUNE SYSTEM: DEFENDING THE BODY
A. The immune system is very similar to the nervous and endocrine systems: all three systems
are capable of responding to their environment, and all have cells that communicate with one
another. The function of the immune system is to monitor the internal body for the presence
of foreign or harmful material and to eliminate it. In autoimmune disorders the immune
system attacks normal body cells.
B. Five lines of evidence demonstrate the interaction of the immune, nervous, and endocrine
systems.
1. Stress has a negative impact on the functioning of the immune system.
2. Immune responses can be learned. Learning involves the nervous system.
3. Stimulating or damaging certain parts of the nervous system, such as the hypothalamus
and portions of the cortex and brainstem, cause changes in the ways in which the
immune system functions.
4. Immune system activity causes changes in neurotransmitter activity, hormonal
secretion, and behavior.
5. Some of the same chemical messengers are found in the brain and in the immune
system.