Chapter 2: The Brain- The Psychological Organ
I. Brain Circuits: Making connections
A. The Neuron: A Powerful Computer. All brain activity hinges on neurons.
1. There are three types of neurons:
a. Sensory neurons that respond to input from sense organs.
b. Motor neurons that send signals to muscles to control movement.
c. Interneurons that are connected to other neurons.
2. Some neurons excite other neurons while others inhibit them.
3. The average brain contains 100 billion neurons and 10 times as many glial cells.
4. Brain circuits are sets of neurons that affect one another.
5. Each neuron has the following structures:
a. A cell body which contains the nucleus.
b. A cell membrane which is the outer covering of the cell.
c. Channels, small holes in the cell membrane which can open and close.
d. An axon, which sends signals to other neurons, muscles, or organs.
(1) At the end of each axon is a terminal button that releases neurotransmitters into the synaptic
cleft when the neuron “fires.”
(2) Most axons are covered with myelin to help impulses move down the axon more efficiently.
However, disorders such as multiple sclerosis cause
deterioration of myelin and impaired functioning.
e. A dendrite that receives messages from the axons of other neurons.
6. With enough stimulation, ions pass through the cell membrane and initiate an action potential.
a. An action potential obeys the all or none law.
b. In the fastest neurons, a neural impulse travels about 120 meters per second.
7. Each neuron is typically connected to 10,000 other neurons (and may be connected to up to 100,000
other neurons). Thus, the number of possible neural connections in your brain is greater than the
number of atoms in the universe.
8. The site where neurons communicate with each other is the synapse.
a. This communication results from the release of neurotransmitters across the
synaptic cleft.
b. The effects of neurotransmitters can be altered by chemicals called
neuromodulators.
c. Neurotransmitters attach themselves to receptor sites on the dendrite of the
neuron on the other side of the synaptic cleft.
(1) Specific neurons will only attach at specific receptor sites, like a lock and key.
(2) Neurotransmitters can either be excitatory or inhibitory.
(3) The same neurotransmitter or neuromodulator can have very different effects depending on
which type of receptor is present.
(4) Some neurotransmitters released by the terminal button are reabsorbed
(reuptake) into vesicles in the terminal button.
d. Drugs can affect this process: some drugs are agonists while others are
antagonists.
II. The Nervous System: An Orchestra with Many Members
A. Overview
1. The brain sends and receives information from 12 cranial nerves.
2. The spinal cord
a. The largest conduit of information going to and from the brain is the spinal cord.
b. The spinal cord and brain are collectively known as the central nervous system.
c. The spinal cord can initiate reflexes.
3. The peripheral nervous system links the central nervous system to the rest of the body. It is made up of
two parts: the autonomic nervous system and the skeletal
nervous system.
a. The autonomic nervous system controls smooth muscles and some glands.
b. The autonomic nervous system controls many self-regulating functions not
under conscious control such as digestion and circulation.
c. The autonomic nervous system has two parts: the sympathetic nervous
system and the parasympathetic nervous system.
(1) The sympathetic nervous system is involved in arousal reactions that
prepare an animal for “fight or flight.”
(2) The parasympathetic nervous system tends to slow things down.
(e) The skeletal muscle system is involved in voluntary control of the striated
muscles.
4. As viewed from above, the brain is divided neatly into two cerebral hemispheres, covered with bulges
and folds. Each bulge is called a gyrus and each fold is called a sulcus.
a. Each hemisphere is divided into four lobes.
(1) The occipital lobe is entirely involved in processing visual information.
(2) The temporal lobe is involved in hearing, comprehending language, and
storing new memories.
(3) The parietal lobe is involved in spatial awareness, attention, and motor
control.
(4) The frontal lobe is the center of planning, memory search, motor control, and reasoning.
b. Each hemisphere controls the functions of the opposite side of the body.
c. The two hemispheres are connected by a thick bundle of nerve fibers called the corpus callosum.
d.
Each hemisphere performs distinct functions.
(1) Evidence from split-brain patients has helped identify the unique functions of each hemisphere.
(2) While language is usually controlled by the left hemisphere, in most cases
hemispheric differences are more of degree than kind.
(3) Usually both hemispheres operate simultaneously to carry out different aspects of a task.
5. The cerebral cortex is where most mental processes take place.
6. Underneath the cortex is an “inner brain” made up of subcortical structures.
a. The thalamus receives input from sensory and motor systems and plays a crucial role in attention
and sleep.
b. The hypothalamus is located under the thalamus and plays a central role in
controlling eating and drinking, and in regulating the body’s temperature, blood pressure, and heart
rate, and contains the so-called “pleasure centers.”
c. The hippocampus plays a crucial role in allowing new information to be stored in memory.
d. The amygdala plays a special role in a variety of emotions, particularly fear and anger.
e. The basal ganglia are involved in planning and producing movement, as well as forming new
habits.
f.
The brainstem is a set of neural structures at the base of the brain that connect the brain to the
spinal cord. It is made up of the medulla, the pons, and the reticular formation.
(1) The medulla is the lowest part of the brainstem and is crucially involved in the automatic
control of breathing, swallowing, and blood circulation.
(2) The reticular formation has two parts:
(a) Its “ascending” part helps keep a person awake and alert.
(b) Its “descending” part is involved in autonomic nervous system reactions.
(3) The pons is a bridge between the brainstem and the cerebellum. It is involved in sleep and
control of facial muscles.
g. The cerebellum is concerned with physical coordination, estimating time, and
paying attention.
h. The cerebellum, pons, and medulla together are considered the hindbrain.
7. The neuroendocrine system is regulated by the central nervous system.
a. It is comprised of endocrine glands which make hormones (such as testosterone, estrogen, and
cortisol) that affect many bodily functions.
b. The pituitary gland is the so-called “master gland” which is itself controlled by the hypothalamus,
but controls the other endocrine glands.
8. The brain plays a role in regulating the immune system.
a. Psychoneuroimmunology focuses on the ways in which mental and emotional
states affect the immune system.
b. The hypothalamus has a role, which has not yet been clarified, in this process.
III. Probing the Brain
A. Studying the damaged brain.
1. The first evidence of how specific parts of the brain control different functions came from natural
experiments, studies of people with brain damage due to accidents.
a. These accidents produce lesions.
b. The most frequent source of damage is a stroke.
c. Natural experiments a limited because the damage they cause is rarely neat or
confined to a specific area of the brain.
2. Lesioning studies involve the removal of the specific parts of the brains of animals to observe the
consequences on behavior.
a. It can be difficult to generalize from animals to humans in these studies.
b. They do not provide a complete picture of brain function.
B. Recording techniques can provide information about the activity of a normal brain.
1. The electroencephalograph (EEG) records electrical current produced by the brain.
a. The electroencephalogram is a recording from the scalp of electrical activity in the brain over
time.
b. This type of recording allowed scientists to discover that people go through
distinct stages of sleep characterized by different brain waves.
c. This technique is limited in that it cannot detect the precise locations of the brain’s electrical
currents and the current is distorted as is passes through the skull or along the surface of the brain
and scalp.
2. The problems with EEG can be avoided with single-cell recording, but this technique is very limited in
the range of what it can record.
C. Three kinds of neural stimulation have been used to help researchers see what various
parts of the brain do.
1. Mild electricity can be delivered to a part of the brain and then the participant can be asked to report
what he or she experienced but it can be difficult with this technique to separate actual memories from
made-up stories.
2. In transcranial magnetic stimulation (TMS), researchers stimulate the brain by
putting a wire coil on the person’s head and discharging a large current through the coil to create a
magnetic field.
a. This field makes neurons under the coil fire.
b. The stimulated neurons may stimulate other neurons to fire, making it difficult
to sort out which neurons caused the observed effects.
3. Electrodes can be placed on the surface of or inside the brain and researchers observe which activities
are disrupted when a current is applied, but it is limited by problems similar to those created by TMS.
D. Neuroimaging yields actual pictures of neuronal structure and functioning and has
revolutionized psychology by answering questions impossible to answer before.
1. Computer-assisted tomography (CT) uses a series of X-rays to build up a threedimensional image of the brain, slice-by-slice.
2. Magnetic resonance imaging (MRI) uses the magnetic properties of atoms to take much sharper
pictures of the structure of the brain.
3. Positron emission tomography (PET) uses small amounts of radiation to track
Blood or energy consumption in the brain and provide an image of neural activity in different parts of
the brain, but this technique is slow, requires radiation, and is expensive (up to $2,000 per person).
4. The most popular type of imaging today is functional magnetic resonance imaging (fMRI) which
images neural activity by measuring the amount of oxygen being brought to a particular part of the
brain.
a. It is quicker the MRI and does not require radiation.
b. However, fMRI machines are noisy and require that the participant lie in a narrow tube, which
many people find uncomfortable.
IV. Genes, Brain, and Environment: The Brain in the World
A. Genes as Blueprints: Born to be Wild?
1. Deoxyribonucleic acid (DNA) is the molecule that contains our genes.
2. Genes affect us from the instant of conception and are in turn affected by the
environment.
3. Genes affect behavior as well as physical traits.
a. A small genetic change can have a large impact on physiology and behavior.
b. About 99% of the genetic material in humans and chimpanzees is the same.
4. Your genes cannot program your entire brain in advance, they interact with the
environment.
5. Your brain contained far more neural connections at birth than it does now.
a. Through pruning certain unused or rarely used neural connections are eliminated.
b. In heavily used areas of the brain, new connections are formed.
c. Even in adults, new connections and even new neurons can be created.
6. Many genes keep working your entire life. Some change their operation constantly, being turned on
and off as needed to produce needed substances.
7. There are three ways in which genes and the environment interact.
a. Passive interaction occurs when the parents’ or sibling’s genetically shaped
tendencies produce an environment that is passively received by the child.
b. Evocative (or reactive) interaction occurs when genetically influenced
characteristics draw out behavior from other people.
c. Active interaction occurs when people choose, partly based on genetic
tendencies, to put themselves in specific situations and to avoid others.
B. Behavioral genetics tries to determine how much of the differences among people in a given environment,
are due to their genes and how much to that environment.
1. 90% of the variability in heights among people in Western countries is determined by genes. This is not
necessarily true in other countries with different diets, etc.
2. Twin studies compare identical (monozygotic) and fraternal (dizygotic) twins to
determine the relative contributions of genes to variability in a behavior or
characteristic.
3. Adoption studies provide an even better way to examine heritability by studying the ways in which
characteristics of children, often twins, adopted at birth are compared to those of their adoptive parents
or siblings versus their biological parents or siblings.
C. Evolution occurs largely through the process of natural selection and at times by
accident.
1. Creatures born with characteristics that are favorable to survival (adaptations) in a given environment
are likely to survive, reproduce, and have offspring who will
survive. Barring some major environmental change, creatures with these adaptations will eventually
become dominant members of their species in that environment.