5 Steps to a 5 AP Psychology, 2010-2011 Edition (20 page)

Your extraordinarily complex brain is composed of trillions of neurons and glial cells.
Glial cells
guide the growth of developing neurons, help provide nutrition for and get rid of wastes of
neurons
, and form an insulating sheath around neurons that speeds conduction. The neuron is the basic unit of structure and function of your nervous system. Neurons perform three major functions: receive information, process it, and transmit it to the rest of your body. Three major regions of a neuron enable the cell to communicate with other cells (see
Figure 7.3
). The
cell body
(a.k.a. cyton or soma) contains cytoplasm and the
nucleus, which directs synthesis of such substances as neurotransmitters. The
dendrites
are branching tubular processes capable of receiving information. The
axon
emerges from the cyton as a single conducting fiber (longer than a dendrite) which branches and ends in tips called
terminal buttons
, axon terminals, or synaptic knobs. The axon is usually covered by an insulating
myelin sheath
(formed by glial cells).
Neurogenesis
, the growth of new neurons, takes place throughout life.

Figure 7.3 Typical neurons
.

Neurotransmitters
are chemicals stored in structures of the terminal buttons called synaptic vesicles. Different neurotransmitters have different chemical structures and perform different functions. For example,
acetylcholine (ACh)
causes contraction of skeletal muscles, helps regulate heart muscles, is involved in memory, and also transmits messages between the brain and spinal cord. Lack of ACh is associated with Alzheimer’s disease.
Dopamine
stimulates the hypothalamus to synthesize hormones and affects alertness and movement. Lack of dopamine is associated with Parkinson’s disease; too much dopamine is associated with schizophrenia.
Glutamate
is a major excitatory neurotransmitter involved in information processing throughout the cortex and especially memory formation in the hippocampus. Both schizophrenia and Alzheimer’s may involve glutamate receptors.
Serotonin
is associated with sexual activity, concentration and attention, moods, and emotions. Lack of serotonin is associated with depression. Opioid peptides such as
endorphins
are often considered the brain’s own pain killers.
Gamma-aminobutyric acid (GABA)
inhibits firing of neurons. Benzodiazepine (Valium) and anticonvulsant drugs increase activity of GABA. Huntington’s disease is associated with insufficient GABA-producing neurons in parts of the brain involved in coordination of movement. Seizures are associated with malfunctioning GABA systems. Other chemicals, such as drugs, can interfere with the action of neurotransmitters.
Agonists
may mimic a neurotransmitter and bind to its receptor site to produce the effect of the neurotransmitter.
Antagonists
block a receptor site inhibiting the effect of the neurotransmitter or agonist.

Figure 7.4 Action potential
.

All your behavior begins with the actions of your neurons. A neuron gets incoming information from its receptors spread around its dendrites. That information is sent to its cell body, where it’s combined with other incoming information. Neural impulses are electrical in nature along the neuron. The neuron at rest is more negative inside the cell membrane relative to outside of the membrane. The neuron’s resting potential results from the selective permeability of its membrane and the presence of electrically charged particles called ions near the inside and outside surfaces of the membrane in different concentrations. When sufficiently stimulated (to threshold), a net flow of sodium ions into the cell causes a rapid change in potential across the membrane, known as the
action potential
(see
Figure 7.4
). If stimulation is not strong enough, your neuron doesn’t fire. The strength of the action potential is constant whenever it occurs. This is the “all-or-none principle.”

The wave of depolarization and repolarization is passed along the axon to the terminal buttons, which release neurotransmitters. Spaces between segments of myelin are called nodes of Ranvier. When the axon is myelinated, conduction speed is increased since depolarizations jump from node to node. This is called
saltatory conduction
. Chemical neurotransmitters
are released into the
synapse
where they attach to specific receptor sites on membranes of dendrites of your postsynaptic neurons, like a key fitting into the tumbler of a lock (the lock and key concept). Some of your synapses are
excitatory
, the neurotransmitters cause the neuron on the other side of the synapse to generate an action potential (to fire); other synapses are
inhibitory
, reducing or preventing neural impulses. The sum of all excitatory and inhibitory inputs determines whether your next neuron will fire and at what rate. The constant flow of these neurochemical impulses gives your behavior its amazing complexity. It regulates your metabolism, temperature, and respiration. It also enables you to learn, remember, and decide.

The simplest form of your behavior, called a
reflex
, involves impulse conduction over a few (perhaps three) neurons. The path is called a
reflex arc
. Sensory or
afferent neurons
transmit impulses from your
sensory receptors
to the spinal cord or brain.
Interneurons
, located entirely within your brain and spinal cord, intervene between sensory and motor neurons. Motor or
efferent neurons
transmit impulses from your sensory or interneurons to muscle cells that contract or gland cells that secrete. Muscle and gland cells are called
effectors
. Examples of your reflexes include your pupillary reflex, knee jerk, sneezing, and blinking. Neural impulses travel one way along the neuron from dendrites to axons to terminal buttons, and among neurons from the receptor to the effector.

Your endocrine system interacts with your nervous system to regulate your behavior and body functions. Your
endocrine system
consists of glands that secrete chemical messengers called
hormones
into your blood. The hormones travel to target organs where they bind to specific receptors. Endocrine glands include the pineal gland, hypothalamus, and pituitary gland in your brain; the thyroid and parathyroids in your neck; the adrenal glands atop your kidneys; pancreas near your stomach; and either testes or ovaries.

Why do you behave the way you do? To what extent is your behavior determined by your heredity? To what extent is it determined by your life history or environment? The
nature-nurture controversy
deals with the extent to which heredity and the environment each influence behavior.
Evolutionary psychologists
study how natural selection favored behaviors that contributed to survival and spread of our ancestors’ genes, and may currently contribute to our survival into the next generations. Evolutionary psychologists look at universal behaviors shared by all people. They look at behaviors conserved across related species to understand how we are adapted to maximize our success in our environments. Charles Darwin pointed out the similarities of the expressions of emotions in people and other animals, suggesting that expressions shared across cultures and species are biologically determined.

Behavioral geneticists
study the role played by our genes and our environment in mental ability, emotional stability, temperament, personality, interests, etc.; they look at the causes of our individual differences. Your genes predispose your behavior. Studies of twins have been helping to separate the contributions of heredity and environment.
Identical twins
are two individuals who share all of the same genes/heredity because they develop from the same fertilized egg or
zygote
; they are
monozygotic twins. Fraternal twins
are siblings that share about half of the same genes because they develop from two different fertilized eggs or zygotes; they are
dizygotic twins. Heritability
is the proportion of variation among individuals in a population that is due to genetic causes. For the special case of identical twins, you might want to say that the heritability for traits of identical twins is zero, but that is not exactly correct. Like evolution, heritability is a concept applied to the population rather than the individual. When twins grow up in the same environment, the extent to which behaviors of monozygotic twins are behaviorally more similar than dizygotic twins reveals the contribution of heredity to behavior. Schizophrenia and general intelligence are more similar in monozygotic twins than dizygotic twins. If monozygotic twins are separated at birth and raised in different environments (adoption studies), behavioral differences may reveal the contribution of environment to behavior; similarities may reveal the contribution of heredity.

Adoption studies assess genetic influence by comparing resemblance of adopted children to both their adoptive and biological parents. The children must have been adopted as infants without contact with their biological parents. If the children resemble their biological parents, but not their adoptive families, with respect to a given trait,
researchers infer a genetic component for that trait. Such constellations of behaviors as alcoholism, schizophrenia, and general intelligence have shown both genetic and environmental components.

Transmission of hereditary characteristics is achieved by biological processes, including formation of sex cells, fertilization, embryonic development, and protein synthesis. Each DNA segment of a
chromosome
that determines a trait is a
gene
. Chromosomes carry information stored in genes to new cells during reproduction. Normal human body cells have 46 chromosomes, except for eggs and sperms that have 23 chromosomes. Males have 44 chromosomes, plus X and Y. Females have 44 chromosomes, plus X and X. At fertilization, 23 chromosomes from the sperm unite with 23 chromosomes from the egg to form a zygote with 46 chromosomes. If the male contributes an X chromosome, the baby is female; if the male contributes a Y chromosome, the baby is male. The presence of a Y chromosome makes the baby a male. All of the cells of the embryo/baby have the same 23 pairs of chromosomes, which carry genes for the same traits. Fertilization that includes a sperm or egg with the wrong number of chromosomes results in a zygote, and subsequently an individual, with chromosomal abnormalities. Turner syndrome females have only one X sex chromosome (XO). Girls with
Turner syndrome
are typically short with a webbed neck, lack ovaries, and fail to develop secondary sex characteristics at puberty. Although usually of normal intelligence, they typically evidence specific cognitive deficits in arithmetic, spatial organization, and visual form perception.
Klinefelter’s syndrome
males arise from an XXY zygote. The syndrome becomes evident at puberty when male secondary sex characteristics fail to develop, but breast tissue does. Klinefelter’s males tend to be passive. The presence of three copies of chromosome-21 results in the expression of
Down syndrome
. Down syndrome individuals are typically mentally retarded and have a round head, a flat nasal bridge, a protruding tongue, small round ears, a fold in the eyelid, and poor muscle tone and coordination.