MCAT Behavioral Sciences Review - Kaplan Test Prep 2021–2022

1.5 Influences on Behavior
A Brief History of Neuropsychology

LEARNING OBJECTIVES

After Chapter 1.5, you will be able to:

· Associate major neurotransmitters with their common functions

· Detail the links between the endocrine system and the brain

· Explain the nature vs. nurture debate and the different study types used to explore this question

Merely describing the functions of brain regions does not fully explain the wide variety of human behaviors that are possible. Other influences on behavior include chemical controls (neurotransmitters, hormones in the endocrine system), heredity, and the environment.

NEUROTRANSMITTERS

A neurotransmitter is a chemical used by neurons to send signals to other neurons; more than 100 neurotransmitters have been identified. Seven of the most important are described in this section and are summarized in Table 1.3. Some drugs mimic the action of neurotransmitters by binding to the same receptor to produce the same biological response. A drug that mimics the action of some neurotransmitter is called an agonist. Drugs can also act by blocking the action of neurotransmitters, and such drugs are called antagonists.

Acetylcholine

Acetylcholine is a neurotransmitter found in both the central and peripheral nervous systems. In the peripheral nervous system, acetylcholine is used to transmit nerve impulses to the muscles. It is the neurotransmitter used by the parasympathetic nervous system and a small portion of the sympathetic nervous system (in ganglia and for innervating sweat glands). In the central nervous system, acetylcholine has been linked to attention and arousal. In fact, loss of cholinergic neurons connecting with the hippocampus is associated with Alzheimer’s disease, an illness resulting in progressive and incurable memory loss.

Key Concept

Acetylcholine is the neurotransmitter used by the efferent limb of the somatic nervous system and the parasympathetic nervous system. Acetylcholine can act as an excitatory or inhibitory neurotransmitter in muscle cells, dependent on the type of receptor found on the cell. For example, acetylcholine will transmit an inhibitory response in cardiac muscle cells, but it can also transmit an excitatory response if acting on skeletal muscle cells. Acetylcholine within the central nervous system largely functions as an excitatory neurotransmitter.

Epinephrine and Norepinephrine

Epinephrine, norepinephrine, and dopamine are three closely related neurotransmitters known as catecholamines. Due to similarities in their molecular composition, these three transmitters are also classified as monoamines or biogenic amines. The most important thing to know about the catecholamines is that they all play important roles in the experience of emotions.

Epinephrine (adrenaline) and norepinephrine (noradrenaline) are involved in controlling alertness and wakefulness. As the primary neurotransmitter of the sympathetic nervous system, they promote the fight-or-flight response. Whereas norepinephrine more commonly acts at a local level as a neurotransmitter, epinephrine is more often secreted from the adrenal medulla to act systemically as a hormone. Low levels of norepinephrine are associated with depression; high levels are associated with anxiety and mania.

Dopamine

Dopamine is another catecholamine that plays an important role in movement and posture. High concentrations of dopamine are normally found in the basal ganglia, which help smooth movements and maintain postural stability.

Imbalances in dopamine transmission have been found to play a role in schizophrenia. An important theory about the origin of this mental illness is called the dopamine hypothesis of schizophrenia. The dopamine hypothesis argues that delusions, hallucinations, and agitation associated with schizophrenia arise from either too much dopamine or from an oversensitivity to dopamine in the brain. Although the dopamine hypothesis of schizophrenia is an important theory, it does not account for all of the findings of the disease.

Parkinson’s disease is associated with a loss of dopaminergic neurons in the basal ganglia. These disruptions of dopamine transmission lead to resting tremors and jerky movements, as well as to postural instability.

Real World

The role of dopamine in both schizophrenia and Parkinson’s disease can be seen in their treatment. Antipsychotic medications used in schizophrenia are dopamine blockers, and can cause motor disturbances (“extrapyramidal symptoms”) as a side effect. Parkinson’s disease can be treated with L-DOPA, which increases dopamine levels in the brain; an overdose of L-DOPA can lead to psychotic symptoms similar to schizophrenia.

Serotonin

Along with the catecholamines, serotonin is classified as a monoamine or biogenic amine neurotransmitter. Serotonin is generally thought to play roles in regulating mood, eating, sleeping, and dreaming. Like norepinephrine, serotonin is thought to play a role in depression and mania. An oversupply of serotonin is thought to produce manic states; an undersupply is thought to produce depression.

GABA, Glycine, and Glutamate

The neurotransmitter γ-aminobutyric acid (GABA) produces inhibitory postsynaptic potentials and is thought to play an important role in stabilizing neural activity in the brain. GABA exerts its effects by causing hyperpolarization of the postsynaptic membrane.

Glycine may be better known as one of the twenty proteinogenic amino acids, but it also serves as an inhibitory neurotransmitter in the central nervous system by increasing chloride influx into the neuron. This hyperpolarizes the postsynaptic membrane, similar to the function of GABA.

Finally, glutamate, another of the twenty proteinogenic amino acids, also acts as a neurotransmitter in the central nervous system. In contrast to glycine, however, it is an excitatory neurotransmitter.

Peptide Neurotransmitters

Studies suggest that peptides are also involved in neurotransmission. The synaptic action of these neuromodulators (also called neuropeptides) involves a more complicated chain of events in the postsynaptic cell than that of regular neurotransmitters. Neuromodulators are therefore relatively slow and have longer effects on the postsynaptic cell than neurotransmitters. The endorphins, which are natural painkillers produced in the brain, are the most important peptides to know. Endorphins (and their relatives, enkephalins) have actions similar to morphine or other opioids in the body.

THE ENDOCRINE SYSTEM

We’ve already discussed the relatively fast communication network—the nervous system—that uses chemical messages called neurotransmitters. The endocrine system is the other internal communication network in the body, and it uses chemical messengers called hormones. The endocrine system is somewhat slower than the nervous system because hormones travel to their target destinations through the bloodstream. The endocrine system is covered extensively in Chapter 5 of MCAT Biology Review, so our focus here will be on the role of certain endocrine organs on behavior.

Bridge

The entire endocrine system is covered in Chapter 5 of MCAT Biology Review.

The hypothalamus links the endocrine and nervous systems and, in addition to the roles described earlier, regulates the hormonal function of the pituitary gland. The hypothalamus and pituitary gland are spatially close to each other, and control is maintained through endocrine release of hormones into the hypophyseal portal system that directly connects the two organs, as shown in Figure 1.12.

Figure 1.12. The Hypophyseal Portal System

The pituitary gland, sometimes referred to as the “master” gland, is located at the base of the brain and is divided into two parts: anterior and posterior. It is the anterior pituitary that is the “master” because it releases hormones that regulate activities of endocrine glands elsewhere in the body. However, the anterior pituitary itself is controlled by the hypothalamus. The pituitary secretes various hormones into the bloodstream that travel to other endocrine glands located elsewhere in the body to activate them. Once activated by the pituitary, a given endocrine gland manufactures and secretes its own characteristic hormone into the bloodstream.

The adrenal glands are located on top of the kidneys and are divided into two parts: the adrenal medulla and adrenal cortex. The adrenal medulla releases epinephrine and norepinephrine as part of the sympathetic nervous system. The adrenal cortex produces many hormones called corticosteroids, including the stress hormone cortisol. The adrenal cortex also contributes to sexual functioning by producing sex hormones, such as testosterone and estrogen.

The gonads are the sex glands of the body—ovaries in females and testes in males. These glands produce sex hormones in higher concentrations, leading to increased levels of testosterone in males and increased levels of estrogen in females. These sex hormones increase libido and contribute to mating behavior and sexual function. Higher levels of testosterone also increase aggressive behavior.

GENETICS AND BEHAVIOR

Just as physical traits are inherited from parents, behavioral traits can be inherited as well. Evidence for the inherited nature of behavior comes from the fact that many behaviors are species specific. For example, many animals exhibit mating behaviors only seen within their species. Behaviors can also be bred into a species; many breeds of dog have been bred for certain traits and behaviors. Behaviors are also seen to run in families. Often times, violence and aggression are observed passing along a family line, as are mental illnesses.

Real World

Bipolar disorder is considered one of the most heritable disorders, including medical illnesses. In one study, having a monozygotic (identical) twin with bipolar disorder was associated with a 43% risk of being diagnosed with the same disorder.

Innate behavior is genetically programmed as a result of evolution and is seen in all individuals regardless of environment or experience. In contrast, other behaviors are considered learned. Learned behaviors are not based on heredity but instead are based on experience and environment. Adaptive value is the extent to which a trait or behavior positively benefits a species by influencing the evolutionary fitness of the species, thus leading to adaptation through natural selection.

Bridge

Natural selection is discussed in greater detail in Chapter 12 of MCAT Biology Review.

How much of an individual’s behavior is based on genetic makeup and how much is based on environment and experiences? This controversial question is often referred to as the nature vs. nurture question. Here, nature is the influence of inherited characteristics on behavior. Nurture refers to the influence of environment and physical surroundings on behavior. There is no easy answer to this long-debated question. An individual's behavior is not only influenced by both genetics and environment, but also by how these two factors may influence each other. For example, hereditary traits may make a certain person more likely to have an addictive personality. But, the individual would still have to be exposed to drugs, alcohol, or gambling to develop an addiction.

To determine the degree of genetic influence on behavior, researchers often use one of three methods: family studies, twin studies, and adoption studies. Family studies rely on the fact that genetically related individuals are more similar genotypically than unrelated individuals. Researchers may compare rates of a given trait among family members to rates of that trait among unrelated individuals. For example, family studies have determined that the risk of developing schizophrenia for children of schizophrenics is 13 times higher than in the general population. For siblings of a schizophrenic patient, the rate is 9 times higher. Observations such as these have led psychologists to conclude that schizophrenia has a hereditary component. Family studies are limited, however, because families share both genetics and environment. Family studies cannot distinguish shared environmental factors from shared genetic factors. For example, what if the increased rates of schizophrenia in families are a result of experiencing the same emotional climate in the home rather than genetically shared characteristics?

Twin studies, comparing concordance rates for a trait between monozygotic (MZ; identical) and dizygotic (DZ; fraternal) twins, are better able to distinguish the relative effects of shared environment and genetics. Concordance rates refer to the likelihood that both twins exhibit the same trait. MZ twins are genetically identical, sharing 100 percent of their genes, whereas DZ twins share approximately 50 percent of their genes. The assumption made by twin studies is that the two individuals in each MZ or DZ twin pair share the same environment; thus, differences between MZ and DZ twins are thought to reflect hereditary factors. Twin studies can also be used to measure genetic effects relative to environmental effects. In this version of the twin study, researchers compare traits in twins raised together versus twins raised apart. For example, one study of personality characteristics showed that MZ (identical) twins raised in separate families were still more similar than DZ (fraternal) twins raised together. Such a result offers convincing evidence for a strong genetic component to personality.

Finally, adoption studies also help us understand environmental and genetic influences on behavior. These studies compare the similarities between biological relatives and the adopted child to similarities between adoptive relatives and the adopted child. For example, researchers have found that the IQ of adopted children is more similar to their biological parents’ IQ than to their adoptive parents’ IQ. This research suggests that IQ is heritable. Criminal behavior among teenage boys shows a similar pattern of heritability.

MCAT Concept Check 1.5:

Before you move on, assess your understanding of the material with these questions.

1. Match the neurotransmitters below to their functions:

2. Which endocrine organs influence behavior? What hormones do they use, and what do they accomplish?

3. Briefly discuss the influence of nature vs. nurture on behavior.

4. In each of the study types below, what is the sample group? The control group?