Chapter 2: The Biological Basis of Behavior
Chapter Review

Neurons: The Messengers
The Synapse
Synapses and Drugs
The Central Nervous System
The Brain
The Limbic System
The Cerebral Cortex
Hemispheric Specialization
Tools for Studying the Nervous System
The Spinal Cord
The Peripheral Nervous System
The Endocrine System
Genes, Evolution and Behavior
Behavior Genetics

This module presents the basic biological processes that are at the root of our thoughts, feelings, and actions. The body possesses two systems for coordinating and integrating behavior: the nervous system and the endocrine system.

The billions of neurons, or nerve cells, that underlie all the activity of the nervous system form a communication network that coordinates all the systems of the body and enables them to function. Neurons usually receive messages from other neurons through short fibers, called dendrites, that pick up messages and carry them to the neuron's cell body. The axon carries outgoing messages from the cell. A group of axons bundled together makes up a nerve. Some axons are covered with a myelin sheath, made up of glial cells. The myelin sheath increases neuron efficiency and provides insulation.

A typical myelinated neuron .

myelinated neuron

Neurons that carry messages from the sense organs to the brain or spinal cord are called sensory (afferent) neurons. Neurons that carry messages from the brain or spinal cord to the muscles and glands are called motor (efferent) neurons. Interneurons (association neurons carry messages from one neuron to another. When the neuron is at rest, or at its resting potential, a slightly higher concentration of negative ions exists inside the membrane surrounding the cell body than outside, so there is a negative electrical charge inside relative to outside. At rest, a neuron is in a state of polarization. When an incoming message is strong enough, the electrical charge is changed, an action potential(neural impulse) is generated, and the neuron is depolarized. Incoming messages cause graded potentials, which, when combined, may exceed the minimum threshold of excitation and make the neuron fire. After firing, the neuron goes through the absolute refractory period, when it will not fire again, and then enters the relative refractory period, when firing will only occur if the incoming message is much stronger than usual. However, according to the all-or-none law, the impulse sent by a neuron does not vary in strength.

The neural impulse --communication within the neuron.

the neural impulse

Electrical changes during the action potential.

electrical changes

Neurotransmitter molecules, released by synaptic vesicles, cross the tiny synaptic space (or cleft) between the axon terminal (or synaptic knob) of the sending neuron and the dendrite of the receiving neuron, where they latch on to a receptor site, much the way a key fits into a lock. This is how they pass on their excitatory or inhibitory messages.

Synaptic transmission – communication between neurons.

synaptic transmission

Click here to view the Major Neurotransmitters and Their Effects table

Synapses and Drugs
Certain drugs produce psychological effects by increasing or decreasing the quantity of neurotransmitters at the synapse. Other drugs work at the receptor sites, blocking the receptors or interfering with the removal or reabsorption of the neurotransmitters. Drugs that block the dopamine receptors, for example, reduce the symptoms of schizophrenia.

Experience and Neurons
The brain has plasticity, that is, it can be physically and chemically altered by experience. In a pioneering study of the influence of the environment on the brain, researchers found that rats that had been raised in a stimulating environment had more synaptic connections than rats that had been raised in cages that offered them no opportunities to explore or to manipulate objects.

Brain growth and experience.

brain growth

The billions of neurons in the brain are connected to neurons throughout the body by trillions of synapses. The nervous system is organized into two parts: the central nervous system, which consists of the brain and the spinal cord, and the peripheral nervous system, which connects the central nervous system to the rest of the body.

A schematic diagram of the divisions of the nervous system and their various subparts .

nervous  system

The Brain
The brain contains more than 90 percent of the body's neurons. Physically, the brain has three more or less distinct areas: the hindbrain, the midbrain, and the forebrain.

The hindbrain is found in even the most primitive vertebrates. It is made up of the cerebellum, the pons, and the medulla. The medulla is a narrow structure nearest the spinal cord; it is the point at which many of the nerves from the left part of the body cross to the right side of the brain and vice versa. The medulla controls such functions as breathing, heart rate, and blood pressure. The pons, located just above the medulla, connects the top of the brain to the cerebellum. Chemicals produced in the pons help maintain our sleep-wake cycle. The cerebellum is divided into two hemispheres and handles certain reflexes, especially those that have to do with balance. It also coordinates the body's actions.

The midbrain lies between the hindbrain and forebrain and is crucial for hearing and sight. The forebrain is supported by the brain stem and buds out above it, drooping somewhat to fit inside the skull. It consists of the thalamus, the hypothalamus, and the cerebral cortex. The thalamus relays and translates incoming messages from the sense receptors—except those for smell. The hypothalamus governs motivation and emotion and appears to play a role in coordinating the responses of the nervous system in times of stress.

Parts of the Brain and Their Functions

Click here to view the Parts of the Brain and Their Functions table

The Limbic System
The limbic system encompasses structures that are critical for forming memories and experiencing pleasure, as well as for various motivational and emotional activities. In evolutionary terms, the limbic system is more recent than the central core and is fully developed only in mammals. The limbic system includes the hippocampus and amygdala, as well as other structures. It appears to play a central role in times of stress. The limbic system .

the limbic system

The Cerebral Cortex
The cerebral hemispheres, located above the thalamus and hypothalamus, take up most of the room inside the skull. The outer covering of the cerebral hemispheres is known as the cerebral cortex. The cerebral hemispheres are what most people think of when they think of the brain. They are the most recently evolved portion of the brain, and they regulate the most complex behavior. Each cerebral hemisphere is divided into four lobes, delineated by deep fissures on the surface of the brain. The occipital lobe of the cortex, located at the back of the head, receives and processes visual information. The temporal lobe, located roughly behind the temples, is important to the sense of smell; it also helps us perform complex visual tasks, such as recognizing faces. The parietal lobe, which sits on top of the temporal and occipital lobes, receives sensory information, in the sensory projection areas, from all over the body and figures in spatial abilities. The ability to comprehend language is concentrated in two areas in the parietal and temporal lobes. The frontal lobe is the part of the cerebral cortex responsible for voluntary movement and attention as well as goal-directed behavior. The brain starts response messages in the motor projection areas, from which they proceed to the muscles and glands. The frontal lobe may also be linked to emotional temperament.

These four lobes are both physically and functionally distinct. Each lobe contains areas for specific motor sensory function as well as association areas. The association areas—areas that are free to process all kinds of information—make up most of the cerebral cortex and enable the brain to produce behaviors requiring the coordination of many brain areas.

The four lobes of the cerebral cortex .

cerebral cortex

Hemispheric Specialization The two hemispheres of the cerebral cortex are linked by the corpus callosum, through which they communicate and coordinate. Nevertheless, they appear to have some separate functions. The right hemisphere of the cortex excels at nonverbal and spatial tasks, whereas the left hemisphere is usually more dominant in verbal tasks such as speaking and writing. The right hemisphere controls the left side of the body, and the left hemisphere controls the right side.

The cerebral hemispheres .

cerebral hemispheres

Tools for Studying the Nervous System
In recent decades science has developed increasingly sophisticated techniques for investigating the brain and nervous system. Among the most important tools are microelectrode techniques; macroelectrode techniques (ERP); structural imaging (CAT scanning, MRI); functional imaging (EEG imaging, MEG, MSI), and tools such as PET scanning that use radioactive energy to map brain activity. Scientists often combine these techniques to study brain activity in unprecedented detail.

EEG recording of one person's brain waves; EEG electrode attachment .




The Spinal Cord
The spinal cord is a complex cable of nerves that connects the brain to most of the rest of the body. It is made up of bundles of long nerve fibers and has two basic functions: to permit some reflex movements and to carry messages to and from the brain.

The spinal cord and reflex action .

spinal cord

The second major division of the nervous system, the peripheral nervous system, carries messages to and from the central nervous system. It comprises two parts: the somatic and the autonomic nervous systems.

The Somatic Nervous System
The somatic nervous system is composed of the sensory (afferent) neurons that carry messages to the central nervous system and the motor (efferent) neurons that carry messages from the central nervous system to the skeletal muscles of the body.

The Autonomic Nervous System
The autonomic nervous system carries messages between the central nervous system and the internal organs. It is broken into two parts: the sympathetic and parasympathetic divisions. The first acts primarily to arouse the body; the second, to relax and restore the body to normal levels of arousal.

The sympathetic and parasympathetic divisions of the autonomic nervous system .

spinal cord

The endocrine system—the other communication system in the body—is made up of endocrine glands that produce hormones, chemical substances released into the bloodstream to guide such processes as metabolism, growth, and sexual development. Hormones are also involved in regulating emotional life.

The Thyroid Gland
The thyroid gland secretes thyroxin, a hormone that can reduce concentration and lead to irritability when the thyroid is overactive, and cause drowsiness and a sluggish metabolism when the thyroid is under active.

The Parathyroid Glands
Within the thyroid are four tiny pea-shaped organs, the parathyroids, that secrete parathormone to control and balance the levels of calcium and phosphate in the blood and tissue fluids. This, in turn, affects the excitability of the nervous system.

The Pineal Gland
The pineal gland is a pea-sized gland that apparently responds to exposure to light and regulates activity levels over the course of the day.

The Pancreas
The pancreas lies in a curve between the stomach and the small intestine and controls the level of sugar in the blood by secreting insulin and glucagon.

The Pituitary Gland
The pituitary gland produces the largest number of different hormones and therefore has the widest range of effects on the body's functions. The posterior pituitary is controlled by the nervous system. It produces two hormones: vasopressin, which causes blood pressure to rise and regulates the amount of water in the body's cells, and oxytocin, which causes the uterus to contract during childbirth and lactation to begin. The anterior pituitary, often called the "master gland," responds to chemical messages from the bloodstream to produce numerous hormones that trigger the action of other endocrine glands.

The Gonads
These reproductive glands—the testes in males and the ovaries in females, and, to a lesser extent, the adrenal glandssecrete androgens (including testosterone) and estrogens.

The Adrenal Glands
The two adrenal glands are located above the kidneys. Each has two parts: an outer covering, the adrenal cortex, and an inner core, the adrenal medulla. Both influence the body's responses to stress. For example, in response to a stressful situation, the pituitary gland may release beta endorphin and ACTH, which, in turn, prompt the adrenal cortex to release hormones. Meanwhile, the autonomic nervous system stimulates the adrenal medulla to secrete hormones such as epinephrine into the bloodstream.

The glands of the endocrine system .


The nature versus nurture question refers to the interactive role that heredity (nature) and environment (nurture) play in human behavior. Although no contemporary psychologist would take either a pure nature or a pure nurture view of human behavior, the extent to which many traits are influenced by genetics and environment is still debated. The related fields of behavior genetics and evolutionary psychology help psychologists explore the influence of heredity on human behavior.

Genetics is the study of how plants, animals, and people pass on traits from one generation to the next through genes. The transmission of traits is referred to as heredity. Each gene is lined up on tiny threadlike bodies called chromosomes, which are made up predominantly of deoxyribonucleic acid (DNA). Members of a gene pair can be either dominant or recessive genes. In polygenic inheritance, several genes interact to produce a certain trait.

Transmission of eye color by dominant (B) and recessive (b) genes .

eye color

Behavior Genetics
Psychologists use a variety of methods to study the relationships between genes and various behaviors. Strain studies help to determine the heritability of certain traits in inbred animals; selection studies estimate the heritability of a trait by breeding animals with other animals that have the same trait. Through family studies, scientists examine genetic influences on human behavior, whereas twin studies probe identical twins who share identical genetic makeup, as opposed to fraternal twins who are only as genetically similar as regular siblings. Adoption studies are useful in determining the influence of heredity and environment on human behavior.

In 1859 Charles Darwin proposed the theory of natural selection to account for evolution—the idea that groups of organisms change over time. In modern terms, the theory of natural selection states that organisms best adapted to their environment tend to survive, transmitting their genetic characteristics to succeeding generations, whereas organisms with less adaptive characteristics tend to disappear.

Evolutionary Psychology
Evolutionary psychology analyzes human thoughts, traits, and behaviors by examining their adaptive value from an evolutionary perspective. It has proved useful in explaining many cross-cultural commonalities in human behavior.

Social Implications
The study of behavior genetics and evolutionary psychology makes many people uneasy. With the development of amniocentesis and chorionic villus sampling, prospective parents can often detect genetic abnormalities in a fetus, leading to questions about the rights of a child versus those of the parents. Some fear that research in evolutionary psychology will undermine movements toward social equality by attempting to justify the adaptive value of certain forms of social injustice. Others fear that it will make people feel that genetics is destiny -- that who we are is written in some kind of permanent ink before we are born.

Average risk of schizophrenia among biological relatives of people with schizophrenia .

risk of schizophrenia

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