Chapter 2: The Biological Basis of Behavior Chapter ReviewNeurons: The MessengersThe SynapseSynapses and DrugsThe Central Nervous SystemThe BrainThe Limbic SystemThe Cerebral CortexHemispheric SpecializationTools for Studying the Nervous SystemThe Spinal CordThe Peripheral Nervous SystemThe Endocrine SystemGenes, Evolution and BehaviorBehavior GeneticsThis module presents the basic biological processes that are at the root of our thoughts, feelings, andactions. The body possesses two systems for coordinating and integrating behavior: the nervoussystem and the endocrine system.The billions of neurons, or nerve cells, that underlie all the activity of the nervous system form acommunication network that coordinates all the systems of the body and enables them to function. Neuronsusually receive messages from other neurons through short fibers, called dendrites, that pick up messagesand carry them to the neurons 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 glialcells. The myelin sheath increases neuron efficiency and provides insulation.A typical myelinatedneuron .NEURONS: THE MESSENGERSNeurons 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 glandsare called motor (efferent) neurons. Interneurons (association neurons carry messages from oneneuron to another. When the neuron is at rest, or at itsresting potential, a slightly higher concentration ofnegative 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 incomingmessage is strong enough, the electrical charge is changed, an action potential(neural impulse)isgenerated, and the neuron is depolarized. Incoming messages cause graded potentials, which, whencombined, may exceed the minimum threshold of excitation and make the neuron fire. After firing, theneuron goes through the absolute refractory period, when it will not fire again, and then enters the relativerefractory 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.
Electrical changes during the action potential.
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 receivingneuron, where they latch on to a receptor site, much the way a key fits into a lock. This is how they pass ontheir excitatory or inhibitory messages.Synaptic transmission – communication between neurons.
Click here to view the Major Neurotransmitters and Their Effects tableSynapses and DrugsCertain drugs produce psychological effects by increasing or decreasing the quantity of neurotransmitters atthe synapse. Other drugs work at the receptor sites, blocking the receptors or interfering with the removal orreabsorption of the neurotransmitters. Drugs that block the dopamine receptors, for example, reduce thesymptoms of schizophrenia.Experience and NeuronsThe brain has plasticity, that is, it can be physically and chemically altered by experience. In a pioneeringstudy of the influence of the environment on the brain, researchers found that rats that had been raised in astimulating environment had more synaptic connections than rats that had been raised in cages that offeredthem no opportunities to explore or to manipulate objects.Brain growth and experience.
THE CENTRAL NERVOUS SYSTEMThe 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 brainand the spinal cord, and the peripheral nervous system, which connects the central nervous system to therest of the body.A schematic diagram of the divisions of the nervous system and their various subparts .
The BrainThe brain contains more than 90 percent of the bodys neurons. Physically, the brain has three more or lessdistinct 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 ofthe nerves from the left part of the body cross to the right side of the brain and vice versa. The medullacontrols such functions as breathing, heart rate, and blood pressure. The pons, located just above themedulla, connects the top of the brain to the cerebellum. Chemicals produced in the pons help maintain oursleep-wake cycle. Thecerebellum is divided into two hemispheres and handles certain reflexes, especiallythose that have to do with balance. It also coordinates the bodys actions.The midbrain lies between the hindbrain and forebrain and is crucial for hearing and sight. The forebrain issupported by the brain stem and buds out above it, drooping somewhat to fit inside the skull. It consists ofthe thalamus, the hypothalamus, and the cerebral cortex. The thalamus relays and translates incomingmessages from the sense receptors—except those for smell. The hypothalamus governs motivation andemotion 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 tableThe Limbic SystemThe limbic system encompasses structures that are critical for forming memories and experiencingpleasure, as well as for various motivational and emotional activities. In evolutionary terms, the limbicsystem is more recent than the central core and is fully developed only in mammals. The limbic systemincludes the hippocampus and amygdala, as well as other structures. It appears to play a central role intimes of stress. The limbic system .The Cerebral CortexThe cerebral hemispheres, located above the thalamus and hypothalamus, take up most of the room insidethe skull. The outer covering of the cerebral hemispheres is known as thecerebral cortex. The cerebralhemispheres 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 intofour lobes, delineated by deep fissures on the surface of the brain. The occipital lobe of the cortex, locatedat the back of the head, receives and processes visual information. The temporal lobe, located roughlybehind the temples, is important to the sense of smell; it also helps us perform complex visual tasks, such asrecognizing faces. The parietal lobe, which sits on top of the temporal and occipital lobes, receives sensoryinformation, in the sensory projection areas, from all over the body and figures in spatial abilities. Theability to comprehend language is concentrated in two areas in the parietal and temporal lobes. The frontallobe 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 theyproceed 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 motorsensory function as well as association areas. The association areas—areas that are free to process allkinds of information—make up most of the cerebral cortex and enable the brain to produce behaviorsrequiring the coordination of many brain areas.The four lobes of the cerebral cortex .
Hemispheric Specialization The two hemispheres of the cerebral cortex are linked by the corpuscallosum, through which they communicate and coordinate. Nevertheless, they appear to have someseparate functions. The right hemisphere of the cortex excels at nonverbal and spatial tasks, whereas theleft hemisphere is usually more dominant in verbal tasks such as speaking and writing. The right hemispherecontrols the left side of the body, and the left hemisphere controls the right side.The cerebral hemispheres .
Tools for Studying the Nervous SystemIn recent decades science has developed increasingly sophisticated techniques for investigating the brainand nervous system. Among the most important tools are microelectrode techniques; macroelectrodetechniques (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 combinethese techniques to study brain activity in unprecedented detail.EEG recording of one persons brain waves; EEG electrode attachment .
The Spinal CordThe spinal cord is a complex cable of nerves that connects the brain to most of the rest of the body. It ismade up of bundles of long nerve fibers and has two basic functions: to permit some reflex movements andto carry messages to and from the brain.The spinal cord and reflex action .
THE PERIPHERAL NERVOUS SYSTEMThe second major division of the nervous system, the peripheral nervous system, carries messages to andfrom the central nervous system. It comprises two parts: the somatic and the autonomic nervous systems.The Somatic Nervous SystemThe somatic nervous system is composed of the sensory (afferent) neurons that carry messages to thecentral nervous system and the motor (efferent) neurons that carry messages from the central nervoussystem to the skeletal muscles of the body.The Autonomic Nervous SystemThe autonomic nervous system carries messages between the central nervous system and the internalorgans. It is broken into two parts: the sympathetic and parasympatheticdivisions. The first acts primarilyto 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 .
THE ENDOCRINE SYSTEMThe endocrine system—the other communication system in the body—is made up of endocrine glands thatproduce hormones, chemical substances released into the bloodstream to guide such processes asmetabolism, growth, and sexual development. Hormones are also involved in regulating emotional life.The Thyroid GlandThe thyroid gland secretes thyroxin, a hormone that can reduce concentration and lead to irritability whenthe thyroid is overactive, and cause drowsiness and a sluggish metabolism when the thyroid is under active.The Parathyroid GlandsWithin the thyroid are four tiny pea-shaped organs, the parathyroids, that secrete parathormone to controland balance the levels of calcium and phosphate in the blood and tissue fluids. This, in turn, affects theexcitability of the nervous system.
The Pineal GlandThe pineal gland is a pea-sized gland that apparently responds to exposure to light and regulates activitylevels over the course of the day.The PancreasThe pancreas lies in a curve between the stomach and the small intestine and controls the level of sugar inthe blood by secreting insulin and glucagon.The Pituitary GlandThe pituitary gland produces the largest number of different hormones and therefore has the widest rangeof effects on the bodys functions. The posterior pituitary is controlled by the nervous system. It producestwo hormones: vasopressin, which causes blood pressure to rise and regulates the amount of water in thebodys 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 thebloodstream to produce numerous hormones that trigger the action of other endocrine glands.The GonadsThese reproductive glands—the testes in males and the ovaries in females, and, to a lesser extent, theadrenal glandssecrete androgens (including testosterone) and estrogens.The Adrenal GlandsThe two adrenal glands are located above the kidneys. Each has two parts: an outer covering, the adrenalcortex, and an inner core, the adrenal medulla. Both influence the bodys responses to stress. Forexample, 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 systemstimulates the adrenal medulla to secrete hormones such as epinephrine into the bloodstream.The glands of the endocrine system .
GENES, EVOLUTION, AND BEHAVIORThe 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 ora pure nurture view of human behavior, the extent to which many traits are influenced by genetics andenvironment is still debated. The related fields of behavior genetics and evolutionary psychology helppsychologists explore the influence of heredity on human behavior.GeneticsGenetics is the study of how plants, animals, and people pass on traits from one generation to the nextthrough genes. The transmission of traits is referred to as heredity. Each gene is lined up on tiny threadlikebodies called chromosomes, which are made up predominantly of deoxyribonucleic acid (DNA).Members of a gene pair can be either dominantor recessive genes. In polygenic inheritance, severalgenes interact to produce a certain trait.Transmission of eye color by dominant (B) and recessive (b) genes .
Behavior GeneticsPsychologists use a variety of methods to study the relationships between genes and variousbehaviors. Strain studies help to determine the heritability of certain traits in inbred animals; selectionstudies 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 twinstudies probe identical twins who share identical genetic makeup, as opposed to fraternal twins who areonly as genetically similar as regular siblings. Adoption studies are useful in determining the influence ofheredity and environment on human behavior.EvolutionIn 1859 Charles Darwin proposed the theory of natural selection to account for evolution—the idea thatgroups of organisms change over time. In modern terms, the theory of natural selection states thatorganisms best adapted to their environment tend to survive, transmitting their genetic characteristics tosucceeding generations, whereas organisms with less adaptive characteristics tend to disappear.Evolutionary PsychologyEvolutionary psychology analyzes human thoughts, traits, and behaviors by examining their adaptive value