|Biological Foundations of Behavior: Evolution, Genetics, and The Brain|
This chapter covers the relationship between evolutionary theories and behavior, genetics and behavior, neural bases of behavior, the nervous system (including the structures and functions of the brain), nervous system interactions with the endocrine and immune systems, and genetic influences on behavior.
Evolutionary psychology focuses on biologically-based mechanisms that evolved as solutions to a species' problems of adaptation. Evolution is a change over time in the frequency with which particular genes occur within an interbreeding population. The cornerstone of Darwin's theory of evolution is the principle of natural selection, which posits that biologically-based characteristics that contribute to survival and reproductive success increase in the population over time because those who lack the characteristic are less likely to pass on their genes. Genetically-based traits often occur in "packages." As in the case of sickle cell disease, a factor that impairs survival may persist in a population if another trait in the package is even more important for survival.
Hereditary potential is carried within the DNA portion of the 23 pairs of chromosomes in units called genes, whose commands trigger the production of proteins that control body structures and processes. Genotype (genetic structure) and phenotype (outward appearance) are not identical, in part because some genes are dominant while others are recessive. Many characteristics are influenced by the interactions of multiple genes. Behavior geneticists study the contributions of genetic and environmental factors to psychological traits and behaviors. Adoption and twin studies are the major research methods used in an attempt to disentangle hereditary and environmental factors. Especially useful is the study of identical (monozygotic) and fraternal (dizygotic) twins who were separated early in life and raised in different environments. These studies suggest that many psychological characteristics have appreciable genetic contributions. Genetic engineering allows scientists to duplicate and alter genetic material or, potentially, to repair dysfunctional genes. These procedures promise groundbreaking advances in treatment of diseases, but they also raise momentous ethical and moral issues.
Specialized cells called neurons are the building blocks of the nervous system. Each neuron has three main parts: dendrites, which collect information from neighboring neurons and send it on to the cell body; the cell body, which contains the biochemical structures that keep the neuron alive; and the axon, which conducts electrical impulses away from the cell body to other neurons, muscles, and glands. Glial cells support neurons by holding them in place, manufacturing nutrient chemicals, and absorbing toxins and waste materials. An action potential is a sudden reversal in the neuron's membrane voltage. The shift from negative to positive voltage is called depolarization. The depolarization process occurs when the dendrites of the cell are stimulated, resulting in small shifts in the cell membrane's electrical potential, a shift called a graded potential. If the graded potential is large enough to reach the action potential threshold, an action potential occurs. Either an action potential occurs or it does not, according to the all-or-none law. When a neuron is stimulated, tiny protein structures called ion channels are activated. Sodium ion channels allow positively charged sodium ions to enter the interior of the cell, leading to the process of depolarization. Immediately after an impulse passes any point on the axon, a time period called a refractory period occurs, during which another action potential cannot occur. The myelin sheath is a tubelike insulating substance covering some axons in the brain and spinal cord.
Neurons communicate through synaptic transmission. The synapse is a tiny gap between the axon terminal and the next neuron. Chemical substances called neurotransmitters, which are stored in synaptic vesicles, carry messages across the synapse and bind to receptor sites. Once a neurotransmitter molecule binds to its receptor, it continues to activate or inhibit the neuron until deactivation occurs. One method of deactivation is reuptake, in which the transmitter molecules are taken back into the presynaptic neuron. There are many types of neurotransmitters. One involved in memory and muscle activity is acetylcholine. Various drugs function as agonists (increasing the activity of a neurotransmitter) or antagonists (decreasing neurotransmitter activity).
There are three major types of neurons in the nervous system. Sensory neurons input messages from the sense organs to the spinal cord and brain, motor neurons carry impulses from the brain and spinal cord to the muscles and organs, and interneurons perform connective or associative functions within the nervous system. The division of the nervous system containing the brain and spinal cord is called the central nervous system. The division that consists of all neurons connecting the CNS with the muscles, glands, and sensory receptors is called the peripheral nervous system. In turn, the PNS is divided into two systems. The somatic nervous system consists of sensory and motor neurons while the autonomic nervous system regulates the body's glands and involuntary functions such as breathing, circulation, and digestion. The autonomic nervous system consists of two branches. The sympathetic branch activates or arouses bodily organs while the parasympathetic branch slows down body processes. Most nerves enter and leave the CNS via the spinal cord. Some simple stimulus-response sequences such as pulling away from a hot stove typically don't involve the brain and are known as spinal reflexes.
Psychologists use a number of methods to study the brain. Neuropsychological tests measure verbal and nonverbal behaviors that are known to be affected by brain damage. Researchers may destroy neurons under controlled conditions or stimulate them with electrical current or with chemicals. The activity of large groups of neurons is often studied via an electroencephalogram (EEG). The newest tools of discovery involve brain imaging. X-ray technology used to study brain structures are called computerized axial tomography (CT) scans. Pictures of brain activity involve the use of positron emission tomography (PET) scans. A technique to measure both brain structures and functions is called magnetic resonance imaging (MRI); functional MRI (fMRI) yields snapshots of brain activity taken less than a second apart.
The brain historically has been divided into three main divisions: the hindbrain, midbrain, and forebrain. The hindbrain consists of the brain stem and cerebellum. The brain stem, which contains the medulla and the pons, is involved in life support. The medulla plays a major role in vital body functions such as heart rate and respiration. The pons is a bridge carrying nerve impulses between higher and lower levels of the nervous system. The cerebellum, attached to the brain stem above the pons, is concerned primarily with muscular coordination. The midbrain, lying just above the hindbrain, is an important relay center for the visual and auditory systems. Within the midbrain is the reticular formation, which is involved in brain arousal, sleep, and attention.
The forebrain's size and complexity distinguishes humans from lower animals. An important sensory relay station in the forebrain is the thalamus, while the hypothalamus plays a major role in motivational and emotional behavior. The limbic system helps to coordinate memory, emotion, and motivational urges. Within the limbic system are the hippocampus, which is involved in the formation and storage of memories; and the amygdala, which is linked to aggression and fear.
The outermost layer of the brain, constituting 80 percent of human brain tissue, is the cerebral cortex. Each hemisphere of the cortex is divided into the frontal, parietal, occipital, and temporal lobes, each of which is associated with particular sensory and motor functions. Lying at the rear of the frontal lobe is the motor cortex, which is involved in controlling muscles. The somatic sensory cortex receives sensory input. Two specific speech areas are also located in the cortex. Wernicke's area is involved in speech comprehension while Broca's area is involved in the production of speech. The association cortex is involved in the highest levels of mental functions. People who suffer from agnosia, the inability to identify familiar objects, often have suffered damage to their association cortex. Executive functions such as goal setting, judgment, and planning may be controlled by the prefrontal cortex. The brain is also divided into two hemispheres: the left and the right. The corpus callosum is a bridge that helps the two hemispheres communicate and work together. The location of a function primarily in a single hemisphere is known as lateralization. "Split-brain" research designed to look at the relative functions of the hemispheres involves studying the roles of the corpus callosum and the optic chiasma.
The ability of neurons to change in structure and function is known as neural plasticity. Environmental factors, particularly early in life, have notable effects on brain development. There are often critical periods during which environmental factors have their greatest (or only) effects on plasticity. A person's ability to recover from brain damage depends on several factors. Other things being equal, recovery is greatest early in life and declines with age. When neurons die, surviving neurons can sprout enlarged dendritic networks and extend axons to form new synapses. Neurons can also increase the amount of neurotransmitter substance they release and the number of receptors on postsynaptic neurons so that they are more sensitive to stimulation. Recent findings suggest that the brains of mature primates and humans are capable of producing new neurons (neurogenesis). Current advances in the treatment of neurological disorders include experiments on neuron regeneration and the injection of neural stem cells into the brain, where they find and replace diseased or dead neurons.
The endocrine system consists of numerous glands distributed throughout the body. The system conveys information via hormones, which are chemical messengers secreted by the glands into the bloodstream. The adrenal glands secrete stress hormones, which mobilize the body's immune system. When foreign substances known as antigens invade the body, the immune system produces antibodies to destroy them. Problems arise with both an underactive and an overactive immune system. An overactive response known as an autoimmune reaction results when the immune system incorrectly identifies part of the body as an enemy and attacks it.
Biological factors allow a range of effects, depending on the environment in which they function. Thus, cultural factors, learning experiences, interpersonal relations, and other environmental factors combine with biological factors to influence behavior. One might think that if the heritability of a particular trait is high, then society can have little effect on the trait. In actuality, however, if a particular society were to put pressure on everyone to conform to a particular behavior, then environmental influences would be similar for everyone; consequently, the causal weight for that behavior would shift toward biological factors.