Biological Psychology
Biological processes are the basis of our thoughts, feelings, and actions. All of our behaviors are kept in tune with our surroundings and coordinated with one another through the work of two interacting systems: the nervous system and the endocrine system.
Neurons: The Messengers
What types of cells are found in the nervous system?
The basic building block of the nervous system is the neuron, or nerve cell. Neurons have several characteristics that distinguish them from other cells. Neurons receive messages from other neurons through short fibers, called dendrites. A longer fiber, called an axon, carries outgoing messages from the cell. A group of axons bundled together forms a nerve or tract. Some axons are covered with a myelin sheath, made up of glial cells; this increases neuron efficiency and provides insulation.
The Neural Impulse
What "language" do neurons speak?
When a neuron is at rest, a state called the resting potential, there is a slightly higher concentration of negatively charged ions inside its membrane than there is outside. The membrane is said to be polarized, that is, the electrical charge inside it is negative relative to its outside. When an incoming message is strong enough, this electrical imbalance abruptly changes (the membrane is depolarized), and an action potential (neural impulse) is generated. Incoming messages cause graded potentials, which, when combined, may exceed the minimum threshold of excitation and make the neuron "fire.” After firing, the neuron briefly goes through the absolute refractory period, when it will not fire again, and then through the relative refractory period, when firing will occur only if the incoming message is much stronger than usual. According to the all-or-none law, every firing of a particular neuron produces an impulse of equal strength. More rapid firing of neurons is what communicates the strength of a message.
The Synapse
What happens as information moves from one neuron to the next?
Neurotransmitter molecules, released by synaptic vesicles, cross the tiny synaptic space (or cleft) between an axon terminal (or terminal button) of a sending neuron and a dendrite of a receiving neuron. Here they latch on to receptor sites, much as keys fit into locks, and pass on their excitatory or inhibitory messages. Psychologists need to understand how synapses function because neurotransmitters affect an enormous range of physical and emotional responses. Certain drugs produce psychological effects by increasing or decreasing the amount of neurotransmitters at synapses. Other drugs work on receptor sites, blocking the receptors or interfering with the removal or re-absorption of neurotransmitters.
Neural Plasticity and Neurogenesis
How does experience change the brain?
Research demonstrates that experiences in our environments can produce changes in the brain, a principle called neural plasticity. Human brains also are capable of neurogenesis, the production of new brain cells.
The Central Nervous System
The Organization of the Nervous System
How is the nervous system organized?
The nervous system is organized into two parts: the central nervous system (CNS), which consists of the brain and spinal cord, and the peripheral nervous system (PNS), made up of nerves that radiate throughout the body, linking all of the body's parts to the CNS.
The Brain
What are the major structures and areas of the brain, and what functions do they serve?
Physically, the brain has three more or less distinct areas: the central core, the limbic system, and the cerebral cortex.
The central core consists of the hindbrain, cerebellum, midbrain, thalamus and hypothalamus, and reticular formation. The hindbrain is made up of the medulla, a narrow structure nearest the spinal cord that controls breathing, heart rate, and blood pressure, and the pons, which produces chemicals that maintain our sleep-wake cycle. The medulla 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 cerebellum controls the sense of balance and coordinates the body's actions. The midbrain, which is above the cerebellum, is important for hearing and sight and is one of the places in which pain is registered. The thalamus is a relay station that integrates and shapes incoming sensory signals before transmitting them to the higher levels of the brain. The hypothalamus is important to motivation and emotional behavior. The reticular formation, which is woven through all of these structures, alerts the higher parts of the brain to incoming messages.
The Limbic system, a ring of structures located between the central core and the cerebral hemispheres, is a more recent evolutionary development than the central core. It includes the hippocampus, which is essential to the formation of new memories, and the amygdala, which together with the hippocampus governs emotions related to self-preservation. Other portions of the limbic system heighten the experience of pleasure. In times of stress, the limbic system coordinates and integrates the nervous system's response.
The cerebrum takes up most of the room inside the skull. The outer covering of the cerebral hemispheres is known as the cerebral cortex. They are the most recently evolved portions 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, 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 from all over the body and oversees in spatial abilities. Messages from sensory receptors are registered in the primary somatosensory cortex. The frontal lobe receives and coordinates messages from the other lobes and keeps track of past and future body movement. It is primarily responsible for goal-directed behavior and is key to the ability to lead a mature emotional life. The primary motor cortex is responsible for voluntary movement. 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.
Hemispheric Specialization
How are the left and right hemispheres specialized for different functions?
The two cerebral hemispheres are linked by the corpus callosum, through which they communicate and coordinate their activities. Nevertheless, each hemisphere appears to specialize in certain tasks (although they also have overlapping functions). The right hemisphere excels at visual and spatial tasks, nonverbal imagery, and the perception of emotion, whereas the left hemisphere excels at language and perhaps analytical thinking, too. The right hemisphere controls the left side of the body, and the left hemisphere controls the right side.
Tools for Studying the Brain
What methods have been developed to study the brain?
An increasingly sophisticated technology exists for investigating the brain. Among the most important tools are microelectrode techniques, macroelectrode techniques (EEG), structural imaging (CT scanning and MRI), and functional imaging (EEG imaging, MEG, and MSI). Two new functional imaging techniques, PET scanning and MRI, allow us to observe not only the structure but also the functioning of parts of the brain. Scientists often combine these techniques to study brain activity in unprecedented detail information that can help in the treatment of medical and psychological disorders.
The Spinal Cord
What does the spinal cord do? How does it work with the brain to sense events and act on them?
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. A break in the cord disrupts the flow of impulses from the brain below that point, causing paralysis.
The Peripheral Nervous System
How does the brain communicate with the rest of the body? How is the autonomic branch of the PNS involved in controlling emotions?
The peripheral nervous system (PNS) contains two types of neurons: afferent neurons, which carry sensory messages to the central nervous system, and efferent neurons, which carry messages from the CNS. Neurons involved in making voluntary movements of the skeletal muscles belong to a part of the PNS called the somatic nervous system, whereas neurons involved in governing the actions of internal organs belong to a part of the PNS called the autonomic nervous system. The autonomic nervous system is itself divided into two parts: the sympathetic division, which acts primarily to arouse the body when it is faced with threat, and the parasympathetic division, which acts to calm the body down, restoring it to normal levels of arousal.
The Endocrine System
Why are psychologists interested in hormones?
The endocrine system is the other communication system in the body. It is made up of endocrine glands that produce hormones, chemical substances released into the bloodstream to either trigger developmental changes in the body or to activate certain behavioral responses. The thyroid gland secretes thyroxin, a hormone involved in regulating the body's rate of metabolism. Symptoms of an overactive thyroid are agitation and tension, whereas an underactive thyroid produces lethargy. The parathyroids control and balance the levels of calcium and phosphate in the blood and tissue fluids. This process in turn affects the excitability of the nervous system. The pineal gland regulates activity levels over the course of the day and also regulates the sleep-wake cycle. The pancreas controls the level of sugar in the blood by secreting insulin and glucagon.
When the pancreas secretes too much insulin, the person can suffer hypoglycemia. Too little insulin can result in diabetes mellitus. Of all the endocrine glands, the pituitary gland regulates the largest number of different activities in the body. It affects blood pressure, thirst, uterine contractions in childbirth, milk production, sexual behavior and interest, and the amount and timing of body growth, among other functions. Because of its influences on other glands, it is often called the 'master gland.' The gonads, the testes in males and the ovaries in females, secrete hormones called androgens, (including testosterone) and estrogens. Testosterone has long been linked to aggressive behavior, and recent research suggests that estrogen may also play a role in aggression, as well as cognitive ability and cardiovascular health. Each of the two adrenal glands has two parts: an outer covering, the adrenal cortex, and an inner core, the adrenal medulla. Both affect our response to stress, although the adrenal cortex affects other body functions, too. One stress-related hormone of the adrenal medulla is epinephrine, which amplifies the effects of the sympathetic nervous system.
Genes, Evolution, and Behavior
How are traits passed from one generation to the next?
The related fields of behavior genetics and evolutionary psychology explore the influences of heredity on human behavior. Both are helping to settle the nature-versus-nurture debate over the relative contributions of genes and the environment to human similarities and differences. Genetics is the study of how traits are passed on from one generation to the next via genes. This process is called heredity. Each gene is lined up on tiny threadlike bodies called chromosomes, which in turn are made up predominantly of deoxyribonucleic acid (DNA). Each member of a gene pair can be either dominant or recessive. In polygenic inheritance a number of genes interact to produce a trait.
Behavior Genetics
What methods do psychologists use to study the effects of genes on behavior?
Psychologists use a variety of methods to study heritability, that is, the contribution of genes in determining variations in certain traits. Strain studies approach the problem by observing strains of highly inbred genetically similar animals, whereas selection studies try to determine the extent to which an animal's traits can be passed on from one generation to another. In the study of humans, family studies tackle heritability by looking for similarities in traits as a function of biological closeness. Also useful in studying human heritability are twin studies and adoption studies. Molecular genetics focuses on mapping the genes on the chromosomes. The Human Genome Project has produced a rough map of the genes on the 23 pair of human chromosomes.
Evolutionary Psychology
How might the process of natural selection influence human social behaviors?
The theory of evolution by natural selection states that organisms best adapted to their environment tend to survive, transmitting their genetic characteristics to succeeding generations, whereas organisms with fewer adaptive characteristics tend to die off. Evolutionary psychology analyzes human behavioral tendencies by examining their adaptive value from an evolutionary perspective. It has proved useful in helping to explain some of the commonalities in human behavior that occur across cultures.
Social Implications
What are some of the ethical issues that arise as society gains more control over genetics?
Manipulating human genes in an effort to change how people develop is a new technology that makes many people uneasy, but their concerns may be exaggerated because genes are not all-powerful. Both heredity and environment play a part in shaping most significant human behaviors and traits
|
|
