CHAPTER 3
3.1 The nervous system is the interacting network of nerve cells that underlies all psychological activity. Neurons are the basic units of the nervous system. Sensory (afferent) neurons carry sensory information from sensory receptors to the brain. Motor (efferent) neurons transmit commands from the brain to the glands and muscles of the body. Interneurons connect neurons with one another. Neurons generally have a cell body, dendrites which receive information from other neurons, an axon that carries information down the neuron, and terminal branches which trasmit information to adjacent neurons. Neurons connect at synapses.
3.2 When a neuron is at rest (its resting potential of -70 mVs), it is polarized, with a negative charge inside the cell membrane and a positive charge outside. When a neuron is stimulated by another neuron, its cell membrane is either depolarized or hyperpolarized. The spreading voltage changes along the cell membrane that occur as one neuron is excited by other neurons are called graded potentials. If the cell membrane is depolarized by enough graded potentials (to about
-50 mVs), the neuron will fire, sending the information down its axon to potentially transmit to other neurons. This depolarization process is called an action potential, or nerve impulse.
3.3 Within the terminal buttons of the presynaptic neuron are neurotransmitters, such as glutamate, GABA, dopamine, serotonin, acetylcholine, and endorphins. Neurotransmitters transmit information from one neuron to another as they are released into the synapse from the synaptic vesicles. They bind with receptors in the membrane of the postsynaptic neuron, which produces graded potentials that can either excite or inhibit the postsynaptic neuron from firing. Levels of neurotransmitters affect behavior and cognitions such as sleep and memory. Medications can affect neurotransmitter levels in a number of ways.
3.4 The nervous system consists of the central nervous system (CNS: the brain and spinal cord) and the peripheral nervous system (PNS: neurons in the rest of the body). Neurons of the PNS carry messages to and from the CNS. The PNS has two subdivisions: the somatic nervous system and the autonomic nervous system. The somatic nervous system consists of sensory neurons that carry sensory information to the brain and motor neurons that direct mostly voluntary actions of skeletal muscles. The autonomic nervous system controls basic, mostly involuntary life processes such as the beating of the heart, workings of the digestive system, and breathing. It consists of two parts, the sympathetic nervous system, which is activated primarily in response to threats (but is also involved in general emotional arousal) readying the body for flight or fight, and the parasympathetic nervous system, which is involved in more routine activities such as maintaining the body’s energy resources and restoring the system to an even keel following sympathetic activation.
3.5 Scientists can study the brain by using brain scans. EEGs read electrical activity (brain waves), PET scans show activity in different regions, and CT scans and MRIs are primarily used to identify tumors and other structural abnormalities. The central nervous system (CNS) consists of the brain and spinal cord. The spinal cord carries out reflexes (automatic motor responses), transmits sensory information to the brain, and transmits messages from the brain to the muscles and organs. Each of its 31 paired segments controls sensation and movement in a different part of the body. Because the spinal cord is the source of communication between the brain and the body, severance of the cord results in paralysis at all levels below the injury.
3.6 The hindbrain includes the medulla oblongata, the cerebellum, and parts of the
reticular formation. The medulla regulates vital physiological functions, such as heartbeat, circulation, and respiration, and forms a link between the spinal cord and the rest of the brain. The cerebellum is the brain structure involved in movement (in particular, fine motor movements), but parts of it also appear to be involved in learning and sensory discrimination. The reticular formation is most centrally involved in consciousness and arousal. The midbrain consists of the tectum and tegmentum. The tectum is involved in orienting to visual and auditory stimuli. The tegmentum is involved in, among other things, movement and arousal.
3.7 The subcortical forebrain consists of the hypothalamus, thalamus, the limbic system, and the basal ganglia. The hypothalamus helps regulate a wide range of behaviors, including eating, sleeping, sexual activity, and emotional experience. Among its other functions, the thalamus processes incoming sensory information and transmits this information to higher brain centers. The limbic system includes the septal area, amygdala, and hippocampus. The precise functions of the septal area are unclear, although it appears to be involved in learning to act in ways that avoid pain and produce pleasure. The amygdala is crucial to the experience of emotion. The hippocampus plays an important role in committing new information to memory. Basal ganglia structures are involved in the control of movement and also play a part in “automatic” responses and judgments that may normally require little conscious attention.
3.8 The cerebral cortex is convoluted and comprises 80 percent of the brain’s mass. The “hills” are referred to as gyri and the valleys as sulci. It includes primary areas, which usually process raw sensory data (except in the frontal lobes), and association areas, which are involved in complex mental processes such as perception and symbolic thinking. The cortex consists of the right and left hemispheres, each of which has four lobes. The occipital lobes are involved in vision. The parietal lobes are involved in the sense of touch, perception of movement, and location of objects in space. The frontal lobes serve a variety of functions, such as coordinating and initiating movement, attention, planning, social skills, abstract thinking, memory, and aspects of personality. Sections of the temporal lobes are important in hearing, language, and recognizing objects by sight. Damage to Broca’s area, located in the left frontal lobe causes Broca’s aphasia (difficulty speaking language). Damage to Wernicke’s area, located in the left temporal lobe, causes Wernicke’s aphasia (difficulty understanding language).
3.9 Some psychological functions are lateralized, or processed primarily by one hemisphere.
Although both hemispheres are involved in the processing of all information, the left is more involved in processing verbal and analytic information, and the right is more involved in processing nonlinguistic functions, such as recognizing faces and forming visual maps. The corpus collosum is responsible for communication between the two hemispheres. Split-brain studies (on patients in whom the corpus collosum has been severed) have provided a wealth of information about lateralization. Although the differences tend to be relatively small, female brains tend to show less lateralization than males.
3.10 Positive psychology defines happiness as subjective feelings of well-being which appear to involve activation of the frontal cortex. Frequent feelings of well-being are associated with career success, more satisfactory and stable romantic relationships, and improved physical health. Psychologists interested in genetics study the influence of genetic blueprints (genotypes) on observable qualities (phenotypes). Genes are arranged along chromosomes and consist of two alleles (one each from the biological mother and father). Usually dominant alleles (R) are dominant over recessive (r) alleles. Behavioral genetics research often involves comparing monozygotic (MZ) twins (who share 100% of their genetic material) and dizygotic (DZ) twins (who share about 50% of their genetic material). Research in behavioral genetics suggests that a surprisingly large percent of the variation among individuals on psychological attributes such as intelligence and personality reflects genetic influences, which interact with environmental variables in very complex ways. Heritability refers to the proportion of variability among individuals on an observed characteristic (phenotypic variance) that can be accounted for by genetic variability (genotypic variance).
3.11 Evolution refers to a change in gene frequencies over many generations. Evolutionary theory examines the adaptive significance of human and animal behavior. Known for his conceptualization of evolutionary theory, Charles Darwin proposed natural selection as the mechanism through which changes in organisms’ appearance and behavior change over time. As evolutionary theory has taken root, its applications to a number of phenomena within cognitive psychology (e.g., language acquisition) and other areas within psychology have flourished.
3.12 The design of the human nervous system, like that of other animals, reflects its evolution. Early precursors to the first vertebrates (animals with spinal cords) probably reacted with reflexive responses to environmental stimulation at specific points of their bodies. The most primitive vertebrate brain, or brain stem, included a forebrain (specialized for sensing nearby stimuli, notably smells and tastes), a midbrain (specialized for sensation at a distance, namely vision and hearing), and a hindbrain (specialized for control of movement). This rough division of labor persists in contemporary vertebrates, including humans. The forebrain of humans and other contemporary vertebrates includes an expanded cerebrum, with a rich network of cells comprising its outer layers or cortex, which allows much more sophisticated sensory, cognitive, and motor processes. While a sheep has been successfully cloned (duplicated in genetic material), the system has not been perfected; thus the clone was recently euthanized because of health problems. Human cloning has not yet been successful and poses a number of ethical considerations.