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• The brain is the bedrock of consciousness, perception, memory, emotion, and self-awareness. The Nervous System: A Basic Blueprint • The function of the nervous system is to gather and process information, produce responses to stimuli, and coordinate the workings of different cells. Scientists divide it into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS, which includes the brain and spinal cord, receives, processes, interprets, and stores information and sends messages destined for muscles, glands, and organs. The PNS transmits information to and from the CNS by way of sensory and motor nerves.
• The peripheral nervous system consists of the somatic nervous system, which permits sensation and voluntary actions, and the autonomic nervous system, which regulates blood vessels, glands, and internal (visceral) organs. The autonomic system usually functions without conscious control, although some people can learn to heighten or suppress autonomic responses, using biofeedback techniques. • The autonomic nervous system is divided into the sympathetic nervous system, which mobilizes the body for action, and the parasympathetic nervous system, which conserves energy.
Communication in the Nervous System • Neurons are the basic units of the nervous system and are held in place by glial cells.
Each neuron consists of dendrites, a cell body, and an axon. In the peripheral nervous system, axons (and sometimes dendrites) are collected together in bundles called nerves. Many axons are insulated by a myelin sheath that speeds up the conduction of neural impulses and prevents signals in adjacent cells from interfering with one another.
Recent research has challenged the old assumption that neurons in the human central nervous system cannot be induced to regenerate or multiply. Scientists have also learned that precursor cells in brain areas associated with learning and memory continue to divide and mature throughout adulthood. A stimulating environment enhances this process of neurogenesis. • Communication between two neurons occurs at the synapse. Many synapses have not yet formed at birth. During development, axons and dendrites continue to grow as a result of both physical maturation and experience with the world, and throughout life, new learning results in new synaptic connections in the brain. Thus, the brain’s circuits are not fixed and immutable but are continually changing in response to information, challenges, and changes in the environment. • When a wave of electrical voltage (action potential) reaches the end of a transmitting axon, neurotransmitter molecules are released into the synaptic cleft. When these molecules bind to receptor sites on the receiving neuron, that neuron becomes either more or less likely to fire. The message that reaches a final destination depends on how frequently particular neurons are firing, how many are firing, what types are firing, and where they are located.
• Through their effects on neural circuits, neurotransmitters play a critical role in mood, memory, and psychological well-being. Abnormal levels of neurotransmitters have been implicated in several disorders, including depression, Alzheimer’s disease, and Parkinson’s disease. • Endorphins, which act primarily as neuromodulators that affect the action of neurotransmitters, reduce pain and promote pleasure. Endorphin levels seem to shoot up when an animal or person is afraid or is under stress. Endorphins may also be linked to the pleasures of social contact. • Hormones, produced mainly by the endocrine glands, affect and are affected by the nervous system. Psychologists are especially interested in melatonin, which promotes sleep and regulates a "biological clock" that coordinates bodily rhythms; adrenal hormones such as epinephrine and norepinephrine, which are involved in emotions, memory, and stress; and the sex hormones, which are involved in the physical changes of puberty, the menstrual cycle (estrogens and progesterone), sexual arousal (testosterone), and some nonreproductive functions—including, many researchers believe, mental functioning. Mapping the Brain • Researchers study the brain by observing patients with brain damage, by using the lesion method with animals, and by using such techniques as electroencephalograms (EEGs), positron-emission tomography (PET scans), and magnetic resonance imaging (MRI). A Tour Through the Brain • All modern brain theories assume localization of function. In the lower part of the brain, in the brain stem, the medulla controls automatic functions such as heartbeat and breathing, the pons is involved in sleeping, waking, and dreaming, and the reticular activating system (RAS)
screens incoming information and is responsible for alertness. The cerebellum contributes to balance and coordination, and may also play a role in some higher mental operations.
• The thalamus directs sensory messages to appropriate higher centers. The hypothalamus
is involved in emotion and in drives associated with survival. It also controls the operations of the autonomic nervous system and sends out chemicals that tell the pituitary gland when to "talk" to other endocrine glands. • The limbic system is involved in emotions that we share with other animals, and it contains pathways involved in pleasure. Within this system, the amygdala is responsible for evaluating sensory information and quickly determining its emotional importance, and for the initial decision to approach or withdraw from a person or situation. The hippocampus has been called the "gateway to memory" because it plays a critical role in the formation of long-term memories for facts and events.
• Much of the brain’s circuitry is packed into the cerebrum, which is divided into two cerebral hemispheres and is covered by thin layers of cells known collectively as the cerebral cortex.
The occipital, parietal, temporal, and frontal lobes of the cortex have specialized (but partially overlapping) functions. The association cortex appears to be responsible for higher mental processes. The frontal lobes, particularly areas in the prefrontal cortex, are involved in social judgment, the making and carrying out of plans, and decision making. The Two Hemispheres of the Brain
• Studies of split-brain patients, who have had the corpus callosum cut, show that the two cerebral hemispheres have somewhat different talents, a phenomenon known as lateralization. In most people, language is processed mainly in the left hemisphere, which generally is specialized for logical, symbolic, and sequential tasks. The right hemisphere is associated with spatial–visual tasks, facial recognition, the creation and appreciation of art and music, and the processing of negative emotions. In most mental activities, however, the two hemispheres cooperate as partners, with each making a valuable contribution.
Two Stubborn Issues in Brain Research • Sleep appears to be necessary not only for bodily restoration but also for normal brain function. During sleep, periods of rapid eye movement, or REM, alternate with non-REM sleep. Dreams are reported most often during the REM periods. Freud argued that dreams arise because of unconscious wishes and longings, whereas many contemporary sleep researchers believe that dreams express the conscious conflicts of waking life. Biological theories, however, emphasize the physiological origins of dreams. For example, the activation-synthesis theory holds that dreams occur when the cortex tries to make sense of spontaneous neural firing initiated in the pons.
• Sex differences have been observed in anatomical and biochemical studies of animal brains. Sex differences in human brains, however, have been more elusive, and there is controversy about their existence and their meaning. Biological differences do not necessarily explain behavioral ones, and sex differences in experience could affect brain organization rather than the other way around.
• In evaluating research on the brain and behavior, it is important to remember that findings about the brain are most illuminating when they are integrated with psychological and cultural ones.
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