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Chapter 2© Jones and Bartlett Publishers. NOT FOR SALE OR DISTRIBUTION Physiology To understand the stress response, we must possess of Stress a fundamental knowledge ans Selye’s discovery of a direct relationship between not only of psychology chronic stress and the excessive wear and tear throughout the body laid the foundation for a clearer understand- but of physiology as well. Hing of how physiological systems work in an extremely —George Everly © Jones and Bartlett Publishers. NOT FOR SALE OR DISTRIBUTION Physiology of Stress complex and integrative way. Perhaps be- Cingulate gyrus cause of this discovery and the fact that Neocortex physical deterioration is so noticeable, much attention has been directed toward the Level Frontal lobe physiology of stress. This chapter will take Neocortical you through some basic concepts that ex- plain the physiological dynamics involved Septal with the stress response—specifically, the area immediate, intermediate, and prolonged Hypothalamus Limbic System Thalamus effects on the body. These processes will be Pituitary gland explained in terms of “pathways,” which Amygdala set in action the systematic and integrative Hippocampus steps of the stress response. Because physi- Spinal cord Reticular formation ology involves specific nomenclature out- Level side the realm of your everyday vocabulary, Vegetative you may find the nature of this chapter to be very specific and its contents very de- FIGURE 2.1 Three levels of the human brain: vegetative tailed. Most likely it will merit more than level, limbic system, and neocortical level. one reading to fully grasp, understand, and appreciate how the body responds to stress. The importance of a strong familiarity with The Central Nervous System human physiology as influenced by stressful stimuli The nervous system can be divided into two parts: the becomes evident when the necessary steps are taken to central nervous system (CNS), which consists of the effectively deal with the symptoms they produce, espe- brain and spinal cord, and the peripheral nervous system cially when using relaxation techniques. For example, (PNS), comprising all neural pathways to the extremities. it is important to know how the body functions when The human brain is further divided into three levels: the using specific imagery, visualization, music therapy, vegetative level, the limbic system, and the neocortical and biofeedback. level (FIG. 2.1 ᮡ ). In many circles, this topic of study is referred to as psy- The Vegetative Level chophysiology. This term reflects the fact that a sensory stimulus that prompts the stress response must be The lowest level of the brain consists of both the reticu- processed at the mental level before it can cascade down lar formation and the brain stem. The reticular forma- one or more physiological pathways. In other words, the tion, or more specifically the fibers that make up the term psychophysiology suggests that there is a mind- reticular activating system (RAS), is the link connect- body relationship and supports the theory that many dis- ing the brain to the spinal cord. Several stress physiolo- eases and illnesses are psychosomatic, meaning that their gists believe that it is the bridge joining the mind and the origins lie in the higher brain centers. Although the mind-body dualism suggested by Descartes is no longer Psychophysiology: A field of study based on the a viable model for a complete understanding of human principle that the mind and body are one, where physiology, to hold an appreciation of the “whole per- thoughts and perceptions affect potentially all as- son” we must first examine the parts to understand how pects of physiology. they connect to that whole. Central nervous system (CNS): Consists of the brain, spinal column, and peripheral nervous system Three systems are directly involved with the physiology (PNS), comprising all neural pathways to the of stress: the nervous system, the endocrine system, and extremities. the immune system, all of which can be triggered by per- Reticular activating system (RAS): The neural ceived threats. Because the immune system is so closely fibers that link the brain to the spinal column. linked to the disease process, it will be dealt with sepa- rately in Chapter 3. 35 Chapter 2 © Jones and Bartlett Publishers. NOT FOR SALE OR DISTRIBUTION body as one; this organ functions as a communications (ADH) or vasopressin; and (4) it stimulates the thyroid link between the mind and the body. The brain stem, gland to produce thyroxine. All of these will be dis- consisting of the pons, medulla oblongata, and mes- cussed in greater detail later. encephalon, is responsible for involuntary functions of the human body, such as heartbeat, respiration, and vaso- The Neocortical Level motor activity. It is considered the automatic-pilot con- The neocortex is the highest and most sophisticated level trol center of the brain, which assumes responsibility for of the brain. It is at this level that sensory information is keeping the vital organs and vegetative processes func- processed (decoded) as a threat or a nonthreat and where tioning at all times. This level is thought to be the most cognition (thought processes) takes place. Housed within primitive section of the human brain, as this portion is the neocortex are the neural mechanisms allowing one to similar to those of all other mammals. employ analysis, imagination, creativity, intuition, logic, memory, and organization. It is this highly developed The Limbic System area of brain tissue that is thought to separate humans The second or mid-level portion of the brain is called the from all other species. limbic system. The limbic system is the emotional con- As Figure 2.1 illustrates, the positions of these structures trol center. Several tissue centers in this level are directly are such that a higher level can override a lower level of responsible for the biochemical chain of events that con- the brain. Thus, conscious thought can influence emo- stitute the stress response Cannon observed. The limbic tional response, just as conscious thought can intercede system consists of the thalamus, the hypothalamus, the in the involuntary control of the vegetative functions to amygdala, and the pituitary gland, also known as the control heart rate, ventilation, and even the flow of master endocrine gland. These four glands work in uni- blood. This fact will become important to recognize son to maintain a level of homeostasis within the body. when learning coping skills and relaxation techniques For example, it is the hypothalamus that controls ap- designed to override the stress response and facilitate petite and body-core temperature. The hypothalamus physiological homeostasis. also appears to be the center that registers pain and plea- Separate from the CNS is a network of neural fibers that sure; for this reason it is often referred to as the seat of feed into the CNS and work in close collaboration with emotions. The combination of these functions in the hy- it. This neural tract, the peripheral nervous system pothalamus may explain why hunger decreases when (PNS), comprises two individual networks. The first is body-core temperature increases in extreme ambient the somatic network, a bidirectional circuit responsible heat, or why appetite diminishes when you are ex- for transmitting sensory messages along the neural path- tremely worried. Research evidence is clear that fear is ways between the five senses and the higher brain cen- first registered in the amygdala. When a threat is en- ters. These are called the efferent (toward periphery) countered, the hypothalamus carries out four specific and afferent (toward brain) neural pathways. The sec- functions: (1) it activates the autonomic nervous system; ond branch of the PNS is called the autonomic nervous (2) it stimulates the secretion of adrenocorticotropic hor- system (ANS). The ANS regulates visceral activities mone (ACTH); (3) it produces antidiuretic hormone and vital organs, including circulation, digestion, respi- ration, and temperature regulation. It received the name autonomic because this system can function without con- scious thought or voluntary control, and does so most, if not all, of the time. limbic system: The mid-level of the brain, including the hypothalamus and amygdala, which is thought to Research conducted by endocrinologist Bruce McEwen be responsible for emotional processing. indicates that initially a stressful encounter is etched into Autonomic nervous system: Often referred to as the the memory bank (so as to avoid it down the road), but automatic nervous system, the ANS consists of the that repeated episodes of stress decrease memory by sympathetic (arousal) and parasympathetic (relaxed) weakening hippocampal brain cells. Chronic stress is nervous systems. This part of the central nervous sys- thought to wither the fragile connection between neurons tem requires no conscious thought; actions such as in this part of the brain, resulting in “brain shrinkage.” breathing and heart rate are programmed to function automatically. Until recently it was believed that, unlike the voluntary somatic system involved in muscle movement, the ANS 36 © Jones and Bartlett Publishers. NOT FOR SALE OR DISTRIBUTION Physiology of Stress Parasympathetic Sympathetic Division Division Pupils constricted Pupils dilated Salivary Salivary secretion secretion decreased increased Heart rate increased Heart rate • decreased • • • Bronchioles • • constricted • • Bronchioles dilated Intestinal • • secretions
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