In this section, we will present an overview of the nervous system and the other major communication system of the body, the endocrine glandular system. We will also discuss the three types of neurons in the nervous system. An overview diagram of the major subdivisions of the nervous system is given in Figure 2.3. The two major parts are the central nervous system (CNS), made up of the brain and spinal cord, and the peripheral nervous system (PNS), the remainder of the nervous system throughout the body, linking the CNS with the body’s sensory receptors, muscles, and glands. One part of the peripheral nervous system, the autonomic nervous system, plays an important role in emotional experiences. We will learn how it does so once we learn more about the general nature of the nervous system and its major divisions and the body’s other major communication system, the endocrine glandular system.

There are three types of neurons in the nervous system. Interneurons, which integrate information within the CNS by communicating with one another, are only in the CNS. Interneurons also intervene within the spinal cord between sensory and motor neurons that are only in the PNS. Sensory neurons carry information to the CNS from sensory receptors (such as the rods and cones in the eyes), muscles, and glands. Motor neurons carry movement commands from the CNS out to the rest of the body. Bundles of sensory neurons are called sensory nerves. Most enter the CNS through the spinal cord, while some (from the head) enter the brain directly through holes in the cranium (the skull around the brain). Collections of the motor neurons are called motor nerves. They exit the CNS through the spinal cord or, for the head, from the cranium. To begin to understand the role that these neurons play, we’ll next discuss the CNS and the role of the spinal cord.

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Both parts of the CNS, the spinal cord and the brain, are totally encased in bone—the spinal cord within the spinal column and the brain within the cranium—for protective reasons. In addition, for further protection, both are surrounded by cerebrospinal fluid for cushioning. We will detail the spinal cord’s role in the CNS here, and then discuss the brain in the last section of this chapter.

The spinal cord spans from the stem of the brain down through the neck and the center of the spinal column. It has two main functions. First, it serves as the conduit for both incoming sensory data and outgoing movement commands to the muscles in the body. This is why a spinal cord injury, such as that suffered by actor Christopher Reeve, may result in paralysis and in difficulty breathing. Second, it provides for spinal reflexes. A spinal reflex is a simple automatic action not requiring involvement of the brain. A good example is the knee-jerk reflex in which a leg jerks forward when the knee is tapped. Only sensory neurons and motor neurons are involved in this reflex. The knee-jerk reflex might appear to have no use, but it is an example of a stretch reflex that is important to maintaining our posture and for lifting objects.

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For most spinal reflexes, however, interneurons are also involved. Sensory neurons connect with interneurons in the spinal cord, which then connect with motor neurons. The sensory information is also sent up to the brain by way of interneurons, but the brain is usually not involved. A good example of such a spinal reflex is the withdrawal reflex. When we touch something extremely hot or painful, we jerk away. These reflexes normally occur without interference from the brain so that they can occur rapidly. The brain can stop a spinal reflex by sending commands to override the reflexive response, but for obvious reasons it usually chooses not to interfere.

The brain is the control center for the entire nervous system, but it couldn’t perform this job without agents to carry out its commands and to provide information about the activities of the rest of the body and the world outside. We will next discuss the PNS, the system that provides these supportive functions for the brain.

Not only does the PNS gather information for the brain about the external environment and the body’s internal environment through the sensory nerves, it also serves as the conduit for the brain’s commands to the rest of the body through the motor nerves. To carry out these tasks, the PNS has two parts working in concert with each other—the somatic (or skeletal) nervous system and the autonomic nervous system.

The somatic (skeletal) nervous system carries sensory input from receptors to the CNS and relays commands from the CNS to the skeletal muscles to control their movement. Skeletal muscles are those that are attached to bone, such as the muscles of the arms and legs. As we have already discussed, the primary neurotransmitter for these muscles is acetylcholine. The autonomic nervous system regulates the functioning of our internal environment (glands and organs such as the stomach, lungs, and heart). The somatic nervous system is thought of as voluntary, but the autonomic system is usually thought of as involuntary, operating on automatic (hence the name, autonomic) to maintain our internal functioning such as heartbeat, respiration, and digestion. This is why we are generally unaware of what the autonomic nervous system is doing.

The autonomic system has two parts—the sympathetic nervous system and the parasympathetic nervous system. These two systems normally work together to maintain a steady internal state. However, the sympathetic nervous system is in control when we are very aroused, as in an emergency, and prepares us for defensive action. The parasympathetic nervous system takes over when the aroused state ends to return the body to its normal resting state. Some effects of each part of this dual autonomic system on the glands and muscles that they impact are shown in Table 2.2. In general, the sympathetic nervous system expends energy, and the parasympathetic nervous system conserves energy. The two systems are connected to essentially the same glands and organs, but generally lead to opposite effects. For example, the sympathetic leads to pupil dilation, accelerated heartbeat, and inhibited digestion; the parasympathetic leads to pupil contraction, slowed heartbeat, and stimulated digestion. Given these actions, the sympathetic nervous system is usually referred to as the “fight-or-flight” system; it prepares us for one of these two actions in times of emergency. In contrast, the parasympathetic nervous system has sometimes been called the “rest-and-digest” system (Dowling, 1998).

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The endocrine glandular system, the body’s other major communication system, is not part of the nervous system, but these two systems are connected in order to maintain normal internal functioning. The endocrine glandular system works with the autonomic nervous system in responding to stress, and it also plays an important role in such basic behaviors as having sex and eating and in normal bodily functions, such as metabolism, reproduction, and growth.

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The endocrine glandular system communicates through messengers in the bloodstream. Endocrine glands secrete chemical substances within the body into the bloodstream; exocrine glands (such as sweat and tear glands) secrete substances outside the body. The substances secreted by the endocrine glands are hormones. A hormone is a chemical messenger produced by the endocrine glands and carried by the bloodstream to target sites throughout the body. Some hormones (adrenalin and noradrenalin secreted by the adrenal glands) are chemically similar to some neurotransmitters (epinephrine and norepinephrine), but neurotransmitters are released at their targets (other neurons), while hormones have to travel through the bloodstream to reach their targets. For example, the male sex hormone, testosterone, travels from the male sex glands through the bloodstream to target sites in facial skin to stimulate hair growth.

The endocrine glandular system is controlled by its connection with a part of the brain called the hypothalamus (to be discussed in the next section). The hypothalamus controls the most influential gland in the endocrine system, the pituitary gland, which releases hormones essential for human growth and also releases hormones that direct other endocrine glands to release their hormones. For example, it is the pituitary gland that directs a male’s sex glands to secrete testosterone. The pituitary gland, located near the base of the brain, thus functions like the master of the endocrine system, which is why it is sometimes referred to as the “master gland.” The hypothalamus, through its control of the pituitary gland, is thus able to regulate the endocrine glandular system.

Other endocrine glands—such as the thyroid gland, the adrenal glands, the pancreas, and the sex glands—are located throughout the body. The thyroid gland, which affects our growth and maturation among other things, is located in the neck. The adrenal glands, which are involved in our metabolism and help to trigger the “fight-or-flight” response with commands from the autonomic nervous system, are situated above the kidneys. The pancreas, located between the kidneys, is involved in digestion and maintaining our blood-sugar levels. The testes (in males) and ovaries (in females) secrete sex hormones. The major endocrine glands along with their main functions are depicted in Figure 2.4.

Most of us experience anger, fear, joy, love, hate, and many other emotions. Emotions play an important part in our lives, and the autonomic nervous system, especially the sympathetic division, plays an important role in how we experience and express emotions. Think about the heightened state of arousal and the bodily feelings that accompany various emotions. For example, we may feel our blood pressure rising, our heart pounding, our body trembling, or butterflies in our stomach. Such bodily arousal and feelings stem from autonomic nervous system activity, which prepares us for our emotional reactions. But exactly what is an emotion, and how do emotions arise?

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The three components of emotion. Psychologists usually define an emotion as a complex psychological state that involves three components: (1) a physical component—a state of physiological arousal triggered by the autonomic nervous system; (2) a behavioral component—outward expression of the emotion, including facial expressions, movements, and gestures; and (3) a cognitive component—our appraisal of the situation to determine which emotion we are experiencing and how intensely we are experiencing it.

First, let’s consider the physical component of emotion. In emotional situations the autonomic nervous system increases our physiological arousal. The sympathetic nervous system goes into its “fight-or-flight” mode—our heart rate and breathing increase, our blood pressure surges, we start sweating, our pupils dilate, we begin trembling, our digestion stops, and so on. This aroused state prepares us to react emotionally to the situation, whether our reaction is to run from an attacker, hug a loved one, or laugh at a roommate’s joke. Different emotions seem to lead to subtly different patterns of autonomic nervous system arousal (Levenson, 1992). The language we use to describe different emotional states reflects the different patterns in arousal states for these emotions (Oatley & Duncan, 1994). For example, we say we are “hot under the collar” when angry, but that we have “cold feet” when afraid. These descriptions parallel differences in body temperature for these two basic emotions; we have a higher than normal body temperature when we are angry and a lower one when we are afraid.

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The behavioral component of emotion is the product of the somatic nervous system. It provides the nonverbal, expressive behavior for the emotion—our facial expressions, such as smiles and frowns, and our body language, such as clenching our fists. Some emotion researchers have proposed a facial-feedback hypothesis, which assumes that the facial muscles send messages to the brain, allowing the brain to determine which emotion is being experienced (Izard, 1990; Soussignan, 2002). For example, when we see someone we truly care for and haven’t seen for some time, we automatically start smiling; signals about this facial expression are sent to the brain to help the brain determine what emotion is being experienced. In fact, there is even evidence that watching another person’s facial expressions causes a reciprocal change in our own facial muscles (Dinberg & Thunberg, 1998). If they smile, we smile. Signals are sent not only from the facial muscles but also from the entire motor nervous system to the brain, contributing to the cognitive appraisal component of the emotion.

The cognitive component of emotion appraises the situation to determine what emotion we are experiencing. We perceive the changes in bodily arousal and behavioral responses within the situational context, and then, using the knowledge we have stored in memory about emotions, we determine what emotion we are experiencing. For example, if your hands are sweating, you feel butterflies in your stomach, your mouth is dry, and you are about to give a speech to a large crowd, then your brain will easily deduce that you are anxious.

Next, we will consider how this cognitive component interacts with the physical and behavioral components to produce our emotional experiences as we contrast some of the major theories of emotion.

Theories of emotion. Many theories have been proposed over the last century to explain emotion. We’ll consider three of them—the James-Lange theory, the Cannon-Bard theory, and the Schachter-Singer two-factor theory. To see how these theories differ, we’ll contrast them with what is referred to as the “commonsense” explanation of emotion, which proposes that the subjective experience of the emotion triggers the physiological arousal and behavioral response. For example, you are out hiking and you see a bear (an emotion-provoking stimulus). The commonsense view would say that you cognitively recognize a dangerous situation and react emotionally by realizing you are afraid. This emotional feeling of fear triggers the arousal of the autonomic nervous system—for example, your heart races and you start sweating—and a behavioral response—you back away slowly. (Don’t run from a bear. It triggers their chase instincts.) The commonsense view proposes the following order of events—you realize you are afraid, and then the physiological arousal and behavioral response follow. Your heart races and you start sweating and back away because you are afraid.

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American psychologist William James and Danish psychologist Carl Lange developed one of the earliest theories of emotion (Lange & James, 1922). They disagreed with the order of events proposed by the commonsense theory and argued that the emotional feeling does not precede, but rather follows, the physiological arousal and behavioral response. According to the James-Lange theory, physiological arousal and backing away are responses to the stimulus of seeing the bear. You then interpret these autonomic and behavioral responses as the emotion fear. You determine that you are afraid because you are sweating, your heart is racing, and you are backing away. The emotional feeling occurs after, and as a result of, the arousal and behavioral responses. The particular emotion experienced depends upon the specific arousal and response patterns.

Walter Cannon, the physiologist most responsible for discovering the functions of the autonomic nervous system, had different ideas about the role of arousal in emotion. He and another physiologist, Philip Bard, developed what is called the Cannon-Bard theory (Bard, 1934; Cannon, 1927). This theory argues that arousal patterns for different emotions are too physiologically alike to be used to determine which emotion is being experienced. Instead, they proposed that an emotion-provoking stimulus (the bear) sends messages simultaneously to the peripheral nervous system and the brain. The autonomic nervous system produces the physiological arousal responses such as our heart racing; the motor nervous system produces the behavioral response (backing away); and the brain produces the emotional feeling (fear). These three responses occur simultaneously, but independently. So, according to the Cannon-Bard theory, the arousal and behavioral responses do not cause the emotional feeling, and the emotional feeling does not cause the responses. They all happen at the same time.

Subsequent research on these two theories has not strongly supported either one but has indicated that the cognitive component may play a larger role than previously thought. These early theories did not emphasize this component. The last theory of emotion that we will discuss, Stanley Schachter and Jerome Singer’s two-factor theory, illustrates the emerging importance of the cognitive component (Schachter & Singer, 1962). Schachter and Singer agreed with the James-Lange theory that arousal was a central element in our experience of emotion, but also with the Cannon-Bard theory that physiological arousal is too similar to distinguish different emotions. According to the Schachter-Singer two-factor theory, there are two important ingredients in determining the emotion—physiological arousal and cognitive appraisal. The physiological arousal tells us how intense the emotion is, while the cognitive appraisal of the entire situation (the arousal responses such as our heart racing, the behavioral response of backing away, and the situational aspect of being alone with the bear) allows us to identify (label) the emotion, leading to the emotional feeling (fear). In the two-factor theory, cognitive appraisal precedes the emotion. This is different from the Cannon-Bard assumption that arousal and the emotion occur simultaneously. The two-factor theory is contrasted with the three earlier theories in Figure 2.5.

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Schachter and Singer’s experimental work to test their theory only provided partial support, but their theory has spurred much discussion and research on the role of cognitive appraisal in emotion. As mentioned earlier, the autonomic arousal patterns for some emotions are not the same, but some argue that these pattern differences may be too small to allow the differentiation of most emotions (Lang, 1994). However, these emotions also lead to different patterns of brain activity (Damasio et al., 2000). Could these patterns be used to differentiate emotions?

To help resolve this question, Joseph LeDoux (1996, 2000) proposes that there are different brain systems for different emotions. Some emotional responses might be the product of a brain system that operates as a reflex system without cognitive appraisal. Fear is probably such an emotion. LeDoux actually has some evidence that the fear response can be generated almost instantaneously—before higher-level cognitive appraisal of the situation could occur. Such fast responding would have survival value from an evolutionary point of view by allowing us to make quick, defensive responses to threats without having to wait on slower, cognitive appraisal.

Other emotions, however, may entail using a brain system that relies on cognitive appraisal and the use of past emotional experiences in this appraisal. This system would be responsible for more complex emotions, such as love or guilt, that do not require an instant response. In sum, this theory not only accommodates much of what we know about emotion but also provides a general framework for research on the roles of various brain structures in generating our emotions. We now turn to a discussion of these brain structures and what we presently know about them and their roles not only in emotion but also in thought, language, memory, and the many other information-processing abilities we possess.