Back 49 The Autonomic Nervous System and the Hypothalamus Susan Iversen Leslie Iversen Clifford B. Saper WHEN WE ARE FRIGHTENED our heart races, our breathing becomes rapid and shallow, our mouth becomes dry, our muscles tense, our palms become sweaty, and we may want to run. These bodily changes are mediated by the autonomic nervous system, which controls heart muscle, smooth muscle, and exocrine glands. The autonomic nervous system is distinct from the somatic nervous system, which controls skeletal muscle. As we shall learn in the next chapter, even though the neural control of emotion involves several regions, including the amygdala and the limbic association areas of the cerebral cortex, they all work through the hypothalamus to control the autonomic nervous system. The hypothalamus coordinates behavioral response to insure bodily homeostasis, the constancy of the internal environment. The hypothalamus, in turn, acts on three major systems: the autonomic nervous system, the endocrine system, and an ill-defined neural system concerned with motivation. In this chapter we shall first examine the autonomic nervous system and then go on to consider the hypothalamus. In the next two chapters, we shall examine emotion and motivation, behavioral states that depend greatly on autonomic and hypothalamic mechanisms. P.961 The Autonomic Nervous System Is a Visceral and Largely Involuntary Sensory and Motor System In contrast to the somatic sensory and motor systems, which we considered in Parts IV and V of this book, the autonomic nervous system is a visceral sensory and motor system. Virtually all visceral reflexes are mediated by local circuits in the brain stem or spinal cord. Although these reflexes are regulated by a network of central autonomic control nuclei in the brain stem, hypothalamus, and forebrain, these visceral reflexes are not under voluntary control, nor do they impinge on consciousness, with few exceptions. The autonomic nervous system is thus also referred to as the involuntary motor system, in contrast to the voluntary (somatic) motor system. The autonomic nervous system has three major divisions: sympathetic, parasympathetic, and enteric. The sympathetic and parasympathetic divisions innervate cardiac muscle, smooth muscle, and glandular tissues and mediate a variety of visceral reflexes. These two divisions include the sensory neurons associated with spinal and cranial nerves, the preganglionic and postganglionic motor neurons, and the central nervous system circuitry that connects with and modulates the sensory and motor neurons. The enteric division has greater autonomy than the other two divisions and comprises a largely self-contained system, with only minimal connections to the rest of the nervous system. It consists of sensory and motor neurons in the gastrointestinal tract that mediate digestive reflexes. The American physiologist Walter B. Cannon first proposed that the sympathetic and parasympathetic divisions have distinctly different functions. He argued that the parasympathetic nervous system is responsible for rest and digest, maintaining basal heart rate, respiration, and metabolism under normal conditions. The sympathetic nervous system, on the other hand, governs the emergency reaction, or fight-or-flight reaction. In an emergency the body needs to respond to sudden changes in the external or internal environment, be it emotional stress, combat, athletic competition, severe change in temperature, or blood loss. For a person to respond effectively, the sympathetic nervous system increases output to the heart and other viscera, the peripheral vasculature and sweat glands, and the piloerector and certain ocular muscles. An animal whose sympathetic nervous system has been experimentally eliminated can only survive if sheltered, kept warm, and not exposed to stress or emotional stimuli. Such an animal cannot, however, carry out strenuous work or fend for itself; it cannot mobilize blood sugar from the liver quickly and does not react to cold with normal vasoconstriction or elevation of body heat. Figure 49-1 Anatomical organization of the somatic and autonomic motor pathways. A. In the somatic motor system, effector motor neurons in the central nervous system project directly to skeletal muscles. B. In the autonomic motor system, the effector motor neurons are located in ganglia outside the central nervous system and are controlled by preganglionic central neurons. The relationship between the sympathetic and parasympathetic pathways is not as simple and as independent as suggested by Cannon, however. Both divisions are tonically active and operate in conjunction with each other and with the somatic motor system to regulate most behavior, be it normal or emergency. Although several visceral functions are controlled predominantly by one or the other division, and although both the sympathetic and parasympathetic divisions often exert opposing effects on innervated target tissues, it is the balance of activity between the two that helps maintain an internal stable environment in the face of changing external conditions. The idea of a stable internal environment in the face of changing external conditions was first proposed in the nineteenth century by the French physiologist Claude Bernard. This idea was developed further by Cannon, who put forward the concept of homeostasis as the complex P.962 physiological mechanisms that maintain the internal milieu. In his classic book The Wisdom of the Body published in 1932, Cannon introduced the concept of negative feedback regulation as a key homeostatic mechanism and outlined much of our current understanding of the functions of the autonomic nervous system. Figure 49-2 Anatomical organization of the sympathetic preganglionic and postganglionic axons. (Adapted from Loewy and Spyer 1990.) If a state remains steady, it does so because any change is automatically met by increased effectiveness of the factor or factors that resist the change. Consider, for example, thirst when the body lacks water; the discharge of adrenaline, which liberates sugar from the liver when the concentration of sugar in the blood falls below a critical point; and increased breathing, which reduces carbonic acid when the blood tends to shift toward acidity. Cannon further proposed that the autonomic nervous system, under the control of the hypothalamus, is an important part of this feedback regulation. The hypothalamus regulates many of the neural circuits that mediate the peripheral components of emotional states: changes in heart rate, blood pressure, temperature, and water and food intake. It also controls the pituitary gland and thereby regulates the endocrine system. Each of the Three Divisions of the Autonomic Nervous System Has a Distinctive Anatomical Organization The Motor Neurons of the Autonomic Nervous System Lie Outside the Central Nervous System In the somatic motor system the motor neurons are part of the central nervous system: They are located in the spinal cord and brain stem and project directly to skeletal muscle. In contrast, the motor neurons of the sympathetic and parasympathetic motor systems are located outside the spinal cord in the autonomic ganglia. The autonomic motor neurons (also known as postganglionic neurons) are activated by the axons of central neurons (the preganglionic neurons) whose cell bodies are located in the spinal cord or brain stem, much as are the somatic motor neurons. Thus, in the visceral motor system a synapse (in the autonomic ganglion) is interposed between the efferent neuron in the central nervous system and the peripheral target (Figure 49-1). The sympathetic and parasympathetic nervous systems have clearly defined sensory components that provide input to the central nervous system and play an important role in autonomic reflexes. In addition, some sensory fibers that project to the spinal cord also send a branch to autonomic ganglia, thus forming reflex circuits that control some visceral autonomic functions. The innervation of target tissues by autonomic nerves also differs markedly from that of skeletal muscle by somatic motor nerves. Unlike skeletal muscle, which has specialized postsynaptic regions (the end-plates; see Chapter 14), target cells of the autonomic nerve fibers have no specialized postsynaptic sites. Nor do the postganglionic nerve endings have presynaptic specializations such as the active zones of somatic motor neurons. Instead, the nerve endings have several swellings (varicosities) where vesicles containing transmitter substances accumulate (see Chapter 15). Synaptic transmission therefore occurs at multiple sites along the highly branched axon terminals of autonomic nerves. The neurotransmitter may diffuse for distances P.963 as great as several hundred nanometers to reach its targets. In contrast to the point-to-point contacts made in the somatic motor system, neurons in the autonomic motor system exert a more diffuse control over target tissues, so that a relatively small number of highly branched motor fibers can regulate the function of large masses of smooth muscle or glandular tissue. Sympathetic Pathways Convey Thoracolumbar Outputs to Ganglia Alongside the Spinal Cord Preganglionic sympathetic neurons form a column in the intermediolateral horn of the spinal cord extending from the first thoracic spinal segment to rostral lumbar segments. The axons of these neurons leave the spinal cord in the ventral root and initially run together in the spinal nerve. They then separate from the somatic motor