Control of the Autonomic Nervous System by Higher Brain Centers

Visceral functions are largely regulated by autonomic reflexes. In most autonomic reflexes, sensory input is transmitted to brain centers that integrate this information and respond by modifying the activity of preganglionic autonomic neurons. The neural centers that directly control the activity of autonomic nerves are influenced by higher brain areas, as well as by sensory input.

The medulla oblongata of the brain stem is the area that most directly controls the activity of the autonomic system. Almost all autonomic responses can be elicited by experimental stimulation of the medulla, where centers for the control of the cardiovascular, pulmonary, urinary, reproductive, and digestive systems are located. Much of the sensory input to these centers travels in the afferent fibers of the vagus nerve—a mixed nerve containing both sensory and motor fibers. The reflexes that result are listed in table 9.8.

Although it directly regulates the activity of autonomic motor fibers, the medulla itself is responsive to regulation by higher brain areas. One of these areas is the hypothalamus, the brain region that contains centers for the control of body temperature, hunger, and thirst; for regulation of the pituitary gland; and (together with the limbic system and cerebral cortex) for various emotional states.

As described in chapter 8, the limbic system is a group of fiber tracts and nuclei that form a ring around the brain stem. It includes the cingulate gyrus of the cerebral cortex, the hypothalamus, the fornix (a fiber tract), the hippocampus, and the amygdaloid nucleus (see fig. 8.14). The limbic system is involved in basic emotional drives, such as anger, fear, sex, and hunger. The involvement of the limbic system with the control of autonomic function is responsible for the visceral responses that are characteristic of these emotional states. Blushing, pallor, fainting, breaking out in a cold sweat, a racing heartbeat, and "butterflies in the stomach" are only some of the many visceral reactions that accompany emotions as a result of autonomic activation.

The autonomic correlates of motion sickness—nausea, sweating, and cardiovascular changes—are eliminated by cutting the motor tracts of the cerebellum. This demonstrates that impulses from the cerebellum to the medulla oblongata influence activity of the autonomic nervous system. Experimental and clinical observations have also demonstrated that the frontal and temporal lobes of the cerebral cortex influence lower brain areas as part of their involvement in emotion and personality.

ft Traditionally, the distinction between the somatic system and the autonomic nervous system was drawn on ^ Ji i~i the basis that the former is under conscious control whereas the latter is not. Recently, however, we have learned that conscious processes in the cerebrum can influence autonomic activity. In biofeedback techniques, data obtained from devices that detect and amplify changes in blood pressure and heart rate, for example, are "fed back" to patients in the form of light signals or audible tones. The patients can often be trained to consciously reduce the frequency of the signals and, eventually, to control visceral activities without the aid of a machine. Biofeedback has been used successfully to treat hypertension, stress, and migraine headaches.

Test Yourself Before You Continue

1. Define adrenergic and cholinergic and use these terms to describe the neurotransmitters of different autonomic nerve fibers.

2. List the effects of sympathoadrenal stimulation on different effector organs. In each case, indicate whether the effect is due to alpha- or beta-receptor stimulation.

3. Describe the effects of the drug atropine and explain these effects in terms of the actions of the parasympathetic system.

4. Explain how the effects of the sympathetic and parasympathetic systems can be antagonistic, cooperative, or complementary. Include specific examples of these different types of effects in your explanation.

5. Explain the mechanisms involved when a person blushes. What structures are involved in this response?

Fox: Human Physiology, 9. The Autonomic Nervous Text © The McGraw-Hill

Eighth Edition System Companies, 2003

HPer Links of the Nervous System with Other Body Systems

Integumentary System

• The skin houses receptors for heat, cold, pain, pressure, and vibration (p. 244)

• Afferent neurons conduct impulses from cutaneous receptors (p. 245)

• Sympathetic neurons to the skin help to regulate cutaneous blood flow . . .(p. 428)

Skeletal System

• The skeleton supports and protects the brain and spinal cord (p. 190)

• Bones store calcium needed for neural function (p. 623)

• Afferent neurons from sensory receptors monitor movements of joints . . . .(p. 242)

Muscular System

• Muscle contractions generate body heat to maintain constant temperature for neural function (p. 608)

• Afferent neurons from muscle spindles transmit impulses to the CNS . . . .(p. 348)

• Somatic motor neurons innervate skeletal muscles (p. 347)

• Autonomic motor neurons innervate cardiac and smooth muscles (p. 220)

Endocrine System

• Many hormones, including sex steroids, act on the brain (p. 304)

• Hormones and neurotransmitters, such as epinephrine and norepinephrine, can have synergistic actions on a target tissue (p. 290)

• Autonomic neurons innervate endocrine glands such as the pancreatic islets (p. 613)

• The brain controls anterior pituitary function (p. 301)

• The brain controls posterior pituitary function (p. 301)

Circulatory System

• The circulatory system transports O2 and CO2, nutrients, and fluids to and from all organs, including the brain and spinal cord (p. 366)

• Autonomic nerves help to regulate cardiac output (p. 411)

• Autonomic nerves promote constriction and dilation of blood vessels, helping to regulate blood flow and blood pressure (p. 420)

Immune System

• Chemical factors called cytokines, released by cells of the immune system, act on the brain to promote a fever (p. 448)

• Cytokines from the immune system act on the brain to modify its regulation of pituitary gland secretion (p. 462)

• The nervous system plays a role in regulating the immune response . .(p. 462)

Respiratory System

• The lungs provide oxygen for all body systems and eliminate carbon dioxide (p. 480)

• Neural centers within the brain control breathing (p. 499)

Urinary System

• The kidneys eliminate metabolic wastes and help to maintain homeostasis of the blood plasma (p. 524)

• The kidneys regulate plasma concentrations of Na+, K+, and other ions needed for the functioning of neurons (p. 544)

• The nervous system innervates organs of the urinary system to control urination (p. 525)

• Autonomic nerves help to regulate renal blood flow (p. 531)

Digestive System

• The GI tract provides nutrients for all body organs, including those of nervous system (p. 561)

• Autonomic nerves innervate digestive organs (p. 563)

• The GI tract contains a complex neural system, called an enteric brain, that regulates its motility and secretions (p. 585)

• Secretions of gastric juice can be stimulated through activation of brain regions (p. 583)

• Hunger is controlled by centers in the hypothalamus of the brain (p. 606)

Reproductive System

• Gonads produce sex hormones that influence brain development (p. 640)

• The brain helps to regulate secretions of gonadotropic hormones from the anterior pituitary (p. 640)

• Autonomic nerves regulate blood flow into the external genitalia, contributing to the male and female sexual response (p. 643)

• The nervous and endocrine systems cooperate in the control of lactation (p. 677)

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