NERVOUS SYSTEM OUTLINE 18.1 Comparison of the Somatic and Autonomic Nervous Systems 540 18.2 Overview of the Autonomic Nervous System 542 18 18.3 Parasympathetic Division 545 18.3a Cranial Nerves 545 18.3b Sacral Spinal Nerves 545 18.3c Effects and General Functions of the Parasympathetic Division 545 Autonomic 18.4 Sympathetic Division 547 18.4a Organization and Anatomy of the Sympathetic Division 547 18.4b Sympathetic Pathways 550 Nervous 18.4c Effects and General Functions of the Sympathetic Division 550 18.5 Other Features of the Autonomic Nervous System 552 System 18.5a Autonomic Plexuses 552 18.5b Neurotransmitters and Receptors 553 18.5c Dual Innervation 554 18.5d Autonomic Reflexes 555 18.6 CNS Control of Autonomic Function 556 18.7 Development of the Autonomic Nervous System 557 MODULE 7: NERVOUS SYSTEM mck78097_ch18_539-560.indd 539 2/14/11 3:46 PM 540 Chapter Eighteen Autonomic Nervous System n a twisting downhill slope, an Olympic skier is concentrat- Recall from figure 14.2 (page 417) that the somatic nervous O ing on controlling his body to negotiate the course faster than system and the autonomic nervous system are part of both the anyone else in the world. Compared to the spectators in the viewing central nervous system and the peripheral nervous system. The areas, his pupils are more dilated, and his heart is beating faster SNS operates under our conscious control, as exemplified by vol- and pumping more blood to his skeletal muscles. At the same time, untary activities such as getting out of a chair, picking up a ball, organ system functions not needed in the race are practically shut walking outside, and throwing the ball for the dog to chase. (We down. Digestion, urination, and defecation can wait until the race have already seen that some SNS activities, such as swinging the is over. The skier exhibits a state of heightened readiness, called the arms while walking, occur at the subconscious level.) By contrast, “fight-or-flight” response, because the sympathetic division of the ANS functions are involuntary, and we are usually unaware of autonomic nervous system is dominant. them. For example, we are oblivious to the muscular actions of the The autonomic (aw-tō -nom ́ik; auto = self, nomos = law) stomach during digestion or changes in blood vessel diameter to nervous system (ANS) is a complex system of nerves that govern adjust blood pressure. involuntary actions. The ANS works constantly with the somatic Both the SNS and the ANS use sensory and motor neurons nervous system (SNS) to regulate body organs and maintain (figure 18.1). In the SNS, somatic sensory neurons conduct normal internal functions. We begin this chapter by comparing stimulus information from a sensory receptor, such as a tactile the SNS and the ANS. receptor in the skin, while somatic motor neurons innervate skel- etal muscle fibers. The ANS, by contrast, is activated by visceral sensory neurons. For example, some of these sensory neurons 18.1 Comparison of the Somatic detect pressure by monitoring stretch in blood vessels and organ and Autonomic Nervous Systems walls, while others measure carbon dioxide concentration in the blood. Some somatosensory receptors, such as those that detect Learning Objectives: temperature and light, also activate specific ANS responses (e.g., 1. Compare and contrast the anatomy and functions of the pupil constriction in response to bright light). In addition, auto- SNS and the ANS. nomic motor neurons innervate smooth muscle cells, cardiac 2. Explain how the two-neuron chain facilitates muscle cells, or glands. These motor neurons can either excite or communication and control in the ANS. inhibit cells in the viscera. Somatic Nervous System Autonomic Nervous System Autonomic Posterior root ganglion ganglion Anterior root Somatic sensory neuron Preganglionic autonomic motor neuron receives sensory sends nerve impulses to a ganglionic information from skin, motor neuron skeletal muscle, joints, and special senses Ganglionic autonomic motor neuron (vision, hearing, etc.) sends nerve impulses to smooth muscle, cardiac muscle, and glands Somatic motor neuron Visceral sensory neuron sends nerve impulses to receives sensory information from skeletal muscle blood vessels and smooth muscle in the viscera Smooth muscle in trachea Sensory receptor in skin Sensory receptor in viscera Skeletal muscle Figure 18.1 Comparison of Somatic and Autonomic Motor Nervous Systems. The somatic nervous system extends a single motor neuron to its effector, while the autonomic nervous system uses two motor neurons, which meet in an autonomic ganglion, to reach its effector. However, both systems use a single sensory neuron to convey impulses to the CNS. mck78097_ch18_539-560.indd 540 2/14/11 3:46 PM Chapter Eighteen Autonomic Nervous System 541 Autonomic ganglion Postganglionic axon Preganglionic axon Preganglionic neuron cell body Ganglionic neuron cell body Spinal cord Effector organ Figure 18.2 Components of the Autonomic Nervous System. The autonomic nervous system employs a preganglionic neuron, which is housed in the CNS (brain or spinal cord). The preganglionic axon synapses with a ganglionic neuron in an autonomic ganglion. The postganglionic axon (from the ganglionic neuron) travels to the effector. The motor neurons of the SNS innervate skeletal muscle within the ganglion, it is also known as a presynaptic neuron. The fibers, typically causing conscious, voluntary movements. A single axon of a ganglionic neuron, which extends to the effector, is also lower motor neuron axon extends uninterrupted from the spinal known as the postsynaptic axon. cord to one or more muscle fibers (figure 18.1). The impulses con- ducted by these motor neurons stimulate skeletal muscle fibers, WHAT DO YOU THINK? causing them to contract. Contraction continues until neuron ●1 Why does the autonomic motor nervous system use two neurons impulses cease to stimulate the muscle fiber. By contrast, the ANS (preganglionic and ganglionic) in a chain to an effector? (For the uses a pathway that includes a two-neuron chain to innervate answer, read the next section.) muscles and glands (figure 18.2). The first of the two ANS motor neurons is the preganglionic (prē gang-l ́ ē -on ́ik) neuron. Its cell The two-neuron chain vastly increases communication and body lies within the brainstem or the spinal cord. A preganglionic control in the ANS. Neuronal convergence (kon-ver ́jens; con = axon extends from this cell body and exits the CNS in either a with, vergere = to incline) occurs when axons from numerous cranial nerve or a spinal nerve. This axon projects to the cell body preganglionic cells synapse (converge) on a single ganglionic cell. of the second neuron, which is housed within an autonomic gan- In contrast, neuronal divergence (di-ver ́jens; di = apart) occurs glion in the peripheral nervous system. The second neuron in this when axons from one preganglionic cell synapse on numerous pathway is called a ganglionic neuron, and a postganglionic axon ganglionic cells. extends from its cell body to effector cells or an effector organ. Table 18.1 summarizes the characteristics of the somatic Since a preganglionic neuron synapses with a ganglionic neuron and autonomic nervous systems. Table 18.1 Comparison of Somatic and Autonomic Motor Nervous Systems Feature Somatic Nervous System Autonomic Nervous System Type of Control Voluntary control (from cerebral cortex; input from basal Involuntary control (from brainstem, hypothalamus, nuclei, brainstem, cerebellum, and spinal cord) limbic system, and spinal cord) Number of Neurons in Pathway One neuron in pathway; somatic motor neuron axon Two neurons in pathway; preganglionic neuron in extends from CNS to effector CNS projects preganglionic axon to ganglionic neuron; ganglionic neuron projects postganglionic axon to effector Ganglia Associated with Motor None Autonomic ganglia: sympathetic trunk ganglia; Neurons prevertebral ganglia; terminal or intramural ganglia Sensory Input General somatic senses, proprioceptors; special senses Some somatic and visceral senses Ganglia Associated with Sensory Posterior root ganglia; sensory ganglia of cranial nerves Posterior root ganglia; sensory ganglia of cranial nerves Input Effector Organs Skeletal muscle fi bers Cardiac muscle fi bers, smooth muscle fi bers, glands Response of Effector Excitation only Either excitation or inhibition of effectors Neurotransmitter Released Acetylcholine (ACh) ACh from all preganglionic axons and parasympathetic postganglionic axons, and a few sympathetic postganglionic axons; norepinephrine (NE) from most sympathetic postganglionic axons Axon Properties Myelinated, thick; fast conduction Preganglionic axons are thin, myelinated; postganglionic axons are thinner, unmyelinated, have slow conduction mck78097_ch18_539-560.indd 541 2/14/11 3:46 PM 542 Chapter Eighteen Autonomic Nervous System Study Tip! A good way to understand the two-neuron ANS chain is to compare it to the U.S. airline system, which uses connecting flights and “airport hubs” to transport the maximum number of people in the most cost-effective way. 1 “Preganglionic” flight from Indianapolis Imagine that you are flying from Indianapolis to Miami for spring break: to Chicago (autonomic ganglion). Your first flight from Indianapolis to Chicago is the preganglionic neuron. Although flying north to Chicago is out of your way, the airline wants you to go to an airport hub because it is more efficient to send all Indianapolis passengers to this main location before they take different Chicago flights throughout the United States. The airport hub in Chicago is the autonomic ganglion, the point where Indianapolis preganglionic and postganglionic flights meet up. Other preganglionic flights are meeting up at the airport hub, and here all these passengers will connect with other flights. Your connecting flight from Chicago to Miami is your postgangli- onic neuron. This flight will take you to your final destination, just as a postganglionic neuron sends a nerve impulse to an effector organ. On the Miami plane with you are people from other preganglionic flights who all want 2 “Postganglionic” flight to go to Miami as well.