Autonomic Nervous System

Fourteen Autonomic Nervous System

CHAPTER OBJECTIVES

. To understand the structure and physiology of the autonomic nervous system

A forty-six-year-old man underwent a right-sided pneumonectomy for carcinoma of the bronchus and is seen by his thoracic surgeon for follow-up. The patient says that he feels surprisingly fit and is gaining some of the weight that he had lost prior to the operation. His wife comments that the upper lid of his right eye tends to droop slightly when he gets tired at the end of the day.

During a careful physical examination, the surgeon notices that in addition to the ptosis of the right eye, the patient's right pupil is constricted and that his face is slightly flushed on the right side. Further examination reveals that the skin on the right side of the face appears to be warmer and drier than normal. Palpation of the deep

The autonomic nervous system and the endocrine system control the internal environment of the body. The autonomic nervous system provides a fine discrete control over the functions of many organs and tissues, including heart muscle, smooth muscle, and the exocrine glands. The endocrine system, by means of its blood-borne hormones, exerts a slower more diffuse control.

The autonomic nervous system, like the somatic nervous system, has afferent, connector, and efferent neurons. The afferent impulses originate in visceral receptors and travel via afferent pathways to the central nervous system, where they are integrated through connector neurons at different levels and then leave via efferent pathways to visceral effector organs. The majority of the activities of the autonomic system do not impinge on consciousness.

The efferent pathways of the autonomic system are made up of preganglionic and postganglionic neurons. The cell bodies of the preganglionic neurons are situated in the lateral gray column of the spinal cord and in the motor nuclei of the third, seventh, ninth, and tenth cranial nerves. The axons of these cell bodies synapse on the cell bodies of the postganglionic neurons that are collected together to form ganglia outside the central nervous system.

. To understand the pharmacologic differences between the sympathetic and parasympathetic nervous systems cervical group of lymph nodes reveals a large, hard, fixed node just above the right clavicle.

Based on his clinical findings, the surgeon makes the diagnosis of a right-sided Horner syndrome. These findings were not present before the operation. The presence of the enlarged right-sided deep cervical lymph node indicates that the bronchial carcinoma has metastasized to the lymph node in the neck and is spreading to involve the cervical part of the sympathetic trunk on the right side. This observation explains the abnormal eye and facial skin findings.

Knowledge of the sympathetic innervation of the structures of the head and neck enables the surgeon to make an accurate diagnosis in this patient.

The control exerted by the autonomic system is extremely rapid; it is also widespread, since one preganglionic axon may synapse with several postganglionic neurons. Large collections of afferent and efferent fibers and their associated ganglia form autonomic plexuses in the thorax, abdomen, and pelvis.

The visceral receptors include chemoreceptors, baroreceptors, and osmoreceptors. Pain receptors are present in viscera and certain types of stimuli, such as lack of oxygen or stretch, can cause extreme pain.

The information provided in this chapter is extensively used in clinical practice. The examples of autonomic innervations given are important and are commonly used by examiners to construct good questions.

ORGANIZATION

The autonomic nervous system is distributed throughout the central and peripheral nervous systems. It is divided into two parts, the sympathetic and the parasympathetic and, as emphasized earlier, consists of both afferent and efferent fibers. This division between sympathetic and parasympathetic is made on the basis of anatomic differences, differences in the neurotransmitters, and differences in the physiological effects.

Both the sympathetic and parasympathetic divisions produce opposite effects in most organs and are thus considered as physiologic antagonists. However, both divisions operate in conjunction with one another and the balance in the activities maintains a stable internal environment.