A 49-year-old man with a history of congenital heart disease had a successful cardiac transplant 6 months ago. He is now admitted to the hospital in severe agitation. He is found to have a blood pressure of 170/110 mm Hg, heart rate 130, respirations 35, sweating, and cutaneous vasoconstriction. He admits to self-injecting methamphetamine 4 hours previously. How does methamphetamine increase blood pressure? Normally, heart rate would be greatly reduced with this degree of drug-induced hypertension. Why is this patient’s heart rate elevated?
The nervous system is conventionally divided into the central nervous system (CNS; the brain and spinal cord) and the peripheral nervous system (PNS; neuronal tissues outside the CNS). The motor (efferent) portion of the nervous system can be divided into two major subdivisions: autonomic and somatic. The autonomic nervous system (ANS) is largely independent (autonomous) in that its activities are not under direct conscious control. It is concerned primarily with visceral functions such as cardiac output, blood flow distribution, and digestion, which are necessary for life. Evidence is accumulating that the ANS, especially the vagus nerve, also influences immune function and some CNS functions such as seizure discharge. Remarkably, recent evidence indicates that autonomic nerves also influence prostate cancer development and progression. The somatic subdivision is largely concerned with consciously controlled functions such as movement, respiration, and posture. Both systems have important afferent (sensory) inputs that provide information regarding the internal and external environments and modify motor output through reflex arcs of varying size and complexity.
The nervous system has several properties in common with the endocrine system. These include high-level integration in the brain, the ability to influence processes in distant regions of the body, and extensive use of negative feedback. Both systems use chemicals for the transmission of information. In the nervous system, chemical transmission occurs between nerve cells and between nerve cells and their effector cells. Chemical transmission takes place through the release of small amounts of transmitter substances from the nerve terminals into the synaptic cleft. The transmitter crosses the cleft by diffusion and activates or inhibits the postsynaptic cell by binding to a specialized receptor molecule. In a few cases, retrograde transmission may occur from the postsynaptic cell to the presynaptic neuron terminal and modify its subsequent activity.
By using drugs that mimic or block the actions of chemical transmitters, we can selectively modify many autonomic functions. These functions involve a variety of effector tissues, including cardiac muscle, smooth muscle, vascular endothelium, exocrine glands, and presynaptic nerve terminals. Autonomic drugs are useful in many clinical conditions. Unfortunately, a very large number of drugs used for other purposes have unwanted effects on autonomic function (see Case Study).
ANATOMY OF THE AUTONOMIC NERVOUS SYSTEM
The ANS lends itself to division on anatomic grounds into two major portions: the sympathetic (thoracolumbar) division and the parasympathetic (craniosacral)...