The nervous system is anatomically divided into the central nervous system (CNS; the brain and spinal cord) and the peripheral nervous system (PNS; neuronal tissues outside the CNS). Functionally, 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 control and integration of visceral functions necessary for life such as cardiac output, blood flow distribution, and digestion. Evidence is accumulating that the ANS, especially the vagus nerve, also influences immune function and some CNS functions such as seizure discharge. Remarkably, some evidence indicates that autonomic nerves can also influence cancer development and progression. The motor portion of the somatic subdivision is largely concerned with consciously controlled functions such as movement, respiration, and posture. Both the autonomic and the somatic systems have important afferent (sensory) inputs that provide information regarding the internal and external environments and modify motor output through reflex arcs of varying 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 (eg, allergies, mental illness) have unwanted effects on autonomic function.
Medulla NE, D ˜, D1 Spinal cord ACh  ACh  ACh  ACh  ACh  ACh  ACh  ACh  N N N N N N M M NE Adrenal medulla Epi, NE Voluntary motor nerve ˜, °
The ANS lends itself to division on anatomic grounds into two major portions: the sympathetic (thoracolumbar) division and the parasympathetic (traditionally “craniosacral,” but see Box: Sympathetic Sacral Outflow) division (Figure 6–1). Motor neurons in both divisions originate in nuclei within the CNS and give rise to preganglionic efferent fibers that exit from the brain stem or spinal cord and terminate in motor ganglia. The sympathetic preganglionic fibers leave the CNS through
Cardiac and smooth muscle, gland cells, nerve terminals
Cardiac and smooth muscle, gland cells, nerve terminals