DRUGS ACTING ON THE AUTONOMIC NERVOUS SYSTEM

CHAPTER TWO

Objectives

After reading this chapter the students is expected to:

• Correctly  identify  the  different  classes  of  drugs  affecting  the  autonomic nervous system (autonomic drugs)

• Discuss the effects and therapeutic uses of various drugs

• Identify  side  effects  and  contraindications  of  commonly  used  autonomic drugs.

• Prescribe autonomic drugs in clinical practice rationally.

INTRODUCTION

The nervous system controls all the major functions of the body. It is divided into central and peripheral nervous systems. The peripheral nervous system includes the somatic and autonomic nervous systems which control voluntary and involuntary functions respectively.

The ANS controls the vegetative functions of the body. These include functions like circulation, respiration, digestion and the maintenance of body temperature.

The ANS is subdivided into two major sub-divisions; this classification is based on  both anatomic and physiologic grounds; the two subdivisions are sympathetic (thoracolumbar) and parasympathetic (craniosacral). Autonomic nerves are actually composed of two neuron systems, termed preganglionic and postganglionic, based on anatomical location relative to the ganglia. A preganglionic neuron has its cell body in the spinal cord or brain.

The sympathetic nervous system arises from the thoracic and lumbar areas of the spinal cord and the preganglionic fibers for the parasympathetic nervous system arise from the cranial and sacral nerves. The postganglionic neurons send their axons directly to the effector organs (peripheral involuntary visceral organs). Autonomic innervation, irrespective of whether it belongs to the parasympathetic or the sympathetic nervous system, consists of a myelinated preganglionic fiber which forms a synapse with the cell body of a non-myelinated second neuron termed post-ganglionic fiber. The synapse is defined as a structure formed by the close apposition of a neuron either with another neuron or with effector cells.