WHY INCLUDE THIS MATERIAL?
Why include a review of the autonomic nervous system in a book on pharmacology? The main reason is that autonomic pharmacology is easiest if you have an understanding of the anatomy and physiology of the autonomic nervous system. Therefore, a quick review of the autonomic nervous system should simplify the pharmacology. In addition, autonomic pharmacology forms a basis for cardiovascular and central nervous system pharmacology. Consequently, learning the autonomics thoroughly will save you time and effort later on.
The nervous system is divided into two main parts: the central and the peripheral nervous systems. The central nervous system is made up of the brain and spinal cord. The peripheral nervous system contains everything else. The peripheral nervous system is divided into two branches: the somatic and autonomic nervous systems. The somatic nervous system is mainly the motor system that includes all of the nerves to the muscles. The other branch, the autonomic nervous system, is the part we’re interested in here.
The autonomic nervous system is responsible for maintaining the internal environment of the body (homeostasis).
Knowing the role of the autonomic nervous system in homeostasis makes it easy to remember the target organs served by this system. It is clear that the cardiovascular system needs regulation, but the smooth muscle of the gastrointestinal (GI) tract and the various glands throughout the body also need to be constantly monitored. Let’s first consider some points that are true about the entire autonomic nervous system before we break the system down into parts.
Within the autonomic nervous system, two neurons are required to reach a target organ: a preganglionic neuron and a postganglionic neuron.
The preganglionic neuron originates in the central nervous system. It forms a synapse with the postganglionic neuron, the cell body of which is located in autonomic ganglia.
All preganglionic neurons release acetylcholine as their transmitter. The acetylcholine binds to nicotinic receptors on the postganglionic cell.
The preceding statement is a general rule. We’ll come back to the transmitter and receptors in more detail later.
The autonomic nervous system is divided into the sympathetic and parasympathetic systems (Figure 6–1). The sympathetic system is catabolic, meaning that it burns energy. It is the one involved in the fight-or-flight response. If you remember this, most of the effects of the sympathetic nervous system make sense. The sympathetic nervous system is also called the thoracolumbar system because the ganglia are located lateral to the vertebral column in the thoracic and lumbar regions. In addition, because the ganglia are fixed along the back, the postganglionic fibers can be quite long. Within the sympathetic system, the preganglionic axons form synapses with many postganglionic cells, thus giving this system widespread action. Note that this is consistent with the fight-or-flight response.