Introduction

The peripheral nervous system (PNS) is divided into two main subdivisions: the somatic motor system and the autonomic nervous system (ANS). The somatic nervous system controls voluntary muscle movement, and the autonomic nervous system is responsible for regulating numerous involuntary functions, such as the heart, secretory glands, and smooth muscle. The ANS is further divided into the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS) (see Figure 10.2). The PSNS relaxes the body and maintains certain life-sustaining properties such as digestion and excretion. The SNS is involved in the fight-or-flight response by preparing the body to deal with stressors. There are two primary types of receptors within the PNS: cholinergic receptors and adrenergic receptors. Cholinergic receptors are activated by the neurotransmitter acetylcholine (ACh) and can be activated by endogenous or exogenous substances. Adrenergic receptors are stimulated by the neurotransmitter norepinephrine (Bekdash, 2021). In addition, these primary receptors have subtypes.

Figure 10.2 The autonomic nervous system consists of the parasympathetic nervous system (PSNS) and the sympathetic nervous system (SNS). The PSNS relaxes the body and maintains certain life-sustaining properties. The SNS is involved in the fight-or-flight response. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

This chapter will focus on only the cholinergic receptors with no further discussion of adrenergic receptors. There are three major subtypes of cholinergic receptors: muscarinic, nicotinic_M, and nicotinic_N. Table 10.1 describes the functions of each of these. In some organs, the PSNS and SNS work in opposition to each other (such as increasing or decreasing the heart rate). In other organs, the two systems have a complementary effect where both divisions are necessary for a physiological function to occur; for example, the PSNS is involved in males obtaining an erection, and the SNS is responsible for ejaculation. Both systems are required to work together for reproduction to successfully take place.

Notably, cholinergic receptors are not associated with the nervous system in any way. Regardless, the cholinergic receptors on blood vessels do have pharmacologic efficacy because drugs that are able to activate these receptors will cause blood pressure to decrease.