10.0: 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.