9.1 Introduction to the Endocrine System

Introduction^[1]

Maintaining homeostasis throughout the body requires constant input, analysis, and communication by the nervous system and the endocrine system. As discussed in Chapter 8, the nervous system uses two types of communication – electrical signals and chemical messengers called neurotransmitters. Both types of communication produce quick, brief actions, such as movement, sensation, and cognition.

In contrast, the endocrine system uses chemical messengers called hormones that are carried through the bloodstream to receptors on target cells throughout the body to stimulate a certain response. Because hormones have to travel through the body and bind to receptors, the endocrine system requires more time to create a response than the nervous system. The amount of time for a response varies with different hormones. For example, the adrenal hormones released when a person senses a real or perceived threat, called the fight-or-flight response, stimulates a response within seconds. In contrast, reproductive hormones may take up to 48 hours for target cells to respond. The endocrine system is also typically less specific than the nervous system because the same hormone may play a role in a variety of different physiological processes, depending on the target cells involved. For example, the hormone oxytocin promotes uterine contractions in labor, but it is also involved in breastfeeding, the sexual response, and feelings of emotional attachment. See Table 9.1 for a comparison of the characteristics of the endocrine and nervous systems.

Table 9.1. Comparison of the Endocrine and Nervous Systems^[2]