- Understand the integrated response to a meal and the need for mechanisms that regulate the function of the gastrointestinal tract as a whole
- Describe modes of communication in the gastrointestinal tract
- General features of neurohumoral regulation
- Characteristics of chemical signals
- Understand principles of endocrine regulation
- Definition of a hormone
- Identify established and candidate GI hormones and their mechanisms of action
- Understand the design of the enteric nervous system and neurocrine regulation
- Describe immune and paracrine regulatory pathways
- Understand how the GI tract and the brain cooperate to regulate food intake
As we have learned from the previous chapter, the gastrointestinal system subserves several functions that are critical for whole-body homeostasis. For nutrient assimilation in particular, specific tissues and regions of the gastrointestinal system must sense, signal, and respond to the ingestion of a meal (Figure 2–1). By cooperating with the central nervous system, the gastrointestinal system is also intimately involved in the control of food intake. To conduct the business of the gastrointestinal system most efficiently, the various segments must communicate. Thus, the activities of the gastrointestinal tract and the organs that drain into it are coordinated temporally via the action of a series of chemical mediators, with the system being referred to collectively as neurohumoral regulation, implying the combined action of soluble and neuronal pathways. The integrated regulation of gastrointestinal function underlies the efficiency of the system as described in Chapter 1, and its ability to provide for the effective uptake of nutrients even when they are in short supply.
Overview of neural control of the gastrointestinal system. Nutrients activate both special senses (smell, taste) as well as specific sensory nerve endings that exist within the wall of the gut. These responses are conveyed via the autonomic nervous system and enteric nervous system (ENS) to alter the function of the gastrointestinal tract and organs draining into it, resulting in changes in secretion and motility. Such functional changes may additionally feedback on neural control to allow for appropriate homeostasis of the system.
General Features of Neurohumoral Regulation
The gastrointestinal tract stretches from mouth to anus, implying that communication that rests simply on diffusion of locally released signals will not be adequate for the timely transfer of information from one segment to another. Likewise, the gastrointestinal tract also needs to communicate its status to organs that drain into it, such as the pancreas and gallbladder, and to distant organs, such as the brain. Thus, the system has evolved mechanisms for communication over significant distances, although local messengers also play a role in fine-tuning information delivery or, in some cases, amplifying or antagonizing it. Overall, information is carried between the various sites by chemical entities possessing specific physicochemical properties. Another general principle underlying communication in the gastrointestinal system is that of functional redundancy. Several different mediators may ...