Paracrine agonists act locally within the tissue. For example, histamine is released from cells in the stomach wall and regulates the secretion of HCl. The GI immune system also releases Paracrine agents. The mass of immune cells in the GI is equivalent to that in the rest of the body. They release antibodies in response to specific food antigens and mount immune responses to pathogens.
Autonomic innervation is either sympathetic or parasympathetic .
The enteric nervous system is made up of nerve fibers and ganglion cell bodies. Interneurons connect incoming afferent fibers with efferent neurons innervating smooth muscle and secretory cells, forming reflex arcs, shown below, right. The extrinsic and intrinsic elements of nervous control of the GI is illustrated in Text Fig. Amplitude and frequency can be modulated by Intrinsic and Extrinsic mechanisms. For example, sympathetic activity decreases slow wave amplitude.