Physiology 5th Ed.

INNERVATION OF THE GASTROINTESTINAL TRACT

The gastrointestinal tract is regulated, in part, by the autonomic nervous system, which has an extrinsic component and an intrinsic component. The extrinsic component is the sympathetic and parasympathetic innervation of the gastrointestinal tract. The intrinsic component is called the enteric nervous system. The enteric nervous system is wholly contained within the submucosal and myenteric plexuses in the wall of the gastrointestinal tract; it communicates extensively with the parasympathetic and sympathetic nervous systems.

Parasympathetic Innervation

Parasympathetic innervation is supplied by the vagus nerve (cranial nerve [CN] X) and the pelvic nerve (see Chapter 2Fig. 2-3). The pattern of parasympathetic innervation of the gastrointestinal tract is consistent with its function. The vagus nerve innervates the upper gastrointestinal tract including the striated muscle of the upper third of the esophagus, the wall of the stomach, the small intestine, and the ascending colon. The pelvic nerve innervates the lower gastrointestinal tract including the striated muscle of the external anal canal and the walls of the transverse, descending, and sigmoid colons.

Recall from Chapter 2 that the parasympathetic nervous system has long preganglionic fibers that synapse in ganglia in or near the target organs. In the gastrointestinal tract, these ganglia actually are located in the walls of the organs within the myenteric and submucosal plexuses. Information relayed from the parasympathetic nervous system is coordinated in these plexuses and then relayed to smooth muscle, endocrine, and secretory cells (Figs. 8-2 and 8-3).

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Figure 8–2 The extrinsic nervous system of the gastrointestinal tract. Efferent neurons of the parasympathetic and sympathetic nervous systems synapse in the myenteric and submucosal plexuses, in the smooth muscle, and in the mucosa. ACh, Acetylcholine; NE, norepinephrine.

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Figure 8–3 Intrinsic nervous system of the gastrointestinal tract.

Postganglionic neurons of the parasympathetic nervous system are classified as either cholinergic or peptidergic. Cholinergic neurons release acetylcholine (ACh) as the neurotransmitter. Peptidergic neurons release one of several peptides including substance P and vasoactive inhibitory peptide (VIP); in some instances, the neuropeptide has not yet been identified.

The vagus nerve is a mixed nerve in which 75% of the fibers are afferent and 25% are efferent. Afferent fibers deliver sensory information from the periphery (e.g., from mechanoreceptors and chemoreceptors in the wall of the gastrointestinal tract) to the central nervous system (CNS). Efferent fibers deliver motor information from the CNS to target tissues in the periphery (e.g., smooth muscle, secretory, and endocrine cells) (see Fig. 8-2). Thus, mechanoreceptors and chemoreceptors in the gastrointestinal mucosa relay afferent information to the CNS via the vagus nerve, which triggers reflexes whose efferent limb is also in the vagus nerve. Such reflexes, in which both afferent and efferent limbs are contained in the vagus nerve, are called vagovagal reflexes.

Sympathetic Innervation

Preganglionic fibers of the sympathetic nervous system are relatively short and synapse in ganglia outside the gastrointestinal tract. (Contrast the preganglionic fibers of the parasympathetic nervous system, which are long and synapse in ganglia inside the wall of the gastrointestinal tract.) Four sympathetic ganglia serve the gastrointestinal tract: celiac, superior mesenteric, inferior mesenteric, and hypogastric (see Chapter 2Fig. 2-2). Postganglionic nerve fibers, which are adrenergic (i.e., release norepinephrine), leave these sympathetic ganglia and synapse on ganglia in the myenteric and submucosal plexuses, or they directly innervate smooth muscle, endocrine, or secretory cells (see Fig. 8-2).

Approximately 50% of the sympathetic nerve fibers are afferent and 50% are efferent. Thus, as with the parasympathetic innervation, sensory and motor information is relayed back and forth between the gastrointestinal tract and the CNS, coordinated by the submucosal and myenteric plexuses.

Intrinsic Innervation

The intrinsic or enteric nervous system can direct all functions of the gastrointestinal tract, even in the absence of extrinsic innervation. The enteric nervous system is located in ganglia in the myenteric and submucosal plexuses and controls the contractile, secretory, and endocrine functions of the gastrointestinal tract (see Fig. 8-3). As shown in Figure 8-2, these ganglia receive input from the parasympathetic and sympathetic nervous systems, which modulate their activity. These ganglia also receive sensory information directly from mechanoreceptors and chemoreceptors in the mucosa and send motor information directly to smooth muscle, secretory, and endocrine cells. Information is also relayed between ganglia by interneurons.

A large number of neurochemicals, or neurocrines, have been identified in neurons of the enteric nervous system (Table 8-1). Some of the substances listed are classified as neurotransmitters and some are neuromodulators (i.e., they modulate the activity of neurotransmitters). Most neurons of the enteric nervous system contain more than one neurochemical, and upon stimulation, they may cosecrete two or more neurocrines.

Table 8–1 Neurotransmitters and Neuromodulators in the Enteric Nervous System

Substance

Source

Actions

Acetylcholine (ACh)

Cholinergic neurons

Contraction of smooth muscle in wall

Relaxation of sphincters

↑ Salivary secretion

↑ Gastric secretion

↑ Pancreatic secretion

Norepinephrine (NE)

Adrenergic neurons

Relaxation of smooth muscle in wall

Contraction of sphincters

↑ Salivary secretion

Vasoactive Intestinal Peptide (VIP)

Neurons of mucosa and smooth muscle

Relaxation of smooth muscle

↑ Intestinal secretion

↑ Pancreatic secretion

Gastrin-Releasing Peptide (GRP), or Bombesin

Neurons of gastric mucosa

↑ Gastrin secretion

Enkephalins (opiates)

Neurons of mucosa and smooth muscle

Contraction of smooth muscle

↓ Intestinal secretion

Neuropeptide Y

Neurons of mucosa and smooth muscle

Relaxation of smooth muscle

↓ Intestinal secretion

Substance P

Cosecreted with ACh

Contraction of smooth muscle

↑ Salivary secretion