Gastrointestinal Physiology

I. Structure and Innervation of the Gastrointestinal Tract

A. Structure of the gastrointestinal (GI) tract (Figure 6-1)

1. Epithelial cells

- are specialized in different parts of the GI tract for secretion or absorption.

2. Muscularis mucosa

-Contraction causes a change in the surface area for secretion or absorption.

3. Circular muscle

- Contraction causes a decrease in diameter of the lumen of the GI tract.

4. Longitudinal muscle

- Contraction causes shortening of a segment of the GI tract.

5. Submucosal plexus (Meissner's plexus) and myenteric plexus

- comprise the enteric nervous system of the GI tract.

_ - Lamina propria

C^^Ss^c^S^-Circular muscle

Myenteric plexus .--Longitudinal muscle yyyyyyy^yyyyyyy—serosa

_ - Lamina propria

C^^Ss^c^S^-Circular muscle

Myenteric plexus .--Longitudinal muscle yyyyyyy^yyyyyyy—serosa

Figure 6-1. Structure of the gastrointestinal (GI) tract.

- integrate and coordinate the motility, secretory, and endocrine functions of the GI tract.

B. Innervation of the GI tract

- The autonomic nervous system (ANS) of the GI tract comprises both extrinsic and intrinsic nervous systems.

1. Extrinsic innervation (parasympathetic and sympathetic nervous systems)

- Efferent fibers carry information from the brain stem and spinal cord to the GI tract.

-Afferent fibers carry sensory information from chemoreceptors and mechanoreceptors in the GI tract to the brain stem and spinal cord.

a. Parasympathetic nervous system

- is usually excitatory on the functions of the GI tract.

- is carried via the vagus and pelvic nerves.

- Preganglionic parasympathetic fibers synapse in the myenteric and submucosal plexuses.

- Cell bodies in the ganglia of the plexuses then send information to the smooth muscle, secretory cells, and endocrine cells of the GI tract.

(1) The vagus nerve innervates the esophagus, stomach, pancreas, and upper large intestine.

- Reflexes in which both afferent and efferent pathways are contained in the vagus nerve are called vagovagal reflexes.

(2) The pelvic nerve innervates the lower large intestine, rectum, and anus.

b. Sympathetic nervous system

- is usually inhibitory on the functions of the GI tract.

- Fibers originate in the spinal cord between T-8 and L-2.

- Preganglionic sympathetic cholinergic fibers synapse in the prevertebral ganglia.

- Postganglionic sympathetic adrenergic fibers leave the prevertebral ganglia and synapse in the myenteric and submucosal plexuses. Direct postganglionic adrenergic innervation of blood vessels and some smooth muscle cells also occurs.

- Cell bodies in the ganglia of the plexuses then send information to the smooth muscle, secretory cells, and endocrine cells of the GI tract.

2. Intrinsic innervation (enteric nervous system)

- coordinates and relays information from the parasympathetic and sympathetic nervous systems to the GI tract.

- uses local reflexes to relay information within the GI tract.

- controls most functions of the GI tract, especially motility and secretion, even in the absence of extrinsic innervation.

a. Myenteric plexus (Auerbach's plexus)

- primarily controls the motility of the GI smooth muscle.

b. Submucosal plexus (Meissner's plexus)

- primarily controls secretion and blood flow.

- receives sensory information from chemoreceptors and mechanoreceptors in the GI tract.

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