Figure 624

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Neural control of the gut wall, showing (1) the myenteric and submucosal plexuses (black fibers); (2) extrinsic control of these plexuses by the sympathetic and parasympathetic nervous systems (red fibers); and (3) sensory fibers passing from the luminal epithelium and gut wall to the enteric plexuses, then to the prevertebral ganglia of the spinal cord and directly to the spinal cord and brain stem (dashed fibers).

To prevertebral ganglia, spinal cord, and brain stem

Sympathetic

Parasympathetic

(mainly postganglionic)

(preganglionic)

Sensory neurons

To prevertebral ganglia, spinal cord, and brain stem

(mainly postganglionic)

(preganglionic)

Sensory neurons

Myenteric plexus yy i

Submucosal plexus

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Myenteric plexus yy i

Submucosal plexus

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in the wall of the gut, beginning in the esophagus and extending all the way to the anus. The number of neurons in this enteric system is about 100 million, almost exactly equal to the number in the entire spinal cord. This highly developed enteric nervous system is especially important in controlling gastrointestinal movements and secretion.

The enteric nervous system is composed mainly of two plexuses, shown in Figure 62-4: (1) an outer plexus lying between the longitudinal and circular muscle layers, called the myenteric plexus or Auerbach's plexus, and (2) an inner plexus, called the submucosal plexus or Meissnefs plexus, that lies in the submucosa. The nervous connections within and between these two plexuses are also shown in Figure 62-4.

The myenteric plexus controls mainly the gastrointestinal movements, and the submucosal plexus controls mainly gastrointestinal secretion and local blood flow.

Note especially in Figure 62-4 the extrinsic sympathetic and parasympathetic fibers that connect to both the myenteric and submucosal plexuses. Although the enteric nervous system can function on its own, independently of these extrinsic nerves, stimulation by the parasympathetic and sympathetic systems can greatly enhance or inhibit gastrointestinal functions, as we discuss later.

Also shown in Figure 62-4 are sensory nerve endings that originate in the gastrointestinal epithelium or gut wall and send afferent fibers to both plexuses of the enteric system, as well as (1) to the pre-vertebral ganglia of the sympathetic nervous system, (2) to the spinal cord, and (3) in the vagus nerves all the way to the brain stem. These sensory nerves can elicit local reflexes within the gut wall itself and still other reflexes that are relayed to the gut from either the prevertebral ganglia or the basal regions of the brain.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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