A reflex circuit known as the vago-vagal reflex underlies moment-to-moment adjustments required for optimal digestive function in the upper digestive tract (see Clinical Focus Box 26.1). The afferent side of the reflex arc consists of vagal afferent neurons connected with a variety of sensory receptors specialized for the detection and signaling of mechanical parameters, such as muscle tension and mucosal brushing, or luminal chemical parameters, including glucose concentration, osmolality, and pH. Cell bodies of the vagal afferents are in the nodose ganglia. The afferent neurons are synaptically connected with neurons in the dorsal motor nucleus of the vagus and in the nucleus of the tractus solitarius. The nucleus of the tractus solitarius, which lies directly above the dorsal motor nucleus of the vagus (see Fig. 26.8), makes synaptic connections with the neuronal pool in the vagal motor nucleus. A synaptic meshwork formed by processes from neurons in both nuclei tightly links the two into an integrative center. The dorsal vagal neurons are second- or third-order neurons representing the efferent arm of the reflex circuit. They are the final common pathways out of the brain to the enteric circuits innervating the effector systems.
Efferent vagal fibers form synapses with neurons in the ENS to activate circuits that ultimately drive the outflow of signals in motor neurons to the effector systems. When the effector system is the musculature, its innervation consists of both inhibitory and excitatory motor neurons that participate in reciprocal control. If the effector systems are gastric glands or digestive glands, the secretomotor neurons are excitatory and stimulate secretory behavior.
The circuits for CNS control of the upper GI tract are organized much like those dedicated to the control of skeletal muscle movements (see Chapter 5), where fundamental reflex circuits are located in the spinal cord. Inputs to the spinal reflex circuits from higher order integrative
| CLINICAL FOCUS BOX 26.1
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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.