Vagal fibers

The parasympathetic nervous system, through the vagus nerve, inhibits the spontaneous rate of depolarization of pacemaker cells. The release of acetylcholine from cholinergic vagal fibers increases potassium conductance (gK+) in pacemaker cells, and this enhanced outward movement of K+ results in a more negative po-... 

Control of transmitter release is not limited to modulation by the transmitter itself. Nerve terminals also carry regulatory receptors that respond to many other substances. Such heteroreceptors may be activated by substances released from other nerve terminals that synapse with the nerve ending. For example, some vagal fibers in the myocardium synapse on sympathetic noradrenergic nerve terminals and inhibit norepinephrine release. Alternatively, the ligands for these receptors may diffuse to the receptors from the blood or from nearby tissues. Some of the transmitters and receptors identified to date are listed in Table 6-4. Presynaptic regulation by a variety of endogenous chemicals probably occurs in all nerve fibers. 

In addition, it has been found that 5-HT4-RS localized on enterochromaffin cells inhibit 5-HT release [81]. This inhibition could reduce S-HTg-R-mediated excitation of vagal fibers and may be one of the factors to inhibit the vomiting reflex elicited by this cascade of events [109], 5-HT4-R antagonists could offer a novel therapeutic approach in the treatment of emesis. 

If both the pneumotaxic center and the vagal fibers are transected, prolonged inspiratory apneusis develops. An apneustic center seems to be located in the lower pons, between the inspiratory and pneumotaxic centers. The discharge of the apneustic center stimulates the inspiratory center. Under normal conditions, the apneustic center is periodically restrained by vagal reflexes and pneumotaxic impulses, when uncontrolled apneustic breathing takes place. 

The above observations were later criticized on two groimds the nodose ganglion normally contains the perikarya of the chemosensory fibers of the aortic nerve (20), as well as of bronchopulmonary C-fibers responsive to CO2 (21). Thus, responses described by De Castro (19) in his double anastomosed preparations could result from reinnervation of the earotid body by originally chemosensitive vagal fibers. 

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