| | Initiation
and coordination of emesis
The gastrointestinal
tract is the first, and sometimes the only, defense against ingested
poisons. Ingested toxins can stimulate chemosensitive vagal afferent
nerve endings directly. Any substance or bolus that produces obstruction
or distension of the gastrointestinal tract can excite vagal mechanosensitive
afferents, and these also may initiate emesis (Andrews
et al., 1990). As discussed previously, vagal sensory neurons
terminate in the nucleus of the solitary tract.
Adjacent to
and medial to the nucleus of the solitary tract in the caudal
medulla is the area postrema. This area, which lies in the floor
of the fourth ventricle, does not have a well-developed blood-brain
barrier. This means that blood-borne pathogens have direct access
to brain tissue, which does not occur in most areas of the brain.
Thus, these pathogens excite neurons in the area postrema, ultimately
triggering emesis (Hornby,
2001; Page et al., 1997).
The "vomiting
center" is located in the dorsolateral medullary reticular
formation. In general, this region is bordered laterally by the
inferior cerebellar peduncle and spinal trigeminal nucleus, ventrally
by the nucleus ambiguus and inferior olivary nuclei, medially
by the gigantocellular reticular nucleus, and dorsally by the
dorsal motor nucleus of the vagus nerve and the solitary nucleus
and tract. Neurons in the area postrema project to the vomiting
center.
|