960 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY' II 



is a "pneumotaxic" center located in the pons. Tliese 

 two systems are mutually replaceable and their ac- 

 tivity causes periodic inhibition of the inspiratory sys- 

 tem and hence rhythmicity in breathing. The location 

 of the pontine pneumotaxic center has recently been 

 explored by Baxter & Olszewski (18) and by Cohen & 

 Wang (42) in 1957, who located it in the dorsolateral 

 portion of the pons. Neurons in this region fire syn- 

 chronously with respiratory movements in vagoto- 

 mized animals. Other factors are also involved in the 

 control of respiration, including cortical, hypotha- 

 lamic and widespread reflex influences of mmierous 

 types. There may also be hormonal influences on 

 respiration as shown by Hiestand & NeLson (76, 77). 



OTHER REFLE.XES. Besides neural organizations in the 

 medulla which are necessary for the control of res- 

 piration and cardiovascular activity, this small area 

 contains visceral afferent and efferent mechanisms for 

 various other reflexes. These include the following, a) 

 The coughing reflex is mediated by afferent fibers in 

 the vagus nerve and efferent fibers in the nerves to 

 the respiratory and laryngeal muscles, h) The sneezing 

 reflex is evoked by afferent fibers which enter the 

 brain stem via the trigeminal nerve, c) The swallow- 

 ing refle.x is aroused by stimulation of the trigeminal 

 and glos.sopharyngeal nerves, the glossopharyngeal 

 and vagus nerves carrying the efferent fibers which 

 arise in the nucleus ambiguus. d) The salivary reflex 

 depends on afferent fibers which are borne by the 

 trigeminal and glossopharyngeal nerves and which 

 may be activated by stimulation of oral or olfactory 

 surfaces or by strong psychic factors. Efferent fibers 

 from the salivatory nuclei pass by way of the glosso- 

 pharyngeal and facial nerves, e) The afferents of the 

 sucking reflex are borne by the trigeminal and glosso- 

 pharyngeal nerves and the efferent fibers by the 

 facial, glossopharyngeal and hypoglossal nerves. /) 

 Hyperglycemia may be evoked reflcxly. It has been 

 shown by Brooks (34) that the integrity of an area in 

 the lower part of the medulla oblongata is necessary 

 for the occurrence of the elevations in blood sugar 

 which ordinary take place upon the stimulation of 

 afferent nerves. Higher regulatory centers are not 

 necessary for this response, g) The vomiting reflex de- 

 pends on neural inechanisms which have recently 

 been the subject of extensive study; this work has been 

 reviewed by Wang & Borison {170) and Brizzee (32). 

 Vomiting is usually said to be set up through afferent 

 fibers of the vagus, glossopharyngeal, vestibular and 

 possibly splanchnic nerves, by abnormal stimuli in 

 the stomach, pharynx, intestine or inner ear. The 



efferent fibers are said to originate in the dorsomotor 

 nucleus of the vagus, plus the nuclei for somatic out- 

 flow to muscles of the abdominal wall and diaphragm. 

 This reflex concept, however, is evidently too narrow 

 because it is known that vomiting may have other 

 causes. Thus, vomiting occurs in uremia and other 

 disorders which increa.se the accumulation of metabo- 

 lites in the body, and also after x-irradiation. Wang, 

 Borison, Brizzee and their colleagues propose that 

 there is a chemoreceptor trigger zone for vomiting in 

 the lower part of the medulla probably in the area 

 postrema. They have found that destruction of this 

 chemoreceptor zone reduces the incidence of vomiting 

 in nephrectomized dogs and cats (30). In studying 

 x-irradiation emesis Brizzee found that severe damage 

 to the dorsal vagal sensory nuclei alone did not 

 eliminate the vomiting. Similar lesions, however, to- 

 gether with involvement of the area postrema did 

 effectively eliminate the response. In vagotomized 

 animals the emesis did not occur, probably because 

 of loss of afferent fibers. Brizzee believes that the area 

 postrema, besides iieing a chemoreceptor trigger 

 zone, is also a central mediator for incoming sensory 

 fillers. Brizzee & Neal (33) reviewed the cellular 

 morphology of the area postrema, which they found 

 to contain glialoid cells and some small neurons. 

 Some nerve fibers from the area passed toward ad- 

 jacent nuclei, especially toward the nucleus of the 

 solitary tract. They also observed sinusoids and thick 

 walled arterioles in this region. They postulate that 

 the glialoid cells act as chemoreccptors while the 

 nerve fibers connect with the emetic center in the 

 lateral part of the reticular area in the medulla, 



Midhrani 



The midbrain appears to contain relativeh' few 

 autonomic mechanisms although it serxes as a route 

 for fibers of passage which are concerned with auto- 

 nomic activities. 



CENTR.-^L GRAY. The Central gray of the aqueduct has 

 been thought to contain centers for circulatory and 

 respiratory regulation, since Sachs (143) in igii 

 produced elevation of arterial pressure and increase 

 in respiratory rate from stimulation in this region. 

 More recently Kaisat and his collaborators (91, 93, 

 94) confirmed these findings and also produced 

 pupillary dilatation and contraction of the bladder 

 from stimulation of the central gray and from other 

 points in the tegmentum of the midiirain. Still more 

 recently McQueen et al. (114) also obtained pressor 



