82 VERTEBRATE RESPIRATION 



Apparently there is a region in the caudal two-thirds of 

 the pons which must be present if apneustic breathing is to 

 persist and it has been called the apneustic centre. Removal of 

 this centre reduces the inspiratory cramp and leads to the 

 development of a more normal respiratory rhythm. The centre 

 is apparently located diffusely in the network of intermingled 

 fibres and cells which comprise the reticular formation rather 

 than in a clearly-defined group of cells within the pons. 



It has been concluded that the fundamental respiratory 

 rhythm can develop within the medulla without the assistance of 

 reflex excitation or stimulation from higher centres. It is spon- 

 taneously periodic (i.e., comes in bursts in the absence of any 

 input), does not require any sensory input to produce successive 

 alternation of inspiration and expiration, nor does it require the 

 inhibitory effects from the pontine centres. Evidence for this 

 medullary autonomy is also derived from isolation experiments, 

 such as those of Adrian and Buytendijk on the goldfish. They 

 recorded slow rhythmic potentials of about the same fre- 

 quency (20-60/min.) as the fish breathing movements. Recently, 

 similar slow potential changes have been recorded from intact 

 specimens. Evidence for the central autonomy has also been 

 derived from experiments in which unit activity recorded by fine 

 electrodes within the medulla has been shown to persist when all 

 cranial nerves were cut except the trigeminal (V) nerve on one 

 side. Moreover, after cutting this nerve it was still possible to 

 record bursts of action potentials from the sites where they were 

 present in the whole animal preparation. Similar observations 

 have also been made on the mammalian medulla. In other experi- 

 ments with mammals and fishes the respiratory movements were 

 abolished by the injection of curare-like drugs which block 

 neuromuscular transmission. Consequently the afferent inflow 

 normally produced by these rhythmic movements was cut off, 

 yet rhythmic bursts of impulses from the motor neurones 

 continued. 



These bursts do not persist for long periods, so it appears 

 that some non-specific input to the medulla is necessary for 

 the full expression of the respiratory rhythm. 



