80 



VERTEBRATE RESPIRATION 



which are known to be intrinsic to most central nervous systems, 

 (ii) Other workers stress the latter properties and suppose that 

 the rhythm is generated by periodic inhibition of the continuous 

 discharge of neurones which innervate inspiratory muscles. This 

 inhibition could come either from other regions of the brain or 

 from inputs derived from the movements themselves, (iii) An 

 even more central view considers that the rhythmicity results 

 entirely from intrinsic properties of the region of the c.n.s. most 

 concerned with respiratory control and that inputs from the 

 peripheral sense organs serve only to modify or regulate the 

 rhythm. 



Higher 

 centres 



From 



To respiratory 

 muscles 



(i) Peripheral 

 driving 



(ii) Inhibition of 

 tonically 

 active neurones 



Fig. 24. 



\C02 in 

 blood 



(iii) Interaction of respiratory 

 neurones generates 

 rhythm which can be 

 modified by peripheral 

 and central inputs 



Diagram of three possible mechanisms for the generation of the 

 respiratory rhythm in the medulla of a vertebrate. The influence of 

 the input from O2 receptors and proprioceptors excited by the move- 

 ments have a driving action in (i), but in (ii) it serves to inhibit a 

 tonically active inspiratory centre. In (iii) the rhythm is generated 

 entirely by the interaction of spontaneously active Q medullary 

 neurones but it is influenced by the inputs from peripheral receptors 

 and centres in other parts of the brain. The respiratory neurones are 

 sensitive to changes in COg tension of the circulating blood. 



Evidence that neurones concerned with respiration lie within 



