CHAPTER XVIII 

 TONUS 



THIS word really implies a state of persistent excitation and is appropriate enough 

 as applied to the condition of a nerve centre when sending out a constant stream 

 of impulses which maintain some effector organ in a state of activity. As we shall 

 see presently, however, it does not so well apply 'to the case of smooth muscle, 

 which may, as it seems, remain in a shortened state without necessarily being in a 

 state of excitation. Since it is to this case that the name has most commonly 

 been applied, and the last word has not yet been said as to the nature of the 

 process, we may retain the name for both kinds of phenomena. 



As indicated, there are three sets of phenomena to be taken account of, 

 although perhaps only for convenience of description, (i.) The prolonged state of 

 contraction of smooth muscle, which is automatic, or independent of the receipt of 

 excitatory impulses from nerve centres, (ii.) That shown under certain conditions 

 by the cross-striated, skeletal muscle, of which mention has already been made as 

 "decerebrate rigidity" (page 417), and is dependent on stimuli from the centres, 

 disappearing when these are cut off. (iii.) The state of some nerve centres 

 themselves, in which they appear to give out constantly nerve impulses apart 

 from the receipt of messages from receptor organs. The discharges of such centres 

 are frequently rhythmic, as in the case of the respiratory centre. 



We will take first the case of smooth muscle, with its natural " tonus." 



TONUS OF SMOOTH MUSCLE OF VARIOUS KINDS 



It is a general property of this kind of tissue, wherever met with, to maintain 

 itself in a certain degree of shortening apart from impulses from nerve centres. It 

 is also, almost invariably, provided with two kinds of nerves a set which increase 

 the tone, excitatory, and a set which diminish it, inhibitory. 



This peripheral tonus may also show itself as rhythmic changes, as in the case 

 of the heart muscle. As has been remarked above, this structure behaves as 

 smooth muscle. 



In the first place, what is the evidence of a natural, inherent tonus in smooth 

 muscle, apart from the obvious activity of the heart 1 



The Blood Vessels. Goltz was the first to point out that the dilatation of the 

 blood vessels, which results from section of their constrictor nerves, on account 

 of the cutting off of continuous impulses from the vaso-constrictor centre, is not 

 so great as that produced by stimulation of dilator nerves (Goltz, Freusberg, and 

 Gergens, 1875, p. 62). Thus, after section of the vaso-constrictors, a state of 

 moderate contraction still remains, which can be further reduced by stimulation 

 of dilator nerves. This moderate tonus, left by section of constrictors, increases 

 in a few days, and becomes nearly equal to the original one in some weeks, 

 although the nerves may not have regenerated (Goltz and Freusberg, 1876, 

 p. 175). 



In the frog, after pithing, rhythmic contraction of arterioles is present, 

 while the vessels can still be dilated by the action of carbon dioxide (Bayliss, 

 1901, 1), showing that they were previously in a state of contraction. 



In the mammal, after destruction of the spinal cord, although all nervous 

 influence is thereby cut off, the arterial pressure remains at 30 to 50 mm. of 

 mercury. 



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