8. THE REGULATION OF BREATHING
327
branchial processes lining the inner opening of the gill slits and were slowly
adapting sense organs with both static and dynamic characteristics. The
effective stimulus was thought to be displacement of the pharyngeal wall
rather than water flow itself. The branchial processes were thus deflected
by being pressed against the gill bar in front. Satchell and Way suggested
that the receptors were involved in a breath by breath regulation of
breathing rate, the duration of the inspiratory period and the following
pause being determined by the level of activity during the preceding
expiratory period. Thus in an almost completely curarized animal the
breathing rate became largely independent of imposed ventilation
volume, suggesting that the mechanoreceptors were not excited by
abnormally small breathing movements. In the normal animal, however,
the decrease in rate as ventilation was progressively restricted resulted
from greater excitation of the mechanoreceptors because the amplitude
of breathing increased. The original observations of the relationship between water flow and breathing rate must now be explained in terms of
a change in amplitude of breathing. The receptors and pathways involved in this response are not known. The inhibitory reflex is unusual
in that the receptors are excited during orobranchial contraction and
decrease breathing rate by delaying the onset of the next expiration
phase after a longer inspiration and pause.
Section of the branchial branches of the IXth and Xth cranial
nerves in the shark caused an increase in breathing rate; inspiration was
quicker and the pause disappeared (Satchell, 1959). This correlates well
with the proposed inhibitory role of the mechanoreceptors. In teleost fish
the effects of IXth and Xth nerve section are not so well established.
Shelton (1959) showed that the immediate change caused by sectioning
the nerves in tench was an increase in amplitude of the breathing movements. The nature of the receptors and the function of the afferent
pathways involved in this response were not defined. Powers and Clark
(1942) had earlier done similar experiments and had found that breathing failed after section of nerves IX and X. The gill blood vessels were
also sectioned in their experiments. Effects other than the direct ones of
nerve section may therefore have been complicating the results, although
the respiratory center seems to be quite resistant to circulatory failure
( Serbenyuk, 1965). Serbenyuk et al. ( 1959) and Serbenyuk ( 1965)
found that rhythmic activity persisted in the center after section of
the branchial nerves or novocaine anesthesia of the gills in carp, although there was a considerable change in breathing pattern. There can
be little doubt that important afferents, probably from mechanoreceptors as well as other sense organs, are carried in the branchial nerves
of teleosts although their role cannot at present be precisely defined.
327
branchial processes lining the inner opening of the gill slits and were slowly
adapting sense organs with both static and dynamic characteristics. The
effective stimulus was thought to be displacement of the pharyngeal wall
rather than water flow itself. The branchial processes were thus deflected
by being pressed against the gill bar in front. Satchell and Way suggested
that the receptors were involved in a breath by breath regulation of
breathing rate, the duration of the inspiratory period and the following
pause being determined by the level of activity during the preceding
expiratory period. Thus in an almost completely curarized animal the
breathing rate became largely independent of imposed ventilation
volume, suggesting that the mechanoreceptors were not excited by
abnormally small breathing movements. In the normal animal, however,
the decrease in rate as ventilation was progressively restricted resulted
from greater excitation of the mechanoreceptors because the amplitude
of breathing increased. The original observations of the relationship between water flow and breathing rate must now be explained in terms of
a change in amplitude of breathing. The receptors and pathways involved in this response are not known. The inhibitory reflex is unusual
in that the receptors are excited during orobranchial contraction and
decrease breathing rate by delaying the onset of the next expiration
phase after a longer inspiration and pause.
Section of the branchial branches of the IXth and Xth cranial
nerves in the shark caused an increase in breathing rate; inspiration was
quicker and the pause disappeared (Satchell, 1959). This correlates well
with the proposed inhibitory role of the mechanoreceptors. In teleost fish
the effects of IXth and Xth nerve section are not so well established.
Shelton (1959) showed that the immediate change caused by sectioning
the nerves in tench was an increase in amplitude of the breathing movements. The nature of the receptors and the function of the afferent
pathways involved in this response were not defined. Powers and Clark
(1942) had earlier done similar experiments and had found that breathing failed after section of nerves IX and X. The gill blood vessels were
also sectioned in their experiments. Effects other than the direct ones of
nerve section may therefore have been complicating the results, although
the respiratory center seems to be quite resistant to circulatory failure
( Serbenyuk, 1965). Serbenyuk et al. ( 1959) and Serbenyuk ( 1965)
found that rhythmic activity persisted in the center after section of
the branchial nerves or novocaine anesthesia of the gills in carp, although there was a considerable change in breathing pattern. There can
be little doubt that important afferents, probably from mechanoreceptors as well as other sense organs, are carried in the branchial nerves
of teleosts although their role cannot at present be precisely defined.