3. AUTONOMIC NERVOUS SYSTEMS
119
such fibers exist in fish. Campbell and Burnstock (1968) have pointed
out that many of the excitatory responses to vagal stimulation recorded
by earlier workers on both selachian and teleostean gut do not start
until some time after the period of stimulation of the nerves is over (see
Fig. 4). In this respect the contractions appear more like the “rebound
or recovery contractions which follow responses to inhibitory nerve
stimulation in mammalian gut than like the virtually immediate primary
contractions caused by stimulation of the excitatory innervation. In other
words, there is still considerable doubt as to whether the contractions
seen in fish stomach preparations following vagus nerve stimulation are
mediated by excitatory or inhibitory nerves. The absence of records of
actual inhibition could be easily explained under the conditions of experimentation for the nerves have been stimulated for relatively short periods,
the spontaneous contractions occur at a very low rate, if at all, and the
musculature has little or no tonus. Pharmacological investigations have
been of little help in answering this question so far. For instance, vagal
excitation of the stomach is not inhibited by atropine in the brown trout
(Burnstock, 1958b) or in Raja (Babkin et al., 1935), whereas vagal excitation of the smooth muscle of the intestine of the tench is prevented
by atropine (MBhBs and Wolsky, 1932); in all three preparations,
atropine prevents the excitatory action of acetylcholine. Only further
experimentation can settle this question.
There have been no further investigations of neural control of gastric,
hepatic, or pancreatic secretion in any fish since the subject was reviewed by Barrington in 1957. One can only agree with Barrington
that there is no evidence for such a nervous control, while remarking
that there have been too few investigations to allow any conclusion
to be reached.
The processes causing filling and emptying of the swim bladder of
teleosts are complex (see chapter by Steen, this volume). At least
three processes seem to be implicated in determining whether a net
secretion or absorption of gases occurs. First, the relative amounts of
secretory and absorptive epithelium exposed to the lumen of the swim
bladder can be varied by differential contraction of the muscularis
mucosae or by variable closure of the oval (Fange, 1953). Second, the
rates of blood perfusion of the secretory epithelium, and therefore of
the retia mirabilia, and of the resorptive epithelium are probably independently controlled. Third, the activity of the secretory cells themselves, which appear to act by adding metabolites to the blood leaving
the secretory epithelium (see Kuhn et aZ., 1963), is probably under
nervous control.
Bohr (1894) first showed that following section of the vagi, experi-
119
such fibers exist in fish. Campbell and Burnstock (1968) have pointed
out that many of the excitatory responses to vagal stimulation recorded
by earlier workers on both selachian and teleostean gut do not start
until some time after the period of stimulation of the nerves is over (see
Fig. 4). In this respect the contractions appear more like the “rebound
or recovery contractions which follow responses to inhibitory nerve
stimulation in mammalian gut than like the virtually immediate primary
contractions caused by stimulation of the excitatory innervation. In other
words, there is still considerable doubt as to whether the contractions
seen in fish stomach preparations following vagus nerve stimulation are
mediated by excitatory or inhibitory nerves. The absence of records of
actual inhibition could be easily explained under the conditions of experimentation for the nerves have been stimulated for relatively short periods,
the spontaneous contractions occur at a very low rate, if at all, and the
musculature has little or no tonus. Pharmacological investigations have
been of little help in answering this question so far. For instance, vagal
excitation of the stomach is not inhibited by atropine in the brown trout
(Burnstock, 1958b) or in Raja (Babkin et al., 1935), whereas vagal excitation of the smooth muscle of the intestine of the tench is prevented
by atropine (MBhBs and Wolsky, 1932); in all three preparations,
atropine prevents the excitatory action of acetylcholine. Only further
experimentation can settle this question.
There have been no further investigations of neural control of gastric,
hepatic, or pancreatic secretion in any fish since the subject was reviewed by Barrington in 1957. One can only agree with Barrington
that there is no evidence for such a nervous control, while remarking
that there have been too few investigations to allow any conclusion
to be reached.
The processes causing filling and emptying of the swim bladder of
teleosts are complex (see chapter by Steen, this volume). At least
three processes seem to be implicated in determining whether a net
secretion or absorption of gases occurs. First, the relative amounts of
secretory and absorptive epithelium exposed to the lumen of the swim
bladder can be varied by differential contraction of the muscularis
mucosae or by variable closure of the oval (Fange, 1953). Second, the
rates of blood perfusion of the secretory epithelium, and therefore of
the retia mirabilia, and of the resorptive epithelium are probably independently controlled. Third, the activity of the secretory cells themselves, which appear to act by adding metabolites to the blood leaving
the secretory epithelium (see Kuhn et aZ., 1963), is probably under
nervous control.
Bohr (1894) first showed that following section of the vagi, experi-