8. THE REGULATION OF BREATHING
321
the motor neurons, so that there is only slight justification for making a
distinction on functional grounds. The central respiratory drive potentials
(Eccles et al., 1962) recorded intracellularly in spinal motor neurons are
almost certainly derived directly or via interneurons from activity within
the medullary center. To the extent that the latter is independently
rhythmic it is therefore possible to treat it separately.
In the teleosts examined, rhythmic discharges of a respiratory nature
also occurred in the smaller cells of the motor nuclei (von Baumgarten
and Salmoiraghi, 1962) and in the reticular cells around the nuclei
(Shelton, 1961). There was evidence that the active cells were not uniformally distributed throughout the respiratory area. In general, the
region which was most likely to result in a successful insertion of the
searching electrode was immediately behind the cerebellum slightly
to either side of the midline (Shelton, 1961). If the search was carried
on anterior or posterior to this region, the chances of making a successful electrode insertion were decreased. This could account for some of
the discrepancies in different investigators’ results. The full extent of the
area defined would depend very greatly on the number of electrode insertions made and, if the distribution varies between animals, on the
number of fish studied. There was, in fact, reason to suggest that the
respiratory area was not constant from fish to fish or in the same fish
at different times (von Baumgarten and Salmoiraghi, 1962; Shelton,
1961). For example, it was often the case that activity once located could
not be relocated by positioning the electrode in the same region at a
later time. Von Baumgarten and Salmoiraghi (1962) suggested that
the extent of the area depended upon the depth of respiration. Certainly
there is good evidence that the behavior of neurons can alter with this
type of variation. Electrodes chronically implanted in the brains of
tench or African perch detected changes both in activity, and in the
number of units active, when breathing was stimulated by lack of oxygen
or high levels of carbon dioxide (Young, 1969). It seems very likely
that the number of neurons, if not the general area they occupy in the
medulla, reflects changes in ventilation, both tending to increase or decrease together.
There is very little information available on the central coordination
of respiration in elasmobranchs. The early work of Hyde (1904) and
Springer ( 1928), based on ablation and transection experiments, has not
been extended. Hyde believed that independent segmental units associated with the VIIth, IXth, and Xth nuclei existed in the skate medulla
but this was questioned by Springer. Both workers showed that the
medulla was capable independently of coordinating rhythmic breathing,
although Springer claimed that some supramedullary influences were
321
the motor neurons, so that there is only slight justification for making a
distinction on functional grounds. The central respiratory drive potentials
(Eccles et al., 1962) recorded intracellularly in spinal motor neurons are
almost certainly derived directly or via interneurons from activity within
the medullary center. To the extent that the latter is independently
rhythmic it is therefore possible to treat it separately.
In the teleosts examined, rhythmic discharges of a respiratory nature
also occurred in the smaller cells of the motor nuclei (von Baumgarten
and Salmoiraghi, 1962) and in the reticular cells around the nuclei
(Shelton, 1961). There was evidence that the active cells were not uniformally distributed throughout the respiratory area. In general, the
region which was most likely to result in a successful insertion of the
searching electrode was immediately behind the cerebellum slightly
to either side of the midline (Shelton, 1961). If the search was carried
on anterior or posterior to this region, the chances of making a successful electrode insertion were decreased. This could account for some of
the discrepancies in different investigators’ results. The full extent of the
area defined would depend very greatly on the number of electrode insertions made and, if the distribution varies between animals, on the
number of fish studied. There was, in fact, reason to suggest that the
respiratory area was not constant from fish to fish or in the same fish
at different times (von Baumgarten and Salmoiraghi, 1962; Shelton,
1961). For example, it was often the case that activity once located could
not be relocated by positioning the electrode in the same region at a
later time. Von Baumgarten and Salmoiraghi (1962) suggested that
the extent of the area depended upon the depth of respiration. Certainly
there is good evidence that the behavior of neurons can alter with this
type of variation. Electrodes chronically implanted in the brains of
tench or African perch detected changes both in activity, and in the
number of units active, when breathing was stimulated by lack of oxygen
or high levels of carbon dioxide (Young, 1969). It seems very likely
that the number of neurons, if not the general area they occupy in the
medulla, reflects changes in ventilation, both tending to increase or decrease together.
There is very little information available on the central coordination
of respiration in elasmobranchs. The early work of Hyde (1904) and
Springer ( 1928), based on ablation and transection experiments, has not
been extended. Hyde believed that independent segmental units associated with the VIIth, IXth, and Xth nuclei existed in the skate medulla
but this was questioned by Springer. Both workers showed that the
medulla was capable independently of coordinating rhythmic breathing,
although Springer claimed that some supramedullary influences were