M. V. L. BENNETT
480
(and then cease to be common). The command to a particular organ
may be expressed in terms of frequency.
Several of these modifications are already required for ordinary
motor systems. For example, some muscle fibers are innervated by
more than one nerve fiber, which is analogous to paired command
nuclei. If tension of a muscle fiber rather than any contraction at all
is considered, the command may be coded in terms of frequency of
firing. Furthermore, it is obvious that we do not understand all about
the control of an electric organ or a muscle fiber when we know the
final common path to it. We also need to know all the fibers afferent
to neurons in that path. We have taken the decision to fire the organ as
meaning the firing of the highest level nucleus of the final common
path. The decision could also be defined as activity in any subset of
fibers afferent to the highest nucleus that can cause the nucleus to fire.
Similarly, the relevant output of a whole muscle, its tension, may be
achieved (or coded for) by many different combinations of fibers active
at different frequencies.
For freshwater weakly electric fish and for the electric catfish the
neural circuitry guarantees that every electrocyte receives neural excitation in every discharge. The operation of the system is essentially
all-or-none and an organ discharge is present or absent. It should be
recognized that amplitude can vary minorly as a result of refractoriness
(and perhaps facilitation), but these changes must be secondary to
changes in frequency. The situation is similar in the eel except that
the amplitude normally varies over a wide range. In all these fish and
to a good approximation in Torpedo and the stargazers as well, the
behavior of the animal with respect to its discharge can be characterized
by a single series of time intervals, a simplicity which gives the system
some attractiveness for quantitative study.
Two additional characteristics are frequently found in electric organ
control systems. Where a group of neurons fire synchronously there
is likely to be positive feedback between the cells. In command nuclei
in which a highly synchronous volley arises, positive feedback must
be present and must be electrically mediated because chemically
mediated transmission is too slow. In relay nuclei there is no absolute
requirement for feedback, but cells are often coupled electrotonically,
presumably to increase synchronization. Another characteristic often
found in electric organ control systems is a tendency toward reduced
numbers of cells at higher levels. Apparently the decision to discharge the electric organ is always reached in a group of cells that
are few in number compared to the final generating cells. The most
striking example is found in the electric catfish, in which two neurons
480
(and then cease to be common). The command to a particular organ
may be expressed in terms of frequency.
Several of these modifications are already required for ordinary
motor systems. For example, some muscle fibers are innervated by
more than one nerve fiber, which is analogous to paired command
nuclei. If tension of a muscle fiber rather than any contraction at all
is considered, the command may be coded in terms of frequency of
firing. Furthermore, it is obvious that we do not understand all about
the control of an electric organ or a muscle fiber when we know the
final common path to it. We also need to know all the fibers afferent
to neurons in that path. We have taken the decision to fire the organ as
meaning the firing of the highest level nucleus of the final common
path. The decision could also be defined as activity in any subset of
fibers afferent to the highest nucleus that can cause the nucleus to fire.
Similarly, the relevant output of a whole muscle, its tension, may be
achieved (or coded for) by many different combinations of fibers active
at different frequencies.
For freshwater weakly electric fish and for the electric catfish the
neural circuitry guarantees that every electrocyte receives neural excitation in every discharge. The operation of the system is essentially
all-or-none and an organ discharge is present or absent. It should be
recognized that amplitude can vary minorly as a result of refractoriness
(and perhaps facilitation), but these changes must be secondary to
changes in frequency. The situation is similar in the eel except that
the amplitude normally varies over a wide range. In all these fish and
to a good approximation in Torpedo and the stargazers as well, the
behavior of the animal with respect to its discharge can be characterized
by a single series of time intervals, a simplicity which gives the system
some attractiveness for quantitative study.
Two additional characteristics are frequently found in electric organ
control systems. Where a group of neurons fire synchronously there
is likely to be positive feedback between the cells. In command nuclei
in which a highly synchronous volley arises, positive feedback must
be present and must be electrically mediated because chemically
mediated transmission is too slow. In relay nuclei there is no absolute
requirement for feedback, but cells are often coupled electrotonically,
presumably to increase synchronization. Another characteristic often
found in electric organ control systems is a tendency toward reduced
numbers of cells at higher levels. Apparently the decision to discharge the electric organ is always reached in a group of cells that
are few in number compared to the final generating cells. The most
striking example is found in the electric catfish, in which two neurons