474
M. V. L. BENNETT
system “informs” the sensory system that an organ discharge is coming
(Bennett and Steinbach, 1969; see Chapter 11, this volume). The pathways mediating this activity and the effects on afferent volleys are just
being explored.
6. TORPEDINIDS
In Torpedo there is a command nucleus on each side that is located
deep in the medulla (Szabo, 1954; Albe-Fessard and Buser, 19%).
These nuclei activate the very large electromotor nuclei that lie on the
dorsal surface of the medulla (Fig. 8). The electric organ discharges
are synchronous on the two sides, and it is probable that either command nucleus can activate all the relay cells on both sides. NO data are
available about interaction between the two command nuclei, but some
coordinating mechanism is likely to be found. Preliminary data indicate
that the electromotor cells, which comprise a relay, are not coupled
to each other and that transmission from the command nucleus is
chemically mediated (cf. Saito, 1966). Single volleys from the command
nucleus excite single volleys in the relay cells that produce the single
PSPs in the electrocytes comprising the individual organ discharges.
7. THE ELECTRIC EEL
The mechanism of control in the eel shows an interesting variation.
All the electrocytes fire together during the large discharges, but only
Sachs’ organ and posterior part of Hunter’s organ are active during
the weak discharges. As in weakly electric gymnotids, the organs are
controlled by a single midline relay nucleus in the medulla (Bennett
et al., 1964; Bennett, 1968a). The pacemaker nucleus has not yet been
found in the eel. Each command volley from the medullary relay excites
all the spinal neurons that innervate the electrocytes, and the command
volley passes out ventral roots to anterior as well as posterior parts of
the organ. At low frequencies PSPs in the main organ and the anterior
part of Hunter’s organ are small and do not excite the cells, but they
are large enough to excite cells in Sachs’ organ and in the posterior
part of Hunter’s organ (Albe-Fessard and Chagas, 1954). When the
interval between command volleys becomes as small as several milliseconds, the PSPs in the main organ and anterior part of Hunter’s
organ greatly increase in amplitude and become adequate to excite the
cells of these organs. Thus the command to excite the main organ consists of high frequency activity in the same neurons that control Sachs’
organ. Only a single bulbospinal relay system is required to control
both organs. This increase in simplicity is obtained at the cost of several
M. V. L. BENNETT
system “informs” the sensory system that an organ discharge is coming
(Bennett and Steinbach, 1969; see Chapter 11, this volume). The pathways mediating this activity and the effects on afferent volleys are just
being explored.
6. TORPEDINIDS
In Torpedo there is a command nucleus on each side that is located
deep in the medulla (Szabo, 1954; Albe-Fessard and Buser, 19%).
These nuclei activate the very large electromotor nuclei that lie on the
dorsal surface of the medulla (Fig. 8). The electric organ discharges
are synchronous on the two sides, and it is probable that either command nucleus can activate all the relay cells on both sides. NO data are
available about interaction between the two command nuclei, but some
coordinating mechanism is likely to be found. Preliminary data indicate
that the electromotor cells, which comprise a relay, are not coupled
to each other and that transmission from the command nucleus is
chemically mediated (cf. Saito, 1966). Single volleys from the command
nucleus excite single volleys in the relay cells that produce the single
PSPs in the electrocytes comprising the individual organ discharges.
7. THE ELECTRIC EEL
The mechanism of control in the eel shows an interesting variation.
All the electrocytes fire together during the large discharges, but only
Sachs’ organ and posterior part of Hunter’s organ are active during
the weak discharges. As in weakly electric gymnotids, the organs are
controlled by a single midline relay nucleus in the medulla (Bennett
et al., 1964; Bennett, 1968a). The pacemaker nucleus has not yet been
found in the eel. Each command volley from the medullary relay excites
all the spinal neurons that innervate the electrocytes, and the command
volley passes out ventral roots to anterior as well as posterior parts of
the organ. At low frequencies PSPs in the main organ and the anterior
part of Hunter’s organ are small and do not excite the cells, but they
are large enough to excite cells in Sachs’ organ and in the posterior
part of Hunter’s organ (Albe-Fessard and Chagas, 1954). When the
interval between command volleys becomes as small as several milliseconds, the PSPs in the main organ and anterior part of Hunter’s
organ greatly increase in amplitude and become adequate to excite the
cells of these organs. Thus the command to excite the main organ consists of high frequency activity in the same neurons that control Sachs’
organ. Only a single bulbospinal relay system is required to control
both organs. This increase in simplicity is obtained at the cost of several