10. ELECTRIC ORGANS
385
pulses are emitted at quite low frequencies down to a few per minute.
When actively swimming, the pulse rate is usually increased to about
30 or more per second. Large pulses are emitted only in high frequency
bursts at a frequency of several hundred per second. The first pulse
in such a burst is small, but the amplitude increases to the maximum
within 2 or 3 pulses ( Albe-Fessard, 1950a). The weak pulses are implicated in the electrosensory system (see Chapter 11, this volume); the
strong pulses have offensive or defensive value. The emission of moderate
to high frequency pulses by one eel attracts other eels, and they also
increase their rates of discharge (Bullock, 1970).
The electric tissue occupies much of the cross section of the body
in the posterior three-fourths of the fish, and it is divided into three
(bilateral) organs. There is axial musculature dorsal to the organs and
a smaller amount of muscle ventrally that controls the anal fin (Fig. 17).
In the dorsal and posterior region is the organ of Sachs, and in the
ventral and anterior region is the main organ (Fig. 1). Beneath these
organs is Hunter’s organ. The electrocytes are more widely separated
and somewhat larger in Sachs’ organ than in the main organ, but otherwise the two organs are similar (Luft, 1957; Couceiro and Akerman,
1948). The main organ generates the greater part of the high voltage
discharge used offensively or defensively. The organ of Sachs generates
the greater part of the low voltage pulses involved in the electrosensory
system but also contributes in a minor way to the high voltage discharge.
Hunter’s organ apparently functions like main organ anteriorly and
Sachs’ organ posteriorly ( Albe-Fessard and Chagas, 1954).
The single electrocytes are flattened in the anterior posterior axis
(Fig. 17). They are ribbon-shaped and tend to run from the medial
septum to the lateral margin of the fish. In an adult there are dorsoventrally about 25 in the main organ and 10 in Hunter’s organ. There
are some 6000 in the anteroposterior axis ( Albe-Fessard, 1950a). Each
cell is contained in a connective tissue chamber, and successive layers
of cells in the anterior posterior direction are fairly accurately aligned,
one behind the other. The alignment is somewhat less good in the
dorsoventral direction.
The electrocytes of the main organ occupy perhaps one-fifth to onehalf the chamber volume; the fraction is smaller in Sachs’ organ (Luft,
1957; Couceiro and Akerman, 1948). The cells are innervated on their
posterior faces by spinal nerves. The posterior faces have a moderate
number of short papilli or stalks protruding from them that increase
the surface area; there are also some tubules in this face (Fig. 17). The
innervation is primarily on the stalks and is much less dense than in
electrocytes of torpedinids. The anterior faces have a large number of
385
pulses are emitted at quite low frequencies down to a few per minute.
When actively swimming, the pulse rate is usually increased to about
30 or more per second. Large pulses are emitted only in high frequency
bursts at a frequency of several hundred per second. The first pulse
in such a burst is small, but the amplitude increases to the maximum
within 2 or 3 pulses ( Albe-Fessard, 1950a). The weak pulses are implicated in the electrosensory system (see Chapter 11, this volume); the
strong pulses have offensive or defensive value. The emission of moderate
to high frequency pulses by one eel attracts other eels, and they also
increase their rates of discharge (Bullock, 1970).
The electric tissue occupies much of the cross section of the body
in the posterior three-fourths of the fish, and it is divided into three
(bilateral) organs. There is axial musculature dorsal to the organs and
a smaller amount of muscle ventrally that controls the anal fin (Fig. 17).
In the dorsal and posterior region is the organ of Sachs, and in the
ventral and anterior region is the main organ (Fig. 1). Beneath these
organs is Hunter’s organ. The electrocytes are more widely separated
and somewhat larger in Sachs’ organ than in the main organ, but otherwise the two organs are similar (Luft, 1957; Couceiro and Akerman,
1948). The main organ generates the greater part of the high voltage
discharge used offensively or defensively. The organ of Sachs generates
the greater part of the low voltage pulses involved in the electrosensory
system but also contributes in a minor way to the high voltage discharge.
Hunter’s organ apparently functions like main organ anteriorly and
Sachs’ organ posteriorly ( Albe-Fessard and Chagas, 1954).
The single electrocytes are flattened in the anterior posterior axis
(Fig. 17). They are ribbon-shaped and tend to run from the medial
septum to the lateral margin of the fish. In an adult there are dorsoventrally about 25 in the main organ and 10 in Hunter’s organ. There
are some 6000 in the anteroposterior axis ( Albe-Fessard, 1950a). Each
cell is contained in a connective tissue chamber, and successive layers
of cells in the anterior posterior direction are fairly accurately aligned,
one behind the other. The alignment is somewhat less good in the
dorsoventral direction.
The electrocytes of the main organ occupy perhaps one-fifth to onehalf the chamber volume; the fraction is smaller in Sachs’ organ (Luft,
1957; Couceiro and Akerman, 1948). The cells are innervated on their
posterior faces by spinal nerves. The posterior faces have a moderate
number of short papilli or stalks protruding from them that increase
the surface area; there are also some tubules in this face (Fig. 17). The
innervation is primarily on the stalks and is much less dense than in
electrocytes of torpedinids. The anterior faces have a large number of