10. ELECTRIC ORGANS
441
including resistance changes and weak electric pulses (Szabo and
Fessard, 1965; Moller, 1971 ) . Accelerations can be classically conditioned
as well as operantly conditioned in an avoidance paradigm (Mandriota
et al., 1965, 1968). Brief interruptions of discharge can also occur in
response to novel stimuli and in interaction with other electric fish.
The electric organs lie just anterior to the caudal fin (Fig. 1). They
are made up of four columns of cells and each column contains 100-200
cells in series. The cells are accurately aligned one behind the other.
In many species the body narrows close to the tail to form a “caudal
peduncle” and almost the entire cross section of the fish in this region
is made up of electric organ. Each column of electrocytes occupies one
quadrant and the only other structures are spinal column, skin, and
tendons to the caudal fin.
The cells are innervated by spinal neurons that lie in three segments
in the central region of the electric organ that extends over 8 or 10
segments. The nerve fibers end on stalks as in Mahpterurus and some
gymnotids, but the stalks are much more complex (Szabo, 1958, 1961a;
Bennett and Grundfest, 1961~). A large number of stalks arise from
the posterior faces (Fig. 61). These stalks fuse repeatedly (binarily )
to form a greatly reduced number of stalks before they are innervated.
In electrocytes of Movmyrus there may be more than 10 separate stalk
systems, each with its own site of innervation. In Mormyrops there are
one or two. In Gnuthonemus, Petrocephalus, and Marcusenius there is
only one.
There is a further complication in some species of Gnathonemus.
After varying numbers of fusions the stalks turn anteriorly and pass
Fig. 61. Anatomy of mormyrid electrocytes. ( A-C) Single isolated cells stained
with methylene blue, ( A ) Caudal surface of a cell from Mormyrus ruma The
darkly stained, thick, branching nerve trunk innervates ( i ) at least eleven separate
systems of stalks ( s ) . ( B ) Rostral surface of an electrocyte of Gnathonemus compressirostris with penetrating stalks. The regions surrounding the penetrations ( p )
are darkly stained, elsewhere the stalks ( s ) and body of the cell are only lightly
stained. A few stalks are seen running to the site where they are all fused and
innervated ( i ) . Under high magnification details of the stalk system could be
seen, and the stalks on half the rostral surface of this cell are drawn in Fig. 67C.
( C ) Nonpenetrating stalk system torn off an electrocyte of G. compressirostris
and heavily stained. The innervation ( i ) may be seen surrounding the stalk ( s )
at the central region. ( D ) Parasagittal section through the electric organ of G.
tamndua stained with hematoxylin and eosin. Rostral surface up. Bodies of two
electrocytes, about 30 p thick and heavily stained, run horizontally across the figure.
Fine stalks ( s ) , about 10 p in diameter, arise from the caudal surface, fuse, and
pass through penetrations ( p ) in the bodies of the cells to the rostral surfaces
where they leave the plane of section. The regions of innervation ( i ) were included in the section and lie rostral to the cell bodies. Darkly stained nuclei are
seen in the stalk and in the bodies of the cells. From Bennett and Grundfest ( 1 9 6 1 ~ ) .
441
including resistance changes and weak electric pulses (Szabo and
Fessard, 1965; Moller, 1971 ) . Accelerations can be classically conditioned
as well as operantly conditioned in an avoidance paradigm (Mandriota
et al., 1965, 1968). Brief interruptions of discharge can also occur in
response to novel stimuli and in interaction with other electric fish.
The electric organs lie just anterior to the caudal fin (Fig. 1). They
are made up of four columns of cells and each column contains 100-200
cells in series. The cells are accurately aligned one behind the other.
In many species the body narrows close to the tail to form a “caudal
peduncle” and almost the entire cross section of the fish in this region
is made up of electric organ. Each column of electrocytes occupies one
quadrant and the only other structures are spinal column, skin, and
tendons to the caudal fin.
The cells are innervated by spinal neurons that lie in three segments
in the central region of the electric organ that extends over 8 or 10
segments. The nerve fibers end on stalks as in Mahpterurus and some
gymnotids, but the stalks are much more complex (Szabo, 1958, 1961a;
Bennett and Grundfest, 1961~). A large number of stalks arise from
the posterior faces (Fig. 61). These stalks fuse repeatedly (binarily )
to form a greatly reduced number of stalks before they are innervated.
In electrocytes of Movmyrus there may be more than 10 separate stalk
systems, each with its own site of innervation. In Mormyrops there are
one or two. In Gnuthonemus, Petrocephalus, and Marcusenius there is
only one.
There is a further complication in some species of Gnathonemus.
After varying numbers of fusions the stalks turn anteriorly and pass
Fig. 61. Anatomy of mormyrid electrocytes. ( A-C) Single isolated cells stained
with methylene blue, ( A ) Caudal surface of a cell from Mormyrus ruma The
darkly stained, thick, branching nerve trunk innervates ( i ) at least eleven separate
systems of stalks ( s ) . ( B ) Rostral surface of an electrocyte of Gnathonemus compressirostris with penetrating stalks. The regions surrounding the penetrations ( p )
are darkly stained, elsewhere the stalks ( s ) and body of the cell are only lightly
stained. A few stalks are seen running to the site where they are all fused and
innervated ( i ) . Under high magnification details of the stalk system could be
seen, and the stalks on half the rostral surface of this cell are drawn in Fig. 67C.
( C ) Nonpenetrating stalk system torn off an electrocyte of G. compressirostris
and heavily stained. The innervation ( i ) may be seen surrounding the stalk ( s )
at the central region. ( D ) Parasagittal section through the electric organ of G.
tamndua stained with hematoxylin and eosin. Rostral surface up. Bodies of two
electrocytes, about 30 p thick and heavily stained, run horizontally across the figure.
Fine stalks ( s ) , about 10 p in diameter, arise from the caudal surface, fuse, and
pass through penetrations ( p ) in the bodies of the cells to the rostral surfaces
where they leave the plane of section. The regions of innervation ( i ) were included in the section and lie rostral to the cell bodies. Darkly stained nuclei are
seen in the stalk and in the bodies of the cells. From Bennett and Grundfest ( 1 9 6 1 ~ ) .