10. ELECTRIC ORGANS
417
quency of any known electric fish. Depending on the species, individual,
and temperature the frequency ranges from about 700 to 1700 impulses
per second ( Steinbach, 1970). The discharge shape varies somewhat with
the species, and will be treated separately below. The frequency is
generally highly stable and is not affected by ordinary stimuli. Weak
electric stimulation at closely neighboring frequencies does evoke small
shifts in frequency, the “jamming avoidance” response ( Larimer and
MacDonald, 1968; Bullock, 1970; see Chapter 11, this volume). Several
species have been observed to accelerate their discharge briefly, emitting
a “chirp” (Bullock, 1970). These changes may function in intraspecific
communication and in species recognition.
The electrocytes of the sternarchids turn out to be spinal neurons
rather than cells of myogenic origin (de Oliveira Castro, 1955; Bennett,
1966, 1970). The best-studied species is Sternarchus albifrons because
this is the one most commonly obtained by tropical fish dealers who call
it the black ghost knife fish. The fish, electric organ, and electrocytes are
shown diagrammatically in Fig. 42. The organ runs longitudinally just
ventral to the spinal column over most of the length of the body. The
axons of the spinal neurons descend from the cord and enter the electric
organ (Fig. 43). They then run anteriorly for several segments, turn
around and run posteriorly to end blindly at about the same level that
they entered the organ. This course is established by dissection of single
fibers from their point of entry into the organ until their termination.
Evidently the organ has lost its myogenic electrocytes and enlarged the
axons that formerly innervated them. This origin is confirmed by the
existence of a nerve plexus in Gymnotus, Hypopomus, Steatogenys, and
Gymnorhumphichthys in the same location as the sternarchid electric
organ. In Gymnotus axons in this plexus have been traced to the (myogenic) electrocytes. The plexus is absent in Sternopygus, Eigenmannia,
and Electrophorus, a fact which has implications for the phylogenetic
relations within the gymnotid group.
There are characteristic changes in the nerve fibers as they run along
in the electric organ. In both its anteriorly and posteriorly running parts
it becomes greatly dilated and can exceed 100p in diameter. Before it
enters the organ and where it turns around to run posteriorly, it is of the
usual diameter for a large myelinated fiber, 10-2Op. It tapers gradually
before it terminates. The nodal structure also changes characteristically
along the fiber (Fig. 44). The nodes become somewhat enlarged just
after the fiber enters the organ. As the fiber dilates, the nodes become
very narrow (although area is difficult to estimate because of the increased fiber diameter). As the fiber tapers near its most anterior part,
there are several very long nodes. The nodes become of ordinary size
again as the fiber turns around. This sequence repeats itself in the
417
quency of any known electric fish. Depending on the species, individual,
and temperature the frequency ranges from about 700 to 1700 impulses
per second ( Steinbach, 1970). The discharge shape varies somewhat with
the species, and will be treated separately below. The frequency is
generally highly stable and is not affected by ordinary stimuli. Weak
electric stimulation at closely neighboring frequencies does evoke small
shifts in frequency, the “jamming avoidance” response ( Larimer and
MacDonald, 1968; Bullock, 1970; see Chapter 11, this volume). Several
species have been observed to accelerate their discharge briefly, emitting
a “chirp” (Bullock, 1970). These changes may function in intraspecific
communication and in species recognition.
The electrocytes of the sternarchids turn out to be spinal neurons
rather than cells of myogenic origin (de Oliveira Castro, 1955; Bennett,
1966, 1970). The best-studied species is Sternarchus albifrons because
this is the one most commonly obtained by tropical fish dealers who call
it the black ghost knife fish. The fish, electric organ, and electrocytes are
shown diagrammatically in Fig. 42. The organ runs longitudinally just
ventral to the spinal column over most of the length of the body. The
axons of the spinal neurons descend from the cord and enter the electric
organ (Fig. 43). They then run anteriorly for several segments, turn
around and run posteriorly to end blindly at about the same level that
they entered the organ. This course is established by dissection of single
fibers from their point of entry into the organ until their termination.
Evidently the organ has lost its myogenic electrocytes and enlarged the
axons that formerly innervated them. This origin is confirmed by the
existence of a nerve plexus in Gymnotus, Hypopomus, Steatogenys, and
Gymnorhumphichthys in the same location as the sternarchid electric
organ. In Gymnotus axons in this plexus have been traced to the (myogenic) electrocytes. The plexus is absent in Sternopygus, Eigenmannia,
and Electrophorus, a fact which has implications for the phylogenetic
relations within the gymnotid group.
There are characteristic changes in the nerve fibers as they run along
in the electric organ. In both its anteriorly and posteriorly running parts
it becomes greatly dilated and can exceed 100p in diameter. Before it
enters the organ and where it turns around to run posteriorly, it is of the
usual diameter for a large myelinated fiber, 10-2Op. It tapers gradually
before it terminates. The nodal structure also changes characteristically
along the fiber (Fig. 44). The nodes become somewhat enlarged just
after the fiber enters the organ. As the fiber dilates, the nodes become
very narrow (although area is difficult to estimate because of the increased fiber diameter). As the fiber tapers near its most anterior part,
there are several very long nodes. The nodes become of ordinary size
again as the fiber turns around. This sequence repeats itself in the