10. ELECTRIC ORGANS
465
activity can normally be conducted in either direction across some of
these synapses, pre- and postsynaptic denote the usual morphological
relations rather than the direction of impulse propagation. ) Coupling can
also be mediated by both presynaptic fibers and dendrodendritic
synapses in the same nucleus ( Pappas and Bennett, 1966).
A. Pathways and Patterns of Neural Activity
The circuitry of electric organ command systems in teleosts that have
been studied physiologically is diagrammed in Fig. 73. The physiological
analysis has generally involved microelectrode recording from the entire
animal paralyzed or anesthetized.
1. THE ELECTRIC CATFISH
The simplest control system is in the electric catfish (Bennett et al.,
1967b). Two neurons lie in the first spinal segment, one on either side
(Fig. 72A). Each neuron innervates all the electrocytes on its side and
each impulse fires the electrocytes once. The two neurons are closely
coupled electrotonically, and hyperpolarization produced in one cell
spreads to the other (Fig. 74C ) . The coupling is so close that an impulse
initiated in one cell propagates into the other (Fig. 74B). The pathway
of coupling is by way of presynaptic fibers (Fig. 72). Excitatory inputs
gradedly depolarize the cells (Fig. 74A,D), and when one cell is excited
the other must fire also. This explains why no stimulus can be found that
excites one cell without exciting the other.
That the two cells comprise the command nucleus is indicated by the
gradual rise in potential when organ discharge is evoked by cutaneous
stimulation (Fig. 74D). When PSPs in one cell exceed threshold, both
cells are depolarized since the presynaptic fibers end on both; once initiated the impulse rapidly propagates between the cells. The conduction
Fig. 71. Morphological basis of electrotonic coupling: dendrodendritic junctions
between electromotor neurons of the mormyrid Gnathonemus. ( A ) In a silver
stained preparation ( Romanes’ method) of the medullary relay nucleus, a thick
bridge appears to connect the two cell bodies without there being any intervening
membrane. ( B ) With the electron microscope such cells are seen not to have
cytoplasmic continuity. The cell bodies ( s ) are separated by membranes the ends
of which are indicated by the arrows. Blood vessels ( b v ) and myelinated nerve
fibers are seen. Axon terminals ( a ) make contact with the cells; one on the lower
right shows a portion of the myelin sheath. ( C ) At a similar region of apposition
between spinal electromotor neurons, higher magnification shows large regions where
the membranes appear fused. In this very thin section the central dark region
appears as a series of dots. Osmic acid fixation. From Bennett et al. (1967a).
465
activity can normally be conducted in either direction across some of
these synapses, pre- and postsynaptic denote the usual morphological
relations rather than the direction of impulse propagation. ) Coupling can
also be mediated by both presynaptic fibers and dendrodendritic
synapses in the same nucleus ( Pappas and Bennett, 1966).
A. Pathways and Patterns of Neural Activity
The circuitry of electric organ command systems in teleosts that have
been studied physiologically is diagrammed in Fig. 73. The physiological
analysis has generally involved microelectrode recording from the entire
animal paralyzed or anesthetized.
1. THE ELECTRIC CATFISH
The simplest control system is in the electric catfish (Bennett et al.,
1967b). Two neurons lie in the first spinal segment, one on either side
(Fig. 72A). Each neuron innervates all the electrocytes on its side and
each impulse fires the electrocytes once. The two neurons are closely
coupled electrotonically, and hyperpolarization produced in one cell
spreads to the other (Fig. 74C ) . The coupling is so close that an impulse
initiated in one cell propagates into the other (Fig. 74B). The pathway
of coupling is by way of presynaptic fibers (Fig. 72). Excitatory inputs
gradedly depolarize the cells (Fig. 74A,D), and when one cell is excited
the other must fire also. This explains why no stimulus can be found that
excites one cell without exciting the other.
That the two cells comprise the command nucleus is indicated by the
gradual rise in potential when organ discharge is evoked by cutaneous
stimulation (Fig. 74D). When PSPs in one cell exceed threshold, both
cells are depolarized since the presynaptic fibers end on both; once initiated the impulse rapidly propagates between the cells. The conduction
Fig. 71. Morphological basis of electrotonic coupling: dendrodendritic junctions
between electromotor neurons of the mormyrid Gnathonemus. ( A ) In a silver
stained preparation ( Romanes’ method) of the medullary relay nucleus, a thick
bridge appears to connect the two cell bodies without there being any intervening
membrane. ( B ) With the electron microscope such cells are seen not to have
cytoplasmic continuity. The cell bodies ( s ) are separated by membranes the ends
of which are indicated by the arrows. Blood vessels ( b v ) and myelinated nerve
fibers are seen. Axon terminals ( a ) make contact with the cells; one on the lower
right shows a portion of the myelin sheath. ( C ) At a similar region of apposition
between spinal electromotor neurons, higher magnification shows large regions where
the membranes appear fused. In this very thin section the central dark region
appears as a series of dots. Osmic acid fixation. From Bennett et al. (1967a).