M. V. L. BENNETT
394
trocytes can be clearly demonstrated by stimulating with externally
applied currents that run along the axis of the cell; these currents hyperpolarize one face and depolarize the other. An experiment of this kind is
illustrated in Fig. 21. When a stimulating cathode is placed external to
the uninnervated face, the applied current tends to depolarize this face
and hyperpolarize the innervated face ( Fig. 21B,). The uninnervated
face fires first as indicated by initial positivity of the response recorded
external to the innervated face. The spike recorded across the innervated
face arises from a hyperpolarized level of membrane potential confirming
that this face is not initiating the spike. The innervated face is excited by
the activity of the uninnervated face, and negativity external to this face
follows the positive phase. When the stimulating electrode external to the
uninnervated face is an anode, the innervated face is depolarized and
fires first; there is initial negativity outside this face (Fig. 21A1). Since
the uninnervated face is hyperpolarized by the applied current, its
firing is delayed compared to that evoked by neural or intracellular
stimulation and the spike recorded across the innervated face has two
quite distinct components. (Thc two spikc components are only barely
recognizable in the monopolar recordings of Fig. 20. )
Usually if stimulation only moderately above threshold is applied as
in Fig. 21A,,B,, the initially active face can excite the opposite face that
is being hyperpolarized by the stimulus. However, strong stimuli can
cause sufficient hyperpolarization of this face that the spike of the
initially firing face cxcitvs it only partially if at all. Correspondingly,
there is failure of the second spikc component recorded intracellularly
and disappearance or reduction of the second phase of the extcrnally
recorded responses (Fig. 21A2,B2).
Although in Gymnotus the single cells generate diphasic extcrnal
potentials, the overall organ discharge is triphasic. This configuration
results from the opposite orientation and earlier firing of the most dorsal
electrocytes (Fig. 22). These cells fire about ?d msec earlier than the
more ventral cells, all of which fire synchronously. The activity of thc
anterior faces of the dorsal cells results in the initial head negativity. The
activity of their uninnervated faces is simultaneous with the activity of
thc innervated faccs of the ventral cclls and summates with it to cause
the second, head negative phase. Activity of the uninnervated faces of the
ventral cclls causes the final, head negative phase. Corresponding to the
number of cells active, the initial phasc is the smallest, the second phase
is the largest, and the final phase is somewhat smaller than the second
phase.
In Gymnotus the electric organ has at its rostra1 region a small
number of modified cells that fire earlier than the main organ and
394
trocytes can be clearly demonstrated by stimulating with externally
applied currents that run along the axis of the cell; these currents hyperpolarize one face and depolarize the other. An experiment of this kind is
illustrated in Fig. 21. When a stimulating cathode is placed external to
the uninnervated face, the applied current tends to depolarize this face
and hyperpolarize the innervated face ( Fig. 21B,). The uninnervated
face fires first as indicated by initial positivity of the response recorded
external to the innervated face. The spike recorded across the innervated
face arises from a hyperpolarized level of membrane potential confirming
that this face is not initiating the spike. The innervated face is excited by
the activity of the uninnervated face, and negativity external to this face
follows the positive phase. When the stimulating electrode external to the
uninnervated face is an anode, the innervated face is depolarized and
fires first; there is initial negativity outside this face (Fig. 21A1). Since
the uninnervated face is hyperpolarized by the applied current, its
firing is delayed compared to that evoked by neural or intracellular
stimulation and the spike recorded across the innervated face has two
quite distinct components. (Thc two spikc components are only barely
recognizable in the monopolar recordings of Fig. 20. )
Usually if stimulation only moderately above threshold is applied as
in Fig. 21A,,B,, the initially active face can excite the opposite face that
is being hyperpolarized by the stimulus. However, strong stimuli can
cause sufficient hyperpolarization of this face that the spike of the
initially firing face cxcitvs it only partially if at all. Correspondingly,
there is failure of the second spikc component recorded intracellularly
and disappearance or reduction of the second phase of the extcrnally
recorded responses (Fig. 21A2,B2).
Although in Gymnotus the single cells generate diphasic extcrnal
potentials, the overall organ discharge is triphasic. This configuration
results from the opposite orientation and earlier firing of the most dorsal
electrocytes (Fig. 22). These cells fire about ?d msec earlier than the
more ventral cells, all of which fire synchronously. The activity of thc
anterior faces of the dorsal cells results in the initial head negativity. The
activity of their uninnervated faces is simultaneous with the activity of
thc innervated faccs of the ventral cclls and summates with it to cause
the second, head negative phase. Activity of the uninnervated faces of the
ventral cclls causes the final, head negative phase. Corresponding to the
number of cells active, the initial phasc is the smallest, the second phase
is the largest, and the final phase is somewhat smaller than the second
phase.
In Gymnotus the electric organ has at its rostra1 region a small
number of modified cells that fire earlier than the main organ and