520
M. V. L. BENNETT
causes the release of transmitter that leads to the tonic discharge in the
nerve. Each nerve fiber synapses with many receptor cells; thus, the relationship between receptor cell and nerve frequency is generally not one
to one. A small cathodal stimulus increases the frequency of activity of
the receptor cells. The initial effect of a cathodal stimulus on the inner
faces of the receptor cells is to hyperpolarize them, but the later and
predominant effect is depolarization because of increased activity of the
outer faces. Thus, more transmitter is secreted and the nerve frequency
is increased. Conversely an anodal stimulus decreases the frequency of
firing of the receptor cells and leads to a decrease in nerve discharge. (It
is the decrease of nerve response evoked by anodal stimuli that requires
the hypothesis that the receptor cells are tonically active.) As strength
of a cathodal stimulus is increased, more and more receptor cells are
brought into activity at the onset of the stimulus and a larger negativity
is evoked in the lumen of the canal. In this manner the response can become sufficiently regenerative to lead to an all-or-none impulse.
This theory predicts that strong cathodal stimuli would overcome the
depolarizing effect of the spikes generated in the outer faces of the
receptor cells and hyperpolarize the inner faces, thus blocking transmitter release and nerve discharge. Conversely strong anodal stimuli
would directly depolarize the inner faces and lead to excitation of the
nerve at a shorter latency than do cathodal stimuli. Both of these predictions have been verified (S. Obara and M. V. L. Bennett, unpublished
data).
Microelectrode studies of the receptor cells will be required to establish the proposed mechanisms. They explain several of the peculiarities
of the Lorenzinian receptors, namely, the opposite polarity of excitatory
stimuli and the long synaptic delay as compared to tonic receptors of
freshwater species. The action of the stimulus on an oscillating electrically
excitable system rather than on secretory membrane may be a factor in
the greater sensitivity of the ampullae of Lorenzini.
IV. PHASIC ELECTRORECEPTORS
The morphology of phasic receptors is distinctively different from
that of tonic receptors. The receptor lumen is not connected to the exterior by an obvious canal, and for this reason the phasic receptors have
been called tuberous organs (Szabo, 1965). Instead of a canal there is a
plug of loosely packed epithelial cells between the lumen of the ampulla
and the superficial layers of the epidermis (Figs. 19-21). Electron
microscopy reveals clefts between cells of the epithelial plug and super-
M. V. L. BENNETT
causes the release of transmitter that leads to the tonic discharge in the
nerve. Each nerve fiber synapses with many receptor cells; thus, the relationship between receptor cell and nerve frequency is generally not one
to one. A small cathodal stimulus increases the frequency of activity of
the receptor cells. The initial effect of a cathodal stimulus on the inner
faces of the receptor cells is to hyperpolarize them, but the later and
predominant effect is depolarization because of increased activity of the
outer faces. Thus, more transmitter is secreted and the nerve frequency
is increased. Conversely an anodal stimulus decreases the frequency of
firing of the receptor cells and leads to a decrease in nerve discharge. (It
is the decrease of nerve response evoked by anodal stimuli that requires
the hypothesis that the receptor cells are tonically active.) As strength
of a cathodal stimulus is increased, more and more receptor cells are
brought into activity at the onset of the stimulus and a larger negativity
is evoked in the lumen of the canal. In this manner the response can become sufficiently regenerative to lead to an all-or-none impulse.
This theory predicts that strong cathodal stimuli would overcome the
depolarizing effect of the spikes generated in the outer faces of the
receptor cells and hyperpolarize the inner faces, thus blocking transmitter release and nerve discharge. Conversely strong anodal stimuli
would directly depolarize the inner faces and lead to excitation of the
nerve at a shorter latency than do cathodal stimuli. Both of these predictions have been verified (S. Obara and M. V. L. Bennett, unpublished
data).
Microelectrode studies of the receptor cells will be required to establish the proposed mechanisms. They explain several of the peculiarities
of the Lorenzinian receptors, namely, the opposite polarity of excitatory
stimuli and the long synaptic delay as compared to tonic receptors of
freshwater species. The action of the stimulus on an oscillating electrically
excitable system rather than on secretory membrane may be a factor in
the greater sensitivity of the ampullae of Lorenzini.
IV. PHASIC ELECTRORECEPTORS
The morphology of phasic receptors is distinctively different from
that of tonic receptors. The receptor lumen is not connected to the exterior by an obvious canal, and for this reason the phasic receptors have
been called tuberous organs (Szabo, 1965). Instead of a canal there is a
plug of loosely packed epithelial cells between the lumen of the ampulla
and the superficial layers of the epidermis (Figs. 19-21). Electron
microscopy reveals clefts between cells of the epithelial plug and super-