548
M. V. L. BENNETT
tion of the field. For longitudinally oriented stimuli the receptors at the
rostral and caudal ends of the fish are most affected. There is a smaller
potential across the skin near the center of the fish, and receptors in this
region are less affected. Since current enters the fish at one end and
leaves at the other, the polarity of stimulation is opposite at the two ends.
Near the center of the fish there is a point or rather a line around the
fish where current is parallel to the body surface and there is no potential across the skin. Most fish are considerably longer than they are
wide, and the maximum voltage across the animal is greater for longitudinally oriented fields than for transversely oriented fields. Thus, it
would be expected that the fish's sensitivity is considerably greater for a
field oriented in the longitudinal direction.
The effect of receptor location on sensitivity to longitudinally oriented
stimuli is illustrated for large receptors of a mormyrid in Fig. 36. There is
no consistent variation along the body of thresholds to locally applied
stimuli. However, longitudinally oriented stimuli excite receptors at the
ends of the fish at much lower thresholds than they excite the receptors
in the center of the body. Moreover, there is a sharply localized transition
region anterior to which the receptors respond to onset of head-positive
stimuli and caudal to which the receptors respond to onset of headnegative stimuli. These data indicate that when head-positive stimuli are
applied, current enters the fish anterior to the transition region and exits
posterior to it. The lower sensitivity near the center of the body indicates
that a much smaller fraction of the applied voltage is developed across
the skin in this region. The greater sensitivity of the receptors in the head
region as compared to the most caudal ones may mean that the skin is
of higher resistance rostrally; alternatively, the most caudal receptors are
located well rostral to the tail (Fig. l ) , which also could account for
their lower sensitivity.
In freshwater electric fish the arrangement of the receptors and accessory structures probably represents a compromise between the requirements of detecting weak fields of extrinsic origin, of establishing the field
distribution, and of setting up and measuring the field of the fish's own
organ. The distribution of skin capacity may be important in receptor
responses, particularly since tonic and phasic receptors are sensitive to
different frequency ranges. Accurate measurements must be obtained
to distinguish the resistance and capacity of the receptors from those of
surrounding skin, and more complete studies of regional differences are
required. The flattened epidermal cells of mormyrids ( F in Fig. 10) are
present only in the receptor regions; the skin resistance is not much lower
on the sides of the fish (Bennett, 1965), but presumably the capacity is
greater. In Torpedo the skin resistance over the electric organs is lower
than that over the remainder of the body, which tends to maximize cur-
M. V. L. BENNETT
tion of the field. For longitudinally oriented stimuli the receptors at the
rostral and caudal ends of the fish are most affected. There is a smaller
potential across the skin near the center of the fish, and receptors in this
region are less affected. Since current enters the fish at one end and
leaves at the other, the polarity of stimulation is opposite at the two ends.
Near the center of the fish there is a point or rather a line around the
fish where current is parallel to the body surface and there is no potential across the skin. Most fish are considerably longer than they are
wide, and the maximum voltage across the animal is greater for longitudinally oriented fields than for transversely oriented fields. Thus, it
would be expected that the fish's sensitivity is considerably greater for a
field oriented in the longitudinal direction.
The effect of receptor location on sensitivity to longitudinally oriented
stimuli is illustrated for large receptors of a mormyrid in Fig. 36. There is
no consistent variation along the body of thresholds to locally applied
stimuli. However, longitudinally oriented stimuli excite receptors at the
ends of the fish at much lower thresholds than they excite the receptors
in the center of the body. Moreover, there is a sharply localized transition
region anterior to which the receptors respond to onset of head-positive
stimuli and caudal to which the receptors respond to onset of headnegative stimuli. These data indicate that when head-positive stimuli are
applied, current enters the fish anterior to the transition region and exits
posterior to it. The lower sensitivity near the center of the body indicates
that a much smaller fraction of the applied voltage is developed across
the skin in this region. The greater sensitivity of the receptors in the head
region as compared to the most caudal ones may mean that the skin is
of higher resistance rostrally; alternatively, the most caudal receptors are
located well rostral to the tail (Fig. l ) , which also could account for
their lower sensitivity.
In freshwater electric fish the arrangement of the receptors and accessory structures probably represents a compromise between the requirements of detecting weak fields of extrinsic origin, of establishing the field
distribution, and of setting up and measuring the field of the fish's own
organ. The distribution of skin capacity may be important in receptor
responses, particularly since tonic and phasic receptors are sensitive to
different frequency ranges. Accurate measurements must be obtained
to distinguish the resistance and capacity of the receptors from those of
surrounding skin, and more complete studies of regional differences are
required. The flattened epidermal cells of mormyrids ( F in Fig. 10) are
present only in the receptor regions; the skin resistance is not much lower
on the sides of the fish (Bennett, 1965), but presumably the capacity is
greater. In Torpedo the skin resistance over the electric organs is lower
than that over the remainder of the body, which tends to maximize cur-