11. ELECTRORECEPTlON
51 7
D. Ampullae of Lorenzini
The ampullae of Lorenzini have been most completely studied in the
skate, but the properties are probably the same in all elasmobranchs.
The ampullae themselves tend to lie deep beneath the surface, but they
are connected to the exterior by canals that may be many centimeters long
(Figs. 6 and 7). It has been found that the canal wall is of very high resistance, as high as 10 M a cm2, and that the space constant of the canal
for low frequency signals is long compared to their length (Waltman,
1966). The capacity of the canal wall is appreciable, and there is considerable attenuation of higher frequency signals in longer canals. As in
the tonic receptors of freshwater fish, the receptor cells are embedded in
the wall of the ampullary cavity with only a small portion of their surface
exposed to the lumen. In the skate there may be a single cilium extending into the lumen, a feature lacking in other electroreceptors studied
ultrastructurally ( Waltman, 1966). At their bases the cells form typical
receptor synapses with the innervating nerve fibers, where there are presynaptic vesicles and ribbons. A minor difference from the freshwater
species is that several nerve fibers innervate the receptor cells in a single
ampulla.
As at the other tonic receptors, nerve fibers innervating the ampullae
of Lorenzini are tonically active (Fig. 17A; Murray, 1962, 1965, 1967).
However, at these receptors, the sign of stimulating currents is opposite
to that in the freshwater species; cathodal stimuli applied in the canal
lumen increase and anodal stimuli decrease the tonic discharge (Figs.
17B-D ) . The changes are approximately linear for small signals ( Murray,
1965). The receptors are also more sensitive than those in the freshwater
species. A microvolt or two has been reported to produce a perceptible
change in impulse frequency (Murray, 1967). The receptors show considerable accommodation to maintained stimuli, and a stimulus causing a
strong acceleration is followed by a silent period (Fig. 17). The accommodation is associated with approximately constant sensitivity in terms
of incremental stimuli; that is, a small brief stimulus evokes the same
change in impulse frequency if it is given alone or if it is superimposed on
a long-lasting stimulus after the impulse frequency has reached a new
steady state plateau level.
Receptor cells of ampullae of Lorenzini appear electrically excitable
( Obara and Bennett, 1968; Waltman, 1968 ). In many ampullae cathodal
stimuli can evoke a rather long-lasting all-or-none impulse that recorded
in the canal lumen is negative-going (Fig. 17D). In other preparations
only a graded response is obtained. The polarity of the impulse is what
one would observe if the outer membrane of the receptor cells generated
51 7
D. Ampullae of Lorenzini
The ampullae of Lorenzini have been most completely studied in the
skate, but the properties are probably the same in all elasmobranchs.
The ampullae themselves tend to lie deep beneath the surface, but they
are connected to the exterior by canals that may be many centimeters long
(Figs. 6 and 7). It has been found that the canal wall is of very high resistance, as high as 10 M a cm2, and that the space constant of the canal
for low frequency signals is long compared to their length (Waltman,
1966). The capacity of the canal wall is appreciable, and there is considerable attenuation of higher frequency signals in longer canals. As in
the tonic receptors of freshwater fish, the receptor cells are embedded in
the wall of the ampullary cavity with only a small portion of their surface
exposed to the lumen. In the skate there may be a single cilium extending into the lumen, a feature lacking in other electroreceptors studied
ultrastructurally ( Waltman, 1966). At their bases the cells form typical
receptor synapses with the innervating nerve fibers, where there are presynaptic vesicles and ribbons. A minor difference from the freshwater
species is that several nerve fibers innervate the receptor cells in a single
ampulla.
As at the other tonic receptors, nerve fibers innervating the ampullae
of Lorenzini are tonically active (Fig. 17A; Murray, 1962, 1965, 1967).
However, at these receptors, the sign of stimulating currents is opposite
to that in the freshwater species; cathodal stimuli applied in the canal
lumen increase and anodal stimuli decrease the tonic discharge (Figs.
17B-D ) . The changes are approximately linear for small signals ( Murray,
1965). The receptors are also more sensitive than those in the freshwater
species. A microvolt or two has been reported to produce a perceptible
change in impulse frequency (Murray, 1967). The receptors show considerable accommodation to maintained stimuli, and a stimulus causing a
strong acceleration is followed by a silent period (Fig. 17). The accommodation is associated with approximately constant sensitivity in terms
of incremental stimuli; that is, a small brief stimulus evokes the same
change in impulse frequency if it is given alone or if it is superimposed on
a long-lasting stimulus after the impulse frequency has reached a new
steady state plateau level.
Receptor cells of ampullae of Lorenzini appear electrically excitable
( Obara and Bennett, 1968; Waltman, 1968 ). In many ampullae cathodal
stimuli can evoke a rather long-lasting all-or-none impulse that recorded
in the canal lumen is negative-going (Fig. 17D). In other preparations
only a graded response is obtained. The polarity of the impulse is what
one would observe if the outer membrane of the receptor cells generated