Electric Organ Discharge and Electrosensory Reafference in Skates

12 Pages·1994·4.7 MB·English

by  J G New

Reference: Biol. Bull 187: 64-75. (August, 1994)
Electric Organ Discharge and Electrosensory
Reafference in Skates
JOHN G. NEW
Marine Biological Laboratory, Woods Hole, Massachusetts 02543, andDepartment ofBiologyand
The FamilyHearing Institute, Loyola University ofChicago, Chicago. Illinois 60626
Abstract. Skates possess bilateral electric organs that sincethelastcentury (Stark, 1844;Ewart 1888a, b, 1892),
produce intermittent, weak discharges of relatively long the behavioral function ofthese organs hasyet to be con-
durationcomparedtothedischargesofotherweaklyelec- clusively determined. Owingto itssmall amplitude (Ben-
EOD
tric fish. They, like all elasmobranchs, also have an elec- nett, 1971; Bratton and Ayres, 1987), the of the
trosensory system capable of detecting weak, low-fre- skate is unlikely to be used in prey capture, unlike the
quency electric fields. Several studies have suggested that discharge ofthe larger and more powerful electric organ
the discharge is used in some type of social communi- found in rays ofthe genus Torpedo. Furthermore, unlike
cation. This study measured the strength and nature of the nonhomologous electric organs offreshwater teleost
the response oftheskate electrosensory system to electric fishes (notably the gymnotids and mormyrids), the skate
organ discharge. electric organ is only intermittently active and is more
Electricorgandischarge(EOD)waselicited viaelectrical variable in amplitude and temporal discharge pattern
stimulation of the medullary command nucleus in two (Bennett, 1971; Bratton and Ayres, 1987). Although an
speciesofskates. Thetemporal structure and powerspec- intra- or interspecific communicative function for this
traoftheEODsdemonstrated that theyshould beeffective discharge has been suggested (Mikhailenko, 1971; Mor-
stimuli fortheskateelectrosensory system. Theresponses tenson and Whitaker, 1973; Bratton and Ayres, 1987),
ofelectrosensory afferent fibers in theanterior lateral line the precise role ofsuch communication in theskate'seth-
nerve (ALLN) to EODs were variable depending upon ogram remains to be conclusively determined.
thelocation and orientation ofthereceptor. The responses The electric organs of the skate are spindle-shaped
of most ALLN fibers were very weak compared to the structures that extend bilaterally along the length ofthe
strong reafference produced by the skate's ventilatory ac- longitudinal axis of the tail (Stark, 1844; Ewart, 1888a;
tivity. Unlike the common-mode ventilatory reafference, Sanderson and Gotch, 1888) (Fig. 1). The organ consists
EOD reafference was variable in terms of excitation or ofapproximately 8,000-10,000 cup- ordisk-shaped elec-
inhibition, depending upon receptor orientation. Despite trocytes arranged anteroposteriorly in series (Ewart, 1892;
thelowsignal-to-noise ratio observed in ALLN responses Brock eta/.. 1953; D. M. Koester, Univ. ofNew England,
to EODs, it is likely that EODsserveasa communicative pers. comm.). Each electrocytedevelopsaweak potential
signal over moderate distances. whendepolarized bytheinnervating nervefibers(Bennett,
1971). Previousstudieshavedemonstrated thattheelectric
Introduction organ discharge ofvariousskate speciesisofvariable am-
plitude and duration, but generally consists ofa mono-
Marine elasmobranchs of the skate family (Rajidae) phasic, head-negative wave of60-500 ms duration (Ben-
possesselectric organsthat generate aweakelectric organ nett, 1971; Bratton and Ayres, 1987). The electrocytes
discharge (EOD) (for reviews of electric organs see themselves are not electrically excitable overall, although
Grundfestand Bennett, 1961; Bennett, 1971; Bass, 1986). theposteriorfaceofthecup-shaped electrocytespossesses
Although the existence ofthese organs has been known anelectricallyexcitablecomponentthatinvolvesadelayed
rectification (Bennett, 1971). Electrocytes in the skate
Received 15July 1993;accepted 19 May 1994. electric organ are innervated by motor neurons in the
64
ELECTRIC ORGAN DISCHARGE IN SKATES 65
Figure 1. Location and innervation ofthe electric organs in Raja. (A) Location ofelectric organs (in
black)inthetailofRaja (B)TransversesectionthroughthetailofRajaatthelevelindicatedinA,showing
the electric organs, muscles, and vertebral column. (C) Innervation ofelectrocytes by spinal electromotor
nerves. (Modified from Ewart, 1892;and Bratton and Ayers, 1987.) Abbreviations: d, dorsal musclegroup
ofepaxial musculature;dl,dorsolateral musclegroupofepaxialmusculature;dr,dorsalrootofspinal nerve;
eo.electricorgan;sc,spinal cord;v,ventral musclegroupofhypaxial musculature;vl,ventrolateral muscle
groupofhypaxial musculature; vr, ventral root ofspinal nerve.
ventral spinal cord (Koester, 1987; Baron et al., 1992), order electrosensory nucleus of the medulla, the dorsal
which in turn are believed to receive direct descending octavolateralis nucleus(DON). The ampullaethemselves
input from an electric organ command nucleus (EOCN) consist ofa longtubelike canal that terminates internally
located in the basal midline ofthe medulla (Albe-Fessard in a bulbous alveolus and externally through a pore in
and Szabo, 1955; Szabo, 1955, 1961). The location and the epidermal surface (for reviews ofampullary electro-
nature ofany higher level descending projections to the receptor structure and function, see Murray, 1974; Ben-
EOCNare unknown, although reflexive EODsinresponse nett and Clusin, 1978; Sejnowski and Yodlowski. 1982;
totactilestimulationcanstillbeelicited in skatesfollowing Bodznick and Boord, 1986). The apical membranes of
ablation of the telencephalon or the cerebellum (Albe- the receptorcellsthemselvesare embedded in the base of
Fessard and Szabo, 1955). the alveolusand connected tothewallsofthe alveolus by
Skates, like other elasmobranchs, also possess an elec- tightjunctions. The wallsofthe alveolus and canals have
trosensorysystemcapableofdetectingtheveryweakelec- a high electrical resistance, and the internal lumen ofthe
K+
tric fields produced by physicochemical and biological ampulla is filled with a highly conductive, -rich,jelly-
phenomena in aquatic environments (for reviews, see like matrix (Murray and Potts, 1961). The ampulla thus
Bodznick and Boord, 1986; Kalmijn, 1988). The electro- functionsasan insulated coreconductor, andthe receptor
sensory receptor organs, the ampullae of Lorenzini, are cells measure the voltage drop across the length of the
innervated by fibers of the anterior lateral line nerve ampulla; i.e.. the receptors measure the potential differ-
(ALLN), which project to and terminate within the first- ence between the internal lumen, which is isopotential
66 J. G. NEW
withtheexternalenvironment at theepidermal pore, and trawl in the vicinity of Woods Hole and the Elizabeth
the internal field gradient established within the animal Islands, Massachusetts. Thespecificidentityofindividual
by the low skin resistance and high conductivity ofthe skateswasdetermined bycountingtheteeth seriesineach
internal tissues. jaw(Bigelowand Schroeder, 1953). Theanimalswere held
The electrosensory system of elasmobranchs is ex- in temperature-controlled (16C)runningseawatertanks
tremely sensitive; the behavioral threshold for responses prior to experimentation.
to dipole sources is less than 5 nV/cm (Murray, 1974; Individual skates were anesthetized in approximately
Kalmijn. 1982). The electrosensory system is most sen- 0.025% tricaine methanesulfonate (MS-222) and thedor-
sitive to low-frequency electric fields, from near direct sal aspect of the brain and anterior lateral line nerve
current (DC) (<0.01 Hz) to approximately 10-15 Hz (ALLN) exposed. In some cases, thethoracic spinal cord,
(Montgomery. 1984a: New. 1990;Tricasand New, unpub. the rostral and caudal poles of the electric organ, or all
obs.). Behavioral studieshavedemonstrated thattheelec- three, were exposed at one or two locations for implan-
trosensory system is used in directing prey-catching be- tation ofstimulating electrodes. The animal was decere-
havior;elasmobranchswill strikeatelectrical sourcesthat brated byacompletetransection ofthediencephalon and
mimic the bioelectric fields produced by livingorganisms caudal telencephalon at the level oftheopticchiasm. The
inwater(Kalmijn, 1971, 1982).Otherstudiessuggestthat animal was then placed in an acrylic holder designed to
elasmobranchs may also use their electrosense to detect hold the animal immobilized during theexperiment. Im-
electric fieldsinduced by movement relativetotheearth's mobilizing drugs such as curare were not employed in
magnetic field (Kalmijn, 1978). Additionally, recent ex- thisstudybecausetheyblock synaptictransmission at the
perimentsinamatingpopulation ofstingrays(which lack spinal motor nerve-electric organ electrocyte junction.
an electricorgan) demonstrated that malessearch forand The holder was positioned in an acrylic tank (61 cm X
locate females hidden beneath a sand substrate by de- 12.5 cm X 45.5 cm),andcooled 12C), recirculated, aer-
(
tecting the weak ventilator/ potentials produced by the ated seawater was passed over the gills by means of an
females(T. Tricas, Florida Inst. Tech., pers. comm.). The oral tubetoensurean adequate supplyofoxygen. A small
ability of male stingrays to detect the very weak signals pulse offast green dye was injected into the oral tube to
of females (approximately 5 ^V intensity at 1 cm from ensure that water was passing over the gills and not back
the spiracle) at a distance (about 1 m) suggests that the out ofthe mouth or spiracle.
electricorgan inskatesmayserveauseful purposeinsocial The discharges of the electric organ were measured
communication (Mikhailenko, 1971). through Ag-AgCl wireelectrodes bilaterally or unilaterally
The purpose ofthis study is to characterize the EOD implanted subcutaneously at aboutthe level ofthe rostral
andexaminetheresponsesoftheskateelectrosensorysys- and caudal poles of the electric organ in the tail. The
tem to discharge ofthe animal's own electric organ, with discharges were amplified and recorded on tape with an
specific regard to the usefulness ofthis organ in signaling instrumentation recorder (Vetter, Model B) for subse-
otherskates and to the nature and amount ofreafference quentanalysis. Thespread and orientation oftheinternal
it produces. Previous studies have indicated that theven- and external fields generated by discharge ofthe electric
tilatory potentials produced by elasmobranchs are a sig- organ were measured directly by electrodes either in the
nificant source of electrosensory reafference (Montgo- seawater bath or implanted in the ampullary clusters
mery, 1984a; New and Bodznick, 1990; Bodznick and within the body ofthe skate (see below). External fields
Montgomery, 1992; Bodznick eta/., 1992). Thisinternally weremeasured usingan Ag-AgClwireelectrodepositioned
generated reafference modulates the activity of electro- at various locations around the animal's body in the sea-
receptors by changing the voltage at the internal face of water bath. A reference electrode was positioned at some
theapical membrane ofthe receptorcells. These internal distance (approximately 25 cm) from the animal. Both
potentials thus provide a source ofcommon-phase reaf- electrodes were connected to a Grass P-15 differential
ference; all receptorsare excited orinhibited in phasewith preamplifier, filtered between 0.3 Hz and 1.0 kHz (time
oneanotherregardless ofreceptororientation or position constant >150 ms), and measured directly on an oscil-
on thebody surface. Whethertheelectric organdischarge loscope. Internal potentialswere measuredbyimplanting
oftheskatealsoconstitutesasignificant sourceofinternal an insulated Ag-AgCl tipped wire electrode into the sub-
reafference, viaasimilarlow-resistance internal pathway, dermal clusters ofampullary alveoli in which theelectro-
is also investigated in this study. receptors are located. The recording electrodes were in-
troduced through a fine hypodermic needle inserted
Materials and Methods through the skin ata point on theanimal abovethewater
line in the tank and then withdrawn. The reference elec-
Specimens oflittle skate. Raja criuacca (n = 18), and trodewasthen positionedat variouslocationsaround the
=
winterskate. Rajaocel/ata(n 2), werecollectedbyotter skate'sbodyto measurethevoltagedropacrossthe length
ELECTRIC ORGAN DISCHARGE IN SKATES 67
ofthe receptors (in a manner similar to the voltage mea- Tactile Stimulation
sured by the receptors). The potential difference between
recording(internal)and reference(external)electrodeswas
amplified by a Grass P-15 preamplifier and measured on
an oscilloscope. Left
The activity of individual ALLN fibers was recordMed
by means ofglass micropipette electrodes filled with 2
NaCl saturated with fastgreen dyeand with input imped-
ancesof12-20 megohms. Responses ofALLN fiberswere Right
recordeddirectly from thenerve, neartheALLNganglion.
Electrosensory ALLN fibers were initially identified by
their responses to a uniform electric field oriented along
the longitudinal axis ofthe tank between two carbon rod
electrodes connected to a Grass Instruments S-88 stim-
ulator. The field stimulus consisted ofDC-step pulses of
approximately lOO^V/cm amplitude and 700 ms dura-
tion. The intensity ofthe field was measured directly in
the tank by means oftwo Ag-AgCl wire electrodes posi-
tioned 10 cm apart and parallel with the orientation of EOCN Stimulation
the field. The threshold receptive fields ofthe individual
ALLN fibers were subsequently identified with a hand-
positioned local dipole source consisting oftwo 2.0-mm
(o.d.)glass tubes filled with 2% agarin seawaterand con- Left
nected to a Grass Instruments S-88 stimulator via Ag-
AgCl wire electrodes. The initial intensity ofthe local DC-
step field, measured directly at one of the poles, was
<50 f/V, and the average distance between the poles of Right
thelocaldipolewas4.0cm. Uponencounteringampullary
poresrespondingtolocalelectricfieldstimuli, thestimulus
was systematically decreased to identify as nearly as pos-
ALLN
sible the threshold receptive field ofthe recorded
fiber. In thisstudy, the threshold receptive field isdefined
as the location of the epidermal pore of the ampullary
electroreceptor, but in actuality the "receptive field" of
the electroreceptor is the distance between the pore and
the electroreceptor cells within the internal alveoli across
which the voltage drop is measured. Action potentials re-
corded from ALLN fibers were transformed into digital O.I s
logic pulses through a WPI window discriminator and Figure 2. Electricorgan discharges(EODs) measuredsimultaneously
stored as peristimulus time histograms on a Tracor in nght and left electric organs. Upper photo: EODs elicited by tactile
Northern signal analyzer. Additionally, spikeactivityand stimulation (gentle tap ofdorsal surface). Arrows indicate electromyo-
electric organ discharges were in some cases recorded as gtarcatimlseosftismpuilnuasl.reLfolwexeirvepwhoittoh:drEawOaDlsofrepcelovridcedfinisnftohleloswaimnegdaenliimvaerlyboyf
analog signals on a Vetter Model B reel-to-reel instru- electricpulsetrainstimulationofthemedullaryelectricorgancommand
mentation tape recorder. Electric organ discharges re- nucleus (EOCN). Stimulus delivery was 10ms following initiation of
corded in this manner were converted to digital signals, theoscilloscopesweep.
and the power spectra ofthe signals were analyzed using
a Zenith 286 microcomputer and ASYSTANT PLUS
(Keithley Metrabyte) analytical software. Aliased fre- via two monopolar stimulating electrodes located several
quencies represented in thegenerated powerspectrawere centimetersdistant from each otheralongthe longitudinal
removed by software filtering. axis of the cord. Alternatively, monopolar stimulating
Electricorgandischarges(EODs)wereelicitedinskates electrodes were placed at the rostral and caudal pores of
by several different techniques. The methods employed theelectric organ. Finally, EODscould beelicited through
included stimulation ofthe thoracic spinal cord eitherby stimulationofthemedullaryelectricorgancommand nu-
insertion ofa concentric bipolar stimulating electrode or cleus (EOCN) by means ofan implanted concentric bi-
68 J. G. NEW
Raja erinacea however,onlythelast methodwasfoundtobeconsistently
reliable. Stimulation ofthespinal cordwitheitherasingle
concentricbipolarelectrode ortwo monopolarelectrodes
placed at locations several centimeters distant from each
other along the longitudinal axis of the cord was only
occasionally successful in eliciting repeatable EODs, de-
spite numerous repositionings ofthe stimulatingelectrode
or electrode pairs. Neither single pulse stimuli nor trains
ofpulses up to 400 Hz elicited consistent EODs, despite
thefactthat theyrepeatedly andconsistentlyelicitedlarge
motor neuron responses, as indicated by movement and
electromyograms (EMGs) recorded via the subcutaneous
electrodes planted in the tail. Similarly, concentric bipolar
or paired monopolar electrodes implanted in the electric
organ itselfwere only occasionally successful in eliciting
EODs, presumably by means ofthe excitation ofthe pre-
synaptic elements ofthe neural-electric organ junction.
Stimulation of the electric organ command nucleus
(EOCN) was the most reliable method for eliciting con-
sistent and repeatable EODs. Brief(50 ms) trains of0.5-
ms pulsesdelivered at 40-75 Hz and 3-5 V intensitywere
found to be sufficient stimuli. These were much lower
frequencies and intensities (up to 15V and 500 Hz) than
Raja ocellata were required to occasionally elicit EODs by spinal cord
or electric organ stimulation. The threshold ofthe EOD
responsetostimulation ofthe EOCN wasdependent upon
the location ofthestimulatingelectrode;optimum results
were found within an approximately 3-mm length ofthe
20 mV
basal medullaatthe level ofentryofthefacial(VII) nerve.
0. S The electric organ discharges recorded following stim-
Figure3. ElectricorgandischargesrecordedfrIom Ra/aerinacea(A, ulation ofthe EOCN were strongly similar in amplitude,
B)and Rajaocellata(C). Thetracesin(B)and (C)exhibit the multiple duration, and waveform to those elicited by tactile stim-
small peaksseen in many EODsevoked by both tactileand command ulation (gently brushing or tapping the dorsal surface of
nucleusstimulation. theanimal)andtotheoccasionallyrecorded spontaneous
EODs generated by the animal (Fig. 2). Discharges in all
cases were bilateral (both organs discharged) and syn-
polarelectrode. In those cases in which the latter method chronous, although occasionally an asymmetry in am-
ofstimulation was employed, the trigeminal nerves were plitude ofthe discharge was observed between the right
bilaterally transected to reduce movement caused by vol- and left electric organs. Such asymmetries occurred fol-
ume conduction ofelectrical current from the stimulating lowing both EOCN stimulation and tactile stimuli and
electrode to the nuclei ofthe branchiomeric motor col- persisted in multiple trials.
umn. Stimuli consisted ofsingle DC step pulses or, more When measured across the rostrocaudal length ofthe
commonly, brieftrains ofpulses (3-5 pulses/train). The electric organ by implanted subcutaneous electrodes, the
duration ofthe stimulus pulses was 0.5 ms, pulse ampli- EODofboth R. erinaceaandR. ocellataconsistsprimarily
tude 3-12 V; the pulse rate ofthe trains varied from 50- ofa large peak that is negative with respect to the rostral
400 Hz. depending upon location; and thetrain duration pole ofthe organ, followed in most cases by a shallower
was 40-70 ms. and longerlastingpositivecomponent (Fig. 3). The mean
amplitude ofthe negative peak forall wavesrecordedwas
Results -27.9 mV (SEM 2.58, n = 35), and the mean duration
ofthe negative peak was 53.5 ms (SEM 2.16). The mean
Electric organ discharges amplitude ofthe positive component ofthe EOD wave-
Discharges of the electric organ (EODs) in the skate form was +11.12 mV (SEM 2.16. n = 33) and the mean
could be elicited by stimulation of the spinal cord, the durationwas 192.35 ms(13.6SEM). In bothspontaneous
electric organ, or the electric organ command nucleus; EODsand thoseelicited bytactileorelectrical stimulation.
ELECTRIC ORGAN DISCHARGE IN SKATES 69
R. erinacea
5.kB 15.8 25.B
Frequency (Hz)
B
R. ocellata
Frequency (Hz)
C
Log Frequency (Hz)
Figure4. Powerspectraofelectricorgandischargesrecorded from Rajaerinacea(A)andRajaocellala
(B)comparedwithfrequencyresponsecurvesgeneratedforanteriorlaterallinenerve(ALLN)electrosensory
afferent fibers and medullary ascending electrosensory neurons(AENs) (C). Bottom graph modified from
New(1990).
therewere frequently oneorseveral small, sharp, positive 26). The latency of EODs elicited by tactile stimulation
peaks superimposed upon the slower negative wave (Fig. was considerably longer than the responses of muscles
3B). However, the shape ofthe waveform appeared to be that appeared to be involved in spinal reflexes, such as
veryconsistent foraspecific individual, showingonly mi- the pelvic tin withdrawal reflex. Figure 2 illustrates an
nor variations, ifany, between spontaneous and elicited EODelicited bylightly tappingtheanimal'sdorsalsurface.
EODs. The mean latency ofthe EOD ofR erinacea fol- The EMG recorded astheanimal withdrew itspelvic fins
lowing EOCN stimulation was 89.4 ms (SEM 2.7, n = can be observed at aconsiderably shorter latency (90 ms)
70 J. G. NEW
,0 0, ,0 Q
ulpopweerr:: 11000mVuV ,
00 mS
I
Figure5. Fieldstrength oftheelectricorgandischargeelicitedby EOCN stimulation measuredaround
thebodyoftheskate. Each pairoftracesrepresentsthefield measured locally atthepoint indicatedbythe
dot associated with each letter(top trace) and the EOD measured subcutaneously along the length ofthe
electric organ (bottom trace). In traces A-F, the recordingelectrode was placed at the indicated locations
and the reference electrode at a distant location. In tracesG-L, the recording electrode was implanted in
theampullarytissuesatthelocationmarked"r"andthereferenceelectrodeatthepositionsindicatedaround
the perimeter ofthe animal. In the latter examples, the voltage measured approximated the voltage drop
alongthe length ofthe ampullary electroreceptorssooriented.
than the elicited EOD (235 ms), suggesting that somato- erinacea (mean duration, negative peak = 50 ms, mean
sensory information must first ascend, presumably at least amplitude = -35 mV), and there appeared to be little
to medullary levels, rather than eliciting an EOD via a difference in either the EOD waveform or power spectra
direct spinal reflex pathway to the electric organ motor between the two species.
neurons in the spinal cord. The spread ofthe EOD around and through the skate
The EODsofmature maleand female specimens ofR. was measured with external and internally implanted
erinacea and R. ocellata were recorded on tape and dig- electrodes (Fig. 5). The externally measured field created
itized for frequency composition analysis. The power by the EOD reverses its polarity across the length ofthe
spectra obtained from EODs of all specimens indicate electric organ anddiminishes sharply with increasingdis-
that the majority ofthe power in the EOD signal is in a tance around the surface ofthe animal from the rostral
single range offrequencies between less than 1.0 Hz and terminus ofthe organ, diminishing to less than millivolt
10-15 Hz, with the peak of the power spectra between intensity at the rostral end ofthe skate's body (Fig. 5A-
1.8and3.0Hz(Fig.4). AlthoughthenumberofR. ocellata F). The amplitude of the externally measured EOD at
used in this study was too small for statistical analysis (n the rostral terminus ofthe organ, at a lateral distance of
= 2), the mean duration and amplitude ofthe EOD of 1 cm from thetail, was 180^V peakto peak. Atadistance
R. ocellata werewithin the range ofthose observed for R. of4.6 cm from the rostral tip ofthe electric organ along
ELECTRIC ORGAN DISCHARGE IN SKATES 71
100
fiV
B
Figure6. Responsesofasingleelectrosensory ALLN afferent fibertoan elicited EOD. Thelocationof
the epidermal pore ofthe ampullary electroreceptor innervated by this particular afferent is shown at the
locationmarked"(v)"(theporewaslocatedupontheventralsurfaceoftheskate). Thesmallseriesofwaves
immediately preceding the EOD-invoked reafferent burst isa stimulus artifact ofthe train pulsedelivered
tothe EOCN. The bursts ofactivity indicated with the letter"r" indicate reafferent activity caused by the
animal'sventilation.
the pectoral disk, the amplitude had decreased to 50 ^V, instantaneous frequency ofthe action potential response
and at 8.7 cm (at thewidestdiameterofthe pectoral disk) recorded from these afferents was approximately 12
the peak-to-peak amplitude had decreased to less than spikes/second. Ampullary organs with more rostrally lo-
5 fiV. A recording electrode was subsequently implanted cated pores showed responses with a much lower signal-
inside the animal in the region of the superficial to-noise ratio (Fig. 8). The responsesofthe most rostrally
ophthalmic ampullary cluster. The EOD thus measured located and directed organs (the majority of ampullary
as the difference across the skin ofthe skate (i.e., across electroreceptors in theskate)showed veryweak responses
the length ofthe receptors) was similar to the externally to electric organ discharges, or no detectable response
measured field in amplitude and decrease in amplitude above the normal resting activity ofthe ALLN fiber.
with distance from the electric organ, but the polarity of Additionally, the temporal pattern ofthe measurable
therecordeddischargewasreversed(Fig. 5G-L). Thelatter responses of identified electrosensory ALLN afferents
results indicate that the electric organ discharge does not varied depending upon the location and orientation of
generate a significantly strong internal field contrary to theampullary organ canal and epidermal pore. Generally,
that generated during the animal's ventilatory activity. fibers innervating the most caudally oriented and located
ampullary organs responded with a briefburst ofaction
EOD
Responses ofelectrosensoryALLNafferentfibers potentials during the positive-going phase of the
(Fig. 7). The response pattern forunits innervating more-
A total of 32 individual ALLN fibers with identified rostral ampullary organs is less clear owing to the low
ampullae were recorded in this study. The responses of signal-to-noise ratio ofthe response and the ongoing ac-
electrosensory ALLN fibers innervating different ampul- tivityofthe fiber. However, several ofthese neuronsdem-
lary electroreceptors displayed considerable variability, onstrated a response that was out ofphase with the more
depending upon the location ofthe ampullary pore. Am- caudal units, i.e., an inhibition ofactivity associated with
pullary organs opening through caudally directed canals the EOD. In no case was the response ofelectrosensory
to pores located upon the caudal surface ofthe pectoral ALLN fibers to discharge ofthe electric organ as robust
fin showedthegreatest responses(Figs. 6, 7). The maximal as the response ofthese same units to the potentials gen-
72 J. G. NEW
B
Figure 7. Penstimulus time histograms (PSTHs) ofEOD-invoked reafference in three ALLN electro-
sensoryafferentfibersinnervatingcaudallyorientedampullaryreceptors.Theampullaryporesofthereceptors
are indicated by the open circles; "v" indicates a pore on the ventral surface ofthe pectoral disk. The
horizontal time scaleequals 100 ms; the vertical scale indicates instantaneous frequencies of 17.5 spikes/s
(A, B)and 7.5 spikes/s(C).
crated by the animal's ventilatory activity (Fig. 6). The both in the intensity ofthe response and in the temporal
maximal spike rate ofthe responses to respiratory poten- pattern. Invariably, thereafferenceproducedbydischarge
ALLN
tials in fibers approached 91 spikes/second. ofthe electric organ is much weaker than that generated
by ventilation; the maximal response ofthe most caudally
Discussion directed ampullary organs drives the fibers through less
Reafference produced asa result ofmotoractivity plays than 10% oftheirdynamic range. Indeed, for most ofthe
an important role in the activity and organization ofner- electroreceptors, which are located in the rostral third of
vous systems. Ventilatory activity in skates produces a thepectEoOraDldisk,theresponsesofinnervatingALLN fibers
strong reafference, driving the electrosensory ALLN af- to the are difficult to detect against the ongoing ac-
ferentsthroughapproximately63%oftheirdynamicrange tivity ofthe nerve.
(New and Bodznick, 1990). Reafference ofthis intensity ALTLhNe variability in intensity and temporal pattern of
may well interfere with the animal's ability to detect ex- fiber responses indicates that the reafference pro-
ternally generated fields ofinterest to the animal. In the duced by electric organ discharge is fundamentally dif-
central nervous system ofelasmobranchs, electrosensory ferent from that produced when the animal ventilates.
reafference produced by ventilatoryactivity (gilling)isre- Respiratory potentials are produced by the modulation
DC
duced through a common-mode rejection mechanism of potentials generated by the different electrical
based upon commissural and unilateral local circuit neu- properties of various body surfaces in contact with the
rons in the first-order medullary nucleus (Montgomery, surrounding seawater, particularly ion-exchange surfaces.
1984b; New and Bodznick, 1990; Bodznick and Mont- Potentials thus produced are probably modulated by
gomery, 1992). Unlike the common-mode reafference changes in the resistance ofthe current pathway caused
produced by ventilation, in which all fibers are excited by the opening and closing ofmouth and gill slits during
and inhibited in phase together, the response ofelectro- ventilation (Kalmijn, 1988; Bodznicketa/., 1992). These
ALLN EOD
sensory afferents to the is highly variable potentials modulate the activity ofampullary organs by
among fibers innervating different ampullary receptors. changing the voltage at the internal surface ofthe apical
ELECTRIC ORGAN DISCHARGE IN SKATES 73
iliilillliiU iIJI,l.ll, lilt ,iMn -1
11. . ^,.,...4.J , i 1.1i.l.U..JH..1i.
00 mS
I
Figure8. Penstimulustime histograms(PSTHs) ofEOD-invoked reafference in ALLN electrosensory
afferent fibers innervating rostrally located ampullary receptors. The ampullai-y pores ofthe receptors are
indicated by the open circles; the poreswere located on eitherthe dorsal or ventral surfaceofthe pectoral
disk.The horizontal timescaleequals 100ms;thevertical scaleindicatesinstantaneousfrequenciesof20.0
spikes/s(A, B, F)and 17.1 spikes/s(C, D, E).
membrane ofthe receptor cells by means ofcurrent flow the voltage drop along the length ofthe receptor is mea-
through a low-resistance pathway in the visceral tissues sured by moving the reference electrode to various loca-
ofthe skate. All receptorcellsarethus modulated in phase tions around the animal's epidermis, a similar decrease
together from the inside, regardless ofthe orientation of in voltage with distance from the electric organ was ob-
the receptor'scanal orthe location ofthe ampullary pore served, although the polarity ofthe recorded EOD was
EOD
on the surface of the body. Such is clearly not the case reversed. These results indicate that the does not
for reafference produced by the EOD. modulate the activity ofelectroreceptors internally, asdo
ThevariabilityoftheresponsesofALLN fiberstoEOD the potentials produced during ventilation, but rather
reafference suggeststhat the EOD spreads overthe outside through an external and internal spread ofthe EOD over
and inside of the animal and modulates the activity of the surface and through the tissues of the animal. It is
ampullary electroreceptorsdifferentially depending upon possible that the electric organs are insulated from the
receptor location and orientation with respect tothegen- low-resistance tissues of the viscera by the surrounding
erated external electric field. The experiments measuring layers of muscle and connective tissue sheathing of the
EOD EOD
the shape and intensity ofthe fieldgenerated by the in the skate's tail.
supportthisview. When measuredwith respecttoadistant Although it isdifficult todiscern the responsesofmany
ALLN EOD
(approximately 25 cm) reference electrode, the strength individual electrosensory fibersto the against
EOD
ofthefielddropsoffrapidlywith increasingdistancefrom the ongoing activity of the fiber, the peaks of the
the electric organ, possibly dropping offto below the de- power spectrum fall within the bandwidth to which am-
tectable range of elasmobranch electroreceptors within pullary electroreceptors are most sensitive (1-5 Hz) and
several body lengths. When the recordingelectrode is im- suggest that the EOD should be an effective stimulus at
planted within the tissues ofthe ampullary cluster, and short ranges. Although the signal-to-noise ratio ofprimary