Non-Nervous Conduction in Invertebrates and Embryos
ANDREW N. SPENCER
Institute of Biology, University of Odense, 5000 Odense, Denmark
SYNOPSIS. Propagated electrical events in epithelial tissues have been recorded from a
number of invertebrates and from vertebrate embryos. Those groups in which such
events have been recorded are the-Cnidaria, Ctenophora. Echinodermata. Urochordata.
and Amphibia. In several cases these non-nervous action potentials mediate escape or
protective responses. In other cases they cause ciliary reversal or arrest of ciliary beat.
Propagation appears to occur via low resistance intercellular pathways involving direct
current flow. The ontogeny of such epithelial impulses is described in an amphibian.
INTRODUCTION
The propagation of action potentials in
epithelial cells is slowly becoming accepted
as a phenomenon common to a wide variety
of animals. This acceptance has taken
nearly a century. Students of coelenterate
physiology and anatomy had suggested the
existence of neuroid mechanisms late in the
last century (Kleinenberg, 1872; Romanes,
1876; Chun, 1897); however, it is only
within the last 10 years, with the advent of
suitable electrical recording techniques,
that workers have confirmed these suspicions (Mackie, 1965; Mackie and Passano,
1968). Research on neuroid conduction and
its evolutionary implication was reviewed
by Mackie in 1970.
This paper will concentrate on epithelial
events which function in reception and
transmission over long distances, like
nerves, rather than pure myoid events
which are solely concerned with the spread
of excitation within an effector. Although
all the events described below are propagated electrical changes across membranes,
the possible existence and functions of
decremental events should not be ignored;
these will be referred to later. In the text
the term "neuroid conduction" refers to
conduction of potentials in non-nervous,
non-muscular tissues; "myoid conduction"
I would like to thank Dr. G. O. Mackie for
contributing the recordings in Figure 3 and 7 and
for other information he provided. The author's
work on development of Xenopus skin impulses
was supported by a M.R.C. grant to Dr. A. Roberts,
University of Bristol.
917
to conduction of potentials in muscular tissues; "epithelial conduction" to conduction
of potentials in epithelial tissues; "epithelial pulses" to propagated action potentials
in epithelial tissues.
EPITHELIAL CONDUCTION IN CN1DARIA
Of all the invertebrate phyla, epithelial
conduction has been most closely studied in
the Cnidaria (Mackie, 1965; Josephson and
Macklin, 1967, 1969; Mackie et al., 1967;
Mackie and Passano, 1968; Kass-Simon,
1972; Spencer, 1974). Now that a number
of examples have been found a clear picture
is emerging of the role of epithelial conducting systems in the behavioral repertoire
of an individual. Generalizing one may say
that these systems mediate escape or protective responses of an all-or-nothing character.
The hydrozoan Proboscidactyla fiavicirrata demonstrates the properties of epithelial conducting systems in both its polypoid
and medusoid generation (Spencer, 1971a,
1974). In addition it shows some other,
rather unexpected, properties which may
give some clues as to the mechanism of epithelial conduction.
This gymnoblastic hydroid is colonial
and found encrusting the rim of the tube
of sabellid worms. With the feeding polyps
located close to the feeding tracts of the
worm's fan (brachioles), it is an easy matter
for them to steal food. However, their position while feeding from the brachiolar
grooves puts them in danger when the host
retracts the fan rapidly, since they are liable
918
ANDREW N. SPENCER
to be trapped between the inside of the tube
and the retreating fan of the worm. The
colony is able to anticipate withdrawal of
the fan by detecting the insertion of setae
into the interior wall of the tube just before
the worm contracts its longitudinal musculature. Mechanical stimulation of the colony by the host or by experimental
prodding of the colony causes a wave of
contraction to spread to all individuals of
the colony. Individuals of the colony (gastrozooids, gonozooids, dactylozooids, and
medusa buds) are joined by a network of
stolons consisting of simple ecto-endodermal
tubes invested with a thin perisarc. Neither
neurons nor muscle cells have been seen in
these stolons except for a short distance beneath the gastrozooid (Spencer, 1971a).
Plastic suction electrodes attached to the
stolons can record the passage of a propagated action potential, which I have called
the colonial pulse, through the stolons and
individuals of the colony. It spreads over
the colony at a velocity of between 1 and
4cm/sec, depending on the time after the
passage of a previous colonial pulse. Each
colonial pulse causes gastrozooids and dactylozooids to bend and medusa buds to
swim or "crumple" (a protective radial muscle contraction described later). Whether
the attached medusae swim or crumple depends on their developmental stage, and
hence type of tissue connection made with
the colony. Since crumpling, like colonial
retraction, is also a protective response mediated by an epithelial pathway we can
trace the ontogeny of these homologous
events (Spencer, 1971a, 1974).
The properties of the colonial pulse system in Proboscidactyla may be summarized
as follows:
1) Colonial pulses are conducted in epithelial cells of the stolons and epitheliomuscular cells of the polyps.
2) The conducting system is unpolarised.
3) The conducting system shows no consistent facilitating properties and generates
an action potential to the first suprathreshold stimulus.
4) Multiple firing to a single stimulating
shock is not seen.
5) In preparations not recently stimu-
electrode 1
FIG. 1. Fatiguing of the colonial pulse system in
the hydroid of Proboscidactyla. Both suction electrodes are attached to the bases of gastrozooids on
opposite sides of a colony. Six shocks were given at
a frequency of 0.5/sec close to electrode 1. Notice
that the conduction velocity decreases with successive pulses and also that the last pulse fails to
reach electrode 2. Superimposed oscilloscope traces.
Calibration: 50 msec and 1 mV. (From Spencer,
1974.)
lated, the conduction velocity is between 1
and 2.5-cm/sec over long stolonal tracts.
6) The conduction velocity falls rapidly
with repetitive stimulation when the period
between shocks is less than 3 sec (Fig. 1).
Colonial retraction has been previously
reported in cnidarians (Horridge, 1957;
Josephson, 1961), but recordings of spontaneous events were not obtained. At that
time the properties of such systems, in for
example Cordylophora and perforate
madreporarian corals, could be most easily
explained by the presence of a nerve net.
Such conducting systems were shown to
be capable of facilitated conduction, decremental conduction and through conduction. This is probably the correct interpretation for coral colonies, but hydroid
colony retraction is probably not always
due to activity in a through conducting system of large axons. In this context it is interesting to note some recent
work by Mackie (personal communication)
who has shown that in the stem of another
colony, the siphonophore Nanomia, a giant
fibre system is present which conducts action potentials causing stem contraction and
synchronized swimming of nectophores.
In the free-swimming and attached medusae of the Hydrozoa we find another
dramatic use of an epithelial conducting
system. This time it serves to initiate a response both for escape and protection. This
response is called "crumpling" (Hyman,
1940) and has been described in detail by
NON-NERVOUS CONDUCTION
919
FIG. 2. Three suction electrodes, each attached to
the base of a tentacle of the hydromedusa of
Proboscidactyla. A spontaneous crumpling pulse
(Cr.P) appears simultaneously at the three elec-
trades. Other pulses are pacemaker potentials conducted in the margin (two top channels) and muscle
potentials of a contracting tentacle (bottom channel). Calibration: 5 sec and 1 mV.
Mackie et al. (1967) and Mackie and Passano (1968) in Sarsia.
If any part of the external surface of the
jellyfish of Proboscidactyla is stimulated
strongly either mechanically or electrically,
an action potential, the crumpling pulse, is
generated (Fig. 2). This pulse totally invades the ectodermal epithelium and then
crosses over into the endodermal lamella.
The exact route has not been determined,
but it is probably via an epithelial bridge
or neurons crossing the mesogloea which
originate in the subumbrella nerve ring
(Spencer, 1971a). The crumpling pulse
causes contraction of the smooth radial
muscles of the endoderm lying along each
of the radial canals and their tributaries.
Contraction of these muscles and the contiguous longitudinal muscles of the tentacles
causes the margin of the bell to be drawn
up into the subumbrella cavity and the tentacles to contract and curl orally. Graded
responses are not normally seen in these
muscles when an epithelial pulse initiates
contraction, though summation may exist
in some cases (Fig. 3). This is surprising
since these same muscles capable of fine
changes of length when exicted through another, presumably nervous, pathway during feeding (Horridge, 1955a; Josephson,
1965). The "crumpling" posture serves two
functions: firstly, it protects the sensory and
nervous regions of the margin, and secondly,
it reduces the surface area of the jellyfish so
that it sinks more rapidly since the medusa
is negatively buoyant.
The properties of the crumpling pulse
system parallel those of the colonial pulse
system except that:
1) Crumpling pulses are conducted in
FIG. 3. Crumpling in the anthomedusan Stomotoca
atra. a, Stimulated crumpling pulses recorded with
a suction electrode from the subumbrella. b,
Mechanogram (RCA 5734 mechano-electric transducer) showing retraction of the margin. Calibration: 10 sec and 5 mV. (From Mackie, unpublished.)
920
ANDREW N. SPENCER
electrode 1
FIG. 4. Fatiguing of the crumpling pulse system of
the medusa of Proboscidactyla. Both suction electrodes are attached to the exumbrella. 12 shocks
were given at a frequency of 2/sec close to electrode
1. The conduction velocity decreases with successive
pulses as does the amplitude. Superimposed oscilloscope traces. Calibration: 20 msec and 1 mV.
pure ectodermal epithelium on the exumbrella surface but in epitheliomuscular
cells of the subumbrella endoderm.
2 )The conduction velocity is greater
than that of the colonial pulse system
(values between 7 and 13-cm/sec at 14 C)
probably because the conducting cells are
larger.
3) With repetitive stimulation at frequencies greater than 1 shock/sec, a decrease in the amplitude and conduction
velocity of crumpling pulses can be seen
with each successive shock (Fig. 4).
It should be noted that crumpling is a
common behavior pattern of hydromedusae.
Recordings of crumpling pulses have been
obtained from Sarsia, Euphysa, Bougainvillea, Slomotoca, and the leptomedusan Phialidium (Mackie and Passano, 1968;
Spencer, 1971a; Mackie, personal communication).
In the Siphonophora the operation of an
epithelial conducting system is slightly modified so that epithelial excitation activates
muscles responsible for shaping the funnel
through which water is forced during a
swimming contraction of a bell, and thus
enables the siphonophore to reverse its normal swimming direction by deflecting the
exhaust water (Mackie, 1964, 1965).
EPITHELIOMUSCULAR CONDUCTION IN CNIDARIA
One of the features of non-nervous conduction in the Cnidaria is that the pulses
may be conducted in sequence through pure
epithelium (e.g., exumbrella epithelium)
and epithelial cells which contain myofilaments. Conduction of a pulse in an epitheliomuscular sheet should strictly be termed
myoid conduction; however, epithelial
4IUFIG. 5. Propagation of pulses in epitheliomuscular
cells without observable, associated contraction of
myofibrils (gastrozooid of Proboscidactyla). Electrode 1 attached to one tentacle, electrode 2 to the
remaining tentacle and electrode 3 attached to the
column. Tentacle contractions are recorded from
the tentacle contracting as large positive-going
pulses (2 such pulses are marked by triangles);
from the passive tentacle and column as smaller,
negative-going pulses (3 such pulses are marked
by squares). A burst of contractions of tentacle 2 is
indicated by an arrow. The first pulse recorded on
all channels is a colonial pulse. Calibration: 5 sec
and 1 mV.
NON-NERVOUS CONDUCTION
921
pulses often pass through areas lacking lar junctions one can even find a subsurface
myofilaments to muscular regions without cisterna of endoplasmic reticulum running
any obvious changes in conduction charac- parallel to the synaptic cleft. The experiteristics. In addition, it is probable that ments of a number of workers on physiologiepithelial pulses can be conducted in epi- cal mechanisms in coelenterates over many
theliomuscular cells without causing mea- years have shown that innervation of swimsurable contraction of the myofilaments of ming muscle is by nerves located in or conthat cell (Fig. 5).
nected with a marginal nerve ring(s) or
Horridge (1955a) showed that in the center(s) in hydro- and schyphomedusae
swimming muscle of the hydromedusa respectively (Romanes, 1876, 1877, 1880;
Geryonia the conducting pathway could be Bullock, 1943; Horridge, 1954, 1955a, 1959;
functional while the contractile system was Passano, 1965; Mackie and Passano, 1968;
refractory. Since Geryonia has an extensive Spencer, 1971a).
subumbrella nerve net it is apparent that
The author was fortunate enough to be
the spread of excitation is due to nervous able to make intracellular recordings of
activity. In Sarsia, however, there is no epitheliomuscular potentials in the stem of
nerve net over the striated swimming mus- the siphonophore Nanomia bijuga largely
cle and conduction must be myoid (Mackie because of the considerable thickness of
and Passano, 1968).
these cells (45 ^m) (Spencer, 19716). Resting
The properties of a conducting sheet of potentials of between — 50-mV and —80epitheliomuscular cells is best known in mV were measured with the cells showing
Hydra from the studies of Josephson and spontaneous depolarizations of 17 to 20Macklin (1967, 1969), Macklin and Joseph- mV conducted at approximately 20 cm/sec
son (1971), and Kass-Simon (1972) and in (13 C) along the stem. Each stimulated and
the hydromedusae (Mackie and Passano, spontaneous action potential caused a
1968).
small contraction in the longitudinal musOne problem when interpreting electro- cle of the stem. Figure 6 shows these potenphysiological recordings from eptheliomus- tials recorded simultaneously with an
cular tissues is the frequent presence of an external suction electrode and an intracelluassociated nerve net which may or may not lar microelectrode. The extracellular reparticipate in propagation. Kass-Simon cording is typical of the pulses that have
(1972) believes that, in the case of the con- been recorded from cnidarians where an
traction burst pulse of Hydra, neurons do epithelial pulse was associated with connot participate and also that the pulse is traction.
transmitted wholly within the ectoderm via
closely apposed junctions stabilized by
desmosomes. Previously Josephson and
Macklin (1967, 1969) had shown that the
contraction burst pulse could be recorded
superimposed on a transepithelial potential
and that possibly the nerve net facilitates
the propagation of the pulse at the inner
ectodermal cell membrane, though they
thought this unlikely.
From a variety of ultrastructural work we
can say that the possibilities of interaction
15 msec
between epithelial cells, both with and
without myofilaments, and neurons are FIG. 6. Contraction pulse conducted in the stem
both numerous and obvious. There are of the siphonophore Nanomia. a, Intracellular mirecording of one contraction pulse, b,
typical neuromuscular synapses in hydro- croelectrode
Suction electrode recording of the same pulse at the
zoans, scyphozoans, and anthozoans (West- same site. Arrow shows stimulus artefact. (From
fall, 1973), and in scyphozoan neuromuscu- Spencer, 19716.)
922
ANDREW N. SPENCER
INTERACTION OF EPITHELIAL PULSES WITH
NEURONAL POTENTIALS IN CNIDARIA
COORDINATION OF COMB-PLATE BEATING IN
CTENOPHORA
. One example of interaction of an epithelial pulse with a neuronal potential has just
recently been recorded by Mackie (personal
communication). He discovered that if the
anthomedusan Stomotoca atra is stimulated
to crumple repeatedly while it swims, then
within 5 sec, swimming is inhibited completely. If the stimulation is then stopped
and crumpling ceases, swimming is resumed
but at a new frequency (Fig. 7). Furthermore, he showed that crumpling pulses
inhibited the primary nervous events responsible for swimming, i.e., the swimming
pacemaker potentials which are conducted
in the nerve ring. Mackie suggests that inhibition may be electrical, or due to hyperpolarisation created by an altered external
ionic milieu that results from the passage of
a crumpling pulse. The morphology of the
marginal region is compatible with this
explanation, since the nerve rings are completely surrounded by ectodermal epithelium. Horridge (19556) recorded inhibition
of swimming during a radial response in
Aequorea, but the radial response in question is part of feeding behavior which is
quite distinct from crumpling and probably
does not involve epithelial conduction pathways at all.
Research in the past on the coordination
of ciliary beating of the comb plates of
ctenophores had resulted in a very confused
picture. Experimental evidence demonstrated both neuroid coordination and coordination by viscous coupling (Verworn,
1890; Parker, 1905; Lillie, 1908; Child,
1933; Horridge, 1965a,b, 1966; Sleigh, 1963,
1966, 1974). However, recently Tamm
(1973) has resolved some of these apparently
conflicting observations.
Tamm's experiments have clearly shown
that in the cydippids and beroids, in which
there is no interplate ciliated groove, the
beating of comb plates is coordinated by
mechanical interaction. If the comb plates
were prevented from moving, metachronal
waves were blocked. It is apparent that coordination cannot be accounted for by the
conduction of a depolarising pulse in epithelial cells joining comb plates.
The lobate ctenophores, which possess an
interplate ciliated groove, present a rather
different picture. Here it seems that the
comb plates do not mechanically trigger
the power stroke of their neighbor, but that
coordination is maintained by the interplate ciliated groove. Surprisingly, coordination between ciliated cells of the groove is
not maintained by viscous drag but appears
to be an internal mechanism. Tamm
showed this to be the case by removing a
number of comb plates and hence increasing the spacing between comb plates; he observed that the time interval between successive beats of two widely separated plates
is the same as that between two normally
spaced plates. In addition it must be remembered that in all ctenophores beating
of the plates in a comb row is triggered by
activity in the ciliated groove which passes
down from the balancers, and that the cilia
of the first plate are some 100 times larger
than those of the ciliated groove. Thus,
mechanical triggering by viscous drag seems
unlikely. One is thus left with the possibility of a non-nervous conducting system being present in the ciliated groove of some
ctenophores.
FIG. 7. Inhibition of a neuronal pacemaker by an
epithelial pulse (Stomotoca). a, Suction electrode
attached to marginal region recording swimming
pulses (muscle potentials) and large biphasic
crumpling pulses (epithelial pulses) which were
produced by electrical stimulation, b, Mechanogram
(RCA 5734 transducer) showing movements of
margin; both swimming contractions and the slow
rise in tension associated with crumpling can be
seen. Note that crumpling pulses inhibit swimming
and that when inhibition is removed swimming
recommences at a new frequency. Calibration: 10
sec and 5 mV. (From Mackie, unpublished.)
923
NON-NERVOUS CONDUCTION
CONTROL OF CILIARY BEATING
IN ECHINODERMATA
Tracts of ciliated epithelium are possible
candidates for the possession of a nonnervous conduction system, provided neurons are absent. Recordings of electrical
activity in the ciliated epithelium of an
echinoid plutens larva, Strongylocentrotns
droebachiensis, showed that when ectodermal cilia reversed the direction of their
active stroke, low amplitude potentials
could be recorded which were conducted
between densely ciliated regions (Mackie
et al., 1969). Nervous elements could not be
positively identified and removal of the
neuronal, apical complex did not prevent
the appearance of these reversal potentials,
thus suggesting that an epithelial conducting system may be responsible. Should neurons prove to be present however, echinoderm larvae would become candidates for
inclusion in the list of animals which have
ciliated epithelium in which the initiation
and maintenance of coordination resides in
the epithelial cells themselves while reversal
or arrest of beating involves nervous inhibition.
CONTROL OF CILIARY BEATING IN
UROCHORDATA
In the larvacean Oikopleura, Gait and
Mackie (1971) showed that the direction of
flow water into the mouth is controlled by
rings of ciliated cells in the stigmatal openings. Whenever ciliary reversals were seen,
potentials were recorded in ciliated cells of
the ring and also at the surface of the body.
Although these ciliated rings are under
nervous control, not all the ciliated cells are
innervated; thus, it is likely that "neuroid"
conduction between ciliated cells and between the two ciliated rings exists.
Ciliary arrest in the branchial sac of Corella (Ascidiacea) is also controlled by a
similar mechanism. The innervation of ciliated cells is very sparse and yet large numbers of ciliated cells undergo arrest
simultaneously (Mackie and Paul, 1973). In
this animal it seems likely that arrest is
initiated at neurociliary junctions and then
is coordinated by a signal which spreads
through groups of coupled cells. It is not
certain whether the depolarization that can
be recorded intracellularly in the ciliated
cells during ciliary arrest spreads passively
or is propagated within the ciliated epithelium, but the evidence suggests the latter.
ACTION POTENTIALS IN THE SKIN OF
AMPHIBIAN EMBRYOS
In 1971 Roberts and Stirling showed that
the skin of Xenopus laevis embryos and
larvae can propagate an action potential.
The skin impulse is all-or-nothing in character, is sodium dependent, and is conducted in all directions at a velocity of
approximately 8 cm/sec (Fig. 8). The superficial cells of the skin are joined by "tight"
or "gap" junctions at the outer surface and
elsewhere by simple appositions. Experiments in which current was passed between
two intracellular electrodes indicated that
direct current flow between cells could be
the mechanism responsible for propagation.
Recent work by Roberts and Smyth (personal communication) has shown that
Xenopus embryos possess two types of skin
sensitivity. Firstly, an overall sensitivity to
strong stimuli, such as pokes and pinches,
is present as early as stage 20 (Nieuwkoop
and Faber, 1956). A motor response to these
stimuli is apparent at stage 24 (27 hr). In
stage 24-27 embryos these movements involve flexion of the trunk to one side
(Roberts, 1971), while stage 28-33 embryos
L
J
FIG. 8. Intracellular recording of a skin impulse of
a stage 36 Xenopus larva. Upper trace shows bath
potential. Calibration: 100 msec and 20 mV per
division. (From Roberts and Stirling, 1971.)
924
ANDREW N. SPENCER
RESTING
10
50
0
20
i
30
r
T
POTENTIAL - m V
40
•f—i
50
r
Stage
13
i
60
70
14
CO
CO
Skin active
QC
19
20
i
21'
22
C_3
23
24
FIG. 9. Percentage histograms of resting potentials
of skin cells for stage 13-24 Xenopus embryos. Those
stages in which the skin is active are indicated by
triangles.
exhibit alternating flexion which becomes
more rapid as development proceeds, so that
by stage 33 sustained swimming is seen following a single strong stimulus to the skin.
This overall sensitivity is due to the initiation of a skin impulse at any point on the
ectoderm which then excites the central
nervous system, probably via Rohon-Beard
sensory cells, to cause contraction of the
segmental myotomes (Roberts, 1971).
Secondly, a sensitivity to light touch (stroking with a fine hair) develops a little later
(stage 26) with only the skin over the most
rostral myotomes at first showing irritability. Gradually the field of sensitivity spreads
caudally over the myotomes. At stage 31
the head, dorsal fin, and most of the belly
are sensitive. Roberts and Smyth believe
that this sensitivity resides in the RohonBeard sensory cells which gradually inner-
NON-NERVOUS CONDUCTION
vate areas of the skin further from their
origins in the central nervous system. Thus,
in older embryos strong stimuli evoke a
skin impulse and cause a sustained muscular response whereas light stimuli excite
sensory neurons and produce simple trunk
flexions.
When does the skin impulse first appear
and how does it develop? Palmer and Slack
(1970) measured several bioelectric parameters of early Xenopus laevis embryos from
stages 1-8. They showed that a dramatic
increase in the membrane potential of surface cells from a mean value of — 6.5 ± 2.0
mV at stage 1 to —57.0 ± 8.0 mV at stage 8
is associated with changes in the input resistance and junctional resistance between
skin cells. Palmer and Slack suggest that
this is due to a change in the relative permeability of the surface membrane to sodium
and potassium ions. They do not report any
rapid, spontaneous changes in potential
across the membranes of these cells.
The following experiment was performed
in order to follow changes in resting potential of skin cells in older embryos and to
discover at what stage the skin becomes
excitable. 1 removed the vitelline membrane of embryos under Holtfreter's solution (Na+ 62-mM, K+ 0.67-mM, Ca++ 0.9iriM, Cl- 64-mM and HCO 3 - 2.4-mM) and
used glass micropipettes filled with 3-M
KC1 to measure the resting potential of skin
cells and to monitor skin impulses on the
flank of whole embryos. Microelectrodes
had resistances between 30 to 70-Mfi and tip
potentials less than 12-mV. Embryos not
showing spontaneous skin impulses were
stimulated with 1 msec square waves delivered via an Ag/AgCl suction electrode.
The results are presented in Figure 9 as
percentage histograms of resting potential
in stage 13-24 embryos; measured values
may be below the true value because of the
small size of the cells and the resulting
clamage caused by the microelectrode. The
following developmental changes can be
seen.
Between stages 13-18 when the skin is
inexcitable, only 10% of cells have resting
potentials greater than — 25-mV, but by the
time the skin is active at stage 19 this per-
Stage
925
19
FIG 10. Typical skin impulses of stage 19-26
Xenopus embryos. Calibration: 50 msec and 20 mV.
centage rises to 39. At stage 21 the skin
frequently becomes spontaneously excited
and gives repetitively firing skin impulses
which may continue for several minutes at
a frequency of 2 to 3/sec. This repetitive
firing is rarely seen at stage 23 when 87%
of cells have resting potentials greater than
-25-mV.
Changes in the shape of the skin impulse can be observed as development proceeds (Fig. 10). In younger stages (19 and
20) when the impulse is "immature" the
ratio of the time taken for the impulse to
decay to half peak amplitude to the rise
time is about 3:1; during stages 22-24 it is
over 4:1, and by stage 25 and 26 the ratio
is well over 5:1. The "mature" skin impulses recorded by Roberts and Stirling
(1971) give ratios of about 10:1 for embryos
between stages 33-40.
CHARACTERISTICS AND PROPERTIES OF NONNERVOUS CONDUCTING SYSTEMS
Cells forming a layer of conducting tissue
aie likely to have specialized junctions such
that the nonjunctional membrane resistance
is far larger than the intercellular resistance.
In addition it can be expected that communicating areas of low resistance, joining
neighboring cells, are surrounded by a perijunctional insulation (Loewenstein, 1966).
In freshwater animals where the outer
ectoderm is the likely conducting element,
for example Hydra (Kass-Simon and Passano, 1969) and Xenopus larvae, some
926
ANDREW N. SPENCER
modifications are present so as to make the
outer membrane surfaces which are in contact with the bathing medium relatively
impermeable, thus preventing loss of essential ions. In these cases the inner ectodermal
membrane is probably the active membrane
since current flow through the external
membrane is precluded (Josephson and
Macklin, 1969; Roberts and Stirling, 1971).
Although the skin impulse of amphibians
is sodium dependent, this ion probably does
not play such an important role in electrogenesis in excitable epithelia of cnidarians. For example, tetrodotoxin (at what
should be physiologically adequate levels)
does not prevent the conduction of epithelial pulses in hydromedusae (Mackie and
Passano, 1968). Marine hydrozoans can
propagate epithelial pulses in 1:1 solutions
of sea-water and isotonic magnesium chloride for considerable lengths of time after
all nervous and muscular activity has
ceased (Mackie and Passano, 1968; Spencer,
1971a, 1974). Macklin and Josephson (1971)
have suggested that divalent cations play
an important role in generating contraction
burst pulses in Hydra.
The excitable epithelia of amphibians
and hydrozoans can be excited by mechanical stimulation with no specific areas or
cells acting as receptor sites, though areas
having lower thresholds may exist. In hydromedusae, for example, marginal regions
have lower thresholds for initiation of
crumpling than exumbrella regions. No
epithelial conducting system has yet been
shown to be photosensitive, and though
the ectoderm of animals having an epithelial impulse often demonstrates chemosensitivity, the transduction sites probably
reside in nerve cells.
Excitation from a single point of stimulation spreads out in every direction apparently invading all the cells of that
particular tissue layer. In this way the
motor site is eventually reached where an
escape or protective response is triggered,
but the route and mechanism of excitationcoupling has not been determined. At first
sight this may appear to be an inefficient
mechanism; however, it does have advantages. Fiistly, the shortest route is auto-
matically used, and secondly, the conducting system can be damaged considerably
without preventing conduction. Since the
animals using epithelial conducting systems are fairly small, conduction velocities
of between 1 to 20-cm/sec are adequate.
From the above remarks we can see that
the selective pressures for developing tracts
of conducting cells in an epithelial sheet of
inexcitable cells are not likely to be very
strong, especially since the information
carried is so simple that problems of cross
talk do not exist. Only in the ctenophore
comb rows have we the possibility of conducting tracts of epithelial cells. The evolution of these tracts could be accounted for
by the advantages given to an animal having independent control over cilia in each
comb row.
In the past where there was a "need" to
carry more "bits" of information in welldefined tracts, nerve cells evolved. Since the
functions of nervous and non-nervous conducting systems barely overlap they have
existed side by side to the present day in
several phyla. In other phyla the loss of
epithelial conducting systems, assuming it
to be a primitive feature, was probably consequent on the appearance of giant axons
and myelinated axons.
Although well-defined tracts of conducting epithelial cells may be rare, preferential conduction in certain directions may
be of importance. In Hydra, conduction of
the contraction burst pulse has a longitudinal bias (Kass-Simon, 1972), and in
Proboscidactyla conduction velocities of
crumpling pulses conducted parallel to the
longer axis of exumbrella cells are higher
than pulses conducted parallel to the
shorter axis (Spencer, 1971a). It may be
that the number of intercellular junctions
per unit distance along the direction of
conduction is a major determinant of conduction velocity.
An intriguing characteristic of some
epithelial conducting systems is their rapid
fatiguing with repetitive firing (Spencer,
1971a, 1974). The decrease in conduction
velocity coincident with the passage of a
number of closely spaced impulses through
a sheet of epithelial cells could be explained
927
NON-NERVOUS CONDUCTION
in the following way. If it is assumed that
the conducting sheet is formed by a network of cells coupled by low resistance
bridges (Mackie, 1970; Roberts and Stirling,
1971), then current flow from a number of
neighboring cells might be required to depolarize any cell sufficiently for it to fire.
Thus, any increase in the threshold of cells
in such a sheet will increase the delay at
each intercellular junction, and so reduce
the conduction velocity. The phenomenon
of fatiguing confers an additional property
on some epithelial conducting systems, that
of adaptation, since the system eventually
fails to conduct with continued high frequency stimulation (approx. 0.3 to 10/sec).
ONTOGENY OF NON-NERVOUS CONDUCTING
SYSTEMS
It is not surprising that researchers have
been able to trace an increase in the resting
potential of cells during their development
to form a system capable of regenerative
electrogenesis (Ito and Hori, 1966; Palmer
and Slack, 1970; Takahashi et al., 1971)
since excitable cells generally have higher
resting potentials than non-excitable ones.
The skin cells of Xenopus embryos also
show this rise in resting potential, but the
skin does not become excitable until a large
proportion of cells have resting potentials
over — 25-mV. Before the skin reaches a
stable state it passes through a period of
instability with the whole system firing
spontaneously and regularly for many seconds. Is this phenomenon functionally significant or is it just an accidental consequence of reaching an active but stable
situation with a built-in safety factor? Do
similar events occur in nervous systems
during development? Certainly the behavior of vertebrate embryos suggests that
this is the case, since all vertebrates go
through a period of spontaneous, uncoordinated movement usually in the form of
trunk flexions (Coghill, 1929; Hamburger,
1963; Hughes et al., 1967).
It is difficult to say what importance
should be attached to changes in the shape
of the skin impulse as the embryo of
Xenopus develops. However, very similar
changes have been seen in the developing
muscles of tunicate tadpoles (Takahashi et
al., 1971). In both Xenopus and tunicate
tadpoles the "action potential" is first seen
as a simple pulse lacking a plateau, then as
the duration of the pulses increases a plateau becomes more prominent.
FUTURE RESEARCH
We seem to be entering the second phase
of research into non-nervous conducting
systems, having left behind those first uncertain but exciting steps. Intracellular
recordings of long duration using a number
of electrodes should prove rewarding since
it will enable experimenters to pass current
and study the effects of drugs and the ionic
environment on propagation. Attempts
should also be made to discover whether
non-regenerative potentials coordinate activity such as secretion or ciliary beating
within small groups of cells as suggested by
Mackie (1970). Recent work hints of direct
interaction between nervous and epithelial
conducting systems and perhaps such work
deserves the greatest experimental effort because it is possible that non-excitable
epithelia modify nervous activity in higher
animals by controlling the milieu of
neurons.
REFERENCES
Bullock, T. H. 1943. Xeuromuscular facilitation in
scyphomcdusae. J. Cell. Comp. Physiol. 22:251272.
Child, C. M. 1933. The swimming plate rows of
the ctenophore, Pleurobrachia, as gradients: with
comparative data on other forms. J. Comp.
Neuiol. 57:199-252.
Chun, C. 1897. In H. G. Bronn [ed.], Klassen und
Ordnungen des Thierreichs, Bd. 2, Abt. 2:323327. C. F. Winter, Leipzig.
Coghill, E. G. 1929. Anatomy and the problem of
behavior. Cambridge Univ. Press, London.
Gait, C. P., and G. O. Mackie. 1971. Electrical
correlates of ciliary reversal in Oikopleura. J.
Exp. Biol. 55:205-212.
Hamburger, V. 1963. Some aspects of the embryology of behavior. Quart. Rev. Biol. 38:342-365.
Horridge, G. A. 1954. The nerves and muscles of
medusae. I. Conduction in the nervous system
of Aurellia aurita. J. Exp. Biol. 31:594-600.
Horridge, G. A. 1955a. The nerves and muscles of
medusae. II. Geryonia proboscidalis. J. Exp. Biol.
32:555-568.
928
ANDREW N. SPENCER
Horridge, G. A. 1955b. The nerves and muscles of
medusae. IV. Inhibition in Aequorea forskalea.
J. Exp. Biol. 32:642-648.
Horridge, G. A. 1957. The coordination of the
protective retraction of coral polyps. Phil. Trans.
Roy. Soc. London B 675:495-529.
Horridge, G. A. 1959. The nerves and muscles of
medusae. VI. The rhythm. J. Exp. Biol. 36:72-91.
Horridge, G. A. 1965a. Intracellular action potentials associated with the beating of the cilia
in ctenophore comb plate cells. Nature (London)
205:602.
Horridge, G. A. 1965b. Relations between nerves
and cilia in ctenophores. Amer. Zool. 5:357-375.
Horridge, G. A. 1966. Pathways of coordination in
ctenophores. Symp. Zool. Soc. London 16:247-260.
Hughes, A., S. Bryant, and A. Bellairs. 1967. Embryonic behaviour in the lizard, Lacerta viviparu.
J. Zool. London 153:139-152.
Hyman, L. H. 1940. Observations and experiments
on the physiology of medusae. Biol. Bull. 79:
282-296.
Ito, S., and Y. Hori. 1966. Electrical properties of
Trilurus egg cells during cleavage. J. Gen.
Physiol. 49:1019-1027.
Josephson, R. K. 1961. Colonial responses of
hydroid polyps. J. Exp. Biol. 38:559-577.
Josephson, R. K. 1965. Mechanisms of pacemaker
and effector integration in coelenterates. Symp.
Soc. Exp. Biol. 20:33-47.
Josephson, R. K., and M. Macklin. 1967. Transepithelial potentials in Hydra. Science 156:16291631.
Josephson, R. K., and M. Macklin. 1969. Electrical
properties of the body wall of Hydra. J. Gen.
Physiol. 53:638-665.
Kass-Simon, G. 1972. Longitudinal conduction of
contraction burst pulses from hypostomal excitation loci in Hydra attemiata. J. Comp.
Physiol. 80:29-49.
Kass-Simon, G., and L. M. Passano. 1969. Conduction pathways in Hydra. Amer. Zool. 9:1113.
(Abstr.)
Kleinenberg, N. 1872. Hydra: eine anatomischentwicklungs-geschichtliche Untersuchung. Engelmann, Leipzig.
Lillie, R. S. 1908. The relation of ions to contractile
processes. II. The role of calcium ion in the
mechanical inhibition of the ctenophore comb
plate. Amer. J. Physiol. 21:200-220.
Loewenstein, W. R. 1966. Permeability of membrane
junctions. Ann. N.Y. Acad. Sci. 137:441-472.
Mackie, G. O. 1964. Analysis of locomotion in a
siphonophore colony. Proc. Roy. Soc. London B
159:366-391.
Mackie, G. O. 1965. Conduction in the nerve-free
epithelia of siphonophores. Amer. Zool. 5:439453.
Mackie, G. O. 1970. Neuroid conduction and the
evolution of conducting tissues. Quart. Rev. Biol.
45:319-332.
Mackie, G. O., and L. M. Passano. 1968. Epithelial
conduction in hydromedusae. J. Gen. Physiol.
52:600-621.
Mackie, G. O., L. M. Passano, and M. Pavans de
Ceccatty. 1967. Physiologic du comportement de
l'Hydromeduse Sarsia tubulosa Sars. Les systemes
a conduction aneurale. C. R. Acad. Sci. Paris
264:466-469.
Mackie, G. O., and D. H. Paul. 1973. Electrical
recordings from tunicate ciliated cells. Amer.
Zool. 13:1298. (Abstr.)
Mackie, G. O., A. N. Spencer, and R. Strathmann.
1969. Electrical activity associated with ciliary
leversal in an echinoderm larva. Nature (London)
223:1384-1385.
Macklin, M., and R. K. Josephson. 1971. The ionic
requirements of transepithelial potentials in
Hydra. Biol. Bull. 141:299-318.
Nieuwkoop, P. D., and J. Faber. 1956. Normal tables
of Xenopus laevis (Daudin). North Holland Publ.
Co., Amsterdam.
Palmer, J. !•'., and C. Slack. 1970. Some bioelectiic
parameters of early Xenopus embryos. J. Embryol.
Exp. Morphol. 24:535-553.
Pantin, C. F. A. 1935. The nerve net of the
Actinizoa IV. Facilitation and the staircase. J.
Exp. Biol. 12:389-396.
Parker, G. H. 1905. The movements of the swimming-plates in ctenophores, with reference to the
theories of ciliary metachronism. J. Exp. Zool.
2:407-423.
Passano, L. M. 1965. Pacemakers and activity patterns in medusae: homage to Romanes. Amer.
Zool. 5:465-481.
Roberts, A. 1971. The role of propagated skin impulses in the sensory system of young tadpoles.
Z. Vergl. Physiol. 75:388-401.
Roberts, A., and C. A. Stirling. 1971. The properties
and propagation of a cardiac-like impulse in the
skin of young tadpoles. Z. Vergl. Physiol. 71:
295-310.
Romanes, G. J. 1876. Preliminary observations on
the locomotor system of medusae. Phil. Trans.
Roy. Soc. London 166:269-313.
Romanes, G. J. 1877. Further observations on the
locomotor system of medusae. Phil. Trans. Roy.
Soc. London 167:659-752.
Romanes, G. J. 1880. Concluding observations on
the locomotor system of medusae. Phil. Trans.
Roy. Soc. London 171:161-202.
Sleigh, M. A. 1963. Movements and coordination of
the ciliary comb plates of the ctenophores Beroe
and Pleurobrachia. Nature (London) 199:620-621.
Sleigh, M. A. 1966. Some aspects of the comparative
physiology of cilia. Amer. Rev. Resp. Dis. 93:
16-31.
Sleigh, M. A. 1974. Features of ciliary movement
of the ctenophores Beroe, Pleurobrachia and
Cestus. (In press)
Spencer, A. N. 1971a. Behaviour and electrical activity in Proboscidactyla flavicirrata (Hydrozoa).
Ph.D. Thesis. Univ. of Victoria, B.C.
NON-NERVOUS CONDUCTION
Spencer, A. N. 1971b. Myoid conduction in the
siphonophore Xanomia bijuga. Nature (London)
233:490-491.
Spencer, A. N. 1974. Behaviour and electrical activity in the hydrozoan Proboscidactyla flavicirrata (Brandt). I. The hydroid colony. Biol. Bull.
(In press)
Takahashi, K., S. Miyazaki, and Y. Kidokoro. 1971.
Development of excitability in embryonic muscle
929
cell membranes in certain tunicates. Science 171:
415-418.
Tamm, S. 1973. Mechanisms of ciliary co-ordination
in ctenophores. J. Exp. Biol. 59:231-245.
Verworn, M. 1890. Studien zur Physiologie der
Flimmerbewegung. Pfliigers Arch. Ges. Physiol.
48:149180.
Westfall, J. A. 1973. Ultrastructural evidence for
neuromuscular systems in coelenterates. Amer.
Zool. 13:237-246.