Bioelectric Basis of Behavior in Protozoa
YUTAKA NAITOH
Department of Biology, University of California, Los Angeles, California 90024
SYNOPSIS. Locomotor responses of ciliate protozoans to external stimuli primarily depend
on changes in ciliary motion evoked by the stimuli. Certain regions of the protozoan
cell jjiuuuce a iecepiur potential in response to stimulation. The receptor potential
electrotronically spreads to the entire cell membrane and generates an overall electric
response due to the electrical excitability of the general membrane. The ionic mechanisms for electrogensis are basically identical to those in nerves, muscles, and receptors of
metazoan organisms. The ionic movements across the membrane associated with the
electrogensis modify directly and/or indirectly the concentration of certain cations within the cell. The catonic concentration change brings about a modification of the contractile activity of cilia, which in turn results in a change in the ciliary movements.
Cilia on different locations of the cell have intrinsically ionic concentrations. This fact,
together with the morphological specialization of cilia in different locations on the cell,
contributes to the complexity and adaptiveness of locomotor responses found in the
ciliated protozoa.
potentials). Depolarizing receptor potentials are amplified by regenerative conducThe locomotor behavior of many proto- tance changes to evoke a large electric
zoans depends on the movement of their response over the entire surface memcilia or flagella. The cilia change their beat- brane. The ionic bases for these electric
ing direction (direction of effective power responses are similar in principle to those
stroke), beating frequency, and beating found in nerves, muscles, and receptor cells
form spontaneously or in response to many of multicellular organisms. Activities of the
kinds of external stimuli. These changes in cilia depend much on their cationic enciliary motion are responsible for much of vironment. Therefore, the movement of
the locomotor behavior of these protozoans ions through the membrane associated with
(Jennings, 1906). Since a protozoan is uni- excitation influences the movements of the
cellular, detection of environmental changes cilia and thus controls the behavior of the
and signaling of these changes to effector organism.
organs (cilia) to evoke an adaptive locoSome locomotor behaviors other than
motor behavior of the organism must all be those involving ciliary motion, such as conperformed without benefit of the specialized traction of the cell body in Paramecium
cellular components or network of a nerv- (Kinosita et al., 1964), tentacle movement
ous system.
of Noctiluca (Eckert and Sibaoka, 1967),
Our electrophysiological studies on proto- and protoplasmic streaming in Amoeba
zoan cells (Naitoh and Eckert, 1974) have (Tasaki and Kamiya, 1964) are also found
revealed that changes in ciliary motion are to be associated with membrane excitation.
closely correlated with electrogenesis in the Bioluminescence of Noctiluca is controlled
surface membrane. Certain regions of the by an action potential which is triggered by
membrane are functionally differentiated mechanical stimulation of the cell body
to receive external stimuli (receptor (Eckert and Sibaoka, 1968). The bioelectric
regions). At those regions stimuli are control of behavior, therefore, seems to be a
transduced into electric signals (receptor very wide-spread phenomenon in the
Protozoa.
Suport for this work came from U.S. Public
This paper will deal with the mechanism
Health Service grant NS 08364 and National Science
Foundation grant GB-30499. I thank Dr. R. Eckert by which bioelectric events in the membrane of the common ciliate Paramecium
for comments on the manuscript.
INTRODUCTION
883
884
YUTAKA NAITOH
govern the locomotor responses of the specimen to mechanical stimulation.
BEHAVIORAL RESPONSES OF
Paramecium TO
STIMULI
Avoiding response
In pond water or in culture medium, the
beating direction of cilia of Paramecium
caudatum is largely toward the posterior, so
that the specimen swims forward. When the
forward-swimming specimen bumps against
a solid object (Fig. 1/4,1) it temporarily reverses the beating direction of cilia on the
cell surface, so that the specimen swims
backward for a short distance (Fig. 1/4,2).
The cilia gradually resume their original
normal (posteriorly pointing) direction, so
that the backward swimming halts, and the
specimen begins to swim forward again.
Before resuming forward swimming, the
specimen rotates around its posterior end
due to strong beating of oral cilia (Fig.
1/4,3). Consequently, the direction of resumed forward swimming is different from
the original direction (Fig. 1/4,4). Thus, the
specimen, by one or more trials, can avoid
the mechanical obstacle with which it
es:=:
es - =
FIG. 2. Responses of cilia to intracellularly applied
electric current (AJ3) and to mechanical stimulation (C,D). An outward current through the membrane induces reversed beating of cilia on the whole
cell surface (A), which is identical with the ciliary
response evoked by mechanical stimulation of the
anterior (a) membrane (C). An inward current
through the membrane induces an increase in beating frequency of the cilia in the normal direction
(B), which is identical with the ciliary response to
mechanical stimulation of the posterior membrane
(D). Small arrows across the membrane shown in A
and B indicate the general direction of electric current, r, Glass rod for mechanical tap of the membrane.
originally collided. Jennings (1906) called
this response the "avoiding reaction." The
tactic behaviors of ciliates depend primarily
on this locomotor response.
An avoiding response can be induced exFIG. 1. Behavorial responses of Paramecium. A, perimentally by touching the anterior
Avoiding response following collision with an ob- region of the specimen with a fine glass
stacle. The specimen temporarily reverses the beating direction of its cilia to swim backward for a needle (Fig. 2C). A strong touch induces
short distance, then resumes forward locomotion in strong and long-lasting backward swima different direction, thus avoiding the obstacle. B, ming, while a weak touch induces only a
Escape response following entrapment in a narrow brief halting of forward swimming. Close
chink of debris in the culture. The cilia beat
vigorously in forward swimming direction, allowing observation of ciliary motion upon the apthe specimen to escape from the chink, a, Anterior plication of a mechanical stimulus revealed
end of the specimen.
that the direction of effective power stroke
BIOELECTRIC BASIS OF BEHAVIOR IN PROTOZOA
885
of cilia changed clockwise to fully reversed
direction with increasing intensity of the
stimulus (Dryl and Grebecki, 1966; Machemer, 1972). Therefore, a weak stimulus
induces only slight clockwise shift of the
beating direction, which causes a slight
reduction of forward momentum of the
specimen.
Escape response
When a mechanical touch by a glass
needle is applied to the posterior region of
the specimen, all the cilia on the cell surface temporarily beat at a higher frequency
than before the stimulus, but the beating
direction does not change (Fig. 2D). This
results in rapid forward swimming of the
ciliate, facilitating its escape from the
source of stimulus ("escape response"). The
increase in beating frequency becomes more
prominent with stronger stimuli.
In cultures, the escape response is most
readily observable when the specimen is
trapped in a narrow channel within rather
soft debris (Fig. 15). Mechanical stimulation of the posterior region of the specimen
by the debris results in an increase in the
beating frequency, resulting in rapid headlong progress through the channel until
the specimen swims out of it. Under these
conditions the mechanical stimulus provided to the anterior region does not induce
an avoiding response, because the threshold
for the avoiding response is higher than
that for the escape response (Naitoh and
Eckert, 1969). For the same reason, a light
tap of the culture vessel induces a sudden
increase in the forward-swimming velocity
of all the specimens in the culture.1
Responses to electric current
It has long been known that when an
electric current is applied to a medium in
which paramecia are suspended, the specimens change their orientation so as to
i Some ciliate protozoa, such as Urostyla sp., show
an avoiding response with their culture vessel is
lightly tapped (Naitoh, unpublished). In these
species the mechanical sensitivity of the head region
may be higher than that of the posterior region.
FIG. 3. Responses of cilia to an electric current applied externally to Paramecium. 1, Forward-swimming specimen before electric stimulation. 2, Upon
an application of electric current, the cilia nearest
the cathode (-) reverse their beating direction, while
the cilia nearest the anode ( + ) beat in the normal
direction. The beating of cilia in opposite direction
acts to rotate the specimen until its anterior end
(a) points toward the cathode (3).
swim toward the cathode. It was soon discovered that cilia on the cell surface near
the cathode show reversed beating, whereas
the cilia on the surface near the anode beat
in the normal direction but with a higher
frequency (Kamada, 1931). The opposite
beating direction of cilia on the two sides
of a single specimen apparently yields a
torque which rotates the specimen until its
anterior end points toward the cathode
(Verworn, 1889; Jennings, 1906) (Fig. 3).
An electric current enters the cell through
the membrane near the anode and leaves
the cell through the membrane near the
cathode. Thus, it was concluded that an
outward current through the membrane induces reversed beating, while an inward
current evokes an increase in beating frequency of the cilia in normal direction.
In the case of an external application of
electric current, the distribution of the current density and direction across the mem-
886
YUTAKA NAITOH
brane are complicated due to a non-linear
change in the membrane resistance in response to the current (Naitoh and Eckert,
1968) and complexity of cell shape. An
application of electric current to the cell
membrane through a microelectrode inserted into the cell makes an analysis far
simpler. The applied current produces an
almost uniform change in the electric potential across the entire membrane (see
later section) and causes a ciliary response
of a given type depending on the polarity
of the current. As a matter of fact, an application of sufficient outward current produces reversed beating of cilia on the entire
membrane (Naitoh, 1958) (Fig. 2A). The
direction of effective power stroke changes
clockwise to fully reversed direction with
increasing current intensity. The ciliary response to an outward current is quite similar to that evoked by mechanical stimulation of the anterior membrane (Fig. 2C).
On the other hand, application of inward
current through the membrane through the
microelectrode induces an increase in the
beating frequency of cilia on the entire cell
membrane (Naitoh, 1958) (Fig. 25). The
degree of the increase is larger when the
current intensity is higher. The ciliary response to an inward current is similar to
that evoked by mechanical stimulation of
the posterior membrane (Fig. 2D).
ANALYSIS OF THE BIOELECTRIC MECHANISMS
Resti?ig membrane potential
Paramecium has an internally negative
resting potential as do nerve and muscle
cells. This was first found by Kamada (1934)
and confirmed later by others (Yamaguchi,
1960; Naitoh and Eckert, 1968). The magnitude of the negative potential decreases
with increasing external concentration of
various cations (K+, Rb+, Na+, Ca2+, Mg2+,
etc.), although the degree of decrease is different for different cation species (Fig. 4).
This indicates that the membrane shows
relatively little specificity for any of the
various cations tested. K+ and Rb+ are the
most permeable among the cations tested,
and Na+, Ca2+, and Mg2+ are less perme-
(mV)
0
-10
-20
-30
-40
0.5
1.0
2.0
4.0
IONIC CONC.
8.0
16
(mM)
FIG. 4. Concentration effects of various cations on
the resting membrane potential of Paramecium. The
elfects of cations other than Ca2t were all determined in I he presence of 1 ITIM Ca2f.
able. Therefore, according to Hodgkin and
Horowicz (1959), the level of the membrane
potential of Paramecium in a mixture of
KC1 (4 mM) and CaCl2 (IITIM) is presumably between the equilibrium potential for
K+ (EK) and that for Ca2+ (ECn), but closer
to EK due to higher permeability of the
resting membrane to K+. The internal K+
concentration of Paramecium is approximately 20 mM KC1 (Naitoh and Eckert,
1973), which corresponds to an EK value of
about —40 mV. The Ca2+ concentration of
the cytoplasm of Paramecium appears to be
less than 1 0 - 7 M (Naitoh and Kaneko,
1973). Accordingly, ECa approximates +120
mV. The actual membrane potential measured in this mixture is about —20 mV.
Based on these data together with the value
of the resting conductance (4 x 10~8 mho;
Eckert and Naitoh, 1970) the ratio of permeability of the membrane for K+ to that
for Ca2+ can be calculated to be about 10.
Response of the membrane to electric
current
When a small outward (depolarizing)
electric current pulse is applied to Paramecium through an inserted microelec-
887
BIOELECTRIC BASIS OF BEHAVIOR IN PROTOZOA
16
(mM)
FIG. 5. A, Electric responses of the membrane (Vm)
to outward current pulses of different intensities
(I). dVm/dt is the first-time derivative of the elec-
trie response. B, Concentration effects of Ca"* on the
peak value of the response (Vp). C, Concentration
elfects of K+ on Vp.
trode, the membrane behaves as an ohmic
resistance in parallel with a capacitance.
That is, the potential change due to the
pulse exhibits a simple exponential time
course (Fig. 5A,a). If the depolarization
exceeds a certain level, the passive electric
change is accompanied by a further depolarization which may continue to develop
even after cessation of the pulse (Fig. 5A,b).
This indicates the onset of an active membrane response by the depolarization. The
peak value (Fig. 5A,Vp), as well as the maximum rate of rise (Fig. 5.<4,dVm/dt) of the
active response increases to saturated levels
with increasing intensities of the stimulus
pulse (Fig. 5A,c).
The ionic mechanism of the active response was investigated by examining the
effects of cation concentrations on the saturated peak value (Naitoh et al., 1972). This
value increases with a logarithmic increase
in the external Ca2+ concentration with a
slope of about + 25 mV per tenfold increase
in concentration (Fig. 5B). This approaches
the predicted Nernst slope (29 mV) for a
divalent cation specific electrode. These
findings strongly suggest that a depolarization of the membrane produces an increase
in the permeability of the membrane to
Ca-+ ions, which leads to an inward current
carried by Ca2+ down its electrochemical
gradient in accordance with the ionic hypothesis of Hodgkin, Huxley, and Katz
(Hodgkin, 1957). The inward Ca2+ current,
which is responsible for the upstroke of the
active response can be demonstrated more
directly with the voltage-clamp technique
(Fig. 6). This also reveals that depolariza-
Vrrv
40 mV
5X IO"9A
10 ms
FIG. 6. Membrane current (Im) in the voltageclamped Paramecium. The intensity of the initial
inward (Ca2*) current increases with increasing depolarization of the membrane (Vm). Delayed outward (K+) current markedly increases with the
membrane depolarization over a certain level.
888
YUTAKA NAITOH
tion leads to a delayed outward K+ current
across the membrane as in muscle and nerve
cells. The K+ current is responsible for the
downstroke of the active response (Fig.
5A,c).
The electric response of the membrane
to an inward (hyperpolarizing) electric current is relatively passive. However, recent
evidence indicates that in response to a
hyperpolarization, Ca2+ permeability of the
membrane decreases, while K+ permeability
gradually increases (Naitoh, unpublished).
This gradual increase in K+ permeability
might be a cause for a delayed anomalous
rectification found in Paramecium upon an
application of electric current (Naitoh and
Eckert, 1968).
anterior region of Paramecium evokes a
transient depolarization of the membrane
(Fig. 1A, upper part), while the same mechanical stimulus evokes a transient hyperpolarization of the membrane when it is
applied to the posterior region (Naitoh and
Eckert, 1969) (Fig. 1A, lower part). Both
potential responses increase in amplitude to
saturated values with increasing intensities
of stimulation.
In order to examine the electric conductance change during these potential responses, a train of small electric pulses was
injected into the cell during the potential
response (Fig. 8). The fact that superimposed deflections produced by the current
pulses during the early phase of each potential response are smaller than those on
the resting membrane potential indicates
that the membrane conductance is higher
during the early portion of the electric
response to mechanical stimulation (Eckert
Response of the membrane to mechanical
stimulation
A mechanical stimulus applied to the
(mV)
30
-c
• B
20
Vpo
10
- o
?..
Vpa
•
-10
-20
-30
-40
Vpp
-50
[K]=2mM
tCa] = I mM
-60
0.016 0.063
0.25
[Co]
FIG. 7. A, Electric responses of the membrane (Vm)
to mechanicial stimulation of the anterior (a) (upper figures) and the posterior (lower figures) ends
of Paramecium. Sm, Electric pulses applied to a
piezoelectric phonocartridge (T) which drives a
1.0
4.0
1.0
2.0
4.0
CK3
8.0
16
glass rod against the cell surface. B, Concentration
effects of Ca2+ on the peak values of the responses
(Vpa and Vpp). C, Concentration effects of K+ on
Vpa and Vpp.
BIOELECTRIC BASIS OF BEHAVIOR IN PROTOZOA
Vm
100
mt
FIG. 8. Conductance changes during the electric responses of the membrane to mechanical stimulation (Sm). A train of current pulses (Si) was injected into the cell to superinpose resulting IR drops
on the responses. Membrane potential change (Vm)
in response to anterior stimulation (A) and posterior
stimulation (B).
et al., 1972; Naitoh and Eckert, 1973). This
means that mechanical stimulation makes
the membrane more permeable to certain
ions.
The identity of the ionic species responsible for the membrane responses was
determined by examining concentration effects of various cations on the saturated
peak values of the responses. The peak
value of posterior hyperpolarizing response
(Fig. 7/4/Vpp). increases linearly with
logarithmic increase in K+ concentration
with a slope of about 50 mV (Fig. 7C),
which approximates the value of 58 mV
predicted from the Nernst K+-diffusion potential. When the membrane is kept hyperpolarized by inward current injection, the
posterior response diminishes its amplitude
and reverses its polarity (to depolarizing
direction) if the level of hyperpolarization
is beyond a certain potential. This reversal
potential is consistent with the equilibrium
potential for potassium, EK. This evidence
indicates that mechanical stimulation of
889
the posterior membrane produces a local
increase in permeability to K+ ions. Since
EK is more negative than the resting potential level, K+ current flows outward
across the membrane, producing a hyperpolarization. The peak value of the anterior
depolarizing response (Fig. 7/4,Vpa) increases linearly with a logarithmic increase
in Ca2+ concentration with a slope of 25
mV (Fig. IB), which approximates the 29
mV slope predicted for a Nernst Ca2+diffusion potential.
Examination of the depolarizing response
to mechanical stimulation of the anterior
end revealed that it consists of two components (Eckert et al., 1972). The first is a
relatively slow depolarization, which is followed by the second, a faster spike-like
depolarization (Fig. 1A). The rate of rise
and amplitude of the early slow component
also increases with increasing stimulus intensity. General features of the fast component as well as its Ca2+ dependency are
very similar to those of the Ca2+ response
evoked by a depolarizing current. The fast
component, therefore, seems to be a regenerative Ca2+ response evoked by the initial
slow depolarization induced directly by mechanical stimulation.
Although direct evidence is still lacking
for the identity of the ionic species carrying
the initial inward mechanoreceptor current,
Ca2+ is the most probable candidate.
Cable properties
The inside of Paramecium is virtually
isopotential. This was demonstrated by the
injection of an electric current into one end
of the specimen while the potential response
of the membrane was recorded separately
from both ends of the specimen (Eckert
and Naitoh, 1970). The two recorded potentials were compared with each other to
determine the decay and the time delay of
the response along the longitudinal axis of
the specimen. The membrane responses recorded from both ends of the specimen are
virtually identical in their magnitude and
time course. This result is consistent with
the relatively large value of space constant
of Paramecium (1400 ;uxn; 5 times longer
890
YUTAKA NAITOH
than the long axis), which can be calculated
according to the standard cable equation of
Hodgkin and Rushton (1946) by introducing the measured values of membrane
resistance and cytoplasmic resistance.
Because of the isopotential condition of
the cell interior the mechanoreceptor potential evoked at one end of a paramecium
spreads electronically to the rest of the cell
membrane to evoke a distributed electric
response by the entire membrane.
CATIONIC CONTROL OF CILIARY MOTION
IN DETERGENT-EXTRACTED CILIA
Electron microscopic studies on cilia revealed that ciliary apparatus is tightly covered by a membrane which is continuous
with the surface membrane of the cell body
(Fawcett, 1961). It is, therefore, not unreasonable to suspect that ciliary activity
may be influenced by the ionic fluxes which
occur across the membrane during its electric activity. In this context it is important
to know the effects of the ciliary apparatus
of the ions involved in the membrane electric events. For this reason we examined the
effects of various cations on the ATPreactivated cilia of detergent (Triton X100)-extracted models of Paramecium. Since
the detergent destroys the diffusion-limiting
properties of the membrane, the externally
applied cations have direct access to the
ciliary apparatus without membrane intervention (Naitoh and Kaneko, 1973).
In a mixture of ATP and Mg2+, models
of Paramecium swim forward by their reactivated ciliary beating in normal direction
as live specimens do in the absence of depolarizing stimuli (Fig. 9B). Ca2+ ions are
unnecessary for the reactivation of ciliary
beating. The beating frequency and, therefore, the forward swimming velocity of the
model depend on the concentrations of both
ATP and Mg2+.
The direction of the effective stroke
gradually shifts clockwise to the fully reversed direction with increasing Ca2+ concentration in the ATP-Mg2+ mixture up to
a calcium concentration of 5 x 10~ 5 M. A
slight change in the beating direction of
V
ATP, Mg
ATP.Mg.Co
ATP, Co
FIG. 9. Schematic illustrations of ciliary motion in
detergent-extracted models of Paramecium. A, An
extracted model in a reference (50 DIM KC\) solution. Cilia in this solution remain immobile. The
position of the non-beating cilia approximates the
end of the effective power stroke of beating cilia in
normal direction. B, In a mixture of ATP and Mg2*
all the cilia on the model are reactivated to beat
metachronously in the normal direction. The model
swims forward. C, The ATP-Mg2+ reactivated cilia
beat in reversed direction when Ca2+ ions are added
to the ATP-Mg2* mixture. The model swims backward. D, On application of Ca2+ together with ATP
(without Mg2*) the non-beating cilia swing once to
point anteriorly, a, Anterior end of the model.
cilia evoked by low Ca2+ concentration results in only a slowing down of forward
swimming velocity. Reversed beating of
cilia evoked by higher Ca2+ concentration
makes the models swim backward (Fig. 9C).
The effect of Ca2+ ions on the motion of
the reactivated cilia is similar to the effect
of membrane depolarization on the motion
of live cilia.
If Ca2+ ions are applied to the models
together with ATP, but without Mg2+ ions,
ciliary beating does not occur, but the nonbeating cilia swing once clockwise so as to
point anteriorly, the direction corresponding to the end of the effective power stroke
of fully reversed beating cilia (Fig. 9D).
These findings lead to the conclusion that
the ciliary apparatus of Paramecium has
two kinds of motile components: one is
responsible for cyclic ciliary bending and
requires Mg2+ as a cofactor for its activation. The other, which is calcium dependent, governs the orientation of the cilium
in its cyclic movements. Each of these components is believed to include an ATPase,
activated by the corresponding divalent
BIOELECTRIC BASIS OF BEHAVIOR IN PROTOZOA
cation, responsible for the transduction of
chemical into mechanical energy.
COUPLING OF THE ELECTRIC RESPONSES IN THE
MEMBRANE TO CILIARY ACTIVITY
891
motile apparatus, but through an indirect
mechanism which might be located in one
of the detergent-extractable components of
the cell.
SUMMARY
The ionic requirements for reversed beating of cilia in response to electric stimulation has lung been studied (Bancroft, 1906;
Kinosita and Murakami, 1967). It was concluded that only Ca2+ ions in the external
solution are essential for the response. Other
cations antagonize the effect of Ca2+.
Simultaneous recording of both electric
and ciliary responses clearly demonstrated
that reversed beating of cilia is initiated by
depolarization only if the regenerative calcium response is evoked in the membrane
(Machemer and Eckert, 1973). The degree
of ciliary response is larger when the calcium
response is greater. Based on these findings
together with the fact that ATP-Mg reactivated model cilia beat in reversed direction
in the presence of Ca2+ ions, it is concluded
that Ca2+ ions carried across the membrane
during the regenerative depolarization of
the membrane might bring about an increase in the cytoplasmic Ca2+ concentration, which activates the Ca-sensitive motile
component in the ciliary apparatus. The
activation of this component, in turn, results in a reversed beating of cilia (Eckert,
1972; Eckert and Naitoh, 1972).
The mechanism by which hyperpolarization of the membrane activates the Mg2+sensitive beating mechanism in ciliary
apparatus to increase the beating frequency
remains unknown.
A tremendous increase in the beating
frequency occurs in association with reversed beating of cilia evoked by calcium
response in live specimens (Kinosita et al.,
1965). Therefore, it might be conjectured
that the beating frequency is also controlled
by the Ca2+ concentration. However, reversed beating of ATP-reactivated model
cilia evoked by Ca2+ ions is not associated
with an increase in the beating frequency
(Naitoh and Kaneko, 1972). This suggests
that the increase in the beating frequency
associated with the calcium response is not
due to the direct action of Ca2+ on the
Although our experimental evidence
does not yet provide a complete explanation of the control mechanisms underlying
the behavioral response of Paramecium to
mechanical stimulation, we can tentatively
summarize (Fig. 10) the mechanisms based
on the foregoing findings as follows:
1) Avoiding response: A mechanical stimulus to the anterior membrane evokes
an increase in the permeability of that
membrane to ion j+ (probably Ca2+). j +
ions move into the cell to make the membrane depolarized (a depolarizing mechanoreceptor potential). The depolarization
spreads to the whole membrane due to the
cable properties of the cell. The calcium
conductance (GQ, 2 *) of the entire membrane increases in response to the depolarizing receptor potential. External calcium
ions move into the cell. This makes the
membrane more depolarized and more
permeable to calcium ions. This results in a
large regenerative depolarizing response.
Ca2+ inflow associated with the response
increases the cytoplasmic Ca2+ concentration ([Ca2+]j). The calcium-sensitive mechanism for reversed beating of the cilia is
activated by the increased [Ca2+]i. The activation makes the cilia beat in the reversed
direction and produces backward swimming
of the specimen. The increase in cytoplasmic Ca2+ concentration due to Ca2+
inflow indirectly (through an unknown
Mg-+-related mechanism) stimulates the
beating mechanism of cilia. This results in
a large increase in the frequency during
reversed beating. The large depolarization
of the membrane due to the regenerative
increase in GCa2+ induces a delayed increase in the K+ conductance (GK+) of the
membrane. K+ ions move out of the cell
according to their electrochemical gradient.
The outflow of K+ ions acts to repolarize
the membrane, turning off the regenerative
calcium response. [Ca2+]j is decreased by
892
YUTAKA NAITOH
Mechanical stimulation
I
I
i
Posterior membrane
Anterior membrane
GR* increased
Gj* increased (possibly GQQ*)
K* moved out of the cell
j + moved into the cell
Hyperpolarizing receptor potential
Depolarizing receptor potential
Electrotonically spread
Electrotonlcally spread
General membrane hyperpolarized
General membrane depolarized
r->
Regenerative 1
•
Co response = I Ca+Viewed into the cell
I—Further depolarizationCCa++l|
increased
Reversal mechanism
activated
Beating mechanism
activated
Beating direction
reversed
Beating frequency
increased
I
Beating frequency increased
Quick backward swimming
II
| Avoiding response!
i
Forward swimming accelerated
| Escape response |
I
Gcn'decreased
Ca response
terminated =
Beating mechanism activated
i
GK*increosed
G«+ decreased
K+moved out of the cell
Resting potential resumed
-•Membrane repolarized
Beating mechanism deactivated
Normal beating resumed
Normal forward swimming restored
Reversal mechanism
deactivated
Beating mechanism
deactivated
I
I
J
Normal beating resumed
Forward swimming restored
FIG. 10, Summary of mechanisms controlling loco-
motor behavior of Parmecium.
pumping out and/or sequestering of Ca2+,
and reversed beating ceases. The cilia resume their normal beating direction, and
forward swimming is restored.
2) Escape response: A mechanical stimulus applied to the posterior membrane
evokes an increase in the permeability of
that membrane to K+ ions. K+ ions move
out of the cell, thereby producing a hyperpolarization of the membrane (hyperpolar-
izing receptor potential) which electrotonically hyperpolarizes the entire cell
membrane. The hyperpolarization activates
the beating mechanism of cilia through an
unknown Mg2+-related coupling mechanism. The activation of a beating mechanism results in an increase in the beating
frequency of cilia. Thus, forward swimming
is accelerated. The GK+ increase of the
membrane by a mechanical stimulus is
BIOELECTRIC BASIS OF BEHAVIOR IN PROTOZOA
temporary. The membrane potential soon
resumes its resting level, and the accelerated
forward movement of the specimen slows
down to normal velocity.
REFERENCES
Bancroft, F. W. 1906. On the influence of the relative concentration of calcium ions on the reversal
of the polar effects of the galvanic current in
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