Isometric Responses of the Somatic Musculature
of Cryotochiton stelleri
(Phylum: Mollusca, Class: Polyplacophora)
Abstract
Electrophysiological data were gathered on an isolated seg-
ment of the somatic musculature of Cryptochiton stelleri, a sub¬
tidal Polyplacophoran.
1. The muscles were found to be iterative, single stimuli
evoking no response.
2. Both steady D.C. (2 sec) and trains of short duration
pulses evoke a short-term "phasic" response, although the time
relationships differ slightly.
3. Steady D.C. of polarity opposite to the above yields a
longer "tonic" response.
4. Both the tonic and phasic contractions will summate, but
only the phasic will sustain a state of tetanus.
5. The tonic response is hypothetically attributed to mem¬
brane hyperpolarization and subsequent Cat influx, and as such.
probably does not occur in vivo.
Polyp
Harrison
isculature
page 2
INTRODUCTION
The musculature of mollusks has attracted the attention
of phsiologists for many years largely because molluscan
smooth muscle exhibits two unusual properties. One is
the unusually great capacity for development of tension
by smooth muscles of bivalves. The other is the occurrence
of "catch", a change in physical state, first described
in the clam Pecten maximus by von Uexkull (1912), and since
extensively investigated in Mytilus edulis (e.g., Twarog, 1967)
Molluscan muscles exhibit a wide variety of ultrastructural
organization varying from classic smooth muscles to
highly complex striated fibers and including smooth muscles
containing paramyosin that posses catch properties. Thus
the phylum cannot be characterized by any one type of
physiological response of somatic muscles.
Even though a comprehensive anatomical characterization
of the musculature of Tonicella is available (Henrici,
1912), there appear to have been no attempts to isolate
preparations of somatic muscles of polyplacophorans for
study of their physiological properties. The procedures
described here were designed to investigate the contractive
properties of one group of somatic muscles of one species of
Polyplacophora, Cryptochiton stelleri (Middendorff 1846).
These preliminary results of isometric responses of the
dorsal oblique muscle show that the responses resemble those
found not only in other mollusks, but in Crustacea as well.
Harrison
Polyplacophoran Somatic Musculature
MATERIALS AND METHODS
Adult specimens of Cryptochiton stelleri were collected
subtidally at Mussel Point, California, and amintained in a
large seawater system tank prior to use. From each animal,
an anterior dorsal oblique muscle was isolated and tied off
with silk thread at the dorsal insertion. A ten to fiteen
millimeter segment was then excised and the untied end
secured in a plexiglass muscle clamp having imbedded Ag/AgCl¬
electrode, and the entire system was maintained at about
15°C for the duration of the experiment. The muscle was
secured at its working length as measured in the intact animal
and allowed to equilibrate for about twenty minutes.
Isometric responses were coupled by a Grass FT.O3 Force
Transducer to a Grass Model 5 Polygraph through a Model
5PIC Low Level D.C. Preamplifier.
RESULTS
A respresentative sample of isometric responses from
anterior oblique muscle preparations of Cryptochiton stelleri
to various regimes of electrical stimulation is illlustrated
in Figure 1. The contractions may be generalized into two
basic types of waveform: a rapidly ascending and decaying
"phasic" form, and a slower "tonic" response that is characterized
by a period of latency preceding the development of tension and
page 3
Harrison
Polyplacophoran Somatic Musculature
page 4
by a more linear and elongate curve of relaxation.
The waveforms shown in Fig. 1A are typical of the response
to D. C. stimulation with a duration of a few seconds. Peak
contraction is attained in from 3 to 6.5 seconds, depending
upon the strength of stimulus, while the decay from peak to
half-peak ranges from 4.5 to 11 seconds in different
individual preparations. The latter variation is not
dependent upon stimulus intensity. By contrast, tonic
contractions such as those in Fig. IB average 27 seconds to
peak and 44 seconds to half-decay, neither value showing any
dependence upon stimulus strength. The stimulus mode which
elicits the tonic response is also moderate duration D.C..
the only difference being the reversal of polarity at the
stimulator. In addition, both responsese may be evoked
by various regimes of pulsed stimulation, as long as the
appropriate polarity'is employed for the phasic or tonic
waveform. Fig. 1C illustrates a phasic waveform elicited by
a train of pulses of varying duration.
Both the phasic and the tonic responses are iterative
in that they cannot be induced by single brief stimuli but
require repetitive or continuous activation. It is also
evident that for the phasic waveform both the rate of development
of tension and the degree of tension achieved are dependent
upon the amount of charge passed across the membrane. Thus,
the increasing phasic summation (Fig. 1D) reflects the decrease
Harrison
Polyplacophoran Somatic Musculature
page 5
in interval between pulses and can be extra-polated to approximate
the type of waveform in Fig. 1C. By contrast, summation in
the tonic mode is accompanied by the latency of response and
retains a characteristic sawtooth appearance (Fig. IE).
In the phasic mode, a tetanic contraction can be achieved
which, for a muscle or working length 9 mm under a rest tension
of 17.5 g, peaks at a maximum tension of 24.6 g.
As in other smooth muscles, a constant tension produces
a gradual elongation which, in the isometric preparation,
is recorded as a steady decrease in resting tension.
Furthermore, a protracted regime of occasional low level
phasic stimulation will cause the rest tension to stabilize.
In preliminary experiments on the dorsal oblique muscle
of Cryptochiton stelleri, reproducible responses could not
be elicited with acetylcholine (10 M), 5-hydroxytryptamine
(10 2 to 10 M), or norepinephrine.
Teased muscle fibers examined by phase contrast micro¬
scopy exhibit a diameter of 3.5 to 5 microns and appear
several millimeters long. Under high magnification, contractile
fibrils are visible and of uniform diameter, with no indication
of striation. Thus light microscopy revealed only one type of
fiber. Other details of fine structure are not readily
apparent although the presence of occasional membrane processes
is indicated. The entire muscle is encased in a sheet of
connective tissue.
page 6
Polyplacophoran Somatic Musculature
Harrison
DISCUSSION
Although the characteristics of the phasic waveform from
a dorsal oblique muscle of Cryptochiton stelleri are
generally comparable with those of other molluscan non-striated
muscles such as the opaque adductor of Pecten (Prosser, 1973,
p. 747), both the character and the means of elicitation of the
tonic response are anomalous. Significantly, there is no
evidence of differentiation between fibers or contractile
fibrils that would indicate fast and slow response capabilities.
Consequently, the source of the variation in response must
be sought elsewhere.
Preliminary reports on the development of tension in
crayfish motor fibers as a consequence of membrane hyper¬
polarization (Reuben et al., 1967, p. 634) have recently
been confirmed in other crustaceans (Uchitel & Garcia, 1974,
p. 111). The characteristics of the evoked waveform include
a period of latency and a time to peak and time to half
decay considerably longer than those of normally elicited
responses. Furthermore, the contraction following hperpolar¬
ization has been proven independent of normal intracellular
mechanisms for release and sequestration of Ca' and is a
function of inward diffusion of Ca from the bathing medium
in response to the imposed electrochemical gradient.
On the basis of the waveform similarities, the tonic
response of the chiton smooth muscle is tentatively suggested
Harrison
Polyplacophoran Somatic Musculature
page 7
to be attributable to a similar hyperpolarization mechanism.
The individual muscle fibers are known to be several millimeters
in length, and in the process of isolation many fibers have been
cut, leaving a channel for the establishment of intracellular
variation in potential. The reversal of polarity is suggested
to set up an electrochemical gradient which promotes the
diffusion of Ca" inward across the membranes of intact
cells as a result of hyperpolarization. The discrepencies
in time course from those reported by Uchitel and Garcia
are to be expected because of significant ultrastructural
differences between the two muscles, and are equally reflected
in the respective phasic responses.
The similarity of the phasic and tonic waveforms to those
obtained on a faster time scale for crustacean striated
muscle points out the phylogenetic transcendence of the basic
functional principles of muscular response. Further characteri¬
zation, both electrophysiological and pharmacological, of this
preparation will enhance our appreciation of its adaptive
structural properties.
SUMMARY
Electrophysiological data were gathered on an isolated
segment of the somatic musculature of Cryptochiton stelleri,
a subtidal Polyplacophoran.
1. The muscles were found to be iterative, single short
stimuli evoking no response.
2. Both steady D.C. (2 sec.) and trains of short
Harrison
Polyplacophoran Somatic Musculature
page 8
duration pulses evoke a short term phasic response, although
the time relations differe slightly.
3. Steady D.C. of polarity opposite to the aboye
yields a longer tonic contraction.
4. Both the tonic and the phasic responses will summate.
but only in the phasic mode will the muscle sustain a state
of tetanus.
5. The tonic response is hypothetically attributed to
membrane hyperpolarization and subsequent ca influx, and
as such probably does not occur in vivo.
Harrison
Polyplacophoran Somatic Musculature
LITERATURE CITED
Henrici, P.
1912. Muskulatur und Fussdrusen bei Tonicella.
Ark. f. Zool., 7(35): 1-17.
Prosser, C. L.
1973. Comparative Animal Physiology. i-xxii + 966
pp: illus. Philadelphia, Pa. (W. B. Saunders Co.).
Reuben, J. P.; Brandt, P. W.; Garcia, H.; Grundfest, H.
1967. Excitation-Contraction Coupling in Crayfish.
Am. Zool., 7: 623-645.
Twarog, B. M.
1967. Excitation of Mytilus Smooth Muscle.
J. Physiol. (Lond.), 192: 857-868.
Uchitel, O. D. & Garcia, H.
1974. Muscle Contraction During Hyperpolarizing Currents
in the Crab. J. Gen. Physiol., 63(1): 111-122.
Uexküll, J. von
1912. Studien über den Torrus. 6: Die Pilgermuschel.
Zs. Biol. Munchen 58: 305-322.
page 9
FIGURE CAPTIONS
Fig. 1A-E. A composite of isometric responses of somatic
muscle preparations from Cryptochiton stelleri, horizontal
axis in 10 sec. intervals. A--phasic responses to 2 sec.
D.C. stimuli, voltages indicated. B--tonic waveforms
elicited by 5 sec. stimuli of 5 volts, polarity reversed from
A. C--phasic response to normal polarity D. C. pulses,
10 pulses per second, duration as indicated. D--phasic
summation: D.C. (4v, 2 sec.) separated by intervals of 10,
8, 6, 4, and 2 seconds. E-tonic summation: reverse polarity
D.C. (20v, 3 sec.) at points indicated.
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