Fast muscle in squid (Loligo pealei): contractile properties of a specialized muscle fibre type
William M. Kier1 and
Nancy A. Curtin2
1 Department of Biology, CB 3280 Coker Hall, University of North Carolina,
Chapel Hill, NC 27599-3280, USA
2 Biological Structure and Function Section, Division of Biomedical
Sciences, Faculty of Medicine, Fleming Building, Imperial College, London SW7
2AZ, UK
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Fig. 1. Schematic diagram of cross sections of the tentacular stalk (A) and arm (B)
of the squid Loligo pealei. The core of the tentacle and the arm
consists of a densely packed mass of transverse muscle fibres (T) oriented
perpendicular to the long axis of the appendages. The axial nerve cord (Ax)
and longitudinal muscles (L) are also visible in the diagram. The muscle fibre
bundle preparations (shaded) were obtained by cutting a small section from a
transverse slice of the arm or tentacle. The foil clips used to hold the
preparation in the testing apparatus were attached to the ends of the
preparation (shaded and cross-hatched) in such a way that only transverse
muscle fibres were present between the clips.
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Fig. 2. Transmission electron micrographs of longitudinal sections of the
transverse muscle fibres of the tentacle (A) and arm (B) of Loligo
pealei. The extensive sarcoplasmic reticulum (SR) of the cross-striated
tentacle fibres is visible. Note the short sarcomeres and short thick
filaments of the tentacle fibres compared with the much longer thick filaments
apparent in the obliquely striated arm fibres. Scale bar, 1 µm.
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Fig. 3. Twitch force (A) and length (B) of a muscle fibre bundle preparation from
the transverse muscle mass of the tentacle of Loligo pealei. The
recordings of twitch force and length are superimposed after a stimulus at
time zero. The highest force recording was obtained during an isometric
(constant-length) twitch, and the others (a, b, c, d) were obtained during
isotonic twitch with the force clamped to four different levels. The vertical
lines mark the point at which force was measured. The slopes of the dashed
lines on the length recordings give the velocity of shortening, and the
vertical lines are the same times at which force was measured and mark the
centre of the section from which the slopes were measured.
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Fig. 4. Force/velocity relationship for a single tentacle transverse muscle bundle
preparation (filled circles) and a single arm transverse muscle bundle
preparation (open circles). Force is expressed relative to the isometric force
of the preparation (mean of repeat twitches for the tentacle and repeat 100
ms, 50 Hz tetani for the arm). Velocity is expressed in L0
s-1, where L0 is the length of the preparation
at which peak isometric force is produced. The lines were fitted to the data
using Hill's single hyperbolic function (see text and
Table 1, tentacle preparation 6
and arm preparation 6).
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Fig. 5. Summaries of the length/active force relationships for tentacle transverse
muscle fibre bundle preparations (A) during twitch stimulation (N=10)
and arm transverse muscle fibre bundle preparations (B) during tetanic
stimulation for 100 ms at 50 Hz (filled circles, N=6) and twitch
stimulation (open circles, N=5). Force is expressed relative to the
preparation's peak force for the same pattern of stimulation, and length is
expressed relative to that giving this peak force.
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Fig. 6. Length/force relationship of a single arm (A) and a single tentacle (B)
transverse muscle fibre bundle preparation. Force is expressed relative to the
peak force P0 during twitch stimulation, and the
preparation length is expressed relative to the optimal length for active
force production, L0. Filled circles represent the active
force produced in response to twitch stimulation and open circles represent
the passive force produced in the absence of stimulation.
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Fig. 7. Superimposed recordings of force produced in response to stimulation at
frequencies of 1 (twitch), 5, 10, 20, 40, 80, 120 and 160 Hz for 0.2 s
(indicated by the horizontal bar) for a tentacle transverse muscle fibre
bundle preparation (A) and an arm transverse muscle fibre bundle preparation
(B).
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Fig. 8. Pooled stimulus frequency/force relationship data for tentacle muscle fibre
bundle preparations (filled circles, N=10) and arm muscle fibre
bundle preparations (open circles, N=10). Force is expressed relative
to the preparation's maximum observed force. Values are means ±
S.D.
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© The Company of Biologists Ltd 2002
