Muscle contraction is caused by the action of myosin motors within the structural confines of contractile unit arrays. When the force generated by cyclic interactions between myosin crossbridges and actin filaments is greater than the average load shared by the crossbridges, sliding of the actin filaments occurs and the muscle shortens. The shortening velocity as a function of muscle load can be described mathematically by a hyperbola; this characteristic force–velocity relationship stems from stochastic interactions between the crossbridges and the actin filaments. Beyond the actomyosin interaction, there is not yet a unified theory explaining smooth muscle contraction, mainly because the structure of the contractile unit in smooth muscle (akin to the sarcomere in striated muscle) is still undefined. In this review, functional and structural data from airway smooth muscle are analyzed in an engineering approach of quantification and correlation to support a model of the contractile unit with characteristics revealed by mathematical analyses and behavior matched by experimental observation.

References

References
1.
Harris
,
D. E.
, and
Warshaw
,
D. M.
,
1993
, “
Smooth and Skeletal Muscle Actin Are Mechanically Indistinguishable in the In Vitro Motility Assay
,”
Circ. Res.
,
72
(
1
), pp.
219
224
.
2.
Guilford
,
W. H.
,
Dupuis
,
D. E.
,
Kennedy
,
G.
,
Wu
,
J.
,
Patlak
,
J. B.
, and
Warshaw
,
D. M.
,
1997
, “
Smooth Muscle and Skeletal Muscle Myosins Produce Similar Unitary Forces and Displacements in the Laser Trap
,”
Biophys. J.
,
72
(
3
), pp.
1006
1021
.
3.
Hill
,
A. V.
,
1938
, “
The Heat of Shortening and the Dynamic Constants of Muscle
,”
Proc. R. Soc. London B Biol. Sci.
,
126
(843), pp.
136
195
.
4.
Kamm
,
K. E.
, and
Stull
,
J. T.
,
1985
, “
Myosin Phosphorylation, Force, and Maximal Shortening Velocity in Neurally Stimulated Tracheal Smooth Muscle
,”
Am. J. Physiol.
,
249
(
3
), pp.
C238
C247
.
5.
Seow
,
C. Y.
, and
Stephens
,
N. L.
,
1986
, “
Force-Velocity Curves for Smooth Muscle: Analysis of Internal Factors Reducing Velocity
,”
Am. J. Physiol.
,
251
(
3
), pp.
C362
C368
.
6.
Warshaw
,
D. M.
,
1987
, “
Force: Velocity Relationship in Single Isolated Toad Stomach Smooth Muscle Cells
,”
J. Gen. Physiol.
,
89
(
5
), pp.
771
789
.
7.
Squire
,
J. M.
,
1975
, “
Muscle Filament Structure and Muscle Contraction
,”
Annu. Rev. Biophys. Bioeng.
,
4
(
00
), pp.
137
163
.
8.
Craig
,
R.
, and
Megerman
,
J.
,
1977
, “
Assembly of Smooth Muscle Myosin Into Side-Polar Filaments
,”
J. Cell Biol.
,
75
(
3
), pp.
990
996
.
9.
Cooke
,
P. H.
,
Fay
,
F. S.
, and
Craig
,
R.
,
1989
, “
Myosin Filaments Isolated From Skinned Amphibian Smooth Muscle Cells Are Side-Polar
,”
J. Muscle Res. Cell Motil.
,
10
(
3
), pp.
206
220
.
10.
Xu
,
J. Q.
,
Harder
,
B. A.
,
Uman
,
P.
, and
Craig
,
R.
,
1996
, “
Myosin Filament Structure in Vertebrate Smooth Muscle
,”
J. Cell Biol.
,
134
(
1
), pp.
53
66
.
11.
Hodgkinson
,
J. L.
,
Newman
,
T. M.
,
Marston
,
S. B.
, and
Severs
,
N. J.
,
1995
, “
The Structure of the Contractile Apparatus in Ultrarapidly Frozen Smooth Muscle: Freeze-Fracture, Deep-Etch, and Freeze-Substitution Studies
,”
J. Struct. Biol.
,
114
(
2
), pp.
93
104
.
12.
Hill
,
A. V.
,
1964
, “
The Effect of Load on the Heat of Shortening of Muscle
,”
Proc. R. Soc. London B Biol. Sci.
,
159
(975), pp.
297
318
.
13.
Abbott
,
B. C.
, and
Wilkie
,
D. R.
,
1953
, “
The Relation Between Velocity of Shortening and the Tension-Length Curve of Skeletal Muscle
,”
J. Physiol.
,
120
(
1–2
), pp.
214
223
.
14.
Seow
,
C. Y.
,
2013
, “
Hill's Equation of Muscle Performance and Its Hidden Insight on Molecular Mechanisms
,”
J. Gen. Physiol.
,
142
(
6
), pp.
561
573
.
15.
Huxley
,
A. F.
,
1957
, “
Muscle Structure and Theory of Contraction
,”
Prog. Biophys. Biophys. Chem.
,
7
, pp.
255
318
.
16.
Huxley
,
A. F.
, and
Simmons
,
R. M.
,
1971
, “
Proposed Mechanism of Force Generation in Striated Muscle
,”
Nature
,
233
(
5321
), pp.
533
538
.
17.
Piazzesi
,
G.
,
Reconditi
,
M.
,
Linari
,
M.
,
Lucii
,
L.
,
Bianco
,
P.
,
Brunello
,
E.
,
Decostre
,
V.
,
Stewart
,
A.
,
Gore
,
D. B.
,
Irving
,
T. C.
,
Irving
,
M.
, and
Lombardi
,
V.
,
2007
, “
Skeletal Muscle Performance Determined by Modulation of Number of Myosin Motors Rather Than Motor Force or Stroke Size
,”
Cell
,
131
(
4
), pp.
784
795
.
18.
Edman
,
K. A.
,
1988
, “
Double-Hyperbolic Force-Velocity Relation in Frog Muscle Fibres
,”
J. Physiol.
,
404
, pp.
301
321
.
19.
Wang
,
J.
,
Jiang
,
H.
, and
Stephens
,
N. L.
,
1994
, “
A Modified Force-Velocity Equation for Smooth Muscle Contraction
,”
J. Appl. Physiol.
,
76
(
1
), pp.
253
258
.
20.
Devrome
,
A. N.
, and
MacIntosh
,
B. R.
,
2007
, “
The Biphasic Force-Velocity Relationship in Whole Rat Skeletal Muscle in Situ
,”
J. Appl. Physiol.
,
102
(
6
), pp.
2294
2300
.
21.
Bond
,
M.
, and
Somlyo
,
A. V.
,
1982
, “
Dense Bodies and Actin Polarity in Vertebrate Smooth Muscle
,”
J. Cell Biol.
,
95
(
2 Pt. 1
), pp.
403
413
.
22.
Herrera
,
A. M.
,
McParland
,
B. E.
,
Bienkowska
,
A.
,
Tait
,
R.
,
Paré
,
P. D.
, and
Seow
,
C. Y.
,
2005
, “
Sarcomeres' of Smooth Muscle: Functional Characteristics and Ultrastructural Evidence
,”
J. Cell Sci.
,
118
(
11
), pp.
2381
2392
.
23.
Gordon
,
A. M.
,
Huxley
,
A. F.
, and
Julian
,
F. J.
,
1966
, “
The Variation in Isometric Tension With Sarcomere Length in Vertebrate Muscle Fibres
,”
J. Physiol.
,
184
(
1
), pp.
170
192
.
24.
Harris
,
D. E.
, and
Warshaw
,
D. M.
,
1990
, “
Slowing of Velocity During Isotonic Shortening in Single Isolated Smooth Muscle Cells. Evidence for an Internal Load
,”
J. Gen. Physiol.
,
96
(
3
), pp.
581
601
.
25.
Syyong
,
H. T.
,
Raqeeb
,
A.
,
Paré
,
P. D.
, and
Seow
,
C. Y.
,
2011
, “
Time Course of Isotonic Shortening and the Underlying Contraction Mechanism in Airway Smooth Muscle
,”
J. Appl. Physiol.
,
111
(
3
), pp.
642
656
.
26.
Liu
,
J. C.
,
Rottler
,
J.
,
Wang
,
L.
,
Zhang
,
J.
,
Pascoe
,
C. D.
,
Lan
,
B.
,
Norris
,
B. A.
,
Herrera
,
A. M.
,
Paré
,
P. D.
, and
Seow
,
C. Y.
,
2013
, “
Myosin Filaments in Smooth Muscle Cells Do Not Have a Constant Length
,”
J. Physiol.
,
591
(
23
), pp.
5867
5878
.
27.
Ip
,
K.
,
Sobieszek
,
A.
,
Solomon
,
D.
,
Jiao
,
Y.
,
Paré
,
P. D.
, and
Seow
,
C. Y.
,
2007
, “
Physical Integrity of Smooth Muscle Myosin Filaments Is Enhanced by Phosphorylation of the Regulatory Myosin Light Chain
,”
Cell. Physiol. Biochem.
,
20
(
5
), pp.
649
658
.
28.
Rovner
,
A. S.
,
Fagnant
,
P. M.
,
Lowey
,
S.
, and
Trybus
,
K. M.
,
2002
, “
The Carboxyl-Terminal Isoforms of Smooth Muscle Myosin Heavy Chain Determine Thick Filament Assembly Properties
,”
J. Cell Biol.
,
156
(
1
), pp.
113
123
.
29.
Mahajan
,
R. K.
,
Vaughan
,
K. T.
,
Johns
,
J. A.
, and
Pardee
,
J. D.
,
1989
, “
Actin Filaments Mediate Dictyostelium Myosin Assembly In Vitro
,”
Proc. Natl. Acad. Sci. U. S. A.
,
86
(
16
), pp.
6161
6165
.
30.
Applegate
,
D.
, and
Pardee
,
J. D.
,
1992
, “
Actin-Facilitated Assembly of Smooth Muscle Myosin Induces Formation of Actomyosin Fibrils
,”
J. Cell Biol.
,
117
(
6
), pp.
1223
1230
.
31.
Katayama
,
E.
,
Scott-Woo
,
G.
, and
Ikebe
,
M.
,
1995
, “
Effect of Caldesmon on the Assembly of Smooth Muscle Myosin
,”
J. Biol. Chem.
,
270
(
8
), pp.
3919
3925
.
32.
Seow
,
C. Y.
,
2005
, “
Myosin Filament Assembly in an Ever-Changing Myofilament Lattice of Smooth Muscle
,”
Am. J. Physiol. Cell Physiol.
,
289
(
6
), pp.
C1363
C1368
.
You do not currently have access to this content.