Muscle and Nonmuscle Contractile Systems
TABLE 21-1
Comparison o f Actin-Based and Tubulin-Based Motility Systems
Actin/ Myosin
Structural arrangement
Actin (thin) filaments and myosin
(thick) filaments; myosin heads form
bridges during contraction; filaments
are stable in sarcomeres and stereocilia,
but often labile in cytoplasm.
Static element/dynamic
Actin filament/myosin filament,
Type of motion
Sliding of actin and myosin filaments
past each other.
Energy transducer
Properties of static
Properties of dynamic
Regulatory factors
Myosin is an actin-activated ATPase.
Actin is globular, MW 42,000; monomer
binds MgATP; ATP and calcium
promote polymerization; polymerizes
as a two-stranded helix 6-7 nm
across; each subunit interacts with
four adjacent ones; monomer can be
added or lost at either end: encoded by
multigene family.
Myosin: MW 475,000; globular and
helical regions; heterohaxamer;
ATPase activity; functional unit is
bipolar aggregate of 10 to 500 myosin
monomers; some types are activated
by phosphorylation; encoded by multi-
gene family.
Cytoplasmic calcium concentration;
phosphorylation of myosin light chains
and (less important) other proteins;
interactions with actin-binding proteins.
Tubulin/ Dynein or Kinesin
Microtubules with dynein arms or cytoskeletal
microtubules to which cytosolic dynein or
kinesin may attach; dynein forms intertubule
bridges during movement; tubules are stable in
cilia and flagella, but often labile in cytoplasm.
Microtubule/dynein or kinesin.
Sliding of microtubules restrained at one
end produce bending in cilia and flagella;
dynien or kinesin translate organelles or
other cargo along cytoskeletal microtubules.
Dynein and kinesin are tubulin-activated ATPases.
Tubulin is heterodimer of globular proteins,
MW 50,000 each; dimer binds two GTP,
calcium may inhibit polymerization;
polymerizes in protofilaments which bond in
parallel to form a cylinder of 13 filaments;
each dimer interacts with 4 or
dimers; dimer can be added or lost at either
end; encoded by a multigene family.
Dynein: MW 1,500,000+; functional unit is
heteromultimer; ATPase activity; encoded
by a multigene family.
Kinesin: MW 380,000; heteromultimer of
2 heavy and 2 intermediate chains; ATPase
activity; encoded by a multigene family.
Cytoplasmic calcium concentration;
phosphorylation of several proteins;
interaction with MAPs, tau and other
by growth or shortening of bundles of actin filaments;
and in several microtubule systems, the motor protein is
kinesin rather than dynein. However, muscle and cilia have
many features in common with most other actin and tubu-
lin systems, and are vital to normal functioning of the
21.1 Muscle Systems
Mammals have four types of cells specialized for con-
traction: skeletal muscle, cardiac muscle, smooth muscle,
and myoepithelial cells (Table 21-2). Skeletal and car-
diac muscles contract with more force and much more
speed, shorten less, and consume much more ATP during
maintenance of tension than smooth muscle or myoepithe-
lium. Characteristics of these cell types reflect the func-
tional roles of the tissues in which they occur; however, the
fundamental mechanism of contraction is the same in all
Structure and Development of Skeletal Muscle
The skeletal muscles of the torso and limbs arise from
the mesoderm of the somites, while those of the head
arise from the mesoderm of the somitomeres which contri-
bute to the branchial (pharyngeal) arches. They form by the
fusion and elongation of numerous precursor cells called
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