462
chapter
21
Muscle and Nonmuscle Contractile Systems
TABLE 21-4
Equivalent Light Chain Nomenclature
Alkaline
Essential
Ai
LCif
light chains
light chains
a 2
LC3f
A
L C ls
DTNB
Regulatory
DTNB
LC2f
light chains
light chains
l c 2s
There is also a designation based on the source of the
MLC, with the subscripts “f” and “s” indicating the MLCs
are from fast or slow muscle fibers, respectively. The re-
lationship of one nomenclature to another is shown in
Table 21-4. As explained under control of contraction, the
regulatory light chain (rather than troponin) is the major
regulatory protein under some circumstances in smooth
muscle.
The physiologically active form of myosin is the
thick myofilament (Figure 21-8), which is typically about
1.6
p
.m long and is formed by aggregation of myosin tails
to create a bundle with the heads sticking outward. Head
pairs are spaced linearly 14.3 nm apart axially, and 60°
apart angularly around the filament, which corresponds
to the spacing of the thin filaments in a hexagonal pattern
around the thick filaments. The axial repeat distance of the
thick filaments is 43 nm. Apparently, myosin molecules
initially bind tail to tail, and thereafter assemble head to
tail, so that there is a “central bare zone” where there are
only tails and no heads. Spontaneous polymerization is an
intrinsic property of actin, myosin, and tubulin.
In vitro,
myosin can self-assemble into filaments similar to those
found
in vivo.
About 500 myosin molecules aggregate to
form one skeletal muscle thick filament.
Filament assembly depends on characteristics of the
myosin tail, especially the LMM, in which much of the
amino acid sequence exhibits a heptapeptide repeat. If
the repeat is represented as ABCDEFG, residues A and
D are hydrophobic and lie at points where the a-helices
1.6 ktm
Clear zone
Heads of
(tails of
myosin
myosin
molecules
molecules)
FIGURE 21-8
Organization of myosin in striated-muscle thick filaments. Filament
formation begins with tail-to-tail (antiparallel) binding of myosin
molecules, with subsequent parallel binding of myosin molecules to the
ends of the initial nucleus, leaving the central clear zone. There are
approximately 500 myosin molecules per striated thick filament.
in the coiled-coil touch. E, which is usually acidic, and G,
which is usually basic, probably create ionic interactions
that help bind the myosins into a filament.
Organization and Properties of Muscle Fibers
With few exceptions, a vertebrate skeletal muscle fiber is
innervated at one region along its length by a branch of an
axon from an a-motor neuron (a-MN). Each a-MN may
innervate many fibers, from 10 or fewer in small muscles
used for finely graded movements, to 2000 or more in
proximal limb muscles. The group of fibers innervated by
a single a-MN is called a muscle unit, and the muscle unit
and the a-MN driving it form a motor unit. The muscle
unit is the basic functional unit of skeletal muscle, since
normally all fibers in a muscle unit are stimulated when
their a-MN is active. In contrast, cardiac muscle contracts
in response to action potentials generated spontaneously
in the heart and transmitted from cell to cell through gap
junctions. Contraction of the heart is modified, but not ini-
tiated, by its innervation. Organization of smooth muscle
varies considerably by tissue.
Skeletal muscle fibers have different mechanical and
metabolic properties, and fiber classification schemes are
based on these differences. All fibers within a muscle unit
are similar (but not identical) with respect to twitch char-
acteristics, as these are largely determined by the innerva-
tion, and all fibers in a muscle unit are innervated by the
same a-MN. However, even within a muscle unit, there
may be appreciable interfiber differences in metabolic pro-
file. Consequently, the following classifications should be
viewed as useful simplified categories rather than a literal
description of populations of fibers.
Contractile Properties
Most fibers can be classified as slow-twitch (ST) or fast-
twitch (FT). The latter have a shorter time to peak tension,
shorten faster, and have a shorter relaxation time than the
former, and are used for movements requiring high speed
or power. ST fibers are used for sustained activity and con-
trol of posture. The speed of shortening is related to the
ATPase activity of the myosin, while the twitch duration
parameters are related to the extent of the SR, and so these
two are not directly connected. However, fiber histology
and myosin gene expression have some controlling influ-
ences in common, so that speed of contraction and twitch
duration vary inversely.
pH Dependence o f Myosin ATPase Activity
The myosins in muscle differ in their inhibition by high
or low pH, and can be classified on this basis. Serial thin
