Page 73 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Contractile Apparatus of Striated changes in the head–neck segment allow the
Muscle myosin head to “tilt” when interacting with
actin (sliding filaments; ! p. 62).
The muscle cell is a fiber (! A2) approximately Actin is a globular protein molecule (G-
10 to 100 µm in diameter. Skeletal muscles actin). Four hundered such molecules join to
fibers can be as long as 15 cm. Meat “fibers” form F-actin, a beaded polymer chain. Two of
visible with the naked eye are actually bundles the twisted protein filaments combine to form
of muscle fibers that are around 100 to an actin filament (! B), which is positioned by
1000 µm in diameter (! A1). Each striated the equally long protein nebulin.
Nerve and Muscle, Physical Work mitochondria (sarcosomes), substances in- 40 nm or so (! B). Each troponin molecule
Tropomyosin molecules joined end-to-end
muscle fiber is invested by a cell membrane
called the sarcolemma, which surrounds the
(40 nm each) lie adjacent to the actin filament,
and a troponin (TN) molecule is attached every
sarcoplasm (cytoplasm), several cell nuclei,
consists of three subunits: TN-C, which has
volved in supplying O 2 and energy (! p. 72),
and several hundreds of myofibrils.
two regulatory bindings sites for Ca
at the
2+
amino end, TN-I, which prevents the filaments
So-called Z lines or, from a three-dimen-
sional aspect, Z plates (plate-like proteins;
from sliding when at rest (! p. 62), and TN-T,
! B) subdivide each myofibril (! A3) into ap-
which interacts with TN-C, TN-I, and actin.
prox. 2µm long, striated compartments called
The sarcomere also has another system of
sarcomeres (! B). When observed by (two-di-
protein titin (connectin). Titin is more than
nating light and dark bands and lines (hence
1000 nm in length and has some 30 000 amino
2 mensional) microscopy, one can identify alter- filaments (! B) formed by the filamentous
acids (M r ! 3000 kDa). It is the longest known
the name “striated muscle”) created by the
thick myosin II filaments and thin actin fila- polypeptide chain and comprises 10% of the
ments (! B; for myosin I, see p. 30). Roughly total muscle mass. Titin is anchored at its carb-
2000 actin filaments are bound medially to the oxyl end to the M plate and, at the amino end,
Z plate. Thus, half of the filament projects into to the Z plate (! p. 66 for functional descrip-
two adjacent sarcomeres (! B). The region of tion).
the sarcomere proximal to the Z plate contains The sarcolemma forms a T system with
only actin filaments, which form a so-called several transverse tubules (tube-like invagina-
I band (! B). The region where the actin and tions) that run perpendicular to the myofibrils
myosin filaments overlap is called the A band. (! p. 63 A). The endoplasmic reticulum
The H zone solely contains myosin filaments (! p. 10 ff.) of muscle fibers has a characteris-
(ca. 1000 per sarcomere), which thicken tic shape and is called the sarcoplasmic reti-
towards the middle of the sarcomere to form culum (SR; ! p. 63 A). It forms closed cham-
the M line (M plate). The (actin) filaments are bers without connections between the intra-
anchored to the sarcolemma by the protein and extracellular spaces. Most of the chambers
dystrophin. run lengthwise to the myofibrils, and are
Each myosin filament consists of a bundle of therefore called longitudinal tubules
ca. 300 myosin-II molecules (! B). Each (! p. 63 A). The sarcoplasmic reticulum is
molecule has two globular heads connected by more prominently developed in skeletal
flexible necks (head and neck = subfragment muscle than in the myocardium and serves as a
S1; formed after proteolysis) to the filamen- Ca 2+ storage space. Each T system separates
tous tail of the molecule (two intertwined α- the adjacent longitudinal tubules, forming tri-
helices = subfragment S2) (! C). Each of the ads (! p. 63 A, B).
heads has a motor domain with a nucleotide
binding pocket (for ATP or ADP + P i) and an
actin binding site. Two light protein chains are
located on each neck of this heavy molecule
(220 kDa): one is regulatory (20 kDa), the
60 other essential (17 kDa). Conformational
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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