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Skeletal muscle part 1

Biological Sciences
Course Code
Stephen Reid

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Skeletal Muscle – Lecture 8
Structure of Skeletal Muscle
Bands of skeletal muscle are composed of bundles called fasciles. Each fascicle is surrounded by
connective tissue and contains numerous muscle fibres (cells). Each muscle cell runs the entire
length of the muscle and contains many nuclei. Muscle cells, in turn, are composed of bundles of
myofibrils. Myofibrils are bundles of contractile filaments (actin and myosin) plus other
The plasma membrane of a muscle fibre (cell) is called the sarcolemma. The sarcoplasmic
reticulum is an endoplasmic reticulum-like compartment that contains stores of intracellular
calcium. T-tubules penetrate the sarcoplasmic reticulum; connecting the sarcolemma to the
sarcoplasmic reticulum. T-tubules are important in conducting waves of depolarisation from the
sarcolemma to the sarcoplasmic reticulum.
Longitudinal and cross sections of a myofibril illustrate specific patterns that have helped to
elucidate the structure of skeletal muscle.
In cross section, you see an orderly arrangement of large circular structures each surrounded by
six smaller circular structures. The larger circular structures represent myosin molecules (thick
filaments) while the smaller circular structures represent actin molecules (thin filaments). There
are globular heads on the thick filaments (myosin) that form cross-bridges with the thin filaments
(actin molecules).
In a longitudinal section, you various lines, bands and zones. The lines, bands and zones
illustrate areas of overlap (or lack of overlap) between the thick and thin filaments or (in the case
of the Z-line) proteins.
The sarcomere is the “basic unit of muscle structure that repeats itself along the length of the
muscle from Z-line to Z-line.
The Thin Filament
The thin filaments consist of actin molecules. Actin monomers (G-actin) form double-helical
actin strands (i.e., two actin strands, composed of monomers, wound around each other). Each
actin monomer also has a myosin-binding site. There are several regulatory proteins associated
with the thin filaments. Tropomyosin is a long, thin fibrous protein that extends over many actin
monomers and blocks the myosin binding site (at least under resting conditions). Troponin, the
second regulatory protein associated with the thin filaments, binds actin, tropomyosin and
calcium. Its role is to assist in the removal of tropomyosin from the myosin-binding sites on actin.
The Thick Filament
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The thick filaments consist of myosin molecules. A myosin molecule, in turn, consists of two
intertwined myosin subunits (i.e., a dimer). Each of the myosin sub-units in the dimer has a long
fibrous tail (intertwined together) and a globular head. The globular heads (which form the cross-
bridges with the thin filaments) has an actin-binding site and an ATPases site. Within the thick
filaments, the myosin molecules are oriented such that the tails are located together in the centre
of the thick filament while the globular heads are at the opposite ends. Thick filaments are made
up of many myosin molecules that are staggered such that the globular heads also protrude in a
staggered, offset manner.
Sarcomere Structure (Z-line to Z-line)
The A-band consists of the area occupied by the thick filaments. Within the A-band there two
sections (one at either end) in which the thick filaments overlap the thin filaments and an area in
the middle where there is no thick-thin filament overlap. This middle region is called the M-line.
Within the area of thick-thin filament overlap there is an area in which the thick filament globular
heads overlap the thin filaments and an area in which the thick filament fibrous tails overlap the
thin filaments. This later area is called the H-zone.
The I-band consists of the area occupied by the thin filaments in which there is no overlap with
the thick filaments.
The Z-line (the border of two consecutive sarcomeres) consists of proteins that anchor the thick
and thin filaments.
Thick filaments are anchored to the Z-line by an elastic protein called titin.
Nebulin, an elastic protein that runs the length of the thin filament, assists in anchoring the thin
filaments and establishes their length.
Force Generation in Muscle
The sequence of events outlined in the Power Point slides illustrates how the energy of ATP is
used to cause the thick and thin filaments to slide across each other, thereby generating muscle
contraction, as well as how ATP is also required to cause muscles to relax.
It is easiest to look at this sequence starting from the point at which one instance of thick and thin
filament sliding has finished and the thick and thin filaments are still attached to each other.
Note, this whole sequence takes place many times during the course of a muscle contraction.
Begin in the state of rigor in which the thick and thin filaments are still bound to each other and
the myosin head is in its low energy state. The binding of ATP to the ATPase site on myosin
causes the thick and thin filaments to detach from each other (step 5 to step 1 in the figures).
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