Skeletal Muscle And Its Contraction
Muscles make up about 50% of the adult body mass. There are three
types of muscle: skeletal (muscle
attached to the skeleton); cardiac (muscle involved in cardiac function;
Chapter 15) (both of these are morphologically striated or striped and are
commonly called striated muscles); and smooth (muscle involved in
many involuntary processes in the blood vessels and gut; this type is not structurally
striated, hence its name; Chapter 15). A comparison of the properties of the
three muscle types is shown in Appendix I.
Skeletal muscle
The skeletal muscles and the
skeleton function together as the musculoskeletal system. Skeletal
muscle is sometimes referred to as voluntary muscle because it is under
conscious control. It uses about 25% of our oxygen consumption at rest and this
can increase up to 20-fold during exercise.
General mechanisms of skeletal
muscle contraction
The functions of muscle tissue are
the development of tension and shortening of the muscle. Muscle fibres have the
ability to shorten a considerable amount, which is brought about by the
molecules sliding over each other. Muscle activity is transferred to the
skeleton by the tendons, and the tension developed by the muscles is graded and
adjusted to the load.
Fine structure of skeletal muscle
(Fig. 12a) The connective tissue surrounding the whole
muscle is called the epimysium. The connective tissue that extends
beyond the body of the muscle eventually blends into a tendon, which is
attached to bone or cartilage. Skeletal muscle is composed of numerous
parallel, elon- gated, multinucleated (up to 100) cells, referred to as muscle
fibres or myofibres, which are between 10 and 100 μm in diameter and
vary in length, and are grouped together to form fasciculi. Each
fasciculus is surrounded by the perimysium. Each myofibre is encased by connective
tissue called the endomysium. Beneath the endomysium is the sarcolemma
(an excitable membrane). This is an elastic sheath with infoldings that
invaginate the fibre interior, particularly at the motor end plate of the
neuromuscular junction (Chapter 13). Each myofibre is made up of myofibrils 1
μm in diameter separated by cytoplasm and arranged in a parallel fashion along
the long axis of the cell. Each myofibril is further subdivided into thick and
thin myofilaments (thick, 10–14 nm in width and 1.6 μm in length;
thin, 7 nm in width and 1 μm in length). These are responsible for the
cross-striations. Thin filaments consist primarily of three proteins, actin,
tropomyosin and troponin, in the ratio 7 : 1 : 1, and thick
filaments consist primarily of myosin. The cytoplasm surrounding the
myofilaments is called the sarcoplasm. Each myofibre is divided
at regular intervals along its length into sarcomeres separated by Z-discs
(in longitudinal sections, these are Z-lines). To the Z-lines are
attached the thin filaments held in a hexagonal array. The I-band extends
from either side of the Z-line to the beginning of the thick filament (myosin).
The myosin filaments make up the A-band.
The H-zone is at the centre
of the sarcomere, and the M-line is a disc of delicate filaments in the
middle of the H-zone that holds the myosin
filaments in position so that each one is surrounded by six actin filaments.
The thin filaments consist of two
intertwining strands of actin with smaller strands of tropomyosin and troponin
between the intertwining strands. Each strand of actin is made up of about 200
units of globular or G-actin. It is on these globules that there is a site for
myosin to bind during contraction.
The thick filaments are made up of
about 100 myosin molecules; each molecule is club shaped, with a thin tail
(shaft) comprising two coiled peptide chains and a head made up of two heavy
peptide chains and four light peptide chains that have a regulatory function.
The ATPase activity of the myosin molecule
is concentrated in the head.
The thin tails of the myosin
molecules form the bulk of the thick filaments, whereas the heads are ‘hinged’
and project outward to form cross-bridges between the thick filaments and their
neighbouring thin filaments. Six thin filaments surround each thick filament.
Between the myofibrils are a large
number of mitochondria and glycogen granules, as found in other cells, but
muscle cells have regular invaginations which project from outside the cell and
wrap around the sarcomeres, particularly where the thin and thick filaments
overlap. These invaginations are called transverse or T-tubules and
contain extracellular fluid. The specialized smooth endoplasmic reticulum, the sarcoplasmic
reticulum, which is close to the T-tubules, is enlarged to form terminal
cisternae which actively transport Ca2+ into the lumen from the sarcoplasm.
Like fingers of the hands sliding
over one another, actin and myosin molecules slide past each other. The myosin
heads bind to the actin chain and tilt. There is a constant process of binding,
tilting, releasing and rebinding of cross-bridges, as well as rotation of the
myosin filaments as they interact with the actin filaments and bind with the
alternate myofibril in the hexagonal structure. This results in the contraction of the whole muscle. The cross-bridges are formed asynchronously so
that some are active, whilst others are resting.
