Muscle Contraction

1. Sliding filament mechanism of muscle
contraction has been accepted as the explanation of
how a muscle cell shortens on contraction.
a. Filament length is constant. In the
shortening of muscles during contraction,
myofilaments maintain constant lengths because
thin filaments slide past thick filaments.
b. Movement of myofilaments. The
sliding movement is due to cross-bridges formed
between the myosin heads (motor proteins) and
binding sites on the actin monomers.
i. Myosin motors, which connect
the thick and thin filaments during muscle
contraction, move along the actin tracks in a
ratcheting fashion until the sarcomere is shortened.
ii. Myosin filaments are fixed. In
each sarcomere, the thick filaments remain
stationary. However, the ratcheting movements of
their myosin heads cause the thin filaments to slide
past the thick filaments toward the center of the
sarcomere.
c. Tension development is proportional
to the number of myosin heads overlapped by thin
filaments.
2. Actin-myosin interaction. At the onset
of muscle contraction, myosin heads move out from
the thick filament backbone to interact with ATP and
specific binding sites on actin molecules.
a. Bipolarity of thick filaments. The
myosin heads at each end of the bipolar thick
filament pulls thin filaments deeply into the
sarcomere. As the filaments slide, the sarcomeres
and myofibrils shorten. The myosin heads then
detach from the actin molecules and the cycle is
repeated.
b. Contractile appearance. As a
consequence of the actin-myosin interaction, which
causes thin filaments to slide into the A band, the
following results are observed (important to know).
1. Sliding filament mechanism of muscle
contraction has been accepted as the explanation of
how a muscle cell shortens on contraction.
a. Filament length is constant. In the
shortening of muscles during contraction,
myofilaments maintain constant lengths because
thin filaments slide past thick filaments.
b. Movement of myofilaments. The
sliding movement is due to cross-bridges formed
between the myosin heads (motor proteins) and
binding sites on the actin monomers.
i. Myosin motors, which connect
the thick and thin filaments during muscle
contraction, move along the actin tracks in a
ratcheting fashion until the sarcomere is shortened.
ii. Myosin filaments are fixed. In
each sarcomere, the thick filaments remain
stationary. However, the ratcheting movements of
their myosin heads cause the thin filaments to slide
past the thick filaments toward the center of the
sarcomere.
c. Tension development is proportional
to the number of myosin heads overlapped by thin
filaments.
2. Actin-myosin interaction. At the onset
of muscle contraction, myosin heads move out from
the thick filament backbone to interact with ATP and
specific binding sites on actin molecules.
a. Bipolarity of thick filaments. The
myosin heads at each end of the bipolar thick
filament pulls thin filaments deeply into the
sarcomere. As the filaments slide, the sarcomeres
and myofibrils shorten. The myosin heads then
detach from the actin molecules and the cycle is
repeated.
b. Contractile appearance. As a
consequence of the actin-myosin interaction, which
causes thin filaments to slide into the A band, the
following results are observed (important to know).