Molecular
mechanisms of contraction. Excitation–contraction coupling
The
nerve impulse that ultimately results in muscle contraction
appears as an action potential at the sarcolemma, the membrane
that surrounds the muscle fibre. This electrical signal
is communicated to the myofilaments inside the fibre in
the following way. When the action potential reaches the
opening of the transverse tubules (channels that open through
the sarcolemma to the space outside the fibre; see above
The myofi
) in the surface of the fibre, it travels down
into the fibre along the tubular membranes, which are continuous
with the surface membrane, to within a fraction of a micrometre
of each functional contractile unit (Figure 7). In frog
muscle the transverse tubules surround the myofi
at
the level of each Z line, and in mammalian muscles they
are located at the edge of the A bands and I bands. At the
triads (the three-element complex consisting of one transverse
tubule and two cisternae, which are enlarged saclike membranes)
the transverse tubule walls are close to the membranes of
the terminal cisternae of the sarcoplasmic reticulum.
By
some as yet unknown mechanism, the change in the electrical
properties of the transverse tubule during an action potential
causes the rapid release by the terminal cisternae of relatively
large amounts of calcium ions into the sarcoplasm. As the
concentration of calcium ions increases in the sarcoplasm,
they become bound to the troponin in the thin filaments.
This releases (or removes) the troponin–tropomyosin-mediated
inhibition of the myosin-actin interaction. As the stimulation
of the muscle continues, the terminal cisternae continue
to release calcium ions. At the same time, however, some
of the calcium ions are beingremoved from the sarcoplasm
by another portion of the sarcoplasmic reticulum, the longitudinal
tubules. Once the calcium ions are inside the lumen (cavity)
of the longitudinal tubules, many of them slowly diffuse
back to the terminal cisternae, where they are bound to
a protein, calsequestrin, as a storage site. The removal
of calcium ions from the sarcoplasm by the sarcoplasmic
reticulum is energy requiring. The breakdown of ATP is the
chemical reaction that supplies the energy, and two calcium
ions are apparently removed from the sarcoplasm for each
ATP molecule that is split.
