Muscle Filaments -
The actin filament is anchored to vertical bands called Z
lines. The part of a fibril from one Z line to the next is called
a sarcomere. The myosin filament is located between each
actin filament in the sarcomere and together they operate like a sliding
hatch (Figure 6).
muscle fiber is relaxed, the hatch lies "open" and the sarcomere is at its
greatest combined length. But when the myosin receives the power command,
it pulls the hatch "closed." That shortens the combined length of the
sarcomere and provides each filament's tiny share of the muscle
Your genes may not have endowed you with a professional athlete's muscle make-up, but you make similar calculations every time you lift a box or open a door. The brain's orders reach the muscles because a nerve from each motor unit is plugged into the spinal cord like a telephone plugged into a wall socket. But unlike the telephone, if you unplug the muscle, it still works. The reason is that muscles take orders from more than the brain. Some nerves from motor units go to the spinal cord and up to the brain, but others loop back and connect to other motor units, to the skin and to other body tissues. Through these loop back circuits, muscles and skin can communicate among themselves, allowing muscles to react faster than the brain can. The classic example is touching a hot surface. Your skin sounds the alarm, and the muscles pull your hand away before the news even reaches your brain! In either case, muscles require nutrients in order to supply the energy for muscular contractions.
A combination of mechanical and chemical mechanisms play a part in
providing the nutrient and energy supply for these muscular activities.
Chemical changes occur in the muscle, which causes a conversion of
chemical into mechanical energy. This produces the actual movement of
muscle contraction. In order to sustain the movement for any length of
time, however, a supply of nutrients is required so that sufficient energy
can be maintained in the muscle as it continues to work. Let's examine the
chemical reactions that create the energy for the muscles to move our