Muscular Contraction

The following information details the actions involved in muscle contraction. Refer to the page on muscle tissue for muscle structure information.


Contraction of a Skeletal Muscle Cell

Before a muscle cell can contract it must be stimulated by a nerve impulse.
A nerve cell stimulates a muscle cell by secreting a chemical substance called Acetylcholine (ACh). Acetylcholine permits a nerve cell to communicate chemically with a muscle cell at a site called the neuromuscular junction.

Upon its release, ACh diffuses across the junction and binds to specific receptors on the cell membrane of the muscle cell. This binding initiates an electrical impulse which travels down the T tubules into the interior of the cell.

 

When the impulse reaches the SR, Ca+2 stored in the SR is released. The released Ca+2 forms a bond with the troponin-tropomyosin protein complex (t-t complex) which is associated with the actin filaments. When Ca+2 forms this bond, the t-t complex shifts its position on the actin filament, exposing sites on the filament for attachment to the head of the myosin filaments.

After exposing sites on the actin for attachment, a series of events cause a shortening or contraction of a muscle cell. Each myosin head projecting from the myosin filament bonds with the actin filament. After this bonding the myosin head chemically changes its shape, pulling the actin filament towards the center of the sacromere.

This reaction requires energy, which comes from the ATP attached to each myosin head. Myosin acts as an enzyme (when Ca+2 is present) and breaks the ATP into ADP + P + energy. This energy is used to change the shape of the myosin head, which causes the actin filament to slide a little towards the center the center of the sacromere.

After sliding, a new ATP attaches to the myosin head, breaking the myosin-actin bond. When this bond is broken, the myosin head returns to it's original position. Immediately, the myosin bonds to amother site on the actin filament. Then it changes shape again, and the filaments slide a little farther past each other.

The sequence of myosin-actin bonding, sliding, and releasing is repeated rapidly over and over until the muscle has shortened sufficiently. This is the sliding filament mechanism for muscle contraction, and it consumes a great deal of ATP.

The sliding filament mechanism can be compared to walking on a non-motorized treadmill. Each time your foot (myosin head) lands on the treadmill's belt (actin filament), your foot changes position a little and the belt moves a short distance. A series of rapid steps will move the belt a longer distance.

How does the sliding of actin and myosin filaments cause the contraction of a muscle cell? When an individual muscle cell is stimulated, the actin filament in every sacromere unit slide to the middle of the sacromere. As a result, the myofibrils shorten. A muscle cell shortens (contracts) when all its myofibrils shorten. An entire muscle contracts when many of its muscle cells contract simultaneously.

Contraction ends when the muscle ceases to be stimulated by a nerve cell. If an impulse no longer travels down the T tubules to a cells interior, Ca+2 is actively transported back into the SR. Without Ca+2, the t-t complex shifts back into its original position, masking the sites on the actin filament and preventing further bonding by the myosin heads. When this happens, the muscle relaxes and returns to its original shape.


Summary of Actions of Muscle Cell Contraction


1. Nerve impulse cause nerve to release acetylcholine.
2. ACh travels across neuromuscular junction, binding to muscle cell membrane.
3. ACh binding initiates an electrical impulse which travels across membrane and into T tubules.
4. Impulse stimulates release of Ca
+2 from SR.
5. Ca
+2 binds with t-t complex of the actin filaments, shifting it's position, exposing myosin binding sites.
6. Myosin binds to actin; Ca
+2 presence also causes enzymatic actions of myosin to breakdown ATP into ADP + P + energy.
7. Energy of ATP degradation causes shape change of myosin head, pulling actin molecule toward center of sacromere.
8. After sliding, a new ATP binds to myosin, breaking the myosin-actin bond, releasing the myosin head.
9. If Ca
+2 is still present, the process repeats itself until sacromere has shortened completly.
10. If a nerve impulse ceases, the Ca
+2 is reabsorbed by the SR and the muscle relaxes.


Muscle Energy or Muscle/Nerve interaction