Skeletal Muscle Fiber | MUSCLE CONTRACTION |

SKELETAL MUSCLE FIBER

Each muscle consists of muscle bundles. Each muscle bundle is further composed of muscle fibers or muscle cells. Each muscle fiber is long cylindrical cell The diameter of muscle fiber cells is 10 - 100 um, It has following structures 

1. Nuclei:

Muscle fiber cell has multiple (many) oval nuclei. These nuclei are arranged just beneath the sarcolemma.

2. Sarcolemma:

The outer membrane of the muscle cells is called sarcolemma.

3. Sarcoplasm:

The sarcoplasm of muscle fiber is similar to the cytoplasm of other cells. But it contains usually a large amount of stored glycogen and myoglobin Myoglobin is a unique oxygen-binding protein. It has a red pigment that stores oxygen.

4. Myofibrils:

Each muscle fiber contains a large number of myofibrils 1-2 um in diameter These fibers run in the parallel direction to the muscle fiber. They extend in the entire length of the cell. Sarcolemma encloses bundles of myofibrils.

5. Sarcomere:

The small contractile units of the myofibrils are called a sarcomere. Each sarcomere has a series of dark and light bands. These bands can be seen along the length of each myofibril. There are following parts of a sarcomere:

➤ A band:

The dark bands are called A band because they are anisotropic. An anisotropic compound can polarize visible light.

➤ I band:

The light band called I band. They are isotropic or non-polarizing. These give the cell a striped appearance.

➤ H - Zone:

Each A band has a lighter stripe in its midsection. This lighter strip is called H - zone. H stands for "hele". It means bright.

➤ M - line:

The H-zone is bisected by a dark line called M - line.

➤ Z- line:

The I bands have midline called Z - line. Z stands for zwish means between. A sarcomere is the region of a myofibril between two successive Z - line. It is the smallest contractile unit of muscle fiber. The myofibril contains myofilaments.

ENERGY FOR MUSCLE CONTRACTION

There are the following sources of energy for muscle contraction:

1. ATP:

Energy for muscle contraction comes from ATP. The supply of ATP is maintained by the aerobic break down of glucose in the muscle cells. Glucose comes from stored glycogen in the cell.

2. Creatine phosphate:

When more energy is required due to high metabolism, it is provided by other energy-storing substances called creatine phosphate.

3. Lactic acid formation:

Sometimes, a lesser number of ATPs are produced due to oxygen deficiency or very high metabolism (prolonged or strenuous muscular activity). Thus ATP requirement is met by the anaerobic breakdown of glucose into lactic acid. Lactic acid accumulation causes muscle fatigue. At rest, 1/5 of the lactic acid is broken aerobically and its energy is used to change the remaining 4/5 lactic acid into glucose.

EFFECT OF EXERCISE ON MUSCLES

1. Active use of muscles:

The muscle shows how much work it has done. When muscles are used actively they are increased in size or strength. So they become more efficient and fatigue resistant. Aerobic exercise such as swimming, joggings, and fast walking causes several changes in skeletal muscles. Capillary surrounding the muscle fibers and mitochondria within them increase in number. The muscle fibers synthesize more myoglobin. These changes make the muscles more efficient. Their metabolism is increased. Thus these muscles resist fatigue.

2. Inactivity of muscles:

Complete immobilization weakens the muscles. Thus they undergo severe atrophy.

Muscle Fatigue

The state of physiological inability to contract is called muscle fatigue. There are two causes of muscle fatigue:

1. Muscle fatigue is caused due to deficiency of ATP. The cross-bridges are unable to detach from actin in the absence of ATP So there are contractures or states of continuous contractions in muscles.

2. Excess accumulation of lactic acid and ionic imbalances also cause muscle fatigue. Lactic acid reduces the muscle pH (and the muscle to ache) It causes extreme fatigue by breaking glucose.

Tetany

The continuous state of contraction of muscles is called tetany. Tetany is caused by low calcium in the blood. Low level increases the excitability of neurons 11 results in loss of sensations. It causes muscle twitches (rapid movement of muscles) and convulsion. If it remains untreated, it causes spasm of the larynx, respiratory paralysis and ultimately death occurs.

Cramp

The tetanic contraction of the entire muscle is called a cramp. It lasts for just a few seconds or several hours. It causes the muscles to become taut (tightly stretch) and painful. It is most common in the thigh and hip muscles it usually occurs at night or after exercise.

Tetanus 

Tetanus is an acute infectious disease. It is caused by the anaerobic bacterium Clostridium tetani. It causes persistent painful spasms of some skeletal muscles, It begins with gradual stiffness of jaws and neck muscles. It then progressively make the jaws rigid (lockjaw). It then causes the spasms of trunk and limb muscles. It can cause respiratory failure. So it is usually a fatal disease, It is rare in developed countries. But tetanus is the major killer in developing countries. The mortality rate is 40 percent due to this disease in these countries.