Ultra Structure of Muscle

The muscles of a fully developed adult male can produce sufficient power to lift a weight of about 25 tonnes.

• In males, all the muscles make up 42% of the body weight. In women, the muscles make up 36% of the body weight.
• The body consists of 650 muscles. 620 muscles work together to move 206 bones of the skeleton.
• The remaining 30 muscles are needed to ensure the passage of food through the alimentary canal, circulate the flood around the body, and operate certain internal organs.
• The muscles by themselves do not contract.
• They are under the control of the central nervous system [CNS].
• For example, if we wish to pick up an object, our CNS must know the tension, length, and position of the arm muscles in order to cause the correct amount of contraction to move the arm.
• The CNS must stop the contractions when the arm reaches the object, and maintain sufficient contraction to hold the object. Then CNS causes the muscle contractions required to move the arm away.
• The muscles are biological machines that convert chemical energy into force and mechanical work.
• The energy for contraction, as for other life activities, comes from the chemical reaction between the foodstuffs that we eat and the oxygen that we breathe.
• Muscles therefore need a good blood supply to bring essential food and oxygen.
• The main property of the muscle is contraction.
• The muscles contract due to mechanical, thermal, chemical, and electrical stimuli.
• The muscles receive stimuli through motor nerves.
• Before studying muscle contraction, it is better to learn about the structure of a muscle.

Muscles and Its Structural Organisation

• For the movement of the body and its parts in an animal, muscular tissue is present.
• The muscular tissue shows myocytes or muscle fibers. Individual muscle fibre is surrounded by fine fibrous connective tissue called endomycium.
• The parallel bundles of muscle fibers are called fasciculi or fascicles.
• Each fasciculus is surrounded by perimycium. The entire muscle is usually wrapped with a fibrous connective tissue called epimycium.
• The cell wall or plasma membrane of muscle fiber is called "sarcolemma".
• The cytoplasm is called "sarcoplasm", which consists of many nuclei nearer to the sarcolemma.
• Hence muscle cells or muscle fibers are called "syncytial" cells.
• The sarcoplasm also consists of many, longitudinally arranged fibrils called "myofibrils".
• These are the contractile structures. The details of myofibrils can be seen only under the electron microscope.

Muscle - Ultra Structure

The ultra-structure of muscle is well known by the structure of longitudinally arranged fibrils called myofibrils.

Myo Fibril

• Each myofibril consists of "light" and "dark" bands.
• The light band is called "isotropic" or "I-band". The dark band is called "anisotropic" or "A-band".
• These bands are arranged alternately.
• The bands give a striated appearance to the myofibrils.
• Hence the muscles are called "striated muscles".
• The bands are made up of filaments, which in turn are made up of muscle proteins.

"The muscle proteins are Actin and Myosin".

• The light T-band is made up of thin Actin filaments.
• Whereas the Dark 'A'-band is made up of thick myosin filaments. The atomic weight of the actin atom is 70,000 A.M.U.
• Hence the l-band is light. The atomic weight of a myosin atom is 4,50,000 A.M.U. Hence the 'A'-band is dark.
• The protein actin normally exists with calcium ions. Whereas the protein myosin exists with magnesium ions.
• The light l-band is divided into two equal parts by a black line called the "Z" membrane or "Krause's membrane".
• The region of myofibril between the two successive 'Z' membranes is' called "sarcomere".
• One full A-band and two halves of two l-bands lie on either side of the 'A' band.
• The sarcomeres are the fundamental units of myofibrils.
• The Dark 'A'-band is also divided into two equal parts by a light membrane called the "H" zone or "Henson's disk", which consists of a black line at its center, called the 'M' line.
• Glutei, the buttock muscle of a Frog is taken and its contents like, 'A.T.P.' are extracted by pressing.
• When such muscle is kept in A.T.P. liquid, it contracts. The above experiment shows that Actin, myosin, A.T.P, and calcium ions are essential for muscle contraction. A.T.P. and CP are synthesized by the oxidation of the food materials.
• So the mitochondria play an important role in muscle contraction.
• Hence the mitochondria in muscle cells are called "sarcosomeres". The Ca⁺⁺ and Mg⁺⁺ are secreted by the endoplasmic reticulum.
• Hence E.R. in muscle cells is called sarcoplasmic reticulum.

Chemistry of Muscle

1. The muscle contains 75% of water and 25% of solids.
2. Most of the solids are soluble proteins such as actin, myosin, mysinogen, paramysinogen.
3. Non-protein organic substances present in the muscles are ATP, phosphocreatine, creatine, and urea.
4. Important in the muscle is glycogen.
5. Mineral elements present in a muscle are sodium, potassium, calcium, phosphorus, and magnesium, out of which potassium occurs abundantly.
6. Muscles have their own oxygen-carrying iron-protein pigment myoglobin or muscle hemoglobin.
7. Three kinds of muscles are seen in animals.
   1. Striped muscle or striated,
   2. Unstriped or unstriated, and
   3. Cardiac muscle.
8. The cardiac muscles are striated, but they are involuntary.
9. The actin and myosin proteins are arranged as bands like in striped muscles.
10. The smooth muscles also have actin and myosin, which are not aggregated into filaments.