Altered Physiology and Diseases of the Muscles

The skeletal muscles or flesh are essential for the motor functions as well as the shape and features of our body. The muscular atrophy (shrinkage of muscle mass) may lead to a variety of diseases of the muscles. These diseases are distinguished with respect o the site of origin of a disorder as mentioned below:

1. Origin in motor neurons: Polymyelitis, progressive muscular atrophy

2. Origin in the nerve fibers: Polyneuritis

3. Origin in the myoneural junction: Myasthenia gravis, myotonia congenital, and

4. Origin in the muscle only: Primary muscular dystrophy.

All voluntary muscle have innervation by motor neurons. The muscle fibers are composed of myofibrils and these fibrils appear to be the ultimate functional units of the muscle and have alternate light and dark bands. The contractile protein of the muscle is called actomycin (AM) and is composed of actin and myosin. The actomycin is considered to be the structural protein of the resting myofibrils and contain many enzymes necessary the muscle metabolism. The organic phosphate compound called adenosine triphosphate (ATP) is an energy rich compound and is attached to actomycin of myofibrils. The neuronal spark of the motor neurons fires the adenosine triphosphate-actomycin linkage and breaks it. The adenosine triphosphate (ATP) is liberated and its breakdown provides energy for the muscle contraction. During contraction of the muscle, the actin threads could be shortened to 40% of its original relaxed phase size. The physiology of muscle contraction involves a series of molecular interactions and rearrangements within the complex structure of actin, myosin, and adenosine triphosphate.

The change in the functions of any part of the above cited metabolic pathway may lead muscle weakness. The microscopic examination of muscle biopsy may help to achieve a proper diagnosis. There muscle biopsy could reveal an involvement of a whole bundle of muscle fibers in the cases with neurogenic origin. I cases affected by primary muscular dystrophy only some the fibrils may be involved. The ultrastructural study of the muscle biopsy by a transmission electron microscope is always helpful in achieving an accurate diagnosis of the diseases of the muscles.

Physiology of Muscle Contraction

The essential function of the skeletal muscle is contraction and relaxation in response to a command from a motor neuron. The muscle is very unique organ and is capable converting stored chemical energy into mechanical energy through metabolic processes. The skeletal muscles constitute from 40 to 45 percent of the body weight in an adult. There are 434 voluntary muscles in our body and about 250 million muscle fibers constitute these muscles. The muscle cells or sarcomeres are very complex in structure, metabolism and functions.

All voluntary muscle have innervation by motor neurons. The muscle fibers are composed of myofibrils within the sarcoplasm. The myofibrils appear to be the ultimate functional units of the muscle and have alternate light and dark bands. The contractile protein of the muscle is called actomycin (AM) and is composed of two components: (1) actin and (2) myosin. The actomycin is considered to be the structural protein of the resting myofibrils and contain many enzymes necessary the muscle metabolism. The organic phosphate compound called adenosine triphosphate (ATP) is an energy rich compound and is attached to actomycin of myofibrils. The neuronal spark of the motor neurons fires the ATP-AM linkage and breaks it. The ATP is liberated and its breakdown provides energy for the muscle contraction. During contraction of the muscle, the actin threads could be shortened to 40% of its original relaxed phase size. Physiology of muscle contraction involves a series of molecular interactions and rearrangements within the system composed of actin, myosin, and adenosine triphosphate.

Tissues of Human Body: Distribution of Various Tissues

Distribution of Various Tissues:

Epithelial Tissue:

Covering and lining epithelia has been classified according to shape and arrangement of their cells. Simple epithelium composed of flat cells as single layer attached to a basement membrane is called pavement epithelium. These are found lining the blood vessels and peritoneum. Cuboidal epithelium is characterized by cube shaped cells and is found covering the ovaries. Columnar epithelium is composed of taller cells supported by a basement membrane. This type of epithelium is found where wear and tear is little more. It is found lining the stomach and intestine. Ciliated columnar epithelium show hair like projections at the free surface of cells under microscopes and is found in the respiratory tract. A brush border is found in the cells specialized for absorption. These cells have minute fingerlike projections called microvilli. Such cells are found lining the small intestine.

Stratified epithelium is made up of many layers of cells. The deepest layer of columnar cells rests on the basement membrane and is called germinal layer. Cells of the germinal layer keep on dividing frequently and as they divide the parent cells are pushed nearer the surface and become flattened. The cells on the surface are rubbed off frequently and are replaced by new cell from below. Skin is the dry stratified epithelium and the cells of surface layer have keratin that made our skin water proof. In moist surfaces of cavities like mouth, the cells of surface layer survive until they are rubbed off and keratin is not developed in these cells.

Transitional epithelium is also like stratified epithelium. The only difference in transitional epithelium and stratified epithelium is that the surface cells are round in shape and are capable of spreading out when the organ expands. Transitional epithelium is found lining the urinary bladder.

Glands: Glands are a special type of epithelial tissue. Glands have ability to manufacture essential substances from the basic material supplied by blood. The substances produced by glands are called as secretions of glands. Gastric glands can produce hydrochloric acid from sodium chloride provided by the blood. Exocrine glands pour their secretions through a duct. Secretions of majority of exocrine glands are enzymes. Endocrine glands are ductless glands and pour their secretions into the blood stream. Secretions of endocrine glands are hormones and reach the target sites through the blood stream. Pituitary gland in the skull and thyroid gland in the neck are two important examples of endocrine glands.

Connective Tissue:

Connective tissue supports and binds other tissues. There are three main components of connective tissue: Cells, intercellular substances called matrix and fibres. Matrix and fibres form the supporting material of our body. Fibres may be collagenous fibres or elastic fibres. Collagenous fibres originate from fibroblast cells. These coarse fibres occur in bundles. Elastic fibres are fine branching fibres with elastic like property. These are found layers surrounding the organs and in fibrous tendons joining the muscles.

Areolar tissue is loose connective tissue and is composed of loose network of collagenous fibres and elastic fibres with scattered groups of fat cells and fibroblasts. Areolar tissue forms a very thin, transparent and tough layer and is found between and around the organs of our body.

Adipose tissue is a fatty tissue. It is similar to areolar tissue but the spaces of the network of fibres are filled with fat cells. It is known for its food reserve in the form of fat globules in fat cells. It retains body heat and protects our delicate organs like kidneys and eyes being the poor conductor of heat.

Dense connective tissue or fibrous tissue is composed mainly of bundles of collagenous fibres embedded with fibroblasts. It is found in the ligaments and fascia.

Cartilage consists of cells known as chondrocytes, separated by fibres. There are no blood vessels in cartilage tissue. Cartilage is tough and flexible tissue and found in the trachea, covering heads of bones, joints, between body of vertebrae, and auricles of the ear and the epiglottis.

Bone is known as a specialized type of cartilage. In bone, the collagen is impregnated with calcium. It is tough and rigid due to collagen and calcium respectively and gives support to soft tissues of our body. The cells between the fibres are called osteocytes. Bone has very rich blood supply through blood vessels.

Muscular Tissue:

Muscular tissue provides movement to our body through its specialized function of contraction. Where there is movement in the body there must be muscular tissue. Voluntary or striped muscular tissue forms the flesh of our body and supports movement of our body. It consist long cells varying in size from a few millimeters to 30 centimeters depending on the length of muscle. Each muscle cell contains numerous threads like fibres called myofibrils. The diameter of myofibrils varies from 10 micrometers to 100 micrometers. Myofibrils are symmetrically striped in alternate dark and light bands throughout their length. Each fibril is enclosed in connective tissue sheath called sarcolemma. This muscle is under the control will and need energy for contraction. Energy for contraction is supplied by conversion of adenosine triphosphate (ATP) to adenosine diphosphate (ADP). Adenosine diphosphate (ADP) is again converted into ATP by utilization of energy provided by oxygenation of glycogen. Oxygen for energy generation is supplied by the blood supply to the muscle. Blood capillaries run through voluntary muscle cells to ensure adequate blood supply. Contraction of muscle is controlled through nerves. Ultrastructural view of voluntary muscle has been depicted in Fig-1.

Fig-1: Ultrastructural view of voluntary muscle, 6000x

Cardiac muscle is involuntary but irregularly striped also. It is found in the heart wall only and is different from any other skeletal muscle. Cardiac muscle is composed of short cylindrical fibres with centrally placed nuclei. Muscle fibres of cardiac muscle have no sheath but are bound together by connective tissue. It is not under the control of will but contracts automatically in rhythmic fashion throughout life. Rhythmic contractions are controlled by nerves.

Nervous Tissue:

Nervous tissue receives information from inside as well as outside of our body through network of nerves. It is specially designed tissue to carry impulses. Nervous tissue is composed of nerve cells called neurons and supporting network called neuroglia. Each neuron has a large cell body and short processes called dendrites. Dendrites bring impulses from other cells and tissues. There is a long process called axon, which carries impulses away from the cell body.