Muscle tissue

Muscle tissue is specialized for producing movement. There are two types of muscle, and these are classified on both a structural and a functional basis.

1. STRIATED MUSCLE:

a. skeletal muscle

b. cardiac muscle

2. SMOOTH MUSCLE.

The two types of muscle are:

Striated muscle

1. Striated voluntary or skeletal muscle, attached to bones or fascia and constituting the flesh of the limbs and body wall. Because of its close relationship to the bony skeleton, this variety is called skeletal muscle. When examined under a microscope fibres of skeletal muscle show prominent transverse striations. Skeletal muscle is, therefore, also called striated muscle. Skeletal muscle can normally be made to contract under our will (to perform movements we desire). It is, therefore, also called voluntary muscle. Skeletal muscle is supplied by somatic motor nerves. .

2. Striated involuntary or cardiac muscle, forming the wall of the heart. Cardiac muscle is involuntary. The fibres of cardiac muscle show transverse striations. It is supplied by autonomic nerves.

Smooth muscle

3. Smooth involuntary muscle, present chiefly in hollow organs. Contraction of smooth muscle is not under our control; and smooth muscle is, therefore, also called involuntary muscle. It is supplied by autonomic nerves.

SKELETAL MUSCLE

Skeletal muscle is made up essentially of long, cylindrical “fibres”. The length of the fibres is highly variable, the longest being as much as thirty centimetres in length. The diameter of the fibres also varies considerably (ten to sixty micrometers). Each “fibre” is really a symplast with hundreds of nuclei along its length. The nuclei are elongated and lie along the periphery of the fibre, just under the cell membrane (which is called sarcolemma). The cytoplasm (or sarcoplasm) is filled with numerous longitudinal fibrils that are called myofibrils.

The most striking feature of skeletal muscle fibres is the presence of transverse striations in them. The striations represent alternate dark and light bands that strech across the muscle fibre. The dark bands are called A-bands, while the light bands are called I-bands. (“A” and “I” correspond to the second in the words dark and light.

Running across the middle of each I-band there is a thin dark line called the Z-band. The centre of the A-band is traversed by a lighter band called H-zone. Running through the centre of the H-band a thin dark line can be made out. This is M band.

Connective tissue framework of muscles. Muscles are pervaded by a network of connective tissue fibres that support muscle fibres and unite them to each other. Individual muscle fibres are surrounded by delicate connective tissue that is called the endomysium. Individual fasciculi are surrounded by stronger sheath of connective tissue called the perimysium. Connective tissue that surrounds the entire muscle is called the epimysium. At the junction of a muscle with a tendon the fibres of the endomysium, the perimysium and the epimysium become continuous with the fibres of the tendon.

CARDIAC MUSCLE

The structure of cardiac muscle has many similarities to that of skeletal muscle; but there are important differences as well.

Similarities between cardiac and skeletal muscle. These are the follows. Like skeletal muscle, cardiac muscle is made up of elongated “fibres” within which there are numerous myofibrils. The myofibrils (and, therefore, the fibres) show transverse striations similar to those of skeletal muscle. A, I, Z and H bands can be made out in the striations. The connective tissue framework, and the capillary network around cardiac muscle fibres are similar to those in skeletal muscle.

Differences between cardiac and skeletal muscle. These are as follows.

1. The fibres of cardiac muscle do not run in strict parallel formation, but branch and anastomose with other fibres to form a network.

2. Each fibre of cardiac muscle is not a multinucleated symplast as in skeletal muscle, but is a chain of cardiac muscle cells (or cardiac myocytes) each having its own nucleus. Each myocyte is about eighty micrometers long and about fifteen micrometers broad.

3. The nucleus of each myocyte is located centrally (and not peripherally as in skeletal muscle).

4. At places, the myofibrils merge with each other. As a result of these factors, the myofibrils and striations of cardiac muscle are not as distinct as those of skeletal muscle.

5. With the light microscope the junctions between adjoining cardiac myocytes are seen as dark staining transverse lines running across the muscle fibre. These lines are called intercalated discs.

SMOOTH MUSCLE

Smooth muscle is made up of long spindle shaped cells (myocytes) having a broad central part and tapering ends. The nucleus, which is oval or elongated, lies in the central part of the cell. The length of smooth muscle cells is highly variable (fifteen micrometers to five hundred micrometers). The cells are so arranged that the thick central part of one cell is opposite the thin tapering ends of adjoining cells. Each myocyte is surrounded by a network of delicate fibres, that holds the myocytes together. The fibres between individual myocytes become continuous with the more abundant connective tissue that separates fasciculi or layers of smooth muscle.

Each smooth muscle cell is bounded by a plasma membrane. Adjacent smooth muscle cells communicate through nexuses (gap junctions).

Questions:

1. Classification of muscle tissue.

2. Classification of striated muscle.

3. Structural features of skeletal muscle.

4. Structural features of cardiac muscle .

5. Structuiral features of smooth muscle.

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THE BLOOD

Blood is a specialized form of connective tissue consisting of formed elements, or blood cells, and fluid intercellular substance, the blood plasma. The volume of blood in the healthy adult human is about five litres, and quantitatively blood constitutes about eight per cent of the body weight.

The cellular or formed elements of blood are of three main types. These are:

1. red blood corpuscles or erythrocytes;

2. white blood corpuscles or leucocytes;

3. blood platelets.

Erythrocytes. The erythrocytes, or red blood corpuscles, are highly differentiated cells that functionally are specialized for the transportation of oxygen. In mammals the erythrocyte is a cell that has lost its nucleus and its cytoplasmic organelles during development. Each cell is shaped like a biconcave disc. The corpuscles are elastic and are capable of considerable distortion, as is evident in their ability to pass through capillaries of small calibre. They average about 8,5 micrometers in diameter and 1,9 micrometers in thickness.

The erythrocytes are much more numerous than any of the other formed elements of blood. In human males there are 5 to 5,5 million erythrocytes per cubic millimetre; in females, 4,5 to 5 million.

Each erythrocyte is bounded by a delicate plasma membrane which is a lipoprotein complex. Below the plasma membrane is a cytoskeletal complex of two layers: a vertical granular and a horizontal filamentous meshwork. The meshwork is mainly constructed of a contractile protein distinctive to erythrocytes, called spectrin. Spectrin maintains the biconcave shape and allows for efficient flow of oxygen and carbon monoxide within interior.

Chemically the content of the erythrocyte consists of a lipid and protein colloidal complex, principally hemoglobin. It is because of the presence of hemoglobin the erythrocytes (and blood as a whole) are red in colour. Hemoglobin plays an important role in carrying oxygen from the lungs to all tissues of the body. In healthy person there are about 15 grammes of hemoglobin in every 100 millilitres of blood.

Erythrocytes are formed in bone marrow from where they enter the blood stream. Each erythrocyte has a life of about one hundred to one hundred and twenty days.

Leukocytes (white blood corpuscles).

Differences between erythrocytes and leucocytes. Leucocytes are different from erythrocytes in several ways.

a. They are true cells, each leucocyte having a nucleus, mitochondria, Golgi complex, and other organelles.

b. They do not contain hemoglobin and, therefore, appear colourless in unstained preparations.

c. Unlike erythrocytes which do not have any mobility of their own, leucocytes can move actively.

d. Erythrocytes do not normally leave the vascular system, but leucocytes can move out of it to enter surrounding tissues.

Features of different types of leukocytes. Leucocytes are of various types. Some of them have granules in their cytoplasm and are, therefore, called granulocytes. Depending on the staining characters of their granules granulocytes are further divided into neutrophil leucocytes (or neutrophils), eosinophil leucocytes (or eosinophils), and basophil leucocytes (or basophils).

Apart from these granulocytes there are two types of agranular leucocytes. These are lymphocytes and monocytes.

Relative number. There are about seven thousand leucocytes in every cubic millimetre of blood. Of these about two thirds (sixty to seventy per cent) are neutrophils, and about one fourth (twenty to thirty per cent) are lymphocytes. The remaining types are present in very small numbers. The eosinophils are about three per cent, the basophils about one per cent, and monocytes about five per cent.

Nuclei. In lymphocytes the nucleus is spherical, but may show an indentation on one side. In monocytes the nucleus is ovoid and may be indented, it is placed eccentrically. In basophils the nucleus is S-shaped. The nucleus of the eosinophil leucocyte is made up of two or three lobes that are joined by delicate strands. In neutrophil leucocytes the nucleus is very variable in shape and consists of several lobes: that is why these cells are also called polymorphonuclear leucocytes, or simply polymorphs. The number of lobes increases with the life of the cell.

Life span. The life of a neutrophil leucocyte is only about fifteen hours. Eosinophils live for a few days, while basophils can live for nine to eighteen months. The life span of lymphocytes is variable. Some live only a few days (short-lived lymphocytes) while others may live several years (long-lived lymphocytes).

Blood platelets. Blood platelets are round, oval, or irregular discs about three micrometers in diameter. They are also known as thrombocytes. The discs are biconvex. Each disc is bounded by a plasma membrane within which there are mitochondria and membrane bound vesicles. There is no nucleus. The platelets appear to have a clear outer zone (hyalomere) and a granular central part (granulomere).

Platelets are concerned with the clotting of blood. As soon as blood is shed from a vessel, platelets stick to each other and to any available surfaces (specially to collagen fibres). Platelets break down into small granules and threads of fibrin appear around them.

The life of a platelet is about ten days.

Plasma. Plasma is a homogenous, slightly alkaline fluid that transports all nutritive materials.

Questions:

1. What does the blood consist of?

2. Three main types of formed elements of blood.

3. Structural an functional features of the erythrocytes.

4. Differences between erythrocyte and leucocytes.

5. Features of different types of leucocytes.

6. Structural and functional features of blood platelets.

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