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MUSCLE TISSUE

Muscle tissue allows for the movements of the body and its parts. Muscle cells are specialized for contractility and electrical conductivity.

  1. The unit of muscle tissue is the muscle fiber.
  1. Muscle cells are elongated. This shape functions well for contraction.
  2. The cytoplasm of muscle cells is called the sarcoplasm.
  3. Myofibrils are thread-like contractile structures in these cells.
  4. Myofibrils are composed of myofilaments of the contractile proteins actin and myosin.
  1. Muscle cells can be placed into three categories:

            a.  Smooth Involuntary Muscle
                i.    found in hollow visceral organs such as the gut, uterus and blood vessels
                ii.   associated with various exocrine glands.

           b.  Striated Involuntary Muscle - found in the heart (cardiac muscle)

           c.  Striated Voluntary Muscle - makes up the skeletal muscles of the body

I.  Skeletal Muscle - is found attached to the vertebrate skeleton. It produces most of the body’s movements.

Histological Characteristics:

     1. The cells are called skeletal muscle fibers (SMF). They are long and cylindrical in shape.

  1. The fibers can range from several millimeters to several inches in length, in width from 10 to 100 um.

     3. The fibers are tapered or bifurcated at the ends.

      4. Each fiber is covered with a delicate connective tissue called the endomysium. Collagenou fibers of the
      endomysium interconnect each skeletal muscle fiber with adjacent cells in a bundle called the fascicle. The
      collagen fibers from each cell become incorporated into the tendon joining the cells with the periosteum of the
      bone.

      5. The cell membrane of the SMF is called the sarcolemma.

      6. Within the SMF

    1. the sarcoplasm contains myoglobin. This pigment is a close analogue of hemoglobin. It has an even stronger affinity for oxygen than the erythrocyte pigment.
    2. Each SMF is multinucleated. This results from the fusion of many embryonic myoblasts.
    3. The sarcoplasm contains contractile threads called myofibrils. The SMF may contain hundreds of myofibrils.
    4. Each myofibril appears striated. This is due to the presence of repeating subunits called sarcomeres on the myofibrils. Under light microscopy, the entire cell appears striped or striated.
    5. Glycogen granules are abundant in the sarcoplasm.
    6. Mitochondria are also abundantly present in the SMF. They are generally located peripherally in the cell with the nuclei due to the crowding effect of the many myofibrils.
    7. The sarcoplasm contains a specialized system of intracellular sacs called the sarcoplasmic reticulum. These sacs serve to store calcium.

     7. Each SMF receives an end branch of a motor neuron. The end branch closely approximates the SMF
       across a narrow space called the synapse. The sarcolemma across the synapse is specialized to response to
       the nerve message and is called the motor end plate.

  1. Within a whole muscle, the fibers are arranged in bundles called fascicles.
  2. The endomysium around each fiber continues into the fibrous sheath around the fascicle which is called the perimysium.

    10. The perimysia around each fascicle are continuous with the outermost sheath around the entire muscle, the        epimysium.

    11. Blood vessels follow the connective tissue wrappings into skeletal muscle tissue. A rich network of
      capillaries surrounds each SMF within the endomysium.

     12. Lymphatic channels are found in the epimysium and perimysium only.

II.  Smooth Muscle   - is often called involuntary. It is present in the walls of many visceral organs. Its activity is controlled by the autonomic nervous system. Peristalsis and the contractions of birth are examples of its activity.

A. Histological Characteristics:

     1. Cells or fibers are elongated with tapered ends.

      2.  The fibers range in width from 3 to 8 um and in length from 18 to 200 um. In the pregnant uterine wall they         enlarge from 70 to 500 um.

      3. Each fiber is surrounded by a basal lamina and fine reticular fibers.

    4. Elastic fibers also form close associations between adjacent smooth muscle cells.

      5. Gap junctions are commonly seen between adjacent fibers. These junctions, similar to those seen in the 
      intercalated discs between  cardiac muscle fibers, are responsible for the coordinated action of sheets or
      bundles of smooth muscle cells.

      6. Within the smooth muscle fiber:

    1. the single nucleus is elongated (ovoid) and lies midway in the fiber where the cell is widest. The nucleus is also positioned centrally in the cell. When the fiber contracts the nucleus assumes a spherical shape.
    2. The sarcoplasm appears homogeneous under the light microscope. Under the electron microscope, myofilaments can be seen running the length of the cell. The myofilaments are composed of actin and myosin.
    3. Various other organelles are generally confined to the conical regions at each pole of the nucleus.

     7. Smooth muscle fibers are grouped in different numbers and orientations depending on their  location and          function:
                   a. Solitary fibers are found around the small arterioles.
                   b. The cells can be grouped in small isolated bundle forming a fascicle, for example, the arrector pili of a                      hair follicle.  The cells in the bundle are closely packed so that the wide midsections are adjacent to the                      tapered ends of nearby cells.
                   c. In sheets, the bundles may show the same orientation, as in the wall of the intestine or vas
                  deferens.  They may also be arranged  in different orientations as seen in the stomach wall, urinary
                  bladder or uterus.

     8. Smooth muscle tissue has only a moderate blood supply. Capillaries run through the connective tissue
      around bundles but do not enter  the bundles.

       9. Autonomic nerve end branches terminate on some smooth muscle fibers. The excitation of  fibers lacking          innervation can be due to diffusion of neurotransmitters through the interstitial spaces or propagation of    
      excitation via gap junctions.

B.  Function:

  1. Smooth muscle produces a slow, sustained and fatigue resistant contraction.
  2. A constant muscle tension (tone) is maintained in smooth muscle due to the action of the ANS.
  3. At times, contraction of smooth muscle may produce a vigorous propulsive force, e.g., labor.
  4. Sheets and bundles of smooth muscle fibers are capable of expansion allowing dilation of hollow organs, e.g., urinary bladder, pregnant uterus, mammary glands.

III.  Cardiac Muscle Tissue -  is located only in the heart and is responsible for moving the blood and maintaining the blood pressure.

Histological Characteristics

1. Cardiac muscle fibers (CMF) range from 9 to 22um in diameter.

2. They are arranged in columns with cells in adjacent columns connected by thin slanting cellular bridges.

3. The cells are discrete but the "bridges" give the impression of a syncitium.

4. Connective tissue (endomysium) is found between columns of cells.

5. The cells are uninucleated and show some striation.

6. Within the CMF’s:

    1. the nuclei are ovoid and centrally located.
    2. Sarcoplasm is abundant and extends above and below the nucleus forming a spindle-shaped arrangement.
    3. Myofibrils are finer and seem less distinctly banded than in SMF. In some regions of the same cell, the myofibrils fuse giving a more striated appearance.
    4. Mitochondria are abundant and arranged in rows. These rows are separated from one another by myofibrils.
    5. Lipid droplets can be seen in the sarcoplasm. With age, lipofuchsin, a brown pigment, will accumulate in CMF’s. This may occur to such an extent that the myocardium will have a brown color. This condition is referred to as "brown atrophy of the heart".
    6. The sarcoplasmic reticulum is less developed than in SMF’s.

 7.  The intercalated discs are specialized cell junctions between CMF’s. They are located At Z lines and carry out important functions for the cell:

    1. The disc contains gap junctions which permit the rapid conduction of chemical and electrical activities between CMF’s.
    2. The disc also represents the location of firm adhesions between CMF’s. This allows these cells to "pull together" during a contraction.
    3. Myofibrils anchor themselves via their Z lines on the discs. This allows the myofibrils to transmit the tension of their contraction through the entire CMF.

8. Purkinje fibers are CMF’s specialized to function primarily for conduction of the electro-chemical waves of depolarization and repolarization through the myocardium.

     a.  These cells form a network close to the endocardium. They are particularly evident near the
      interventricular septum.
     b.  Purkinje fibers are larger and thicker, about 50um in diameter.
     c.  They are paler than the surrounding CMF’s.
     d.  They contain abundant sarcoplasm and relatively few myofibrils.
     e.  Purkinje fibers are seen to contain large numbers of glycogen granules.