What is Muscular Tissue ? Histology | Anjani Mishra

Muscular tissue 

                             Written By Anjani Mishra           

Muscular tissue is a collection of specialised cells which have the ability to contract and expand to produce movement within certain organs and the body as a whole.

Muscle fiber

The structural and functional unit of muscular tissue is a muscle fiber. Muscle cell is much longer than it’s width, therefore muscle cells are often called muscle fiber.

These are cylindrical cells about 1mm to 4cm long in short muscle and upto 30cm in sartorius muscle. They vary in diameter from 0.01mm to 0.1mm or more. It has single to several nuclei. It is a single cell in skeletal and smooth muscle, but in cardiac muscle it is a collection of few cells. 

A.  An entire skeletal muscle enclosed within a dense connective tissue layer called epimysium
B.  Each fascicle of muscle fibers is enclosed within a thin septa of C.T. called perimysium

C. Individual muscle fibers is surrounded by a more delicate C.T. called endomysium.

Myofibrils

Each muscle fibre (Cells) has bundles of thread-like contractile fibrils called myofibrils, made of actin and myosin myofilaments.

These are microscopic in size, varying from 0.2mm to 2mm in diameter. There are many bundles of myofibrils in a muscle fiber. Each bundle is isolated from its neighbours by sarcoplasm.

Myofilaments

Each myofibril is again composed of bundles of needle-like thin and thick myofilaments which are actually protein molecules.

Actin filaments                       - Long and thin

Myosin filaments                    - Short and thick

Thin filaments are 1.0 mm long and 8 nm wide

Thick filaments are 0.6 mm long and 15 nm wide

These filaments lie parallel to the long axis of the myofibrils in a symmetrical pattern.

Sarcoplasm

The cytoplasm of muscle cells is called sarcoplasm.

Sarcolemma

The covering of muscle cells is the sarcolemma/plasmalemma/cell membrane.

Endomysium

The more delicate C.T., composed of a basal lamina surrounding each muscle fiber is called the endomysium.

Perimysium

The thin septa of C.T. surrounding each bundles of fibre within the muscle is called the perimysium.

Epimysium

The masses of muscle fibers are arranged in regular bundles surrounded by an external sheath of dense C.T., called the epimysium.

Types of muscular tissue

There are three basic types of muscular tissue on the basis of structural and functional characteristics:

•          Skeletal muscle                              -Voluntary, Striated

•           Smooth muscle                               -Involuntary, non-striated

•          Cardiac muscle                               -Involuntary, striated

1.      Skeletal muscle

Shape                    - Cylindrical

Size                       - Very long (upto 30cm)

Diameter               - 0.01 to 0.1 mm (10 um to 100 um)

Nuclei                    - Long, oval nuclei, located peripherally under the cell membrane. It is   

                                  multinucleated

Sarcoplasm            - It contains sarcosomes (mitochondria), golgicomplex and  

                                  sarcoplasmic reticulum (S.E.R.)

  Figure: Skeletal muscle, tongue, horse.

1.   Muscle cells
2.   Capillary
3.   Perimysium
4.   Endomysium
5.   Muscle cell

Myofibrils

• These are long cylindrical filamentous bundles, running parallel to the long axis of the muscle fiber

•    Diameter: 1-2 mm

• Consist of an end to end repetitive arrangement of sarcomeres

• Each myofibrils shows alternating light and dark bands along its length, (responsible for  crossstriations). The darker bands are called A bands (anisotropic); the lighter bands are called I bands (isotropic)

• Under an electron microscope, each I band is bisected by a dark transverse line, the Z line (Z disc), and a pale narrow H band (H disc) is seen across the middle of the A band. A very narrow band (M band or M disc) can be distinguish in the middle of the H band)

Sarcomere

The portion of a myofibril from one Z line to next Z-line is called a sarcomere. It is about 2.5mm long in resting stage. A myofibril, therefore, is composed of a number of sarcomeres arranged end to end. There are about 300 light and dark bands in each millimetre of myofibril.

Figure: Skeletal muscle and tendon, tongue, horse

1. Muscle cell
2. Muscle cell, oblique cut
3. Tendon 

 

2.      Smooth muscle

Shape                                - Spindle/fusiform

Size                                   - 20mm in small blood vessels to 500mm in pregnant uterus

Diameter                           - 3-8mm

Nucleus                             - Elongated oval-shape located centrally in the cell. It is 

                                              uninucleated.

Sarcoplasm                        - It contains mitochondria, golgicomplex, polyribosome and  

                                              rough endoplasmic reticulum (R.E.R.)

Myofibril

These are longitudinal

Diameter- upto 1mm

 

 Figure: Smooth muscle, jejunum, sheep

1. Muscle cell, l.s.
2. Muscle cell, x.s.
3. Connective tissue

  Figure: Smooth muscle, urinary bladder, pig

1. Arteriole
2. Muscle cell, l.s. 
3. Connective tissue
4. Muscle cell, x.s.
5. Venule 

3.      Cardiac muscle

Shape                    - long cylindrical, branched

Size                       - 85-100 mm (length)

Diameter               -15mm

Nucleus                 - Barrel Shape, located centrally contain one or two nuclei

Sarcoplasm            - it  contains mitochondria, sarcoplasmic reticulum (S.E.R.)

Myofibrils

More or less similar to those of skeletal muscle

Intercalated disc:

Presence of dark transverse lines that cross the chain of cardiac cell at irregular intervals. These are specialized intercellular junction between adjacent cells, gives a staircase like appearance 

Figure: Cardiac muscle, heart, cat

1. Intercalated disc
2. Bifurcation
3. Muscle cell, l.s.

Figure: Cardiac muscle, heart, cat

1. Intercalated disc
2. Muscle cell, nucleus

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