Muscles and muscle tissue: Types and functions

Muscles and muscle tissue: Types and functions

Classify muscles with examples

1. Muscles are classified as:

1. Functionally, and
2. Structurally.

1. Functionally it is either

1. Voluntary muscles: They are under the control of will, e.g. all skeletal muscles.
2. Involuntary muscles: These are not under the control of will, e.g. all smooth muscles.

2. Structurally it is either

1. Striated (cross-striped)—shows serial banding of fibres, e.g. all skeletal muscles.
2. Smooth (unstriped)—shows no banding of fibres, e.g. all smooth muscles.

Read And Learn More: Anatomy Notes And Important Question And Answers

2. The two classifications can be combined as follows

1. Smooth involuntary muscle: It is smooth muscle, mostly present in hollow organs.
2. Striated involuntary muscle: It is cardiac muscle of the heart wall.
3. Striated voluntary muscle: It is skeletal muscle, attached to the various bones.

 

Describe the transverse section (TS) of skeletal muscle

TS of skeletal muscle

1. It is multinucleated.

2. The nuclei are situated peripherally and immediately below the sarcolemma of each muscle fibre.

3. The sarcolemma is not visible in TS.

4. Each skeletal muscle fibre shows cross-striations.

5. They are visible as alternating dark A bands and light I band.

6. With higher magnification and transmission electron microscopy, additional details of the cross-striations are visible.

7. Skeletal muscle fibres are aggregated into bundles or fascicles.

8. They are surrounded by fibres of connective tissue.

9. The connective tissue sheath around each muscle fascicle is called the perimysium.

10. From each perimysium, thin partitions of connective tissue extend into each muscle fascicle.

11. These partitions invest individual muscle fibres with a connective tissue layer called the endomysium.

12. Small blood vessels and capillaries are present in the connective tissue around each muscle fibre.

Describe the longitudinal section (LS) of skeletal muscle

LS of skeletal muscle

It shows light and dark cross-striations.

The muscle fibres that were sectioned transversely show cross-striations of myofibrils and peripheral nuclei.

 

What is an intercalated disc? What are its functions?

Intercalated discs

1. Features

1. They are characteristic features of cardiac muscles.
2. These are densely stained.
3. They have special attachment sites that cross the cardiac cells in a stepwise fashion at irregular intervals.
4. They represent the specialized junctional complexes between cardiac muscle fibres.
5. The intercalated discs appear as either straight bands or staggered across individual fibres.

2. Functions

1. They connect individual cardiac muscle fibres.

2. They allow a rapid spread of stimuli throughout the entire muscle mass.

3. Conduction of excitatory impulses to the cardiac sarcomeres is through the

1. T tubules, and
2. Sarcoplasmic reticulum.

4. There are ions present in the gap junctions. They are diffused through the pores. The diffusion of ions coordinates heart function.

5. They allow the cardiac muscle to act as a functional syncytium.

6. They allow the stimuli for contraction to pass through the entire cardiac muscle.

 

Difference between smooth, cardiac and skeletal muscles

Cardiac muscle

1. It is a striated and involuntary muscle.

2. Fibres: Each fibre of the muscle is called myocyte. It branches and anastomoses to form syncytium.

3. Nucleus: Each myocyte contains one nucleus which is placed centrally.

4. Length of myocyte: 50 μm.

5. Width of myocyte: 18 μm.

6. Sarcoplasm: Contains mitochondria.

7. Striations: Not very prominent, as a few myofibrils are present.

8. Peculiarity: It contains a large amount of glycogen, myoglobin and is rich in capillary network.

9. Intercalated disc: It is junction between myocyte. It appears dark-stained transverse lines. It lies opposite the ‘I’ band. It has two parts.

1. Transverse portion
2. Lateral portion

Due to transverse and lateral portions, intercalated disc appears stepwise.

10. Gap junction: Cell membrane and adjacent myocytes are connected by desmosome and is called gap junction.

 

Sarcomere

Introduction: It is the segment of myofibril between two ‘Z’ discs.

1. Features

1. A sarcomere is segment of the fibre between successive Z bands.
2. It is the smallest structural and functional contractile unit of the muscle.
3. It is the repeating contractile units seen along the entire length of each myofibril.
4. It is highly characteristic feature of the sarcoplasm of skeletal and cardiac muscle fibres.
5. It is about 2.5 μm long.
6. It contains A band and the 1/2 of “I” band.
7. Mild trypsin digestion destroys the Z lines. The fibre then fractures into sarcomere segments.
8. Myofibril is the finer structural and functional unit of the sarcomere.

2. Each myofibril

1. Diameter: 1–2 μ
2. Length: As long as muscle fibre.
3. Character: Light and dark zone under light microscope, causes difference in the refractive index.
4. Filaments  Actin, and Myosin.

3. Dark zone: Anisotropic (A band). H band is clear area in A band. M line is thickened central part of H band.

4. Light zone: Isotropic (I band). The centre of the light zone is called Z disc (Krause’s membrane).

5. Sarcomere: ½ of I + A + ½ of I

6. During contraction: Actin filaments slide between myosin filaments. They slide towards the centre of sarcomere. Z discs are brought closer and shorten the contractile unit. A band remains the same.