Cardiac Muscle Fibres :

Occurrence : They are present in myocardium of heart.

Structure : Each cardiac muscle fibre is bounded by plasmalemma. They are separated from each other by intercalated discs. These intercalated discs act as boosters. The adjacent muscle fibres are connected by oblique cross walls. The cardiac muscle fibre has deep in centre a slightly oval nucleus. The myofibrils are present and have light and dark bands. The cardiac muscle fibres have abundant blood supply that is why they are of dark red colour. These fibres contract quickly and take double the time to relax and that is why they never feel fatigued.

Control : They are controlled by central and autonomic nervous system.

Mechanism of muscular contraciton :

The Sliding filament theory explains the movement of the muscles. The theory was given by A.F. Huxley

According to this theory actin and myosin filaments slide past each other with the help of cross bridges to reduce the length of sarcomere and actin filaments which meet in centre of sarcomere.

1. When the stimulus of a motor nerve reaches neuromuscular junction then the vesicles present in the synaptic knobs secrete Acetyl choline which diffuses into sarcomere through membrane channel of muscle fibre.

Na⁺ also flow through channel into sarcomere and results action potential.

2. The muscle fibre which is polarised, after receiving acetyl choline becomes depolarised at that end.

3. The wave of depolarisation spreads due to invagination of sarcolemma called T tubes and Z lines.

4. Due to depolarisation, the sarcoplasmic reticulum secretes Ca⁺² ions which bind to specific site present on troponin component of actin filament. Conformational changes occur in Troponin molecule and active sites of F actin molecules are exposed where the cross bridge of myosin filaments gets joined to actin filaments.

5. The cross-bridges take energy from ATP and get rotated inward. Thus sarcomeres contract as Z-lines come closer.

6. Finally whole muscle contracts and H-lines disappear.

7. ADP molecules take energy from creatine~ phosphate and get converted into ATP.

8. The cross-bridges which have been rotated inward may rotate more after taking more energy from ATP molecule. Thus it brings more contraction of sarcomere.

Relationship between the actin and myosin myofilaments in the (A) relaxed and (B) contracted states

Skeletal muscle is made of a number of muscle bundles or fascicles that our held together by a common collagenous connective tissue layer called fascia.

ii. Each muscle bundle contains a number of muscle fibres.

iii. Each muscle fibre is lined by the plasma membrane known as sarcolemma enclosing the sarcoplasm.

iv. Since the muscle fibre's sarcoplasm contains many nuclei it is caleed as a is a syncitium.

v. The  sarcoplasmic reticulum of the muscle fibres is the store house of calcium ions.

vi. The muscle fibre has a large number of parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils.

vii. Each myofibril has alternate dark and light bands on it. The striated appearance is due to the distribution pattern of  Actin and Myosin proteins. The light bands contain actin and is called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin. 

viii. In the centre of each ‘I’ band is an elastic fibre called ‘Z’ line . The thin filaments are firmly attached to the ‘Z’ line. The thick filaments in the ‘A’ band are also held together in the middle of this band by a thin fibrous membrane called ‘M’ line.

ix. The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils. The portion of the myofibril between two successive ‘Z’ lines is considered as the functional unit of contraction and is called a sarcomere. 

Smooth Muscle Fibres :

Occurrence : They are present in the walls of hollow visceral organs like stomach, intestine, lungs, arteries, veins, urinary bladder etc.

Shape : Spindle shaped with narrow or pointed ends.

Structure : TheThey do not exhibit any striations and are snooth in appearance. The unstriped muscle fibres may occur singly or may occur in groups and form sheets. Each unstriped muscle fibre is bounded by plasmalemma. The sarcoplasm contains an oval nucleus in centre. It also has number of myofibrils but without light and dark bands. These muscle fibres contract slowly and relax slowly that is why they do not get fatigued. 

The muscles are not under the control of the nervous system and hence are called involuntary muscles.

Functions - They muscle assist, for example, in the transportation of food through the digestive tract and gametes through the genital tract.