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.