NONSTRIATED (= SMOOTH) MUSCLES -

1. Non-striated muscles are found in the posterior part of oesophagus, stomach, intestine, lungs, urinogenital tract, urinary bladder, blood vessels, iris of eye, dermis of skin and arrector pili muscle of hair. Smooth muscles never connect with skeleton.
2. These muscle fibres or cells are elongated and spindle shaped.
3. Each fibre contains a single oval nucleus surrounded by the cytoplasm (sarcoplasm). In the cytoplasm the myo- fibrils are arranged longitudinally.
4. There is no sarcolemma, however, the fibre is enclosed by plasma membrane.

FUNCTION -

These muscles help in peristalsis which happens in tubular viscera.

Action of these muscles is controlled by autonomic nervous system and hence they are not under the control of the animal's will.

TYPES -

Functionally smooth muscles are of two types : single-unit smooth muscles and multi-unit smooth muscles.

(i)       Single-unit smooth muscles are composed of muscle fibres closely joined together. All the fibres of the single smooth muscle contract together as a single unit. These muscles are found in the walls of hollow visceral organs like gastrointestinal tract and urinary bladder.

(ii)      Multi-unit smooth muscles are composed of independent muscle fibres and are not closely joined together. Their fibres contract as separate units. Arrector pili muscles of skin dermis, ciliary and iris muscles in the eyes, and muscles of the walls of the large blood vessels are some examples of multi-unit smooth muscles.