It is also known as unstriated muscle. These muscles are found in the well of alimentary canal. Kidney and its tubules, ureter and urinary bladder and blood vessels. It is formed by long, thin and spindle shaped cells. Each cells itself forms a muscle fibre, in the centre of which is large nucleus.
The cytoplasm present around the nucleus is known as sarcoplasm. Many fine fibres known as myofibrils, are present in sarcoplasm. These fibrils possess much power of contraction and expansin. They are surrounded by a fine membrane or placed in many layers. Animals possessing them cannot contract and expand the unstriped muscles at their will, therefore, these muscles are called involuntary. The nerve fibres present in these muscles arise from sympathetic nervous system.