Smooth muscles show sharp contrast with skeletal and cardiac muscles. Moreover, the smooth muscles present in different parts of our body show considerable variety in their size, structural organization, innervation and functional behavior.
Structure of smooth muscle :
Smooth muscles are made up of numerous extended muscle fibers which considerably vary in size. The smooth muscle fibers are in general shorter than skeletal muscle fibers. Each smooth muscle fiber is a single cell surrounded by sarcolemma. The cells are fusiform in shape that means spindle shaped having a wider middle portion called belly and the tapering ends. Each cell contains a single, oval nucleus placed at the center of the belly. The muscle fibers are arranged in such a way that the wider middle portion of one fibre is placed in-between the narrowing ends of adjacent fibers. Each fiber is externally covered by a basement membrane like structure which separates the fibers from each other, some smooth muscles, the basement membrane is discontinuous at places where sarcolemma of adjacent cells fuse together to form 'tight junctions' or low electrical resistance bridges comparable to the inserted discs of cardiac muscle.
Smooth muscle fibers contain myofilaments i.e. actin and myosin but no organized myofibrils. Due to lack of an orderly arrangement of myofilaments, alternate dark and light bands or cross-striations are not seen. However, some faint longitudi striations may be seen because the myofilaments are indiscriminately arranged parallel to the long axis of the fibers. The actin filaments are attached to specialized structures called dense bodies. Some dense bodies are attached to the cell membrane while others are detached in the sarcoplasm. The dense bodies are held in position due to presence of cross-attachments between them. Thus, dense bodies are analogous to the Z-lines of skeletal and cardiac muscles. Myosin filaments are less in number and they remain scattered among the actin filaments. In smooth muscle fibers, tropomyosin is present but troponin is absent. Cell organelles and sarcotubular system are poorly developed and T-tubules are absent.
Types of smooth muscles :
Smooth muscles into two types—single unit smooth muscle and multi-unit smooth muscle which differ in various features. Now let us go for some discussion:
Single unit smooth muscle :
It is commonly called visceral smooth muscle because it is found in the walls of hollow visceral organs like gut, gall bladder, urinary bladder, ureter, uterus etc. In this type of muscle, the muscle fibers are usually arranged in sheets or bundles and they remain unified through low electrical resistance bridges forming a functional syncytium. Thus, when one fiber is excited, the excitation or impulse spreads to the neighboring fibers and as a result large number of muscle fibers contact simultaneously as if they belong to a single unit. For this reason, this type of muscle is known as single unit smooth muscle. Due to the presence of functional connection between the fibers, each fiber is not innervated by a distinct nerve ending. The muscle fibers show pacemaker like activity and they generate action potentials when stretched. So they contract spontaneously in response to stretch even when de-enervated. However, the nerves may modify their shrinkages.
Multi-unit smooth muscle :
This type of smooth muscle found in ciliary and iris muscles of eye, piloerector muscles of skin and in larger blood vessels. This type of muscle composed of distinct muscle fibers without low electrical resistance bridges in between them. Each fiber is completely covered by the basement membrane and remains separated from the neighboring fibers. Each fiber is innervated by a separate nerve ending and operates independently of the others. Since, the individual fibers behave as separate units, this type of smooth muscle is called multi-unit smooth muscle. These muscles do not show pacemaker activity and they do not contract instinctively in response to stretch. Muscle fibers are activated only when stimulated by nervous or chemical agents like hormones.
Contraction and relaxation of smooth muscles
The basic mechanism of contraction and relaxation of smooth muscles is more or less same as that in skeletal and cardiac muscles. But there are some differences. When a smooth muscle fiber is activated either impulsively by elongating or by excitation of the motor nerve, an action potential is generated. As the action potential spreads through the sarcolemma, the intracellular Ca++ concentration rises which causes sliding of myofilaments and contraction of the muscle fiber. The source of Ca++required for excitation-contraction coupling differs in smooth muscles because the sarcoplasmic reticulum is poorly developed and T-tubules are absent. In smooth muscle, most of the Ca++ ions enter the muscle fiber from the extracellular fluid through the sarcolemma during the flow of action potential. Since troponin is absent in smooth muscles, another Ca++ binding protein called calmodulin helps in excitation-contraction coupling. So, smooth muscles contract involuntarily because they are innervated by autonomic nerves. Here the contractions are slow and continuous.