Smooth Muscles

Smooth muscles are a category of muscular connective tissues present in the human body. They do not possess any striations. This occurs because these tissues do not possess any sarcomeres; hence, there are no dark or light bands present on their surface. Smooth muscles can be divided into two units, namely, single-unit smooth muscles and multi-unit smooth muscles. In the case of the single-unit smooth muscle, the bundle sheets that are the constituting units of the muscles are contracted into a single structure called the syncytium. Smooth muscles cannot be controlled willingly. They are found in several organs in the human body such as the stomach, intestines, urinary bladder and uterus. 

Anatomy of smooth muscles

Smooth muscles are spindle-shaped. This means that their ends are tapered in comparison to their centre. These cells possess a single nucleus, and their size can range between thirty to two hundred micrometres. The tissue that connects all the smooth muscles is called the endomysium, and it is produced by the cells themselves. Even though the smooth muscle fibres do not possess any striations, they possess muscle proteins such as actin and myosin. They also have thick and thin filaments. The thin filaments are held in place by certain dense bodies. These dense bodies perform the same function as the Z bands that are present in the skeletal and cardiac muscles of the human body. They also help in the transfer of calcium ions to the entirety of the muscle. Smooth muscles are controlled by several neural elements. A continuous form of communication also takes place between the smooth muscles and their transmitters that are produced locally. These muscles are different from skeletal and cardiac muscles not only because of their structure but also because of their functions and contractile mechanisms. These muscles show greater levels of elasticity and can also withstand higher levels of tension. 

Contractile mechanism of smooth muscles

Smooth muscles do not contain a protein known as troponin. Due to this, the formation of cross-bridges in these muscles is not controlled by the troponin-tropomyosin complex but rather by an enzyme known as calmodulin. In smooth muscles, the calcium ions that are present in the external environment pass the surface of the smooth muscles through specific calcium ions channels and then they bind to the calmodulin enzyme. The complex that is formed by the combination of the calcium ions and the calmodulin enzyme leads to the production of another enzyme called the myosin kinase. This enzyme leads to the activation of the myosin protein that is present in these muscles. This leads to the activation of the myosin heads that can further attach themselves to the actin leads and bind themselves with the thin filaments present in the muscles. When the actin molecules bind to the thin filaments, they pull the surface of the thin filaments. This, in turn, leads to the contraction of the dense bodies that the thin filaments are attached to. The dense bodies are attached to the sarcolemma in the deeper ends of the smooth muscles. The dense bodies pull on the intermediate filaments that cause the contraction of the entire muscle fibre. During the contraction of the smooth muscle fibres, their midsection is contracted in a corkscrew motion. 

The organisation of smooth muscles

Smooth muscles can be organised into two categories, namely, single-unit smooth muscles and multi-unit smooth muscles. The single-unit smooth muscles are more commonly found in the body. The locations of these muscles are different and help in performing different functions. In the single-unit smooth muscle fibres, the muscle fibres are attached with the help of gap junctions. Due to this, all the muscle fibres contract as a single unit. This type of muscle is found in the walls of several visceral organs and is often referred to as the visceral muscle. Most of these muscles are used for the process of stress relaxation. In this process, if any hollow organ is filled with a fluid, the muscles would automatically contract, but the process of contraction will be followed by the relaxation of the muscles. The process of relaxation is useful because it helps in preventing the premature emptying of the contents of the organ. In the case of the multi-unit smooth muscle fibres, the muscle fibres are not attached by gap junction, and due to this, they are not connected electrically. If one unit of the muscle contracts, the contraction is not forwarded to the following muscular unit. The stimuli for the activation of multi-unit smooth muscle fibres originate from the autonomic nervous system. These muscles are found in several blood vessels and also in the human eye.

Conclusion

Hence, it can be concluded that smooth muscles are an important type of muscle present in the human body. The smooth muscles are non-striated, and they are also known as involuntary muscles because their functioning is controlled by the autonomic nervous system. They are found in several body organs and have a wide array of functions. They can be further divided into two categories, namely, single-unit and multi-unit depending on the connection between multiple muscle fibres. They have a unique procedure for muscle contraction.