What are the major types of muscles in the human body?

Based on function, muscles in the human body can be classified into three types - skeletal, cardiac, and smooth muscle. Each type of muscle has unique cellular components, physiology, specific functions, and pathology. Based on the muscle action, muscles in the human body can be further classified into two types – Voluntary and Involuntary muscles. 

Muscle Types Based on Function

Skeletal muscles

Skeletal muscles are the most abundant type of muscle in the body. They are composed of long, cylindrically shaped, multinucleated fibers and are typically attached to bones by collagen fibers called tendons. Skeletal muscles appear striated or striped under a microscope due to the regular repetitive arrangement of contractile proteins within their cells. Skeletal muscles are voluntary and can be consciously contracted and relaxed. Contraction of skeletal muscles typically occurs in response to nerve impulses. These muscles are responsible for all types of body movement including breathing, locomotion, extending an arm and writing. They are also responsible for facial expressions, maintaining posture, regulating temperature, and providing support and protection to delicate internal organs and tissues. 

Cardiac Muscle

Cardiac muscle is exclusively found in the heart. It has a similar striated appearance as skeletal muscles but has other distinct characteristics, including branched fibers, intercalated discs, and transverse tubules that allow for coordinated contractions. Unlike skeletal muscles, cardiac muscle is not part of the musculoskeletal system. Cardiac muscle is involuntary. It contracts continuously and repeatedly without conscious control. The main functions of cardiac muscle are to regulate the functioning of the heart and pump blood throughout the circulatory system, ensuring oxygen and nutrients are delivered to tissues and organs.

Smooth Muscle (Non-striated Muscle)

Smooth muscles are found in the walls of various organs, including the digestive tract, blood vessels, respiratory airways, and reproductive organs. They lack the striations seen in skeletal and cardiac muscles because their contractile proteins are arranged differently. In smooth muscles, the proteins actin and myosin appear as sheets, which give them a smooth, non-striated appearance. Smooth muscles are also involuntary and contract in response to various stimuli, such as hormones and neural signals. These muscles are responsible for regulating the diameter of blood vessels, which helps to regulate blood pressure and oxygen flow throughout the body. They also play key roles in moving fluids through organs, producing connective tissue proteins such as elastin and collagen, contracting the irises, and contracting the sphincter. 

Muscles Based on Type Of Action

Voluntary muscles 

Voluntary muscles are controlled by the somatosensory nervous system. They contract in response to motor nerve signals that originate in the central nervous system. Voluntary muscles are attached to the bones by tendons and play a key role in facilitating precise and coordinated body movements of all types. The skeletal muscles are a type of voluntary muscle. 

Involuntary muscles

Involuntary muscles are not under conscious control. They operate without direct input from the conscious mind. Cardiac muscle and smooth muscle are both involuntary muscles.