What is the Muscular System?

The human body is majorly classified into eleven organ systems. The muscular system is one among them which is specifically responsible for movements. The muscular system is specifically made up of specialized cells termed muscle cells. The significant function of the muscular system is its ability to contract. The muscles attached to bones, blood vessels, and internal organs aid in body movements. Therefore, every movement in the body occurs as a result of muscle contraction.

Muscle Types

Muscle tissues are considered one of the four primary types of tissues in the body. There are three types of muscle tissues in the body of a human. It includes cardiac muscle, smooth muscle, and skeletal muscle. All three muscles exhibit a common property called excitability due to the modification in the plasma membrane's polarization, from polarized to depolarized. The excited muscle cell sends action potential, an electric wave along the total length of the membrane. Each tissue types are unique and are essential for various specific roles in the body.

1. Smooth muscle

These muscles are spindle-shaped and elongated cells that taper at both ends. It is commonly found in the hollow organs' walls, including the stomach, uterus, and intestine, and the passageway walls, including the veins and arteries of the circulatory system, and the urinary tract, respiratory tract, and reproductive tract. The smooth muscle is an involuntary non-striated muscle. These muscles play a significant role in the organs such as the bladder and stomach as they change the shape of the organ to facilitate functions like digestion with the help of wave-like movement or peristalsis. The smooth muscle is formally otherwise referred to as visceral muscles. Its smooth appearance is the major contrasting feature compared to other muscles.

2. Cardiac muscle

Muscles that are confined to the heart are termed cardiac muscles. These muscles are highly organized and contain various cell types such as smooth muscle cells, cardiomyocytes, and fibroblasts. The cardiac muscle cells are vital for performing highly coordinated action responsible for the pumping of the heart and circulation of blood throughout the body. The cardiac muscle produces an involuntary movement in contrast to the skeletal muscle type. Thus, a person cannot control the functioning of the cardiac muscles.

3. Skeletal muscle

The muscle tissue attached to the bone is termed the skeletal muscle. These muscles are involved in voluntary movements as their movements are under the control of the nervous system. It performs several functions, including posture and balance maintenance and protection of several important organs in the body. Walking, writing, and speaking are also certain other regular physical functions that an individual performs consciously with the help of these muscles. The skeletal system combined with the skeletal muscle is referred to as the musculoskeletal system.

Structure of the Muscles

Muscles are made of a basic unit termed myofibril and several other proteins. The muscle fibers comprise the tubular form of myofibrils that are made of repeating sections of sarcomeres. The sarcomere appears as alternating light and dark bands under the microscope. The proteins that exist in the muscle fibers are myosin, troponin, and actin. Myofibril is a multinucleated cell containing all the cell components, including nuclei, cell membrane (sarcolemma), Golgi apparatus, cytoplasm (sarcoplasm), and sarcoplasmic reticulum, and mitochondria.

The muscles are attached to bones by the collagen fibers named tendons that are composed of connective tissues. The skeletal muscle comprises a muscle fibers' bundle called fascicle, which is in the shape of a cylinder. The three connective tissue layers usually enclose this bundle. The fascicle provides the muscle its structure and creates compartments for the muscle fibers within the whole muscle. Each muscle is wrapped inside a dense irregular collagen fiber layer named epimysium. Perimysium surrounds each fascicle, whereas the endomysium surrounds each muscle fiber.

The functions of the Muscular System

Different types of muscles work as a muscle group to achieve a specific function. For example, during running, the sekleltal muscles allow leg movements; the cardiac muscle increases the heart's pumping capacity to continue blood supply to the skeletal muscles; the smooth muscles in the lungs facilitate heavy breathing to ensure adequate oxygen intake and carbon dioxide release.

The significant functions of the muscular system are listed below:

• Stability
• Posture control
• Mobility
• Respiration
• Digestion
• Urination
• Circulation
• Childbirth
• Vision
• Organ protection
• Regulation of temperature

Muscle Contraction

Muscle contraction is defined as the activation of sites required to generate tension within the muscle fibers. This site activation occurs either due to an elevation in the tension or decreased muscle length. The muscle exerts a certain force termed muscle tension. In terms of physiology, muscle contraction does not essentially signify muscle shortening. The muscle tension can also be produced with any modification in the muscle's length, which includes holding heavy objects like the book at the same position. Once the muscle contraction is terminated, muscle relaxation begins, due to which the fibers return to their initial states.

A sarcomere is defined as the muscle's fundamental contractile unit, which is vital for its contraction. It is broken down into several sections: the M line, A band, Z line, H zone, and I band. For muscles to get contracted, the sarcomere must be shortened. However, during the shortening of the sarcomere, the distance between Z lines and the length of the H band and I band is reduced.

Formation of cross-bridges

Myofilaments are small structures that form the muscle's basic unit. Myofilaments are of two significant types: thin and thick filaments, and each filament has a varied location and composition. The thick filaments exist in the A band and are majorly made up of myosin proteins. The thin filaments attach to the Z disc's protein termed alpha-actinin, which comprises tropomyosin, actin, and troponin. The binding occurs across the I band's complete length and partway into the A band. The binding of myosin to actin can explain the contraction mechanism. The tails of myosin filaments bind together to leave the head portion exposed to the interlaced thin filaments. The myosin heads are vital for the formation of cross-bridges.

Role of calcium ion

When the nerve fibers stimulate the muscle fibers, the sarcoplasmic reticulum releases calcium ions into the sarcoplasm. These ions bind to the protein complex known as a troponin-tropomyosin complex, which changes the conformation of troponin, thereby pulling the complex aside. As a result, the active site on the actin filament is exposed to form cross-bridges with myosin, resulting in muscle contraction.

Context and Applications    

This topic is significant in the professional exams for both undergraduate and graduate courses that specifically deal with the concepts of physiology and anatomy. Understanding the functions and anatomy of the muscular system helps in a better treatment for various conditions related to muscles.

Practice problems

• Which term is used for naming (based on the action) the muscle that moves the leg away from the body?

When the muscles are named based on the movements they perform, action words are preferred. Flexion, extension, adduction or adductor, and abduction are some examples of these action words. Among them, the abductor is used to represent the muscle's movement away from the midline.

• Which band does not modify in length during the process of contraction? 

The A band is not modified during muscle contraction, and its length remains the same.