Muscle Tissues in Human: Characteristics, Types, and Functions

Tissues, in the context of biology, are a group of cells that perform the same kind of functions. They are classified based on their cells’ size, shape, and functions. One such type is muscle tissues, which we will delve into further. 

Muscle tissues are highly elastic groups of cells that provide movement throughout and inside the body. The blood supply in the muscle tissues needs to be adequate for proper contraction because blood provides oxygen, calcium, nutrients, and waste removal from the cells.   

Muscle tissues are further classified into three specialized tissues: cardiac, skeletal, and smooth muscle tissues. As the name suggests, cardiac muscle tissues are present in the heart, and skeletal muscles help move bones. Smooth muscle tissues are present in the uterus, bladder, ureters, glands, etc. The main function of these tissues is moving bodily parts. 

Different types of Muscle cells

Characteristics of Muscle Tissue

Muscle tissues have specific physiological characteristics, which are as follows:

  1. Muscles comprise actin and myosin protein filaments, which move past one another to create muscle contraction and expansion.
  2. Different types of tendons, nerves, and elastic tissue form the muscle tissues. 
  3. The muscle tissues can excite or generate electrical signals in response to action potential (external stimuli). 
  4. Muscles can contract (shorten and thicken) in response to action potentials. It can also extend or expand without any damage to the tissue. 
  5. These tissues can return to their original shape after expansion and contraction. 

Types of Muscle Tissues

The muscle tissues are a specialized group of tissues. These tissues are further divided into three types: skeletal, smooth, and cardiac muscle tissues. The structure of skeletal, smooth, and cardiac muscle tissues have different structures, physiological functions, and locations. 

Skeletal Muscle

Skeletal muscle tissue is also called voluntary muscle because there is conscious control over it. It forms the muscles responsible for the movement of the bones or skeleton. Skeletal muscles have stripes or striations (a characteristic banded pattern) that are visible under a microscope. 

The movements are finely coordinated, like writing. However, they may also be controlled subconsciously, like maintaining an upright posture, which does not require thought unless a new locomotor skill is being learned, like skating or cycling.

The fibers of the skeletal muscle tissues are cylindrical with several nuclei and can be up to 35 cm long. Motor nerve impulses originating from the brain or spinal cord and ending at the neuromuscular junction stimulate the contraction of skeletal muscle. 

Organization of Skeletal Muscle

This muscle tissue consists of a large number of muscle fibers, and a connective tissue sheath called epimysium covers the entire muscle. Inside the muscle, the cells are collected into separate bundles called fascicles, and each fascicle is covered in its perimysium. Individual muscle cells are wrapped in a delicate connective tissue layer called endomysium inside the bundles.

Each of these connective tissue layers runs the length of the muscle. The connective tissues bind the fibers into a highly organized structure. These organized structures blend together at each end of the muscle to form the tendon, which secures the muscle to the bone. These tendons are usually rope-like structures, but sometimes they may take the form of a broadsheet called aponeurosis. 

The multiple connective tissue layers throughout the muscle are essential for transmitting the force of contraction from each muscle cell to its points of attachment to the skeleton. The fleshy part of the muscle is the belly, and during muscle contracts, it bulges and becomes shorter.

Smooth Muscle Tissue 

Smooth muscle tissues, a fascinating aspect of our anatomy, are involuntary, meaning their contraction or movement is not under conscious control. These muscles possess the unique ability to contract and relax, a function not found in other muscle types. In addition to autonomic nerve impulses, some hormones like adrenaline dilate the airways, and local metabolites stimulate the contraction of these muscles, showcasing their intricate response mechanisms. 

Unlike skeletal and cardiac muscle tissues, smooth muscle does not have stripes or striations when viewed under the microscope. The smooth muscle forms sheets in the walls of the hollow organs and tubular structures to regulate diameter and propel substances through the tracts. Under microscopic examinations, the cells of this type are seen to be spindle-shaped with only one central nucleus. 

Smooth muscle tissues are present in the walls of hollow organs, such as the diameter of blood vessels, parts of the respiratory tract, gland ducts, the alimentary t, tract, the urinary bladder, and the uterus, the diameter of the ureters. 

Cardiac Muscle Tissue

These muscle tissues are present only in the heart wall. Like smooth muscles, their control is not under conscious control. And like skeletal muscle tissues, these tissues have striations when viewed under a microscope. 

Each muscle fiber (cell) in the heart has a nucleus and one or more branches. The cells’ ends and branches are in very close contact with the ends and branches of adjacent cells. These connections, known as intercalated discs, are not just lines but are thicker and darker than ordinary cross strips when viewed under a microscope. This unique arrangement demonstrates that cardiac muscle appears to be a sheet of muscle instead of many individual fibers, a crucial feature for the heart’s coordinated contraction. 

The end-to-end continuity of cardiac muscle cells is significant in relation to the way the heart contracts. A wave of contraction begins from an individual cell and transfers from cell to cell across the intercalated discs, meaning that cells do not need individual stimulation.  

The heart, a marvel of our body, has an intrinsic pacemaker system. This means it beats in a coordinated manner without external nerve stimulation, a testament to the self-regulating nature of our body. Although autonomic nerve impulses, some hormones, local metabolites, and other substances influence the at which it beats. The heart’s ability to maintain its rhythm is a testament to its internal control. The end-to-end continuity of cardiac muscle cells further enhances this, as a wave of contraction transfers from cell to cell in the intercalated discs, meaning cells do not need to be stimulated individually. 

Functions of Muscle Tissues

Although muscle tissues are of different types, their most important function is movement. Besides movement, muscle tissue also creates force and has other functions in the body. These functions are as follows:

  1. Skeletal muscles support the bones in maintaining posture. 
  2. The skeletal muscle also controls the voluntary movement of the body. 
  3. Likewise, skeletal muscle also aids in the metabolism and storage of energy. 
  4. Similarly, smooth muscle tissues achieve the contractile function for removing and shortening various contents in different organs throughout the body. 
  5. Cardiac muscles aid in propelling blood and proper oxygenation in each cell of the human body. 

Diseases of Muscle Tissues

Myasthenia gravis

It is an autoimmune condition whose origin is unknown. It affects more women than men, usually between 20 and 40. Body produces antibodies that bind to the acetylcholine receptors of neuromuscular junctions. This blocks the transmission of nerve impulses to muscle fibers, causing progressive and extensive muscle weakness. Extrinsic and eyelid muscles (drooping of an eyelid or double vision) are affected first, followed by the neck (difficulty in chewing, speech, and swallowing) and limbs. Strenuous exercise, pregnancy, or infection can cause periods of relapse and remission. 

Muscular Dystrophies

It is an inherited disease leading to the progressive degradation of skeletal muscle cells. It may differ in type due to differences in the age of onset, muscle group involved, or rate of progression.

  1. Duchenne muscular dystrophy: This condition is linked to gender. The is carried on the long X chromosome of female carriers. This condition mainly affects Male children (50% chance), and female children usually become carriers (50% chance). Complete or partial dystrophin protein is present in the pathologic mechanism of both Becker and Duchenne muscular dystrophy. The protein is associated with the filaments of skeletal muscle and provides structure and support to the sarcolemma of the monofilament. The lack of dystrophin protein can damage the supporting sarcolemma, causing weakness and eventually atrophy of healthy muscle fibers. The signs and symptoms may only be seen once the child is five. It starts in the muscles of the lower limbs and then expands to the upper limbs, progressing rapidly without remission. Usually, death occurs in adolescence, often by respiratory failure, cardiomyopathy, or cardiac arrhythmias.  
  2. Facioscapulohumeral dystrophy: The disease affects both sexes. It starts during adolescence, and the younger the age of onset is, the faster it progresses. The muscles affected first are the face and shoulders. This is a chronic condition and progresses slowly and may not cause complete disability. Life expectance is normal. 
  3. Myotonic dystrophy: This disease usually begins in adulthood and affects both genders. Contraction and muscle relaxation occur slowly, traditionally seen during the release of an object held in hand. The tongue and the face are affected first. Cataracts, cardiomyopathy, glucose intolerance, and atrophy of the gonads are the systemic conditions associated with this. There is no remission, and the disease progresses with increasing disability. Death occurs in middle age from respiratory or cardiac failure.

Crush Syndrome 

Long-term pressure in the trunk or limb causes ischemia (a condition where blood flow is restricted in a part of the body), which results in massive muscle necrosis. When the pressure is relieved and circulation is restored, myoglobin and other necrotic products are released from the damaged muscle and enter the blood. These products are highly toxic to the kidneys, and acute renal failure may develop. A common complication is infection, especially by anaerobic microbes like Clostridium perfringer, which causes gas gangrene.

Sarcopenia

It is the loss in muscle mass and atrophy associated with progressed age. It can reduce muscle size and decrease mitochondrial numbers, satellite cells, and elasticity. Sarcopenia varies in degree of physical activity, race, and gender and leads to the loss of muscle power and immobility issues. 

Rotatory cuff injury

The rotator cuff muscles are the tissues that strengthen and stabilize the shoulder joint. Injury in this area is shared, leading to pain and restricted shoulder mobility. Although the condition is not fatal, recovery can take months to years. 

Hypertrophic Obstructive Cardiomyopathy (HOCM) 

It is a genetic inherited disorder in which genetic variants code for a portion of the contractile element of the cardiac muscle cells. This leads to heightened calcium sensitivity, which thickens the interventricular septum, obstructing blood flow. Although the condition is asymptomatic, sometimes shortness of breath, exertion, tachycardia, and cardiac arrest can be seen. It has no cure and is the leading cause of death in young people. 

References

  1. Dave HD, Shook M, Varacallo M. Anatomy, Skeletal Muscle. 2023 Aug 28. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan–. PMID: 30725921. 
  2. Noto RE, Leavitt L, Edens MA. Physiology, Muscle. [Updated 2023 May 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK532258/
  3. Waugh, A., Grant, A. and Ross, J.S. (2010) Ross and Wilson Anatomy and Physiology in health and illness. 11th edn. Edinburgh: Churchill Livingstone.

Ashma Shrestha

Hello, I am Ashma Shrestha. I had recently completed my Masters degree in Medical Microbiology. Passionate about writing and blogging. Key interest in virology and molecular biology.

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