Muscular System

Session: Systems and sub-systems – Muscular System

Key Concepts

• Muscular System

Introduction: 

The muscular system is responsible for the movement of the human body. Muscle cells, commonly known as myocytes, are the cells that makeup muscle tissue. There are 3 types of muscle cells in the human body; cardiac, skeletal, and smooth. Cardiac and skeletal myocytes are sometimes referred to as muscle fibers due to their long and fibrous shape. Cardiac muscle cells, or cardiomyocytes, are the muscle fibers that comprise the myocardium, the middle muscular layer of the heart. 

Connective tissues in the muscular system: Endomysium, perimysium, epimysium, deep fascia, myofascial, tendon, tendon sheath, aponeurosis, retinacula. 

Endomysium: Connective tissue layer that surrounds individual muscle fibers. 

Perimysium: Connective tissue layer that surrounds fasciculi. 

Epimysium: Connective tissue layer surrounding an entire muscle. 

Deep fascia: Connective tissue layer that surrounds muscle groups. 

Tendon: Cord-like structure anchoring the end of a muscle to a bone. 

Myofascial: Refers to skeletal muscles and related fascia in the muscular system. 

Tendon sheath (synovial sheath): Tube-like structure lined with a synovial membrane that surrounds long tendons. 

Muscular system 

The muscular system is an organ system involved majorly in the movement of the body. There are nearly 700 muscles that are connected to the bones of the skeletal system, which roughly make up half of the human’s body weight. Every muscle is a different organ made of blood vessels, skeletal muscle tissue, nerves, and tendons. Muscle tissues are found in the heart, blood vessels, and digestive system. The muscular system consists of various types of muscles, each of which plays a crucial role in the function of the body. 

Muscles allow a person to move, speak, and chew. They control heartbeat, breathing, and digestion. Other unrelated functions such as temperature regulation and vision also rely on the muscular system. 

There are 3 types of muscle cells in the body: 

1.Skeletal Muscle cells:  

Skeletal muscle cells make up the muscle tissues connected to the skeleton and are important in locomotion. Smooth muscle cells are responsible for involuntary movement, like that of the intestines during peristalsis (contraction to propel food through the digestive system). 

Characteristics 

Skeletal muscle cells are long, cylindrical, and striated. They are multi-nucleated, meaning that they have more than one nucleus. This is because they are formed from the fusion of embryonic myoblasts. Each nucleus regulates the metabolic requirements of the sarcoplasm around it. Skeletal muscle cells have high energy requirements, so they contain many mitochondria in order to generate sufficient ATP. 

Structure of Skeletal Muscle Cells 

The cell membrane of a muscle cell is known as the sarcolemma, and the cytoplasm is called sarcoplasm. The sarcoplasm contains myoglobin, an oxygen storage site, as well as glycogen in the form of granules in the cytosol, which provides  energy to muscle cell.   

Sarcoplasm also contains many tubular protein structures called myofibrils, which are made up of myofilaments. There are 3 types of myofilaments; thick, thin, and elastic. Thick myofilaments are made from myosin, a type of motor protein, while thin myofilaments are made from actin, another type of protein used by cells for structure. Elastic myofilaments are composed of a protein known as titin. Together these myofilaments work to create muscle contractions. The basic unit of striated (striped) muscle is a sarcomere comprised of actin (light bands) and myosin (dark bands) filaments. 

Myofilaments: Thick and thin protein strands within each sarcomere. They consist of actin and myosin. 

Sarcomere: A muscle’s contractile unit. It is found within myofibrils. 

Myofibrils: Thin strands within each muscle fiber. It contains myofilaments. 

Muscle fiber: Thread-like muscle cell. 

Fasciculi: Groups of muscle fibers or neurons. Singular is fascicle. 

Contraction 

The muscle contractions of striated muscle cells are regulated by calcium ion concentration, which in turn is regulated by a structure known as the sarcoplasmic reticulum. This structure is similar to the smooth endoplasmic reticulum of other types of cells. 

2.Smooth Muscle Cells (visceral muscles): 

Characteristics: 

Smooth muscle cells are spindle-shaped and contain a single central nucleus. They range from 10 to 600 μm (micrometers) in length and are the smallest type of muscle cell. They are elastic and, therefore, important in the expansion of organs such as the kidneys, lungs, and vagina. The myofibrils of smooth muscle cells are not aligned like in cardiac and skeletal muscle, meaning that they are not striated. Hence, the name smooth muscles. Smooth muscle cells are arranged together in sheets, and this organization means that they can contract simultaneously. 

Smooth muscle cells found in the walls of the hollow internal organs such as blood vessels, the gastrointestinal tract, bladder, and uterus, is under control of the autonomic nervous system. Smooth muscle cannot be controlled consciously and thus, acts involuntarily. The non-striated (smooth) muscle cell is spindle-shaped and has one central nucleus. Smooth muscle contracts slowly and rhythmically. 

Smooth Muscle Tissue 

3.Cardiac Muscle Cells: 

Characteristics: 

Cardiomyocytes (cardiac muscle cells) are short, narrow and fairly rectangular in shape. They are around 0.02 mm wide and 0.1 mm (millimeters) long. Cardiomyocytes contain many sarcosomes, which provide the required energy for contraction. Cardiomyocytes normally contain a single nucleus. Cardiomyocytes generally contain the same cell organelles as skeletal muscle cells, although they contain more sarcosomes. 

Cardiomyocytes are large, muscular and are structurally connected by intercalated discs which have gap junctions for diffusion and communication. The key role of cardiomyocytes is to generate enough contractile force for the heart to beat effectively. They contract together, causing enough pressure to force blood around the body. Satellite cells (SCs) are the main muscle stem cells responsible for their regenerative capacity. Satellite cells are also present in skeletal muscle cells. 

Cardiac muscle, found in the walls of the heart, is also under the control of the autonomic nervous system. The cardiac muscle cell has one central nucleus, like smooth muscle, but it also is striated, like skeletal muscle. The cardiac muscle cell is rectangular in shape. The contraction of cardiac muscle is strong and rhythmical. 

The major muscles of the body (location and function): 

• Pectoral (Chest muscle): connect the bones of the chest to the shoulder and upper arm. 
• Bicep (Muscle): located between the shoulder and the elbow; allows for arm movement. 
• Abdominal (Belly muscle): located under the ribcage; support the trunk, allow for movement, and hold your organs in place. 
• Quadricep (Thigh muscle): located in the front of the thighs; allow for running, walking, jumping, and squatting 

• Deltoid (Shoulder muscle): located in the shoulder; allows movement of the shoulder. 

Functions of the Muscular System: 

1. Movement 

The main function of the muscular system is movement. Muscles are the only tissue in the body that has the ability to contract and, therefore, move the other parts of the body. 

2. Stability 

• Muscle tendons stretch over joints and contribute to joint stability. Muscle tendons in the knee joint and the shoulder joint are crucial in stabilization. 
• The core muscles are those in the abdomen, back, and pelvis, and they also stabilize the body and assist in tasks such as lifting weights. 

3. Posture 

• Skeletal muscles help keep the body in the correct position. 
• Good posture relies on strong, flexible muscles. Stiff, weak, or tight muscles contribute to poor posture and misalignment of the body. 

• Long-term, bad posture leads to joint and muscle pain in the shoulders, back, neck, and elsewhere. 

4. Circulation 

• The heart is a muscle that pumps blood throughout the body. The movement of the heart is outside of conscious control, and it contracts automatically when stimulated by electrical signals. 
• Smooth muscle in the arteries and veins play a further role in the circulation of blood around the body. These muscles maintain blood pressure and circulation in the event of blood loss or dehydration. 
• They expand to increase blood flow during times of intense exercise when the body requires more oxygen. 

5. Respiration 

• Breathing involves the use of the diaphragm muscle. 
• The diaphragm is a dome-shaped muscle located below the lungs. The contraction and relaxation of the diaphragm muscle helps in the movement of air in and out of the lungs. 
• When someone wants to breathe more deeply, it requires help from other muscles, including those in the abdomen, back, and neck. 

6. Digestion 

• Smooth muscles in the gastrointestinal or GI tract control digestion. The GI tract stretches from the mouth to the anus. 
• Food moves through the digestive system with a wave-like motion called peristalsis. Muscles in the walls of the hollow organs contract and relax to cause this movement, which pushes food through the esophagus into the stomach. 
• The upper muscle in the stomach relaxes to allow food to enter, while the lower muscles mix food particles with stomach acid and enzymes. 

7. Urination 

• The urinary system comprises both smooth and skeletal muscles, including those in the bladder, kidneys, penis or vagina, prostate, ureters, and urethra. 

• The muscles and nerves must work together to hold and release urine from the bladder. 

8. Vision 

• Six skeletal muscles around the eye control its movements. These muscles work quickly and precisely and allow the eye to maintain a stable image, scan the surrounding area, and track moving objects. 

9. Organ protection 

• Muscles in the torso protect the internal organs at the front, sides, and back of the body. The bones of the spine and the ribs provide further protection. 
• Muscles also protect the bones and organs by absorbing shock and reducing friction in the joints. 

10. Temperature regulation 

• Maintaining normal body temperature is an important function of the muscular system. 

Muscular System Diseases 

Muscle diseases are any disease that affects the human muscle system. A muscle disease will be either primary or secondary. 

Common primary diseases of the muscular system include inflammatory myopathies, such as polymyositis and dermatomyositis, muscular dystrophy, myasthenia gravis, amyotrophic lateral sclerosis, rhabdomyolysis, and cardiomyopathy, among others. 

Primary Muscular Diseases 

• Polymyositis (PM): This is a rare type of inflammatory myopathy (also called myosotis), a group of muscle diseases that causes inflammation of muscle and their associated tissues, including blood vessels. 
• Dermatomyositis (DM): DM is a rare inflammatory muscle disease affecting people of any age or sex, although it is more often seen in women. 

• Muscular dystrophy (MD): MD is a group of inherited muscle diseases. These conditions all cause muscle loss and weakness. 
• Myasthenia gravis (MG): MG is a neuromuscular disease that causes weakness of the skeletal muscles, resulting from impaired communication between nerve cells and muscles. 
• Amyotrophic lateral sclerosis (ALS): ALS, also called Lou Gehrig’s disease, is a group of rare neurological diseases affecting the nerve cells in the brain and spinal cord. 
• Rhabdomyolysis: Rhabdomyolysis is a muscle disease that causes the breakdown of skeletal muscle. 

Secondary Muscle Diseases 

• Geriatric muscle disease—Sarcopenia  

Geriatric muscle disease or Sarcopenia is a secondary muscle disease common in older adults, while other types of muscle diseases are less common in older adults. Sarcopenia causes muscle mass loss and muscle strength loss. 

• Muscle weakness  

Muscle weakness is one of the first signs of a problem with our muscles. This means there is a lack of strength in the affected muscle, and it cannot do its job. Many different diseases can cause muscles to become weak. In addition to muscle pain and weakness, a person with muscle disease may experience muscle spasms, cramping, or twitching. Other symptoms of muscle diseases include muscle wasting or muscle loss, problems with movement and balance, numbness, tingling, painful sensations, double vision, or droopy eyelids, problems with swallowing—dysphagia, breathing troubles, especially dyspnea (shortness of breath). 

Summary:

• One of the most predominant characteristics of skeletal muscle tissue is its contractility, and nearly all movement in the body is the result of muscle contraction.
• Three types of muscle are skeletal, smooth, and cardiac.
• Muscles are attached to bones by tendons.
• Functions of the muscular system are movement, posture, joint stability, heat production, stability, organ protection, temperature regulation.
• Polymyositis, dermatomyositis, muscular dystrophy are muscular diseases.
• Muscle weakness is the first sign of a problem with muscles.