Muscular System in Humans

Muscular System in Humans

  • Muscle tissue, originating from mesoderm, is a specialized tissue vital for movement and bodily functions. 
  • Constituting 40-50% of the body weight in adults, muscles possess unique properties such as excitability, contractility, extensibility, and elasticity. 
  • Muscles exhibit distinctive properties essential for their function:

- Excitability: Ability to respond to stimuli, such as nerve impulses. 

- Contractility: Capacity to contract and generate force, leading to movement. 

- Extensibility: Ability to streatch and lengthen beyond their resting state. 

- Elasticity: Capability to return to their original shape and length after contraction or extension. 

  • Classification: Muscles are classified based on various criteria, including: 

- Location: Divided into skeletal, visceral, and cardiac muscles. 

- Appearance: Distinguished by striated (striped) or non-striated appearance. 

- Regulation: Categorized by voluntary or involuntary control of their activities. 

  • Types of Muscles:

- Skeletal Muscles: Attached to bones and responsible for voluntary movements such as walking, running, and lifting weights. 

- Visceral (Smooth) Muscles: Found in the walls of internal organs like the digestive tract, performing involuntary functions such as peristalsis. 

- Cardiac Muscles: Exclusive to the heart, facilitating involuntary contractions to pump blood throughout the body. 

  •  Muscles play a crucial role in various bodily functions, including movement, posture maintenance, heat production, and organ support. 
  • Understanding the structure, function, and regulation of muscles is essential for comprehending human anatomy, physiology, and overall health and well-being.  

 

 

 

Skeletal Muscles

  • Skeletal muscles are closely linked to the bones of the body, forming a functional unit for movement and support. 
  • Striated Appearance: Under a microscope, skeletal muscles exhibit a striped or striated appearance due to the arrangement of their contractile proteins. 
  • Voluntary Control: These muscles are under voluntary control of the nervous system, allowing individuals to consciously initiate and regulate their movements. 
  • Primary Functions:

- Locomotion: Skeletal muscles play a key role in various forms of movement, including walking, running, and jumping. 

- Posture Maintenance: They contribute to maintaining body posture and stability, enabling individuals to stand, sit, and maintain balance. 

Structure and Mechanism of Contraction in Skeletal Muscles

  • Muscle Organization: Skeletal muscles are composed of muscle bundles or fascicles held together by a collagenous connective tissue layer called fascia. 
  • Muscle Fibers: Each muscle bundle contains multiple muscle fibers, each surrounded by a plasma membrane called sarcolemma. 
  • Syncitium: Muscle fibers are syncytial, containing multiple nuclei within their sarcoplasm. 
  •  Sarcoplasmic Reticulum: Endoplasmic reticulum within muscle fibers stores calcium ions crucial for muscle contraction. 
  • Myofibrils: Parallelly arranged filaments within muscle fibers, known as myofilaments or myofibrils, give skeletal muscles their striated appearance. 
  • Actin and Myosin: Two important proteins present in myofibrils responsible for muscle contraction. Actin filaments, thinner and lighter, are found in the I-band, while myosin filaments, thicker and darker, are located in the A-band. 
  • Sarcomere: Functional unit of contraction, the portion of myofibril between two Z lines. It consists of alternating dark A-bands and light I-bands. 
  • Z Line: Elastic fibers that bisect the I-band, serving as attachment points for thin filaments. 
  • M Line: Thin fibrous membrane in the middle of the A-band, holding thick filaments together. 
  • H Zone: Central part of the A-band not overlapped by thin filaments in a resting state.

 

 

Smooth Muscles 

  • Visceral muscles also known as smooth muscles are situated within the inner walls of hollow visceral organs such as the alimentary canal (digestive tract) and reproductive tract. 
  • Appearance: Unlike skeletal muscles, visceral muscles lack striations and appear smooth under a microscope. Hence, they are referred to as smooth muscles or nonstriated muscles. 
  • Involuntary Control: The activities of visceral muscles are not under voluntary control of the nervous system. Instead, they are regulated involuntarily by automatic processes within the body. 
  • Functions:

- Visceral muscles assist in the movement of substances, such as food through the digestive tract and gametes through the reproductive tract.

 

Cardiac Muscles

  • Cardiac muscles form the muscular tissue of the heart, enabling it to contract and pump blood throughout the body. 
  • Structure: Cardiac muscle cells assemble in a branching pattern, creating a network that constitutes the cardiac muscle tissue. 
  • Appearance: Similar to skeletal muscles, cardiac muscles exhibit a striated appearance when viewed under a microscope. 
  • Involuntary Nature: Contrary to skeletal muscles, the activities of cardiac muscles are involuntary. They are regulated by specialized cells within the heart itself and are not directly controlled by the nervous system. 
  • Function:

- Pumping Action: Cardiac muscles contract rhythmically to pump blood from the heart's chambers (atria and ventricles) into the arteries, supplying oxygen and nutrients to the body's tissues and organs. 

- Maintaining Circulation: Through coordinated contractions, cardiac muscles ensure continuous circulation of blood throughout the cardiovascular system, maintaining blood pressure and distributing oxygenated blood to tissues and organs. 

- Electrical Conduction: In addition to mechanical contraction, cardiac muscles generate electrical impulses that regulate the heart's rhythm and coordinate its pumping action, ensuring efficient blood flow.

 

Structure of Contractile Proteins

  • Actin Filaments (Thin Filaments):

- Each actin filament consists of two filamentous (F) actins helically wound to each other. 

- Filamentous actin (F-actin) is a polymer of monomeric globular (G) actins. 

- Tropomyosin proteins run parallel to actin filaments, regulating access to the active binding sites. 

- Troponin, a complex protein, is distributed regularly along tropomyosin, with a subunit masking the active binding sites for myosin in the resting state. 

 

 

 

  • Myosin Filaments (Thick Filaments):

- Myosin filaments are polymerized proteins composed of many monomeric proteins called meromyosins. 

- Meromyosins consist of a globular head with a short arm (heavy meromyosin - HMM) and a tail (light meromyosin - LMM). 

- The globular head, also known as the cross arm, projects outward at regular intervals and angles from the surface of the myosin filament. 

- The globular head contains an active ATPase enzyme and binding sites for ATP, as well as active sites for binding to actin during muscle contraction.

 

 

 

 

Mechanism of Muscle Contraction 

  •  Muscle contraction is a vital process enabling movement and essential physiological functions in the body. 
  • Understanding the intricate mechanism of muscle contraction offers insights into how muscles generate force and motion. 
  • The sliding filament theory serves as the foundational framework for elucidating the sequence of events underlying muscle contraction. 
  • Thin and Thick Filaments:

- Thin filaments are made of actin protein.

- Thick filaments are made of myosin protein. 

  • I Bands and H Zones:

- I bands contain only thin filaments and appear lighter under a microscope.

- H zones contain only thick filaments and appear darker under a microscope.

- During contraction, I bands and H zones change in size due to the sliding of filaments. 

  • Neural signals originating from the central nervous system play a pivotal role in initiating muscle contraction. 
  • The interaction between contractile proteins within muscle fibers orchestrates the process of muscle contraction. 
  • Calcium ion release, ATP hydrolysis, and molecular interactions between actin and myosin filaments are key components driving muscle contraction. 
  • Sarcomere shortening, resulting from the sliding of actin and myosin filaments, is the physical manifestation of muscle contraction. 
  •  Understanding the detailed steps involved in the mechanism of muscle contraction enhances comprehension of muscle physiology and regulation.

 

 

 

 

1. Initiation of Contraction:

  • Neural Signaling:

- Muscle contraction begins with a signal from the central nervous system (CNS) transmitted via motor neurons. 

- Motor neurons, originating from the spinal cord, extend to muscle fibers, forming neuromuscular junctions. 

- At the neuromuscular junction, the motor neuron releases a neurotransmitter called acetylcholine (ACh) into the synaptic cleft. 

  • Action Potential Generation:

- Acetylcholine binds to receptors on the sarcolemma (cell membrane) of the muscle fiber. 

- This binding triggers an action potential, an electrical impulse, that travels along the sarcolemma and into the T-tubules (transverse tubules) of the muscle fiber. 

  • Calcium Ion Release:

- The action potential propagates along the T-tubules, reaching the sarcoplasmic reticulum (SR), a specialized organelle within the muscle fiber that stores calcium ions (Ca2+). 

- The action potential stimulates the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm, the cytoplasm of the muscle fiber. 

2. Calcium Ion Binding and Cross-Bridge Formation:

  • Calcium Ion Binding:

- Upon release from the sarcoplasmic reticulum, calcium ions (Ca2+) bind to troponin molecules located on the actin filaments within the muscle fiber. 

- Troponin is a regulatory protein complex composed of three subunits: TnC (calcium-binding subunit), TnI (inhibitory subunit), and TnT (tropomyosin-binding subunit). 

- Calcium ions bind specifically to the TnC subunit of troponin, causing a conformational change in the troponin-tropomyosin complex. 

  • Tropomyosin Positioning:

- As calcium ions bind to troponin, the troponin-tropomyosin complex undergoes a structural alteration. 

- This change in conformation causes tropomyosin to shift away from its blocking position, exposing the active binding sites on the actin filaments. 

  • Cross-Bridge Formation:

- With the active sites on the actin filaments exposed, myosin heads (from the thick filaments) bind to these sites, forming cross-bridges between actin and myosin. 

- The myosin heads contain ATP-binding sites and act as ATPase enzymes, capable of hydrolyzing ATP into ADP and inorganic phosphate (Pi). 

  • Power Stroke Initiation:

- Upon ATP hydrolysis, the myosin heads undergo a conformational change, extending and pivoting to interact with the actin filaments. 

- This conformational change, known as the power stroke, generates force and causes the actin filaments to slide relative to the myosin filaments, resulting in muscle contraction. 

  • Release and Rebinding:

- After the power stroke, ADP and Pi are released from the myosin heads, which remain attached to the actin filaments in a low-energy state. 

- To detach from the actin filaments, a new molecule of ATP binds to the myosin heads, leading to their dissociation from actin. 

- The hydrolysis of ATP to ADP and Pi provides the energy necessary for the myosin heads to reset and rebind to new active sites on the actin filaments, initiating another cycle of cross-bridge formation and contraction. 

3. Relaxation and Reattachment:

  • Removal of Calcium Ions:

- After muscle contraction, the sarcoplasmic reticulum actively pumps calcium ions (Ca2+) back into its storage vesicles, reducing the cytosolic calcium concentration. 

- This removal of calcium ions from the cytosol is crucial for muscle relaxation, as it allows for the dissociation of calcium ions from troponin, restoring the inhibitory conformation of the troponin-tropomyosin complex. 

  • Tropomyosin Repositioning:

- With the decrease in calcium ion concentration, tropomyosin returns to its original position, covering the active binding sites on the actin filaments. 

- This repositioning of tropomyosin prevents further cross-bridge formation between actin and myosin, effectively inhibiting muscle contraction. 

  • Reattachment of Myosin Heads:

- As calcium ions are sequestered back into the sarcoplasmic reticulum and tropomyosin blocks the active sites on actin, the myosin heads detach from the actin filaments. 

- In the absence of calcium ions and ATP, myosin heads return to their low-energy state and await reattachment to actin during the next contraction cycle. 

  • Energy Restoration:

- ATP, generated through various metabolic pathways, becomes available to the muscle fibers during relaxation. 

- This ATP is utilized for the re-energization of the myosin heads, preparing them for subsequent cross-bridge formation and contraction. 

  • Sarcomere Length Restoration:

- As relaxation occurs, the sarcomeres within the muscle fiber return to their original length due to the elastic recoil of titin proteins and the stretching of surrounding connective tissues. 

- This process restores the muscle to its resting state, ready to respond to new neural stimuli and initiate another contraction cycle. 

  • Metabolic Waste Removal:

- During relaxation, metabolic by-products such as lactic acid are removed from the muscle tissue through the circulatory system. 

- Oxygen replenishment and waste removal help maintain the metabolic balance within the muscle fibers and prevent fatigue during prolonged muscle activity.

 

 

 

 

Joints

  • Joints are crucial for facilitating movement in the skeletal system, enabling various types of bodily motions. 
  • They serve as points of connection between bones or between bones and cartilages, allowing for flexibility and mobility. 
  • Muscular forces are transmitted through joints, where they act as pivot points for movement. 
  • The mobility of joints varies depending on their structural composition and function.

 

Classification of Joints:

  • Fibrous Joints:

- Fibrous joints are immovable and characterized by dense fibrous connective tissue. 

- Examples include the sutures of the skull, where flat bones fuse together to form the cranium. 

 

 

 

  • Cartilaginous Joints:

- Cartilaginous joints are connected by cartilage and permit limited movement. 

- An example is the joint between adjacent vertebrae in the vertebral column, allowing for slight flexion and extension. 

  • Synovial Joints:

- Synovial joints feature a fluid-filled synovial cavity between articulating surfaces, allowing for extensive movement. 

- These joints facilitate locomotion and various other bodily movements.

 

 

 

 

Types of Synovial Joints:

  • Ball and Socket Joint:

- Found between the humerus and the pectoral girdle, enabling a wide range of movements in multiple directions. 

  • Hinge Joint:

- Examples include the knee joint, allowing movement primarily in one plane, like bending and straightening. 

  • Pivot Joint:

- Located between the atlas and axis vertebrae, allowing rotational movement, such as the rotation of the head. 

  • Gliding Joint:

- Present between the carpals of the wrist, facilitating smooth sliding movements in various directions. 

  • Saddle Joint:

- Found between the carpal and metacarpal bones of the thumb, allowing for a wide range of movements, including flexion, extension, abduction, and adduction.

 

Disorder of Muscular and Skeletal System 

Myasthenia Gravis:

  • An autoimmune disorder affecting the neuromuscular junction, resulting in fatigue, weakness, and paralysis of skeletal muscles. 
  • It occurs due to the body's immune system attacking its own neuromuscular junction, leading to impaired communication between nerves and muscles.

 

Muscular Dystrophy:

  • A progressive degenerative disorder of skeletal muscles, often caused by genetic mutations. 
  • It leads to gradual weakening and wasting of muscle tissues over time, affecting mobility and muscle function.

 

Tetany:

  • Tetany refers to rapid and involuntary muscle spasms or contractions caused by abnormally low levels of calcium (Ca++) in the body fluids. 
  • This condition can result in muscle cramps, twitching, and spasms, affecting various parts of the body.

 

Arthritis:

  • Arthritis is the inflammation of joints, resulting in pain, swelling, and stiffness. 
  • It can be caused by various factors such as autoimmune reactions, wear and tear, or infections, leading to joint damage and discomfort.

 

Osteoporosis:

  •  Osteoporosis is an age-related disorder characterized by decreased bone mass and increased susceptibility to fractures.
  •  It often occurs due to hormonal changes, particularly decreased estrogen levels in postmenopausal women, leading to bone weakening and increased risk of fractures.

 

Gout:

  • Gout is a form of arthritis characterized by inflammation of joints due to the accumulation of uric acid crystals. 
  • It typically affects the big toe but can also involve other joints, causing pain, swelling, and redness.