SKELETAL MUSCLES
The contractile tissue derived from the mesodermal (middle) layer of germ cells during embryonic development is known as a muscle. Muscles are basically of three types: 1) Skeletal muscles (voluntary), 2) Smooth muscles (involuntary), and 3) Cardiac muscles (involuntary).
Skeletal muscles are a type of striated muscle tissues under the control of the somatic nervous system. A bundle of collagen fibres, which act as a link between bone and skeletal muscles, is known as a tendon. Individual components of a skeletal muscle are known as muscle fibres, which are formed by the fusion of developmental myoblasts. Long, cylindrical, and multinucleated cells made up of actin and myosin myofibrils are known as myofibres.
HISTOLOGY OF SKELETAL MUSCLE
Muscle fibres are the structural unit of skeletal muscle. They are long cylindrical cells with multiple nuclei (thickness: 10-100 µm; length: 1-3 cm).
Structural Organisation
- Myofibrils: Delicate contractile strands within muscle fibre (diameter ~1 µm)
- Sarcolemma: Plasma membrane covering myofibrils
- Sarcoplasm: Cytoplasm of muscle fibre
- Sarcomeres: Contractile units; repeated banding pattern along myofibril
- Filaments: Thin (actin), thick (myosin), elastin (titin), and inelastic (nebulin)
- Fascicles: Bundles of muscle fibres
- Endomysium: Connective tissue around individual muscle fibre
- Perimysium: Connective tissue wrapping fascicles
- Epimysium: External sheath wrapping the entire muscle
PHYSIOLOGY OF MUSCLE CONTRACTION
Muscle contraction (muscle twitch) involves change in muscle fibre length. The central nervous system (brain and spinal cord) controls the process – voluntary contractions by brain, involuntary reflexes by spinal cord.
Steps of Muscle Contraction
- Stimulus generated in CNS (voluntary from brain or reflex from spinal cord)
- Motor neuron in ventral horn activated; action potential passes to motor end plate (ACh release)
- Action potential passes along muscle fibre surface in both directions
- Action potential spreads inside muscle fibre via transverse tubules
- Ca²⁺ ions released from sarcoplasmic reticulum
- Ca²⁺ causes movement of troponin and tropomyosin on thin filament
- Myosin heads move along thin filament to generate contraction force
Sliding Filament Theory
According to the sliding filament theory, skeletal muscle contraction causes shortening of muscle fibres due to the sliding of thick (myosin) and thin (actin) filaments over one another.
Bands and Lines in Sarcomere
- I-band: Only thin (actin) filaments
- A-band: Myosin filaments overlapping with actin filaments
- H-band: Only myosin filaments (no overlapping with actin)
- Z-line: Actin filaments between two neighbouring sarcomeres
- M-line: Connections between myosin filaments
Steps of Sliding Filament Theory
- Myosin heads have ATP-binding sites and ATPase enzyme
- Conversion of ATP to ADP releases energy and inorganic phosphate
- Energy causes movement of myosin cross-bridges; thin filaments slide inward (Power Stroke)
- Freshly synthesised ATP replaces ADP in myosin molecule
- If Ca²⁺ taken back into SR → cross-bridges detach → relaxation
- If Ca²⁺ still present → process repeats → further contraction
All-or-None Principle: A skeletal muscle fibre will either show maximum contraction or no contraction at all for any stimulus. Threshold stimulus → maximum response; sub-threshold stimulus → no response.
DISORDERS OF SKELETAL MUSCLES
Musculoskeletal disorders (MSDs) affect muscles, bones, and joints of the body.
Types of MSDs
- Tendinitis: Inflammation of tendons; causes discomfort and tenderness
- Carpal Tunnel Syndrome: Compression of median nerve in the hand
- Osteoarthritis: Chronic joint problem; cartilage breakdown causes bones to rub
- Rheumatoid Arthritis (RA): Autoimmune disease causing joint pain and damage
- Fibromyalgia: Chronic condition; frequently misdiagnosed
- Bone Fractures: Painful injuries commonly caused by sports, accidents, or falls
- Myopathies: Diseases affecting skeletal muscle structure, metabolism, or channel function
Symptoms of MSDs
- Persistent pain
- Rigid joints
- Swelling
- Dull aches
- Can affect neck, shoulders, wrists, back, hips, legs, knees, and feet
Causes of MSDs
- Constantly sitting in the same position at a computer
- Performing repetitive motions
- Exercising using high weights
- Maintaining bad posture at work
Diagnosis and Treatment of MSDs
- Diagnosis: Reflex tests, imaging tests (X-rays, MRI), blood testing for rheumatic illnesses
- Treatment: Exercises, over-the-counter pain relievers (ibuprofen, acetaminophen), prescription drugs, physical therapy, occupational therapy
