SKELETAL MUSCLES: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future healthcare professionals!
The contractile tissue that develops from the mesodermal layer (middle layer) of germ cells during embryonic life is known as muscle. Muscles are the organs that help in movement, posture, and various involuntary functions of the body. There are three main types of muscles in the human body: Skeletal muscles (voluntary), Smooth muscles (involuntary), and Cardiac muscles (involuntary).
Dpharmguru’s exam insights:
In my years of teaching muscle physiology, I have observed that students often confuse the three types of muscles. Remember: Skeletal = Voluntary and Striated, Smooth = Involuntary and Non-striated, Cardiac = Involuntary and Striated (but only in the heart). This is a classic exam question!
HISTOLOGY OF SKELETAL MUSCLE
Skeletal muscles are also called striated muscles because they show alternating light and dark bands under the microscope. They are voluntary in nature — meaning their movement is consciously controlled. A tendon is a bundle of strong collagen fibres that connects skeletal muscles to bones, helping in the transmission of muscle contraction force.
Each skeletal muscle is made up of many muscle fibres — the smallest functional units responsible for contraction. These fibres are long, cylindrical, multinucleated cells formed by the fusion of embryonic cells known as myoblasts. Inside each fibre are myofibrils composed of actin (thin) and myosin (thick) filaments that cause muscle contraction.
Structure of a Muscle Fibre
- A muscle fibre is the structural and functional unit of skeletal muscle.
- Each muscle fibre is a long, cylindrical cell containing several nuclei just beneath the sarcolemma (the plasma membrane of the muscle fibre). Length: About 1–3 cm. Thickness: About 10–100 μm.
- Inside each muscle fibre, there are numerous myofibrils — the actual contractile elements of the muscle.
- These myofibrils are arranged parallel to each other in the cytoplasm (called sarcoplasm) and run throughout the length of the cell.
- Each myofibril is made up of repeating units called sarcomeres, which are the basic functional units of contraction.
- The sarcomere shows a distinct banding pattern (alternating dark and light bands), giving the muscle its striated appearance.
Connective Tissue Coverings of Skeletal Muscle
- Endomysium: Surrounds each individual muscle fibre.
- Perimysium: Surrounds a group of muscle fibres called a fascicle.
- Epimysium: The outermost sheath that encloses the entire muscle.
- Fascia: A connective tissue layer that covers and separates muscles from surrounding structures.
Dpharmguru’s exam insights:
Remember the connective tissue layers of skeletal muscle from inside out: Endomysium (around each fibre), Perimysium (around fascicle), Epimysium (around whole muscle). This is a commonly tested concept in histology exams!
PHYSIOLOGY OF MUSCLE CONTRACTION
Muscle contraction (also known as a twitch) is a physiological process where the muscle fibre shortens to produce movement. It is controlled by the central nervous system (CNS) — the brain (for voluntary actions) and the spinal cord (for reflex actions).
Steps of Muscle Contraction
- Initiation of impulse: A nerve impulse (action potential) is generated in the CNS — either voluntarily from the brain or as a reflex from the spinal cord.
- Activation of motor neuron: The impulse travels through a motor neuron to the muscle fibre via the neuromuscular junction (motor end plate).
- Release of Acetylcholine (ACh): At the neuromuscular junction, the neurotransmitter ACh is released into the synaptic cleft and binds to receptors on the muscle fibre membrane, producing an action potential on the sarcolemma.
- Transmission of action potential: The impulse travels along the sarcolemma and enters the muscle fibre through transverse tubules (T-tubules).
- Calcium ion release: The sarcoplasmic reticulum (SR) releases stored Ca²⁺ ions into the sarcoplasm.
- Exposure of binding sites: The calcium ions cause troponin and tropomyosin to move away from the binding sites on actin, allowing myosin heads to attach to actin.
- Cross-bridge formation: The myosin heads bind to the actin filaments, forming cross-bridges.
- Power stroke: Using energy from ATP hydrolysis, the myosin heads pivot and pull the actin filaments toward the centre of the sarcomere, causing contraction.
- Detachment and relaxation: When new ATP binds to the myosin head, it detaches from actin. If calcium ions are pumped back into the SR, the muscle relaxes.
Sliding Filament Theory
The Sliding Filament Theory explains how muscles contract at the molecular level. According to this theory: The thin (actin) and thick (myosin) filaments slide over each other. The sarcomere shortens, but the filaments themselves do not change length. This sliding movement causes the entire muscle fibre to shorten, producing contraction.
Bands and Lines in the Sarcomere
- I-band: Contains only actin filaments (light region).
- A-band: Contains entire length of myosin filaments; overlapping region of actin and myosin.
- H-zone: Central region of A-band containing only myosin (no overlap).
- Z-line: Boundary between two sarcomeres; holds actin filaments together.
- M-line: Middle of sarcomere; connects myosin filaments via proteins like myomesin.
Dpharmguru’s exam insights:
During muscle contraction, the I-band and H-zone shorten, but the A-band remains the same length. This is evidence for the sliding filament theory. This is a frequently tested concept in physiology exams!
Steps in the Sliding Filament Mechanism
- ATP binds to myosin head.
- ATP is hydrolysed to ADP and inorganic phosphate, releasing energy.
- Myosin head binds to actin, forming a cross-bridge.
- The power stroke occurs — myosin pulls actin filaments inward.
- New ATP binds, detaching myosin from actin.
- If Ca²⁺ is still available, the cycle repeats.
- When Ca²⁺ is pumped back into SR, the muscle relaxes.
All-or-None Law
A single skeletal muscle fibre follows the all-or-none principle — It either contracts completely when stimulated above threshold, or does not contract at all if the stimulus is below threshold.
DISORDERS OF SKELETAL MUSCLES
Musculoskeletal disorders (MSDs) affect the muscles, bones, tendons, and joints of the body.
Types of MSDs
- Tendinitis: Inflammation of the tendons due to overuse or injury, causing pain and limited movement.
- Carpal Tunnel Syndrome: Compression of the median nerve in the wrist leading to pain, tingling, and numbness in the hand.
- Osteoarthritis: Degenerative joint disease caused by breakdown of cartilage, leading to pain and stiffness.
- Rheumatoid Arthritis (RA): An autoimmune disorder that causes inflammation and deformity in joints, often on both sides of the body.
- Fibromyalgia: Chronic muscle pain with fatigue and tenderness; often confused with other disorders due to lack of specific tests.
- Bone Fractures: Breaks or cracks in the bones caused by trauma or pressure. They require rest or surgical fixation depending on severity.
- Myopathies: Diseases affecting the muscle fibres directly, often causing weakness, cramps, or stiffness.
Symptoms of MSDs
- Persistent muscle or joint pain
- Stiffness and reduced movement
- Swelling and tenderness
- Dull aches or fatigue in affected areas
Commonly Affected Regions
- Neck, Shoulders, Wrists, Back, Hips, Legs, Knees, Feet
Causes of MSDs
- Repetitive or prolonged strain on muscles
- Poor posture or ergonomics (especially in office work)
- Lifting heavy weights incorrectly
- Trauma or overuse injuries
- Aging and degenerative changes
Diagnosis of MSDs
- X-rays or MRI scans help visualize bone and soft tissue damage.
- Blood tests may be used to detect autoimmune disorders (like RA).
- Reflex testing is done to assess nerve involvement.
Treatment of MSDs
- Rest and physiotherapy for mild cases.
- Pain relievers like ibuprofen or acetaminophen.
- Anti-inflammatory drugs for swelling and stiffness.
- Physical and occupational therapy to improve posture, strength, and flexibility.
- In severe cases, surgery or joint replacement may be needed.
Dpharmguru’s exam insights:
Skeletal muscles are essential for movement and posture. In exams, focus on the structure of the sarcomere, the sliding filament mechanism, and the steps of muscle contraction. Remember: Calcium ions are the key to muscle contraction — they expose the binding sites on actin. This is almost guaranteed to appear in exams!
REFERENCES AND FURTHER READING
- Tortora, G. J., & Derrickson, B. H. (2017). Principles of Anatomy and Physiology (15th ed.). John Wiley & Sons.
- Marieb, E. N., & Hoehn, K. (2019). Human Anatomy & Physiology (11th ed.). Pearson Education.
- Standring, S. (2020). Gray’s Anatomy: The Anatomical Basis of Clinical Practice (42nd ed.). Elsevier.
- Hall, J. E., & Guyton, A. C. (2020). Guyton and Hall Textbook of Medical Physiology (14th ed.). Elsevier.
- National Institutes of Health (NIH). (2022). Musculoskeletal System Resources. Retrieved from https://www.nih.gov.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.
written by:
Dr. N. Sujith Kumar
