Skeletal Muscles – Anatomy and Physiology
Introduction
The muscular system is responsible for movement, posture, and body heat production.
Muscles are made of specialized cells that can contract and relax to produce motion.
Among the three types of muscles (skeletal, smooth, and cardiac), skeletal muscles are the ones attached to bones and responsible for voluntary body movements.
Definition
Skeletal muscles are voluntary, striated muscles that are attached to bones by tendons and help in body movements under conscious control.
Original Notes:
Characteristics of Skeletal Muscles:
- Striated: Show alternate light and dark bands under the microscope.
- Voluntary: Controlled by the somatic nervous system.
- Multinucleated: Each muscle fiber has many nuclei.
- Contractile: Can shorten and generate force.
- Extensible: Can be stretched without damage.
- Elastic: Can return to original shape after stretching.
Structure of Skeletal Muscle
Each skeletal muscle is composed of bundles of long, cylindrical cells called muscle fibers.
- Epimysium: Outer covering of the whole muscle.
- Perimysium: Surrounds each bundle of muscle fibers (fascicle).
- Endomysium: Covers each individual muscle fiber.
Each muscle fiber contains myofibrils, which are made up of smaller units called sarcomeres — the structural and functional units of contraction.
Structure of a Sarcomere
- The sarcomere is the region between two Z-lines in a myofibril.
- It contains two main protein filaments:
- Actin (Thin Filament)
- Myosin (Thick Filament)
- The arrangement of these filaments gives skeletal muscle its striated appearance.
- A Band: Dark area containing myosin.
- I Band: Light area containing actin.
- H Zone: Central part of A band with only myosin.
- Z Line: Boundary between sarcomeres.
Neuromuscular Junction (NMJ)
It is the connection between a motor neuron and a muscle fiber.
- The neuron releases a chemical messenger called acetylcholine (ACh).
- ACh binds to receptors on the muscle membrane, triggering an electrical impulse that leads to muscle contraction.
Mechanism of Muscle Contraction (Sliding Filament Theory)
Muscle contraction occurs when thin and thick filaments slide over each other, shortening the sarcomere.
Steps:
- Nerve Impulse: A motor neuron sends an electrical signal to the muscle.
- Release of Calcium Ions: The sarcoplasmic reticulum releases calcium into the cytoplasm.
- Cross-Bridge Formation: Calcium allows myosin heads to attach to actin filaments.
- Power Stroke: Myosin heads pull actin filaments inward, shortening the sarcomere.
- Detachment: ATP binds to myosin, causing it to release actin.
- Relaxation: When stimulation stops, calcium is reabsorbed, and the muscle relaxes.
Energy for Muscle Contraction
Muscles require ATP (Adenosine Triphosphate) as an energy source.
ATP is regenerated by:
- Creatine Phosphate breakdown
- Aerobic respiration (with oxygen)
- Anaerobic respiration (without oxygen, producing lactic acid)
Functions of Skeletal Muscles
- Movement: Enable body movements such as walking, running, and lifting.
- Posture Maintenance: Keep the body upright and stable.
- Heat Production: Generate heat during contraction to maintain body temperature.
- Joint Stability: Support and protect joints by maintaining tension.
- Protection: Shield internal organs by forming muscular walls.
Types of Skeletal Muscle Fibers
- Slow-Twitch (Type I):
- Contract slowly but resist fatigue.
- Rich in mitochondria and myoglobin.
- Used in endurance activities (e.g., walking, long-distance running).
- Fast-Twitch (Type II):
- Contract quickly but fatigue easily.
- Produce strong, short bursts of power (e.g., sprinting, weightlifting).
Disorders of Skeletal Muscles
- Muscular Dystrophy: Genetic disorder causing progressive muscle weakness.
- Myasthenia Gravis: Autoimmune disorder affecting communication between nerves and muscles.
- Cramps: Sudden involuntary contractions due to dehydration or electrolyte imbalance.
- Atrophy: Wasting of muscle tissue due to inactivity.
- Strain: Overstretching or tearing of muscle fibers.
