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DrX Whiz Niraj
Skeletal System & Skeletal Muscle
Comprehensive notes on Bones, Axial vs Appendicular Skeleton, Neuromuscular Junction, and Physiology of Muscle Contraction.
The adult human skeleton consists of 206 bones. These bones are grouped into two principal divisions: the Axial Skeleton (80 bones) and the Appendicular Skeleton (126 bones).
| Division | Main Parts | Number of Bones |
|---|---|---|
| Axial Skeleton (Central axis of the body) |
|
80 Bones |
| Appendicular Skeleton (Limbs & attachments) |
|
126 Bones |
Types of Bones:
- Long Bones: Greater length than width (e.g., Femur, Humerus).
- Short Bones: Cube-shaped, nearly equal in length and width (e.g., Carpals in wrist, Tarsals in ankle).
- Flat Bones: Thin, provide protection and large area for muscle attachment (e.g., Sternum, Scapula, Cranial bones).
- Irregular Bones: Complex shapes (e.g., Vertebrae, Facial bones).
- Sesamoid Bones: Develop within tendons where there is high friction (e.g., Patella / Kneecap).
A skeletal muscle is a complex organ made up of thousands of individual muscle cells called Muscle Fibers. Connective tissue surrounds and protects these fibers.
Connective Tissue Coverings
- Epimysium: Outermost layer encircling the entire skeletal muscle.
- Perimysium: Surrounds groups of 10-100 muscle fibers, separating them into bundles called Fascicles.
- Endomysium: Separates individual muscle fibers from one another inside the fascicle.
Microscopic Anatomy
- Sarcolemma: The plasma membrane of a muscle cell.
- Sarcoplasm: The cytoplasm containing large amounts of glycogen and myoglobin.
- Myofibrils: Contractile organelles inside the muscle fiber. Contains thin (Actin) and thick (Myosin) filaments.
- Sarcoplasmic Reticulum (SR): Stores Calcium ions (Ca²⁺) necessary for muscle contraction.
Skeletal muscles do not contract on their own; they need a signal from the nervous system. The synapse (connection) between a somatic motor neuron and a skeletal muscle fiber is called the Neuromuscular Junction (NMJ).
Steps at the NMJ:
- 1. Release of ACh: An action potential arrives at the synaptic end bulb. Calcium channels open, causing synaptic vesicles to release Acetylcholine (ACh) into the synaptic cleft.
- 2. Activation of Receptors: ACh binds to its receptors on the motor end plate of the muscle fiber. This opens sodium (Na+) channels.
- 3. Muscle Action Potential: Inflow of Na+ makes the inside of the muscle fiber positively charged, generating a muscle action potential that travels along the sarcolemma.
- 4. Termination: An enzyme called Acetylcholinesterase (AChE) breaks down ACh, ending the muscle action potential.
Muscle contraction is best explained by the Sliding Filament Mechanism. According to this model, skeletal muscle shortens during contraction because the thick (Myosin) and thin (Actin) filaments slide past one another.
The Contraction Cycle (4 Steps):
When the muscle action potential reaches the Sarcoplasmic Reticulum, it releases Calcium ions (Ca²⁺) into the sarcoplasm. Ca²⁺ binds to troponin, which uncovers the myosin-binding sites on actin.
- 1. ATP Hydrolysis: Myosin head hydrolyzes ATP and becomes energized.
- 2. Cross-bridge Formation: Energized myosin head attaches to the actin filament.
- 3. Power Stroke: The cross-bridge rotates, sliding the thin filament toward the center of the sarcomere.
- 4. Detachment: Another ATP binds to the myosin head, causing it to detach from actin. The cycle repeats as long as ATP and Ca²⁺ are available.
Clinical Correlation: Myasthenia Gravis
Myasthenia Gravis is an autoimmune disease that causes chronic, progressive damage of the neuromuscular junction. The immune system inappropriately produces antibodies that bind to and block some ACh receptors, thereby decreasing the number of functional ACh receptors. This leads to severe muscle weakness and fatigue.