Physiological Properties of Skeletal Muscle
All muscle cells share several important physiological properties — contractility, excitability, extensibility, and elasticity. These properties enable muscles to perform movement, maintain posture, and respond to stimuli effectively.
1. Contractility:
It is the ability of muscle cells to shorten forcefully. For example, when you flex your elbow, the biceps brachii contracts to reduce the joint angle. Muscles can only pull; they never push.
2. Excitability:
This refers to the ability of a muscle to respond to a stimulus from a motor neuron or hormone. When stimulated, muscle fibers generate electrical impulses (action potentials) leading to contraction.
3. Extensibility:
The capacity of a muscle to be stretched without being damaged. During elbow flexion, the extensor muscles must lengthen to permit movement. Lack of extensibility results in spasticity.
4. Elasticity:
The ability of the muscle to recoil to its original length after being stretched. This property helps muscles maintain tone and shape.
Disorders of Skeletal Muscle:
Osteoporosis:
A disease causing reduced bone mineral density, leading to fragile bones and higher fracture risk. It develops due to low peak bone mass, increased bone resorption, or reduced new bone formation.
Paget’s Disease:
A chronic disorder causing enlarged, misshapen bones due to abnormal bone remodeling. It leads to pain, deformities, and fractures, usually affecting specific bones.
Arthritis:
Inflammation of joints causing pain, swelling, and stiffness. There are over 100 types of arthritis, commonly involving joint inflammation and degeneration.
Tetany:
A condition resulting from low calcium levels (hypocalcemia), causing spasms in hands, feet, and larynx, along with overactive reflexes.
Muscular Dystrophies:
A group of inherited diseases causing progressive muscle degeneration. Different types vary by age of onset, rate of progression, and affected muscle groups.
Duchenne Muscular Dystrophy:
A sex-linked disorder appearing around 5 years of age. Weakness begins in lower limbs and progresses upward, often leading to respiratory or cardiac failure in adolescence.
Facio-Scapulo-Humeral Dystrophy:
Affects both sexes, starting in adolescence. Muscles of the face and shoulders are first involved, progressing slowly over time. Life expectancy remains normal.
Myotonic Dystrophy:
Begins in adult life, causing slow contraction and relaxation of muscles. Associated symptoms include cataracts, gonadal atrophy, cardiomyopathy, and glucose intolerance. It progresses without remission, often leading to cardiac or respiratory complications.
Myasthenia Gravis:
An autoimmune disorder more common in women aged 20–40. Antibodies block acetylcholine receptors at neuromuscular junctions, leading to progressive muscle weakness. Symptoms include drooping eyelids (ptosis), double vision, difficulty in chewing, swallowing, or speaking. Relapses can be triggered by infections, exertion, or pregnancy.
Rotator Cuff Injury:
Injury to the shoulder stabilizing muscles, resulting in pain and restricted shoulder movement. Recovery may take months or longer.
Electrical and Mechanical Properties of Skeletal Muscle:
Excitability:
- Ability to receive and respond to stimuli (electrical or chemical).
- Resting membrane potential (RMP): –90 mV.
- Action potentials are produced when threshold stimulus is reached.
- Duration: 2–4 ms; Refractory period: 1–3 ms; Conduction velocity: 3–5 m/s.
- Obeys the all-or-none law and has a refractory period.
Conductivity:
The propagation of action potentials along the sarcolemma in both directions from the stimulus point. Conductive velocity is 3–5 m/s, similar to unmyelinated nerve fibers.
Contractility:
Ability to shorten and produce tension when stimulated by an action potential. Muscle contraction types:
| Type | Muscle Length | Work Done | Examples |
|---|---|---|---|
| Isometric | No change | No external work (energy converted to heat) | Postural muscles, pushing a wall |
| Isotonic | Decreases (shortening) | Performs work (lifting or walking) | Flexing arm, leg movement |
Factors Affecting Force of Contraction:
- Strength of stimulus
- Frequency of stimulation
- Initial muscle length (Starling’s law)
- Load applied (preload and afterload)
- Temperature — increases force up to physiological limit
- Type of muscle — faster fibers generate higher force
Extensibility:
The muscle’s ability to be stretched without damage. Essential for smooth movements and prevention of injury.
Elasticity:
The ability to return to normal length after stretching, due to the presence of elastic components within muscle fibers. It contributes to muscle tone and passive tension.
Detailed Notes:
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