Skeletal Muscle
Structural Classification
Skeletal muscles are structurally classified as pennate or non-pennate.
- Pennate muscles are further divided into:
- Unipennate
- Bipennate
- Multipennate
This classification is based on the orientation of muscle fibers relative to the tendon.
Internal Structure
Under microscopy, skeletal muscle shows several landmarks:
- Z line
- M line
- H zone
- A band
- I band
These structures are essential for understanding the contractile mechanics of muscle fibers. In electron microscopy, alternating I and A bands are visible.
- Thick filaments contain myosin.
- Thin filaments contain actin.
Together, myosin and actin form the basis of the sliding filament mechanism of muscle contraction.
Muscle Contraction
According to the Sliding Filament Theory, contraction begins when a nerve impulse reaches the neuromuscular junction.
- Acetylcholine (ACh) is released into the synapse.
- It binds to receptors on the muscle membrane.
- This triggers calcium release from the sarcoplasmic reticulum.
- Calcium binds to troponin, shifting tropomyosin and exposing actin binding sites.
- Myosin heads attach to actin, forming cross-bridges using ATP.
During contraction: - The sarcomere shortens.
- The A band remains constant.
- The I band and H zone narrow as actin and myosin overlap.
Energetics of Muscle
ATP is the primary energy source for muscle contraction. The body generates ATP through three energy systems:
| System | Duration | Description |
|---|---|---|
| ATP-CP (Creatine Phosphate) | Up to 20 seconds | Immediate energy source |
| Anaerobic (Lactic Acid) System | 20–120 seconds | Moderate duration, no oxygen required |
| Aerobic System | Beyond 2 minutes | Sustained activity using oxygen |
Muscle Fiber Types
Muscle fibers differ in contraction speed, fatigue resistance, and metabolism.
| Fiber Type | Color | Speed | Fatigue Resistance | Metabolism | Motor Unit Size |
|---|---|---|---|---|---|
| Type I (Slow-twitch, Red) | Red | Slow | High | Aerobic | Small |
| Type IIA (Fast-twitch Oxidative) | Pink | Fast | Moderate | Aerobic & Anaerobic | Medium |
| Type IIB (Fast Glycolytic) | White | Very Fast | Low | Anaerobic | Large |
Recruitment order: Type II fibers are activated first during high-intensity effort, followed by Type I fibers for endurance.
Muscle Contractions
Muscle contractions are classified as:
- Isometric – Tension without change in length.
- Isotonic – Change in length:
- Concentric: Muscle shortens.
- Eccentric: Muscle lengthens.
- Isokinetic – Constant speed contraction (using special equipment).
Peripheral Nerve Structure and Function
Peripheral nerves contain afferent (sensory), efferent (motor), and sympathetic fibers.
Structure
- Myelinated and unmyelinated fibers are present.
- Nodes of Ranvier facilitate saltatory conduction and nutrient exchange.
Fiber Classification
| Fiber Type | Diameter | Myelination | Speed | Function |
|---|---|---|---|---|
| Type A | Large | Heavy | Fast | Touch, proprioception |
| Type B | Medium | Intermediate | Moderate | Autonomic |
| Type C | Small | None | Slow | Pain |
Resting and Action Potentials
- Resting Membrane Potential: ~ -70 mV
- Maintained by ionic gradients and active transport.
Action Potential Sequence
- Depolarization: Sodium channels open → Na⁺ influx.
- Threshold: Around -55 mV.
- Repolarization: Potassium channels open → K⁺ efflux.
- After-hyperpolarization: Membrane becomes more negative before returning to baseline.
Nerve Injuries
Nerve injuries occur due to compression, stretch, or laceration.
Classification
| Type | Description | Recovery |
|---|---|---|
| Neuropraxia (Sunderland I) | Temporary conduction block | Full recovery |
| Axonotmesis (Sunderland II) | Axonal disruption, sheath intact | Good recovery |
| Sunderland III–V | Progressive sheath damage | Poor recovery |
| Neurotmesis (Sunderland V) | Complete transection | Poor prognosis |
Degeneration and Regeneration
Wallerian Degeneration
- Begins within 24 hours after injury.
- Distal axon and myelin break down.
- Macrophages clear debris.
Regeneration
- Schwann cells proliferate and form Bands of Büngner.
- Axonal sprouts grow at ~1 mm/day.
- Success depends on alignment and trophic support.
Delayed reinnervation (after 12–18 months) leads to irreversible muscle atrophy.
Nerve Conduction Studies (NCS)
NCS evaluate peripheral nerve function by measuring:
- Sensory Nerve Action Potential (SNAP)
- Compound Motor Action Potential (CMAP)
| Parameter | Normal Value | Interpretation |
|---|---|---|
| Velocity (Upper limb) | > 45 m/s | ↓ velocity → Demyelination |
| Velocity (Lower limb) | > 35 m/s | ↑ latency → Demyelination |
| Latency (Wrist) | < 3.5 ms | ↓ amplitude → Axonal loss |
Electromyography (EMG)
Used to study electrical activity of skeletal muscle, typically 3 weeks post-injury.
Key Findings
- Positive sharp waves
- Fibrillations
- Fasciculations
- Myokymic discharges
Fibrillations indicate axonotmesis or neurotmesis. Absence of fibrillations suggests neuropraxia.
Tendons and Ligaments
Tendons
- Connect muscle to bone.
- May be sheathed (with vincula) or covered by paratenon.
- Composed mainly of Type I collagen, with some Type III collagen and elastin.
- Fibroblasts are the main cells.
Ligaments
- Connect bone to bone.
- Covered by epiligament.
- Contain more elastin and less uniform fiber orientation than tendons.
Biomechanical Properties
| Tissue | Tolerable Strain Before Failure |
|---|---|
| Tendons & Ligaments | 5–10% |
| Bone | 1–4% |
Their elasticity supports dynamic musculoskeletal movement.
Healing of Tendons
Tendon healing occurs in three overlapping phases:
| Phase | Duration | Key Events |
|---|---|---|
| Inflammation | 0–5 days | Hematoma, inflammatory cell recruitment |
| Proliferation | 3 days – 6 weeks | Fibroblast proliferation, collagen bridge formation |
| Remodeling | 6 weeks – 1 year | Collagen maturation and alignment |
After surgical repair: - Weakest point: 1 week
- Significant strength gain: 4 weeks
- Near-normal strength: 6 months