Basic Biology
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.
- Concentric: Muscle shortens.
- 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
Self Assessment
Which of the following structures shortens during skeletal muscle contraction?
During contraction, the I band and H zone shorten due to increased overlap between actin and myosin filaments, while the A band remains constant in length.
Which protein binds calcium to initiate the sliding filament mechanism?
Calcium binds to troponin, which triggers a conformational change that moves tropomyosin, exposing actin binding sites for myosin attachment.
Which muscle fiber type is most resistant to fatigue?
Type I fibers are slow-twitch, rich in mitochondria and myoglobin, and rely on aerobic metabolism, making them highly resistant to fatigue.
Which energy system primarily supports a 30-second sprint?
Anaerobic glycolysis is the dominant energy system during activities lasting 20–120 seconds, providing rapid ATP through glucose breakdown without oxygen.
What is the typical resting membrane potential of a neuron?
The neuron maintains a resting potential of about -70 mV due to selective ion permeability and active transport by the sodium-potassium ATPase pump.
Which of the following is first recruited during voluntary muscle contraction?
According to Henneman’s size principle, smaller Type I motor units are recruited first because they require less neural input and provide fine, fatigue-resistant control.
Which of the following best describes neuropraxia?
Neuropraxia involves a temporary conduction block due to myelin damage, but the axon remains intact, allowing full recovery within weeks.
Which EMG finding is most consistent with axonotmesis?
Axonotmesis causes denervation of muscle fibers, leading to fibrillations and positive sharp waves on EMG due to spontaneous activity in affected fibers.