15.07 Muscle Structure and Contraction

Types of Muscle Tissue

1.Striated Muscle (Skeletal Muscle)

• Location: Attached to skeleton; responsible for voluntary movement.
• Appearance: Striated (striped) under a microscope due to regular arrangement of filaments.
• Control: Neurogenic—requires motor neurone impulses to contract.

2.Cardiac Muscle

• Location: Found exclusively in the heart.
• Appearance: Striated, with cells connected by intercalated discs that facilitate synchronized contraction.
• Control: Myogenic—contracts automatically without neural input.

3.Smooth Muscle

• Location: Found in walls of tubular organs (e.g., blood vessels, digestive tract).
• Appearance: Non-striated (smooth).
• Control: Generally neurogenic but can contract in response to stretch, as seen in arteries.

Structure of Striated Muscle

• Muscle Composition: Each muscle (e.g., biceps) consists of thousands of muscle fibres bundled together and connected to bones by tendons.
• Muscle Fibre Structure:
  • Syncytium: Muscle fibres are multinucleated, formed by fusion of cells.
  • Sarcolemma: Cell membrane surrounding each muscle fibre.
  • Sarcoplasm: Cytoplasm within the muscle fibre, containing mitochondria and myofibrils.
  • Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions, crucial for contraction.
  • T-tubules: Infoldings of the sarcolemma that conduct action potentials into the fibre, ensuring coordinated contraction.
• Myofibrils:
  • Composition: Each fibre contains many cylindrical myofibrils made up of thick (myosin) and thin (actin) filaments.
  • Striations: Alternating dark (A bands) and light (I bands) regions due to the arrangement of myosin and actin filaments.

Detailed Myofibril Structure

• Sarcomere: The basic contractile unit of a myofibril, spanning from one Z line to the next.
• Z Line: Anchors actin filaments and marks the boundary of each sarcomere.
• A Band: Dark band containing overlapping actin and myosin filaments.
• I Band: Light band with only actin filaments, no overlap with myosin.
• H Band: Lighter central area within the A band, containing only myosin filaments.
• M Line: Central line within the H band, where myosin filaments are anchored.

Structure and Function of Muscle Filaments

• Thick Filaments (Myosin):
  • Composition: Made up of myosin molecules, each with a fibrous tail and a globular head.
  • Arrangement: Myosin molecules are bundled with heads pointing away from the M line.
  • Myosin Heads: Function as ATPases, breaking down ATP to drive contraction.
• Thin Filaments (Actin):
  • Composition: Primarily actin, with regulatory proteins tropomyosin and troponin.
  • Actin: Globular proteins linked in a double helix structure, providing binding sites for myosin.
  • Tropomyosin: Twists around actin, blocking myosin binding sites in a relaxed muscle.
  • Troponin: Calcium-binding protein that, when bound to Ca²⁺, allows tropomyosin to move and expose myosin-binding sites on actin.

Muscle Contraction: The Sliding Filament Model

1.Muscle Relaxation:

• Tropomyosin and troponin block myosin-binding sites on actin, preventing myosin from attaching.

2.Stimulation and Calcium Release:

• An action potential reaches the neuromuscular junction, where acetylcholine (ACh) is released, causing depolarization of the sarcolemma.
• The action potential travels along T-tubules, triggering Ca²⁺ channels in the SR to open and release calcium ions into the sarcoplasm.

3.Calcium Binding and Cross-Bridge Formation:

• Calcium Ions: Bind to troponin, changing its shape and moving tropomyosin away from myosin-binding sites on actin.
• Cross-Bridge Formation: Myosin heads attach to exposed binding sites on actin.

4.Power Stroke:

• Myosin heads pivot, pulling actin filaments toward the M line and shortening the sarcomere by approximately 10 nm.

5.ATP and Cross-Bridge Cycling:

• ATP Binding: Causes myosin heads to detach from actin.
• ATP Hydrolysis: Resets myosin heads to their original position, ready for the next contraction cycle.
• Continuous Cycle: Repeats as long as Ca²⁺ and ATP are available, further shortening the sarcomere.

6.Muscle Relaxation:

• When stimulation stops, Ca²⁺ ions are pumped back into the SR, tropomyosin re-covers binding sites, and the muscle relaxes.

Neuromuscular Junction and Muscle Activation

• Function: Transmits signals from motor neurones to muscle fibres to initiate contraction.
• Process:

1. An action potential reaches the motor end plate, causing an influx of Ca²⁺.
2. ACh is released from synaptic vesicles and binds to receptors on the sarcolemma.
3. ACh binding opens Na⁺ channels, causing depolarization and triggering an action potential in the muscle fibre.
4. This action potential spreads along T-tubules to the SR, resulting in Ca²⁺ release and initiating contraction.

• Termination: Acetylcholinesterase breaks down ACh to prevent continuous stimulation, allowing the muscle to relax when impulses cease.

Energy for Muscle Contraction

• ATP Sources:
  • Aerobic Respiration: Provides ATP via mitochondria during sustained, moderate activity.
  • Lactate Fermentation: During intense activity, ATP is produced anaerobically, leading to lactate accumulation as a byproduct.

Key Terms

• Sliding Filament Model: Mechanism of muscle contraction where myosin heads pull actin filaments toward the M line, shortening the sarcomere.
• Striated Muscle: Muscle with a striped appearance, due to the arrangement of myofibrils.
• Sarcolemma: Muscle fibre membrane.
• Sarcoplasm: Cytoplasm of muscle fibres, containing mitochondria and myofibrils.
• Sarcoplasmic Reticulum (SR): Stores and releases Ca²⁺ ions essential for muscle contraction.
• Myofibril: Cylindrical bundles of actin and myosin filaments within a muscle fibre.
• Myosin: Thick filament protein, with heads that break down ATP to power contraction.
• Actin: Thin filament protein that interacts with myosin for contraction.
• Sarcomere: Basic contractile unit of a myofibril, between two Z lines.
• Z Line: Anchors actin filaments and separates sarcomeres.
• A Band: Dark band with overlapping actin and myosin.
• I Band: Light band containing only actin.
• H Band: Central part of the A band with only myosin.
• T-tubule: Infolding of sarcolemma that conducts impulses into the muscle fibre.
• Troponin and Tropomyosin: Regulatory proteins controlling actin-myosin binding.
• Neuromuscular Junction: Synapse between a motor neurone and muscle fibre.