1. Types of Muscle:

<aside> 💡 striated - 线条的 uni/bi/multi-nucleated - 单/双/多核的

</aside>

Cardiac Muscle

• Found in heart wall
• Involuntary, uni-binucleated, striated, cylindrical and branched

Function

• Pump blood

Skeletal Muscle

• Attached to bones in legs, arms and torso
• Voluntary, multinucleated, striated and cylindrical

Function

• Holds skeletons together
• Acts against gravity to keep upright
• Enable movements
• Maintain body temperature

Smooth Muscle

• Found in hollow internal organs - intestine, stomach, bladder, blood vessels
• Involuntary, uninucleated, spindle shaped and forms sheets

Function

• Digestion and waste removal
• Controlling blood flow

2. Movement of Limbs - Lever System

• In the elbow joint, the Biceps Branchii and the Triceps Branchii build a pair of antagonistic flexor and extensor muscles, respectively
• They control the joint angle applying the mechanisms of a Lever System
• Lever System in limb joints is a third-class lever - the most common used in human body
• Force is applied between resistance(weight) and axis(fulcrum)

3. Skeletal Muscle Anatomy

Transverse Section

• Subcutaneous Fat Layer
  • Pathway for blood vessels and nerves before they divide and enter the muscles
  • Act as energy storage
• Fascia
  • Layer of tough connective tissue holding together all the muscle elements serving the same function
  • enclose Fascicles (muscle tissues)
• Structure is built up from:
  • Muscle
  • Fascicle
  • Muscle Fibre
  • Myofibril - constituents of cells
  • Sarcomere - active motor units

Muscle Fibre and Myofibrils

• A Fascicle consists of multiple Muscle Fibres
• Each Muscle Fibre consists of multiple Myofibril and build a Syncytium
• Muscle Cells are multinucleated

Muscle $\to$ Fascicle $\to$ Muscle Fibre $\to$ Myofibril $\to$ Actin/Myosin Filament

• Each fibre is surrounded by a cell surface membrane called Sarcolemma
  • Bits of the Sarcolemma fold inwards across the fibre and stick into the Sarcoplasm
  • Folds are called Transverse Tubules
    • Transverse Tubules help spread the electrical impulses throughout the Sarcoplasm
• Muscle Cell Cytoplasm is know as Sarcoplasm, and contains organelles including:
  • Many Mitochondria
    • Provide ATP needed for contraction
  • Extensive Sarcoplasmic Reticulum - a specialised ER
    • Stores and control release of $Ca^{2+}$
  • A number of Myofibrils
• Myofibrils are the contractile elements
  • Each consist of a smaller contractile units called Sarcomeres
  • Sarcomeres contain two types of protein Myofilaments - Thin Actin Filament and Thick Myosin Filament

Sarcomere

Thick Myosin Filament

• Each molecule consists of a tail and two protruding heads that can bind to a site on Actin

Think Actin Filament

• Mainly two strands of F Actin made up of G Actin subunits
• Tropomyosin (Rod-shaped Protein) molecules coil around this reinforcing it
• A Troponin Complex is attached to each Tropomyosin molecule
  • Troponin consists of three 3 polypeptides:
    • one that binds to Actin
    • one that binds to Tropomyosin
    • one that binds to $Ca^{2+}$

• A Myofibril contains a pattern of alternating dark and light bands
• The end of each Sarcomere are marked with a Z-line
• The middle of each Sarcomere/Myosin is the H-zone
• Cross sections of Myofibrils show the hexagonal organisation of Actin and Myosin Filaments

4. Muscle Contraction - Sliding Filament Theory

• Myosin and Actin Filaments slide over one another to make the Sarcomeres contract - the Myofilaments themselves don’t contract
• The simultaneous contraction of lots of Sarcomeres means the Myofibrils and Muscle Fibres contract
• Sarcomeres return to their original length as the muscle relaxes