Muscle Structure

Question 1

What are the 3 types of muscles?

Answer 1

Skeletal Muscle – voluntary, striated (banded) muscle. Short and fast contraction speed with quick fatigue. They have regular arrangement so muscle contracts in one direction.

• Tubular fibres and are multinucleated

Cardiac Muscle – involuntary, specialised striated (fainter striations than in skeletal muscle) muscle. Fairly fast and fairly short contraction speed. Myogenic (contract without nervous stimulus)

• Fibres are branched and uninucleate

Smooth (involuntary) Muscle – involuntary, non-striated muscle. No regular arrangement. Slow contractions and can remain contracted for a long period of time.

• Spindle shaped fibred, uninucleate

Question 2

What is the structure of a skeletal muscle fibre?

Answer 2

• Skeletal muscle made from bundle of muscle fibre, enclosed by a sarcolemma (plasma membrane enclosing the bundles of fibre)
  
  • Some of the sarcolemma folds inwards (called T or transverse tubules) to help spread electrical impulses through the sarcoplasm
• The fibres have many nuclei – multinucleated
  
  • The fibres are longer than normal cells, formed from fusing of embryonic cells
    
    • Makes them stronger as a junction between cells would be a weak point
• Sarcoplasm – shared cytoplasm within a muscle fibre

Question 3

Why do muscle fibres have many mitochondria?

Answer 3

Muscle fibres have lots of mitochondria providing ATP for muscle contraction, contains modified ER called sarcoplasmic reticulum

Extends throughout the muscle fibre, has calcium ions needed for muscle contraction

Question 4

What are myofibrils and what are they made of?

Answer 4

Each muscle fibre has multiple myofibril (long cylindrical organelles made of protein that are specialised for contraction)

• Myofibrils exist in parallel to maximise the contraction force

Made of:

1. Actin – thinner filament – 2 strands twisted around one another
2. Myosin – thicker filament, long rod-shaped fibres with bulbous heads, projecting to one side

Question 5

What causes the striped appearance (striated) in myofibrils?

Answer 5

Has alternating light and dark bands giving a striped appearance:

• Light bands – appear lighter, where actin and myosin do not overlap (known as I-Band)
• Dark bands – presence of thick myosin filaments, the edges are dark as myosin doesn’t overlap with actin (known as A-Band)
• Z-Line – found at centre of each light band, distance between adjacent Z-lines is a sarcomere (one functional unit of a myofibril)
  
  • As muscle contracts, sarcomere shortens
• H-Zone – lighter coloured region at the centre of the dark band, only myosin is present
  
  • As muscle contracts H-Zone gets smaller