Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

2.8.3Inquiry Process

Skeletal Muscle — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 88 of 160

📚 IB Biology HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Part 1 of 5Muscle Fibre Structure

2.8.3 Skeletal Muscle

Skeletal muscle is attached to bones and under voluntary control.

Multinucleate fibres
  • Each fibre is a long cell formed by fusion of many cells, so it contains many nuclei just beneath the cell surface.
Myofibrils
  • Cylindrical bundles of actin and myosin fill the fibre and give it a striped, or striated, appearance.
Sarcoplasmic reticulum
  • This specialised endoplasmic reticulum surrounds the myofibrils and stores calcium ions for release when stimulated.
Mitochondria
  • Many mitochondria between myofibrils generate the ATP needed for contraction and relaxation.
T-tubules
  • Inward folds of the cell membrane carry the action potential rapidly into the middle of the fibre, so all myofibrils contract together.

Part 2 of 5Comparing Muscle Types

Comparing Muscle Types
Skeletal muscle
  • Striated, voluntary and multinucleate; contracts quickly but tires.
Cardiac muscle
  • Striated, involuntary and branched, with intercalated discs linking cells;
    • it contracts rhythmically without tiring.
Smooth muscle
  • Non-striated and involuntary, found in walls of gut and blood vessels; contracts slowly and can be sustained.
Fast and slow fibres
  • Fast fibres contract quickly and rely on anaerobic respiration,
    • while slow fibres have more myoglobin and mitochondria and resist fatigue.
PropertySkeletalCardiacSmooth
StriationsYesYesNo
ControlVoluntaryInvoluntaryInvoluntary
Nuclei per cellManyOne or twoOne

Part 3 of 5Muscle Control

1
A motor neuron releases acetylcholine at the neuromuscular junction.
2
The acetylcholine triggers an action potential in the muscle membrane, which spreads along T-tubules.
3
Calcium is released from the sarcoplasmic reticulum, causing the myofibrils to contract.
4
All the fibres supplied by one motor neuron, a motor unit, contract together.
5
Stronger contractions recruit more motor units, while a continuous stream of impulses maintains tension.

Part 4 of 5Worked Example

Question

  • In relaxed muscle a sarcomere is 2.5 µm long.
  • When contracted it measures 1.8 µm.
  • Calculate the percentage decrease in length.

  1. Decrease = 2.5 − 1.8 = 0.7 µm.
  2. Percentage = 0.7 ÷ 2.5 × 100.
  3. = 28%.

Answer The sarcomere shortens by 28%.

Part 5 of 5Exam Checklist & Tips

2.8.3 – Key Points for the Exam

  • Muscle fibre A single long, multinucleate muscle cell.
  • Myofibril A cylindrical bundle of contractile filaments inside a muscle fibre.
  • Motor unit One motor neuron and all the muscle fibres it stimulates.
  • T-tubule A fold of the plasma membrane that carries action potentials deep into the fibre.

Common Mistakes

  • Calling skeletal muscle fibres single-nucleus cells; they are multinucleate.
  • Using 'striated' for smooth muscle.
  • Confusing myofibrils with muscle fibres; fibres contain many myofibrils.
  • Stating that the sarcoplasmic reticulum makes ATP; it stores calcium.

Last updated:

\n