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B3.3.4 (HL)—Motor units

Motor units connect motor neurons to muscle fibres, where impulses at motor end plates trigger neurotransmitter release and start contraction.

Syllabus
First assessment 2025
Objective
B3.3.4
Level
HL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceNovember 2017
Most common paperPaper1
Typical marks1

Common command terms

  • Identify

Recent exam appearances

November 2017Paper1 ["HL"] · TZ024[ 1 ]B3.3.4 (HL)—Motor units
Practice this objective

Coverage 2017–2017 · Updated 15 Jul 2026

Trigger and reset a muscle

HL only
A compact diagram connecting motor neuron, neuromuscular junction, motor end plate, sarcoplasmic reticulum calcium release, and a titin spring inside one sarcomere.

A motor neuron triggers skeletal muscle contraction at the neuromuscular junction. An action potential reaches the motor end plate, acetylcholine is released and the sarcolemma is depolarized; the signal causes the sarcoplasmic reticulum to release Ca²⁺. Ca²⁺ exposes actin binding sites and cross-bridge cycling shortens the sarcomere. Acetylcholinesterase removes acetylcholine and Ca²⁺ pumps return calcium to the sarcoplasmic reticulum, so tropomyosin covers the binding sites and the fibre relaxes. A motor unit is one motor neuron plus all the fibres it controls. Titin provides elastic recoil, centres myosin and limits overstretching; antagonistic muscles are needed because muscle contraction produces force in one direction, not active extension.

  • One motor neuron and its muscle fibres form a motor unit.
  • Acetylcholine and Ca²⁺ initiate contraction.
  • Acetylcholinesterase and Ca²⁺ reuptake allow relaxation.
  • Titin recoils the sarcomere; antagonistic pairs reverse movement.

Motor units

HL only

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Identify.

Command terms

Identify

What earns marks

Build the answer around this relationship: An action potential at the motor end plate causes neurotransmitter release.

Representative question

Question 1

[Maximum number: 1]

What happens when an action potential reaches motor end plates?

A

Calcium ions are absorbed by the muscle fibres.

B

The sarcomeres relax.

C

Neurotransmitter is released.

D

Action potential is passed to the neuron.

Muscle And Motility

HL only

A strong answer links movement benefit, muscle contraction mechanism, force transfer, joint range, and locomotion adaptations. For contraction, use calcium-troponin-tropomyosin and ATP-driven myosin cross-bridge cycling. For movement, use antagonistic muscles, tendons, ligaments, skeletons as levers, and joint type. For locomotion, link body form to survival or swimming advantage. Locomotion can improve survival and reproductive success.

  • Contraction answers need calcium, actin binding sites, myosin cross-bridges, ATP, and sarcomere shortening.
  • Movement answers need antagonistic pairs because muscles contract but do not actively extend.
  • Joint and skeleton answers need tendon versus ligament, lever action, synovial fluid, and range of motion.
  • Locomotion answers should link examples to food, escape, mate finding, or migration: blackbirds feeding, hares escaping predators, orangutans finding mates, and whales migrating.

Concept essentials

  • An action potential at the motor end plate causes neurotransmitter release.
  • Neuromuscular junctions link motor neurons to skeletal muscle fibres.
  • Excitation of the sarcolemma leads to calcium release inside the fibre.
  • Motor units convert neural signals into coordinated muscle contraction.