Motor units and the all-or-none law
A motor neuron is a nerve cell that carries a signal from the spinal cord to a muscle. One motor neuron and all the muscle fibres it controls form a motor unit.
When the signal (an action potential) is strong enough to pass the threshold, every fibre in that unit contracts fully. If it is too weak, none contract. This is the all-or-none law.
- Small motor units (a few fibres) give fine control, e.g. eye and finger muscles.
- Large motor units (hundreds of fibres) give big force, e.g. thigh muscles.
The signal crosses from nerve to muscle at the neuromuscular junction using a chemical called acetylcholine.
Making more force: recruitment and summation
Since one unit cannot pull "a bit harder", the body has two tricks:
- Spatial summation (recruitment): switch on more motor units, and bigger ones. Small, slow units are used first; large, fast units join when more force is needed.
- Wave summation: send signals so quickly that the fibre has no time to relax. Each twitch adds to the last.
- Tetanic contraction (tetanus): signals so fast that the twitches merge into one smooth, strong, steady pull.
Recruiting units in turn also lets some rest while others work, which delays tiredness.
Muscle fibre types
| Feature | Type I (slow oxidative) | Type IIa (fast oxidative glycolytic) | Type IIx (fast glycolytic) |
|---|---|---|---|
| Speed of contraction | Slow | Fast | Fastest |
| Force | Low | High | Highest |
| Tiredness | Very resistant | Fairly resistant | Tires quickly |
| Mitochondria, myoglobin, capillaries | Many (red) | Medium | Few (white) |
| Main energy | Aerobic (with oxygen) | Both | Anaerobic (without oxygen) |
| Example | Marathon | 800 m, 1500 m | 100 m, shot put |
The mix is mostly set by genes. Endurance training can make Type IIx behave more like IIa, and all fibres can grow (hypertrophy).
PNF: proprioceptive neuromuscular facilitation
Muscles have built-in sensors called proprioceptors:
- Muscle spindles sense stretch. Too much stretch triggers the stretch reflex, which tightens the muscle to protect it.
- Golgi tendon organs sense tension in the tendon. High tension makes the muscle relax: autogenic inhibition.
PNF steps (contract-relax): 1) stretch the muscle to its limit with a partner; 2) contract it against the partner without moving (isometric) for about 6โ10 s; 3) relax and stretch again, now further. Repeat 2โ3 times. It is one of the best ways to improve flexibility, and is usually done after training when muscles are warm.
Try it: feel recruitment
Hold an empty bag with your arm straight out and feel your upper arm. Now ask a friend to add books one by one. The muscle gets firmer: your brain is recruiting more motor units. Notice it shakes when nearly tired: units are dropping out.
Key formulas and definitions
- Key term: Motor unit = one motor neuron + all the muscle fibres it supplies
- Key term: All-or-none law = a motor unit contracts fully or not at all
- Key term: Spatial summation (recruitment) = using more and larger motor units for more force
- Key term: Wave summation = repeated signals before relaxation add up; tetanus = smooth, maximum, sustained contraction
- Key term: Type I / IIa / IIx = slow-endurance / fast-fairly-tireless / fastest-quickly-tired fibres
- Key term: PNF = stretch โ isometric contraction โ stretch further, using Golgi tendon organs (autogenic inhibition)
Worked examples
1. A marathon runner's calf muscle biopsy shows 80% Type I fibres. Why does this help?
Type I fibres have many mitochondria, lots of myoglobin and capillaries, so they make energy aerobically and resist tiredness for hours. This suits a 2โ3 hour race at a steady pace.
2. Why can a weightlifter lift 100 kg once but a person writing uses tiny movements? Explain with motor units.
The lifter's brain recruits many large motor units (spatial summation) and fires them fast (wave summation to tetanus). Writing uses a few small motor units in the hand, each with only a few fibres, giving fine control.
3. Describe how PNF increases range of movement in the hamstring.
Partner stretches the hamstring to its limit; muscle spindles signal the stretch. The athlete then pushes against the partner for about 10 s (isometric). Golgi tendon organs sense the high tension and cause autogenic inhibition, so the muscle relaxes. The partner then stretches it further. Repeating raises flexibility.
4. Which fibre type is used most in a 400 m race and why?
Mostly Type IIa (with IIx at the start and finish): the race lasts about 45โ60 s, needing high force for longer than IIx can last but more power than Type I gives.
Common mistakes
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