A muscle is composed of many motor units.Each motor unit contains many muscle fibers. Each muscle fiber within a motor unit is of the same fiber type. |
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The smallest motor units are in muscles that must produce very fine gradations of force:
The fibers that make up a motor unit are not adjacent to one another, but they inhabit the same general region of the muscle.
| Muscle | number of alpha motor axons | fibers/motor unit |
|---|---|---|
| biceps brachii | 774 | 750 |
| brachioradialis | 330 | 390 |
| first dorsal interosseous | 119 | 340 |
| first lumbrical | 98 | 110 |
| opponens pollicis | 133 | 595 |
| gastrocnemius | 580 | 1720 |
| tibialis anterior | 445 | 610 |
A fiber in a large muscle like the gastrocnemius
Each sarcomere
Each myofibril
Sources:
Goldspink, G. (1985). Malleability of the motor system: A comparative approach. Journal of Experimental Biology, 115, 375-391.
| fiber types | |||
|---|---|---|---|
| SO (I) | FR (IIa) | FF (IIb) | |
| metabolic proteins | oxidative (SO) | oxidative- glycolytic (FOG) | glycolytic (FG) |
| motor neuron characteristics | small cell bodies small axon diameter slow conduction velocity tonic low frequency activity | large cell bodies large axon diameter fast conduction velocity phasic (burst-like) high frequency activity | |
| muscle fiber twitch characteristics | long duration low tension | brief duration high tension | |
| muscle cell characteristics | abundant myoglobin abundant mitochondria abundant capillaries less extensive sarcoplasmic reticulum (SR) | fewer myoglobin molecules fewer mitochondria less extensive capillaries extensive SR | |