Motor unit

An alpha motor neuron and the muscle fibers that it innervates

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.

Motor units are organized according to:

  1. SIZE

    The smallest motor units are in muscles that must produce very fine gradations of force:

    • lumbricals: 100 fibers/unit
    • eye muscles: 5 fibers/unit


    The fibers that make up a motor unit are not adjacent to one another, but they inhabit the same general region of the muscle.

Composition of muscles and motor units

A muscle is composed of motor units while each motor unit is composed of numerous fibers:
Musclenumber of alpha motor axonsfibers/motor unit
biceps brachii774750
first dorsal interosseous119340
first lumbrical98110
opponens pollicis133595
tibialis anterior445610

A fiber in a large muscle like the gastrocnemius

is composed of (on average) 2000 sarcomeres

Each sarcomere

is composed of around 1000 myofibrils

Each myofibril

contains around 1500 myosin and 3000 actin myofilaments


Guyton, A.C. (1991). Medical Physiology. 8th ed. Philadelphia: W.B. Saunders.

Goldspink, G. (1985). Malleability of the motor system: A comparative approach. Journal of Experimental Biology, 115, 375-391.

Muscle fiber types

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
abundant myoglobin

abundant mitochondria

abundant capillaries

less extensive sarcoplasmic reticulum (SR)

fewer myoglobin molecules

fewer mitochondria

less extensive capillaries

extensive SR

Last updated 12-11-00 ©Dave Thompson PT
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