Intro | Golgi Tendon Organ | Motor Neuron | Muscle | Muscle Spindle | Sensory Neurons | Tendon

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Muscle tissue is composed of hundreds of thousands of extrafusal and intrafusal muscle fibers that are held together and anchored to bone by dense membranes called tendons. Multiple extrafusal muscle fibers are innervated by alpha motor neurons at the neuromuscular junction, by activating the motor-end plate on each fiber. Alpha motor neurons release the neurotransmitter, acetylcholine, to activate the muscle fibers. There are two types of extrafusal muscle fibers in skeletal muscle, fast fibers and slow fibers. Fast muscle fibers contract quickly, fatigue quickly, and are capable of great force. These fibers are highly developed in sprint runners for whom speed for short distances is important. Slow muscle fibers are slow, but capable of sustained contraction, and are highly developed in marathon runners for whom sustained effort is important. Intrafusal muscle fibers that form the proprioceptive organ called the muscle spindle are embedded with the extrafusal muscle fibers.


The force of a muscle contraction is determined via two mechanisms that occur simultaneously, according to the size principle and the frequency code (Parent, 1996). According to the size principle, motor units are recruited in an orderly fashion according to size. When a group of motor neurons are activated, the smallest cell bodies with the lowest threshold for activation are affected first. With this arrangement, motor units activated by weak inputs will generate the smallest force of contraction, whereas motor units activated by stronger inputs will generate strong contractions. According to the frequency code, the greater the frequency of the firing rate reaching the motor end plate, the greater the force of contraction.


Parent, A. (1996). Carpenter's human neuroanatomy (9th ed.). London: Williams & Wilkins.