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3. 6. 2012.

Motor response

 Once an action potential reaches an alpha-motor neuron, it travels the length of the neuron to the neuromuscular junction. From there, the action potential spreads to all muscle fibers innervated by that particular alpha-motor neuron. Recall that the alpha-motor neuron and all muscle fibers it innervates form a single motor unit. Each muscle fiber is innervated by only one alpha-motor neuron, but each alpha-motor neuron innervates up to several thousand muscle fibers, depending on the function of the muscle. Muscles controlling fine movements, such as those controlling the eyes, have only a small number of muscle fibers per alpha-motor neuron. Muscles with more general functions have many fibers per alpha-motor neuron.
The muscles that control eye movements(the extraocular muscles) have an innervation ratio of 1:15, meaning that one alpha-motor neuron controls only 15 muscle fibers. In contrast, the gastrocnemius and tibialis anterior muscles of the lower leg have innervation ratios of almost 1: 2,000.
The muscle fibers in a specific motor unit are homogenous with respect to fiber type. Thus, one will not find a motor unit that has both type II and type I fibers. In fact, it is generally believed that the characteristics of the alpha-motor neuron actually determine the fiber type in that motor unit.

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