What is the function of the Ia nerve fiber?
The Ia nerve arises from annulospiral endings within the muscle spindle.
When the muscle spindle is stretched (i.e., when the muscle is relaxed), the Ia sensory nerve, through the dorsal root, monosynaptically stimulates the alpha motor neuron, which fires and contracts (shortens) the muscle.
Thus, the muscle stretch reflex maintains tone and tension in the muscle, by contracting it when it becomes too relaxed.
