Spiral nerve endings in human extraocular muscles terminate in motor end plates.

The long held view that spiral nerve endings in extraocular muscles are sensory, recently shown to be incorrect for monkeys, was tested in man. Muscle samples were taken from orbits of five patients after eye enucleation and prepared for light and electron microscopy. Most spiral endings terminated in the motor end plate band in several well separated clusters of boutons applied to various aspects of individual fibres, in contrast to the single group of boutons of other endings. They displayed a dapple appearance using the acetylcholinesterase technique and possessed fine structural features characteristic of motor end plates. Approximately 5% of motor end plates had spiral endings and most were sheathed by extensions from perineural epithelial cells. Hence, the spiralling of nerve fibre endings in man, as in monkeys, is a device for conveying boutons to dapple motor end plates rather than for monitoring the contraction of muscle fibres. It is suggested that muscle fibres with dapple motor end plates may be responsible for the exceptional speed of contraction of extraocular muscles.