Fibre analysis of the nerve to the inferior oblique muscle in monkeys.

The nerves to the inferior oblique muscles from both sides of four rhesus and seven cynomolgus monkeys were examined by light and electron microscopy. Myelinated fibres averaged slightly over 3000 in rhesus and 2000 in cynomolgus monkeys, with a bimodal distribution of diameters in both, the lower peak being 2.5-3.0 micron and the upper 7-11 micron, the large variation of the latter applying to both species. Unmyelinated fibres were less than 10% of the total. Following intracranial ophthalmic neurectomy in five monkeys, a few unmyelinated and small myelinated fibres were degenerated in the nerve to the inferior oblique muscle (1.8% of all fibres on average). A similar proportion of fibres survived oculomotor neurectomy in three cynomolgus monkeys when the superior cervical ganglion was additionally removed. A variable number of unmyelinated fibres were degenerated in the nerve after superior cervical ganglionectomy in three cynomolgus monkeys. The experiments indicate that ophthalmic and sympathetic branches pass to the nerve to the inferior oblique muscle. The few ophthalmic nerve fibres entering the muscle appear inadequate to serve the large numbers of receptors present and therefore most of the sensory fibres probably enter the brainstem in the oculomotor nerve in both species. The sympathetic pathway to the muscle provided by its motor nerve may be augmented by others.