The role of neural mechanisms in the regulation of intraocular pressure in the cat

The afferent activity evoked by intraocular pressure changes, the effects of stimulation of retrobulbar nerves on the intraocular pressure, and the evidence of the occurrence of central integration were examined in cats over short periods. Afferent activity was recorded from the long ciliary nerves of isolated preparations as well as preparations in situ. Spontaneous activity which could be related to resting pressure levels was not observed in any preparation. An increase of intraocular pressure elicited a discharge of impulses, the frequency of which was directly related to the pressure level. Preparations in situ were generally more stable and yielded more reproducible results. The temporal pattern of the pressure-evoked activity was different from that of activity elicited by mechanical stimulation of the cornea or the iris. Stretching of the nerve endings located in the outer coats of the eye appears to initiate the impulses evoked by raising the intraocular pressure. Stimulation of the ciliary ganglion or its motor root, of isolated perfused eyes or eyes in situ caused a fall in the intraocular pressure. A small increase in the intraocular pressure was observed in the eye without circulation, due to increased inward tension of the chorioid during ciliary muscle contraction. Stimulation of the long ciliary nerves produced a small fall in pressure due to sympathetic nerve activity. This fall is masked in the intact eye due to stimulation of the extraocular structures. Efferent activity recorded from either the long or short ciliary nerves could not be modulated by pressure changes. Retrobulbar injection of lidocaine did not affect the resting intraocular pressure in the cat nor did it affect the rat at which the pressure returned to normal after imposed changes. These observations are discussed in relation to the postulated neural control of the intraocular pressure.