Localization and morphology of cat extraocular muscle afferent neurones identified by retrograde transport of horseradish peroxidase

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Abstract
Afferent neurones that provide proprioceptive innervation of extraocular muscles of the cat have been identified by means of retrograde axonal transport of horseradish peroxidase (HRP). Discrete injections of HRP into the medial rectus, lateral rectus, or retractor bulbi muscles labeled pseudounipolar neurones that were localized exclusively to the ipsilateral semilunar ganglion. The distribution of labeled neurones within the ganglion was consistent with its somatotopic organization with the majority found within the ophthalmic subdivision. Cell counts indicating approximately 90 labeled neurones per horizontal rectus muscle correlated well with earlier quantitative observations regarding the percentage of afferent fibers in oculomotor nerves and the number of proprioceptive terminals in the extraocular muscles. Neither the trigeminal mesencephalic nucleus nor the contralateral semilunar ganglion contained labeled neurones following injections of HRP into extraocular muscles.Consistent with other studies of spinal and cranial ganglia the contingent of pseudounipolar neurones present in the cat semilunar ganglion included both light and dark cell types. Light and electron microscope analysis of HRP‐labeled neurones in combination with acetylcholinesterase (AChE) histochemistry revealed that only one of the two neuronal types, the light cell, subserves extraocular muscle proprioception. Our data support the hypothesis that ganglion neurone type and, more specifically, soma diameter, are important determinants of functional status.