Central representation and somatotopic organization of the jaw muscles within the facial and trigeminal nuclei of the pigeon (Columba livia)

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Abstract
The location of facial and trigeminal brainstem motoneurons innervating the jaw muscles of the pigeon has been determined, using the technique of retrograde transport of horseradish peroxidase. Trigeminal motoneurons innervating the jaw closing muscles are located within the classically defined main trigeminal motor nucleus, and their organization exhibits approximately a mediolateral somatotopy. Trigeminal neurons innervating the opener muscle of the upper jaw are also located in the main trigeminal motor nucleus, but comprise a dorsomedial subnucleus which is continuous caudally with the dorsal facial nucleus whose neurons innervate the opener muscle of the lower jaw. The Vth and VIIth motor nuclei are thus linked in the rostrocaudal plane. The representations of other head muscles are arranged in a dorsoventral manner within the V–VII motor complex, with the lower eyelid muscle represented most dorsally and the hyoid and superficial neck muscles most ventrally. Jaw closing muscles were well represented in both the Gasserian ganglion and trigeminal mesencephalic nucleus. No such representation was found for either of the jaw opening muscles. The HRP data indicate that the V–VII motor complex in birds is far more extensive than has previously been suggested.