Are laws that govern behavior embedded in the structure of the CNS? The case of Hering's law.

The pattern of axonal terminations of individual premotoneuronal medium lead burst neurons (MLBs) has been elucidated with the help of the intraaxonal recording and horseradish peroxidase injection technique in alert behaving monkeys. These findings indicate that individual MLBs do not influence individual muscles; instead they influence groups of muscles. Horizontal medium lead burst neurons (EBNs) project to ipsilateral lateral rectus motoneurons directly, and could contact contralateral medial rectus motoneurons indirectly, through the internuclear neurons of the ipsilateral abducens nucleus. Also, upward MLBs project to motoneurons innervating both the superior rectus and the inferior oblique muscles of both eyes. Finally, downward MLBs project to ipsilateral motoneurons innervating the ipsilateral inferior rectus muscle and the contralateral superior oblique muscle. All in all, the pattern of oculomotor terminations of MLBs provides experimental support for Hering's law of equal innervation. EBNs also project to the nucleus prepositus hypoglossi while vertical MLBs also project to the interstitial nucleus of Cajal (NIC). These structures are thought to participate in the process of "velocity to position integration", in the horizontal and vertical planes respectively. Intracellular recording from individual neurons of the NIC in alert behaving monkeys followed by biocytin or horseradish peroxidase injections demonstrates that their axons pass through the posterior commissure on their way to vertical extraocular motoneurons. It also demonstrates that these fibers carry a phasic signal related to saccades and a tonic signal related to eye position in a precise quantitative manner. Work is in progress to determine whether their pattern of oculomotor terminations is also appropriate for Hering's law.