Figure 8.

Postulated trophic classification of extraocular motoneurons based on their accessibility to different sources of neurotrophins. Different neurotrophins produced by multiply-innervated (MIF) and singly-innervated (SIF) muscle fibers could be secreted and accessible to motoneurons. Complementary gradients of BDNF and NT-3 or homogeneous supply of NGF by the muscle will reach in different proportions to the soma of motoneurons via their complement of Trk receptors. It is postulated that some motoneurons will not be endowed with all three receptors. Trk signaling will lead to a synaptotrophic scaling of the three main afferents to abducens motoneurons generating thus the three semantically distinct groups of motoneurons that could both differ in size and signaling properties. From top to bottom, phasic (red), tonic-phasic (green) and tonic (blue) proposed motoneuronal types.