Figure 1. Differences in the Dorso-ventral Distribution of Spinal Axon Collaterals among the Identified nMLF Neurons.

(A) A schematic of a larval zebrafish viewed from above (A1). The brain area is expanded in A2 to show the retrogradely labeled descending neurons in the midbrain and hindbrain. The red box highlights the nMLF (nucleus of the medial longitudinal fasciculus). In A3, the three large, identified nMLF neurons are marked. Mid = midline. Scale bar = 20 μm.

(B, C) Top-down view of the retrogradely labeled nMLF (white), with two identified nMLF neurons labeled with different colored dyes in the same fish (red and green).

(D) A schematic of a larval zebrafish viewed from the side (D1). Lateral view of the reconstructed main axon and collaterals of a MeM (D2), MeLc (D3), and MeLr (D4) neuron in the spinal cord from the region indicated in red on the schematic.

(E, F) Lateral view of the spinal cord with the main axon and collaterals of two identified nMLF neurons labeled with different colored dyes in the same fish. Cells are the same as those presented in B and C. White tick marks divide the dorso-ventral extent of spinal cord into 10 equal divisions.

(G) Total collateral length from body segments 5–14 at each dorso-ventral division for the MeLr and MeM neurons shown in B and E, normalized to the top (1) and bottom (0) edges of spinal cord.

(H) As in G, but for the MeLr and MeLc neurons shown in C and F.

(I) Contribution of MeM, MeLc, and MeLr neurons to the total amount of their spinal collaterals expressed as a percentage. Analysis was restricted to the dorso-ventral divisions 0.2–0.7, where spinal motoneurons are located (n = 5 for each cell type). The MeM neuron contributes more to the total collateral length at division 0.2 (black arrowhead), due to its ventral commissural collaterals.