Dendrite bundles in Lamina IX of cat spinal cord: A possible source for electrical interaction between motoneurons?

Longitudinally‐oriented bundles of closely‐aligned motoneuron dendrites in Lamina IX of cat spinal cord were examined. These dendrites were interrelated in one of several ways: (1) They were separated by a space of several microns containing small myelinated and unmyelinated fibers, synaptic boutons, and glial processes; (2) Some dendrites were separated by a gap of 0.2–0.5 μ which contained only an attenuated astrocyte process; (3) The plasma membranes of the adjacent dendrites were occasionally found in direct apposition and in most of these instances there was no detectable specialization of the opposing membranes; (4) There were definite modifications along the appositional membranes. These zones were characterized by a gap of ∼ 180 Å, and symmetrical, non‐polarized aggregations of electron‐opaque material were found along the cytoplasmic surfaces of the membranes. It is suggested that the spatial arrangement and appositional relations of these dendrites represent a morphological substrate for the weak electrical facilitation known to occur between motoneurons in the cat spinal cord.