Fine structural evidence of mechanoreception in spinal lumbosacral accessory lobes of pigeons.

In the lumbosacral spinal cord of birds there are accessory lobes which protrude into the vertebral canal. The accessory lobes consist of multipolar neurons and glia-derived glycogen cells. It has been suggested that these lobes function as a sense organ of equilibrium. Therefore the lobes were studied ultrastructurally to look for possible mechanoreceptive structures. Extracellular lacunae extend from the periphery deep into each lobe. The dendrites of neurons ramify into the lacunae where they issue finger-like processes which do not contact any other cells and which are not contacted by boutons. Since finger-like processes are typical of peripheral and central mechanoreceptive neurons it is concluded that the observed processes indicate a mechanoreceptive function of the lobe neurons.