Mechanistic Hypotheses for Proprioceptive Sensing Within the Avian Lumbosacral Spinal Cord.

Animals need to accurately sense changes in their body position to perform complex movements. It is increasingly clear that the vertebrate central nervous system contains a variety of cells capable of detecting body motion, in addition to the comparatively well-understood mechanosensory cells of the vestibular system and the peripheral proprioceptors. One such intriguing system is the lower spinal cord and column in birds, also known as the avian lumbosacral organ (LSO), which is thought to act as a set of balance sensors that allow birds to detect body movements separately from head movements detected by the vestibular system. Here, we take what is known about proprioceptive, mechanosensory spinal neurons in other vertebrates to explore hypotheses for how the LSO might sense mechanical information related to movement. Although the LSO is found only in birds, recent immunohistochemical studies of the avian LSO have hinted at similarities between cells in the LSO and the known spinal proprioceptors in other vertebrates. In addition to describing possible connections between avian spinal anatomy and recent findings on spinal proprioception as well as sensory and sensorimotor spinal networks, we also present some new data that suggest a role for sensory afferent peptides in LSO function. Thus, this perspective articulates a set of testable ideas on mechanisms of LSO function grounded in the emerging spinal proprioception scientific literature.

Molecular Markers of Mechanosensation in Glycinergic Neurons in the Avian Lumbosacral Spinal Cord.

Birds are exceptionally adept at controlling their body position. For example, they can coordinate rapid movements of their body while stabilizing their head. Intriguingly, this ability may rely in part on a mechanosensory organ in the avian lower spinal cord called the lumbosacral organ (LSO). However, molecular mechanotransduction mechanisms have not been identified in the avian spinal cord. Here, we report the presence of glycinergic neurons in the LSO that exhibit immunoreactivity for myosin7a and espin, molecules essential for function and maintenance of hair cells in the inner ear. Specifically, we find glycinergic cell bodies near the central canal and processes that extend laterally to the accessory lobes and spinal ligaments. These LSO neurons are reminiscent of glycinergic neurons in a recently-described lateral spinal proprioceptive organ in zebrafish that detects spinal bending. The avian LSO, however, is located inside a series of fused vertebrae called the synsacrum, which constrains spinal bending. We suggest the LSO may be a modification and elaboration of a preexisting mechanosensory spinal network in vertebrates. A mechanistic understanding of its function may be an important clue to understanding the evolution and development of avian locomotion.