The axonal projections of the Hofmann nuclei in the spinal cord of the late stage chicken embryo

Abstract

The projection pattern of the neurons of the paragriseal Hofmann nuclei was mapped in the chicken embryo using the lipophilic tracer DiI. This report focuses on the pattern of projection from the Hofmann nuclei major observed 1-4 days prior to hatching, at which time the projection appears to be substantially developed. (1) Each neuron extends a commissural axon through the ventral gray matter and across the midline in the ventral commissure. The axons originating from a single Hofmann nucleus cross within a stretch of the cord equivalent to about one spinal segment. There is a small overlap of the axon populations originating from adjacent Hofmann nuclei. After reaching the contralateral ventral columns the individual axons bifurcate and extend rostrally and caudally up to 5 spinal segments in each direction. The rostral and caudal trajectories differ; the rostral axons shift progressively more laterally while the caudal axons tend not to deviate from their initial course. (2) Throughout their longitudinal course the axons give rise to terminal collaterals that are concentrated in lamina 8. Rostrally and caudally the terminals decrease in density and become increasingly scattered. (3) Hofmann neurons are multipolar with 4-5 laterally directed primary dendrites whose arbors are restricted to the Hofmann nucleus major within which the neurons reside. (4) Hofmann neurons receive afferent terminals from a longitudinal column of commissural interneurons located contralaterally in close approximation to the central canal. Each Hofmann nucleus major is innervated by a rostrocaudally restricted subset of these presynaptic neurons. The axon trajectories of the presynaptic neurons are similar to those of the Hofmann neurons. (5) Paragriseal neurons that are not located within Hofmann nuclei major are also commissural intersegmental interneurons and tend to be clustered segmentally. The segmentation is clearest for the Hofmann nuclei minor, which are clusters of neurons iterated along the ventrolateral margin of the thoracic spinal cord but not organized in protruding lobes.

Chick spinal accessory lobes contain functional neurons expressing voltagegated sodium channels generating action potentials

“…The complete action potentials recorded in the current clamp mode under the whole-cell configuration suggest that AL neurons can propagate the action potential along the axon toward the synaptic terminal located far from the soma, and that they can make functional projections to other neurons. It has been reported that AL neurons extend axons projecting to lamina VIII neurons in the contralateral spinal gray matter (4,10), and that lamina VIII neurons project to contralateral ventral horn motoneurons in the pigeon (14). There is little information about functional roles of lamina VIII neurons of birds.…”

A Physical Model Suggests That Hip-Localized Balance Sense in Birds Improves State Estimation in Perching: Implications for Bipedal Robots

“…The presence of a local and distributed balance sensing organ that is directly integrated with hindlimb spinal networks has likely contributed to this modular control organization. The mechanosensing neurons of the LSO project directly to pre-motor neurons in the spinal cord (Eide, 1996;Necker, 2006). This suggests the balance sense information produced by the LSO is likely to contribute to rapid and effective control because it is processed locally.…”

“…Moreover, such rapid and informative sensing is also critical to low-level (distributed, spinal or sub-cortical) sensorimotor processing to control short-latency responses to perturbations (Lawrence et al, 2015a,b) that ultimately supports long-latency control of voluntary function in general. The LSO is directly integrated with the hindlimb spinal motor control networks (Eide, 1996;Necker, 2006), suggesting that hiplocalized balance sense is likely relevant to all hindlimb-mediated behaviors, including perching, standing balance, over-ground locomotion and arboreal locomotion. Birds effectively have two distinct balance sensorimotor processing centers: the "cerebral brain, " responsible for executive function and navigation, and the "sacral brain, " responsible for low-level, short latency control of terrestrial perching, standing and locomotion.…”

Ultrastructural characterization of the accessory lobes of Lachi in the lumbosacral spinal cord of the pigeon with special reference to intrinsic mechanoreceptors

“…Terni (1926) was the first to show that the axons of the accessory lobe neurons cross in the ventral commissure to reach ventromedial aspects of the contralateral gray substance. This was later confirmed by silver impregnation techniques (Matsushita, 1968) and neuronal tracer techniques (Eide, 1996;Necker, 1997).…”