Electrophysiological and Morphological Characterization of Propriospinal Interneurons in the Thoracic Spinal Cord

Abstract

Propriospinal interneurons in the thoracic spinal cord have vital roles not only in controlling respiratory and trunk muscles, but also in providing possible substrates for recovery from spinal cord injury. Intracellular recordings were made from such interneurons in anesthetized cats under neuromuscular blockade and with the respiratory drive stimulated by inhaled CO(2). The majority of the interneurons were shown by antidromic activation to have axons descending for at least two to four segments, mostly contralateral to the soma. In all, 81% of the neurons showed postsynaptic potentials (PSPs) to stimulation of intercostal or dorsal ramus nerves of the same segment for low-threshold (≤ 5T) afferents. A monosynaptic component was present for the majority of the peripherally evoked excitatory PSPs. A central respiratory drive potential was present in most of the recordings, usually of small amplitude. Neurons depolarized in either inspiration or expiration, sometimes variably. The morphology of 17 of the interneurons and/or of their axons was studied following intracellular injection of Neurobiotin; 14 axons were descending, 6 with an additional ascending branch, and 3 were ascending (perhaps actually representing ascending tract cells); 15 axons were crossed, 2 ipsilateral, none bilateral. Collaterals were identified for 13 axons, showing exclusively unilateral projections. The collaterals were widely spaced and their terminations showed a variety of restricted locations in the ventral horn or intermediate area. Despite heterogeneity in detail, both physiological and morphological, which suggests heterogeneity of function, the projections mostly fitted a consistent general pattern: crossed axons, with locally weak, but widely distributed terminations.

Ipsilateral inspiratory intercostal muscle activity after C2 spinal cord hemisection in rats

“…12,32 In the present study, this interneuronal pool did not sufficiently activate the side ipsilateral to the injury. It also does not appear to be a strong input on the side contralateral to the injury, since there were no differences in the contralateral activity in injured animals compared to controls.…”

“…Monosynaptic connections to intercostal muscles are rare. 11,12 The return of some activity in the most rostral intercostal muscles appeared rather quickly indicating that crossed inputs at the thoracic level must be rather common. However, activity in the lower half of the spinal cord did not return as quickly or as robust suggesting that the amount of input or drive is substantially less than the activity in the rostral sections.…”

“…Innervation of external intercostal muscles appears to be largely due to polysynaptic pathways, which cross the midline to affect motoneurons on the contralateral side of the spinal cord. 11,12 Kirkwood et al 9 and Dougherty et al 10 showed that hemisection of the spinal cord caused an immediate reduction in ipsilateral intercostal nerve activity below the injury site. In both studies, intercostal activity returned; however, Dougherty et al 10 only examined the rostral intercostal activity of the first and second rib space after a C2 hemisection and Kirkwood et al 9 hemisected the thoracic spinal cord and also only looked at a few thoracic intercostal spaces below the injury.…”

A Computational Model for Epidural Electrical Stimulation of Spinal Sensorimotor Circuits

“…Myelinated fibers, which included Group Ia/Ib, Group II, and efferent axons, were modeled as cable-axon structures with two compartments: (1) active Nodes of Ranvier and (2) passive myelinated internodes. A McIntyre-RichardsonGrill (MRG) model (Richardson et al, 2000) was used to describe the equation of the active membrane at the nodes of Ranvier and at the passive internodal segments. Myelin was represented as a nonperfect insulator, as used for model B in the study by Richardson et al, 2000.…”

“…A McIntyre-RichardsonGrill (MRG) model (Richardson et al, 2000) was used to describe the equation of the active membrane at the nodes of Ranvier and at the passive internodal segments. Myelin was represented as a nonperfect insulator, as used for model B in the study by Richardson et al, 2000. The nodes of Ranvier's membrane included fast sodium, persistent sodium, and slow potassium channels along with a leakage conductance.…”