Axonal projections of Renshaw cells in the thoracic spinal cord

Abstract

Renshaw cells are widely distributed in all segments of the spinal cord, but detailed morphological studies of these cells and their axonal branching patterns have only been made for lumbosacral segments. For these, a characteristic distribution of terminals was reported, including extensive collateralization within 1–2 mm of the soma, but then more restricted collaterals given off at intervals from the funicular axon. Previous authors have suggested that the projections close to the soma serve inhibition of motoneurons (known to be greatest for the motor nuclei providing the Renshaw cell excitation) but that the distant projections serve mainly the inhibition of other neurons. However, in thoracic segments, inhibition of motoneurons is known to occur over two to three segments (20–40 mm) from the presumed somatic locations of the Renshaw cells. Here, we report the first detailed morphological study of Renshaw cell axons outside the lumbosacral segments, which investigated whether this different distribution of motoneuron inhibition is reflected in a different pattern of Renshaw cell terminations. Four Renshaw cells in T7 or T8 segments were intracellularly labeled with neurobiotin in anesthetized cats and their axons traced for distances ≥6 mm from the somata. The only morphological difference detected within this distance in comparison with Renshaw cells in the lumbosacral cord was a minimal taper in the funicular axons, where in the lumbosacral cord this is pronounced. Patterns of termination were virtually identical to those in the lumbosacral segments, so we conclude that these patterns are unrelated to the pattern of motoneuronal inhibition.

Expressions of VGLUT1/2 in the inspiratory interneurons and GAD65/67 in the inspiratory Renshaw cells in the neonatal rat upper thoracic spinal cord

“…Thus, Renshaw cells generally could receive not only the excitatory synaptic inputs from the axon collaterals of motoneurons but also from central neuronal networks to regulate the motor outputs. Although the present and previous studies' findings showed that there are inspiratory Renshaw cells in the thoracic spinal cord ( Kirkwood et al, 1981 ; Saywell et al, 2013 ), it remains unknown whether these Renshaw cells receive the central respiratory input. Further experiments are necessary to examine the existence and strength of the central respiratory input to elucidate the role of Renshaw cells in the regulation of the inspiratory motor outputs.…”

“…It is well documented that Renshaw cells are excited by axon collaterals from motoneurons and provide recurrent inhibition of synergistic motoneurons ( Renshaw, 1941 ; Eccles et al, 1954 ). This recurrent inhibition has been observed at all spinal levels, i.e., in the cervical ( Lipski et al, 1985 ; Brink and Suzuki, 1987 ), thoracic ( Kirkwood et al, 1981 ; Saywell et al, 2013 ), lumbar ( Renshaw, 1941 ; Eccles et al, 1954 ), and sacral segments ( Jankowska et al, 1978 ). The mapping of the Renshaw cells in the sagittal plane showed the cells distributed at the ventral area of laminar VII, and thus at a slightly medial position of the motoneuron pool in the cat lumbar cord ( Fyffe, 1990 ).…”

“…Renshaw cells receive excitatory inputs from the motoneuron axon collaterals and exert a recurrent inhibition of synergist motoneurons ( Alvarez and Fyffe, 2007 ). Although Renshaw cells are well-characterized inhibitory interneurons and have been studied especially in the lumbar spinal cord in relation to the regulation of locomotor activity ( Nishimaru et al, 2006 ), the Renshaw cells and their location in the thoracic segments have not been fully examined ( Kirkwood et al, 1981 ; Saywell et al, 2013 ). We therefore investigated the existence of mRNA of glutamic acid decarboxylase 65 and/or 67 (GAD65/67) on the recorded cells, which received depolarizing postsynaptic potentials evoked by ventral root stimulation, to verify whether these cells are truly Renshaw cells.…”

Escape from homeostasis: spinal microcircuits and progression of amyotrophic lateral sclerosis

“…Renshaw cells have several targets ( Hultborn et al 1971 ; Ryall 1970 ; Wilson et al 1964 ), but in this review we focus on their MN targets. Renshaw cells project back to and form inhibitory synapses (glycinergic and/or GABAergic) with MNs ( Cullheim and Kellerth 1981 ; Eccles et al 1954 ; Renshaw 1946 ; Schneider and Fyffe 1992 ), for the most part in the same or adjacent segments ( Jankowska and Smith 1973 ; Kirkwood et al 1981 ; Ryall et al 1971 ; Saywell et al 2013 ; van Keulen 1979 ) ( Fig. 1 B , h ).…”

Firing properties of Renshaw cells defined by Chrna2 are modulated by hyperpolarizing and small conductance ion currents Ih and ISK

“…In particular, similar to our findings, spontaneous RC firing at 7 Hz has been reported in the cat spinal cord by Walmsley & Tracey (). More recently, Renshaw cells recorded from the cat thoracic spinal cord were shown to have irregular spontaneous firing of ≤40 impulses/s at rest (Saywell et al ., ). While our in vitro preparation was targeting Renshaw cells in lumbar slices, it is possible that the caudal‐most thoracic Renshaw cells were included when aiming to restrict slices to the lumbar spinal cord.…”