Recruitment Order of Cat Abducens Motoneurons and Internuclear Neurons

Abstract

Abducens neurons undergo a dose-dependent synaptic blockade (either disinhibition or complete blockade) when tetanus neurotoxin (TeNT) is injected into the lateral rectus muscle at either a low (0.5) or a high dose (5 ng/kg). We studied the firing pattern and recruitment order in abducens neurons both in control and after TeNT injection. The eye position threshold for recruitment of control abducens neurons was exponentially related to the eye position and velocity sensitivities. We also found a constancy of recruitment threshold for different eye movement modalities (spontaneous, optokinetic, and vestibular). Exponential relationships were found, as well, for eye velocity sensitivity during saccades and for position and velocity sensitivities during the vestibulo-ocular reflex. Likewise, inverse relationships were found between recruitment threshold or position sensitivity with the antidromic latency in control abducens neurons. These relationships, however, did not apply following TeNT treatment. Neuronal firing after TeNT appeared either disinhibited (low dose) or depressed (high dose), but the relationships between neuronal sensitivities and recruitment still applied. However, the pattern of recruitment shifted toward the treated side as more inputs were blocked by the low-and high-dose treatments, respectively. Nonetheless, although the recruitment-to-sensitivity relationships persisted under the TeNT synaptic blockade, we conclude that synaptic inputs are determinant for establishing the recruitment threshold and recruitment spacing of abducens motoneurons and internuclear neurons.

Nerve Growth Factor Regulates the Firing Patterns and Synaptic Composition of Motoneurons

“…jneurosci.org as supplemental material). Abducens motoneurons show orderly recruitment patterns that correlate with neuronal sensitivity (Pastor and González-Forero, 2003). Thus, when regression lines between firing rate and eye position are plotted for individual cells, the slopes (k) are higher as the cells become recruited further toward ipsilateral eye positions (supplemental Fig.…”

A chronically implantable device for the controlled delivery of substances, and stimulation and recording of activity in severed nerves

“…However, as expected, there were movements in which individual motoneurons entered into cut off, and the activity in the nerve continued, since the nerve presumably contained axons with thresholds lower than those of the recorded unit. These would be active at more eccentric positions in the off direction than the recorded neuron (Pastor and González-Forero, 2003).…”

“…The animal was seated in the feline-restraining system, as previously described (de la Cruz et al, 2000;Pastor and González-Forero, 2003). In brief, the assembly consisted of a sturdy Perspex box with an internal foam cushion.…”

“…Furthermore, we wished to compare the neuronal responses in this system to the administration of diverse potentially therapeutic agents under controlled conditions. In order to do so, we needed to test and validate a method for the isolation of a severed nerve that allows the chronic delivery of substances, as well as the study of the electrophysiological properties (Baker and Highstein, 1975;de la Cruz et al, 1994;Delgado-García et al, 1986;Pastor and González-Forero, 2003).…”

Synaptic structural modification following changes in activity induced by tetanus neurotoxin in cat abducens neurons

“…Experimental procedures were in accordance with the directive of the European Union (86/609/EEC) and the current Spanish legislation for the use and care of laboratory animals (BOE 67/8509-12, 1988). A single low (0.5 ng/kg) or high dose (5 ng/kg) of TeNT was injected in the left lateral rectus muscle following procedures described extensively in detail previously (Gonzá lez Forero et al, 2002a,b, 2003Pastor and Gonzá lez-Forero, 2003). Two animals received low-dose injections, and six animals were injected with high doses.…”