Electrical stimulation of circumscribed areas of the pontine and medullary reticular
formation inhibits muscle tone in cats. In this report, we present an analysis of
the anatomical distribution of atonia-inducing stimulation sites in the brain stem
of the rat. Muscle atonia could be elicited by electrical stimulation of the nuclei
reticularis pontis oralis and caudalis in the pons as well as the nuclei gigantocellularis,
gigantocellularis alpha, gigantocellularis ventralis, and paragigantocellularis dorsalis
in the medulla of decerebrate rats. This inhibitory effect on muscle tone was a function
of the intensity and frequency of the electrical stimulation. Average latencies of
muscle-tone suppressions elicited by electrical stimulation of the pontine reticular
formation were 11.02 +/- 2.54 and 20.49 +/- 3.39 (SD) ms in the neck and in the hindlimb
muscles, respectively. Following medullary stimulation, these latencies were 11.29
+/- 2.44 ms in the neck and 18.87 +/- 2.64 ms in the hindlimb muscles. Microinjection
of N-methyl-D-aspartate (NMDA, 7 mM/0.1 ml) agonists into the pontine and medullary
inhibitory sites produced muscle-tone facilitation, whereas quisqualate (10 mM/ 0.1
ml) injection induced an inhibition of muscle tone. NMDA-induced muscle tone change
had a latency of 31.8 +/- 35.3 s from the pons and 10.5 +/- 0.7 s from the medulla
and a duration of 146.7 +/- 95.2 s from the pons and 55.5 +/- 40.4 s from the medulla.
The latency of quisqualate (QU)-induced reduction of neck muscle tone was 30.1 +/-
37.9 s after pontine and 39.5 +/- 21.8 s after medullary injection. The duration of
muscle-tone suppression induced by QU injection into the pons and medulla was 111.5
+/- 119.2 and 169.2 +/- 145.3 s. Smaller rats (8 wk old) had a higher percentage of
sites producing muscle-tone inhibition than larger rats (16 wk old), indicating an
age-related change in the function of brain stem inhibitory systems. The anatomical
distribution of atonia-related sites in the rat has both similarities and differences
with the distribution found in the cat, which can be explained by the distinct anatomical
organization of the brain stem in these two species.
