Inactivation of the pons blocks medullary-induced muscle tone suppression in the decerebrate cat

Abstract

The pontomedullary region is responsible for the reduction of muscle activity in rapid-eye-movement sleep and contributes to the control of muscle tone in waking. This study sought to clarify the nature of the interaction between the pontine and medullary reticular formation in mediating muscle tone suppression. The degree of medullary-induced neck muscle tone suppression in the decerebrate cat was assessed before and after microinjection of lidocaine into the pontine reticular formation. Medullary stimulation-induced suppression of neck muscle tone was blocked after pontine lidocaine microinjection. The maximum blockade was observed at 16.6 minutes on average after the injection, and recovery occurred within 45 minutes. We conclude that descending mechanisms from the medulla are not sufficient for the triggering of muscle tone suppression. A two-way interaction between the medulla and pons is hypothesized to play a crucial role in the control of muscle tone.

Figure 1.—

Injection sites of lidocaine (A) and stimulating sites in the medulla (B) are shown. Pontine injection sites are transposed onto a section P 3.1 mm, and medullary stimulating sites onto the P 8.5 mm plane. Circles—cat 1; triangles—cat 2; inverted triangles—cat 3. BC—brachium conjunctivum; TRC—central division of the tegmental reticular nucleus; TB—trapezoid body.

Figure 2.—

Two types of blockade of medullary-induced neck muscle tone suppression elicited by pontine lidocaine injection are shown. Long-train stimulation (200 ms trains of 0.2 ms pulses at 100 Hz) was delivered at the closed arrowheads (A-1, B-1). The averaged waveforms of rectified EMG changes induced by short-train stimulation (6.3 ms trains of three 0.2 ms pulses at 330 Hz) are shown in A-2 and B-2. Open arrowheads indicate the beginning of the short train. A constant stimulus intensity (A: 30 μA, B: 40 μA) was used during each trial. Preinjection: Both in A and B, long and short trains suppressed neck muscle tone bilaterally. Blockade: Both in A and B, long trains failed to reduce muscle activity. Short trains produced a reduction of muscle tone, though with a lesser degree of magnitude of suppression as compared with waveforms obtained before lidocaine injection. In B but not in A, long trains elicited excitation, and short trains also contained muscle facilitation (arrows) prior to the attenuated suppression. Recovery: Both in A and B, long and short trains again produced neck muscle tone suppression bilaterally. L: left (ipsilateral), R: right (contralateral). Calibration: the bars below long-train stimulations (1); 5 seconds, below short-train stimulation (2); 50 ms.