The transduction properties of intercostal muscle mechanoreceptors

Intercostal muscles are richly innervated by mechanoreceptors. In vivo studies of cat intercostal muscle have shown that there are 3 populations of intercostal muscle mechanoreceptors: primary muscle spindles (1 degrees ), secondary muscle spindles (2 degrees ) and Golgi tendon organs (GTO). The purpose of this study was to determine the mechanical transduction properties of intercostal muscle mechanoreceptors in response to controlled length and velocity displacements of the intercostal space. Mechanoreceptors, recorded from dorsal root fibers, were localized within an isolated intercostal muscle space (ICS). Changes in ICS displacement and the velocity of ICS displacement were independently controlled with an electromagnetic motor. ICS velocity (0.5 - 100 microm/msec to a displacement of 2,000 microm) and displacement (50-2,000 microm at a constant velocity of 10 microm/msec) parameters encompassed the full range of rib motion.

Both 1 degrees and 2 degrees muscle spindles were found evenly distributed within the ICS. GTOs were localized along the rib borders. The 1 degrees spindles had the greatest discharge frequency in response to displacement amplitude followed by the 2 degrees afferents and GTOs. The 1 degrees muscle spindles also possessed the greatest discharge frequency in response to graded velocity changes, 3.0 spikes x sec(-1)/microm x msec(-1). GTOs had a velocity response of 2.4 spikes x sec(-1)/microm x msec(-1) followed by 2 degrees muscle spindles at 0.6 spikes x sec(-1)/microm x msec(-1).

The results of this study provide a systematic description of the mechanosenitivity of the 3 types of intercostal muscle mechanoreceptors. These mechanoreceptors have discharge properties that transduce the magnitude and velocity of intercostal muscle length.