Quantitative estimation of sensitivity to muscle stretches in the tonic muscle spindle discharges of cats

Abstract

Abstract-Joint interval histogram (JIH) and power spectrum density (PSD) of GIa unitary discharges were obtained while muscle of an anesthetized cat was stretched steadily. The PSD curve was approximated three lines in the following frequency band, below 15Hz, 15 to 35Hz and above 35Hz. The gradient between 15 to 35 Hz was steep and corresponds to how much the intrafusal fibers were activated. In a pattern of JIH, Ia discharges distributed wide in general. But, interval was regular and histogram made a group when intrafusal fibers were activated by electric stimulation on the ventral rootlet. In particular conditions, peaks were observed around 50 to 60 Hz in PSD. Keywords -Joint interval histogram, a power spectrum density, muscle spindle, intrafusal muscle fibers I. INTRODUCTIONMizote has found two types in afferent discharges of a shortened cat muscle by a combination of steady stretch and vibratory stretch. One is a discharge which comes from a dynamic nuclear bag fiber and the other comes from a static nuclear bag fiber or a nuclear chain fiber [1], [2]. But, response pattern of afferent fibers to vibratory stretches is varied as the muscle is extended and the separation in the afferent discharges was impossible because many impulses with several kinds of amplitude were recruited.We have estimated quantitatively the sensitivity of cat intrafusal muscle fibers to the muscle stretch by the joint interval histogram and by the power spectrum density of inter spike interval of spindle discharges. II. METHODOLOGY A. SubjectsExperimental data, which had already been reported [1] and had been recorded on a magnetic tape, were used for analyses of inter spike interval of cats muscle spindle discharges. Cats were anesthetized intraperitoneally and muscles were dissected free of surrounding tissue, while keeping the blood supply intact as much as possible. All other nerves in the hindlimb were cut except the gastrocnemius and soleus nerves. A laminectomy was performed to expose the dorsal and ventral roots between L5 and S1. They were cut at the entry into the spinal cord. The dorsal root was split into fine filaments for the isolation of single primary afferent fiber, which were identified as spindle endings by their behavior during twitch contractions of the muscle elicited by stimulating its nerve. The initial muscle length was determined by the single shock stimulus of ventral root. The muscle tendon through a steel hook was stretched steadily. Muscle was stretched to several mm till spindle discharges kept firing. Inter spike interval of the tonic discharges of muscle spindle was analyzed.Ventral root L7 was separated into six bundles. Each bundle was used as gamma fusimotor stimulation. Extrafusal muscle fibers were immobilized by gallamine injection, while respiration was kept by a respirator.Because original recordings of experimental data included responses to the vibratory stimuli or (and) to the electric stimulation of ventral rootlet, discharges during vibrations or during ventral stimulations were ejected...