Electromyographic Analysis of Reflex and Spastic Activities of Canine Pectineus Muscles in the Presence and Absence of Hip Dysplasia

Abstract

SUMMARY AND CONCLUSIONS Stretching of the pectineus muscles of dogs lightly anesthetized with methoxyflurane induced motor unit potential activity in these muscles. These responses of the pectineus muscles were recorded electromyographcally. Stretching was accomplished by application of abductive tension to the stifle. A minimum tension (200 to 400 Gm.) was required to initiate the pectineal response. During application of sustained maximal abductive tension (about 45 kg.), the pectineal response was usually well maintained for at least 10 seconds. The response of the pectineus muscle to stretch was considered to be a myotatic reflex for the following reasons. Denervation and neuromuscular blocking agents abolished the response, indicating that the muscle fibers were not activated directly by stretch. Spinal anesthesia bolished the response, indicating involvement of the spinal cord. Thus, the functional integrity of a neural arc was required for activation of the pectineal response. General anesthesia suppressed or abolished the response, arther indicating that the central nervous system was involved. Application of a dilute local anesthetic solution to the nerve supplying the pectineus muscle and a phenothiazine tranquilizer each suppressed the response resumably by altering the activity of the gamma efferent not system. Since the gastrocnemius (lateral head), trieps (long head), and masseter muscles of the dog had responses to stretch that were much weaker than the pectineal response, it was concluded that the pectineal response was a hyperactive myotatic reflex. The spindle density in the pectineus muscle was thought to contribute significantly to the hyperactivity of the response. The relationship between the pectineal response and hip vsplasia was evaluated in 11 adult German Shepherd logs and 1 adult Doberman Pinscher. A pectineal response pattern which characterized muscles associated with dysplastic coxofemoral joints was not found. The motor unit potential complexes recorded from muscles associated with dysplastic coxofemoral joints, however, were longer in duration and lower in amplitude than those for muscles associated with normal coxofemoral joints. A poststretching period of electrical activity or spasm occurred in 10 muscles of 6 dogs and had a mean duration of 27 seconds. Spastic activity of pectineus muscles was not a characteristic of the presence of hip dysplasia, since it occurred in muscles associated with normal coxofemoral joints as well as in those associated with dysplastic coxofemoral joints. Spastic activity usually consisted of the repetition of a motor unit potential complex of long duration at a frequency of 14/second. Power spectral analyses of pectineal electromyograms for reflex and spastic responses did not reveal a frequency pattern that was characteristically different for muscles associated with dysplastic coxofemoral joints but did reveal a shift toward higher frequencies in the spastic activity. Abnormal electrical activity, such as fibrillation potentials and positive potentials, was not present in the pectineal electromyograms. Thus, neither neuropathy nor myopathy significantly affected the innervation of the pectineus muscles examined in the present study.