[Mechanical contraction properties and roles in velar movements of tensor veli palatini muscle]

The present study was intended to clarify the mechanical contraction properties and to investigate roles in velar movements of the tensor veli palatini (TVP) muscle in dogs. The dogs used in the experiments were anesthetized intra-peritoneally with sodium pentobarbital. Experiment I: Tension of the TVP muscle was induced by stimulation to the motor nerve and was recorded: 1. The contraction time and half relaxation time of maximal isometric twitch of the muscle were 13.7 +/- 0.7 msec and 10.4 +/- 1.2 msec, respectively. 2. A clear summation of muscle contractions was obtained with stimulation at 20 Hz with maximal tetanic tension at 90 Hz. The tetanus twitch tension ratio was found to be 10.7 +/- 1.1. Experiment II: The role of the TVP muscle in velar movements was investigated from the standpoint of upper airway patency. In addition, the effect of fracture of the pterygoid hamulus and transection of the TVP muscle at the pterygoid hamulus on velar movements were also investigated. The results indicated the following: 1. Upon electrical stimulation of the motor nerve of the TVP muscle, the anterior part of the soft palate was displaced laterally with the subsequent expansion of the epipharynx. 2. Negative pressure in the nasal cavity was observed with the respiratory activity of the TVP muscle after tracheostomy, but not in the oral cavity. 3. Contraction of the TVP muscle decreased nasal airway resistance when intranasal airflow was confirmed at levels higher than 120 ml/sec. 4. After fracture of the pterygoid hamulus, contraction of the TVP muscle produced expansion of the epipharynx but less than that prior to the treatment. The fracture did not change nasal airway resistance. 5. After transection of the TVP muscle at the pterygoid hamulus, the contraction of the TVP muscle did not produce any velar movements. Neither expansion of the epipharynx nor changes in nasal airway resistance were observed.