RESUMEN
1. Small 'step' or sinusoidal displacements were imposed on the mandible while human subjects maintained an average biting force of 10 N. Phase-related changes in the force resisting sinusoidal displacement were used to determine the mechanical stiffness of the human mandibular system as a function of the frequency of stretching. 2. Jaw-muscle electromyographic (e.m.g.) responses to 'step' stretches were of 8 msec latency and generated a very substantial force response. Jaw-muscle e.m.g. responses having longer latency were not observed. 3. The mechanical stiffness of the human mandible was relatively constant as a function of the frequency of stretching, having a typical magnitude of about 15 N/mm (+/- 200 micrometers stretch) or 10 N/mm (+/- 1500 micrometers stretch) at mean biting forces of 10 N. The force resisting displacement was phase-advanced at all frequencies. 4. Modulation of jaw muscle electrical activity evoked by sinusoidal stretches increased in amplitude as a function of increasing stretch frequency. E.m.g. modulation was 60--100 degrees advanced at frequencies of 1--10 Hz, but the phase decreased at higher frequencies, becoming negative (lagging stretch) at frequencies of 30 Hz and above. These characteristics are consistent with the idea that the jaw stretch reflex is dependent on jaw muscle spindle afferent fibres exciting jaw-closing motoneurones by relatively direct (but not necessarily monosynaptic) connexions. 5. The relationship between jaw-muscle activity and voluntary fluctuations of isometric biting force suggests that human jaw muscles can be modelled as a second-order linear 'filter'. The corner frequency for human jaw muscle is about 3 Hz; thus it would appear to be considerably slower than jaw muscle of monkeys. 6. The reflex stiffness of the human mandible, estimated quantitatively on the assumption that human jaw muscle stiffness is similar to the intrinsic stiffness of the gastrocnemius of the cat, ranges between 5 and 9 N/mm at frequencies between 1 and 8 Hz. Since this reflex stiffness is about the same as muscle stiffness in this frequency range, we conclude that the stretch reflex of the human mandible contributes functionally to its postural stability. 7. Reflex stiffness appears to be greater in the monkey mandible relative to muscle stiffness than in the human mandible. The difference is argued to be a manifestation of the difference in jaw muscle contraction speed between the two species. 8. The fact that the mandibular stretch reflex appears to be stronger than the stretch reflex of the limbs of intact animals and humans is discussed in terms of the special anatomical and functional features of the mandible.
Asunto(s)
Mandíbula/fisiología , Músculo Masetero/fisiología , Músculos Masticadores/fisiología , Reflejo de Estiramiento , Adulto , Niño , Electromiografía , Femenino , Humanos , Masculino , Persona de Mediana Edad , Contracción MuscularRESUMEN
This research represents part of a series of investigations into the function of mechanoreceptor feedback in lip motor control for speech. In the present study, precise mechanical stretches were applied to subjects' lips while they held the firing frequency of a given single motor unit relatively constant using audio and visual feedback. Preliminary data indicate that mechanical stimulation produces a range of short and long latency excitatory and inhibitory effects in perioral motor units. Of particular interest are short latency inhibitory effects in the depressor labii inferior and platysma muscles. These short latency inhibitory effects suggest that the central organization of the perioral sensorimotor system may possess a form of reciprocal innervation. The operation of a circuit is described which permits improved experimental control over the timing of stretch application relative to moter unit discharge. Initial data collected with this circuit suggest that it may be effectively used to assess the time course of subthreshold excitability changes associated with individual motor units during their repeated discharge.
Asunto(s)
Labio/fisiología , Neuronas Motoras/fisiología , Habla/fisiología , Adulto , Retroalimentación , Humanos , Masculino , Movimiento , Músculos/fisiología , Tiempo de ReacciónRESUMEN
Language-disordered children were presented with two types of commands in a sentence comprehension task. One 3-sec pause was placed at the same syntactic boundary in one-half of the sentences in Set I and Set II. Results indicate that the subjects' performance improved as a function of the pause in Sentence Set II but not in Sentence Set I.