Bilaterality of the jaw-opening reflex in the cat

Physiological tremor in human jaw-muscle system

Small, quasi-rhythmical tremor of the jaw occurs at rest and during voluntary movements. In peripheral limbs, tremor consists of a component due to mechanical resonance properties of the system, and a neurogenic component mediated by a central pacemaker or neural loops. The present study attempted to determine if these components were present in jaw tremor measured with a position transducer held lightly between the incisors. When weights were suspended from the mandible, the tremor frequency was unaltered; sharp taps delivered to the jaw did not elicit any kind of damped oscillations of the system. These findings indicated the absence of mechanical resonance in the system. No correlation of the tremor signal with the electrocardiogram was found. However, a strong correlation was found between the rectified electromyographic signals recorded over the masseter muscles and the tremor signal, where the electromyographic signal preceded jaw movement by 20-30 msec. Frequency-domain analysis also showed positive peaks in a majority of coherence functions between electromyographic and tremor signals. These results suggested a strong neurogenic component of the tremor.

Human jaw-opening muscle responses elicited by multiple-site electrical stimulation

Electrical stimuli were applied to subjects' upper and/or lower gingivae around the right canines; (i) during maintained relaxation of masticatory muscles; (ii) at an active opening position; (iii) while clenching in an incisal edge-to-edge contact (IEC) position; and (iv) at the wide-open position. Reflex responses of the suprahyoid and jaw-closing muscles were obtained using surface electrodes. The electrical stimulation produced segmented reflex excitation(s) in the suprahyoid muscle and conventional reflex excitation and/or inhibition in the jaw-closing muscles when some background activity was maintained in the muscle(s). The excitatory reflex in the suprahyoid muscle responded to multiple site electrical stimulation which was delivered on both the upper and lower jaw simultaneously rather than to single site stimulation. Also, the responses depended on the intensity of the electrical stimulation. In particular, stronger intensities resulted in longer latencies. The results support the suggestions in our previous studies with mechanical stimuli, i.e. that the human jaw-opening reflex can be obtained only when some background activity is maintained in the jaw openers, perhaps due to low threshold afferent input, and that spatial summation may be effective for the reflex.

Comparison of jaw-opening reflex response elicited by anterior and posterior tooth tapping in the cat

The response elicited by various levels of controlled mechanical stimuli to maxillary teeth was studied in 8 adult cats anaesthetized with alpha-chloralose. When an anterior tooth was tapped, short (4-6 ms) and long (16-20 ms) latency responses were recorded from the mylohyoid nerve. On the other hand, when a posterior tooth was tap-stimulated, only the long-latency response was induced. The effects of ether narcosis on the responses were studied. As the short-latency response was induced only by anterior tooth tapping and its amplitude increased according to increases in stimulus intensity, the response may be related to the protection of the mouth from entrance of harmful materials. The long-latency response was induced by light tapping on both anterior and posterior teeth and its amplitude increased according to increases in stimulus intensity to the posterior teeth rather than the anterior teeth. As it was readily inhibited by ether, the long-latency response may have a reflex centre in an area like the brain stem reticular formation which is particularly sensitive to ether and may be related to the regulation of jaw movement in mastication.

Bilateral post-stimulus electromyographic complexes in human masseter muscles after stimulation of periodontal mechanoreceptors of bi- and unilaterally-innervated teeth

These complexes (PSECs) were studied in full-wave rectified and averaged EMG in 13 subjects, who jaw-clenched at a controlled level. The PSECs were elicited by mechanical and electrical stimulation of receptors or their afferents in the unilaterally- and bilaterally-innervated periodontium of the upper first premolars and an upper central incisor. To exclude any contribution from acoustic receptors, subjects were exposed to high-intensity noise during mechanical stimulation. Comparison of peak amplitude and area from PSEC waves in normalized EMG amplitude-time plots suggests extensive crossing of the midline by periodontal afferent information. The small variation in latency of the first inhibitory wave on the two sides suggests that there are no additional synapses in the crossed pathway. Latency differences and wave incidence on the two sides of the later inhibitory and excitatory periods varied markedly between subjects suggesting that influences from higher centres affect masseteric motoneurones. In five subjects stimulation of periodontal receptors around different teeth resulted in different PSEC wave sequences.

Jaw-opening reflex activity in the inferior head of the lateral pterygoid muscle in man

Jaw-opening reflex excitation and inhibition in this muscle and the anterior digastric were only observed during activation and not when the muscles were relaxed. After electrical stimulation of the palate over the greater palatine nerve, reflex excitation of the lateral pterygoid was observed in all subjects with average latency 19.4 ms. Excitation in the anterior digastric muscle was found in only five of the eight subjects with latency 18.6 ms. Excitation of the jaw-opening muscles occurred during the inhibition of the jaw-closing muscles, and conversely, jaw-closing muscle excitation occurred during a silencing of the jaw-opening muscles. This suggests a central control over the facilitation and inhibition of jaw-opening and jaw-closing muscle activity.

Intracellular responses of raphe magnus neurons during the jaw-opening reflex evoked by tooth pulp stimulation

Neurons in the nucleus raphe magnus (RM) may play an important role in the modulation of nociception. To determine how RM neurons are activated during a nociceptive reflex, the intracellular responses of raphe neurons were studied during the jaw-opening reflex (JOR) elicited by tooth pulp shock in lightly anesthetized cats. Tooth pulp stimulation produces reflex EMG activation of the digastric muscle at a latency of 7-11 ms, resulting in jaw opening. Tooth pulp shock that elicits the JOR also produces an EPSP in a subset of raphe neurons. This EPSP consists of an early small depolarization that occurs at a latency of 10-15 ms followed by a larger depolarization at a latency of 20-60 ms. In all cases the latency to EPSP is longer than the latency to digastric EMG onset. Electrical stimulation of the 4 paws elicits oligosynaptic EPSPs in the same cells at a latency of 16-20 ms. Electrical train stimulation of the midbrain periaqueductal gray region (PAG) suppresses the JOR. Single shock stimulation at the same PAG sites that suppress the JOR evokes monosynaptic EPSPs in the large majority of raphe neurons recorded. In all cases, the threshold for EPSP is below the threshold for suppression of the JOR. The EPSP amplitude is a direct function of PAG stimulus intensity and there is temporal summation of EPSPs evoked by paired PAG shocks. At condition-test intervals of 40-90 ms, train stimulation of PAG suppresses the tooth pulp-evoked EPSP in raphe neurons. The threshold for EPSP suppression occurs at a PAG stimulation intensity below that required for suppression of the JOR. The present findings provide evidence that RM neurons may play an important role in the modulation of the tooth pulp-evoked JOR, but only after the initial withdrawal reflex has occurred.

Is the jaw-opening reflex a valid model of pain?

Tooth pulp shock does not produce only pain; low intensity stimulation results in a non-painful sensation that is termed pre-pain. In animals low intensity tooth pulp shock does not evoke escape behavior; the similarity of the animal escape/detection threshold ratio with the human pain/pre-pain threshold ratio is evidence that pre-pain and pain may be present in animals as in humans. Both pre-pain and pain may arise from the activation of a common afferent modality. The TP-JOR does not correlate with the degree of pain experienced under all conditions. The TP-JOR threshold is at or near the sensory detection threshold, at stimulation intensities which evoke pre-pain. Under normal conditions both the magnitude of the TP-JOR response and the degree of pain experienced increase with increasing stimulation intensity. The TP-JOR and the tooth pulp-evoked pain are affected in parallel by sensory habituation and both appear to relay in the rostral trigeminal complex. There are no cases where the TP-JOR is suppressed and pain is still experienced from tooth pulp shock; the suppression of the TP-JOR may therefore be an accurate index of analgesia. However, in humans treatments that produce analgesia have not been shown to produce suppression of the TP-JOR. Thus, the TP-JOR that persists following analgesic treatments is not a reliable index of either analgesia or pain.

Short and long latency jaw-opening reflex responses elicited by mechanical stimulation in man

Jaw-opening reflex responses elicited by tapping the chin during maximum clenching in incisal edge-to-edge contact position were studied in 10 healthy subjects. Stimuli were also delivered during weak clenching on a rubber stamp, separating the incisors by approx. 10 mm and protruding the mandible to the edge-to-edge incisor relationship. All four central incisors were stimulated simultaneously. With weak stimuli, there was a short-latency (9.5 ms) digastric response which may have had a disynaptic pathway. Taps of moderate strength produced long-latency (20 ms) responses, and sometimes a short-latency (9.5 ms) component as well. Strong (non-painful) taps produced an even longer-latency digastric response, 30 ms or more following the stimulus with less synchronization than earlier responses. Jaw-jerk reflexes occurred 8.5 ms following the tap, independently of the magnitude of the stimulus. Local anaesthesia of the upper and lower incisors abolished the digastric muscle response. Thus large periodontal afferents may be responsible for the early digastric reflex activity and smaller fibres for later effects. Temporal summation of the reflex response probably occurred when all incisors were stimulated simultaneously.

Surgical determination of the site of crossing of jaw-opening reflex evoked by tooth pulp stimulation in the cat

Tooth pulp stimulation in the cat evokes a jaw-opening reflex that is usually bilateral. We defined the location of fibers crossing to activate the contralateral reflex by making midline sagittal transections at peripheral and central levels in 11 anesthesized cats. A local midsagittal transection of the brain stem 2-4 mm above the obex selectively eliminated the contralateral reflex response. Midline sagittal transections of the maxilla, mandible, cervical cord, or upper brain stem did not.