Modulation of human jaw reflexes: Heterotopic stimuli and stress

Facial nerve stimulation as a future treatment for ischemic stroke

Stimulation of the autonomic parasympathetic fibers of the facial nerve system (hereafter simply “facial nerve”) rapidly dilates the cerebral arteries and increases cerebral blood flow whether that stimulation is delivered at the facial nerve trunk or at distal points such as the sphenopalatine ganglion. Facial nerve stimulation thus could be used as an emergency treatment of conditions of brain ischemia such as ischemic stroke. A rich history of scientific research has examined this property of the facial nerve, and various means of activating the facial nerve can be employed including noninvasive means. Herein, we review the anatomical and physiological research behind facial nerve stimulation and the facial nerve stimulation devices that are in development for the treatment of ischemic stroke.

Similar alteration of motor unit recruitment strategies during the anticipation and experience of pain

A motor unit consists of a motoneurone and the multiple muscle fibres that it innervates, and forms the final neural pathway that influences movement. Discharge of motor units is altered (decreased discharge rate and/or cessation of firing; and increased discharge rate and/or recruitment of new units) during matched-force contractions with pain. This is thought to be mediated by nociceptive (pain) input on motoneurones, as demonstrated in animal studies. It is also possible that motoneurone excitability is altered by pain related descending inputs, that these changes persist after noxious stimuli cease, and that direct nociceptive input is not necessary to induce pain related changes in movement. We aimed to determine whether anticipation of pain (descending pain related inputs without nociceptor discharge) alters motor unit discharge, and to observe motor unit discharge recovery after pain has ceased. Motor unit discharge was recorded with fine-wire electrodes in the quadriceps of 9 volunteers. Subjects matched isometric knee-extension force during anticipation of pain (anticipation: electrical shocks randomly applied over the infrapatellar fat-pad); pain (hypertonic saline injected into the fat-pad); and 3 intervening control conditions. Discharge rate of motor units decreased during pain (P<.001) and anticipation (P<.01) compared with control contractions. De-recruitment of 1 population of units and new recruitment of another population were observed during both anticipation and pain; some changes in motor unit recruitment persisted after pain ceased. This challenges the fundamental theory that pain-related changes in muscle activity result from direct nociceptor discharge, and provides a mechanism that may underlie long-term changes in movement/chronicity in some musculoskeletal conditions.

The inhibitory effect of a chewing task on a human jaw reflex

This study was undertaken to investigate whether an inhibitory jaw reflex could be modulated by experimentally controlled conditions that mimicked symptoms of temporomandibular disorders. Reflecting on previous work, we anticipated that these conditions might suppress the reflex. Electromyographic recordings were made from a masseter muscle in 18 subjects, while electrical stimuli were applied to the upper lip. An inhibitory reflex wave (mean latency 47 ms) was identified and quantified. Immediately following an accelerated chewing task, which in most cases produced muscle fatigue and/or pain, the size of the reflex wave decreased significantly by about 30%. The suppression of inhibitory jaw reflexes by fatigue and pain may result in positive feedback, which may contribute to the symptoms of temporomandibular disorders. Future studies of temporomandibular disorder sufferers will help to determine whether such reflex changes reflect the underlying etiology and/or are a result of the temporomandibular disorder itself.

Involvement of ERK phosphorylation in brainstem neurons in modulation of swallowing reflex in rats

In order to evaluate the neuronal mechanisms underlying functional abnormalities of swallowing in orofacial pain patients, this study investigated the effects of noxious orofacial stimulation on the swallowing reflex, phosphorylated extracellular signal-regulated kinase (pERK) and gamma-aminobutyric acid (GABA) immunohistochemical features in brainstem neurons, and also analysed the effects of brainstem lesioning and of microinjection of GABA receptor agonist or antagonist into the nucleus tractus solitarii (NTS) on the swallowing reflex in anaesthetized rats. The swallowing reflex elicited by topical administration of distilled water to the pharyngolaryngeal region was inhibited after capsaicin injection into the facial (whisker pad) skin or lingual muscle. The capsaicin-induced inhibitory effect on the swallowing reflex was itself depressed after the intrathecal administration of MAPK kinase (MEK) inhibitor. No change in the capsaicin-induced inhibitory effect was observed after trigeminal spinal subnucleus caudalis lesioning, but the inhibitory effect was diminished by paratrigeminal nucleus (Pa5) lesioning. Many pERK-like immunoreactive neurons in the NTS showed GABA immunoreactivity. The local microinjection of the GABA(A) receptor agonist muscimol into the NTS produced a significant reduction in swallowing reflex, and the capsaicin-induced depression of the swallowing reflex was abolished by microinjection of the GABA(A) receptor antagonist bicuculline into the NTS. The present findings suggest that facial skin-NTS, lingual muscle-NTS and lingual muscle-Pa5-NTS pathways are involved in the modulation of swallowing reflex by facial and lingual pain, respectively, and that the activation of GABAergic NTS neurons is involved in the inhibition of the swallowing reflex following noxious stimulation of facial and intraoral structures.

Supraspinal modulation of trigeminal nociception and pain

OBJECTIVE

This study examined modulation of trigeminal pain/nociception by 2 supraspinal mechanisms: emotional controls of nociception and diffuse noxious inhibitory controls.

BACKGROUND

Prior research suggests emotional picture viewing (emotional controls) and tonic noxious stimuli (diffuse noxious inhibitory controls) engage supraspinal mechanisms to modulate pain and nociceptive processes. It is currently unknown, however, whether emotional controls modulate trigeminal pain and nociception. Additionally, the influences of emotional controls and diffuse noxious inhibitory controls have not been compared in the same group of participants.

METHODS

Noxious electrodermal stimuli were delivered to the trigeminal nerve using a concentric electrode designed to selectively activate nociceptive fibers. Trigeminal nociception and pain were assessed (34 participants) from the nociceptive blink reflex and pain ratings, respectively. Emotional controls were engaged by presentation of standardized picture stimuli (pleasant, neutral, and unpleasant) shown to reliably evoke pleasure-induced inhibition and displeasure-induced facilitation of pain and nociception. Diffuse noxious inhibitory controls were engaged with a forearm ischemia task.

RESULTS

Trigeminal pain (self-report ratings) and nociception (blinks) were facilitated by unpleasant pictures and inhibited by pleasant pictures. Emotion induction (as assessed from trend analysis) explained 51% of the variance in trigeminal pain and 25% of the variance in trigeminal nociception. Additionally, forearm ischemia inhibited trigeminal pain but not nociception. The baseline vs ischemia comparison explained 17% of the variance in pain report and 0.1% of the variance in blinks. Supraspinal modulation by emotional controls and diffuse noxious inhibitory controls were uncorrelated.

CONCLUSIONS

Emotional controls and diffuse noxious inhibitory controls modulated trigeminal pain and emotional controls modulated trigeminal nociception. These procedures can be used to study supraspinal modulation of nociceptive processing in disorders of the trigeminal pain system, including headache.

Noxious Lingual Stimulation Influences the Excitability of the Face Primary Motor Cerebral Cortex (Face MI) in the Rat

The mechanisms whereby orofacial pain affects motor function are poorly understood. The aims were to determine whether 1) lingual algesic chemical stimulation affected face primary motor cerebral cortex (face MI) excitability defined by intracortical microstimulation (ICMS); and 2) any such effects were limited to the motor efferent MI zones driving muscles in the vicinity of the noxious stimulus. Ketamine-anesthetized Sprague–Dawley male rats were implanted with electromyographic (EMG) electrodes into anterior digastric, masseter, and genioglossus muscles. In 38 rats, three microelectrodes were located in left face MI at ICMS-defined sites for evoking digastric and/or genioglossus responses. ICMS thresholds for evoking EMG activity from each site were determined every 15 min for 1 h, then the right anterior tongue was infused (20 μl, 120 μl/h) with glutamate (1.0 M, n = 18) or isotonic saline ( n = 7). Subsequently, ICMS thresholds were determined every 15 min for 4 h. In intact control rats ( n = 13), ICMS thresholds were recorded over 5 h. Only left and right genioglossus ICMS thresholds were significantly increased (≤350%) in the glutamate infusion group compared with intact and isotonic saline groups ( P < 0.05). These dramatic effects of glutamate on ICMS-evoked genioglossus activity contrast with its weak effects only on right genioglossus activity evoked from the internal capsule or hypoglossal nucleus. This is the first documentation that intraoral noxious stimulation results in prolonged neuroplastic changes manifested as a decrease in face MI excitability. These changes appear to occur predominantly in those parts of face MI that provide motor output to the orofacial region receiving the noxious stimulation.

Modulation of an inhibitory jaw reflex by remote noxious stimulation: effects of spatial conditioning factors

In humans, inhibitory jaw reflexes can be depressed by painful stimulation of remote parts of the body. The underlying mechanisms may involve diffuse noxious inhibitory controls (DNIC). Animal experiments have shown that the neurons which may mediate DNIC show spatial encoding (i.e. their responses vary in relation to the size of the body area being stimulated). The aim of this study was to investigate whether the modulation of an inhibitory jaw reflex shows similar spatial dependency. Electromyographic recordings were made in 9 subjects, from a masseter muscle that was activated to a level equivalent to 10% of that obtained during a maximum voluntary contraction. Reflex inhibitions were evoked by electrical stimuli to the upper lip, either alone (controls) or during the application of conditioning stimuli (47 degrees C water) to the fingers, the hand, the half forearm or the whole forearm. Conditioning stimuli applied to the larger but not to the smaller areas resulted in significant modulations of the reflex. There was a significant correlation between stimulus area and reflex magnitude. These results demonstrate a spatial dependency for the modulation of an inhibitory jaw reflex by painful stimuli -- a further parallel with DNIC as studied on single neurons in animals.

Dissociation of nociceptive modulation of a human jaw reflex from the influence of stress

In human beings, inhibitory jaw reflexes can be depressed by painful stimulation of remote parts of the body. Since similar effects can be produced by the stress of anticipating pain, we wished to investigate whether the effects of remote painful stimuli are dependent on stress. EMG recordings were made from a masseter muscle while subjects maintained activity in the muscle at approximately 12.5% of maximum using visual feedback. The protocols involved three sequences: (1) "standard controls" in which reflexes were evoked by electrical test stimuli applied to the upper lip; (2) "standard conditioning" in which painful electrical conditioning stimuli were applied over the sural nerve 100 ms before the test stimuli; (3) "random sequences" in which test-only and conditioning-test combinations were employed in a double-blind, random, order. Data are presented as means +/- SEMs. In the standard controls, the stimuli evoked clear inhibitory reflexes (latency 37 +/- 1.3 ms, duration 62 +/- 5.6 ms; n = 10) in all the subjects. During standard conditioning, the reflex magnitude was reduced significantly (by 50.0 +/- 8.5%, P = 0.0002, one-sample t-test). When the test-only and conditioning-test responses were extracted from the random sequences, there was also a significant reduction in the reflex magnitude following conditioning (by 34.6 +/- 5.5%, P = 0.0002, one-sample t-test) albeit less so than between the standard sequences (P = 0.03, paired t-test). A second series of experiments suggested that these lesser effects during the random sequences were not substantially due to any loss of temporal summation of the conditioning mechanisms. The evidence for this was that application of pairs of conditioning stimuli did not produce a significantly greater effect than single conditioning stimuli within a random sequence (39.9 +/- 9.6% as opposed to 32.7 +/- 9.1% reductions in the reflex, P = 0.117, paired t-test). Therefore since any stress in the random sequences would not have been "tied" to the conditioned responses alone, the effects of remote painful stimuli on this inhibitory jaw reflex cannot be entirely secondary to stress.