Modulation of masseter exteroceptive suppression by non-nociceptive upper limb afferent activation in humans

The effects induced by non-noxious electrical stimulation of upper limb nerves on exteroceptive suppression (ES) of masseter muscle EMG activity were studied in 15 healthy subjects. EMG activity of masseter muscles was recorded bilaterally and great care was taken to minimise the activation of afferents other than the stimulated ones. Masseter ES was elicited by applying a non-noxious electrical stimulus to the skin above the mental nerve (Mt) of one side, during a voluntary contraction of masseter muscles at a prescribed steady clenching level. Onset and offset latencies and duration of early and late components of masseter ES (ES1 and ES2, respectively) were evaluated in control conditions and compared to those obtained when a non-noxious electrical stimulation was delivered separately to Med or Rad or simultaneously to both nerves (Med-Rad) of one side. Upper limb nerve stimulation could be simultaneous or it could precede or follow Mt stimulation by various time intervals. In control conditions, ES1 latency onset and duration values (mean +/- SD) were 11.3+/-2.9 ms and 16.9+/-2.1 ms, respectively, and ES2 latency onset and duration values were 44.5+/-6.0 ms and 28.6+/-11.1 ms, respectively. No significant differences were observed which were related to the side being recorded. Two types of effects, opposite in nature, were shown on masseter ES, depending on the time intervals between Mt and upper limb nerve stimulation. The first effect, which was facilitatory, consisted of a significant increase in ES1 and ES2 duration. A maximal increase in ES1 duration (134-155% compared to control value) occurred when upper limb nerve stimulation preceded that of Mt by 18-30 ms. Maximal ES2 lengthening (115-145%) was observed when upper limb nerve stimulation followed that of the Mt by 10 ms. The second effect was inhibitory and affected only ES2, which appeared completely eliminated when Med stimulation preceded that of Mt by 40-80 ms. By contrast, ES1 was never suppressed at any interstimulus interval. These data might reflect the different action of the central outflow, following the upper limb-induced effects, on the different neuronal circuits mediating ES1 and ES2.     

Task-dependent control of human masseter muscles from ipsilateral and contralateral motor cortex

Corticotrigeminal projections to human masseter motoneuron pools were investigated with focal transcranial magnetic stimulation (TMS). Responses in left and right masseter muscles were quantified from the surface electromyogram (EMG) during different biting tasks. During bilateral biting, TMS elicited motor evoked potentials (MEPs) in both masseter muscles. On average, the MEP area in the masseter contralateral to the stimulus was 39% larger than in the ipsilateral muscle, despite comparable pre-stimulus EMG in both muscles. MEPs elicited while subjects attempted unilateral activation of one masseter muscle were compared with those obtained in the same muscle during a bilateral bite at an equivalent EMG level. MEPs in the masseter contralateral to the stimulated hemisphere were significantly smaller during unilateral compared with bilateral biting. There was no significant difference in the size of ipsilateral MEPs during ipsilateral and bilateral biting. We conclude that the corticotrigeminal projections to masseter are bilateral, with a stronger contralateral projection. The command for unilateral biting is associated with a reduced excitability of corticotrigeminal neurons in the contralateral, but not the ipsilateral motor cortex. We suggest that this may be accomplished by reduced activity of a population of corticotrigeminal neurons which branch to innervate both masseter motoneuron pools.     

Heterotopic nociceptive EMG-reactions in M. masseter

Peculiarities of EMG-reactions of m. masseter to noxious homo- and heterotopic stimulation were studied on healthy volunteers. Homotopic noxious electrical stimulation of n. mentalis elicited several successive responses in m. masseter consisting of R-component, early exteroceptive suppression period, early excitation period, late exteroceptive suppression period, and late excitation period. Heterotopic noxious stimulation (forehead, ear lobe, index finger) induced only the late exteroceptive suppression period and late excitation period. It is concluded that the excitatory components have a reflex nature, and that the late exteroceptive suppression period is formed under the effect of central cerebral structures, which mediate their influence via the spinal-corticospinal return loop.     

Enhanced temporal summation of pressure pain in the trapezius muscle after delayed onset muscle soreness

Temporal summation of muscle pain is an important factor in musculoskeletal pain as central integration of repetitive nociceptive input can be facilitated in musculoskeletal pain patients. The aim of this study is to evaluate changes in temporal summation of pressure pain after induction of delayed onset muscle soreness (DOMS) of the trapezius muscle. Sixteen healthy volunteers participated in the study. Temporal summation of pain was induced by sequential pressure stimulation by a computer-controlled algometer. Sequential stimulation consisting of ten stimuli (at pressure pain threshold intensity) was applied over the trapezius muscle. Stimulus duration was 1 s and inter-stimulus intervals (ISI) were 1, 5, 10, and 30 s, respectively. The pain was rated on a continuous visual analogue scale (VAS, 10 cm) after each stimulus and normalised to the VAS score from the first stimulus. DOMS was induced in the right trapezius muscle by eccentric shoulder exercises while the left trapezius muscle served as control. Temporal summation of pressure evoked pain was measured before and 24 h after the exercise. At 24 h after exercise, soreness intensity during shoulder elevation was 3.7±0.2 cm, while no soreness was observed on the control side. When sequential pressure stimulation was applied to the DOMS muscle, VAS scores for 1 s ISI progressively increased to a higher level than before exercise (VAS increase for the last stimulus: 0.8±0.2 cm vs. 0.6±0.1 cm, P<0.05), while VAS scores for ISI 5, 10, and 30 s were not increased. On the control side, significant increases in VAS scores was observed for all ISIs but not affected by contralateral DOMS. Facilitation of temporal summation for 1 s ISI indicated that DOMS may increase the central excitability besides involving peripheral sensitisation. During DOMS there was no potential for further nociceptor sensitisation by repeated noxious pressure stimuli, which may account for the diminishment of temporal summation evoked by pressure stimuli with ISI 5, 10, and 30 s. These data indicate that muscle soreness might facilitate the central components of temporal summation to mechanical stimulation.     

Brainstem evaluation in children with primary nocturnal enuresis

We investigated the brainstem integrity in children with primary nocturnal enuresis (PNE) using auditory brainstem responses (ABR), blink reflex and exteroceptive suppression of the masseter muscle. We examined 23 children with PNE (16 male, 7 female; mean age: 10.4 years) and 19 control subjects (11 male, 8 female; mean age: 11.8 years). ABR parameters such as wave latencies, amplitudes and interpeak latencies and blink reflex parameters such as R1 and R2 amplitude and latencies were not significantly different between the 2 groups. Although S2 parameters of the exteroceptive suppression of the masseter muscle were easily and completely obtained from the control subjects, in the PNE group S2 onset latency and duration were not recorded in 26% of the study children (n = 6) (P = 0.01). S2 duration time was significantly lowered in the enuretic group (left side: P = 0.001 and right side: P = 0.003). S2 duration time changes in the enuretic group supports a possible brainstem dysfunction in children with PNE.     

Non-nociceptive upper limb afferents modulate masseter muscle EMG activity in man

Recent electrophysiological data obtained in anaesthetized rats evidenced jaw muscle excitatory responses to the electrical stimulation of type II limb somatosensory afferents. In the present work, we describe an inhibitory reflex evoked in human masseter muscles by stimulation of non-nociceptive fibres travelling in the median and radial nerves (MED and RAD, respectively). Eighteen healthy volunteers participated in the study. Subjects were seated on a comfortable chair, with the complex head-mandible-neck-trunk and the limbs securely fixed to the chair. Attempts were made to minimize possible interferences due to the activation of afferents other than the stimulated ones. The subjects were instructed to contract masseter muscles at a submaximal level and to maintain a stable level of muscle contraction during all trials. EMG voluntary activity was recorded from both masseter muscles by means of coaxial needle electrodes before and after the electrical stimulation of MED and/or RAD at intensities below pain threshold. In all subjects, MED stimulation induced bilaterally a marked depression of masseter EMG activity, which occurred at a latency of 23.6 +/- 2.1 ms and lasted 27.8 +/- 6.6 ms. RAD stimulation also induced a marked reduction in masseter EMG activity, but this effect was clearly observed in 9 out of 18 subjects, and it showed latency (30.2 +/- 7.5 ms) and duration (44.9 +/- 5.4 ms) significantly longer in comparison with the MED-induced effect. All subjects exhibited the inhibitory period in masseter EMG following the simultaneous stimulation of both nerves; this one appeared at a latency not significantly different (25.3 +/- 5.9 ms) and lasted much more (37.4 +/ - 8.2 ms) than EMG depression evoked by MED stimulation. The duration of masseter muscle inhibition, induced by MED and/or RAD stimulation, was inversely related to the level of EMG activity, while latency was not related to it. Significant increases in the inhibitory period duration were also observed by increasing stimulus intensity, within a subthreshold range for the activation of nociceptive fibres. In all cases, the inhibitory period was followed by a later excitatory rebound activity, whose latency and duration depended on the duration of the preceding EMG inhibition and on the background level of masseter activation. In conclusion, results evidenced that the activation of arm somatosensory fibres modulates masseter muscle activity in normal man. This might lead to a coordination between limb and masticatory muscle activity, which is required in several complex motor acts.     

Exteroceptive suppression of temporalis and masseter muscle activity is enhanced in offspring of hypertensives

Exteroceptive suppression of temporalis and masseter muscle activity was examined in young men with and without a parental history of hypertension. Recent clinical studies suggest that the second exteroceptive suppression period is attenuated in several chronic pain disorders and that this brainstem reflex may serve as a noninvasive index of endogenous pain control. In the present study, offspring of hypertensives exhibited a significant protraction of the late exteroceptive suppression period for both muscle sites, suggesting that the decreased pain sensitivity previously observed in individuals at risk for hypertension may be related to enhanced central pain modulation.     

Experimental skin pain and muscle pain induce distinct changes in human trigeminal motoneuronal excitability

Seeking information on the physiological properties of the trigeminal motoneuronal pool we investigated changes in the excitability of trigeminal motor system induced by two types of experimental pain (muscle and skin). In one session, we studied the effect of muscle pain induced by hypertonic saline infusion into the masseter muscle on the recovery cycle of the heteronymous H-reflex in the temporalis muscle and the homonymous silent period (SP) in the masseter muscle, both elicited by stimulation of the masseteric nerve in ten-healthy subjects. In another session, we studied the effect of laser stimuli applied to the perioral region, at conditioning intervals from 20 to 160 ms, on the temporalis H-reflex and masseter SP in nine healthy subjects. Whereas laser-induced skin pain significantly inhibited the temporalis H-reflex and facilitated the masseter SP (P < 0.01), muscle pain left the time course of the temporalis H-reflex and masseter SP unchanged (P > 0.05). The timing of temporalis H-reflex suppression and masseter-SP enhancement induced by laser stimuli indicates that facial skin nociceptors inhibit trigeminal motoneurones via multysynaptic reflex pathways. Hypertonic saline, a stimulus that predominantly activates group III and IV afferents, left both variables reflecting trigeminal motoneuron excitability unchanged. Due to the differences between the two experimental models, we cannot conclude that such inhibitory reflex pathway does not exist from muscle nociceptors to trigeminal motoneurones.     

Experimental jaw-muscle pain does not change heteronymous H-reflexes in the human temporalis muscle

Muscle pain generally has an inhibitory effect on voluntary orofacial motor function. However, it is not known whether muscle pain causes direct or indirect changes in motoneuron excitability. In this study a monopolar needle stimulation technique was used to evoke the direct motor response (M-response) in the left masseter muscle and the heteronymous H-reflex in the left temporalis muscle as an indirect measure of motoneuron excitability. Series of 20 repeated electrical stimuli were delivered at 50% of maximal voluntary contraction (MVC) before, during, and after periods with experimental jaw-muscle pain in 11 healthy subjects. Pain was induced by standardized infusion of hypertonic (5%) saline into the mid-portion of the right masseter muscle. The mean pain intensity rating on a 100-mm visual analog scale was 42±5 mm. The short-latency responses (less than 6 ms) could be evoked in all subjects. Analysis of the latency and amplitude of the temporal H-reflex indicated no significant effect of jaw-muscle pain. The amplitude of the masseteric M-response was significantly smaller in the postpain condition than in the pain conditions (ANOVA, P=0.018), but no differences were found between the prepain and postpain conditions. In nine subjects, poststimulus periods (mean offset latency, 69.6±8.6 ms) with significantly (more than 50%) suppressed EMG activity were detected in the ipsilateral masseter muscle following the M-response (mean offset latency, 5.5±0.2 ms). These reflex responses did not show a systematic change during the pain conditions. In conclusion, acute contralateral jaw-muscle pain does not seem to modulate the motoneuron excitability as measured by the heteronymous H-reflex. Received: 7 November 1997 / Accepted: 16 February 1998     

Task dependence of human masseter motor unit reflex behaviour

Summary Motor unit (MU) firing frequency is an important determinant of reflex inhibition in the human jaw muscles. Masseter MUs may be driven steadily by various intraoral tasks, but their lowest sustainable firing frequency varies according to task. In this study we examined the effect of task on masseter MU reflex behaviour under controlled conditions, in which the prestimulus MU firing frequency and stimulation were constrained. All MUs tested were inhibited by a non-noxious electrical stimulus applied to the oral mucosa, but there were significant differences in the magnitude of single MU inhibition depending on the task employed to drive the MUs. It appears that single masseter MU reflex behaviour can alter according to task, even when the prestimulus excitation of the masseter motoneuron pool is apparently constant. This suggests that masseter MU reflex behaviour may be modulated by task-related peripheral afferent input.     

Participation of mono- and polysynaptic transmission during tonic activation of the stretch reflex arcs.

Through intracellular observations of the cat spinal motoneuron both mono- and polysynaptic reflex arcs were shown to participate in repetitive activation of stretch reflex. Experiments were performed on 25 cats anesthetized by intraperitoneal (i.p.) injection of 3 cc/kg of a mixture of 1% chloralose and 10% urethane. Driving of the motoneuronal discharges accomplished by electric stimulation (16-100 Hz) of the gastrocnemius lateralis and medialis nerves. Repeated electric stimulation of muscle nerves elicited monosynaptic "vibratory" EPSPs and a polysynaptic "augmenting" EPSP in the motoneuron. The firing of the motoneuron occurred when a temporal summation of the "vibratory" EPSPs was sufficiently obtained. The discharge frequency of motoneuron, MNf, was expressed as a product of electrostimulus frequency, Sf, and reciprocal of an integer, n, i.e. MNf=(1/n)Sf. An increase in the stimulus intensity resulted in a dimimution of the vibratory EPSP and a remarkable increase of n. This was assumed to be due to an inhibitory process produced by the Group II fibers. The minimum value of integer n was always a determinant factor for the maximum value of MNf which was referred to as the "preferred" frequency of the motoneuron. Long-lasting electric stimulation of muscle nerves was followed by a more remarkable recruitment of the augmenting EPSP which made the motoneuron attain its critical threshold of firing. Motoneuronal spikes elicited by such a large augmenting EPSP were not locked to the Ia impulses. Unlocked spikes were observed only in a later stage of electric stimulation of Ia fibers.     

Modulation of exteroceptive suppression periods in human jaw-closing muscles induced by summation of nociceptive and non-nociceptive inputs

Convergence of various afferent inputs onto brain-stem neurons may play an important role in the regulation of trigeminal motor function. The present study examined the efficacy of nociceptive and non-nociceptive perioral stimulation for the modulation of the two exteroceptive suppression periods (ES1 and ES2) in human jaw-closing muscles. The inhibitory jaw-reflexes, ES1 (10-15 ms onset) and ES2 (40-50 ms onset), were recorded as the surface electromyogram of masseter and temporalis muscles in 14 healthy subjects. Non-painful electrical stimuli were applied to the right mental nerve while the subject was biting at 50% of the maximal voluntary force. Five conditions were compared: baseline, repetitive tactile stimulation (1 Hz) on the right cheek, topical application of capsaicin (5%) on the right cheek, repetitive tactile stimulation plus capsaicin, and postbaseline. The perceived intensity of the electrical stimuli was evaluated by the subjects on a 0-10 rating scale. Capsaicin alone failed to induce significant changes of ES1 and ES2; tactile stimulation alone induced a significant delay in the onset of ES2 (P < 0.001). During repetitive tactile stimulation plus capsaicin, the duration of ES1 and ES2 was shortened (ES1 and ES2: P < 0.001) and the degree of suppression was reduced (ES1: P < 0.05; ES2: P < 0.005). Perceived intensity of the electrical stimuli was unchanged during the different experimental conditions. The present results suggest that summation of simultaneous nociceptive and non-nociceptive inputs plays an important role in the modulation of the neural pathways involved in the short-latency ES1 and long-latency ES2.     

H-reflexes in masseter and temporalis muscles in man

In contrast with limb muscles, studies on H-reflexes in the trigeminal system are scarce. The present report aimed at reevaluating the responses obtained in the masseter and temporalis muscles after electrical stimulation of their nerves. Twenty-four subjects participated in the experiments. The reflexes were elicited in the masseter and temporal muscles by monopolar stimulation and recorded using surface electrodes. Stimulation of the masseteric nerve evoked an M-response in the masseter and an H-reflex in both the masseter and the temporal muscles. In contrast with the masseter muscle, where the homonymous H-reflex disappeared at higher stimulation intensities, the heteronymous temporal H-reflex remained and reached a plateau. Simultaneous stimulation of the masseteric and deep temporal nerves resulted in an M-response and an H-reflex in both the masseter and temporal muscles. Increasing stimulus intensitites led to disappearance of the H-reflex in both muscles. The results were compared with those obtained by others on limb muscles. As in these muscles, the presence of heteronymous H-reflexes in the jaw muscles can be used in future studies of motoneuronal excitability.     

Jaw reflexes evoked by mechanical stimulation of teeth in humans.

Jaw reflexes evoked by mechanical stimulation of teeth in humans. The reflex response of jaw muscles to mechanical stimulation of an upper incisor tooth was investigated using the surface electromyogram (SEMG) of the masseter muscle and the bite force. With a slowly rising stimulus, the reflex response obtained on the masseter SEMG showed three different patterns of reflex responses; sole excitation, sole inhibition, and inhibition followed by excitation. Simultaneously recorded bite force, however, exhibited mainly one reflex response pattern, a decrease followed by an increase in the net closing force. A rapidly rising stimulus also induced several different patterns of reflex responses in the masseter SEMG. When the simultaneously recorded bite force was analyzed, however, there was only one reflex response pattern, a decrease in the net closing force. Therefore, the reflex change in the masseter muscle is not a good representative of the net reflex response of all jaw muscles to mechanical tooth stimulation. The net response is best expressed by the averaged bite force. The averaged bite force records showed that when the stimulus force was developing rapidly, the periodontal reflex could reduce the bite force and hence protect the teeth and supporting tissues from damaging forces. It also can increase the bite force; this might help keep food between the teeth if the change in force rate is slow, especially when the initial bite force is low.     

Sensory neurons and motoneurons of the jaw-closing reflex pathway in rats: a combined morphological and physiological study using the intracellular horseradish peroxidase technique

Summary Motoneurons and muscle spindle afferents of the rat masseter muscle were physiologically and morphologically characterized. Their soma-dendritic morphology and axonal course were investigated using the intracellular horseradish peroxidase method. Following electrical stimulation of the masseter nerve, individual motoneurons were identified by antidromic all-or-none action potentials and individual sensory neurons by orthodromic action potentials. Using threshold separation an excitatory input from muscle spindles to a masseter motoneuron was demonstrated. The short latency difference of 0.34 ms between the mean orthodromic response in the sensory neurons and the beginning of the synaptic potential in the masseter motoneuron suggests a monosynaptic connection between the spindle afferents and the motoneurons. Following intrasomatic horse-radish peroxidase injection large multipolar cell bodies of masseter motoneurons were found within the motor nucleus. Their positions corresponded to the topographic organization of the motor trigeminal nucleus as described in retrograde tracing studies. Dendrites of masseter motoneurons were complex and could be found far beyond the nuclear borders. Distal dendrites extended to the mesencephalic trigeminal nucleus, the supratrigeminal nucleus, the lateral lemniscus and the reticular formation. Within the reticular formation dendrites were seen in the intertrigeminal nucleus and the peritrigeminal zone. Unipolar cell bodies of muscle spindle afferents were found in the mesencephalic trigeminal nucleus after intra-axonal injection of horseradish peroxidase. For all reconstructed sensory neurons a similar axonal course was found. Axonal terminals were found ipsilateral in the motor trigeminal nucleus, indicating a direct connection between sensory neurons and motoneurons. Further collaterals were found ipsilateral in the supratrigeminal nucleus and caudal to the motor trigeminal nucleus in the parvocellular reticular nucleus alpha. Since the latter termination areas are important for bilateral control of jaw-movements, the muscle spindle afferents are likely to participate not only in a monosynaptic motor reflex, but also in more complex neuronal circuits involved in jaw-movements.Abbreviations EPSP excitatory postsynaptic potential - HRP horseradish peroxidase - Me5 mesencephalic trigeminal nucleus - Mo5 motor trigeminal nucleus - PCRtA parvocellular reticular nucleus alpha - Su5 supratrigeminal nucleus     

Assessing mechanical sensitivity of masseter muscle in lightly anesthetized rats: a model for craniofacial muscle hyperalgesia

In this report, we present a simple and reliable way of assessing mechanical sensitivity of masseter muscle as a model for craniofacial muscle hyperalgesia. Mechanical thresholds that evoke nocifensive hindpaw responses following noxious masseter stimulation were assessed. Masseteric injections of widely used sensitizing agents significantly increased mechanical sensitivity of the muscle in a time dependent manner without affecting other muscles and overlying skin. This lightly anesthetized rodent paradigm allows us to provide calibrated and reliable mechanical stimulus, which is not possible in behaving animals. The technique can be applied to study mechanistic bases for craniofacial muscle tenderness.     

Segmental reflex inputs to motoneurons innervating dorsal neck musculature in the cat

Summary The responses to Stimulation of upper cervical muscle and cutaneous afferents were studied in motoneurons innervating splenius, complexus, and biventer cervicis dorsal neck muscles of cats. Motoneurons innervating complexus and biventer cervicis fibers, which are in the deeper, longitudinally oriented muscles, were monosynaptically excited by ipsilateral Group I afferents from each of these muscles, but they did not receive significant input from splenius Group I afferents. Likewise, splenius motoneurons were not monosynaptically excited by ipsilateral afferents from complexus and biventer cervicis. Stimulation of ipsilateral cutaneous afferents produced predominant excitation in splenius motoneurons, predominant inhibition in biventer cervicis motoneurons, and inhibition or mixed responses in complexus motoneurons.None of the neck motoneurons studied showed postsynaptic potentials following single or multiple shock stimulation of contralateral muscle nerves at stimulus intensities expected to excite exclusively Group I afferents. Higher intensity stimulation of contralateral muscle afferents, as well as fibers in the greater auricular nerves, produced predominant inhibition in all three neck motoneuron pools.Segmentally-excited afferents to neck motoneurons, like those from supraspinal systems, appear to evoke different patterns of synaptic responses in splenius motoneurons than they do in motoneurons innervating fibers in the deeper, longitudinally oriented complexus and biventer cervicis muscles.     

Increases in reflex excitability of monkey masseter motoneurons before a jaw-bite reaction-time response.

Reflex excitability of the motoneurons innervating the masseter muscle of monkeys was tested before a phasic voluntary activation of the jaw-closing muscles (a RT bite response). Single test shocks were delivered to the Mes V which supplies a monosynaptic excitatory input to the jaw-muscle motoneurons. Changes in reflex excitability were assessed by measuring the amplitude of the synchronous muscle potential evoked by the test shock. Amplitudes of the muscle potentials evoked by shocks which occurred just before the beginning of the voluntary EMG response, as judged by the onset of EMG activity of the masseter muscle contralateral to the test shock were many times larger than potentials evoked immediately following the visual RT stimulus. Curves relating the average amplitude of the evoked response to its time before the beginning of the voluntary response suggest that the reflex excitability of the motoneuron pool begins to increase 25-45 ms before the first detectable EMG activity occurs. These results suggest that inputs arrive at the motoneurons of agonist muscles used in rapid RT tasks substantially before changes in the EMG of the muscle are noted. These results, in part, would account for the time interval noted between the beginning of neural activity in suprasegmental structures which presumably excites spinal motoneurons, and the first EMG activity of muscles which are innervated by these motoneurons.     

Behavioral inhibition induced by electrical stimulation of the median raphe nucleus of the rat.

Earlier studies using brain lesions or drugs that impair 5-hydroxytryptamine (5-HT) neurotransmission suggest that the ascending serotonergic system is involved in behavioral inhibition. Confirming this hypothesis, the present study demonstrates that electrical stimulation of the median raphe nucleus (NMR) of the rat induces behavioral inhibition. Rats implanted with brain electrodes in the NMR were trained to lever press under a variable-interval 1 min schedule of water presentation. At variable time intervals averaging 7 min, periods of 90 sec of brain stimulation were superimposed on VI responding without any exteroceptive stimulus change. Electrical stimulation of the NMR caused suppression of ongoing lever-pressing behavior, proportional to the stimulus intensity. In addition, NMR stimulation elicited defecation, also directly related to the stimulus intensity, and a cluster of somatic and autonomic changes, such as crouching, micturition, piloerection and teeth chattering, that are characteristic of the rat's emotional behavior in stressful situations. In four out of six rats, the response-suppressant effect of NMR stimulation was counteracted by the 5-HT synthesis inhibitor, para-chlorophenylalanine (PCPA), three to five days following the administration of a single dose of 500 mg/kg of the drug. The effect of PCPA tended to disappear after fifteen days and in one rat, was reversed by 5-hydroxytryptophan, given after benserazide, on the third day from PCPA injection. These results support the suggestion that the mesolimbic serotonergic pathway originating in the NMR and projecting to the septal area and the hippocampus is a substrate of behavioral inhibition, in the rat.     

Stumbling corrective reaction during fictive locomotion in the cat

An obstacle contacting the dorsal surface of a cat's hind foot during the swing phase of locomotion evokes a reflex (the stumbling corrective reaction) that lifts the foot and extends the ankle to avoid falling. We show that the same sequence of ipsilateral hindlimb motoneuron activity can be evoked in decerebrate cats during fictive locomotion. As recorded in the peripheral nerves, twice threshold intensity stimulation of the cutaneous superficial peroneal (SP) nerve during the flexion phase produced a very brief excitation of ankle flexors (e.g., tibialis anterior and peroneus longus) that was followed by an inhibition for the duration of the stimulus train (10-25 shocks, 200 Hz). Extensor digitorum longus was always, and hip flexor (sartorius) activity was sometimes, inhibited during SP stimulation. At the same time, knee flexor and the normally quiescent ankle extensor motoneurons were recruited (mean latencies 4 and 16 ms) with SP stimulation during fictive stumbling correction. After the stimulus train, ankle extensor activity fell silent, and there was an excitation of hip, knee, and ankle flexors. The ongoing flexion phase was often prolonged. Hip extensors were also recruited in some fictive stumbling trials. Only the SP nerve was effective in evoking stumbling correction. Delivered during extension, SP stimulus trains increased ongoing extensor motoneuron activity as well as increasing ipsilateral hip, knee, and ankle hindlimb flexor activity in the subsequent step cycle. The fictive stumbling corrective reflex seems functionally similar to that evoked in intact, awake animals and involves a fixed pattern of short-latency reflexes as well as actions evoked through the lumbar circuitry responsible for the generation of rhythmic alternating locomotion.