K. Å. Olsson

Mechanoreceptor activity from the human face and oral mucosa

SummaryThe feasibility of adopting the microneurography technique (Vallbo and Hagbarth 1968) as a tool to investigate the mechanoreceptive innervation of peri- and intra-oral tissues was explored. Multi-unit activity and impulses in single nerve fibers were recorded from the infraorbital nerve in healthy volunteers. The innervation territories of individual nerve fascicles were mapped. These varied considerably but most fascicle fields comprised the corner of the mouth. Twenty-four single mechanoreceptive units were recorded. Eighteen innervated the skin of the face, and six innervated the mucous membranes of the lips or cheeks. A majority of the mechanoreceptive afferent units were slowly adapting with small and well defined receptive fields. It is suggested that the various slowly adapting responses may originate from two different types of afferent units. No afferents showed response properties similar to typical Pacinian-corpuscle afferents.

The importance of reflexes and their control during jaw movement

Abstract Movement generates sensory feedback that often has a tendency to trigger reflex responses in the muscles of the moving part or in muscles elsewhere in the body. Although some of these reflex adjustments are potentially useful, others could prevent normal motor performance. As a consequence, motor control systems must solve the problem of integrating the useful reflexes into the pattern of movement while suppressing the potentially disruptive ones. The masticatory system is one model where this clearly occurs.

Mechanoreceptive afferent activity in the infraorbital nerve in man during speech and chewing movements

SummaryThe method of microneurography was used to record activity in trigeminal cutaneous and mucosal mechanoreceptive afferents during natural orofacial behaviors such as speech gestures, chewing, licking and swallowing. Multi-unit activity and impulses in single nerve fibers were recorded from the infraorbital nerve. It appeared that these mechanoreceptors respond to contact between the lips, air pressures generated for speech sounds, and to the deformation/strain changes of the facial skin and mucosa associated with various phases of voluntary lip and jaw movements. The relatively vigorous discharge of cutaneous and mucosal afferents during natural movements of the face are consistent with the claim that mechanoreceptors found within the facial skin provide proprioceptive information on facial movements.

Bulbar neurones with axonal projections to the trigeminal motor nucleus in the cat

SummaryThe location of bulbar neurones with axons projecting to the ipsi- and contralateral trigeminal motor nucleus were investigated in cats anaesthetized with sodium pentobarbital. Wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) was injected in amounts of 5–24 nl. A volume-calibrated microelectrode was used for recording of evoked potentials and pressure injection of WGA-HRP. The injection site was guided by the position where a maximal antidromic response was evoked by electrical stimulation of the masseteric nerve. The survival time was 19–22 h. In preparations with the depot located in the masseteric subnucleus retrogradely stained neurones were found bilaterally in the borderzone of the trigeminal motor nucleus. Dense populations of stained neurones were observed ipsi- and contralaterally in the dorsal division of the main sensory trigeminal nucleus and the subnucleus-γ of the oral nucleus of the spinal trigeminal tract. Clusters of WGA-HRP-neurones were observed bilaterally in the lateral tegmental field at the level of the subnucleus-β of the oral nucleus of the spinal trigeminal tract, bilaterally dorsal to the facial nucleus and contralaterally adjacent to the hypoglossal nucleus. No stained neurones were found in the gigantocellular reticular nucleus. A group of stained neurones was located in the marginal nucleus of brachium conjunctivum and some were found in the raphé nuclei near obex. Cell profiles were of two types: medium-sized neurones with a triangular profile and 30–40 μm diameter, and fusiform neurones 10×50–70 μm. Convergence of descending cortical and trigeminal afferent inputs on interneurones located in the lateral borderzone of the trigeminal motor nucleus, i.e. the intertrigeminal area, is reported in the preceding paper.

Directional sensitivity of human periodontal mechanoreceptive afferents to forces applied to the teeth.

1. Single‐unit impulse activity from thirty‐eight mechanoreceptive afferent fibres was recorded in the human inferior alveolar nerve using tungsten microelectrodes. All afferents responded to mechanical stimulation of the teeth and most likely supplied periodontal mechanoreceptors. 2. All afferents showed their highest sensitivity to forces applied to a particular tooth (the lower incisors, the canine or the first premolar). Forces with ‘ramp‐and‐hold’ shaped profiles of similar magnitudes were applied to that tooth in the following six directions: lingual, labial, mesial and distal in the horizontal plane, and up and down in the axial direction of the tooth. Both static and dynamic response components were analysed. 3. All afferents were slowly adapting’ since they discharged continuously in response to static forces in at least one stimulation direction. Twenty‐five afferents (66%) were spontaneously active in the sense that they showed an on‐going discharge in the absence of external stimulation. 4. Diverse receptive fields were observed. Most afferents (74%) responded to static forces in two or three of the four horizontal directions. Likewise, all units showed excitatory responses to axial loading with a majority (74%) responding in one of the two axial directions and the remainder in both axial directions. Spontaneously active afferents generally decreased their discharge rate when stimulated in directions opposite to the directions exciting the afferent. With regard to population responses, approximately half of the afferents showed excitatory responses to each stimulus direction except for downwards, in which 86% responded. 5. Twenty‐three afferents (61%) exhibited the strongest response to forces in one of the horizontal directions. Of those, a majority were most responsive to the lingual direction (52%) and some to the labial direction (30%). Accordingly, the discharge rates during force application averaged over the whole afferent sample were highest in these directions. Of the remaining afferents, most responsive to one of the axial directions, 60% showed their strongest responses to forces in the downward direction. 6. Forty‐five per cent of the afferents showed wider receptive fields to the dynamic component of the force stimulation than to the static. The direction of maximal sensitivity, however, remained the same with few exceptions. 7. It was demonstrated that even though individual periodontal mechanoreceptive afferents provide ambiguous information regarding the direction of a force applied to a tooth, populations of such afferents are well suited to give detailed directional information. It is suggested that such information may play an important role for the control of mastication.

Localization of evoked potentials in the digastric, masseteric, supra- and intertrigeminal subnuclei of the cat

SummaryExtracellular focal potentials were evoked and mapped in the trigeminal motor nucleus and its surrounding borderzone in the cat. Graded electrical stimulation was used for orthodromic and antidromic excitation of the masseteric and digastric motoneurones and for orthodromic stimulation of the lingual and inferior alveolar nerves. The method of referring Horsley Clarke coordinates of microelectrode recording positions to their location of the actual histological section was studied and the total error affecting the method was calculated for the H, AP and L axes. The characteristics and the distribution of the evoked focal potentials were described and related to the histological section from the actual experiment. A phase reversal of the negative focal potential evoked by the lingual and inferior alveolar nerves in the main sensory nucleus and in the intertrigeminal nucleus was observed to indicate the dorso-lateral border of the motor nucleus. Other borders were given by the antidromic potentials evoked in the nucleus. Digastric motoneurones were found medially in the caudal third and ventro-medially in the middle third of the motor nucleus. The masseteric motoneurones were located laterally in the middle and rostral thirds of the nucleus. Potentials evoked in the supratrigeminal and intertrigeminal subnuclei, adjacent to the motor nucleus, were considered and discussed in relation to the available evidence of interneurones subserving trigeminal reflex arcs.

Location of, and peripheral convergence on, the interneurone in the disynaptic path from the coronal gyrus of the cerebral cortex to the trigeminal motoneurones in the cat

SummaryPrimary afferent and descending corticobulbar convergence on 186 interneurones located in the intertrigeminal area was investigated. The experiments were performed on cats anaesthetized with chloralose. Nerves from the three trigeminal dermatomes were stimulated electrically at intensities below and above twice the threshold level. Nerves from oral, perioral and periorbital structures, and afferents from the masseteric and digastric muscles were included. The surface of the cerebral cortex was stimulated electrically in systematically selected, maximally receptive points within the trigeminal primary projection fields. The intertrigeminal neurones generally responded to stimulation of lowthreshold afferents from periodontal, lingual or perioral cutaneous receptors with a polysynaptic latency. Inputs from 3–5 nerves were common but one afferent input was usually most effective. The neurones were generally discharged from two or more cortical points, as a rule those of the oral and perioral projection fields in areas 3a and 3b of the coronal gyrus. The fastest path from the cerebral cortex to the intertrigeminal area was monosynaptic. However, the median latency was 4–5 ms which indicates an oligosynaptic path. The path went through the pyramide at the pontine level. The discharge pattern of the intertrigeminal neurones was 1–4 spikes in 54% of the neurones and a high frequency train of spikes in 46%. Cortical excitation followed by inhibition of the neurones was observed. The neurones were not discharged by electrical stimulation in the defence-attack area of the hypothalamus. Transsynaptic responses evoked from the mesencephalon were seen in 1/3 of the tested neurones.

Low threshold afferent projections from the oral cavity and the face to the cerebral cortex of the cat

SummaryThe projections of low threshold afferents from the oral cavity and the face to the cerebral cortex of cats anaesthetized with chloralose were investigated. The projection fields of the ipsi- and contralateral lingual, inferior alveolar, mental, superior alveolar, infraorbital, (separate branches from whiskers and nose), ophthalmic, great auricular and the contralateral superficial radial nerves were localized. Surface potentials of short latency and maximal amplitude were recorded and their location traced on photographs of the rostral part of the right cerebral hemisphere. Reference points were indicated with india ink punctures.The extent of the cytoarchitectonic areas was determined on histological serial sections and the borders transferred to the photographs of the hemisphere. The features of the projections were related to the cytoarchitecture and to the pattern of the gyri and sulci.It was observed that the low threshold afferents from the oral cavity and the face projected via fast conducting, presumably three synaptic paths, to separate locations in areas 3a, 3b, 5a and 6aβ. The projections to area 3b were somatotopically organized starting with the auricular and the ophthalmic nerve projections lateral to the 3b projection of the forelimb in the posterior sigmoid gyrus and continuing with the maxillary nose, maxillary whiskers, mental nerve, superior alveolar, inferior alveolar and lingual nerve fields along the coronal gyrus towards the presylvian sulcus. The somatotopy was, however, not isomorphic with the body surface but displayed consecutive, overlapping bands across area 3b. The projections to area 3a were similarly organized. The somatotopy was less obvious in area 5a and 6aβ. Convergent projections with responses of slightly longer latency were observed in area 43 (gyrus orbitalis).

The Effect of Electrical Stimulation in the Hypothalamus on the Monosynaptic Jaw Closing and the Disynaptic Jaw Opening Reflexes in the Cat

SummaryThe effects of electrical stimulation in the hypothalamus on the monosynaptic jaw closing and the disynaptic jaw opening reflexes were investigated in cats anaesthetized with chloralose. The hypothalamic electrodes were located by observation of behavioural attack responses in the unanaesthetized animal and by means of Horsley-Clarke coordinates. The locations were verified in histological serial sections.Hypothalamic conditioning with trains of 3–10 pulses, 0.5 ms duration, 0.5 mA, 500 Hz, evoked a strong facilitation of the jaw closing reflex and a facilitation followed by an inhibition of the jaw opening reflex. These effects differed from those elicited from the cerebral cortex. The hypothalamic effects had a longer latency (11–13 ms) and required a longer train of conditioning stimuli than was the case with those evoked from the cortex. Bilateral ablation of the sensorimotor cerebral cortex or lesion of the pyramids at the lower pontine level diminished but did not abolish the hypothalamic effects. They did, however, disappear after lesions including the ventral midbrain tegmentum.The stimulus positions eliciting the largest hypothalamic effects on the jaw reflexes were located in a region extending medio-laterally from the perifornical area to the entrance of the ansa lenticularis in the lateral hypothalamus. Rostro-caudally the location was found at the level of the ventromedial hypothalamic nucleus and the anterior hypothalamus just rostral to this nucleus. The region corresponds to those parts of the hypothalamus from which agonistic and feeding responses have been evoked.It is suggested that the observed hypothalamotrigeminal mechanism may exercise a tonic influence on the trigeminal motoneurones, thereby controlling the set point of the biting force. The implications of this hypothesis on the etiology of bruxism and the myofascial pain dysfunction are discussed.

Facilitation and inhibition of jaw reflexes evoked by electrical stimulation of the cat's cerebral cortex

SummaryThe effects of electrical stimulation of the cerebral cortex on the monosynaptic jaw closing and the disynaptic jaw opening reflexes were studied in cats anaesthetized with chloralose. The time course of the reflex effects was recorded. Similar rhythmic sequences of facilitation and inhibition were observed in both reflexes (Fig. 3). The sequence could start with facilitation or inhibition. The latency of the initial effects was short (2.5 ms) indicating a minimum of two synapses in the descending path. The period of the rhythmic sequence was approximately 10 ms.Optimal parameters for the conditioning cortical stimuli were found to be: trains of 3–5 surface anodal pulses, 0.5 ms, 400 Hz. The threshold of the cortical effects on the reflexes was 0.3 mA. A single pulse evoked effects.The cortical origin of the effects was located and related to the somatosensory projections, and to the cytoarchitecture. The effects of largest amplitude and most complex time course were evoked from the oral and perioral projections to areas 3a and 3b. Effects evoked from areas 4γ, 5a, and 6aβ were less complex and of lower amplitude.It is suggested that a trigemino-cortico-trigeminal loop via 3a may function in reflex modulation of the jaw movements. In addition area 3a may contribute to cortico-cortical motor elaborations via U-fiber connections to area 4γ.