Rhyuji Sumino

Responses of dorsal column nuclei neurons in rats with experimental mononeuropathy

&NA; To examine the functional role of dorsal column nuclei in neuropathic pain, electrophysiological properties of low‐ and high‐threshold dorsal column nuclei neurons in neuropathic and normal rats were examined. Single‐neuronal activities were recorded from the gracile nucleus (GN) in rats at 10–14 days after application of four loose ligatures around the sciatic nerve (chronic constriction nerve injury; CCI). A total of 190 units were recorded from the GN in naive and CCI rats. The largest population of low‐threshold mechanoreceptive (LTM) neurons recorded from the GN of naive rats were classified as rapidly‐adapting (RA) LTM neurons, whereas those from CCI rats were slowly‐adapting (SA) neurons. Mean orthodromic latencies of GN neurons ipsilateral to the CCI after sciatic nerve stimulation were significantly longer than those of naive animals and those of GN neurons without receptive fields were significantly longer than any other type of neurons. One hundred and eight of 190 GN neurons were also antidromically activated following electrical stimulation of the ventro‐lateral posterior nucleus of the thalamus. Furthermore, when stronger stimuli were applied to the sciatic nerve, some GN neurons also responded with long latencies. GN neurons of sham‐operated naive rats and those contralateral to the CCI had mechanical receptive fields on the paw, whereas 8.0% of the GN neurons in the CCI side did not have any detectable mechanical receptive field. Receptive field size was not significantly different between neurons ipsilateral or contralateral to the CCI and those of naive rats. Spontaneous activity of GN neurons from the ipsilateral side was significantly higher than those from the contralateral side. On the other hand, spontaneous activity of GN neurons both ipsilateral and contralateral to the CCI were significantly higher than those from naive rats. Furthermore, afterdischarges of GN neurons ipsilateral and contralateral to the CCI were significantly higher than those of naive rats. The present data suggest that the dorsal column pathway is involved in CCI‐produced sensory abnormalities by conveying their hyperactivity to thalamic neurons.

Input-output relationships in the jaw and orofacial motor zones of the cat cerebral cortex

Input-output relationships of the jaw and orofacial motor zones in the cerebral cortex of lightly anesthetized cats were studied. These relationships were examined by studying the motor effects produced by intracortical microstimulation (ICMS) and recording from single neuron. Jaw and orofacial motor effects were evoked by ICMS of the anterior part of the coronal and lateral sigmoid gyri (C-S motor zone) and the lateral wall of the presylvian sulcus (P motor zone). ICMS of the P motor zone produced more complex movements than that of the C-S motor zone. Repetitive stimulation of the P motor zone also evoked rhythmic jaw movements. Almost all cortical cells located in the C-S motor zone responded to tactile stimulation of cutaneous skin of the orofacial regions or the tooth, whereas those of the P motor zone received no cutaneous input from the orofacial regions. Cytoarchitectonically, the C-S motor zone was restricted to areas 3a, 6a beta and occasionally to area 4 gamma, whereas the P motor zone was represented to area 6a beta. Therefore, it is concluded that the C-S motor zone might be involved in sensorimotor integration of the jaw and orofacial motor functions, whereas the P motor zone might function only as a command area for jaw and orofacial movements.

Movements of the jaw and orofacial regions evoked by stimulation of two different cortical areas in cats

Functional properties of the jaw and orofacial motor areas in the cerebral cortex of the lightly anesthetized cat were studied on the basis of the motor effects produced by intracortical microstimulation (ICMS). Jaw and orofacial motor effects were evoked by ICMS (less than 30 microA) delivered to the anterior parts of the coronal and lateral sigmoid gyri (C, coronal area), and the anterior part of the orbital gyrus (O, orbital area). Different patterns of movements in the jaw and orofacial regions were evoked from these two areas. In C one or, at most, two types of simple movement were produced by ICMS to one location, while in O more coordinated movements than those in C were produced. Cytoarchitectonically the jaw and orofacial motor areas were restricted to areas 3a, 4 gamma, and 6a beta in C, and to area 43 in O.

Cortical cells driven by the low-threshold tooth pulpal afferent in cats

The input pattern and spatial distribution of tooth pulp-driven neurons (TPNs) in the cat cortex were studied by recording the unitary activities of these neurons. Stimulation was applied to the upper and lower canine and molar tooth pulps. It was possible to record activities of TPNs in the deep layers of the areas 3a and 3b of the coronal gyrus of cerebral cortex. TPNs driven by the ipsilateral tooth pulp stimulation were distributed more anteroventrally than those driven by contralateral stimulation. Cells driven by bilateral pulp stimulation were situated between these two neurons. The threshold intensity for TPNs in the area 3a was lower than that for neurons in 3b. The majority of TPNs in the area 3a of the anterior part of the coronal gyrus received inputs from low-threshold masseteric muscular afferents as well as from tooth pulp afferents. The findings suggest that TPNs in the area 3a may have a relation to orofacial motor functions.

Fifth somatosensory cortex (SV) representation of the whole body surface in the medial bank of the anterior suprasylvian sulcus of the cat

The physiological properties of neurons in the medial bank of the anterior suprasylvian sulcus (ASSS-m) of the cats cortex were studied using unit recording techniques. Receptive fields (RFs) on the face are represented in the most rostral aspects of the ASSS-m. Of these neurons, 84% responded to light touch of the skin on the contralateral region of the face and 12% responded to mechanical stimulation of facial hair. In addition, 4% of the neurons responded to light touch to the skin or mechanical stimulation of the hair on the contralateral face and also to visual stimuli. The RFs of neurons responsive to the hindlimb and tail are located in the most caudal aspects of the ASSS-m. 22% of these neurons responded to the light touch to the skin and 78% responded to movement of hair. The RFs of neurons responsive to the trunk area in the ASSS-m are located between the facial and hindlimb regions. Of these neurons, 2% responded to light touch of the skin and 98% responded to movements of hair. Some neurons which responded to stimulation of hair or skin on the trunk included forelimb and/or hindlimb areas. In addition, some neurons had RFs on both sides of the trunk including the shoulder area. These regions were in area 5a. Various features of representation in ASSS-m distinguish this region from other somatosensory areas. We designate the ASSS-m as the fifth somatosensory cortex (SV).

Distribution and response characteristics of masseteric nerve-driven neurons in two separate cortical projection areas of cats

Cortical projection areas and distribution and response characteristics of masseteric nerve-driven neurons (MDN) were studied by recording surface-evoked potentials and single neuronal activities elicited by stimulation of the contralateral masseteric nerve in cats. Neuronal activities of MDNs could be recorded in two separate cortical areas. One was located in laminae II-III of area 3b of the posterior part of the coronal gyrus (P), and the other in laminae IV-V of areas 3a and 6a beta of the anterior parts of the coronal and lateral sigmoid gyri (A). The majority of MDNs were driven by low-threshold muscle afferents (Group I and II). Peak latencies of MDNs in P were shorter than those in A. Intracortical microstimulation (less than 30 microA) in A produced oro-facial movements while stimulation in P did not produce any motor effects.

Low-threshold motor effects produced by stimulation of the facial area of the fifth somatosensory cortex in the cat

The motor effective sites of the fifth somatosensory cortex (SV) in the cat were mapped in detail by using unit recording and intracortical microstimulation (ICMS) techniques. The motor effective sites for facial muscle contraction were identified using stimulus currents of less than 30 microA. Of the 257 effective sites detected, 49% were activated by stimulus currents of less than 20 microA and of these, 51% responded to stimulus currents of less than 10 microA. ICMS within the facial area of the SV neuron produced contralateral eye-blinking, the lowest threshold current for which was 2 microA and ICMS within the SV neurons produced whisker movements, the minimum threshold current for which was 4 microA. Furthermore, stimulation of some SV neurons at a threshold current as low as 4 microA produced whisker movements and some responded to both visual and cutaneous stimuli. Ablation of areas 6a beta, 3a, SII, SIII and the motor cortex did not eliminate or reduce the low-threshold responses elicited by this weak stimulation of the SV. These motor effective areas receive input from the contralateral cutaneous of the surrounding muscle motor effective region. Our results suggest that the described effect is independent of motor effective areas.

Expression of AMPA receptors in rat superior colliculus and effect of orbital enucleation

We analyzed the distribution and the morphological characteristics of neurons expressing AMPA-type glutamate receptor subunits (GluR1 and GluR2) in the superficial partition (stratum zonale (SZ), stratum griseum superficiale (SGS) and stratum opticum (SO)) of the rat superior colliculus. GluR1-expressing neurons had round or ovoid somata in SGS and round or fusiform somata and primary dendrites extending tangential or horizontal side in SO. On the other hand, GluR2-expressing neurons mainly corresponded to vertical fusiform cells with vertically oriented dendrites in SGS and medium-sized stellate or ovoid cells with many primary dendrites in SO. The results suggest that the expressions of GluR1 and GluR2 are differentially regulated in individual neurons of the superficial partition. To analyze the effect of retinal deafferentation on the expression of the GluRs, we performed unilateral orbital enucleations in rats within a week after birth. Thirty days after retinal lesioning, lower expression of GluR2 mRNA was observed in the neurons of the contralateral side as compared with that of the ipsilateral side in SO, but not in SGS. These results indicate that GluR2 expression in the SO neurons is regulated by the correct afferentation from the retina.

Effect of interstimulus interval on perceived sensation and intradental nerve activity during thermal tooth stimulation in man

The effect of interstimulus interval on nerve responses and subjective sensory ratings evoked by thermal stimulation of the teeth were studied in man. A total of 30 unitary discharges during heat stimulation of the lower incisor teeth were recorded from the inferior alveolar nerves using microneurographic technique. Fifteen of them had threshold sensory ratings above 3 (pain related-units) and 15 had ratings of less than 2 (non-pain-related units). Repetitive heat stimulation was applied to the teeth with interstimulus intervals of 180 s (ISI 180) and 30 s (ISI 30). Repetitive (ISI 180 and ISI 30) non-painful heat stimulation of the teeth did not alter either the intensity ratings or unitary discharge activities of non-pain-related units (P > 0.05). Repetitive painful heat stimulation of the teeth with ISI 180 did not alter the intensity ratings of pain-related units (P > 0.05), whereas that with ISI 30 significantly reduced the intensity ratings of pain-related units during a second session of heat trials (P < 0.05). On the other hand, repetitive heat stimulation of the teeth with ISI 180 caused a slight reduction in the firing frequency of pain-related units (P > 0.05). Repetitive painful heat stimulation with ISI 30 significantly reduced the firing frequency of pain-related units (P < 0.05).

Thin fiber receptors responding to mechanical, chemical and thermal stimulation in the cat tongue

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