Cutaneous reflexes and pathways affecting them in the monkey, Macaca mulatta

Differential fMRI Activation Patterns to Noxious Heat and Tactile Stimuli in the Primate Spinal Cord

Mesoscale local functional organizations of the primate spinal cord are largely unknown. Using high-resolution fMRI at 9.4 T, we identified distinct interhorn and intersegment fMRI activation patterns to tactile versus nociceptive heat stimulation of digits in lightly anesthetized monkeys. Within a spinal segment, 8 Hz vibrotactile stimuli elicited predominantly fMRI activations in the middle part of ipsilateral dorsal horn (iDH), along with significantly weaker activations in ipsilateral (iVH) and contralateral (cVH) ventral horns. In contrast, nociceptive heat stimuli evoked widespread strong activations in the superficial part of iDH, as well as in iVH and contralateral dorsal (cDH) horns. As controls, only weak signal fluctuations were detected in the white matter. The iDH responded most strongly to both tactile and heat stimuli, whereas the cVH and cDH responded selectively to tactile versus nociceptive heat, respectively. Across spinal segments, iDH activations were detected in three consecutive segments in both tactile and heat conditions. Heat responses, however, were more extensive along the cord, with strong activations in iVH and cDH in two consecutive segments. Subsequent subunit B of cholera toxin tracer histology confirmed that the spinal segments showing fMRI activations indeed received afferent inputs from the stimulated digits. Comparisons of the fMRI signal time courses in early somatosensory area 3b and iDH revealed very similar hemodynamic stimulus-response functions. In summary, we identified with fMRI distinct segmental networks for the processing of tactile and nociceptive heat stimuli in the cervical spinal cord of nonhuman primates. Significance statement: This is the first fMRI demonstration of distinct intrasegmental and intersegmental nociceptive heat and touch processing circuits in the spinal cord of nonhuman primates. This study provides novel insights into the local functional organizations of the primate spinal cord for pain and touch, information that will be valuable for designing and optimizing therapeutic interventions for chronic pain management.

Projections from the nucleus reticularis magnocellularis to the rat cervical cord using electrical stimulation and iontophoretic injection methods

The aim of this study is to clarify the fiber distribution of the nucleus reticularis magnocellularis (NRMC) and adjacent areas in the rat spinal cord. Biotinylated dextran amine was injected iontophoretically through a glass capillary into the areas, in which a single cell responded to noxious electrical stimulation of the sciatic nerve and to a pinch of the thigh skin with multiple spikes. Labeled fibers descended bilaterally through the ventral funiculi of the medulla oblongata and then through the ventral and lateral funiculi of the cervical cord with an ipsilateral predominance, and terminated in the spinal gray (laminae I-X). A single fiber sometimes ran through several laminae while bifurcating many short branches with axon varicosities and terminal buttons in one transverse section, that is, through laminae V, VII and X, through laminae V, IIl-IV and I-II, and through laminae VII to I-II. The present study showed that the wide distribution of a single fiber and a mass of fibers descending from the NRMC and adjacent areas might modulate not only somatic sensory and motor functions but also autonomic functions in the spinal cord.

Periaqueductal gray inhibition of viscerointercostal and galvanic skin reflexes

In adult cats anesthetized with urethan-chloralose, effects of descending volleys from the mesencephalic periaqueductal gray (PAG) upon the viscerointercostal and galvanic skin reflexes were studied. The viscerointercostal reflex (VIR) was evoked by electrical stimulation of the greater splanchnic nerve and was recorded from the 11th, 12th or 13th intercostal nerve. Conditioning stimuli applied to the PAG inhibited the VIR. The inhibition was particularly marked when the nucleus raphe dorsalis (NRD) or its immediately adjacent ventromedial PAG was stimulated. A train of pulses was required in order to produce a recognizable PAG/NRD inhibition of the VIR. When the PAG/NRD was stimulated at 300-500 Hz, stimulation-produced inhibition became more pronounced in parallel with increase in number of pulses in each train and levelled off at about 5 pulses. The most effective frequency of PAG/NRD stimulation was found within this frequency range. Degree of PAG/NRD stimulation-produced inhibition of the VIR was dependent upon the strength of the test stimulus applied to the splanchnic nerve; the weaker the test stimulus, the more marked the inhibition. PAG/NRD stimulation-produced inhibition of the VIR was completely eliminated by bilateral section of the dorsolateral funiculi. The same section enhanced the VIR per se. It was suggested that PAG/NRD stimulation-produced inhibition of the VIR is mediated by descending pathways in the ipsi- as well as contralateral dorsolateral funiculi, and that the VIR per se is tonically inhibited by descending impulses in these pathways. PAG/NRD stimulation inhibited the segmental polysynaptic reflex in the intercostal nerve, but had little effect on the segmental monosynaptic reflex in the same nerve. Intravenous administration of morphine suppressed the VIR. The suppression was antagonized by intravenous naloxone. In contrast, PAG/NRD stimulation-produced inhibition of the VIR was unaffected by intravenous naloxone. Electrical stimulation of the splanchnic nerve evoked the galvanic skin reflex (GSR) from the forepaw pad. The GSR was inhibited by electrical stimulation of the PAG/NRD. The PAG/NRD stimulation-produced inhibition of the GSR was completely eliminated by intravenous administration of naloxone (0.4 mg/kg).

Infant lesion effect: I. Development of motor behavior following neonatal spinal cord damage in cats

This study was undertaken to determine the effect of spinal cord damage on motor development, and to determine whether there is greater survival of motor function in those motor patterns with a later onset of function than in those which are present at birth. The postnatal development of postural reflexes and locomotion was examined during the first 4 months of life in normal kittens and in those which had received a spinal cord lesion (at high cervical or low thoracic levels) at birth. The results suggest that there are some similarities in normal development, recovery of function after adult lesions and recovery and/or development of function after neonatal lesions. After neonatal lesions, just as after lesions in adults, reflex recovery appears to underlie recovery of locomotion. After spinal lesions, the pattern and sequence of motor development was identical to that seen in normal animals. Hindlimb motor development was normal for some time after the spinal lesion, but deficits appeared later. These observations suggest that postural reflexes and locomotion are not dependent upon ipsilateral descending input for their onset, but only for their maturation. Unexpectedly, tactile placing developed after neonatal spinal cord lesions. This represents sparing of function, for tactile placing is abolished and does not recover after the same lesion sustained in adulthood. Tactile placing is the last of the series of postural reflexes to develop. It depends on the last of the spinal pathways to develop, the corticospinal tract. Two aspects of this study support the hypothesis that later developing motor patterns will have a greater chance for survival and subsequent development than those which are present at birth. First, the immediate effects of spinal cord lesions on postural reflexes are more severe on those reflexes that are more mature at birth. Second, the spinal cord lesions produce more severe impairment of the more mature forelimb motor function than of the less mature hindlimb motor function. The hypothesis is not supported, however, when the long-term effect of spinal cord lesions on the maturation of motor behavior is considered. All postural reflexes and locomotion fail to mature fully, i.e. they retain characteristics of the immature responses.

Descending influences on receptive fields and activity of single units recorded in laminae 1,2 and 3 of cat spinal cord

Units (108) were isolated in laminae 1,2 and 3 in segments L7-Sl of decerebrate cat spinal cord. For each unit, the size and nature of its receptive field (RF) was delineated. Then the dorsolateral funiculus (DLF) was stimulated for 1 sec with 10 or 50 Hz, 0.1 msec square waves and the response characteristics of the unit were again examined. Of the 108 units, 55 were excited or facilitated, 6 were inhibited (all in lamina 3) and 47 were unaffected. While some of the excited units responded only during the stimulus train, the majority showed prolonged excitation or facilitation lasting over one minute. The excited units were predominantly those responding to pressure or to brush, touch and pressure. Of the pressure units, 73% were excited or facilitated in contrast to only 29% of the brush/touch units. Most of the excited units showed expansion of their RFs. While many units of this type show ongoing variations of excitability and RF size, the evoked responses reported were sufficiently time-locked to the stimulus for it to be apparent that they were caused by the DLF stimulation. The unit's responses still occurred when the DLF was stimulated caudal to a complete cord transection so that the effects did not pass through the brain stem. The major effect of descending systems or of DLF stimulation previously reported on the large cells of laminae 1,4 and 5 has been inhibition. Here we report that a major descending influence on many units of laminae 1,2 and 3 is excitatory. Therefore it is suggested that a population of small interneurons in the superficial laminae could contribute to the descending inhibition of large dorsal horn neurons.