Body Site Variation of Heat Pain Sensitivity

Thirty-two healthy human subjects provided thresholds for the perception of slight and moderate heat pain. Four body sites were tested bilaterally: thenar eminence of the hand, plantar surface of the foot, dorsolateral forearm, and lateral calf. Thresholds for the glabrous skin of the hand and foot were significantly greater than thresholds for the hairy skin of the arm and leg, the average difference being 1.3°c. Laterality was not a statistically significant factor. Thresholds increased progressively over 2–4 weeks of repeated testing, resulting in values averaging 0.6°c higher in the later sessions. The difference between moderate and slight pain thresholds averaged 1.1°c, and was consistent across body sites and with repeated testing.

The threshold values were normally distributed across subjects. Considerable intersubject variability was observed for both slight and moderate pain thresholds, more so on glabrous than on hairy skin sites. In comparison, the distribution of right-left difference values was narrower, demonstrating less intrasubject versus intersubject variability.

The highly significant difference in thresholds between glabrous and hairy skin sites demonstrates the importance of skin type for heat pain sensitivity. In contrast, there was no significant difference in heat pain sensitivity between comparable sites on the upper versus lower extremities, or between left and right sides.     

c-fos Expression in Gracilothalamic Tract Neurons after Electrical Stimulation of the Injured Sciatic Nerve in the Adult Rat

The number of c-fos protein-like immunoreactive (Fos-LI) cells in the gracile nucleus was determined after electrical stimulation at Aα/Aβ-fiber strength of the normal and of the previously injured sciatic nerve in adult rats. No Fos-LI cells were seen after electrical stimulation of the noninjured sciatic nerve, or after sciatic nerve injury without electrical stimulation. However, stimulation 21 days after sciatic nerve transection resulted in numerous Fos-LI cells in the ipsilateral gracile nucleus. Combined Fos immunocytochemistry and retrograde labeling from the thalamus showed that the majority (76%; range = 70–80%) of the cells in the gracile nucleus that expressed Fos-LI after nerve injury projected to the thalamus. The results indicate that morphological, biochemical, and physiological alterations in primary sensory central endings and second-order neurons, which have earlier been demonstrated in the dorsal column nuclei after peripheral nerve injury, are accompanied by changes in the c-fos gene activation pattern after stimulation of the injured sciatic nerve. A substantial number of the c-fos-expressing neurons project to the thalamus.     

Electrotactile stimulation on the tongue: Intensity perception,discrimination, and cross-modality estimation

Due to its high sensitivity and conductivity, electrotactile stimulation (ETS) on the tongue has proven to be a useful and technically convenient tool to substitute and/or augment sensory capabilities. However, most of its applications have only provided spatial attributes and little is known about (a) the ability of the tongue's sensory system to process electrical stimuli of varying magnitudes and (b) how modulation of ETS intensity affects subjects’ ability to decode stimulus intensity. We addressed these questions by quantifying: (1) the magnitude of the dynamic range (DR; maximal comfortable intensity/perception threshold) and its sensitivity to prolonged exposure; (2) subjects’ ability to perceive intensity changes; and (3) subjects’ ability to associate intensity with angular excursions of a protractor's handle. We found that the average DR (17 dB) was generally large in comparison with other tactile loci and of a relatively constant magnitude among subjects, even after prolonged exposure, despite a slight but significant upward drift (p < 0.001). Additionally, our results showed that as stimulus intensity increased, subjects’ ability to discriminate ETS stimuli of different intensities improved (p < 0.05) while estimation accuracy, in general, slightly decreased (increasing underestimation). These results suggest that higher ETS intensity may increase recruitment of rapidly adapting mechanoreceptor fibers, as these are specialized for coding stimulus differences rather than absolute intensities. Furthermore, our study revealed that the tongue's sensory system can effectively convey electrical stimuli despite minimal practice and when information transfer is limited by memory and DR drift.     

The Central Projections of the Monkey Tooth Pulp Afferent Neurons

Transganglionic transport of horseradish peroxidase conjugated to wheatgerm agglutinin (HRP:WGA) entrapped in hypoallergenic polyacrylamide gel was used to study the patterns of termination of primary afferents that innervate the upper and lower tooth pulps within the trigeminal sensory nuclear complex (TSNC) of the monkey. HRP:WGA injections were also made into the lower incisors and molars, in order to examine the topographic arrangement of pulpal afferent projections. HRP-labeled pulpal afferents innervating lower and upper teeth projected ipsilaterally to the rostral subnucleus dorsalis (Vpd) and caudal subnucleus ventralis (Vpv) of the nucleus principalis (Vp); the rostrodorsomedial (Vo.r) and dorsomedial (Vo.dm) subdivisions of the nucleus oralis (Vo); the dorsomedial subdivision of the nucleus interpolaris (Vi); and laminae I—II and/or V of the nucleus caudalis (Vc) at its rostralmost level. The HRP-labeled terminals from upper and lower pulpal afferents formed a rostrocaudal column from the midlevel of Vp to the rostral tip of Vc. The label in Vp and Vo was considerably dense, but the column of terminals was interrupted at the Vpd-Vpv transition. The label in Vi and Vc was much less dense compared to that in the rostral nuclei, and the column of terminals was interrupted frequently. The representation of the upper and lower teeth in TSNC was organized in a somatotopic fashion that varied from one subdivision to the next, though their terminal zones overlapped within Vpd. The upper and lower teeth were represented in Vpv, Vo.r, Vo.dm, Vi, and Vc in a ventrodorsal, dorsoventral, lateromedial, lateromedial, and lateromedial sequence, respectively. Topographic arrangement was also noticed for the projections of pulpal afferents from the lower incisors and molars: The representations of the lower incisors and molars in Vpv, Vo.r, Vo.dm, Vi, and Vc were organized in a lateromedial, dorsoventral, ventrodorsal, ventrodorsal, and lateromedial sequence, respectively. The present results indicating sparse projections from pulpal afferents in the monkey's Vc are in good correspondence with a clinical report that trigeminal tractotomy just rostral to the obex has no significant effect on dental pain perception in patients. Furthermore, the present study indicates that projection patterns of pulpal afferents—which include the termination sites, the density of terminations between nuclei, and topographic arrangement—differ among animal species.     

New techniques in female pudendal somatosensory evoked potential testing

Objective—The primary nerves innervating the female genitalia are the dorsal nerve of the clitoris (DNC) and the perineal nerve, which innervate the clitoris and the external genitalia/distal vagina, respectively. We describe two novel electrodiagnostic techniques for evaluating the integrity of these female genital somatosensory pathways.

Subjects and methods—Seventy-seven healthy women (mean age 29.3 years) were enrolled for this study. We performed DNC somatosensory evoked potentials (SEPs), stimulating through self-adhesive disk electrodes on either side of the clitoris. Perineal nerve SEPs were evoked through a vaginal probe. Cortical responses were measured through cup electrodes affixed to the scalp at Cpz and Fpz. Stimulus parameters were duration 0.1?ms, frequency 4.1?Hz, filters 5–5,000?Hz, at three times sensory threshold.

Results—DNC and perineal nerve SEPs from both the right and left sides were reproducible and easily discerned. The mean P1 latencies were: right DNC 39.4?ms (SD 2.8?ms), left DNC 39.3?ms (SD 3.3?ms), right perineal nerve 37.8?ms (SD 3.4?ms), left perineal nerve 37.6?ms (SD 3.1?ms). We recorded SEP responses from 90 to 92% of subjects for the DNC, and 69% for the perineal nerve.

Conclusions—We are able to evoke somatosensory potentials from the four primary somatic nerves that mediate female genital cutaneous sensation. In healthy subjects, the DNC responses are robust and maintain laterality. The perineal nerve responses are less consistently obtained, but when recorded, are easily discerned. These preliminary data provide a foundation from which to study female genital innervation, particularly as it applies to sexual function.     

Comparative histomorphology of intrinsic vibrissa musculature among primates: implications for the evolution of sensory ecology and “face touch”

Macrovibrissae are specialized tactile sensory hairs present in most mammalian orders, used in maxillary mechanoreception or “face touch.” Some mammals have highly organized vibrissae and are able to “whisk” them. Movement of vibrissae is influenced by intrinsic vibrissa musculature, striated muscle bands that attach directly to the vibrissa capsule. It is unclear if primates have organized vibrissae or intrinsic vibrissa musculature and it is uncertain if they can move their vibrissae. The present study used histomorphological techniques to compare vibrissae among 19 primates and seven non‐primate mammalian taxa. Upper lips of these mammals were sectioned and processed for histochemical analysis. While controlling for phylogenetic effects the following hypotheses were tested: 1) mammals with well‐organized vibrissae possess intrinsic vibrissa musculature and 2) intrinsic vibrissa musculature is best developed in nocturnal, arboreal taxa. Our qualitative analyses show that only arboreal, nocturnal prosimians possess intrinsic musculature. Not all taxa that possessed organized vibrissae had intrinsic vibrissa musculature. Phylogenetic comparative analyses revealed a 70% probability that stem mammals, primates, and haplorhines possessed intrinsic vibrissa musculature and well‐organized vibrissae. These two traits most likely coevolved according to a discrete phylogenetic analysis. These results indicate that nocturnal, arboreal primates have the potential to more actively use their vibrissae in spatial recognition and navigation tasks than diurnal, more terrestrial species, but there is a clear phylogenetic signal involved in the evolution of primate vibrissae and “face touch.” Am J Phys Anthropol, 2013. © 2012 Wiley Periodicals, Inc.     

Conserved roles of ems/Emx and otd/Otx genes in olfactory and visual system development in Drosophila and mouse

The regional specialization of brain function has been well documented in the mouse and fruitfly. The expression of regulatory factors in specific regions of the brain during development suggests that they function to establish or maintain this specialization. Here, we focus on two such factors—the Drosophila cephalic gap genes empty spiracles (ems) and orthodenticle (otd), and their vertebrate homologues Emx1/2 and Otx1/2—and review novel insight into their multiple crucial roles in the formation of complex sensory systems. While the early requirement of these genes in specification of the neuroectoderm has been discussed previously, here we consider more recent studies that elucidate the later functions of these genes in sensory system formation in vertebrates and invertebrates. These new studies show that the ems and Emx genes in both flies and mice are essential for the development of the peripheral and central neurons of their respective olfactory systems. Moreover, they demonstrate that the otd and Otx genes in both flies and mice are essential for the development of the peripheral and central neurons of their respective visual systems. Based on these recent experimental findings, we discuss the possibility that the olfactory and visual systems of flies and mice share a common evolutionary origin, in that the conserved visual and olfactory circuit elements derive from conserved domains of otd/Otx and ems/Emx action in the urbilaterian ancestor.     

Otolith shape lends support to the sensory drive hypothesis in rockfishes

The sensory drive hypothesis proposes that environmental factors affect both signalling dynamics and the evolution of signals and receivers. Sound detection and equilibrium in marine fishes are senses dependent on the sagittae otoliths, whose morphological variability appears intrinsically linked to the environment. The aim of this study was to understand if and which environmental factors could be conditioning the evolution of this sensory structure, therefore lending support to the sensory drive hypothesis. Thus, we analysed the otolith shape of 42 rockfish species (Sebastes spp.) to test the potential associations with the phylogeny, biological (age), ecological (feeding habit and depth distribution) and biogeographical factors. The results showed strong differences in the otolith shapes of some species, noticeably influenced by ecological and biogeographical factors. Moreover, otolith shape was clearly conditioned by phylogeny, but with a strong environmental effect, cautioning about the use of this structure for the systematics of rockfishes or other marine fishes. However, our most relevant finding is that the data supported the sensory drive hypothesis as a force promoting the radiation of the genus Sebastes. This hypothesis holds that adaptive divergence in communication has significant influence relative to other life history traits. It has already been established in Sebastes for visual characters and organs; our results showed that it applies to otolith transformations as well (despite the clear influence of feeding and depth), expanding the scope of the hypothesis to other sensory structures.