A comparative light microscopic analysis of the sensory innervation of the mystacial pad. I. Innervation of vibrissal follicle-sinus complexes

Innervation patterns of sea otter (Enhydra lutris) mystacial follicle-sinus complexes

Sea otters (Enhydra lutris) are the most recent group of mammals to return to the sea, and may exemplify divergent somatosensory tactile systems among mammals. Therefore, we quantified the mystacial vibrissal array of sea otters and histologically processed follicle-sinus complexes (F - SCs) to test the hypotheses that the number of myelinated axons per F - SC is greater than that found for terrestrial mammalian vibrissae and that their organization and microstructure converge with those of pinniped vibrissae. A mean of 120.5 vibrissae were arranged rostrally on a broad, blunt muzzle in 7-8 rows and 9-13 columns. The F-SCs of sea otters are tripartite in their organization and similar in microstructure to pinnipeds rather than terrestrial species. Each F-SC was innervated by a mean 1339 ± 408.3 axons. Innervation to the entire mystacial vibrissal array was estimated at 161,313 axons. Our data support the hypothesis that the disproportionate expansion of the coronal gyrus in somatosensory cortex of sea otters is related to the high innervation investment of the mystacial vibrissal array, and that quantifying innervation investment is a good proxy for tactile sensitivity. We predict that the tactile performance of sea otter mystacial vibrissae is comparable to that of harbor seals, sea lions and walruses.

Characterisation of whisker control in the California sea lion (Zalophus californianus) during a complex, dynamic sensorimotor task

Studies in pinniped whisker use have shown that their whiskers are extremely sensitive to tactile and hydrodynamic signals. While pinnipeds position their whiskers on to objects and have some control over their whisker protractions, it has always been thought that head movements are more responsible for whisker positioning than the movement of the whiskers themselves. This study uses ball balancing, a dynamic sensorimotor skill that is often used in human and robotic coordination studies, to promote sea lion whisker movements during the task. For the first time, using tracked video footage, we show that sea lion whisker movements respond quickly (26.70 ms) and mirror the movement of the ball, much more so than the head. We show that whisker asymmetry and spread are both altered to help sense and control the ball during balancing. We believe that by designing more dynamic sensorimotor tasks we can start to characterise the active nature of this specialised sensory system in pinnipeds.

Anatomy and ontogeny of a novel hemodynamic organ in zebrafish

The zebrafish maxillary barbel can protract and retract in response to stimuli, and appears connected to a prominent blood sinus on the lateral aspect of the maxillary bone. However, the mechanism of barbel movement is not described. Using whole-mount phalloidin staining of the sinus region, we observed long filamentous actin cables, suggesting highly organized vascular smooth muscle cells, surrounding an endothelial chamber. Although the chamber is variably filled by erythrocytes in vivo, cardiac injection of fluorescent dextrans shows that it consistently contains plasma. Full-thickness confocal imaging of dextran-injected adults containing EGFP(+) endothelial cells revealed a vascular complex with three compartments, here named the distal bulb, central chamber, and accessory chamber. The early ontogeny of all three compartments was confirmed in a whole-mount series of Tg(fli1a:EGFP) juveniles. In wild type adults, the fine structure of each chamber was studied using paraffin- and plastic-section histochemistry and transmission electron microscopy. The distal bulb and central chamber have smooth muscle coats with luminally-elongated septa, forming semi-detached blood-filled lacunae. The central chamber walls and septa are extensively innervated by small, unmyelinated axons, as confirmed by immunohistochemical detection of acetylated tubulin, a component of axonal cytoplasm. The accessory chamber appears neither innervated nor muscularized, but is an endothelial cul-de-sac with a thickened elastic adventitia, suggesting an extensible fluid reservoir. We propose that we have identified a new organ in zebrafish, the maxillary barbel blood sinus, whose neurovascular specializations may contribute to zebrafish sensory biology and appendage control.

The number and cycle of rat vibrissae according to topographic arrangement

This study investigated the cycle of rat vibrissae by counting their number relative to their topographic arrangement. The average duration of maintaining single or double vibrissal shafts was analyzed. The ratio of the lifetime of the single and double shafts was around 3:2 weeks. The rostral and caudal vibrissae had relatively short and long cyclic durations, respectively; this difference may be related to their length and function.

Rat whisker movement after facial nerve lesion: evidence for autonomic contraction of skeletal muscle

Vibrissal whisking is often employed to track facial nerve regeneration in rats; however, we have observed similar degrees of whisking recovery after facial nerve transection with or without repair. We hypothesized that the source of non-facial nerve-mediated whisker movement after chronic denervation was from autonomic, cholinergic axons traveling within the infraorbital branch of the trigeminal nerve (ION). Rats underwent unilateral facial nerve transection with repair (N=7) or resection without repair (N=11). Post-operative whisking amplitude was measured weekly across 10weeks, and during intraoperative stimulation of the ION and facial nerves at ⩾18weeks. Whisking was also measured after subsequent ION transection (N=6) or pharmacologic blocking of the autonomic ganglia using hexamethonium (N=3), and after snout cooling intended to elicit a vasodilation reflex (N=3). Whisking recovered more quickly and with greater amplitude in rats that underwent facial nerve repair compared to resection (P<0.05), but individual rats overlapped in whisking amplitude across both groups. In the resected rats, non-facial-nerve-mediated whisking was elicited by electrical stimulation of the ION, temporarily diminished following hexamethonium injection, abolished by transection of the ION, and rapidly and significantly (P<0.05) increased by snout cooling. Moreover, fibrillation-related whisker movements decreased in all rats during the initial recovery period (indicative of reinnervation), but re-appeared in the resected rats after undergoing ION transection (indicative of motor denervation). Cholinergic, parasympathetic axons traveling within the ION innervate whisker pad vasculature, and immunohistochemistry for vasoactive intestinal peptide revealed these axons branching extensively over whisker pad muscles and contacting neuromuscular junctions after facial nerve resection. This study provides the first behavioral and anatomical evidence of spontaneous autonomic innervation of skeletal muscle after motor nerve lesion, which not only has implications for interpreting facial nerve reinnervation results, but also calls into question whether autonomic-mediated innervation of striated muscle occurs naturally in other forms of neuropathy.

The organization of submodality-specific touch afferent inputs in the vibrissa column

The rodent tactile vibrissae are innervated by several different types of touch sensory neurons. The central afferents of all touch neurons from one vibrissa collectively project to a columnar structure called a barrelette in the brainstem. Delineating how distinct types of sensors connect to second-order neurons within each barrelette is critical for understanding tactile information coding and processing. Using genetic and viral techniques, we labeled slowly adapting (SA) mechanosensory neurons, rapidly adapting (RA) mechanosensory neurons, afferent synapses, and second-order projection neurons with four different fluorescent markers to examine their connectivity. We discovered that within each vibrissa column, individual sensory neurons project collaterals to multiply distributed locations, inputs from SA and RA afferents are spatially intermixed without any discernible stereotypy or topography, and second-order projection neurons receive convergent SA and RA inputs. Our findings reveal a "one-to-many and many-to-one" connectivity scheme and the circuit architecture for tactile information processing at the first-order synapses.

Low-dimensional sensory feature representation by trigeminal primary afferents

In any sensory system, the primary afferents constitute the first level of sensory representation and fundamentally constrain all subsequent information processing. Here, we show that the spike timing, reliability, and stimulus selectivity of primary afferents in the whisker system can be accurately described by a simple model consisting of linear stimulus filtering combined with spike feedback. We fitted the parameters of the model by recording the responses of primary afferents to filtered, white noise whisker motion in anesthetized rats. The model accurately predicted not only the response of primary afferents to white noise whisker motion (median correlation coefficient 0.92) but also to naturalistic, texture-induced whisker motion. The model accounted both for submillisecond spike-timing precision and for non-Poisson spike train structure. We found substantial diversity in the responses of the afferent population, but this diversity was accurately captured by the model: a 2D filter subspace, corresponding to different mixtures of position and velocity sensitivity, captured 94% of the variance in the stimulus selectivity. Our results suggest that the first stage of the whisker system can be well approximated as a bank of linear filters, forming an overcomplete representation of a low-dimensional feature space.

Hydrodynamic perception in true seals (Phocidae) and eared seals (Otariidae)

Pinnipeds, that is true seals (Phocidae), eared seals (Otariidae), and walruses (Odobenidae), possess highly developed vibrissal systems for mechanoreception. They can use their vibrissae to detect and discriminate objects by direct touch. At least in Phocidae and Otariidae, the vibrissae can also be used to detect and analyse water movements. Here, we review what is known about this ability, known as hydrodynamic perception, in pinnipeds. Hydrodynamic perception in pinnipeds developed convergently to the hydrodynamic perception with the lateral line system in fish and the sensory hairs in crustaceans. So far two species of pinnipeds, the harbour seal (Phoca vitulina) representing the Phocidae and the California sea lion (Zalophus californianus) representing the Otariidae, have been studied for their ability to detect local water movements (dipole stimuli) and to follow hydrodynamic trails, that is the water movements left behind by objects that have passed by at an earlier point in time. Both species are highly sensitive to dipole stimuli and can follow hydrodynamic trails accurately. In the individuals tested, California sea lions were clearly more sensitive to dipole stimuli than harbour seals, and harbour seals showed a superior trail following ability as compared to California sea lions. Harbour seals have also been shown to derive additional information from hydrodynamic trails, such as motion direction, size and shape of the object that caused the trail (California sea lions have not yet been tested). The peculiar undulated shape of the harbour seals' vibrissae appears to play a crucial role in trail following, as it suppresses self-generated noise while the animal is swimming.

Whisker sensory system - from receptor to decision

One of the great challenges of systems neuroscience is to understand how the neocortex transforms neuronal representations of the physical characteristics of sensory stimuli into the percepts which can guide the animal's decisions. Here we present progress made in understanding behavioral and neurophysiological aspects of a highly efficient sensory apparatus, the rat whisker system. Beginning with the 1970s discovery of "barrels" in the rat and mouse brain, one line of research has focused on unraveling the circuits that transmit information from the whiskers to the sensory cortex, together with the cellular mechanisms that underlie sensory responses. A second, more recent line of research has focused on tactile psychophysics, that is, quantification of the behavioral capacities supported by whisker sensation. The opportunity to join these two lines of investigation makes whisker-mediated sensation an exciting platform for the study of the neuronal bases of perception and decision-making. Even more appealing is the beginning-to-end prospective offered by this system: the inquiry can start at the level of the sensory receptor and conclude with the animal's choice. We argue that rats can switch between two modes of operation of the whisker sensory system: (1) generative mode and (2) receptive mode. In the generative mode, the rat moves its whiskers forward and backward to actively seek contact with objects and to palpate the object after initial contact. In the receptive mode, the rat immobilizes its whiskers to optimize the collection of signals from an object that is moving by its own power. We describe behavioral tasks that rats perform in these different modes. Next, we explore which neuronal codes in sensory cortex account for the rats' discrimination capacities. Finally, we present hypotheses for mechanisms through which "downstream" brain regions may read out the activity of sensory cortex in order to extract the significance of sensory stimuli and, ultimately, to select the appropriate action.

Fused traditional and geometric morphometrics demonstrate pinniped whisker diversity

Vibrissae (whiskers) are important components of the mammalian tactile sensory system, and primarily function as detectors of vibrotactile information from the environment. Pinnipeds possess the largest vibrissae among mammals and their vibrissal hair shafts demonstrate a diversity of shapes. The vibrissae of most phocid seals exhibit a beaded morphology with repeating sequences of crests and troughs along their length. However, there are few detailed analyses of pinniped vibrissal morphology, and these are limited to a few species. Therefore, we comparatively characterized differences in vibrissal hair shaft morphologies among phocid species with a beaded profile, phocid species with a smooth profile, and otariids with a smooth profile using traditional and geometric morphometric methods. Traditional morphometric measurements (peak-to-peak distance, crest width, trough width and total length) were collected using digital photographs. Elliptic Fourier analysis (geometric morphometrics) was used to quantify the outlines of whole vibrissae. The traditional and geometric morphometric datasets were subsequently combined by mathematically scaling each to true rank, followed by a single eigendecomposition. Quadratic discriminant function analysis demonstrated that 79.3, 97.8 and 100% of individuals could be correctly classified to their species based on vibrissal shape variables in the traditional, geometric and combined morphometric analyses, respectively. Phocids with beaded vibrissae, phocids with smooth vibrissae, and otariids each occupied distinct morphospace in the geometric morphometric and combined data analyses. Otariids split into two groups in the geometric morphometric analysis and gray seals appeared intermediate between beaded- and smooth-whiskered species in the traditional and combined analyses. Vibrissal hair shafts modulate the transduction of environmental stimuli to the mechanoreceptors in the follicle-sinus complex (F-SC), which results in vibrotactile reception, but it is currently unclear how the diversity of shapes affects environmental signal modulation.

Anatomical pathways involved in generating and sensing rhythmic whisker movements

The rodent whisker system is widely used as a model system for investigating sensorimotor integration, neural mechanisms of complex cognitive tasks, neural development, and robotics. The whisker pathways to the barrel cortex have received considerable attention. However, many subcortical structures are paramount to the whisker system. They contribute to important processes, like filtering out salient features, integration with other senses, and adaptation of the whisker system to the general behavioral state of the animal. We present here an overview of the brain regions and their connections involved in the whisker system. We do not only describe the anatomy and functional roles of the cerebral cortex, but also those of subcortical structures like the striatum, superior colliculus, cerebellum, pontomedullary reticular formation, zona incerta, and anterior pretectal nucleus as well as those of level setting systems like the cholinergic, histaminergic, serotonergic, and noradrenergic pathways. We conclude by discussing how these brain regions may affect each other and how they together may control the precise timing of whisker movements and coordinate whisker perception.

A unifying framework underlying mechanotransduction in the somatosensory system

Rodents use their whiskers to sense their surroundings. As most of the information available to the somatosensory system originates in whiskers' primary afferents, it is essential to understand the transformation of whisker motion into neuronal activity. Here, we combined in vivo recordings in anesthetized rats with mathematical modeling to ascertain the mechanical and electrical characteristics of mechanotransduction. We found that only two synergistic processes, which reflect the dynamic interactions between (1) receptor and whisker and (2) receptor and surrounding tissue, are needed to describe mechanotransduction during passive whiskers deflection. Interactions between these processes may account for stimulus-dependent changes in the magnitude and temporal pattern of tactile responses on multiple scales. Thus, we are able to explain complex electromechanical processes underlying sensory transduction using a simple model, which captures the responses of a wide range of mechanoreceptor types to diverse sensory stimuli. This compact and precise model allows for a ubiquitous description of how mechanoreceptors encode tactile stimulus.

Electroreception in the Guiana dolphin (Sotalia guianensis)

Passive electroreception is a widespread sense in fishes and amphibians, but in mammals this sensory ability has previously only been shown in monotremes. While the electroreceptors in fish and amphibians evolved from mechanosensory lateral line organs, those of monotremes are based on cutaneous glands innervated by trigeminal nerves. Electroreceptors evolved from other structures or in other taxa were unknown to date. Here we show that the hairless vibrissal crypts on the rostrum of the Guiana dolphin (Sotalia guianensis), structures originally associated with the mammalian whiskers, serve as electroreceptors. Histological investigations revealed that the vibrissal crypts possess a well-innervated ampullary structure reminiscent of ampullary electroreceptors in other species. Psychophysical experiments with a male Guiana dolphin determined a sensory detection threshold for weak electric fields of 4.6 µV cm(-1), which is comparable to the sensitivity of electroreceptors in platypuses. Our results show that electroreceptors can evolve from a mechanosensory organ that nearly all mammals possess and suggest the discovery of this kind of electroreception in more species, especially those with an aquatic or semi-aquatic lifestyle.

Manatee vibrissae: evidence for a "lateral line" function

Aquatic mammals use vibrissae to detect hydrodynamic stimuli over a range from 5 to 150 Hz, similar to the range detected by lateral line systems in fishes and amphibians. Manatees possess ∼5,300 vibrissae distributed over the body, innervated by ∼209,000 axons. This extensive innervation devoted to vibrissae follicles is reflected in enlarged, elaborate somatosensory regions of the gracile, cuneate, and Bischoff's brain-stem nuclei, ventrobasal thalamus, and presumptive somatosensory cortex. Our preliminary psychophysical testing indicates that in Florida and Antillean manatees the Weber fraction for detection thresholds for grating textures ranges from 0.025 to 0.14. At the lower end of this range, sensitivity is comparable to human index finger thresholds. For hydrodynamic stimuli of 5-150 Hz, detection threshold levels for manatees using facial or postfacial vibrissae were substantially lower than those reported for harbor seals and similar to reports of sensitivity for the lateral line systems of some fish. Our findings suggest that the facial and postfacial vibrissae are used to detect hydrodynamic stimuli, whereas only the facial vibrissae are used for direct contact investigation.

Vitamin K supplementation increases vitamin K tissue levels but fails to counteract ectopic calcification in a mouse model for pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE) is an autosomal recessive disorder in which calcification of connective tissue leads to pathology in skin, eye and blood vessels. PXE is caused by mutations in ABCC6. High expression of this transporter in the basolateral hepatocyte membrane suggests that it secretes an as-yet elusive factor into the circulation which prevents ectopic calcification. Utilizing our Abcc6 (-/-) mouse model for PXE, we tested the hypothesis that this factor is vitamin K (precursor) (Borst et al. 2008, Cell Cycle). For 3 months, Abcc6 (-/-) and wild-type mice were put on diets containing either the minimum dose of vitamin K required for normal blood coagulation or a dose that was 100 times higher. Vitamin K was supplied as menaquinone-7 (MK-7). Ectopic calcification was monitored in vivo by monthly micro-CT scans of the snout, as the PXE mouse model develops a characteristic connective tissue mineralization at the base of the whiskers. In addition, calcification of kidney arteries was measured by histology. Results show that supplemental MK-7 had no effect on ectopic calcification in Abcc6 ( -/- ) mice. MK-7 supplementation increased vitamin K levels (in skin, heart and brain) in wild-type and in Abcc6 (-/-) mice. Vitamin K tissue levels did not depend on Abcc6 genotype. In conclusion, dietary MK-7 supplementation increased vitamin K tissue levels in the PXE mouse model but failed to counteract ectopic calcification. Hence, we obtained no support for the hypothesis that Abcc6 transports vitamin K and that PXE can be cured by increasing tissue levels of vitamin K.

Immunohistochemical comparison of whisker pad cutaneous innervation in Swiss Webster and hairless mice

To establish the mouse mutant, hairless (Hr), as a useful model for future analyses of target-ending interactions, we assessed the cutaneous innervation in the whisker pad after loss of primary hair targets. Postnatal (P) development of fur in Hr begins similarly to that of "normal" Swiss Webster (SW) mice. Around P10, hairs are shed and the follicles rendered permanently incompetent. Hair loss progresses rostrocaudally until the entire skin is denuded. Substantial alterations in the distribution and density of sensory and autonomic endings in the mystacial pad vibrissal and intervibrissal fur innervation were discovered. Pilo-neural complexes innervating fur hairs were dismantled in Hr. Epidermal innervation in SW was rich; only a few endings expressed growth-associated protein-43 kdal (GAP), suggesting limited changes in axonal elongation. Innervation in Hr formed a dense layer passing upward through the thickened epidermis, with substantial increases among all types of endings. Vibrissal follicle-sinus complexes were also hyperinnervated. Endings in Hr vibrissae and fur were strongly GAP-positive, suggesting reorganization of innervation. Dermal and vascular autonomic innervation in both strains co-localized tyrosine hydroxylase and neuropeptide Y, but only in Hr did neuropeptide Y co-localize calcitonin gene-related peptide (CGRP) and express GAP immunolabeling. Stereological quantitation of trigeminal ganglia revealed no differences in neuron number between Hr and SW, although there were small increases in cell volume in Hr trigeminal ganglion cells. These results suggested that a form of collateral sprouting was active in Hr mystacial pads, not in response to local injury, but as a result of loss of primary target tissues.

Hydrodynamic trail following in a California sea lion (Zalophus californianus)

The mystacial vibrissae of pinnipeds constitute a sensory system for active touch and detection of hydrodynamic events. Harbour seals (Phoca vitulina) and California sea lions (Zalophus californianus) can both detect hydrodynamic stimuli caused by a small sphere vibrating in the water (hydrodynamic dipole stimuli). Hydrodynamic trail following has only been shown in harbour seals. Hydrodynamical and biomechanical studies of single vibrissae of the two species showed that the specialized undulated structure of harbour seal vibrissae, as opposed to the smooth structure of sea lion vibrissae, suppresses self-generated noise in the actively moving animal. Here we tested whether also sea lions were able to perform hydrodynamic trail following in spite of their non-specialized hair structure. Hydrodynamic trails were generated by a remote-controlled miniature submarine. Linear trails could be followed with high accuracy, comparable to the performance of harbour seals, but in contrast, increasing delay resulted in a reduced performance as compared to harbour seals. The results of this study are consistent with the hypothesis that structural differences in the vibrissal hair types of otariid compared to phocid pinnipeds lead to different sensitivity of the vibrissae during forward swimming, but still reveal a good performance even in the species with non-specialized hair type.

The morphology of the rat vibrissal follicle-sinus complex revealed by three-dimensional computer-aided reconstruction

The vibrissal follicle-sinus complex (FSC) is a sensory receptor of the mammalian integumentary system that is located around the mouth. The purpose of the present study was to identify the actual 3-dimensional structure of the rat vibrissal FSC. Rat skin tissue was serially sectioned at a thickness of 10 μm and then stained with Masson's trichrome. The serial sections were reconstructed 3-dimensionally using Reconstruct software. The rat vibrissal follicle is a spindle-shaped structure that is embedded within a blood sinus and enveloped within a thick collagenous capsule. The vibrissal FSC is innervated by the deep vibrissal and superficial vibrissal nerves. The deep vibrissal nerve, travelling in the basal-to-apical direction, penetrates the thick collagenous capsule of the vibrissal FSC. The sinus system can be divided into a superior portion, known as the ring sinus, and an inferior portion, known as the cavernous sinus. The ring sinus contains a C-shaped structure, the ringwulst, which is suspended from the mesenchymal sheath of the follicle. Collagenous trabeculae can be seen in the cavernous sinus but not in the ring sinus. The ring sinus encircles the follicle obliquely and asymmetrically. The ringwulst encircles the follicle incompletely, in a C-shaped fashion. This study has demonstrated the previously underappreciated 3-dimensional structure of the vibrissal FSC, which differs from previously reported descriptions, and provides data that will enhance the understanding of vibrissal function.

Development of innervation to maxillary whiskers in mice

The maxillary vibrissal pad is a unique, richly innervated sensory apparatus. It is highly evolved in the rodent that it constitutes a major source of sensory information to the somatosensory cortex. In this report, indocarbocyanine tracing and immunofluorescence were used to study the embryonic and early neonatal development of innervation to maxillary vibrissal follicles in mice. The first sign of vibrissal follicle innervation occurred at embryonic day 12 (E12), when the lateral nasal and maxillary processes were penetrated by nerve branches with small terminal plexuses assuming the positions of vibrissal follicle primordia. Between E13 and E15, the nerve plexuses at the presumptive follicles grew in size and became more numerous with no signs of specific receptor subtype formation. By E17, the nerve plexuses had grown further in size and the region-specific receptor subtype specification developed. At birth (P0), the superficial vibrissal nerves began to innervate the apical part of the inner conical body, whereas the deep vibrissal nerve gave off the recurrent cavernous branches. At P3, all of the different sets of receptor subtypes had regional distributions, densities and morphologies comparable to those described in adult mice. A 3-day old mouse had all complements of sensory receptors necessary for somatosensory transduction as revealed not only by neuroanatomic tracing but also with immunofluorescence for several markers of neurosensory differentiation. Our data reveal a hitherto unknown time table for the development of peripheral sensory receptors in the vibrissal follicles. This time table parallels that of their central targets in the somatosensory barrel cortex, which develops at P4.

Texture sensation through the fingertips and the whiskers

Texture sensation in primates, through the fingertips, and in rats, through the whiskers, is rapid and accurate. Recent work has provided enough knowledge to allow comparison of the mechanisms. In primates, coarse textures are sensed through a spatial signal - increasing roughness leads to greater contrast in firing among neurons with nearby receptive fields. Fine textures are sensed by a motion signal - when the fingertip translates across the surface, excitation of rapidly adapting receptors varies according to texture. In rats, passage of a whisker over a surface leads to a motion signal that varies according to roughness. Motion is converted to neuronal spike trains, with progressively rougher surfaces leading to progressively greater firing rates. The extraction of texture by the whiskers shares basic features with the motion-based mechanism in primates. Spatial relationships among whiskers may serve purposes other than texture.

Adaptation of trigeminal ganglion cells to periodic whisker deflections

Trigeminal ganglion neurons in adult rats adapt to periodic whisker deflections in the range of 1-40 Hz, manifested as a reduction in spike counts to progressively later stimuli in a train of pulsatile or sinusoidal deflections. For high velocity, pulsatile deflections, adaptation is time- and frequency-dependent; as in the case of thalamic and cortical neurons, adaptation is greater at higher stimulus frequencies. With slower velocity, sinusoidal movements, trigeminal ganglion cells differ from central neurons, however, by exhibiting strong adaptation even at low frequencies. For both types of stimuli, effects in trigeminal ganglion neurons were more pronounced in rats maintained during the recording session under neuromuscular blockade than in non-paralysed animals. Results are consistent with previous findings in other systems that frequency-dependent adaptation of cutaneous primary afferent neurons is affected by mechanical properties of the skin. Such effects are likely to vary depending on the nature of the whisker stimuli and physiological states that affect skin viscoelasticity.

Hypoglossal nucleus projections to the rat masseter muscle

We investigated in the rat whether hypoglossal innervation extended to facial muscles other than the extrinsic musculature of the mystacial pad. Results showed that hypoglossal neurons also innervate the masseter muscle. Dil injected into the XII nucleus showed hypoglossal axons in the ipsilateral main trunk of the trigeminal nerve. After Gasser's ganglion crossing, the axons entered into the infraorbital division of the trigeminal nerve and targeted the extrinsic muscles of the mystacial pad. They also spread into the masseter branch of the trigeminal nerve to target the polar portions of the masseter muscle spindles. Retrograde double labelling, performed by injecting Dil into the pad and True Blue into the ipsilateral masseter muscle, showed labelled hypoglossal neurons in the medio-dorsal portion of the XII nucleus. The majority of these neurons were small (15-20 microm diameter), showed fluorescence for Dil and projected to the mystacial pad. Other medium-size neurons (25 microm diameter) were instead labelled with True Blue and projected to the masseter muscle. Finally, in the same area, other small hypoglossal neurons showed double labelling and projected to both structures. Functional hypotheses on the role of these hypoglossal projections have been discussed.

Laboratory experience for teaching sensory physiology

The major challenge in laboratory teaching is the application of abstract concepts in simple and direct practical lessons. However, students rarely have the opportunity to participate in a laboratory that combines practical learning with a realistic research experience. In the Bioengineering Department, we started an experiential laboratory physiology to teach graduated students some aspects of sensorial physiology and exposes them to laboratory skills in instrumentation and physiological measurements. Students were able to analyze and quantify the effects of activation of mechanoreceptors in multifiber afferent discharges using equipment that was not overly sophisticated. In consequence, this practical laboratory helps students to make connections with physiological concepts acquired in theoretical classes and to introduce them to electrophysiological research.

Neural coding and contextual influences in the whisker system

A fundamental problem in neuroscience, to which Prof. Segundo has made seminal contributions, is to understand how action potentials represent events in the external world. The aim of this paper is to review the issue of neural coding in the context of the rodent whiskers, an increasingly popular model system. Key issues we consider are: the role of spike timing; mechanisms of spike timing; decoding and context-dependence. Significant insight has come from the development of rigorous, information theoretic frameworks for tackling these questions, in conjunction with suitably designed experiments. We review both the theory and experimental studies. In contrast to the classical view that neurons are noisy and unreliable, it is becoming clear that many neurons in the subcortical whisker pathway are remarkably reliable and, by virtue of spike timing with millisecond-precision, have high bandwidth for conveying sensory information. In this way, even small (approximately 200 neuron) subcortical modules are able to support the sensory processing underlying sophisticated whisker-dependent behaviours. Future work on neural coding in cortex will need to consider new findings that responses are highly dependent on context, including behavioural and internal states.

Response properties of mouse trigeminal ganglion neurons

We used controlled whisker deflections to examine the response properties of 208 primary afferent neurons in the trigeminal ganglion of adult mice. Proportions of rapidly adapting (RA, 47%) and slowly adapting (SA, 53%) neurons were equivalent, and most cells had low or no spontaneous activity. We quantified angular tuning and sensitivity to deflection amplitude and velocity. Both RA and SA units fired more frequently to larger deflections and faster deflections, but RA units were more sensitive to differences in velocity whereas SA units were more sensitive to deflection amplitudes. Almost all neurons were tuned for deflection angle, and the average response to the maximally effective direction was more than fourfold greater than the average response in the opposite direction; SA units were more tuned than RA units. Responses of primary afferent whisker-responsive neurons are qualitatively similar to those of the rat. However, average firing rates of both RA and SA neurons in the mouse are less sensitive to differences in deflection velocity, and RA units, unlike those in the rat, display amplitude sensitivity. Subtle observed differences between mice and rats may reflect greater mechanical compliance in mice of the whisker hairs and of the tissue in which they are embedded.

Biomechanics of the vibrissa motor plant in rat: rhythmic whisking consists of triphasic neuromuscular activity

The biomechanics of a motor plant constrain the behavioral strategies that an animal has available to extract information from its environment. We used the rat vibrissa system as a model for active sensing and determined the pattern of muscle activity that drives rhythmic exploratory whisking. Our approach made use of electromyography to measure the activation of all relevant muscles in both head-fixed and unrestrained rats and two-dimensional imaging to monitor the position of the vibrissae in head-fixed rats. Our essential finding is that the periodic motion of the vibrissae and mystacial pad during whisking results from three phases of muscle activity. First, the vibrissae are thrust forward as the rostral extrinsic muscle, musculus (m.) nasalis, contracts to pull the pad and initiate protraction. Second, late in protraction, the intrinsic muscles pivot the vibrissae farther forward. Third, retraction involves the cessation of m. nasalis and intrinsic muscle activity and the contraction of the caudal extrinsic muscles m. nasolabialis and m. maxillolabialis to pull the pad and the vibrissae backward. We developed a biomechanical model of the whisking motor plant that incorporates the measured muscular mechanics along with movement vectors observed from direct muscle stimulation in anesthetized rats. The results of simulations of the model quantify how the combination of extrinsic and intrinsic muscle activity leads to an enhanced range of vibrissa motion than would be available from the intrinsic muscles alone.

A two-dimensional force sensor in the millinewton range for measuring vibrissal contacts

The rat vibrissal (whisker) array is a common model system in neuroscience used to study sensorimotor integration. Recent work has suggested that during object contact, the forces and moments at the whisker base may serve as important perceptual cues to the rat. To date, however, the force/moment profile that results from a whisker sweeping against an object has yet to be characterized, because it requires the simultaneous measurement of two-dimensional forces on the order of millinewtons. Current technology for these measurements typically involves prohibitively bulky, expensive equipment with complicated fabrication techniques. We have developed a simple, yet effective two-dimensional force sensor with +/-0.02 mN resolution; it is extremely compact, has a highly linear static response with low-noise output, and is inexpensive to build. We demonstrate the advantages and limitations of the sensor in three different experimental protocols, ranging from the precise quantification of forces on isolated (plucked) whiskers, to the detection of whisker-contact times in the awake behaving animal. Given the high fidelity of the sensor, it could have utility in a broad range of applications in which measuring contact/detach occurrence and/or small magnitude forces are important.

Spike-timing in primary sensory neurons: a model of somatosensory transduction in the rat

In previous work, we constructed a simple electro-mechanical model of transduction in the rat mystacial follicle that was able to replicate primary afferent response profiles to a variety of whisker deflection stimuli. Here, we update that model to fit newly available spike-timing response data, and demonstrate that the new model produces appropriate responses to richer stimuli, including pseudo white noise and natural textures, at a spike-timing level of detail. Additionally, we demonstrate reliable distributed encoding of multi-component oscillatory signals. No modifications were necessary to the mechanical model of the physical components of the follicle-sinus complex, supporting its generality. We conclude that this model, and its continued development, will aid the understanding both of somatosensory systems in general, and of physiological results from higher (e.g. thalamocortical) systems by accurately characterising the signals on which they operate.

Hypoglossal nuclei participation in rat mystacial pad control

Recently, we showed that extra-trigeminal axons, originating from the hypoglossal nucleus, travel with the infraorbital division of the trigeminal nerve (ION), which is known to innervate the rat mystacial pad. Dil was monolaterally injected into the rat XII nucleus to analyse the peripheral distribution of hypoglossal axons to the mystacial pad, to evaluate their involvement in facial sensory-motor control. Electromyographic responses of mystacial pad motor units to electrical stimulation of the ION were recorded, along with the evoked responses to electrical stimulation of the ipsilateral XII nucleus. The results showed that hypoglossal axon terminals target the ipsilateral extrinsic musculature of the mystacial pad, but they do not have any contact with the intrinsic muscles. ION electrical stimulation increased electromyographic activity in the ipsilateral pad extrinsic muscles, even following VII nerve transection. Hypoglossal nucleus electrical stimulation induced field potentials and monosynaptic responses in the same motor units that persisted even following VII nerve transection, these disappearing after cooling the ION. We suggest that the small hypoglossal neurons projecting to the extrinsic musculature of the mystacial pad are part of a hypoglossal-trigeminal loop that participates in the sensory-motor control of the rat vibrissae system.

Adaptations in the structure and innervation of follicle-sinus complexes to an aquatic environment as seen in the Florida manatee (Trichechus manatus latirostris)

Florida manatees are large-bodied aquatic herbivores that use large tactile vibrissae for several purposes. Facial vibrissae are used to forage in a turbid water environment, and the largest perioral vibrissae can also grasp and manipulate objects. Other vibrissae distributed over the entire postfacial body appear to function as a lateral line system. All manatee vibrissae emanate from densely innervated follicle-sinus complexes (FSCs) like those in other mammals, although proportionately larger commensurate with the caliber of the vibrissae. As revealed by immunofluorescence, all manatee FSCs have many types of C, Adelta and Abeta innervation including Merkel, club, and longitudinal lanceolate endings at the level of the ring sinus, but they lack other types such as reticular and spiny endings at the level of the cavernous sinus. As in non-whisking terrestrial species, the inner conical bodies of facial FSCs are well innervated but lack Abeta-fiber terminals. Importantly, manatee FSCs have two unique types of Abeta-fiber endings. First, all of the FSCs have exceptionally large-caliber axons that branch to terminate as novel, gigantic spindle-like endings located at the upper ring sinus. Second, facial FSCs have smaller caliber Abeta fibers that terminate in the trabeculae of the cavernous sinus as an ending that resembles a Golgi tendon organ. In addition, the largest perioral vibrissae, which are used for grasping, have exceptionally well-developed medullary cores that have a structure and dense small-fiber innervation resembling that of tooth pulp. Other features of the epidermis and upper dermis structure and innervation differ from that seen in terrestrial mammals.

Responses of Trigeminal Ganglion Neurons during Natural Whisking Behaviors in the Awake Rat

Rats use their whiskers to locate and discriminate tactile features of their environment. Mechanoreceptors surrounding each whisker encode and transmit sensory information from the environment to the brain via afferents whose cell bodies lie in the trigeminal ganglion (Vg). These afferents are classified as rapidly (RA) or slowly (SA) adapting by their response to stimulation. The activity of these cells in the awake behaving rat is yet unknown. Therefore, we developed a method to chronically record Vg neurons during natural whisking behaviors and found that all cells exhibited (1) no neuronal activity when the whiskers were not in motion, (2) increased activity when the rat whisked, with activity correlated to whisk frequency, and (3) robust increases in activity when the whiskers contacted an object. Moreover, we observed distinct differences in the firing rates between RA and SA cells, suggesting that they encode distinct aspects of stimuli in the awake rat.

Texture discrimination and multi-unit recording in the rat vibrissal nerve

Background

Rats distinguish objects differing in surface texture by actively moving their vibrissae. In this paper we characterized some aspects of texture sensing in anesthetized rats during active touch. We analyzed the multifiber discharge from a deep vibrissal nerve when the vibrissa sweeps materials (wood, metal, acrylic, sandpaper) having different textures. We polished these surfaces with sandpaper (P1000) to obtain close degrees of roughness and we induced vibrissal movement with two-branch facial nerve stimulation. We also consider the change in pressure against the vibrissa as a way to improve the tactile information acquisition. The signals were compared with a reference signal (control) – vibrissa sweeping the air – and were analyzed with the Root Mean Square (RMS) and the Power Spectrum Density (PSD).

Results

We extracted the information about texture discrimination hidden in the population activity of one vibrissa innervation, using the RMS values and the PSD. The pressure level 3 produced the best differentiation for RMS values and it could represent the "optimum" vibrissal pressure for texture discrimination. The frequency analysis (PSD) provided information only at low-pressure levels and showed that the differences are not related to the roughness of the materials but could be related to other texture parameters.

Conclusion

Our results suggest that the physical properties of different materials could be transduced by the trigeminal sensory system of rats, as are shown by amplitude and frequency changes. Likewise, varying the pressure could represent a behavioral strategy that improves the information acquisition for texture discrimination.

Molecular determinants of the face map development in the trigeminal brainstem

The perception of external sensory information by the brain requires highly ordered synaptic connectivity between peripheral sensory neurons and their targets in the central nervous system. Since the discovery of the whisker-related barrel patterns in the mouse cortex, the trigeminal system has become a favorite model for study of how its connectivity and somatotopic maps are established during development. The trigeminal brainstem nuclei are the first CNS regions where whisker-specific neural patterns are set up by the trigeminal afferents that innervate the whiskers. In particular, barrelette patterns in the principal sensory nucleus of the trigeminal nerve provide the template for similar patterns in the face representation areas of the thalamus and subsequently in the primary somatosensory cortex. Here, we describe and review studies of neurotrophins, multiple axon guidance molecules, transcription factors, and glutamate receptors during early development of trigeminal connections between the whiskers and the brainstem that lead to emergence of patterned face maps. Studies from our laboratories and others' showed that developing trigeminal ganglion cells and their axons depend on a variety of molecular signals that cooperatively direct them to proper peripheral and central targets and sculpt their synaptic terminal fields into patterns that replicate the organization of the whiskers on the muzzle. Similar mechanisms may also be used by trigeminothalamic and thalamocortical projections in establishing patterned neural modules upstream from the trigeminal brainstem.

Relationship between physiological response type (RA and SA) and vibrissal receptive field of neurons within the rat trigeminal ganglion

Cells within the trigeminal ganglion (Vg) encode all the information necessary for the rat to differentiate tactile stimuli, yet it is the least-studied component in the rodent trigeminal somatosensory system. For example, extensive anatomical and electrophysiological investigations have shown clear somatotopic organization in the higher levels of this system, including VPM thalamus and SI cortex, yet whether this conserved schemata exists in the Vg is unknown. Moreover although there is recent interest in recording from vibrissae-responsive cells in the Vg, it is surprising to note that the locations of these cells have not even been clearly demarcated. To address this, we recorded extracellularly from 350 sensory-responsive Vg neurons in 35 Long-Evans rats. First, we determined three-dimensional locations of these cells and found a finer detail of somatotopy than previously reported. Cells innervating dorsal facial features, even within the whisker region, were more dorsal than midline and ventral features. We also show more cells with caudal than rostral whisker receptive fields (RF), similar to that found in VPM and SI. Next, for each vibrissal cell we determined its response type classified as either rapidly (RA) or slowly (SA) adapting. We examined the relationship between vibrissal RF and response type and demonstrate similar proportions of RA and SA cells responding to any whisker. These results suggest that if RA and SA cells encode distinct features of stimuli, as previously suggested, then at the basic physiological level each whisker has similar abilities to encode for such features.

A Biomimetic Haptic Sensor

The design and implementation of the periphery of an artificial whisker sensory system is presented. It has been developed by adopting a biomimetic approach to model the structure and function of rodent facial vibrissae. The artificial vibrissae have been formed using composite materials and have the ability to be actively moved or whisked. The sensory structures at the root of real vibrissae has been modelled and implemented using micro strain gauges and Digital Signal Processors. The primary afferents and vibrissal trigeminal ganglion have been modelled using empirical data taken from electrophysiological measurements, and implemented in real-time using a Field Programmable Gate Array. Pipelining techniques were employed to maximise the utility of the FPGA hardware. The system is to be integrated into a more complete whisker sensory model, including neural structures within the central nervous system, which can be used to orient a mobile robot.

Responses of trigeminal ganglion neurons to the radial distance of contact during active vibrissal touch

Rats explore their environment by actively moving their whiskers. Recently, we described how object location in the horizontal (front-back) axis is encoded by first-order neurons in the trigeminal ganglion (TG) by spike timing. Here we show how TG neurons encode object location along the radial coordinate, i.e., from the snout outward. Using extracellular recordings from urethane-anesthetized rats and electrically induced whisking, we found that TG neurons encode radial distance primarily by the number of spikes fired. When an object was positioned closer to the whisker root, all touch-selective neurons recorded fired more spikes. Some of these cells responded exclusively to objects located near the base of whiskers, signaling proximal touch by an identity (labeled-line) code. A number of tonic touch-selective neurons also decreased delays from touch to the first spike and decreased interspike intervals for closer object positions. Information theory analysis revealed that near-certainty discrimination between two objects separated by 30% of the length of whiskers was possible for some single cells. However, encoding reliability was usually lower as a result of large trial-by-trial response variability. Our current findings, together with the identity coding suggested by anatomy for the vertical dimension and the temporal coding of the horizontal dimension, suggest that object location is encoded by separate neuronal variables along the three spatial dimensions: temporal for the horizontal, spatial for the vertical, and spike rate for the radial dimension.

Empirically inspired simulated electro-mechanical model of the rat mystacial follicle-sinus complex

In whiskered animals, activity is evoked in the primary sensory afferent cells (trigeminal nerve) by mechanical stimulation of the whiskers. In some cell populations this activity is correlated well with continuous stimulus parameters such as whisker deflection magnitude, but in others it is observed to represent events such as whisker-stimulator contact or detachment. The transduction process is mediated by the mechanics of the whisker shaft and follicle-sinus complex (FSC), and the mechanics and electro-chemistry of mechanoreceptors within the FSC. An understanding of this transduction process and the nature of the primary neural codes generated is crucial for understanding more central sensory processing in the thalamus and cortex. However, the details of the peripheral processing are currently poorly understood. To overcome this deficiency in our knowledge, we constructed a simulated electro-mechanical model of the whisker-FSC-mechanoreceptor system in the rat and tested it against a variety of data drawn from the literature. The agreement was good enough to suggest that the model captures many of the key features of the peripheral whisker system in the rat.

Microstructure and innervation of the mystacial vibrissal follicle-sinus complex in bearded seals,Erignathus barbatus (Pinnipedia: Phocidae)

Vibrissal follicle-sinus complexes (F-SCs) are sensory receptors of the mammalian integument system. They are best developed within Pinnipedia. The objective of this study was to investigate the F-SCs of bearded seals (Erignathus barbatus) for benthic foraging adaptations. Bearded seals possessed approximately 244 mystacial F-SCs. In this species, F-SCs consisted of an outer dermal capsule (DC) surrounding a blood sinus system [upper cavernous sinus (UCS), ring sinus (RS), and lower cavernous sinus (LCS)] and concentric rings of epidermal tissue. The UCS comprised up to 62% of the F-SC length and may function as thermal protection for mechanoreceptors. A large asymmetrical ringwulst was located in the RS. A deep vibrissal nerve penetrated the DC at its base and terminated on mechanoreceptors in the epidermal tissues of the LCS and RS. The mean number of myelinated axons per F-SC was 1,314 (range, 811-1,650) and was among the highest number of axons per F-SC reported to date. An estimated mean number of 320,616 myelinated axons innervate the entire mystacial vibrissal array. Merkel-Neurite complexes (MNCs) and small simple laminated corpuscles were found in the region of the LCS. Myelinated axons also terminated on MNCs and lanceolate endings apical to the ringwulst. The number of F-SCs, their geometry in the mystacial region, the number of myelinated axons per F-SC, and the distribution of mechanoreceptors support the premise that pinniped vibrissae are sensitive active-touch receptor systems, and that structural differences in bearded seals, relative to other phocids, may be adaptations for benthic foraging.

Response properties of whisker-related neurons in rat second somatosensory cortex

In addition to a primary somatosensory cortex (SI), the cerebral cortex of all mammals contains a second somatosensory area (SII); however, the functions of SII are largely unknown. Our aim was to explore the functions of SII by comparing response properties of whisker-related neurons in this area with their counterparts in the SI. We obtained extracellular unit recordings from narcotized rats, in response to whisker deflections evoked by a piezoelectric device, and compared response properties of SI barrel (layer IV) neurons with those of SII (layers II to VI) neurons. Neurons in both cortical areas have similar response latencies and spontaneous activity levels. However, SI and SII neurons differ in several significant properties. The receptive fields of SII neurons are at least five times as large as those of barrel neurons, and they respond equally strongly to several principal whiskers. The response magnitude of SII neurons is significantly smaller than that of neurons in SI, and SII neurons are more selective for the angle of whisker deflection. Furthermore, whereas in SI fast-spiking (inhibitory) and regular-spiking (excitatory) units have different spontaneous and evoked activity levels and differ in their responses to stimulus onset and offset, SII neurons do not show significant differences in these properties. The response properties of SII neurons suggest that they are driven by thalamic inputs that are part of the paralemniscal system. Thus whisker-related inputs are processed in parallel by a lemniscal system involving SI and a paralemniscal system that processes complimentary aspects of somatosensation.

Encoding of vibrissal active touch

Mammals acquire much of their sensory information by actively moving their sensory organs. Yet, the principles of encoding by active sensing are not known. Here we investigated the encoding principles of active touch by rat whiskers (vibrissae). We induced artificial whisking in anesthetized rats and recorded from first-order neurons in the trigeminal ganglion. During active touch, first-order trigeminal neurons presented a rich repertoire of responses, which could not be inferred from their responses to passive deflection stimuli. Individual neurons encoded four specific events: whisking, contact with object, pressure against object, and detachment from object. Whisking-responsive neurons fired at specific deflection angles, reporting the actual whiskers' position with high precision. Touch-responsive neurons encoded the horizontal coordinate of objects' position by spike timing. These findings suggest two specific encoding-decoding schemes for horizontal object position in the vibrissal system.

Somatosensory organization and behavior in naked mole-rats: II. Peripheral structures, innervation, and selective lack of neuropeptides associated with thermoregulation and pain

African naked mole-rats are subterranean rodents that have a robust orienting response to stimulation of unique vibrissa-like body hairs that are widely spaced over an otherwise hairless skin. To determine whether these large body hairs have a specialized organization similar to facial vibrissae, the structure and innervation of facial vibrissa follicles, body hair follicles, and intervening skin in naked mole-rats was compared with that in rats and a furred African mole-rat species (the common mole-rat). Immunofluorescence and lectin-binding analyses revealed that the body hair follicles in naked mole-rats were exceptionally large and well innervated, similar to guard hairs of furred species. However, these body vibrissae lacked the anatomic specializations and unique types of innervation affiliated with follicle sinus complexes of facial vibrissae. In contrast to the furred species, naked mole-rats had a paucity of Abeta-fiber Merkel endings at all peripheral locations. Naked mole-rats also were completely lacking in cutaneous C-fibers immunoreactive for substance P and calcitonin gene-related peptide. In contrast, the hairless skin of the naked mole-rats had an exceptional abundance of presumptive Adelta-fibers. The unusual features of the cutaneous innervation in naked mole-rats are presumably adaptations to their subterranean environment and that they are the only known poikilothermic mammal. The features of this mammalian model system provide unique opportunities to discriminate mechanisms related to tactile spatial orientation, vascular regulation, and nociception.

Vibrissa resonance as a transduction mechanism for tactile encoding

We present evidence that resonance properties of rat vibrissae differentially amplify high-frequency and complex tactile signals. Consistent with a model of vibrissa mechanics, optical measurements of vibrissae revealed that their first mechanical resonance frequencies systematically varied from low (60-100 Hz) in longer, posterior vibrissae to high ( approximately 750 Hz) in shorter, anterior vibrissae. Resonance amplification of tactile input was observed in vivo and ex vivo, and in a variety of boundary conditions that are likely to occur during perception, including stimulation of the vibrissa with moving complex natural stimuli such as sandpaper. Vibrissae were underdamped, allowing for sharp tuning to resonance frequencies. Vibrissa resonance constitutes a potentially useful mechanism for perception of high-frequency and complex tactile signals. Amplification of small amplitude signals by resonance could facilitate detection of stimuli that would otherwise fail to drive neural activity. The systematic map of frequency sensitivity across the face could facilitate texture discrimination through somatotopic encoding of frequency content. These findings suggest strong parallels between vibrissa tactile processing and auditory encoding, in which the cochlea also uses resonance to amplify low-amplitude signals and to generate a spatial map of frequency sensitivity.

Protracted development of responses to whisker deflection in rat trigeminal ganglion neurons

The rodent whisker-to-barrel pathway constitutes a major model system for studying experience-dependent brain development. Yet little is known about responses of neurons to whisker stimulation in young animals. Response properties of trigeminal ganglion (NV) neurons in 2-, 3-, and 4-week-old and adult rats were examined using extracellular single-unit recordings and controlled whisker stimuli. We found that the receptive field size of NV neurons is mature in 2-week-old animals while response latencies, magnitudes, and angular tuning continue to develop between 2 weeks of age and adulthood. At the earliest time recorded, NV neurons respond to stimulation of only one whisker and can be characterized as slowly or rapidly adapting (SA, RA). The proportion of SA and RA neurons remains constant during development. Consistent with known on-going myelination of NV axons, response latencies decrease with age, becoming adult-like during the third and fourth postnatal weeks for RA and SA neurons, respectively. Unexpectedly, we found that evoked response magnitudes increase several-fold during development becoming adult-like only during the fourth postnatal week. In addition, RA neurons become less selective for whisker deflection angle with age. Maturation of response magnitude and angular tuning is consistent with developmental changes in the mechanical properties of the whisker, the whisker follicle, and the surrounding tissues. The findings indicate that whisker-derived tactile inputs mature during the first postnatal month when whisker-related cortical circuits are susceptible to long-term modification by sensory experience. Thus normal developmental changes in sensory input may influence functional development of cortical circuits.

Imaging spatiotemporal dynamics of surround inhibition in the barrels somatosensory cortex

Sensory processing and its perception require that local information would also be available globally. Indeed, in the mammalian neocortex, local excitation spreads over large distances via the long-range horizontal connections in layer 2/3 and may spread over an entire cortical area if excitatory polysynaptic pathways are also activated. Therefore, a balance between local excitation and surround inhibition is required. Here we explore the spatiotemporal aspects of cortical depolarization and hyperpolarization of rats anesthetized with urethane. New voltage-sensitive dyes (VSDs) were used for high-resolution real-time visualization of the cortical responses to whisker deflections and cutaneous stimulations of the whisker pad. These advances facilitated imaging of ongoing activity and evoked responses even without signal averaging. We found that the motion of a single whisker evoked a cortical response exhibiting either one or three phases. During a triphasic response, there was first a cortical depolarization in a small cortical region the size of a single cortical barrel. Subsequently, this depolarization increased and spread laterally in an oval manner, preferentially along rows of the barrel field. During the second phase, the amplitude of the evoked response declined rapidly, presumably because of recurrent inhibition. Subsequently, the third phase exhibiting a depolarization rebound was observed and clear, and approximately 16 Hz oscillations were detected. Stimulus conditions revealing a net surround hyperpolarization during the second phase were also found. By using new, improved VSD, the present findings shed new light on the spatial parameters of the intricate spatiotemporal cortical interplay of inhibition and excitation.