Does Vibrissal Innervation Patterns and Investment Predict Hydrodynamic Trail Following Behavior of Harbor Seals (Phoca vitulina)?

Our understanding of vibrissal function in pinnipeds is poor due to the lack of comparative morphological, neurobiological, and psychophysical performance data. In contrast, the function of terrestrial mammalian vibrissae is better studied. Pinnipeds have the largest vibrissae of all mammals, and phocids may have the most modified vibrissae. The tactile performance for pinniped vibrissae is well known for harbor seals (Phoca vitulina). Harbor seals display at least two types of tactile behavior involving their mystacial vibrissae: a fine discriminatory capability using active touch and hydrodynamic trail following (the ability to detect and follow turbulent trails). This study investigated innervation patterns of harbor seal follicle-sinus complexes (F-SCs) to test the hypothesis that the whiskers used in hydrodynamic trail following possess increased innervation investment compared to other phocids. Therefore, the most lateral vibrissae from five harbor seals were histologically processed so that morphometric measurements and axon counts could be collected. Vibrissae from one harbor seal were immunolabeled with anti-protein gene product (PGP 9.5) to document the pattern of deep vibrissal nerve innervation of the F-SCs. Overall, harbor seals showed an innervation pattern (axons/F-SC and axons/muzzle) similar to other phocids. The ventrolateral vibrissae, involved in hydrodynamic trail following, have greater axon density in harbor seals than harp seals, suggesting harbor seal F-SC innervation patterns could explain their performance at trail following. The combination of microstructural, innervation investment, and behavioral data provides a foundation for functional inference regarding this tactile behavior in harbor seals and also facilitates future comparative work for other pinniped species. Anat Rec, 302:1837–1845, 2019. © 2019 American Association for Anatomy     

Normotopic and heterotopic cortical representations of mystacial vibrissae in rats with subcortical band heterotopia.

The tish rat is a neurological mutant exhibiting bilateral cortical heterotopia similar to those found in certain epileptic patients. Previous work has shown that thalamocortical fibers originating in the ventroposteromedial nucleus, which in normal animals segregate as 'barrel' representations for individual whiskers, terminate in both normotopic and heterotopic areas of the tish cortex (Schottler et al., 1998). Thalamocortical innervation terminates as barrels in layer IV and diffusely in layer VI of the normotopic area. Discrete patches of terminals are also observed in the underlying heterotopic area suggesting that representations of individual vibrissa may be present in the heterotopic somatosensory areas. The present study examines this issue by investigating the organization of the vibrissal somatosensory system in the tish cortex. Staining for cytochrome oxidase or Nissl substance reveals a normal complement of vibrissal barrels in the normotopic area of the tish cortex. Dense patches of cytochrome oxidase staining are also found in the underlying lateral portions of the heterotopic area (i.e. the same area that is innervated by the ventroposteromedial nucleus). Injections of retrograde tracers into vibrissal areas of either the normotopic or heterotopic area produce topographically organized labeling of neurons restricted to one or a small number of barreloids within the ventroposteromedial nucleus of the thalamus. Physical stimulation of a single whisker (D3 or E3) elicits enhanced uptake of [(14)C]2-deoxyglucose in restricted zones of both the normotopic and heterotopic areas, demonstrating that single whisker stimulation can increase functional activity in both normotopic and heterotopic neurons. These findings indicate that the barrels are intact in the normotopic area and are most consistent with the hypothesis that at least some of the individual vibrissae are 'dually' represented in normotopic and heterotopic positions in the primary somatosensory areas of the tish cortex.     

The surface activity of pulmonary surfactant from diving mammals

Pinnipeds (seals and sea lions) have developed a specialised respiratory system to cope with living in a marine environment. They have a highly reinforced lung that can completely collapse and reinflate during diving without any apparent side effects. These animals may also have a specialised surfactant system to augment the morphological adaptations. The surface activity of surfactant from four species of pinniped (California sea lion, Northern elephant seal, Northern fur seal and Ringed seal) was measured using a captive bubble surfactometer (CBS), and compared to two terrestrial species (sheep and cow). The surfactant of Northern elephant seal, Northern fur seal and Ringed seal was unable to reduce surface tension (gamma) to normal levels after 5 min adsorption (61.2, 36.7, and 46.2 +/- 1.7 mN/m, respectively), but California sea lion was able to reach the levels of the cow and sheep (23.4 mN/m for California sea lion, 21.6 +/- 0.3 and 23.0 +/- 1.5 mN/m for cow and sheep, respectively). All pinnipeds were also unable to obtain the very low gamma(min) achieved by cow (1.4 +/- 0.1 mN/m) and sheep (1.5 +/- 0.4 mN/m). These results suggest that reducing surface tension to very low values is not the primary function of surfactant in pinnipeds as it is in terrestrial mammals, but that an anti-adhesive surfactant is more important to enable the lungs to reopen following collapse during deep diving.     

Morphology and Biomechanics of the Pinniped Jaw: Mandibular Evolution Without Mastication

Pinnipeds (seals, sea lions, and walruses) underwent a shift in jaw function away from typical carnivoran mastication to more novel marine behaviors during the terrestrial‐aquatic transition. Here we test the effect of aquatic prey capture and male‐male combat on the morphological evolution of a mammal jaw that does not masticate. Nine three‐dimensional landmarks were taken along the mandible for 25 species (N = 83), and corpus and symphysis external and cortical breadths for a subset of five species (N = 33). Principal components analysis was performed on size‐corrected landmark data to assess variation in overall jaw morphology across pinnipeds. Corpus breadths were input to a beam model to calculate strength properties and estimated bite force of specific species with contrasting behaviors (filter feeding, suction feeding, grip‐and‐tear feeding, and male‐male combat). Results indicate that, although phylogenetic signal in jaw shape is strong, function is also important in determining morphology. Filter feeders display an elongate symphysis and a long toothrow that may play a role in filtering krill. Grip‐and‐tear feeders have a long jaw and large estimated bite force relative to non‐biting species. However, the largest estimated bite forces were observed in males of male‐male combative species, likely due to the high selection pressure associated with male success in highly polygynous species. The suction feeding jaw is weak in biting but has a different morphology in the two suction feeding taxa. In conclusion, familial patterns of pinniped jaw shape due to phylogenetic relatedness have been modified by adaptations to specialized behavior of individual taxa. Anat Rec, 296:1049–1063, 2013. © 2013 Wiley Periodicals, Inc.     

Structure-function relationships in rat medullary and cervical dorsal horns. I. Trigeminal primary afferents

Intracellular recording and horseradish peroxidase (HRP) labeling were used to examine structure-function relationships in the medullary dorsal horn (MDH) and rostral cervical dorsal horn. In Nembutal-anesthetized rats, 78 trigeminal (V) primary afferent fibers were physiologically characterized and injected with HRP. Axons were sufficiently well stained to reconstruct all of their collaterals in the MDH. Many also extended into the cervical dorsal horn. Except for four axons, which responded best to noxious stimuli, all responded at short (mean = 0.50 ms) latencies to V ganglion shocks and to innocuous stimulation. Forty-five of our recovered fibers were associated with facial vibrissae and responded in either a rapidly adapting, slowly adapting type I, slowly adapting type IIa, or slowly adapting type IIb fashion. The adequate stimuli consisted of either slow deflection, high-velocity deflection, or a noxious pinch of the vibrissa follicle. The collaterals of all of the above-described mystacial vibrissa primary afferents proceeded directly to their region of arborization in a plane perpendicular to the lateral border of the medulla to collectively form a largely continuous, circumscribed terminal column. This longitudinally oriented column of terminal and en passant boutons angled from lamina V rostrally to lamina III caudally. In the magnocellular laminae of the MDH, all mystacial vibrissa primary afferents gave rise to similarly shaped arbors, regardless of their functional classification. While morphological variability was observed both within and between individual axons, variance between functional classes was no greater than that within a class. Moreover, number of collaterals, number of boutons, or bouton size did not distinguish functional classes. Nonmystacial vibrissa afferent arbors, with more caudal peripheral fields, had their primary arbor focus in C1 and C2 dorsal horn. These arbors had relatively little rostrocaudal overlap with mystacial vibrissa afferents, though they exhibited the same lamina V-to-III shift as they descended through the cervical cord. Unlike mystacial vibrissa afferents in the MDH, their collaterals followed a tortuous course and often occupied laminae II-V in one transverse section. The relative location of each vibrissa afferent's terminal field could be predicted by the particular vibrissa innervated. Dorsal vibrissae afferents had ventrolateral terminations and ventral vibrissae afferents terminated dorsomedially. Rostral vibrissae were represented in the rostral MDH, whereas caudal vibrissae were represented in the caudal MDH and rostral cervical dorsal horn.(ABSTRACT TRUNCATED AT 400 WORDS)     

Interaction between muscles and fascia in the mystacial pad of whisking rodents

In whisking rodents, the mystacial pad is supplied with vibrissae and contains a collagenous skeleton that is a part of the snout fascia. The collagenous skeleton is composed of three interconnected layers: superficial, deep spongy mesh and subcapsular fibrous mat. We found that the first two layers contain diverse fascial structures, such as sheets of subcutaneous connective tissue, tendons, ligaments and follicular capsules which transmit muscle efforts to vibrissae and are thus involved in whisking. Subcapsular fibrous mat is built of oriented rostro-caudal wavy fibrils. It maintains spatial arrangement of whisker follicles, provides a quick response to deformation and connects entire mystacial pad to the skull. To move vibrissae, the forces of intrinsic muscles are applied directly to the capsules of the vibrissa follicles, whereas the forces of extrinsic muscles are applied to other parts of the collagenous skeleton, which transmit the forces to the capsules. According to the spatial distribution and anchoring sites of the muscles and fascia, extrinsic muscles provide vibrissa protraction or retraction by pulling the superficial layer of the collagenous skeleton rostral or caudal, respectively. Vibrissae can be also retracted when the efforts of extrinsic muscles are applied to the subcapsular fibrous mat. When the muscles relax, fascial structures return the vibrissae to their resting position. The deep spongy layer encompasses vibrissal follicles providing a uniform distribution of stresses and strains during whisking. In the mystacial pad, fascia is a dominant type of tissue that maintains the integrity of the vibrissa motor plant, translates muscular momentum to the vibrissae, and plays a role in vibrissae movements.     

High responsiveness and direction sensitivity of neurons in the rat thalamic reticular nucleus to vibrissa deflections

The thalamic reticular nucleus (Rt) is strategically positioned to integrate descending and ascending signals in the control of sensorimotor and other thalamocortical activity. Its prominent role in the generation of sleep spindles notwithstanding, relatively little is known of Rt function in regulating interactions with the sensory environment. We recorded and compared the responses of individual Rt and thalamocortical neurons in the ventroposterior medial (VPm) nucleus of the rat to controlled deflections of mystacial vibrissae. Transient Rt responses to the onset (ON) and offset (OFF) of vibrissa deflection are larger and longer in duration than those of VPm and of all other populations studied in the whisker/barrel pathway. Magnitudes of ON and OFF responses in Rt were negatively correlated with immediately preceding activities, suggesting a contribution of low-threshold T-type Ca(2+) channels. Rt neurons also respond with high tonic firing rates during sustained vibrissa deflections. By comparison, VPm neurons are less likely to respond tonically and are more likely to exhibit tonic suppression. Rt and VPm populations are similar to each other, however, in that they retain properties of directional sensitivity established in primary afferent neurons. In both populations neurons are selective for deflection angle and exhibit directional consistency, responding best to a particular direction of movement regardless of the starting position of the vibrissal hair. These findings suggest a role for Rt in the processing of detailed sensory information. Temporally, Rt may function to limit the duration of stimulus-evoked VPm responses and to focus them on rapid vibrissa perturbations. Moreover, by regulating the baseline activity of VPm neurons, Rt may indirectly enhance the response selectivity of layer IV barrel neurons to synchronous VPm firing.     

Spatiotemporal organization of fast (>200 Hz) electrical oscillations in rat Vibrissa/Barrel cortex.

A 64-channel electrode array was used to study the spatial and temporal characteristics of fast (>200 Hz) electrical oscillations recorded from the surface of rat cortex in both awake and anesthetized animals. Transient vibrissal displacements were effective in evoking oscillatory responses in the vibrissa/barrel field and were tightly time-locked to stimulus onset, coinciding with the earliest temporal components of the coincident slow-wave response. Vibrissa-evoked fast oscillations exhibited modality specificity and were earliest and of largest amplitude over the cortical barrel, which corresponded to the vibrissa stimulated, spreading to sequentially engage neighboring barrels over subsequent oscillatory cycles. The response was enhanced after paired-vibrissal stimulation and was sensitive to time delays between movement of separate vibrissae. These data suggest that spatiotemporal interactions between fast oscillatory bursts in the barrel field may play a role in rapidly integrating information from the vibrissal array during the earliest cortical response to somatosensory stimulation.     

Somatotopic organization and columnar structure of vibrissae representation in the rat ventrobasal complex

Summary The region of vibrissae representation in the ventrobasal complex (VB) of the rat was systematically mapped, based on receptive fields of many single neurons. Results showed that the ventralmost row of vibrissae projected to the rostral part of VB, that the dorsal-most row projected to the caudal part, and that the caudalmost vibrissae of each row projected to the most dorsolateral part of VB and more rostral vibrissae to the more ventromedial part. Further, it was revealed that the clusters of neurons receiving projections from any individual vibrissae formed corresponding columns extending from the anterodorsomedial to the posteroventrolateral direction, and that these columns piled up dorsoventrally and anteroposteriorly, with ventral ones shifted progressively medially. When cross sections of these columns were viewed on an oblique horizontal section of VB, a group of columns corresponding to each row lined up from the dorsolateral to the ventromedial direction with a rostral convexity, which means that the third or fourth vibrissa in each row projected most rostrally in that row. These results confirmed previous physiological mapping studies of vibrissal representation and are in good agreement with anatomical studies on barreloid structure in VB.This paper is dedicated to the memory of a dear friend, Dr. I. Sumitomo, Professor of the Laboratory of Biological Science, Osaka Keizai University, who died in November 1989     

Single vibrissal cortical column in the mouse labeled with 2-deoxyglucose

Summary Columnar labeling was found in the primary somatosensory cortex of mice after stimulation of a single mystacial vibrissa following 2-deoxyglucose injection. The cortical vibrissal column had a cylindrical shape, passing through all layers of the cortex and was centered upon the appropriate vibrissal barrel. Columnar labeling extended beyond this barrel onto parts of neighbouring barrels, particularly within the same row. The densest labeling was found in layer IV in the barrel hollow. Removal of the non-stimulated vibrissae resulted in a subsequent lowering of 2DG uptake in the barrelfield surrounding the activated column, but did not affect the dimension of the activated column.     

Projections of the common fur of the muzzle upon the cortical area for mystacial vibrissae in rats dewhiskered since birth

In normal adult rats, the mystacial vibrissae and the common fur of the snout project at different loci on the SI cortex. The surface area of the normal fur projection is 0.8 mm2, whereas the vibrissa field amounts to 3-4 mm2. In rats dewhiskered since birth, the vibrissa area can still be identified through the projections from ipsilateral vibrissae (undamaged side). It is shown that in the absence of the vibrissae since birth, the vibrissa area, and this alone, is invaded by projections from the contralateral fur (damaged side).     

Temporal and spatial integration in the rat SI vibrissa cortex

Glass micropipettes were used to record the activity of 124 single units in the somatosensory vibrissa cortex (SI) of 16 rats in response to combined deflections of contralateral vibrissae. Compact multiangular electromechanical stimulators were used to stimulate individual vibrissal hairs alone or in combinations of two or three adjacent whiskers. Each whisker was stimulated independently to produce controlled temporal and spatial patterns of mechanical stimuli. Following displacement of a vibrissa, unit discharges to subsequent deflections of adjacent whiskers are reduced in a time-dependent fashion. Response suppression is strongest at short interdeflection intervals, i.e., 10-20 ms and decreases progressively during the 50-100 ms following the first deflection. In many cases this period also corresponds with a reduction in ongoing unit discharges. Response suppression was not observed for first-order neurons recorded in the trigeminal ganglion of barbiturate-anesthetized rats. In the cortex, the presence and/or degree of response suppression depends on a number of spatial factors. These include 1) the angular direction(s) in which the individual hairs are moved, 2) the sequence in which two whiskers are deflected, that is, which one is deflected first, 3) the particular combination of whiskers stimulated, and 4) the number (2 or 3) of vibrissae comprising the multiwhisker stimulus. Within a vertical electrode penetration, one particular whisker typically elicits the strongest excitatory and inhibitory effects; other, nearby vibrissae elicit variable (or no) excitation or inhibition. Excitatory and inhibitory subregions of a receptive field could thus be distributed asymmetrically around the maximally effective whisker. In these cases, the receptive fields displayed spatial orientations. Quantitative criteria were used to classify 30 cortical units on the basis of the distribution of inhibitory subregions on either side of the maximally effective whisker. Twenty-one of these cells had receptive fields (RFs) with symmetrical inhibitory side regions. Responses of the other nine units were strongly suppressed by a preceding deflection of a vibrissa on one side but relatively unaffected, or even slightly facilitated, by preceding deflection of the whisker on the other side.(ABSTRACT TRUNCATED AT 400 WORDS)     

An electrophysiological and anatomical study of projections to the mouse cortical barrelfield and its surroundings

This study establishes a cortical map of the somatosensory periphery of the mouse head, with emphasis on the whisker pad. Data in the literature on the projection of the common hair follicles are confusing, notably the question whether or not this projection is separated from or overlaps with that of the facial vibrissae, the barrelfield. Microelectrode recordings in the barrelfield and its immediate surroundings upon natural stimulation of the periphery were followed by microlesions and histological reconstruction. Results show that the barrelfield consists of two parts: an anterior part where vibrissal follicles and the skin bearing them are represented, and a posterior part receiving only vibrissa inputs. The skin between these latter vibrissae is represented outside the barrelfield. We conclude that the partially dissociated cortical representation of skin and vibrissae may allow large vibrissae to be used in tasks requiring greater acuity than the shorter ones provide. This hypothesis is supported by the observation that, owing to special muscles, large vibrissae are more mobile than short ones. We also propose that the segregation of inputs from vibrissae and common fur is related to the number of nerve fibers serving one follicle, and we indicate an experimental model to test this possibility.     

Mechanical signals at the base of a rat vibrissa: the effect of intrinsic vibrissa curvature and implications for tactile exploration

Rats actively tap and sweep their large mystacial vibrissae (whiskers) against objects to tactually explore their surroundings. When a vibrissa makes contact with an object, it bends, and this bending generates forces and bending moments at the vibrissa base. Researchers have only recently begun to quantify these mechanical variables. The present study quantifies the forces and bending moments at the vibrissa base with a quasi-static model of vibrissa deflection. The model was validated with experiments on real vibrissae. Initial simulations demonstrated that almost all vibrissa-object collisions during natural behavior will occur with the concave side of the vibrissa facing the object, and we therefore paid particular attention to the role of the vibrissa's intrinsic curvature in shaping the forces at the base. Both simulations and experiments showed that vibrissae with larger intrinsic curvatures will generate larger axial forces. Simulations also demonstrated that the range of forces and moments at the vibrissal base vary over approximately three orders of magnitude, depending on the location along the vibrissa at which object contact is made. Both simulations and experiments demonstrated that collisions in which the concave side of the vibrissa faces the object generate longer-duration contacts and larger net forces than collisions with the convex side. These results suggest that the orientation of the vibrissa's intrinsic curvature on the mystacial pad may increase forces during object contact and provide increased sensitivity to detailed surface features.     

Stimulus—response relationships in first-order sensory fibres from cat vibrissae

1. In the infraorbital nerve of anaesthetized cat, recordings were made from single units serving the vibrissae. The directional sensitivity of individual units was rather gross, but comparison of the outputs of several units from the same vibrissa could provide information that was more precise about the direction of vibrissa deflexion.2. Many of the units showed ;spontaneous' activity. The majority of rates were less than 1 impulse/sec, so that this activity is not likely to be an important carrier of information.3. When sinusoidal deflexions in the frequency range of 0.5-100 Hz were applied, the frequency responses of about half of the units were those to be expected if they were responding to the velocity of the stimulus; most of the remainder responded to both the velocity and the magnitude of the deflexion. This form of information presentation has been shown to be well suited for compensatory tracking tasks, and it is suggested that the vibrissae are important for such tasks in the animal's life, rather than merely serving as obstacle detectors.4. Greatest deflexion sensitivity typically was at higher frequencies than could be attained in this experiment, but would be greater than the median threshold deflexion angle of 12 min found at 50 Hz. It is suggested that these low thresholds are more appropriate for a tracking system than for an obstacle detector.     

A comparison of primary afferent and cortical neurone activity coding sinus hair movements in the cat

Summary Responses in the somatosensory cortical area S I to stimulation of facial sinus hairs were recorded in the anaesthetized cat and compared with activity in primary afferent fibres innervating vibrissae follicles. The specific cortical vibrissa area is somatotopically organized; 39% of the cortical units in that area responded to stimulation of only a single sinus hair but in some cases all maxillary vibrissae activated a single cortical neurone. The responses consisted of three major groups; either a phasic discharge in response to the movement part of a stimulus, or an additional tonic discharge related to the steady period of vibrissa deflection, or a tonic discharge. On the basis of a comparison of response and excitability characteristics of primary afferent and cortical neurones it is concluded that all four kinds of peripheral units innervating sinus hair follicles project to the somatosensory cortical area S I. It appears from these findings that some cortical neurones receive a specific input related to a particular component of the complex primary afferent response in fibres innervating sinus hair follicles. The results are discussed with respect to previous reports on the central representation of facial sinus hairs in different species.     

Parapoxviruses of seals and sea lions make up a distinct subclade within the genus Parapoxvirus

Poxviruses of seals and sea lions have been tentatively identified as both orthopoxviruses and parapoxviruses, but their exact identity remained unconfirmed. Here, poxviral DNA sequences were generated from 39 clinical cases and compared to sequences from earlier poxvirus isolates from seals (Phocidae) and sea lions (Otariidae). Six genetically distinct poxvirus strains were detected, of which three were previously unrecognized. All detected strains were closely related to the parapoxviruses, confirming their classification as members of the genus Parapoxvirus. A phylogenetic analysis showed that pinniped parapoxviruses form a monophyletic group within the genus Parapoxvirus. Parapoxviruses from Atlantic pinnipeds were phylogenetically distant from those of Pacific pinnipeds. Parapoxviruses from phocids and otariids that inhabit the same geographical region were also phylogenetically distant, suggesting that parapoxviruses are not commonly transmitted between free-ranging phocids and otariids. However, one strain was detected in two otariid species, suggesting that pinniped parapoxviruses are capable of infecting multiple species within a phylogenetic family.     

Functional Implications of Variation in Tooth Spacing and Crown Size in Pinnipedimorpha (Mammalia: Carnivora)

Pinnipeds (seals, sea lions, and walruses) show variation in tooth morphology that relates to ecology. However, crown size and spacing are two aspects of morphology that have not been quantified in prior studies. We measured these characters for nearly all extant pinnipeds and three fossil taxa and then determined the principal sources of variation in tooth size and spacing using principal components (PCAs) and hierarchical cluster analysis (HCA). PCA and HCA showed that species sorted into three groups: taxa with small crowns and large diastemata, taxa with large crowns and small diastemata, and taxa that fell between these two extremes. We then performed discriminant function analysis (DFA) to determine if tooth morphology correlated with foraging strategy or diet. DFA results indicated weak correlation with diet, and stronger correlation with prey capture strategies. Tooth size and spacing were most strongly correlated with the importance of teeth in prey acquisition, with tooth size decreasing and tooth spacing increasing as teeth become less necessary in capturing food items. Taxa which relied on teeth for filtering prey from the water column or processing larger or tougher food items generally had larger crowns and smaller tooth spacing then taxa which swallowed prey whole. We found the fossil taxa Desmatophoca and Enaliarctos were most similar in tooth morphology to extant otariids, suggesting that both taxa were generalist feeders. This study established the relationship between tooth size and feeding behavior, and provides a new tool to explore the paleoecology of fossil pinnipeds and other aquatic tetrapods. Anat Rec, 298:878–902, 2015. © 2014 Wiley Periodicals, Inc.     

Cellular mechanisms of motor control in the vibrissal system

In this article we discuss the experimental advantages that the vibrissal motor system offers for analysis of motor control and the specializations of this system related to the unique characteristics of whisker movements. Whisker movements are often rhythmic, fast, and bilateral. Movements of individual whiskers have simple characteristics, whereas, movements of the entire vibrissae array are complex and sophisticated. In the last few years, powerful methods for high precision tracking of whisker movements have become available. The whisker musculature is arranged to permit forward movements of individual whiskers and consists—depending on the species—mainly or exclusively of fast contracting, fast fatigable muscle fibers. Whisker motor neurons are located in the lateral facial nucleus and their cellular properties might contribute to the rhythmicity of whisking. Numerous structures provide input to the lateral facial nucleus, the most mysterious and important one being the putative central pattern generator (CPG). Although recent studies identified candidate structures for the CPG, the precise identity and the functional organization of this structure remains uncertain. The vibrissa motor cortex (VMC) is the largest motor representation in the rodent brain, and recent work has clarified its localization, subdivisions, cytoarchitectonics, and connectivity. Single-cell stimulation experiments in VMC allow determining the cellular basis of cortical motor control with unprecedented precision. The functional significance of whisker movements remains to be determined.