when one observes rodents introduced in a new environment , one immediately notices that they are extremely curious . they run about , stand up on their hind legs , crane their necks forward , and explore the environment by sniffing and whisking vigorously . in a classic paper published in 1964 , welker provided the first descriptive account of the sniffing behavior in rats . using cinematographic technique , he reported that sniffing consists of an integrated sequence of movements that involve ( 1 ) bursts of polypnea ; ( 2 ) recurrent protraction and retraction of mystacial vibrissae ; ( 3 ) repetitive retraction and protraction of the tip of the snout ; and ( 4 ) a rapid series of head movements and fixations . these four components occur at rates between 5 and 11  hz , in bouts of one to many seconds , and this temporal relationship is illustrated in figure 1 , where whisking and sniffing were recorded as a rat explores the exit of a tunnel . the coordination of whisking , head bobbing , and nose motion with sniffing requires a hierarchal organization of the brainstem circuitry so that the occurrence and timing of these actions do not interfere with each other and with fundamental metabolic needs . here we review evidence for phase locking of rhythmic orofacial actions with breathing , particularly in the context of the synaptic mechanisms that coordinate sniffing , whisking , and nose motion , which are predominant activities during exploratory behaviors in rodents [ 2 , 3 ] . nasofacial muscles of rodents that control movement of the vibrissae , the opening of the nares , and deflection of the nose are innervated by facial motoneurons [ 46 ] . several studies have shown that facial motoneurons in rodents are functionally organized into clusters [ 714 ] . figure 2 shows a schematic representation of the facial muscles and the corresponding myotopic map in the facial nucleus . motoneurons that innervate the intrinsic vibrissa muscles that protract the vibrissae are located in the ventral lateral part of the nucleus ; the extrinsic retractor muscles ( nasolabialis and maxillolabialis ) that translate the mystacial pad in the caudal direction are represented dorsolaterally ; the extrinsic protractor muscles ( nasolabialis profundus ) that translate the pad rostrally and open the nares are represented at the lateral edge of the nucleus . muscle deflector nasi , which is not shown in the drawings of figure 2 ( but see figure 5(a ) ) , is represented dorsolaterally . studies using transsynaptic retrograde labeling have allowed the identification of premotor neurons that control each of these muscle groups [ 13 , 14 ] . we thus focus next on how rhythmic activity of these different pools of premotor neurons coordinates with respiration . sniffing in rats is usually defined as rapid , rhythmic respiration [ 16 , 17 ] . rats , unlike humans , always breathe nasally ; lower frequencies ( 13  hz ) correspond to basal respiration and higher frequencies ( 412  hz ) correspond to sniffing . it is now well established that a medullary region called the pre - btzinger complex ( prebtc ) forms the core of the respiratory generator ( see review by feldman and kam ) and that sniffing is associated with an increased rate of bursts in prebtc cells . the neuronal circuits that accelerate the respiratory rhythm upon presentation of an odorant remain unknown . this hypothesis derived from the observation that whisking persists after motor cortex ablation and that bilateral section of the infraorbital nerve , that is , sensory denervation , has little effect on the generation , kinematics , and bilateral coordination of the normal whisking pattern . recently the whisking oscillator was discovered in a medullary region close to the prebtc . transsynaptic labeling studies demonstrated that the prebtc and the parafacial respiratory region project to the facial nucleus [ 13 , 14 ] . neuronal and electromyographic ( emg ) recordings in alert rats further showed that motoneurons that innervate the extrinsic muscles , that is , muscles nasolabialis , maxillolabialis , and nasolabialis profundus , discharge in a phase - locked manner with the respiratory cycle . yet , most of these muscles are differentially active during basal respiration versus sniffing ( figures 3(a)3(d ) ) . one attaches to the plate of the mystacial pad ( pars maxillaris ) , which contributes to dilation of the nares , and another one that attaches to the corium ( pars media ) that translates the mystacial pad rostrally [ 5 , 6 , 22 ] . both muscle components contract at the onset of inspiration . while naris dilation occurs with each inspiration in awake rats , rostral translation of the pad occurs primarily when the rate or amplitude of respiration increases [ 20 , 22 ] . the same is true for the extrinsic retractor muscles ( nasolabialis and maxillolabialis ) and the nasi deflector muscle , which are principally active when the animal sniffs or whisks [ 22 , 23 ] . the preferential recruitment of these muscles during sniffing could depend on modulatory action that enhances the excitability of the motoneurons per se or of the associated premotor circuits . by and large , prebtc cells that constitute the respiratory pattern generator are glutamatergic and express somatostatin or the neurokinin-1 receptor . interestingly , takatoh et al .   reported that few prebtc cells labeled after g - rabies injection in the mystacial pad express these phenotypes . furthermore , injection of an adeno - associated virus that expresses egfp driven by the somatostatin promoter in the prebtc did not label terminal fields in the facial nucleus . since the deletion or silencing of neurokinin-1 receptor and somatostatin - expressing prebtc neurons , respectively , disrupts breathing in the adult rat [ 25 , 26 ] , the prebtc projection to facial motoneurons may arise from cells that are not themselves part of the respiratory rhythm generator . the latter cells may be recruited when the animal sniffs , or the conditional respiratory drive may involve follower neurons intercalated between the prebtc and facial motoneurons . the region immediately caudal to the facial nucleus , referred to here as the retrofacial region , appears as a potential source of this conditional drive . it receives input from the prebtc and contains glutamatergic premotor neurons ( figures 3(e ) and 3(f ) ) . exploratory recordings in this region revealed a number of neurons that are recruited when the breathing rhythm accelerates ( figure 3(g ) ) . however , it remains unclear whether these cells are part of the parafacial respiratory group as delineated in prior studies [ 2729 ] . premotor neurons that generate whisking are located in the vibrissa - related region of the intermediate reticular formation ( virt ) of the medulla adjacent to the prebtc   ( figures 4(a ) and 4(b ) ) . this proximity appears functionally relevant since whisking is tightly coupled to sniffing , yet it is separately gated [ 1 , 19 , 30 ] . cells in the virt fire either in phase or in antiphase with whisker protraction ( figures 4(c ) and 4(d ) ) , selective lesion of the virt abolishes whisking on the side of the lesion ( figures 4(e ) and 4(f ) ) , and activation of the virt by iontophoretic injection of kainic acid induces long periods of continuous whisking in the lightly anesthetized rat . together , these results indicate that the virt is both necessary and sufficient to generate whisking . this finding is consistent with the notion that sustained depolarizing inputs to the facial motoneurons determine the maximum protraction of the vibrissae , while the whisking oscillator rhythmically inhibits these motoneurons . thus , the virt oscillator drives whisking on the retraction phase , opposite from what was long supposed . the inhibitory nature of the whisking oscillator calls for a reappraisal of the control of brainstem circuits by top - down inputs for the control of amplitude , frequency , and set - point of whisking . phase sensitivity analysis of coupling between whisking and breathing has shown that inspiration can reset whisking , but not vice versa . unidirectional phase resetting of whisking by basal respiration and sniffing is mediated by unidirectional connections from the prebtc to the virt . natural whisking is bilaterally synchronous in the absence of external objects or head turning [ 3234 ] . this requires a circuitry that coordinates the activity of the left and right whisking oscillators . yet , tract tracing by means of conventional tracers or virus injection did not reveal connections between the left and right virts . moreover , spectral analysis revealed that the two whisking oscillators begin to drift in phase when respiration is transiently halted during a sigh or during apnea induced by inhalation of ammonia vapor . this is consistent with the notion that the left and right whisking oscillators are independent from one another and therefore begin to drift in the absence of repeated resetting events . together these results indicate that either commissural connections between virts are absent or , if present , they are not strong enough to synchronize whisking bilaterally . alternatively , several studies have established that glutamatergic prebtc cells are interconnected by commissural axons [ 3537 ] . impairment of these connections in robo knockout mice leads to desynchronization of the excitatory drive to the left and right spinal and cranial motoneurons . given that each prebtc projects to virt , commissural connections between the respiratory pattern generators represent the most likely substrate for the bilateral synchronization of whisking . several behavioral studies have reported that the degree of phase coupling between whisking on both sides of the face or between whisking and sniffing depends on context , motor strategies , and sensory feedback [ 33 , 34 , 3941 ] . this indicates that , like other central pattern generators , the whisking oscillator can be modulated to adapt to the organism 's circumstances and needs [ 42 , 43 ] . for the moment , brainstem and top - down inputs that control the virt and the coupling between the virt and the prebtc remain unknown . a seldom - studied aspect of sniffing is nose motion , which is particularly noticeable in high - speed video recordings . the nose of rodents is made of cartilages that form a telescopic connection with the nasal and premaxillary bones and permits bending of the nose in the dorsoventral and lateral directions . nose movements are controlled by muscle deflector nasi , which attaches to the orbital edge of the maxillary bone and to the aponeurosis above nasal cartilages ( figure 5(a ) ) . bilateral contraction of this muscle raises the tip of the nose , while unilateral contraction produces lateral deflection . emg recordings in head - restrained rats reveal that muscle deflector nasi contracts during the late phase of expiration during basal respiration and shifts to early expiration during sniffing ( figure 5(b ) ) , which allows repositioning of the nares for the next inhalation . furthermore , monitoring nasal airflow through each of the nares reveals that nose deflection is associated with difference in airflow in each of the nares ( figures 5(c)5(e ) ) . rabies injection into muscle nasi deflector leads to transsynaptic labeling in the retrofacial region ( unpublished data ) , but physiological evidence that retrofacial premotor neurons actually control nose motion is lacking . the observation of precise phase locking between sniffing and exploratory whisking leads to the hypothesis that the breathing rhythm functions as the reference oscillation for the alignment of commensurate signals . for example , when rodents are actively exploring their environment , phase locking between whisking and sniffing could ensure that spikes induced by tactile and olfactory stimuli occur with a fixed temporal relationship to one another , which corresponds to an object with a particular smell at a particular location relative to the face . this could provide a means to bind olfactory inputs , which enter the brain at its rostral pole , with coincident inputs from touch , which enter the brain at the level of the brainstem . it obviates the need for a direct neuronal projection of respiratory output between these two regions . this scheme may be readily extended to taste through the entrainment of licking and thus covers the full range of stimuli required to assess the shape , odor , texture , and taste of food . although sniffing clearly serves olfaction and strongly patterns olfactory processing , several studies reported that basal respiratory rhythm is sufficient for delivering odorants to olfactory receptors and that rats and mice can perform simple odor discrimination after a single sniff [ 17 , 44 , 45 ] . likewise , rodents can locate objects and gauge aperture widths with a single whisk , and severing the facial nerve to block whisking does not affect performance [ 46 , 47 ] . lastly , rodents can perform many tactile tasks without whisking per se , either by using head and body movements to move the vibrissae or by maintaining their vibrissae still in a region of interest where contact is expected . given the above evidence that olfaction can occur without sniffing and vibrissa touch can be effective without whisking , the questions remain as to why rodents sniff , whisk , and move their nose . it is intriguing that rats deprived of olfactory and vibrissa afferents continue to sniff and whisk in a relatively normal manner . we suggest three reasons as to why rhythmic activity is associated with the use of these sensors . first , as rodents have relatively limited visual capabilities and are the targets of predators , fast sampling of the immediate environment has clear survival value . the few extra hundred milliseconds of warning that is gained by scanning the environment may be sufficient for escape . second , sniffing and whisking not only serve odor sampling and touch , but also constitute activity patterns that are the overt expression of reward expectation [ 4850 ] . perhaps not independent of this , sniffing is commonly displayed during motivated and social behaviors to communicate and convey information about social hierarchy . finally , the frequency of whisking and sniffing lies close to that of the hippocampal theta rhythm . these rhythms are incommensurate during foraging and exploration , yet they lock for a brief epoch during the approach to a stimulus during forced choice tasks [ 53 , 54 ] . thus , rhythmic dynamics may serve to facilitate transient coherence between two regions of the brain . from this standpoint , respiration is more than a rhythm for life , but it also coordinates orofacial motor commands that engage common muscle groups , serves a variety of active sensory and social behaviors , and may coordinate sensory input with memory formation and recall .
the exploratory behavior of rodents is characterized by stereotypical movements of the vibrissae , nose , and head , which are phase locked with rapid respiration , that is , sniffing . here 
 we review the brainstem circuitry that coordinates these actions and propose that respiration may act as a master clock for binding orofacial inputs across different sensory modalities .