Whisking pattern generation
|Phil Zeigler and Asaf Keller (2009), Scholarpedia, 4(12):7271.||doi:10.4249/scholarpedia.7271||revision #149032 [link to/cite this article]|
The delineation of central mechanisms underlying the generation and modulation of rhythmic movement patterns in vertebrates [Central Pattern Generators: CPGs]—including respiration and locomotion, swallowing, chewing and licking—has been an important goal of systems neuroscience. Common to many of these functions is the operation of small ensembles of premotor neurons that generate patterned drive to motoneurons, producing relatively simple repetitive patterns of movement even after the removal of any identifiable phasic sensory inputs. However, while the generation of these patterns in the absence of phasic peripheral influences is one defining criterion for CPGs, another is their susceptibility to modulation by both sensory inputs and descending control mechanisms. An additional commonality is the ubiquity of chemical modulation, often by serotonergic mechanisms. These three sets of factors, endogenous rhythm generation, descending (e.g. cortical and neuromodulatory) influences and peripheral sensory inputs, interact to provide adaptive control of the rhythmic behavior pattern.
Rodent whisking behavior has many features that make it an excellent model for the study of such interactions. It has a relatively simple motor plant, its peripheral innervation is well-described, as are its central sensory and motor pathways, and the system is not complicated by a proprioceptive loop. Although the system lacks proprioceptors, reflex arcs are formed by sensorimotor loops (Nguyen and Kleinfeld, 2005; Deutsch et al., 2012; Matthews et al., 2015; Matthews et al., 2015). In recent years, behavioral studies of whisking in both head-fixed and freely moving rodents have been facilitated by the development of optoelectronic and videographic methods (Bermejo et al., 1998; Knutsen et al., 2005). Together, the relatively simple mechanics and advanced monitoring techniques available for this system have facilitated investigations of the underlying control circuitry.
Here we review recent progress in the analysis of central pattern generation mechanisms in rodents and suggest that its mediating mechanisms, though similar in many respects to those of the more classic models, have a number of novel features of interest, including the active participation of vibrissae motoneurons in the generation of the whisking rhythm.
The mystacial vibrissae are sinus hairs, each emerging from a follicle that is embedded in the mystacial pad. Protraction of the vibrissae is an active process, effected by contraction of the intrinsic muscles. These are small sling-like muscles that wrap around the base of each follicle and attach to the pad surrounding the next caudal vibrissa. Retraction of the vibrissae can occur passively, through the elastic properties of the tissue. Retraction is also aided by active contraction of a set of extrinsic muscles that are involved with movement of not only the vibrissae, but also of several parts of the face, such as the lips and nares. Kleinfeld, Zeigler and colleagues (Hill et al., 2008) have developed a model to describe the interactions between these intrinsic and extrinsic musculature in the rat. This model proposes that, during exploratory whisking, the periodic motion of the vibrissae and mystacial pad 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. The model suggests that the combination of extrinsic and intrinsic muscle activity leads to a more extended range of vibrissa motion than would be available from the intrinsic muscles alone.
A similar organization is found in the mouse vibrissae pad (Haidarliu et al., 2014), where, in the rostral part of the mouse snout, there are both protractors and retractors of the vibrissae.
The patterns of muscle activation described above characterize whisking behavior during exploration, when whisking is synchronous with sniffing. During other behavioral states, when breathing rate is slow, the rate of whisking exceed that of breathing. Thus, the activity of the intrinsic muscles leads protraction for both sniffing and slow respiration, and the extrinsic muscle—which is active for every whisk during sniffing—is only active for inspiratory whisks during basal breathing (Moore et al., 2013). Thus, whisking dynamics change during different behavioral states.
The follicular muscles are structurally homogeneous: In addition, essentially all follicular muscles are of the fast-twitch type and lack proprioceptors. Analyses by Brecht and collaborators (Jin et al., 2004) demonstrate that >90% of the muscle fibers are of type 2B, which have high levels of anaerobic glycolytic enzymes providing a rapid source of ATP and high maximum velocity of contraction but are less fatigue resistant than other muscle fiber types. The high percentage of type 2B fibers distinguishes the intrinsic vibrissa musculature from skeletal muscles and may have evolved for fast scanning of the sensory environment. A comparable analysis of the extrinsic muscles has not been reported, but they are presumed to contain a mix of slow and fast-twitch fibers, as well as proprioceptors (Lazarov, 2007; Burrows et al., 2014).
Both the extrinsic and follicular muscles are innervated by motoneurons whose parent somata reside in the facial nucleus ipsilateral to the innervated vibrissa pad. Relatively little is known about the motoneurons innervating the extrinsic muscles, but the follicular muscles are innervated by motoneurons (“whisking motoneurons”) in the lateral and intermediate subdivision of the facial nucleus (Klein and Rhoades, 1985; Hattox et al., 2002; Nguyen and Kleinfeld, 2005; Matthews et al., 2015). These motoneurons are arranged, roughly, in a somatotopic manner corresponding to the arrangement of the vibrissae. Available evidence suggests that, with rare exceptions, each motoneuron innervates only one sling muscle, and that the motoneurons have no axon collaterals within the facial nucleus or in any other structure. Further, there are no known interneurons in the lateral facial nucleus, which appears to be composed exclusively of motoneurons and glial cells.
Although the facial nucleus contains a large number of gap junctions, these appear to primarily involve glial cells. Attempts to identify gap junctions, or electrical coupling among facial motoneurons have so far been unsuccessful.
Some rhythmic motor functions, including locomotion, breathing and chewing, involve motoneurons that have intrinsic membrane properties that allow them to function as intrinsic or conditional bursters (Lee and Heckman, 1996; Del Negro et al., 1999). For example, they may express plateau potentials, which generate prolonged firing in response to brief current injections, and a hyperpolarization-activated cationic current (Ih) active at or near resting membrane potential. These properties do not characterize whisking motoneurons (Hattox et al., 2003) (although Ih is expressed in facial motoneurons of young rodents (Larkman and Kelly, 1998; Larkman and Kelly, 2001)), suggesting that these motoneurons are not intrinsically bursting and that they require rhythmic synaptic inputs, or a neuromodulatory drive, to generate rhythmic firing. Below we consider potential sources of these synaptic inputs, and hypothetical mechanisms through which they might generate rhythmic whisking.
Afferents to whisking motoneurons
In an attempt to identify potential contributors to rhythm generation in this system Hattox et al. (2002) systematically labeled and identified the origin of afferents to whisking motoneurons. A very large number of brainstem, mesencephalic and midbrain nuclei were found to provide unilateral, and sometimes bilateral, innervation to the lateral facial nucleus. Similar cortical targets were recently identified Petersen and collaborators (Sreenivasan et al., 2015). In addition, Brecht and collaborators (Grinevich et al., 2005), identified a pathway providing a direct, though sparse, connection between the motor cortex and whisking motoneurons.
The plethora of regions innervating the whisking motoneurons suggests that rhythm generation and the regulation of whisking parameters in this system is subject to control by a number of centers that might be active during different behavioral states. Of particular interest were the findings that some of these regions also received direct inputs from the vibrissa representation in the motor cortex, suggesting that these hypothetical rhythm generators could be controlled voluntarily. Subsequent studies have attempted to narrow down this list by seeking to identify causal relationships between activity in these afferents and whisking behaviors.
Brainstem reticular formation
The brainstem reticular formation originates a particularly dense projection to the facial nucleus. Several lines of anatomical and physiological evidence implicate it in a whisking CPG. Motoneurons in this region are involved in a number of rhythmic motor acts in mammals, such as licking, mastication, and locomotion (for review see: Buttner-Ennever and Holstege, 1986; Moore et al., 2014b). Similarly, the reticular formation in birds contains the CPG for rhythmic acts such as pecking and jaw movements (Berkhoudt et al., 1982; Wild et al., 1985). In rodents, microstimulation of neurons in this region evokes rhythmic whisking, suggesting that these neurons may control whisking behaviors by driving the motoneurons. In support of this idea, electrical stimulation of neurons in the reticular formation evokes monosynaptic EPSPs in facial motoneurons in the cat. Below we discuss the potential role of an important subset of these reticular formation neurons: the serotonergic neurons of the raphe and the lateral paragigantocellularis nucleus.
Recent work by Deschênes, Kleinfeld and collaborators (Moore et al., 2013) identified a region within the intermediate band of the reticular formation (IRt) containing neurons that project directly to whisking motoneurons, and that fire in phase with whisking. Further, lesions in this region abolish whisking behaviors. Thus, these neurons appear to function as a premotor pattern generator for whisking (see also below).
A number of other brainstem nuclei have been implicated in regulating and coordinating various facial rhythmic activities, including whisking, sniffing, licking and breathing (Hattox et al., 2002; Cao et al., 2012; Moore et al., 2014b). Of particular interest is the pre-Bötzinger complex, long implicated in respiratory rhythm generation (Ramirez et al., 2012), and recently shown to be involved also in controlling rhythmic whisking (see below, and: Moore et al., 2013).
The trigeminal nerve carries sensory inputs from the vibrissae and innervates the principal trigeminal nucleus as well as the three spinal trigeminal nuclei (oralis, interpolaris and caudalis). Injection of retrograde markers in facial nucleus produced labeling in the spinal trigeminal nuclei which was sparse, exclusively ipsilateral, and observed predominantly in the nucleus caudalis. Nevertheless, as demonstrated by Nguyen and Kleinfeld (2005) trigeminal inputs to facial motoneurons can evoke a rapidly depressing reflex that might provide a positive sensory feedback to the vibrissa musculature during whisking behaviors.
The spinal trigeminal nucleus pars muralis displays anatomical substrates suggesting that it plays a key role in controlling whisking, and, specifically, in sensorimotor reflex arcs (Matthews et al., 2015). This recently defined nucleus is interspersed between spinal trigeminal nuclei caudal and interpolaris. Glutamatergic projection neurons in this nucleus both receive inputs from sensory afferent fibers and send monosynaptic connections to whisking motoneurons in the facial nucleus. These interactions provide for a disynaptic positive feedback for motor output that drives whisking (Matthews et al., 2015).
The dorsal regions of the red nucleus project to the contralateral facial nucleus, suggesting that this structure is involved in relaying inputs from the olivocerebellar system to whisking motoneuorns. This is of interest because the characteristic frequency of rhythmic activity in the olivocerebellar system is similar to the frequency of rhythmic whisking (see, e.g., Lang et al., 1997). However, inactivation of the inferior olive does not affect exploratory vibrissa movements (Semba and Komisaruk, 1984), so this rhythmic activity may not be causally related to these movements. Furthermore, stimulation of the red nucleus does not reliably evoke vibrissa movements (Isokawa-Akesson and Komisaruk, 1987). Thus, the role of the red nucleus in modulating vibrissa movements is at present unclear.
Cholinergic activating system
The pedunculopontine tegmental nucleus (PPTg) is part of the brainstem cholinergic activating system, critical for controlling arousal and states of vigilance. It sends dense, presumably cholinergic inputs to the lateral facial nucleus.
A particularly dense projection to lateral facial motoneurons arises from both (contralateral and ipsilateral) superior colliculi (SC) and electrical stimulation of SC elicits contralateral vibrissa movements. SC also receives dense projections from the vibrissa representation of the motor cortex suggesting its role in rhythmic whisking can be voluntarily regulated. Because SC also forms reciprocal connections with trigeminal nuclei relaying vibrissal information it is likely to be involved in integrating inputs from motor behaviors with inputs from somatosensory, visual, and auditory sensory modalities. For these reasons the superior colliculus was the object of a series of studies to be discussed in a later section.
We argued above that whisking behavior cannot emerge from the intrinsic properties of the pertinent motoneurons, or from interactions among them. Rather, rhythmic whisking must be governed by inputs these motoneurons receive from some or all the myriad of nuclei that project to these motoneurons. Formulating testable hypotheses regarding the nature of these rhythm generators requires a comprehensive description of the whisking behaviors, which we summarize below. (For a more complete description see other chapters in this edition.)
Whisking: Development, kinematics and bilateral coordination
In rat pups, small, uncoordinated movements of the vibrissae are evident as early as days P10-14, a few days before eye opening and before the initial appearance of reliable motor maps to stimulation of the cortical vibrissal motor area (vMcx; AK, unpublished observations). During the next two weeks the movements gradually increase in both amplitude and frequency, maturing at the characteristic modal frequency for whisking in air (5-9 Hz) by the end of the first month (Welker, 1964; Landers and Philip Zeigler, 2006). Over the same period, there is a parallel increase in the bilateral coordination of whisking on the two sides.
The emergence of whisking behavior parallels the development of bilateral excitatory inputs to whisking motoneurons from LPGi, and the development of descending axons from the vMCx to LPGi (Takatoh et al., 2013).
In adult rats, the rhythmic vibrissa movements used in “active sensing” (whisking and palpation) exhibit a range of frequencies from 1-20 Hz, with dominant frequencies of 5-9 Hz in both head-fixed and freely moving animals (Carvell and Simons, 1990; Gao et al., 2001; Hill et al., 2008). [Note that the whisking parameters reported here were computed from observations on the behavior of the laboratory rat. No comparable data are available for rats observed under more natural conditions and mice are reported to whisk at significantly higher frequencies (Jin et al., 2004)]. However, as with other rhythmic movements, whisking patterns are strongly influenced by signals from peripheral receptors. Higher frequencies (15-25 Hz) have been reported during palpation of objects (“foveation”) (Berg and Kleinfeld, 2003a) and, during texture discriminations, modulation of movement parameters (amplitude, frequency and bandwidth) is correlated with discriminanda properties (Carvell and Simons, 1995; Harvey et al., 2001). Whisking rates may also be brought under voluntary control using behavioral contingencies such as operant reinforcement schedules (Gao et al., 2003b). The brain mechanisms thought to mediate such voluntary control are discussed below.
Observations of whisking in air (without vibrissal contacts) convey a strong impression of bilateral synchrony, but even under head fixed conditions the activity of bilaterally homologous vibrissae is not always in phase or identical in amplitude (see Fig. 2 in Gao et al., 2001). Indeed, under natural conditions rats may exhibit considerable bilateral asynchrony, with persistent whisking on one side and no movements on the other (Wineski, 1985; Towal and Hartmann, 2006; Mitchinson et al., 2007). In contrast, during whisking in air, vibrissae movements on the same side of the face are synchronous, with similar protraction amplitudes and topographies (Bermejo et al., 2005; Hill et al., 2011) (but see Sachdev et al., 2002). The mechanisms responsible for either bilateral or unilateral synchrony is presently unknown (see below).
Whisking behavior patterns: effects of deafferentation
The observation that rhythmic movements persist after sensory denervation (Welker, 1964), decerebration (Lovick, 1972) and cortical ablations (Semba and Komisaruk, 1984) suggested the operation of a central pattern generating mechanism. Subsequently, a detailed analysis of whisking in air in head-fixed animals, using high-resolution optoelectronic monitoring methods for kinematic analysis (Gao et al., 2001) demonstrated that deafferentation (infraorbital nerve section: IOx)—when carried out in a single-stage procedure—did not affect the generation, spectral properties, kinematics, or bilateral coordination of the normal rhythmic whisking pattern (although Berg et al. 2003a) report that IOx produced a slight decrease in whisking frequency). After unilateral section, there was an immediate and significant increase in whisking frequency on both sides of the face that was abolished by subsequent section of the contralateral sensory nerve. Taken in conjunction with the observations of bilateral whisking asynchrony in normal rats, these data suggest, first, the existence of distinct right and left rhythm generators with separate outputs to homolateral motoneurons and, second, some degree of coupling of the right and left rhythm generators. It is possible that respiratory nuclei, that have bilateral influences (see below), are involved in bilateral coordination of whisking (Moore et al., 2014a).
Additional support for these conclusions comes from a deafferentation study carried out in developing rat pups (Landers and Philip Zeigler, 2006). Unilateral IO section at P7 (before the emergence of vibrissae movements) has no effect on whisking behavior, while transection at P12 significantly delays the emergence of the normal whisking rhythm, but only on the treated side. Whisking rhythms on the untreated side emerged at the normal time, but with a slightly, but significantly increased frequency. Bilateral IOx delayed the emergence of normal whisking until almost the end of the first postnatal month. Once normal whisking had emerged, re-sectioning of the sensory nerve had no effect on the re-emergence of vibrissae movements. In pups in which unilateral sensory denervation is combined with contralateral motor denervation—thus reducing the afference generated by active whisking—not only is the initial emergence of whisking significantly delayed but whisking frequency remains significantly reduced two months postnatally.
Taken together, the deafferentation data from adults and pups help to delineate some of the functional properties of central pattern generation mechanisms for whisking, including independent, but closely coupled rhythm generators on the two sides, and a sensitive period during, but not after which, trigeminal afference is critical for the normal development of rhythmic movement patterns. Given that the circuitry for such complex motor patterns as locomotion and suckling is constructed during embryonic development (Nishimaru and Kudo, 2000; Kozlov et al., 2003), trigeminal afference during development seems to contribute primarily to the shaping of pre-existing pattern-generating circuitry.
Hypotheses concerning the neural substrate for the whisking CPG
As of this writing, four potential rhythm generators for whisking have been studied in detail: the respiratory-whisking ventral medulla region, the serotonergic brainstem nuclei, the motor cortex, and the superior colliculus. Indeed, it is entirely feasible that whisking is not governed by a conventional CPG, but, rather, that whisking is controlled by one or more rhythm generators that do not have an intrinsic propensity to generate output at a fixed rhythm.
The search for rhythm generators often focuses on identifying putative rhythmic pre-motoneurons, neurons that provide rhythmic inputs to motoneurons responsible for producing the rhythmic movement. Significant progress towards this goal was made by Moore et al. (2013) who identified a small population of neurons in the brainstem region they defined as the vibrissal zone of IRT (intermediate band of the reticular formation; vIRT) whose oscillatory activity co-varies in phase with vibrissae contractions. Some neurons in this region project to whisking motoneurons in the facial nucleus that control intrinsic muscles, and most of them contain either GABA or glycine. Chemical activation of neurons in this region evokes whisking, whereas lesions in this area abolish whisking. Moore et al. (2013) conclude that this vibrissal zone of vIRT functions as a premotor pattern generator for rhythmic whisking (see also Moore et al., 2014b). They also demonstrated that this pattern generator participates in a larger circuit involved in respiration (see Ramirez et al., 2012). Specifically, (Moore et al. 2013; Moore et al., 2014b) posit that neurons in the pre-Bötzinger complex—critically involved in respiratory rhythm generation—reset the phase of vIRT neurons and thus coordinate whisking and breathing. Pre-Bötzinger neurons interact reciprocally with neurons in the ventral respiratory group, and the latter drive the rhythmic activity of vibrissae extrinsic muscles, to control motion of the mystical pad.
Serotonin and rhythmic whisking
Cranial and spinal motor nuclei, including whisking motoneurons in the lateral facial nucleus, receive some of the densest serotonergic inputs in the brain (Li et al., 1993; Hattox et al., 2003). These inputs arise from several brainstem nuclei known to contain serotonergic neurons, including the raphe magnus and the lateral paragigantocellularis nucleus (LPGi). These serotonergic nuclei also receive dense inputs from the vibrissa representation of the motor cortex (Hattox et al., 2003).
Serotonin is an important modulator of many rhythmic motor acts, including locomotion, respiration, chewing, suckling and licking (Das and Fowler, 1995). Like other spinal and cranial motoneurons, facial motoneurons respond to serotonin both in vivo (VanderMaelen and Aghajanian, 1980) and in vitro (Larkman et al., 1989) with an increase in excitability mediated by a membrane depolarization, an increase in input resistance, and a decrease in their firing threshold. These observations provide an anatomical, behavioral and physiological rationale for implicating serotonin in the regulation of rhythmic whisking.
A series of studies from the Keller laboratory has provided direct evidence for a role of serotonin in initiating and regulating whisking(Hattox et al., 2002; Hattox et al., 2003; Cramer and Keller, 2006; Friedman et al., 2006; Cramer et al., 2007). In brief, infusion of serotonin receptor antagonists into the facial nucleus (in vivo) suppresses voluntary whisking, whereas stimulation (electrical or chemical) of LPGi evokes vibrissa movements. In addition, rhythmic whisking evoked by intracortical microstimulation of the rhythmic protraction region of motor cortex is suppressed by serotonin receptor antagonists. In vitro, serotonin, or its receptor agonists, drives facial motoneurons to fire at whisking frequencies, by facilitating a persistent inward current in these motoneurons; The magnitude of this persistent current is positively correlated with the motoneurons’ firing rate.
The motor cortex and rhythmic whisking
Although it is widely assumed that the vibrissal representation of motor cortex (vMCx) participates in voluntary whisking, the mechanisms by which this occurs remain to be established. One line of evidence suggests that whisking might be controlled by the vMCx on a cycle-by-cycle basis. Berg and Kleinfeld (2003b) reported that stimulation of the vMCx at whisking frequencies evokes vibrissae movements entrained to the stimulation frequency. This result, coupled with the recent findings that vibrissa motoneurons receive direct, albeit sparse, projections from vMCx (Grinevich et al., 2005) suggest that vMCx can, in principle, control whisking on a cycle-by-cycle basis.
In contrast, the data described above suggest that rhythmic whisking is generated by a subcortical CPG or rhythm generator, under modulatory control of the vMCx. Whisking persists after decerebration (Lovick, 1972), or cortical ablation (Semba and Komisaruk, 1984; Gao et al., 2003a), indicating that vMCx is not necessary for rhythmic whisking. Furthermore, recordings of cortical activity during voluntary whisking suggest vMCx does not directly generate whisking. Findings that vMCx activity precedes the onset of voluntary whisking and that rhythmic whisking outlasts vMCx activity are also consistent with activation of a whisking rhythm generator by vMCx (Friedman et al., 2006). Indeed, stimulation of the rhythmic subregion of vMCx evokes whisking epochs that are preceded by relatively long onset latencies, occur at frequencies distinct from the stimulation frequency, and can outlast the stimulus (Haiss and Schwarz, 2005; Cramer and Keller, 2006). Sreenivasan et al. (2015) recently demonstrated that high frequency optogenetic stimulation of vMCx results in near-immediate onset of rhythmic whisking, but conclude that this results from indirect activation of whisking motoneurons, through pre-motoneurons in the brainstem. Taken together, these observations are consistent with the hypothesis that vMCx does not directly generate whisking but instead acts through a subcortical whisking CPG that contains an essential serotonergic component.
Friedman et al. (2012) revealed significant coherence between the frequency of units in the rhythmic subregion of motor cortex and vibrissae movements during free-air whisking, but not when animals were using their vibrissae to contact an object. Spike rate in vMCx was most frequently correlated with the amplitude of vibrissa movements, whereas correlations with movement frequency did not exceed chance levels. Similarly, Hill et al. (2011) report that most vMCx units are modulated by slow variations in whisking envelope, and few units report rapid changes in whisker position. These findings suggest that the specific parameter under cortical control may be the amplitude of whisker movements.
Obviously, the two control strategies are not mutually exclusive. Indeed, Brecht et al. (2004) found that stimulation of layer V vMCx neurons evokes vibrissae movements entrained to the stimulation frequency, whereas stimulation of layer VI neurons produces bouts of whisking that are out of phase across trials. Thus vMCx might use different control strategies to produce or modulate rhythmic whisking.
Recording from the same, rhythmic subregion, Gerdjikov et al. (2013) report that single units may encode two aspects of whisker movement: (1) whisker position; (2) speed, intensity, and frequency. Information theory analysis suggested that these firing patterns contain information mostly about position and frequency, while intensity and speed are less well represented. These investigators found no evidence for phase locking, movement anticipation, or contact related responses. Gerdjikov et al. (2013) conclude that vMCx neither programs nor initiates vibrissae trajectories, nor does it process contact information. They suggest that vMX has an indirect role in whisking, and that it may be related to movement monitoring, perhaps using feedback from a whisking CPG. In contrast, Kleinfeld et al. (2002) find that firing of vMCx units is modulated as a sinusoid at the repetition rate of the stimulus for whisking frequencies (5 to 15 Hz).
The superior colliculus and rhythmic whisking
The superior colliculus sends dense and direct projections to the facial nucleus, where the vibrissa motor neurons are located (Hattox et al., 2002; Miyashita and Mori, 1995), and stimulation of the superior colliculus produces movements of the vibrissae (McHaffie and Stein, 1982). These findings suggest that the superior colliculus may also have a role in controlling whisking kinematics. Indeed the superior colliculus may have a unique role in whisking behavior by functioning as a sensorimotor loop. Collicular neurons reliably respond to vibrissae contacts with short-latency spikes reflecting their direct and potent inputs from trigeminal nuclei (Hemelt and Keller, 2007; Drager and Hubel, 1976). This, coupled with its direct projections to the facial nucleus, implies that the superior colliculus functions as part of a closed loop (Kleinfeld et al., 1999) through which vibrissae contacts reliably evoke vibrissae movements.
Consistent with this hypothesis, Hemelt and Keller (2008) found that in anesthetized rats, microstimulation of the colliculus evokes a sustained vibrissa protraction. This suggests that the superior colliculus plays a pivotal role in vibrissa movement – regulating vibrissa set point and whisk amplitude. This result contrasts with the effects of stimulation of vMCx, which produces rhythmic protractions. Movements generated by the superior colliculus are independent of motor cortex and can be evoked at lower thresholds and shorter latencies than those generated by the motor cortex. Thus, with the motor cortex regulating the whisking frequency (through subcortical targets, perhaps vIRT), the superior colliculus control of set point and amplitude would account for the main parameters of voluntary whisking.
The generation of whisking rhythm
As detailed in the sections above, at least four systems are directly involved in generating rhythmic whisking: The vIRT that functions as a premotor pattern generator (Moore et al., 2013); parafacial respiratory pre-motoneurons that may regulate rhythmic movements of the whisker pad (Moore et al., 2013; Moore et al., 2014b); the superior colliculus that may control vibrissae set point and protraction amplitude (Hemelt and Keller, 2008); brainstem serotonergic nuclei that may initiate whisking and regulate its frequency (Hattox et al., 2003; Cramer and Keller, 2006; Friedman et al., 2006; Cramer et al., 2007).
The challenge, of course, is to determine how these diverse systems interact to perform behaviorally relevant vibrissae movements. For example, Kleinfeld, Deschênes and collaborators (Kleinfeld et al., 2014) posit that activity in vIRT, and in related respiratory rhythm generators, fully account for rhythmic whisking, whereas serotonergic inputs trigger whisking and modulate whisking amplitudes. This conclusion is supported by computational work of Golomb (2014), who shows that moderate periodic inputs from the vIRT and Bötzinger nuclei control whisking frequency, whereas serotonergic neuromodulation controls whisking amplitude.
Rhythm generation in the vibrissa system: Some unanswered questions
These relate primarily to mechanisms of synchronization and coordination of neuronal activity at several levels of the vibrissa sensorimotor system, and the role of vMCx in whisking. These questions include:
- The simultaneous generation (in unison) of the whisking rhythm by all facial motoneurons on one side of the animal, possibly by actions of the vIRT. Because the facial nucleus is thought not to contain interneurons, and its neurons do not have axon collaterals, Cramer, et al. (2007) suggested that unilateral synchronization of whisking might result from coordinated discharge of electrically coupled vFMNs, a hypothesis consistent with the presence of gap junction proteins in the facial nucleus (Rohlmann et al., 1993). However, unpublished results (Y. Li and A. Keller) have thus far failed to identify gap junctions or electrical coupling among vFMNs.
- The coupling of whisking activity on the two sides of the face. While bilateral synchronization could be mediated by one of the numerous pre-motoneuron groups identified by Hattox et al. (Hattox et al., 2002), its mechanism remains to be characterized. As noted above, Kleinfeld and collaborators (Moore et al., 2014a) provided intriguing support for the hypothesis that this bilateral coordination is mediated by respiratory pre-motoneurons.
- The role of vMCx—and other descending motor centers, such as the red nucleus—in regulating whisking. As detailed above, there exist conflicting reports regarding covariation of vMCx activity and whisking parameters. These discrepancies may be due, at least in part, to the use of different whisking behaviors to study these correlations, or to recording from different cell types in different vMCx subregions.
Answers to these questions, and to the question of whether there are multiple circuits for multiple forms of whisking, will require further neurobehavioral experiments in awake, behaving rodents, analogous to those common with primates, combining unit recording, experimental control of the whisking response and high resolution, “online” monitoring of the vibrissae movements.
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