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Keywords:

  • facial motor nucleus;
  • comparative neuroanatomy;
  • facial expression;
  • mammals;
  • primates;
  • stereology;
  • motoneuron

Abstract

  1. Top of page
  2. Abstract
  3. SIMILARITIES ACROSS PHYLOGENY
  4. PHYLOGENETIC SPECIALIZATIONS
  5. CONCLUSION
  6. Acknowledgements
  7. NOTE ADDED IN PROOF
  8. LITERATURE CITED

The facial motor nucleus (VII) contains motoneurons that innervate the facial muscles of expression. In this review, the comparative anatomy of this brainstem nucleus is examined. Several aspects of the anatomical organization of the VII appear to be common across mammals, such as the distribution of neuron types, general topography of muscle representation, and afferent connections from the midbrain and brainstem. Phylogenetic specializations are apparent in the proportion of neurons allocated to the representation of subsets of muscles and the degree of differentiation among subnuclei. These interspecific differences may be related to the elaboration of certain facial muscles in the context of socioecological adaptations such as whisking behavior, sound localization, vocalization, and facial expression. Furthermore, current evidence indicates that direct descending corticomotoneuron projections in the VII are present only in catarrhine primates, suggesting that this connectivity is an important substrate for the evolution of enhanced mobility and flexibility in facial expression. Data are also presented from a stereologic analysis of VII neuron numbers in 18 primate species and a scandentian. Using phylogenetic comparative statistics, it is shown that there is not a correlation between group size and VII neuron number (adjusted for medulla volume) among primates. Great apes and humans, however, display moderately more VII neurons that expected for their medulla size. © 2005 Wiley-Liss, Inc.

Therian mammals are characterized by well-differentiated superficial facial muscles (also known as muscles of facial expression) derived from the second branchial arch. Compared to nonmammalian vertebrates whose facial muscle actions are limited to opening and closing the apertures encircling the mouth, eyes, and nostrils, mammals are capable of a much more varied range of facial movements (van Hooff, 1967). Greater mobility of the lips and cheeks may have evolved in stem mammals to facilitate neonatal suckling and more extensive chewing of food (Huber, 1930). Additionally, with the evolution of increased energetic demands related to homeothermy in mammals, facial muscles may have become differentiated to facilitate mobility of the external ears and whisking movements of tactile vibrissae to explore more actively the environment for food items (van Hooff, 1967). Building on these basic mammalian adaptations within particular lineages, subsets of facial muscles have increased in complexity and expanded concomitantly with socioecological adaptations. For example, the mass of musculature surrounding the blowhole in odontocete cetaceans alters the shape of the spermaceti organ during the emission of biosonar (Cranford et al., 1996). In anthropoid primates, a number of tractor muscles (e.g., zygomaticus major, zygomaticus minor, levator labii superioris, depressor anguli oris, depressor labii inferioris, and risorius) surround the mouth to configure the shape of the lips for vocalizations and facial displays (Huber, 1931). Perhaps the most impressive example of facial musculature specialization is the elongated and highly mobile trunk of elephants, which is composed of several layers of differentially oriented muscle bundles derived exclusively from the caninus muscle (also known as levator anguli oris) (Endo et al., 2001).

Neurons within the facial motor nucleus (VII) of the brainstem innervate the superficial facial musculature and hence comprise the final common output for circuits related to various behaviors, including emotional expression, vocal communication, respiration, ingestion, protective reflexes, and sensory exploration of the environment. In addition to the main VII, the accessory facial nucleus (also called the suprafacial nucleus or the dorsal facial nucleus) contains motoneurons of deep facial muscles (i.e., stylohyoid and the posterior belly of the digastric). Axons of motoneurons in the main VII and the accessory facial nucleus exit the brainstem together on the ipsilateral side as the facial nerve (CN VII), then leave the base of the skull via the stylomastoid foramen and enter the parotid gland, where the main trunk of the facial nerve divides into several major branches.

Considering the central involvement of the VII in diverse sensorimotor adaptations of the facial muscles across phylogeny, the comparative neurobiology of the VII is of special interest. This article presents an overview of phylogenetic variation in the neuroanatomical structure and connectivity of the VII in mammals.

SIMILARITIES ACROSS PHYLOGENY

  1. Top of page
  2. Abstract
  3. SIMILARITIES ACROSS PHYLOGENY
  4. PHYLOGENETIC SPECIALIZATIONS
  5. CONCLUSION
  6. Acknowledgements
  7. NOTE ADDED IN PROOF
  8. LITERATURE CITED

General Cytoarchitectural Plan

The VII is composed predominantly of multipolar α-motoneurons (Fig. 1), which express the biochemical markers choline acetyltransferase (Ichikawa and Hirata, 1990; Ichikawa and Shimizu, 1998; Tsang et al., 2000), nonphosphorylated neurofilament protein (Tsang et al., 2000), and calcineurin (Strack et al., 1996). Morphological and tract tracing studies in rats and cats suggest that the VII contains few, if any, interneurons (Courville, 1966a; McCall and Aghajanian, 1979). Injection of horseradish peroxidase (HRP) into the main trunk of the facial nerve, for instance, results in retrograde labeling of 98% of neurons in the VII, indicating the virtual absence of neurons that do not directly innervate facial muscles (McCall and Aghajanian, 1979). In addition, size histograms of facial neurons in macaque monkeys (Welt and Abbs, 1990) and rats (Martin et al., 1977) show a unimodal distribution, indicating that few small γ-motoneurons are found in the VII. This concords with reports that muscle spindles, which are innervated by γ-motoneurons, occur in very low abundance in superficial facial muscles (Bowden and Mahran, 1956; Olkowski and Manocha, 1973; Dubner et al., 1978; Brodal, 1981; Sufit et al., 1984).

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Figure 1. Motoneurons located in the lateral subdivision of the VII of an orangutan (Pongo pygmaeus) stained for (A) Nissl substance and (B) nonphosphorylated neurofilament protein (NPNFP) with SMI-32 antibody. Scale bar = 100 μm.

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As with other motor nuclei (e.g., hypoglossal) (Sokoloff and Deacon, 1992), the neurons of the VII are arranged in subnuclei that lie adjacent to one another in longitudinal cell columns. Because each subnucleus extends rostrocaudally for a different distance, the differentiation of subnuclei is most apparent in coronal sections at the middle third of the VII. Historically, the number of VII subnuclei recognized by researchers has varied considerably depending on species, anatomical methods, and subjective assessment. For example, based on Nissl staining patterns, Welt and Abbs (1990) described six subnuclei in long-tailed macaques (Macaca fascicularis), Jenny and Saper (1987) described four subnuclei in M. fasciularis, and Satoda et al. (1987) described five subnuclei in Japanese macaques (M. fuscata). Some discrepancy may arise from the fact that facial neurons are arranged in irregular clusters and cytoarchitectural boundaries between subnuclei are not well defined in most species (Papez, 1927; Vraa-Jensen, 1942; van Buskirk, 1945; Courville, 1966a; Dom et al., 1973; Martin and Lodge, 1977; Porter et al., 1989; Welt and Abbs, 1990; Yew et al., 1996; Sherwood et al., 2005). Nonetheless, some boundaries among subnuclei can be more clearly delimited in tissue stained for myelin and nonphosphorylated neurofilament protein (Fig. 2). In these preparations, however, there is not clear differentiation between all the subnuclei identifiable based on cytoarchitecture, suggesting that there may be fewer functionally and anatomically distinct subdivisions than usually recognized. Also, it is interesting that VII subnuclei in larger-brained species tend to be more completely separated by interstitial space as compared to their smaller-brained relatives (Fig. 3). This suggests that there is relative elaboration of the dendritic arbors of these motoneurons as a consequence of scaling rules (Sherwood et al., 2005). These allometric trends may contribute to the appearance of more distinct VII subnuclei in some species; however, the functional significance of such differentiation is not known.

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Figure 2. Coronal sections through the midbody of the VII in a long-tailed macaque monkey (Macaca fascicularis) showing architecture as revealed by staining for (A) Nissl substance, (B) myelin, and (C) nonphosphorylated neurofilament protein (NPNFP). Scale bar = 250 μm.

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Figure 3. Cytoarchitecture of the VII from members of the same order that vary in overall size. Order Rodentia: (A) rat (Rattus norvegicus), approximately 2 g brain weight, versus (B) capybara (Hydrochaeris hydrochaeris), approximately 55 g brain weight; Order Carnivora: (C) domestic cat (Felis silvestris), approximately 35 g brain weight, versus (D) leopard (Panthera pardus), approximately 125 g brain weight. Note that the proportionate composition of subdivisions appears similar in members of the same clade; however, subnuclei are more differentiated in the species with larger brains. Scale bar = 250 μm.

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Conserved Musculotopy

The topographic representation of the muscles of facial expression in the VII has been studied in several species. Unfortunately, interpretation of older axotomy studies is hampered by the fact that retrograde cellular pathologic changes are somewhat unpredictable (Martin and Lodge, 1977). In addition, many retrograde cell degeneration studies involved sectioning of a single peripheral nerve branch. Individual facial nerve branches, however, may innervate a number of different muscles and a particular muscle may be supplied by more than one nerve branch (Provis, 1977). Thus, early studies that found a close correspondence between subnuclei of the VII and the innervation territories of peripheral branches of the facial nerve may have been influenced by methodological artifact (van Gehuchten, 1898; Marinesco, 1899; Yagita, 1910; Papez, 1927; Hogg, 1928; Nishi, 1965; Courville, 1966a), not the true anatomical organization of the nucleus.

More recent studies utilizing sensitive retrograde tract tracers such as HRP and fluorescence dyes have provided more detailed and reliable data on the musculotopic organization of facial motoneurons in several species, including brush-tailed possums (Provis, 1977), opossums (Dom and Zielinski, 1977; Dom, 1982), pigs (Marshall et al., 2005), guinea pigs (Hamner et al., 1989), rats (Watson et al., 1982; Hinrichsen and Watson, 1984; Klein and Rhoades, 1985; Klein et al., 1990), mice (Ashwell, 1982; Komiyama et al., 1984; Terashima et al., 1993), rabbits (Baisden et al., 1987; Satoda et al., 1988), macaque monkeys (Satoda et al., 1987; Porter et al., 1989; Welt and Abbs, 1990; VanderWerf et al., 1997, 1998; Morecraft et al., 2001), capuchin monkeys (Horta-Junior et al., 2004), and cats (Kume et al., 1978; Shaw and Baker, 1985). Taken together, the results of these studies suggest that a basic pattern of muscle representation exists in the VII that is common to all mammals (Dom, 1982; Komiyama et al., 1984; Swanson et al., 1999; Horta-Junior et al., 2004; Marshall et al., 2005).

As a rule, the rostrocaudal axis of the facial musculature is represented along the mediolateral axis of the VII, whereas the superoinferior axis of the face is represented along the dorsoventral axis of the nucleus. Thus, muscles surrounding the mouth are represented in lateral regions of the VII, posterior auricular and neck muscles are represented in medial regions, and neurons that are located intermediate innervate muscles around the eyes, the forehead, and anterior auricular muscles (Fig. 4). Discrete injections of retrograde tracers into facial muscles of macaques and capuchins (Welt and Abbs, 1990; Horta-Junior et al., 2004) and individual whisker follicle muscles of rats (Klein and Rhoades, 1985) indicate that different regions of the same facial muscle are represented at all rostrocaudal levels within the nucleus.

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Figure 4. Schematic diagram showing the position of muscle representation relative to the VII that can be generalized across mammals. Although there is some consistency in the nomenclature for subnuclei across studies of the same species, given interspecific variation in the anatomical orientation of the nucleus as a whole, as well as the degree to which subnuclei are differentiated, existing subnucleus classification cannot be easily applied to all mammals.

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Strong evidence for evolutionary conservation of this musculotopic plan in the mammalian VII comes from tracing experiments in the marsupial North American opossum (Didelphis virginiana), which show that despite poor differentiation among subnuclei, the basic mammalian plan of musculotopic representation is present (Dom, 1982). Data from mutant strains such as reeler mice (Terashima et al., 1993) and Shaking Rat Kawasaki (Setsu et al., 2001), furthermore, indicate that the musculotopic organization of VII is preserved in spite of abnormal migration of facial neuroblasts and atypical VII cytoarchitecture. While it is known that ephrins and hepatocyte growth factor are important axon guidance cues that direct developing motor axons to enter the mesenchyme of the appropriate branchial arch (Caton et al., 2000; Küry et al., 2000), the molecules that direct growth cones toward specific target muscles in the periphery are not yet known (Chandrasekhar, 2004). Based on the similarities in VII musculotopy across diverse species, however, it would seem that the molecular cues that guide axons to target muscles in the second branchial arch are evolutionarily conserved.

While the fundamental topographic organization of muscle representation in the VII seems to be similar across mammals, the precise mapping between peripheral innervation territories and anatomical subdivisions of the nucleus have not been fully elucidated within any species, let alone across species. It appears, however, that a certain amount of overlap may exist among motoneuron pools for different muscles in the VII. For example, overlapping motoneuron pools have been described for different perioral muscles in pigs (Marshall et al., 2005), orbicularis oculi and the frontalis of rats (Watson et al., 1982), the anterior and posterior auricular levators of bats (Friauf and Herbert, 1985), upper and lower eyelids in cats (Shaw and Baker, 1985), and mentalis and orbicularis oris in macaques (Welt and Abbs, 1990). This contrasts with the more orderly somatotopic representation of mechanoreceptors in the principal trigeminal sensory nucleus (Belford and Killackey, 1979). However, the apparent intermingling of muscle representation may, to some extent, be due to the experimental artifact of tracer spread to adjacent muscles. Populin and Yin (1995) have demonstrated extremely specific topography of pinna muscle representation in the mediodorsal subnucleus of the cat VII by using very small injections (1–2 μl) of cholera toxin B-HRP into individual pinna muscles. Discrete motoneuron pools have also been shown to innervate different portions of the orbicularis oculi muscle in rhesus macaques (VanderWerf et al., 1998).

Afferent Connections From Brainstem and Midbrain

Studies, mostly in rodents, have shown that the VII receives afferent inputs from diverse brainstem and midbrain sites, reflecting its role in various complex orofacial behaviors. In addition, pharmacological and anatomical data indicate that the responsiveness of facial motoneurons is regulated by several neuromodulatory systems, including serotonin (Takeuchi et al., 1983; Li et al., 1993b; Tallaksen-Greene et al., 1993; Leger et al., 2001; Hattox et al., 2003), substance P (Senba and Tohyama, 1985; Tallaksen-Greene et al., 1993; Yew et al., 1996), enkephalin (Fort et al., 1989; Yew et al., 1996), and acetylcholine (Fort et al., 1989; Ichikawa and Hirata, 1990; Yew et al., 1996; Ichikawa and Shimizu, 1998; Kus et al., 2003). For example, the presence of very high densities of 5-HT2 receptors in the VII suggests that serotonergic facilitation of facial motoneuron excitability is important in the regulation of central pattern motor generating networks for blink reflexes, respiration, rhythmic whisking, and mastication (McCall and Aghajanian, 1979; Pazos et al., 1985; Mengod et al., 1990; Rasmussen and Aghajanian, 1990; LeDoux et al., 1998).

Afferent inputs to the VII have been identified originating in the brainstem, including locations throughout the reticular formation, the nucleus ambiguus, hypoglossal nucleus, sensory trigeminal complex, paralemniscal nucleus, and parabrachial nucleus (Hinrichsen and Watson, 1983; Fay and Norgren, 1997; Pinganaud et al., 1999; Dauvergne et al., 2001; Popratiloff et al., 2001). The medullary reticular formation is the greatest source of afferents to all orofacial cranial nerve motor nuclei, including VII (Travers and Norgren, 1983). Tracing studies have identified several groups of inhibitory GABA and glycinergic premotor neurons in this region, as well as the paralemniscal zone, which project to the VII, trigeminal motor, ambiguus, and hypoglossal nuclei to coordinate mastication and swallowing (Travers and Norgren, 1983; Li et al., 1997). Some individual neurons in the reticular formation have been shown to possess collateral axons that synapse within several different cranial orofacial motor nuclei (Amri et al., 1990; Li et al., 1993a; Popratiloff et al., 2001). It is thought that projections from the sensory trigeminal complex, particularly the magnocellular portion of the subnucleus caudalis, provide sensory feedback to facial motoneurons involved in whisking movements of the vibrissae (Erzurumlu and Killackey, 1979).

The VII also receives projections from the midbrain, including the superior colliculus, red nucleus, periaqueductal gray, and several nuclei involved in oculomotor control (Courville, 1966b; Martin and Dom, 1970; Mizuno et al., 1971; Edwards, 1972; Yu et al., 1972; Panneton and Martin, 1979, 1983; Hinrichsen and Watson, 1983; Vidal et al., 1988; Hattox et al., 2002; Dauvergne et al., 2004). Inputs from the red nucleus, which relay cerebellar information to facial motoneurons (Holstege et al., 1984; Holstege and Tan, 1988), may play a role in the fine adjustment of nasolabial activity during whisking behaviors (Hinrichsen and Watson, 1983). Orientation of the ears to objects of interest detected in the visual field may be mediated by superior colliculus inputs to motoneurons of the pinnae (Dom et al., 1973; Harting et al., 1973; Kilimov and Milev, 1973). Additionally, the presence of inputs from the superior colliculus to palpebrae motoneurons suggests a substrate for saccade-related lid movements (Vidal et al., 1988; Dauvergne et al., 2004). Collectively, these data illustrate that VII motoneurons integrate an array of inputs to subserve adaptive behaviors of the orofacial muscles.

PHYLOGENETIC SPECIALIZATIONS

  1. Top of page
  2. Abstract
  3. SIMILARITIES ACROSS PHYLOGENY
  4. PHYLOGENETIC SPECIALIZATIONS
  5. CONCLUSION
  6. Acknowledgements
  7. NOTE ADDED IN PROOF
  8. LITERATURE CITED

Counts of Facial Neuron Numbers Across Species

Several studies have reported total neuron number for the VII in different species (Table 1). It is difficult, however, to compare neuron numbers among species based on these data because many older studies used assumption-based counting methods. Typically, these studies were performed by counting the two-dimensional projected profiles of neurons from relatively thin histological sections. Because large cells have a greater chance of being sampled within such sections, the number of profiles counted does not have a simple or known relationship to the total number of cells in a given volume (Thune and Pakkenberg, 2000). Corrections for these biases, such as the Abercrombie correction, make assumptions about the shape and orientation of cells that are rarely met by actual biological objects (Mouton, 2002). As a consequence, intergroup differences in the size, shape, and orientation of cells can lead to significant bias in results based on these methods.

Table 1. Counts of VII neuron numbers from previous studies
SpeciesNMeanRangeReference
Homo sapiens155,196–6,270(Maleci, 1934)
 566,8114,500–9,460(van Buskirk, 1945)
 412,500(Blinkov and Ponomarev, 1965)
 86,040–13,640(Blinkov and Glezer, 1968)
 6,000(Welt and Abbs, 1990)
Macaca mulatta44,600(Blinkov and Ponomarev, 1965)
Macaca fascicularis122,2221,600–3,043(Welt and Abbs, 1990)
Macaca sp.43,875–5,540(Blinkov and Glezer, 1968)
Rattus norvegicus5,092(Martin et al., 1977)
 25,5765,332–5,820(Watson et al., 1982)
Rattus rattus4,425(Tsai et al., 1993)
 24,906(Friauf and Herbert, 1985)
 33,3503,178–3,466(Martin et al., 1977)
Mus musculus22,027(Ashwell, 1982)
 56,0605,350–6,600(Nimchinsky et al., 2000)
Canis lupus familiaris208,6136,800–11,510(van Buskirk, 1945)
 415,800(Blinkov and Ponomarev, 1965)
 412,330–19,060(Blinkov and Glezer, 1968)
Felis silvestris159,100–10,376(Maleci, 1934)
 267,7344,610–9,790(van Buskirk, 1945)
Rousettus aegyptiacus24,126(Friauf and Herbert, 1985)
Trichosurus vulpecula65,342(Provis, 1977)

Another limitation of existing data on VII neuron number is that only a few distantly related species have been studied. Such phylogenetically dispersed data pose complications in establishing the relationship between facial neuron number and the evolution of fine motor control of facial movements. Previous comparative studies, for instance, found that dogs and cats have more facial neurons than primates (van Buskirk, 1945; Blinkov and Ponomarev, 1965). These findings led Blinkov and Ponomarev (1965: p. 299) to conclude that “considerable complications of functions is by no means connected with any increase in the number of neurons in the corresponding motor nuclei of the brain stem.” However, carnivores and primates are separated by approximately 79–88 million years of independent evolution (Murphy et al., 2001). The functional significance of differences in neuron numbers becomes obscured in comparisons of such vastly divergent lineages. It is difficult to discern based on these data whether differences in neuron numbers among mammalian lineages are adaptive specializations or have been driven to fixation by neutral drift or pleiotropy.

Stereologic Analysis of Facial Neuron Number in Primates

To address some of these limitations, Sherwood (2003) performed designed-based stereologic analysis of VII neuron numbers within a restricted phylogenetic sample including 18 species of primates and 1 scandentian (Tupaia glis). The total number of neurons in the left-side VII was estimated based on Nissl-stained sections using the optical fractionator method (West et al., 1991).

Table 2 shows species mean, coefficient of variation (CV), and coefficient of error (CE) of VII neuron number estimates. Among primates, species mean VII neuron number varied by a factor of 3.2. This relatively minimal range of variation contrasts with the much wider range over which VII volume (23.6-fold) and medulla volume (44.3-fold) vary across the same species (Sherwood et al., 2005). Due to this narrow range and the considerable amount of intraspecific variation, there was extensive overlap in VII neuron number among individuals of different taxa (Fig. 5A). Notably, values for many monkeys fell within the range of great apes and humans.

Table 2. Total number of VII neurons estimated by the optical fractionator
SpeciesNMeanCVMean CE of estimate
Tupaia glis23,4820.060.09
Loris tardigradus24,2570.060.07
Galago senegalensis13,5170.06
Nycticebus coucang15,3020.05
Saguinus mystax16,0750.04
Saimiri sciureus57,8470.310.06
Aotus trivirgatus59,1200.280.09
Lagothrix lagothricha15,6800.12
Alouatta seniculus14,0220.07
Macaca fascicularis66,0600.140.06
Macaca mulatta24,8570.130.07
Erythrocebus patas69,7210.240.07
Papio cynocephalus110,8980.05
Papio anubis17,6550.05
Hylobates lar15,6190.15
Pongo pygmaeus49,9630.150.06
Gorilla gorilla410,6040.150.06
Pan troglodytes511,1690.300.06
Homo sapiens410,4700.140.06
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Figure 5. A: The results of optical fractionator estimates of total neuron number in VII. B: The double-logarithmic least-squares (LS) regression of VII neuron number on medulla volume1/3 is shown. The LS line is fit to all data and the great ape and human points are depicted as closed circles for comparison. Values for species mean medulla volume were obtained from Sherwood et al. (2005). Because Shapiro Wilk's W-tests showed that variables were not normally distributed, regression analyses used logarithmic (base 10) transformed data. The cube root of medulla volume was used to adjust volumetric measures to the same dimensionality as neuron number.

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Figure 5B shows the allometric relationship between species mean VII neuron number and medulla volume1/3 in this sample. Overall, the relationship between these variables is only moderately strong (r2 = 0.566; P = 0.001; n = 16) and many species deviate substantially from expectations based on the regression function. Of particular interest is that the greatest departure from predicted VII neuron numbers is Aotus trivirgatus, which have 42% more VII neurons than predicted for a primate of their medulla size (studentized deleted residual = 2.33). It is noteworthy that owl monkeys are active in a nocturnal environment and consequently do not rely extensively on the visual channel for social communication (Moyniham, 1967). In this regard, they have among the most poorly differentiated facial muscles of any anthropoid (Huber, 1931) and they use very few facial expressions in their social communication (Moyniham, 1967). This suggests that variation in VII neuron numbers may not be strictly associated with facial muscle mobility.

To investigate further a possible relationship with specializations for facial expression, VII neuron numbers were examined for a correlation with social group size to test the hypothesis that relatively greater facial muscle innervation is necessary in species that live in large gregarious groups that rely on facial displays to mediate their social interactions (Andrew, 1963b). Data on social group size obtained from Barton (1999) were used as an index of social complexity (Dunbar, 1992, 1998). To control for phylogenetic bias in the data set, independent contrasts (Felsenstein, 1985; Garland et al., 1999) were calculated from log-transformed species mean data based on the topology and untransformed branch lengths from a composite phylogeny of primates (Purvis, 1995). Because contrasts in VII neuron number were significantly correlated with contrasts in medulla volume1/3, VII neuron number contrasts were adjusted by calculating residuals from the least-squares regression line on medulla volume1/3. Group size contrasts were not correlated with medulla volume1/3 contrasts and therefore were not size-adjusted. Results indicate that there is no correlation between size-adjusted VII neuron number contrasts and group size contrasts (r = −0.168; P = 0.603; n = 12). This finding is similar to the result obtained when phylogenetic comparative methods were used to test for correlations between VII volume, VII gray level index, and social group size (Sherwood et al., 2005).

Although there was not a correlation between VII neuron number and social group size across primates, behavioral reports suggest that the facial displays and vocalizations of hominids (i.e., great apes and humans) involve a greater range of facial muscle mobility compared to other primates (van Hooff, 1962; Andrew, 1963a, 1965; Chevalier-Skolnikoff, 1973; Preuschoft and van Hooff, 1995). Therefore, hominids were examined to determine whether these species have more facial neurons than expected for nonhominid primates of their medulla volume1/3. The regression line was redrawn excluding the hominid data and hominid values were compared to this prediction. Because of the wide dispersion of the data, the confidence intervals of the prediction were wide and the coefficient of determination was low (r2 = 0.288; P = 0.072). Thus, although hominids on average have 24% more facial neurons than expected for nonhominid primates of their medulla volume1/3 and all hominid points were above the nonhominid line (y = 0.546x + 3.201), they fall within the 95% prediction intervals of the regression.

Taken together, these findings suggest that primates living in large social groups do not require significant modifications of relative VII neuron numbers for greater volitional control and mobility of facial expressions. In this context, however, hominids display a minor departure from allometric expectations, which may be associated with increased differentiation of subsets of facial muscles surrounding the mouth. Nevertheless, across primates there does not appear to be a systematic relationship between the degree of mobility of facial displays and relative VII neuron number. These conclusions are supported by the considerable overlap observed in the distribution of VII neuron numbers across anthropoid primates.

Species Differences in Cytoarchitectural Organization

Aside from total neuron numbers, the VII may exhibit other, more subtle specializations of its cytoarchitectural organization that are associated with phylogenetic adaptations of the facial muscles. Because there is a correspondence between subnuclei of the VII and musculotopic representation, it is possible that facial muscles that are more elaborated or receive greater innervation density are represented by a relatively larger pool of motoneurons. In catarrhine primates, for example, the perioral muscles are particularly well differentiated (Huber, 1931) and retrograde tracing experiments in long-tailed macaques show that they are innervated by proportionally more motoneurons than other facial muscles (Welt and Abbs, 1990). Furthermore, in Nissl-stained coronal sections of VII in catarrhines, the lateral subdivision is the largest in relative size (van Buskirk, 1945; Jenny and Saper, 1987; Satoda et al., 1987; Welt and Abbs, 1990). One study of the VII in fetal humans, for instance, reported that motoneurons of the perioral muscles comprise the greatest percentage of total nucleus volume as compared to other subnuclei (38–46% of total VII volume) (Shindo, 1959). Interestingly, in platyrrhines, the dorsal subdivision (motoneurons of the upper face) is relatively enlarged so that it is roughly equal in size to the lateral subdivision (Horta-Junior et al., 2004; see Fig. 2 in Sherwood et al., 2005).

Among other mammals, such as carnivores and chiropterans, the pinnae are involved in specialization for sound localization and are capable of a high degree of mobility. Several studies have attempted to relate aspects of VII organization to these motor specializations of the pinnae. The medial subdivision of the VII in cats, which innervates the auricular muscles, has been described by several authors to be significantly larger relative to other subdivisions of the nucleus (Papez, 1927; Courville, 1966a; Kume et al., 1978). A comparative retrograde HRP tract tracing study of auricular motoneurons in Egyptian Rousette bats and rats revealed several apparent specializations in the bat, which may relate to enhanced mobility of the ears (Friauf and Herbert, 1985). In these bats, the medial subnucleus (mostly motoneurons of the pinnae) contains 49% of the total number of neurons in the VII. In contrast, the medial subdivision in rats contains 31% of VII neurons. Furthermore, individual pinnae muscles in bats are represented in nonoverlapping motoneuron pools, whereas in rats there is extensive overlap.

Another facial motor specialization that has been linked to phylogenetic variation in VII organization is the exploratory whisking of tactile vibrissae in many mammals. In rats, nasolabial motoneurons were found to constitute the greatest percentage of the VII (Tsai et al., 1993). In brush-tailed possums, another animal that utilizes whisking behavior, the greatest percentage of neurons in the VII is found in the medial (posterior auricular) and lateral (vibrissae) subdivisions (Provis, 1977). In an effort to test the idea that fine motor control of the whiskers in mice is accomplished by more dense innervation of nasolabial muscles, Ashwell (1982) compared the percentage of facial motoneurons innervating the nasolabial region (43%) with the percentage of total facial muscle volume made up of these muscles (40%). These results suggest that there may not be a greater density of innervation to control nasolabial muscles and, instead, variation in the size of subnuclei corresponds to the size of the peripheral muscle fiber population. Figure 6, which shows phylogenetic variation in the proportions of different VII subnuclei, suggests that the relative size of subnuclei is related to specializations of peripheral muscle groups. Matching motoneuron populations to peripheral targets likely occurs because a significant number of the motoneurons produced during neurogenesis are later eliminated by programmed cell death. To a large extent, motoneuron survival during apoptosis depends on neurotrophic factors derived from skeletal muscles (Sendtner et al., 2000; Banks and Noakes, 2002).

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Figure 6. Cytoarchitecture of VII in diverse mammalian species taken from the midbody of the nucleus. A: Koala (Phascolarctos cinereus, Subclass: Marsupiala, Order: Diprotodontia). B: Giant anteater (Myrmecophaga tridactyla, Order: Xenarthra). C: Mustached bat (Pteronotus parnelli, Order: Chiroptera). D: Domestic dog (Canis lupus familiaris, Order: Carnivora). E: Domestic pig (Sus scrofa, Order: Artiodactyla). F: Florida manatee (Trichechus manatus, Order: Sirenia). Along with the primate, rodents, and carnivores shown in Figures 2 and 3, it can be seen that the cytoarchitecture of the VII in mammals is highly variable. L, lateral; M, medial; D, dorsal; V, ventral. Scale bar = 500 μm.

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Another morphometric variable that may correspond to functional specialization is neuronal size. α-motoneurons that supply fast-twitch muscles fibers tend to be larger than motoneurons that supply slow-twitch fibers (Welt and Abbs, 1990). Therefore, variation in motoneuron size across VII subnuclei may reflect neuromuscular specializations of particular subsets of facial muscles. In rats, the motoneurons of the mental and posterior auricular branch of the facial nerve are significantly larger than neurons of the zygomatico-orbital branch (Tsai et al., 1993). In macaques, neurons in the lateral subnucleus (perioral muscles) were found to have the largest mean perikarya area (Welt and Abbs, 1990).

Phylogenetic Variation in Corticofacial Projections

The presence of direct corticofacial projections originating in primary motor cortex (Brodmann's area 4) has been investigated in a number of mammalian species. Older studies using the axon degeneration technique were unable to reveal direct corticofacial connections in opossums (Martin, 1968; Dom et al., 1973), armadillos (Harting and Martin, 1970), phlangers (Martin et al., 1971), goats (Haarsten and Verhaart, 1967), tree shrews (Shriver and Noback, 1967), rats (Valverde, 1962; Zimmerman et al., 1964; Isokawa-Akesson and Komisaruk, 1987), and cats (Walberg, 1957; Kuypers, 1958a). More recently, anterograde tract tracing in cats and rats have also failed to label direct corticofacial projections (Sokoloff and Deacon, 1990; Hattox et al., 2002). In these species, projections from primary motor cortex can be traced from the pyramidal tract to terminations in the parvocellular reticular formation adjacent to the VII. Like the basal dorsal horn and zona intermedia of the spinal cord, the brainstem reticular formation contains central pattern generators that supply motoneurons and whose activity can be modulated by descending cortical inputs. In rats, for example, lesion of the cortical motor whisker area does not abolish rhythmic whisking behavior; however, it dramatically affects the kinematics, coordination, and temporal pattern of these movements (Gao et al., 2003).

Cortical neurons that synapse directly onto cranial nerve motoneurons have been demonstrated to exist only in some primate species (Walberg, 1957; Kuypers, 1958c; Kuypers and Lawrence, 1967; Morecraft et al., 2001; Jürgens and Alipour, 2002; Simonyan and Jürgens, 2003) and direct cortical projections to the VII have been reported only in Old World anthropoid primates. It remains to be known whether direct cortical afferents to VII can be observed in prosimians or New World monkeys. Neurons in the primary motor cortex project to both the parvocellular reticular formation and directly to the VII in macaques (Kuypers, 1958c; Watson, 1973; Jenny and Saper, 1987; Sokoloff and Deacon, 1990; Morecraft et al., 2001), chimpanzees (Kuypers, 1958c), and humans (Kuypers, 1958b; Iwatsubo et al., 1990). In a series of classic studies, Kuypers (1958b, 1958c) used silver impregnation techniques for degenerating axons to reveal the projections from the ventral portion of primary motor cortex to the brainstem in macaques, chimpanzees, and humans. A greater number of degenerating axons was found in the VII of chimpanzees compared to macaques. In addition, compared to macaques, the lateral subnucleus (motoneurons of perioral muscles) of chimpanzees was more densely innervated by cortical axons and there appeared to be a greater number of degenerating axons in the ipsilateral VII, especially in the lateral and dorsal subnuclei. Using the Nauta-Gygax technique to observe degenerating fibers in human cerebral infarction patients, Kuypers (1958b) reported the existence of direct cortical projections to the VII, as well as the hypoglossal nucleus, nucleus ambiguus, and trigeminal motor nucleus. These findings in humans are generally consistent with earlier anatomical studies using the Marchi technique (Weidenhammer, 1896; Hoche, 1898; Barnes, 1901) as well as a more recent Nauta-Gygax study (Iwatsubo et al., 1990). Transcranial magnetic stimulation of unanesthetised human subjects further supports the existence of a direct corticofacial projection (Benecke et al., 1988).

Silver impregnation techniques for degenerating axons, however, are known to produce somewhat inconsistent results (Heimer and RoBards, 1981). Importantly, the existence of a direct corticofacial projection has been verified in macaques using more reliable modern anterograde tract tracing methods (Jenny and Saper, 1987; Sokoloff and Deacon, 1990; Morecraft et al., 2001; Simonyan and Jürgens, 2003) and antidromic recording of primary motor cortex after activation by VII stimulation (Sirisko and Sessle, 1983; Huang et al., 1988). Based on these studies, macaques appear to have only sparse direct cortical projections from the primary motor cortex to the dorsal subnucleus (upper facial motoneurons) and strong projections to the lateral and ventrolateral regions of the VII (perioral motoneurons) (Jenny and Saper, 1987; Sokoloff and Deacon, 1990; Morecraft et al., 2001). In addition to these projections from primary motor cortex, other cortical motor areas have been shown to innervate directly portions of the VII in rhesus macaques (Morecraft et al., 1996, 2001). In particular, the greatest density of labeled axon terminals was found in VII deriving from primary motor cortex and ventral premotor cortex, while a lower density of terminals originates from neurons in the supplementary motor area, anterior cingulate, posterior cingulate, and dorsal premotor cortices (Morecraft et al., 2001). Each cortical motor area was found to innervate preferentially particular subdivisions of the VII. Most cortical motor areas, including primary motor cortex, premotor cortex, and posterior cingulate cortex, predominantly innervate the contralateral perioral motoneurons, whereas the supplementary motor area and anterior cingulate bilaterally innervate the motoneurons of the auricular muscles and upper face muscles, respectively. A recent retrograde transneuronal tracer study of the cortical innervation of orbicularis oculi motoneurons in rhesus macaques largely supports these findings (Gong et al., 2005).

The elaboration of direct cortical innervation of lower motoneurons in VII (as well as other cranial motor nuclei) among catarrhine primates may be a consequence of brain enlargement. With increasing brain size, the dorsal forebrain disproportionately enlarges compared to the spinal cord and brainstem (Finlay and Darlington, 1995), leading to the development of more widespread cortical projections to subcortical targets via activity-dependent axon sorting processes (Deacon, 1997; Striedter, 2005). A correlation between increased brain size and direct corticomotoneuronal connections is also seen in the spinal cord, where cortical axons project to progressively more caudal parts of the cord and penetrate further into the ventral horn to reach motoneurons with increasing brain size in mammals (Striedter, 2005).

Direct corticomotoneuronal connections might enhance the diversity and flexibility of motor behaviors. Increased corticospinal projections are correlated with some measures of manual dexterity in mammals (Heffner and Masterton, 1975, 1983; Iwaniuk et al., 1999) and the ability to learn diverse vocalizations in birds is associated with the presence of direct connections between the telencephalon and brainstem vocal motoneurons (Striedter, 1994). Notably, vocal learning abilities have recently been reported for elephants and dolphins (Janik, 2000; Poole et al., 2005). These mammals have relatively enlarged neocortices, suggesting that they might also have direct cortical innervation of orofacial motoneurons. Current methodological limitations, however, make this prediction difficult to test.

Among catarrhine primates, behavioral observations support the idea that greater direct corticofacial connections subserve enhanced volitional control of facial movements. Great apes, but not monkeys, have been frequently observed to make nonemotional facial expressions seemingly for the purpose of play and self-amusement (van Lawick-Goodall, 1968; Chevalier-Skolnikoff, 1976, 1982). Additionally, although several species of anthropoids appear to have the capacity to inhibit voluntarily emotional vocalizations and facial expressions to deceive social partners (de Waal, 1982, 1986; Goodall, 1986; Byrne and Whiten, 1992), great apes and humans may be more skilled at suppressing affective output for the purposes of tactical deception (Yerkes and Yerkes, 1929; Chevalier-Skolnikoff, 1976, 1982; Whiten and Byrne, 1988; Byrne and Whiten, 1992). Finally, the coordination of orofacial and laryngeal muscles in human speech probably requires the existence of descending cortical control of motoneurons (Deacon, 1997).

CONCLUSION

  1. Top of page
  2. Abstract
  3. SIMILARITIES ACROSS PHYLOGENY
  4. PHYLOGENETIC SPECIALIZATIONS
  5. CONCLUSION
  6. Acknowledgements
  7. NOTE ADDED IN PROOF
  8. LITERATURE CITED

Considering the involvement of superficial facial muscles in diverse behaviors ranging from blinking to the negotiation of social networks, the anatomic organization of the VII within any given species reflects the combination of evolutionarily conservative organizational programs as well as lineage-specific specializations. The general musculotopic plan of the VII is consistent across all mammals, suggesting a link to early target-derived axon guidance cues. Nonetheless, interspecific variation in the relative distribution of motoneurons for different subsets of facial muscles demonstrates that specialization of the periphery has an influence on the cytoarchitectural organization of VII.

Although studies of the brainstem and midbrain afferent connectivity of VII have not employed explicit comparisons among species, these circuits regulate involuntary activity of the facial muscles, such as protective reflexes and orienting movements, and are likely to be similar across species. In contrast, the development of direct neocortical projections to the VII represents an important anatomical substrate for the evolution of voluntary control of the muscles of facial expression in Old World anthropoid primates and may underlie complex facial behaviors in other lineages.

Acknowledgements

  1. Top of page
  2. Abstract
  3. SIMILARITIES ACROSS PHYLOGENY
  4. PHYLOGENETIC SPECIALIZATIONS
  5. CONCLUSION
  6. Acknowledgements
  7. NOTE ADDED IN PROOF
  8. LITERATURE CITED

The author thanks Drs. P.R. Hof, R.L. Holloway, J.M. Erwin, P.J. Gannon, and S.C. McFarlin for discussions that helped to formulate many of the ideas presented in this manuscript. Brain materials used in these studies were generously provided by the Comparative Neurobiology of Aging Resource, Cleveland Metroparks Zoo, the Neuroanatomical Collection of the National Museum of Health and Medicine, Drs. P.R. Hof, J.J. Wenstrup, K. Zilles, H.D. Frahm, K. Semendeferi, and M. Henneberg. Graphic design support was provided by E. Ando-Yeap and B. von Derau. Supported by the National Science Foundation (DBI-9602234 to NYCEP)

NOTE ADDED IN PROOF

  1. Top of page
  2. Abstract
  3. SIMILARITIES ACROSS PHYLOGENY
  4. PHYLOGENETIC SPECIALIZATIONS
  5. CONCLUSION
  6. Acknowledgements
  7. NOTE ADDED IN PROOF
  8. LITERATURE CITED

As this manuscript was going to press, an article was published that reported the presence of monosynaptic connections from neurons in vibrissae motor cortex to facial motoneurons in rats (Grinevich et al., 2005). Using a lentivirus-based anterograde axon tracing methodology, this study revealed synaptic contacts of labeled axons onto lateral subnucleus facial motoneurons by electron microscopy. These findings suggest that direct cortico-motoneuron connections may be essential to the generation of complex whisker movements of rats during tactile exploration. Most importantly, these results challenge the idea that direct corticofacial pathways are found exclusively in anthropoid primates.

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. SIMILARITIES ACROSS PHYLOGENY
  4. PHYLOGENETIC SPECIALIZATIONS
  5. CONCLUSION
  6. Acknowledgements
  7. NOTE ADDED IN PROOF
  8. LITERATURE CITED