Localization of motoneurons innervating the extraocular muscles in the chameleon (Chamaeleo chameleon)

The topography and localization of motoneurons innervating the six extraocular muscles in the chameleon (Chamaeleo chameleon) was studied following HRP injection in each of these individual muscles. Four muscles were innervated ipsilaterally: medial rectus, inferior rectus, inferior oblique and lateral rectus. The medial rectus muscle was innervated by the dorsomedial part of the oculomotor nucleus. The innervation to the inferior rectus muscle arose from the lateral part of the intermediate oculomotor subnucleus, which extended to the lateral part of the dorsal subdivision. The lateral rectus muscle was innervated by the abducens nucleus, which was composed by two subgroups of labeled cells, respectively observed in the principal and accessory abducens subnuclei, whereas efferents to the inferior oblique muscle originated from both the ventral and intermediate oculomotor subnuclei. The contralateral pattern consisted of motoneurons innervating the superior rectus and the superior oblique that were located respectively in the caudal portion of the ventral oculomotor nucleus and in the trochlear nucleus. These results confirmed data reported in most vertebrate species, and were discussed from a comparative and functional point of view. Accepted: 18 June 1999     

眼外肌构筑学研究

对４６侧眼眶标本的眼肌构筑研究表明：四块直肌纤维长度显著大于两次斜肌，而生理横切面积／肌重比率则四块直肌显著小于两块斜肌。上直肌和下斜肌各项数据之和与下直肌和上斜肌各项数据之和相比无显著性差异。比较内直肌，下直肌，上直肌与外直肌，上斜肌，下斜肌各项数据之和，肌纤维长和肌重前者大于后者，而生理横切面积／肌重后者大于前者，提示：眼上下运动的各项数据基本平衡，但内向运动眼球肌肉的肌张力较大。     

Three-dimensional extraocular motoneuron innervation in the rhesus monkey I: Muscle rotation axes and on-directions during fixation

 The rotation axis for each of the six extraocular muscles was determined in four eyes from three perfused rhesus monkeys. Measurements of the locations of muscle insertions and origins were made in the stereotaxic reference frame with the x-y plane horizontal and the x-z plane sagittal. The computed rotation axes of the horizontal recti were close to being in the x-z plane at an angle of about 15° to the z axis. The rotation axes of the vertical recti and the obliques were close to being in the x-y plane at an angle of about 30° to the y axis. In five alert rhesus monkeys, we simultaneously recorded extraocular motoneuron activity and eye position in three dimensions (3D). The activity of 51 motoneuron axons was obtained from the oculomotor (n=34), trochlear (n=11), and abducens nerve (n=6) during spontaneous eye movements. To extend the torsional range of eye position, the animals were also put in different static roll positions, which induced ocular counterroll without dynamic vestibular stimulation. Periods of 100 ms during fixation or slow eye movements (<10°/s) were chosen for analysis. For each motoneuron, a multiple linear regression was performed between firing frequency and 3D eye position, expressed as a rotation vector, in both stereotaxic and Listing’s reference frame. The direction with the highest correlation coefficient (average R=0.94±0.07 SD) was taken as the on-direction. Each unit’s activity could be unequivocally attributed to one particular muscle. On-directions for each motoneuron were confined to a well-defined cone in 3D. Average on-directions of motoneurons differed significantly from the corresponding anatomically determined muscle rotation axes expressed in the stereotaxic reference frame (range of deviations: 11.9° to 29.0°). This difference was most pronounced for the vertical recti and oblique muscles. The muscle rotation axes of the vertical rectus pair and the oblique muscle pair form an angle of 58.3°, whereas the corresponding angle for paired motoneuron on-directions was 105.6°. On-directions of motoneurons were better aligned with the on-directions of semicircular canal afferents (range of deviation: 9.4–18.9°) or with the anatomically determined sensitivity vectors of the semicircular canals (range of deviation: 3.9–15.9°) than with the anatomically determined muscle rotation axes, but significant differences remain to be explained. The on-directions of motoneurons were arranged symmetrically to Listing’s plane, in the sense that the torsional components for antagonistically paired muscles were almost equal, but of opposite sign. Thus, the torsional components of motoneuron on-directions cancel when eye movements are confined to Listing’s plane. This arrangement simplifies the neuronal transformations for conjugate head-fixed voluntary eye movements, while the approximate alignment with the semicircular canal reference frame is optimal for generating compensatory eye movements. Received: 14 January 1998 / Accepted: 4 January 1999     

Effects of age and inactivity due to prolonged bed rest on atrophy of trunk muscles

This study investigated the effects of age and inactivity due to being chronically bedridden on atrophy of trunk muscles. The subjects comprised 33 young women (young group) and 41 elderly women who resided in nursing homes or chronic care institutions. The elderly subjects were divided into two groups: independent elderly group who were able to perform activities of daily living involving walking independently (n = 28) and dependent elderly group who were chronically bedridden (n = 13). The thickness of the following six trunk muscles was measured by B-mode ultrasound: the rectus abdominis, external oblique, internal oblique, transversus abdominis, thoracic erector spinae (longissimus) and lumbar multifidus muscles. All muscles except for the transversus abdominis and lumbar multifidus muscles were significantly thinner in the independent elderly group compared with those in the young group. The thicknesses of all muscles in the dependent elderly group was significantly smaller than that in the young group, whereas there were no differences between the dependent elderly and independent elderly groups in the muscle thicknesses of the rectus abdominis and internal oblique muscles. In conclusion, our results suggest that: (1) age-related atrophy compared with young women was less in the deep antigravity trunk muscles than the superficial muscles in the independent elderly women; (2) atrophy associated with chronic bed rest was more marked in the antigravity muscles, such as the back and transversus abdominis.     

Epaxial muscle fiber architecture favors enhanced excursion and power in the leaper Galago senegalensis

Galago senegalensis is a habitual arboreal leaper that engages in rapid spinal extension during push‐off. Large muscle excursions and high contraction velocities are important components of leaping, and experimental studies indicate that during leaping by G. senegalensis, peak power is facilitated by elastic storage of energy. To date, however, little is known about the functional relationship between epaxial muscle fiber architecture and locomotion in leaping primates. Here, fiber architecture of select epaxial muscles is compared between G. senegalensis (n = 4) and the slow arboreal quadruped, Nycticebus coucang (n = 4). The hypothesis is tested that G. senegalensis exhibits architectural features of the epaxial muscles that facilitate rapid and powerful spinal extension during the take‐off phase of leaping. As predicted, G. senegalensis epaxial muscles have relatively longer, less pinnate fibers and higher ratios of tendon length‐to‐fiber length, indicating the capacity for generating relatively larger muscle excursions, higher whole‐muscle contraction velocities, and a greater capacity for elastic energy storage. Thus, the relatively longer fibers and higher tendon length‐to‐fiber length ratios can be functionally linked to leaping performance in G. senegalensis. It is further predicted that G. senegalensis epaxial muscles have relatively smaller physiological cross‐sectional areas (PCSAs) as a consequence of an architectural trade‐off between fiber length (excursion) and PCSA (force). Contrary to this prediction, there are no species differences in relative PCSAs, but the smaller‐bodied G. senegalensis trends towards relatively larger epaxial muscle mass. These findings suggest that relative increase in muscle mass in G. senegalensis is largely attributable to longer fibers. The relative increase in erector spinae muscle mass may facilitate sagittal flexibility during leaping. The similarity between species in relative PCSAs provides empirical support for previous work linking osteological features of the vertebral column in lorisids with axial stability and reduced muscular effort associated with slow, deliberate movements during anti‐pronograde locomotion.     

Acetylcholine Receptor Organization at the Dystrophic Extraocular Muscle Neuromuscular Junction

Spared extraocular muscles of dystrophic mice are not subjected to regeneration process and can be used to verify whether the lack of dystrophin per se could cause changes in acetylcholine receptor (AChR) distribution. In the present study, rectus and oblique (spared) and retractor bulbi (nonspared) muscles were dissected from adult control (C57Bl/10) and mdx mice. AChRs and nerve terminals were labeled with rhodamine–α‐bungarotoxin and anti–NF200‐IgG‐FITC, respectively, and visualized by confocal microscopy. Rectus and oblique muscles presented 0.5% central nucleation, while retractor bulbi had central nucleation in 45% of muscle fibers. In mdx rectus, AChRs were distributed in branches in 99% of the junctions examined (n = 200), similar to that observed for controls. Nerve terminals covered the AChR branches in 100% of the junctions examined. In control retractor bulbi, AChRs were distributed in regular branches. In mdx retractor bulbi, multiple fragmented islands of receptors were seen in 56% of the endplates examined (n = 200). These results suggest that the lack of dystrophin per se does not influence the distribution of acetylcholine receptors at the neuromuscular junction of spared extraocular muscles. Anat Rec, 2007. © 2007 Wiley‐Liss, Inc.     

Localization of motoneurons innervating the extraocular muscles in the chameleon (Chamaeleo chameleon)

The topography and localization of motoneurons innervating the six extraocular muscles in the chameleon (Chamaeleo chameleon) was studied following HRP injection in each of these individual muscles. Four muscles were innervated ipsilaterally: medial rectus, inferior rectus, inferior oblique and lateral rectus. The medial rectus muscle was innervated by the dorsomedial part of the oculomotor nucleus. The innervation to the inferior rectus muscle arose from the lateral part of the intermediate oculomotor subnucleus, which extended to the lateral part of the dorsal subdivision. The lateral rectus muscle was innervated by the abducens nucleus, which was composed by two subgroups of labeled cells, respectively observed in the principal and accessory abducens subnuclei, whereas efferents to the inferior oblique muscle originated from both the ventral and intermediate oculomotor subnuclei. The contralateral pattern consisted of motoneurons innervating the superior rectus and the superior oblique that were located respectively in the caudal portion of the ventral oculomotor nucleus and in the trochlear nucleus. These results confirmed data reported in most vertebrate species, and were discussed from a comparative and functional point of view.     

Ultrastructural organization of muscle fiber types and their distribution in the rat superior rectus extraocular muscle

Extraocular muscles (EOMs) are unique as they show greater variation in anatomical and physiological properties than any other skeletal muscles. To investigate the muscle fiber types and to understand better the structure-function correlation of the extraocular muscles, the present study examined the ultrastructural characteristics of the superior rectus muscle of rat. The superior rectus muscle is organized into two layers: a central global layer of mainly large-diameter fibers and an outer C-shaped orbital layer of principally small-diameter fibers. Six morphologically distinct fiber types were identified within the superior rectus muscle. Four muscle fiber types, three single innervated fibers (SIFs) and one multiple innervated fiber (MIF), were recognized in the global layer. The single innervated fibers included red, white and intermediate fibers. They differed from one another with respect to diameter, mitochondrial size and distribution, sarcoplasmic reticulum and myofibrillar size. The orbital layer contained two distinct MIFs in addition to the red and intermediate SIFs. The orbital MIFs were categorized into low oxidative and high oxidative types according to their mitochondrial content and distribution. The highly specialized function of the superior rectus extraocular muscle is reflected in the multiplicity of its fiber types, which exhibit unique structural features. The unique ultrastructural features of the extraocular muscles and their possible relation to muscle function are discussed.     

Specific force of the rat extraocular muscles, levator and superior rectus, measured in situ

Extraocular muscles are characterized by their faster rates of contraction and their higher resistance to fatigue relative to limb skeletal muscles. Another often reported characteristic of extraocular muscles is that they generate lower specific forces (sP(o), force per muscle cross-sectional area, kN/m(2)) than limb skeletal muscles. To investigate this perplexing issue, the isometric contractile properties of the levator palpebrae superioris (levator) and superior rectus muscles of the rat were examined in situ with nerve and blood supply intact. The extraocular muscles were attached to a force transducer, and the cranial nerves exposed for direct stimulation. After determination of optimal muscle length (L(o)) and stimulation voltage, a full frequency-force relationship was established for each muscle. Maximum isometric tetanic force (P(o)) for the levator and superior rectus muscles was 177 +/- 13 and 280 +/- 10 mN (mean +/- SE), respectively. For the calculation of specific force, a number of rat levator and superior rectus muscles were stored in a 20% nitric acid-based solution to isolate individual muscle fibers. Muscle fiber lengths (L(f)) were expressed as a percentage of overall muscle length, allowing a mean L(f) to L(o) ratio to be used in the estimation of muscle cross-sectional area. Mean L(f):L(o) was determined to be 0.38 for the levator muscle and 0.45 for the superior rectus muscle. The sP(o) for the rat levator and superior rectus muscles measured in situ was 275 and 280 kN/m(2), respectively. These values are within the range of sP(o) values commonly reported for rat skeletal muscles. Furthermore P(o) and sP(o) for the rat levator and superior rectus muscles measured in situ were significantly higher (P < 0.001) than P(o) and sP(o) for these muscles measured in vitro. The results indicate that the force output of intact extraocular muscles differs greatly depending on the mode of testing. Although in vitro evaluation of extraocular muscle contractility will continue to reveal important information about this group of understudied muscles, the lower sP(o) values of these preparations should be recognized as being significantly less than their true potential. We conclude that extraocular muscles are not intrinsically weaker than skeletal muscles.     

Organization of the extraocular and preganglionic motoneurons supplying the orbit in the lesser galago

The retrograde tracer wheat germ agglutinin-conjugated horseradish peroxidase was used to establish the organization of the extraocular muscle motoneuron pools in a prosimian, Galago senegalensis, for comparison with the organization in monkeys and non-primates. Medial rectus motoneurons were distributed in three subgroups in the ipsilateral oculomotor nucleus, a pattern similar to that of the monkey. Furthermore, the other component of the near response system, the preganglionic parasympathetic motoneurons, were confined within the Edinger-Westphal nucleus, as in the monkey. In contrast, the distribution of the levator palpebrae and superior rectus motoneurons was similar to that of the cat. Specifically, the majority of levator palpebrae motoneurons were located contralaterally, in the caudal central subdivision of the oculomotor nucleus, and the superior rectus motoneurons had a dorsocaudal location in the contralateral oculomotor nucleus. The distributions of motoneurons supplying the superior oblique and lateral rectus muscles were similar to those of other mammals. Unlike previously studied species, the galago was found to have two accessory muscles, that lie beneath the medial and lateral rectus muscles. Motoneurons supplying the accessory rectus muscles were found ventrolateral to the main abducens nucleus, in a position similar to that occupied by the cat accessory abducens nucleus; although others may be present in the main nuclei. Taken together, these results suggest that the organization of extraocular and preganglionic motoneurons in the galago exhibits both monkey and non-primate features. These observations are consistent with the notion that the galago is a primate species whose oculomotor organization is more similar to the general mammalian scheme. © 1993 Wiley-Liss, Inc.     

A new teaching model for demonstrating the movement of the extraocular muscles

The extraocular muscles consist of the superior, inferior, lateral, and medial rectus muscles and the superior and inferior oblique muscles. This study aimed to create a new teaching model for demonstrating the function of the extraocular muscles. A coronal section of the head was prepared and sutures attached to the levator palpebral superioris muscle and six extraocular muscles. Tension was placed on each muscle from a posterior approach and movement of the eye documented from an anterior view. All movements were clearly seen less than that of the inferior rectus muscle. To our knowledge, this is the first cadaveric teaching model for demonstrating the movements of the extraocular muscles. Clin. Anat. 30:733–735, 2017. © 2017Wiley Periodicals, Inc.     

Motoneuronal innervation and mechanical properties of extraocular muscles in the catfish, (Ictalurus punctatus)

Mechanical characteristics and electrical activity were studied in the extraocular muscles of the catfish, Ictalurus punctatus. The contractile properties were determined by stimulation of the individual muscle nerve branches to lateral and medial rectii and the superior and inferior oblique muscles. The speed of contraction was higher than in most other fish muscle, with a twitch contraction time of about 12 ms and a tetanus fusion frequency of 150–170 Hz in all four eye muscles. The fatigue resistance was also high. These properties were the same in fully innervated and partially innervated muscle, largely irrespective of what part of the muscle that was activated. Although different fibre types are known to exist in fish extraocular muscle, it was not possible to obtain functional separation of the mechanical force profile even in the lateral rectus with two distinct motoneuronal innervations. We suggest that polyneuronal innervation of the muscle fibres produces the mechanical responses. Since EMG activity during spontaneous eye movements was similar in the global and the orbital parts of the muscle, all types of fibres in fish extraocular muscle are probably recruited for all types of eye movements.     

Excitation of the extraocular muscles in decerebrate cats during the vestibulo-ocular reflex in three-dimensional space

Summary (1) Vestibulo-ocular reflex excitation of the six extraocular muscles was studied by recording their electromyographic activity in decerebrate cats during oscillations about horizontal and vertical axes, at frequencies from 0.07 to 4 Hz. Animals were oriented in many different positions and rotated about axes that lay in the horizontal, frontal, or sagittal planes defined by our coordinate system. (2) The strengths of modulation (gains) of the responses of all extraocular muscles were a sinusoidal function of the orientation of the rotation axis within a coordinate plane, and this function was nearly independent of rotation frequency. (3) The responses were used to determine an axis of maximal excitation for each of the extraocular muscles by the vestibulo-ocular reflex. Antagonistic muscle pairs were found to have best axes in nearly opposite directions, confirming their operation as pairs. (4) Excitation of the medial and lateral rectus could be explained by input from the paired horizontal semicircular canals, with essentially no convergent input from vertical canals. (5) Excitation of the vertical rectus and oblique muscles could be explained by convergent inputs from the vertical canals with little or no horizontal canal input.     

Extraocular muscles in the microphthalmic rat

The extraocular muscles in a mutant microphthalmic strain of rat were studied. The eyeball of this strain of rat is reduced to about a third in diameter of that of the normal rat. Nevertheless, in the orbit of the mutant rat, every one of the extraocular muscles was identified; their origins and courses were the same as in the normal rat, but differences existed in the insertions. These insertions could be classified into three groups: Group A (retractor bulbi): like normal insertion into the eyeball. Group B (superior rectus and superior oblique): attachment of tendonlike insertions to each other; these muscles come from opposite directions and form a loop. Group C (lateral, medial, and inferior rectus and inferior oblique): insertion into connective tissue surrounding the reduced eyeball. The volume of each muscle of the mutant rat was smaller than that of the normal rat; moreover, significant differences existed in the degree of reduction in the volume of each muscle group classified according to the change of insertion. In the group A muscle the volume was only 33% of the normal volume, whereas group B was 74% and group C was about half of normal.     

The oblique extraocular muscles in cetaceans: Overall architecture and accessory insertions

The oblique extraocular muscles (EOMs) were dissected in 19 cetacean species and 10 non-cetacean mammalian species. Both superior oblique (SO) and inferior oblique (IO) muscles in cetaceans are well developed in comparison to out-groups and have unique anatomical features likely related to cetacean orbital configurations, swimming mechanics, and visual behaviors. Cetacean oblique muscles originate at skeletal locations typical for mammals: SO, from a common tendinous cone surrounding the optic nerve and from the medially adjacent bone surface at the orbital apex; IO, from the maxilla adjacent to lacrimal and frontal bones. However, because of the unusual orbital geometry in cetaceans, the paths and relations of SO and IO running toward their insertions onto the temporal ocular sclera are more elaborate than in humans and most other mammals. The proximal part of the SO extends from its origin at the apex along the dorsomedial aspect of the orbital contents to a strong fascial connection proximal to the preorbital process of the frontal bone, likely the cetacean homolog of the typical mammalian trochlea. However, the SO does not turn at this connection but continues onward, still a fleshy cylinder, until turning sharply as it passes through the external circular muscle (ECM) and parts of the palpebral belly of the superior rectus muscle. Upon departing this “functional trochlea” the SO forms a primary scleral insertion and multiple accessory insertions (AIs) onto adjacent EOM tendons and fascial structures. The primary SO scleral insertions are broad and muscular in most cetacean species examined, while in the mysticete minke whale (Balaenoptera acutorostrata) and fin whale (Balaenoptera physalus) the muscular SO bellies transition into broad fibrous tendons of insertion. The IO in cetaceans originates from an elongated fleshy attachment oriented laterally on the maxilla and continues laterally as a tubular belly before turning caudally at a sharp bend where it is constrained by the ECM and parts of the inferior rectus which form a functional trochlea as with the SO. The IO continues to a fleshy primary insertion on the temporal sclera but, as with SO, also has multiple AIs onto adjacent rectus tendons and connective tissue. The multiple IO insertions were particularly well developed in pygmy sperm whale (Kogia breviceps), minke whale and fin whale. AIs of both SO and IO muscles onto multiple structures as seen in cetaceans have been described in humans and domesticated mammals. The AIs of oblique EOMs seen in all these groups, as well as the unique “functional trochleae” of cetacean SO and IO seem likely to function in constraining the lines of action at the primary scleral insertions of the oblique muscles. The gimble-like sling formed by SO and IO in cetaceans suggest that the “primary” actions of the cetacean oblique EOMs are not only to produce ocular counter-rotations during up-down pitch movements of the head during swimming but also to rotate the plane containing the functional origins of the rectus muscles during other gaze changes.     

An Analysis of Extraocular Muscle Forces in the Piked Dogfish (Squalus acanthias)

Vertebrates utilize six extraocular muscles that attach to a tough, protective sclera to rotate the eye. The goal of the study was to describe the maximum tetanic forces, as well as the torques produced by the six extraocular muscles of the piked dogfish Squalus acanthias to understand the forces exerted on the eye. The lateral rectus extraocular muscle of Squalus acanthias was determined to be parallel fibered with the muscle fibers bundled into discrete fascicles. The extraocular muscles attach to the sclera by muscular insertions. The total tensile forces generated by the extraocular muscles ranged from 1.18 N to 2.21 N. The torques of the extraocular muscles ranged from 0.39 N to 2.34 N. The torques were greatest in the principal direction of movement for each specific muscle. The lateral rectus produced the greatest total tensile force, as well as the greatest torque force component, while the medial rectus produced the second greatest. This is likely due to the constant rotational movement of the eye anteriorly and posteriorly to stabilize the visual image, as well as increase the effective visual field during swimming. Rotational forces in dimensions other than the primary direction of movement may contribute to motion in directions other than the principal direction during multi-muscle contraction that occurs in the vertebrate eye. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 302:837–844, 2019. © 2018 Wiley Periodicals, Inc.     

Anatomy of the cavernous muscles of the kangaroo penis highlights marsupial–placental dichotomy

The mammalian penis is a complex hydraulic organ of cavernous (spongy) tissue supported by both smooth and skeletal muscle structures. In placental mammals, the paired ?Musculus ischiocavernosi anchor the corpora cavernosa to the pelvis (at the ischium), and the paired M. bulbospongiosi converge as they envelop the base of the corpus spongiosum. Male marsupials have a dramatically different anatomy, however, in which both sets of paired muscles remain separate, have a bulbous, globular shape and do not have any direct connection to the pelvis. Here we provide the first detailed anatomical investigation of the muscles of the penis in the western grey kangaroo (Macropus fuliginosus) incorporating dissection, histology, vascular casting and computed tomography. The M. ischiocavernosus and M. bulbospongiosus form massive, multipennate bodies of skeletal muscle surrounding the paired roots of the corpus cavernosum and corpus spongiosum, respectively. Bilateral vascular supply is via both the artery of the penis and the ventral perineal artery. Histological examination reveals cavernous tissues with substantial smooth muscle supported by fibroelastic trabeculae, surrounded by the thick collagenous tunica albuginea. The M. ischiocavernosus and M. bulbospongiosus are known to function during erection of the penis and ejaculation via muscular contraction increasing blood pressure within cavernous vascular tissues. The thick muscular anatomy of the kangaroo would be well suited to this function. The absence of any connection to the bony pelvis in marsupials suggests the possibility of different mechanisms of action of these muscles with regard to reduction of venous return, eversion from the cloaca, or movements such as penile flips, which have been described in some placental mammals. This highlights a greater diversity in form and function in the evolution of the mammalian penis than has been previously considered.     

Mandible strength and geometry in relation to bite force: a study in three caviomorph rodents

The monophyletic group Caviomorpha constitutes the most diverse rodent clade in terms of locomotion, ecology and diet. Caviomorph species show considerable variation in cranio‐mandibular morphology that has been linked to the differences in toughness of dietary items and other behaviors, such as chisel‐tooth digging. This work assesses the structural strength of the mandible of three caviomorph species that show remarkable differences in ecology, behavior and bite force: Chinchilla lanigera (a surface‐dwelling species), Octodon degus (a semi‐fossorial species) and Ctenomys talarum (a subterranean species). Finite element (FE) models of the mandibles are used to predict the stresses they withstand during incisor biting; the results are related to in vivo bite forces and interspecific variations in the mandibular geometries. The study concludes that the mandible of C. talarum is better able to withstand strong incisor bites. Its powerful adducting musculature is consistent with the notorious lateral expansion of the angular process and the masseteric crest, and the enhanced cortical bone thickness. Although it has a relatively low bite force, the mandible of O. degus also shows a good performance for mid‐to‐strong incisor biting, in contrast to that of C. lanigera, which exhibits, from a mechanical point of view, the worst performance. The mandibles of C. talarum and O. degus appear to be better suited to withstand stronger reaction forces from incisor biting, which is consistent with their closer phylogenetic affinity and shared digging behaviors. The contrast between the low in vivo bite force of C. lanigera and the relatively high estimations that result from the models suggests that its adductor musculature could play significant roles in functions other than incisor biting.     

Phasic activity in rats]

At the central level, in the rat, phasic activity has been recorded during paradoxical sleep and in acute conditions after injection of reserpine or parachlorophenylalanine. At the external level, during paradoxical sleep, the extraocular muscles lateral rectus, superior rectus and superior oblique are activated in both plastic and tonic manners. The muscles of the whiskers are also activated; these muscular activations are more often than not synchronous with the eye movements (80%). The time distribution of these ocular movements is homogenous. Reserpine induces phasic muscular activations of the extraocular muscles.