Cells of origin of the branches of the facial nerve: a retrograde HRP study in the rabbit

The origin of different branches of the facial nerve in the rabbit was determined by using retrograde transport of HRP. Either the proximal stump of specific nerves was exposed to HRP after transection, or an injection of the tracer was made into particular muscles innervated by a branch of the facial nerve. A clear somatotopic pattern was observed. Those branches which innervate the rostral facial musculature arise from cells located in the lateral and intermediate portions of the nuclear complex. Orbital musculature is supplied by neurons in the dorsal portion of the complex, with the more rostral orbital muscles receiving input from more laterally located cells while the caudal orbital region receives innervation from more medial regions of the dorsal facial nucleus. The rostral portion of the ear also receives innervation from cells located in the dorsomedial part of the nucleus, but the caudal aspect of the ear is supplied exclusively by cells located in medial regions. The cervical platysma, the platysma of the lower jaw, and the deep muscles (i.e., digastric and stylohyoid) receive input from cells topographically arranged in the middle and ventral portions of the nuclear complex. It is proposed that the topographic relationship between the facial nucleus and branches of the facial nerve reflects the embryological derivation of the facial muscles. Those muscles that develop from the embryonic sphincter colli profundus layer are innervated by lateral and dorsomedial portions of the nuclear complex. The muscles derived from the embryonic platysma layer, including the deep musculature, receive their input from mid to ventral regions of the nuclear complex.     

Length and moment arm of human leg muscles as a function of knee and hip-joint angles

Lengths of muscle tendon complexes of the quadriceps femoris muscle and some of its heads, biceps femoris and gastrocnemius muscles, were measured for six limbs of human cadavers as a function of knee and hip-joint angles. Length-angle curves were fitted using second degree polynomials. Using these polynomials the relationships between knee and hip-joint angles and moment arms were calculated. The effect of changing the hip angle on the biceps femoris muscle length is much larger than that of changing the knee angle. For the rectus femoris muscle the reverse was found. The moment arm of the biceps femoris muscle was found to remain constant throughout the whole range of knee flexion as was the case for the medial part of the vastus medialis muscle. Changes in the length of the lateral part of the vastus medialis muscle as well as the medial part of the vastus lateralis muscle are very similar to those of vastus intermedius muscle to which they are adjacent, while those changes in the length of the medial part of the vastus medialis muscle and the lateral part of the vastus lateralis muscle, which are similar to each other, differ substantially from those of the vastus intermedius muscle. Application of the results to jumping showed that bi-articular rectus femoris and biceps femoris muscles, which are antagonists, both contract eccentrically early in the push off phase and concentrically in last part of this phase.     

The somatotopy of the masticatory neurons in the rat trigeminal motor nucleus as revealed by HRP study

Young adult albino rats of Wistar strain were used for the present study. 0.5 to 15 microliters of 20-50% of horseradish peroxidase (HRP) were injected into each individual muscle of mastication to label neurons in the trigeminal motor nucleus (TMON) for light microscopic study. The results reveal that: (1) Many HRP-labeled, multipolar neurons are observed in the motor nucleus in each jaw-closing muscle (JCM) with less in each the jaw-opening muscle (JOM). (2) The motor neurons innervating each masticatory muscle in the motor nucleus show a somatotopic arrangement: (a) those innervating the temporalis muscle are located in the medial and dorsomedial parts; (b) those innervating the masseter muscle are located in the intermediate and lateral; (c) those innervating the medial and lateral pterygoid muscles are located in the lateral, ventrolateral and ventromedial parts, respectively; and (d) those innervating the mylohyoid and the anterior belly of the digastric muscles are located in the most ventromedial part of the caudal one-third of the nucleus. Axons of most masticatory motor neurons run ventrolaterally in between the motor and the chief sensory nuclei of the trigeminal nerve. However, those of the mylohyoid and anterior belly of the digastric muscles ascend dorsally to the dorsal aspect of the caudal nucleus and then turn ventrolaterally to join the motor root of the trigeminal nerve. Furthermore, the dendrites of the motor neuron of JCM converge dorsocaudally to the supratrigeminal region. The diameters of neurons of each JCM display a bimodal distribution. However, an unimodal distribution is present in the motor neurons from each JCM. It is suggested that the motor nucleus innervating the JCM is comprised of comprised of alpha- and gamma-motor neurons. It, thus, may provide a neural basis for the regulation of the muscle tone and biting force.     

Evolution of high-performance swimming in sharks: transformations of the musculotendinous system from subcarangiform to thunniform swimmers

In contrast to all other sharks, lamnid sharks perform a specialized fast and continuous "thunniform" type of locomotion, more similar to that of tunas than to any other known shark or bony fish. Within sharks, it has evolved from a subcarangiform mode. Experimental data show that the two swimming modes in sharks differ remarkably in kinematic patterns as well as in muscle activation patterns, but the morphology of the underlying musculotendinous system (red muscles and myosepta) that drives continuous locomotion remains largely unknown. The goal of this study was to identify differences in the musculotendinous system of the two swimming types and to evaluate these differences in an evolutionary context. Three subcarangiform sharks (the velvet belly lantern shark, Etmopterus spinax, the smallspotted catshark, Scyliorhinus canicula, and the blackmouth catshark, Galeus melanostomus) from the two major clades (two galeans, one squalean) and one lamnid shark, the shortfin mako, Isurus oxyrhinchus, were compared with respect to 1) the 3D shape of myomeres and myosepta of different body positions; 2) the tendinous architecture (collagenous fiber pathways) of myosepta from different body positions; and 3) the association of red muscles with myoseptal tendons. Results show that the three subcarangiform sharks are morphologically similar but differ remarkably from the lamnid condition. Moreover, the "subcarangiform" morphology is similar to the condition known from teleostomes. Thus, major features of the "subcarangiform" condition in sharks have evolved early in gnathostome history: Myosepta have one main anterior-pointing cone and two posterior-pointing cones that project into the musculature. Within a single myoseptum cones are connected by longitudinally oriented tendons (the hypaxial and epaxial lateral and myorhabdoid tendons). Mediolaterally oriented tendons (epineural and epipleural tendons; mediolateral fibers) connect vertebral axis and skin. An individual lateral tendon spans only a short distance along the body (a fraction between 0.05 and 0.075 of total length, L, of the shark). This span is similar in all tendons along the body. Red muscles insert into the midregion of the lateral tendons. The shortfin mako differs substantially from this condition in several respects: Red muscles are internalized and separated from white muscles by a sheath of lubricative connective tissue. They insert into the anterior part of the hypaxial lateral tendon. Rostrocaudally, this tendon becomes very distinct and its span increases threefold (0.06L anteriorly to 0.19L posteriorly). Mediolateral fibers do not form distinct epineural/epipleural tendons in the mako. Since our morphological findings are in good accordance with experimental data it seems likely that the thunniform swimming mode has evolved along with the described morphological specializations.     

Differential activation of parts of the latissimus dorsi with various isometric shoulder exercises

As no study has examined whether the branches of the latissimus dorsi are activated differently in different exercises, we investigated intramuscular differences of components of the latissimus dorsi during various shoulder isometric exercises. Seventeen male subjects performed four isometric exercises: shoulder extension, adduction, internal rotation, and shoulder depression. Surface electromyography (sEMG) was used to collect data from the medial and lateral components of the latissimus dorsi during the isometric exercises. Two-way repeated analysis of variance with two within-subject factors (exercise condition and muscle branch) was used to determine the significance of differences between the branches, and which branch was activated more with the exercise variation. The root mean squared sEMG values for the muscles were normalized using the modified isolation equation (%Isolation) and maximum voluntary isometric contraction (%MVIC). Neither the %MVIC nor %Isolation data differed significantly between muscle branches, while there was a significant difference with exercise. %MVIC was significantly higher with shoulder extension, compared to the other isometric exercises. There was a significant correlation between exercise condition and muscle branch in the %Isolation data. Shoulder extension and adduction and internal rotation increased %Isolation of the medial latissimus dorsi more than shoulder depression. Shoulder depression had the highest value of %Isolation of the lateral latissimus dorsi compared to the other isometric exercises. Comparing the medial and lateral latissimus dorsi, the medial component was predominantly activated with shoulder extension, adduction, and internal rotation, and the lateral component with shoulder depression. Shoulder extension is effective for activating the latissimus dorsi regardless of the intramuscular branch.     

LDH isoenzymes in the axial muscle of the sharks Galeus melastomus and Etmopterus spinax

The lactate dehydrogenase isoenzyme patterns have been studied in the axial muscles of the sharks Etmopterus and Galeus. Samples from red, intermediate and white muscle fibres were run separately on a polyacrylamide slab-gel. Both sharks have three isoenzymes; all three are present in the red and intermediate fibres, while the white fibres contain only the two slowest-moving isoenzymes. The red fibres of both sharks contain most of the fastest-moving isoenzyme.
The isoenzymes have a high tolerance towards urea; the slow moving isoenzyme is inhibited at about 2 m urea, the next isoenzyme at 4-6 M urea, and some activity of the fast-moving isoenzyme is still present at 10 M urea in the incubation medium. The LDH distribution in the fibre types is studied by histochemistry on frozen sections.     

The nerve supply to the clavicular part of the pectoralis major muscle: an anatomical study and clinical application of the function-preserving pectoralis major island flap

The purpose of this study was to investigate the nerve supply to the clavicular part of the pectoralis major muscle so that the innervation to this part can be maintained in the muscle-preserving pectoralis major island-flap transfer. Although methods have been described that include a limited portion of the muscle while leaving the upper parts undisturbed with an intact motor innervation, reports on anatomical studies of this nerve supply are brief. The distal distribution of the nerves, the spatial relationship to the main vascular pedicle, and the ways to preserve them during surgical procedures remain unclear. Surgically relevant features of the clavicular part of the pectoralis major muscle were studied by dissection. The nerve supply to this part was examined on 11 sides of eight formalin-fixed cadavers. Two fresh cadavers were used for dissection, intraarterial polymer injection, and application of a nerve-preserving surgical technique. In all subjects, a separate nerve innervated the clavicular and upper medial sternocostal portions of the pectoralis major muscle. This nerve arises craniomedial to the main vascular pedicle of the flap and divides into several branches. These branches run in a fascia on the deep surface of the pectoralis major muscle, superficial to the origin and distal course of the vascular pedicle. Most branches to the clavicular part end medial to the coracoid process. The course of the branches to the upper sternocostal part is more medial. Based on their anatomical findings, the authors propose a surgical technique for transfer of the pectoralis major island flap to the head and neck area through a tunnel in the deltopectoral groove, lateral to the origin of the vascular pedicle. Head and neck reconstruction was performed using this technique. The presented method is a muscle-preserving procedure that maintains maximal donor-site function and morphology.     

Red muscle function in stiff-bodied swimmers: there and almost back again

Fishes with internalized and endothermic red muscles (i.e. tunas and lamnid sharks) are known for a stiff-bodied form of undulatory swimming, based on unique muscle-tendon architecture that limits lateral undulation to the tail region even though the red muscle is shifted anteriorly. A strong convergence between lamnid sharks and tunas in these features suggests that thunniform swimming might be evolutionarily tied to this specialization of red muscle, but recent observations on the common thresher shark (Alopias vulpinus) do not support this view. Here, we review the fundamental features of the locomotor systems in lamnids and tunas, and present data on in vivo muscle function and swimming mechanics in thresher sharks. These results suggest that the presence of endothermic and internalized red muscles alone in a fish does not predict or constrain the swimming mode to be thunniform and, indeed, that the benefits of this type of muscle may vary greatly as a consequence of body size.     

Muscle activation patterns of selected lower extremity muscles during stepping and cutting tasks

Lower extremity muscle activations during crossover and side step cut tasks are hypothesized to play an important role in controlling knee motion, and therefore, impact the design of knee injury prevention and rehabilitation programs. However, the contribution of lower extremity muscles to frontal and transverse plane moments during cutting tasks is unclear. The purpose of this study was to compare the muscle activation patterns of selected lower extremity muscles (vastus lateralis, medial/lateral hamstrings and medial/lateral gastrocnemius) of subjects performing a stepping down and side step cut, a stepping down and crossover cut and an equivalent straight ahead task. Ground reaction force was used to determine the cut angle, stance time and compare the lower limb loading during each task. Electromyography data during all tasks were normalized to the average activation during the straight ahead tasks to determine relative changes in muscle activation between the straight ahead and different cut styles (crossover and side step). There were no differences in the pattern of muscle activation of the vastus lateralis, or lateral hamstring muscles when comparing the cutting tasks to the equivalent straight ahead task. However, the crossover cut task resulted in significantly higher muscle activation of the medial hamstrings and lateral gastrocnemius muscles relative to both the side step cut and straight ahead tasks. These results suggest the medial/lateral hamstrings and medial/lateral gastrocnemius play a role in transverse and frontal plane control during cut tasks.     

Somatotopic localization of the eye muscle afferents in the semilunar ganglion.

In the medial dorsolateral portion of the semilunar ganglion of curarized and anaesthetized lambs a cellular pool has been identified which contains the perikarya of the first-order neurons of the eye muscle proprioception. Responses to moderate manual stretch of individual eye muscles were recorded by means of tungsten microelectrodes, from single units of the ganglion. They were of the type induced by muscle spindle excitation. Such responses showed a somatotopic localization. The superior rectus and the superior oblique muscles were represented in the most dorsal layers of the ganglion, while the inferior rectus and the inferior oblique muscles projected on the most ventral portion of the pool. The medial and the lateral recti were represented in the medial and lateral parts and occasionally wedged themselves between the cells innervating the superior and the inferior muscles. Thus a somatotopic arrangement of the eye muscle proprioception has been demonstrated for the first time in the semilunar ganglion.     

The medial sural artery perforator free flap.

The medial sural artery supplies the medial gastrocnemius muscle and sends perforating branches to the skin. The possible use of these musculocutaneous perforators as the source of a perforator-based free flap was investigated in cadavers. Ten legs were dissected, and the topography of significant perforating musculocutaneous vessels on both the medial and the lateral gastrocnemius muscles was recorded. A mean of 2.2 perforators (range, 1 to 4) was noted over the medial gastrocnemius muscle, whereas in only 20 percent of the specimens was a perforator of moderate size noted over the lateral gastrocnemius muscle. The perforating vessels from the medial sural artery clustered about 9 to 18 cm from the popliteal crease. When two perforators were present (the most frequent case), the perforators were located at a mean of 11.8 cm (range, 8.5 to 15 cm) and 17 cm (range, 15 to 19 cm) from the popliteal crease. A series of six successful clinical cases is reported, including five free flaps and one pedicled flap for ipsilateral lower-leg and foot reconstruction. The dissection is somewhat tedious, but the vascular pedicle can be considerably long and of suitable caliber. Donor-site morbidity was minimal because the muscle was not included in the flap. Although the present series is short, it seems that the medial sural artery perforator flap can be a useful flap for free and pedicled transfer in lower-limb reconstruction.     

Cephalic muscle development in the Australian lungfish,Neoceratodus forsteri

Lungfishes are the extant sister group of tetrapods. As such, they are important for the study of evolutionary processes involved in the water to land transition of vertebrates. The evolution of a true neck, that is, the complete separation of the pectoral girdle from the cranium, is one of the most intriguing morphological transitions known among vertebrates. Other salient changes involve new adaptations for terrestrial feeding, which involves both the cranium and its associated musculature. Historically, the cranium has been extensively investigated, but the development of the cranial muscles much less so. Here, we present a detailed study of cephalic muscle development in the Australian lungfish, Neoceratodus forsteri, which is considered to be the sister taxon to all other extant lungfishes. Neoceratodus shows several developmental patterns previously described in other taxa; the tendency of muscles to develop from anterior to posterior, from their region of origin toward insertion, and from lateral to ventral/medial (outside‐in), at least in the branchial arches. The m.protractor pectoralis appears to develop as an extension of the most posterior m.levatores arcuum branchialium, supporting the hypothesis that the m.cucullaris and its derivatives (protractor pectoralis, levatores arcuum branchialium) are branchial muscles. We present a new hypothesis regarding the homology of the ventral branchial arch muscles (subarcualis recti and obliqui, transversi ventrales) in lungfishes and amphibians. Moreover, the morphology and development of the cephalic muscles confirms that extant lungfishes are neotenic and have been strongly influenced via paedomorphosis during their evolutionary history.     

兔心脏左室乳头肌的形态和动脉分布

乳头肌可因缺血、纤维化或梗塞而影响其收缩功能,并产生二尖瓣关闭不全(朱清于等,1980)。致成乳头肌功能不全最常见的原因是心肌供血不足,因此研究乳头肌的动脉,具有实用意义。一些作者(Estes,1966;Farrer-Brown,1968;Ranganathan,1969等)对人心脏乳头肌动脉来源、分支分布作过研究,但关于实验动物家兔乳头肌的动脉的研究文献尚少报道。本文为了开展实验性乳头肌梗塞的研究,对兔心脏左室乳头肌的形态、动脉来源、分布类型以及口径、血管密度等进行了研究。     

The area octavo-lateralis in Xenopus laevis

Summary Central projections of afferents from the lateral line nerves and from the individual branches of the VIIIth cranial nerve in Xenopus laevis and Xenopus mülleri were studied by the application of HRP to the cut end of the nerves.Upon entering the rhombencephalon, the lateral line afferents form a longitudinal fascicle of ascending and descending branches in the ventro-lateral part of the lateral line neuropile. The fascicle exhibits a topographic organization, that is not reflected in the terminal field of the side branches. The terminal field can be subdivided into a rostral, a medial and a caudal part, each of which shows specific branching and terminal pattern of the lateral line afferents. These different patterns within the terminal field are interpreted as the reflection of functional subdivisions of the lateral line area. The study did not reveal a simple topographic relationship between peripheral neuromasts and their central projections.Two nuclei of the alar plate with significant lateral line input were delineated: the lateral line nucleus (LLN) and the medial part of the anterior nucleus (AN). An additional cell group, the intermediate nucleus (IN), is a zone of lateral line and eighth nerve overlap, although such zones also exist within the ventral part of the LLN and the dorsal part of the caudal nucleus (CN). Six nuclei which receive significant VIIIth nerve input are recognized: the cerebellar nucleus (CbN), the lateral part of the anterior nucleus, the dorsal medullary nucleus (DMN), the lateral octavus nucleus (LON), the medial vestibular nucleus (MVN) and the caudal nucleus (CN).All inner ear organs have more than one projection field. All organs project to the dorsal part of the LON and the lateral part of the AN. Lagena, amphibian papilla and basilar papilla project to separate regions of the dorsal medullary nucleus (DMN). There is evidence for a topographic relation between the hair cells of the amphibian papilla (AP) and the central projections of AP fibers. The sacculus projects extensively to a region between the DMN and the LON. Fibers from the sacculus and the lagena project directly to the superior olive. Fibers from the utriculus and the three crista organs terminate predominantly in the medial vestibular nucleus (MVN) and in the adjacent parts of the reticular formation, and their terminal structures appear to be organotopically organised. Octavus fiber projections to the cerebellum and to the spinal cord are also described.     

Evolutionary developmental studies of cyclostomes and the origin of the vertebrate neck

Because they lack some gnathostome-specific traits, cyclostomes have often been regarded as representing an intermediate state linking non-vertebrate chordates and gnathostomes. To understand the evolutionary origins of the jaw and paired fins, lamprey embryos and larvae have been used as comparative models. The lack of the jaw–neck region is a conspicuous feature specific to cyclostomes; however, the absence of these features has been largely neglected both in evolutionary developmental studies and in the field of classical comparative embryology. This review seeks to develop a possible evolutionary scenario of the vertebrate neck muscles by taking the cucullaris (trapezius) muscle as the focus. By combining the comparative embryology of lampreys and gnathostomes, and considering the molecular-level developmental mechanism of skeletal muscle differentiation, this review argues that the establishment of the vertebrate neck deserves to be called an evolutionary novelty based on the remodeling of mesenchymal components between the cranium and the shoulder girdle, which involves both mesodermal and neural crest cell lineages.     

Motoneuron projection patterns in chick embryonic limbs with a double complement of dorsal thigh musculature

During the normal development of the chick, lateral motoneurons within the lumbosacral motor column of the spinal cord consistently project to muscles of dorsal origin within the limb while medial motoneurons project to muscles of ventral origin. To determine if specific cues arising from each type of target are the dominant guidance cues used by lateral and medial motoneurons to create this pattern, I examined motoneuron projections in embryonic chick limbs with a double complement of dorsal thigh musculature and no ventral musculature. Results indicate that cues associated with muscles of a specific developmental origin do not invariably dominate. Before and after the major period of motoneuron death, all muscles in dorsal limb regions (host) were innervated by lateral or dorsal pool neurons. Most ventrally positioned (donor) muscles were innervated by medial or ventral pool neurons. Only the donor iliofibularis, a muscle located very near to its original source of innervation, received projections from some lateral neurons. Within the limb proper, medial or ventral pool neurons projected to donor muscles in a patterned manner suggesting that they were following nonspecific regional cues and perhaps also responding to the availability of uninnervated target tissue. I conclude that axon sorting into distinct lateral and medial classes is independent of limb target complement and that subsequent pathway choice is a separate event governed by both specific target cues and other guidance mechanisms.     

Anatomy and muscle activity of the dorsal fins in bamboo sharks and spiny dogfish during turning maneuvers

Stability and procured instability characterize two opposing types of swimming, steady and maneuvering, respectively. Fins can be used to manipulate flow to adjust stability during swimming maneuvers either actively using muscle control or passively by structural control. The function of the dorsal fins during turning maneuvering in two shark species with different swimming modes is investigated here using musculoskeletal anatomy and muscle function. White‐spotted bamboo sharks are a benthic species that inhabits complex reef habitats and thus have high requirements for maneuverability. Spiny dogfish occupy a variety of coastal and continental shelf habitats and spend relatively more time cruising in open water. These species differ in dorsal fin morphology and fin position along the body. Bamboo sharks have a larger second dorsal fin area and proportionally more muscle insertion into both dorsal fins. The basal and radial pterygiophores are plate‐like structures in spiny dogfish and are nearly indistinguishable from one another. In contrast, bamboo sharks lack basal pterygiophores, while the radial pterygiophores form two rows of elongated rectangular elements that articulate with one another. The dorsal fin muscles are composed of a large muscle mass that extends over the ceratotrichia overlying the radials in spiny dogfish. However, in bamboo sharks, the muscle mass is divided into multiple distinct muscles that insert onto the ceratotrichia. During turning maneuvers, the dorsal fin muscles are active in both species with no differences in onset between fin sides. Spiny dogfish have longer burst durations on the outer fin side, which is consistent with opposing resistance to the medium. In bamboo sharks, bilateral activation of the dorsal in muscles could also be stiffening the fin throughout the turn. Thus, dogfish sharks passively stiffen the dorsal fin structurally and functionally, while bamboo sharks have more flexible dorsal fins, which result from a steady swimming trade off. J. Morphol. 274:1288–1298, 2013. © 2013 Wiley Periodicals, Inc.     

Electrophysiological evidence for a topographical projection of the nasal mucosa onto the olfactory bulb of the frog

Three olfactory nerve branches respectively subserving either a medial, an intermediate, or a lateral region of the dorsal olfactory receptor sheet of the bullfrog Rana catesbeiana were electrically stimulated with bipolar platinum hook electrodes. Extracellular single unit responses from 93 second-order cells in different regions of the olfactory bulb were recorded with metal-filled glass micropipets. The excitatory responsiveness of each unit to the stimulation of each of the three nerve branches (response profile) was determined. Some units were sensitive to stimulation of each of the three nerve branches, thus suggesting a wide projection from the entire receptor sheet. On the other hand, other units were more selective. Of this latter group, units in the lateral bulb were excited by nerve branches subserving the more lateral regions of the receptor sheet; units in the medial bulb were excited by the nerve branches subserving the more medial regions of the receptor sheet. These data provide electrophysiological evidence for a topographical projection of the olfactory receptor sheet onto the olfactory bulb, and further suggest that the projections onto different bulbar cells vary in degree of localization.     

Function of the medial red muscle during sustained swimming in common thresher sharks: Contrast and convergence with thunniform swimmers

Through convergent evolution tunas and lamnid sharks share thunniform swimming and a medial position of the red, aerobic swimming musculature. During continuous cruise swimming these muscles move uniformly out of phase with local body curvature and the surrounding white muscle tissue. This design results in thrust production primarily from the caudal fin rather than causing whole-body undulations. The common thresher shark (Family Alopiidae) is the only other fish known to share the same medial red muscle anatomy as the thunniform swimmers. However, the overall body shape and extremely heterocercal caudal fin of the common thresher is not shared with the thunniform swimmers, which have both fusiform bodies and high aspect-ratio, lunate caudal fins. Our study used sonomicrometry to measure the dynamics of red and white muscle movement in common thresher sharks swimming in the ocean to test whether the medial position of red muscle is associated with uncoupling of muscle shortening and local body bending as characteristic of thunniform swimmers. Common threshers (～ 60–100 kg) instrumented with sonomicrometric and electromyographic (EMG) leads swam alongside of the vessel with a tail-beat frequency of ～ 0.5 Hz. EMG signals confirmed that only the red muscle was active during sustained swimming. Despite the more medial position of the red muscle relative to the white muscle, its strain was approximately 1.5-times greater than that of the overlying white muscle, and there was a notable phase shift between strain trajectories in the red muscle and adjacent white muscle. These results suggest an uncoupling (shearing) of the red muscle from the adjacent white muscle. Although the magnitude of the phase shift between red and white muscle strain was relatively constant within individuals, it varied among sharks, ranging from near zero (red and white in phase) to almost 180° out of phase. This extent in variability has not been documented previously for thunniform swimmers with a medial red muscle position and may be a characteristic of the thresher's unique body and caudal fin morphology. Nonetheless, the uncoupling of red and white muscle strain remains a consistent character associated with fishes having a medially positioned red muscle.     

Morphology and evolution of the jaw suspension in lamniform sharks

The morphology of the jaw suspension and jaw protrusion mechanism in lamniform sharks is described and mapped onto a cladogram to investigate how changes in jaw suspension and protrusion have evolved. This has revealed that several evolutionary modifications in the musculoskeletal apparatus of the jaws have taken place among lamniform sharks. Galeomorph sharks (Carcharhiniformes, Lamniformes, Orectolobiformes, and Heterodontiformes) have paired ethmopalatine ligaments connecting the ethmoid process of the upper jaw to the ethmoid region of the cranium. Basal lamniform sharks also acquired a novel single palatonasal ligament connecting the symphysis of the upper jaw to the cranium mid-ventral to the nasal capsule. Sharks in the family Lamnidae subsequently lost the original paired ethmopalatine ligament while retaining the novel palatonasal ligament. Thus, basal lamniform taxa (Mitsukurina owstoni, Carcharius taurus, Alopias vulpinnis) have increased ligamentous support of the lateral region of the upper jaw while derived species (Lamnidae) have lost this lateral support but gained anterior support. In previous studies the morphology of the jaw suspension has been shown to play a major role in the mechanism of upper jaw protrusion in elasmobranchs. The preorbitalis is the primary muscle effecting upper jaw protrusion in squalean (sister group to galeomorphs) and carcharhiniform (sister group to lamniforms) sharks. The preorbitalis originates from the quadratomandibularis muscle and inserts onto the nasal capsule in squalean and carcharhiniform sharks. Carcharhiniform sharks have evolved a subdivided preorbitalis muscle with the new division inserting near the ethmoid process of the palatoquadrate (upper jaw). Alopid sharks have also independently evolved a partially subdivided preorbitalis with the new division inserting at the base of the ethmoid process and surrounding connective tissue. Lamnid sharks have retained the two preorbitalis divisions but have modified both of the insertion points. The original ventral preorbitalis division now inserts onto the connective tissue surrounding the mid-region of the upper jaw, while the new dorsal preorbitalis division inserts onto the surrounding connective tissue and skin at a more posterior position on the upper jaw. The retractor muscle of the jaws, the levator hyomandibularis, has also been modified during the evolution of lamniform sharks. In most sharks, including basal lamniforms, the levator hyomandibularis inserts onto the hyomandibula and functions to retract the jaws after protrusion. In alopid and lamnid sharks the levator hyomandibularis inserts primarily onto the upper and lower jaws around the jaw joint and is a more direct route for retracting the jaws. Thus, there has been at least one instance of character loss (ethmopalatine ligament), acquisition (palatonasal ligament), subdivision (preorbitalis), and modification (ventral preorbitalis, dorsal preorbitalis, and levator hyomandibularis) in the ligaments and muscles associated with the jaw suspension and jaw protrusion mechanism in lamniform sharks. While derived lamniform sharks (Lamna nasus, Carcharodon carcharius, and Isurus oxyrinchus) lost the ancestral passive lateral support of the ethmoid articulation of the upper jaw, they simultaneously acquired muscular support by way of the levator hyomandibularis, which provides a dynamic mechanism for lateral support. The evolution of multiple divisions of preorbitalis insertions onto the palatoquadrate and modification of the levator hyomandibularis insertion directly onto the jaws provides an active mechanism for multiple protractions and retractions of the upper jaw, which is advantageous in those sharks that gouge or saw pieces from large oversized prey items.