Muscles, Nerves, and Skin

The muscles of giraffes are similar to those of other artiodactyls but differ in order to accommodate the stresses associated with an extraordinary shape. Extensor muscles of the neck are relatively small but flexor muscles of the neck that have to contract against the elasticity of the ligament nuchae are well-developed. The muscle bodies of neck muscles are located at the base of the neck and exert their effects via long tendons, which minimizes neck mass. Giraffes rarely lie down, or sleep, or run. Consequently limb muscles give support more than movement and are more tendinous that muscular. The power to run is generated by hip flexors and extensors, aided by contractions of the gastrocnemius muscles. The forelimb is the main support for body mass and acts as the fulcrum for movement. Chest and forelimb muscles keep the shoulder and elbow joints rigid, but allow drinking and the complications that arise from it. Brain mass forms 0.5% of adult body mass. Eyes are large and the retina has a unique distribution of photoreceptors that provides both clear images of close objects and allows focusing on ground level objects. Peripheral nerves contain more sensory than motor fibers. The skin of giraffes forms ~10% of body mass, and has more collagen than elastic fibers. Skin panniculus muscles are absent, an absence compensated by sebaceous gland secretion of volatile compounds that repel insects. The development of the colors of skin markings begins in the last trimester of pregnancy.

The Evolution of Appendicular Muscles During the Fin-to-Limb Transition: Possible Insights Through Studies of Soft Tissues, a Perspective

The evolution of the appendages during the fin-to-limb transition has been extensively studied, yet the majority of studies focused on the skeleton and the fossil record. Whereas the evolution of the anatomy of the appendicular musculature has been studied, the changes in the muscular architecture during the fin-to-limb transition remain largely unstudied, yet may provide important new insights. The fin-to-limb transition is associated with the appearance of a new mode of locomotion and the associated shift from pectoral to pelvic dominance. Here, we propose ways to investigate this question and review data on muscle mass and muscle architecture of the pectoral and pelvic muscles in extant vertebrates. We explore whether changes in appendage type are associated with changes in the muscular architecture and the relative investment in different muscle groups. These preliminary data show a general increase in the muscle mass of the appendages relative to the body mass during the fin-to-limb transition. The locomotor shift suggested to occur during the fin-to-limb transition appears supported by our preliminary data since in “fish” the pectoral fins are heavier than the pelvic fins, whereas in tetrapods, the forelimb muscles are less developed than the hind limb muscles. Finally, a shift in the investment in different muscle groups with an increase of the contribution of the superficial groups in tetrapods compared to “fish” appears to take place. Our study highlights the potential of investigating quantitative features of the locomotor muscles, yet also demonstrates the lack of quantitative data allowing to test these ideas.

Intrinsic control of neuronal diversity and synaptic specificity in a proprioceptive circuit

Relay of muscle-derived sensory information to the CNS is essential for the execution of motor behavior, but how proprioceptive sensory neurons (pSNs) establish functionally appropriate connections is poorly understood. A prevailing model of sensory-motor circuit assembly is that peripheral, target-derived, cues instruct pSN identities and patterns of intraspinal connectivity. To date no known intrinsic determinants of muscle-specific pSN fates have been described in vertebrates. We show that expression of Hox transcription factors defines pSN subtypes, and these profiles are established independently of limb muscle. The Hoxc8 gene is expressed by pSNs and motor neurons (MNs) targeting distal forelimb muscles, and sensory-specific depletion of Hoxc8 in mice disrupts sensory-motor synaptic matching, without affecting pSN survival or muscle targeting. These results indicate that the diversity and central specificity of pSNs and MNs are regulated by a common set of determinants, thus linking early rostrocaudal patterning to the assembly of limb control circuits.

Functional Suppression of Premotor Activity in a Transient Model of Motor Cortex Injury

ABSTRACTCortical injuries (e.g. – strokes or traumatic brain injuries) can create a host of secondary events that further impair the brain’s sensory, motor, or cognitive capabilities. Here, we attempted to isolate the acute effects of the primary injury – the loss of cortical activity – on rodent motor cortex (caudal forelimb area, CFA) without the secondary effects that arise from damage to cortical tissue. We then observed the effects of this loss of activity on the rodent premotor cortex (rostral forelimb area, RFA). In anesthetized rats, CFA was temporarily inactivated with the GABA-A agonist muscimol, disrupting motor network function while leaving neural connectivity intact. Using intracortical microstimulation (ICMS) techniques, we found that CFA inactivation completely abolished ICMS-evoked forelimb movement from RFA yet spared some CFA evoked-movement. Neural recordings confirmed that neural suppression by muscimol was isolated to CFA and did not spread into RFA. We next observed how CFA inactivation suppressed RFA influence on forelimb muscles by obtaining intramuscular electromyographical (EMG) recordings from forelimb muscles during ICMS. EMG recordings showed that despite the presence of evoked movement in CFA, but not RFA, muscle activation in both areas were similarly reduced. These results suggest that the primary reason for the loss of ICMS-evoked movement in RFA is not reduced forelimb muscle activity, but rather a loss of the specific activity between RFA and CFA. Therefore, within the intact motor network of the rat, RFA’s influence on forelimb movement is mediated by CFA, similar to the premotor and motor organization observed in non-human primates.

Interhemispheric modulations of motor outputs by the rostral and caudal forelimb areas in rats

In rats, forelimb movements are evoked from two cortical regions, the caudal and rostral forelimb areas (CFA and RFA, respectively). These areas are densely interconnected and RFA induces complex and powerful modulations of CFA outputs. CFA and RFA also have interhemispheric connections, and these areas from both hemispheres send projections to common targets along the motor axis, providing multiple potential sites of interactions for movement production. Our objective was to characterize how CFA and RFA in one hemisphere can modulate motor outputs of the opposite hemisphere. To do so, we used paired-pulse protocols with intracortical microstimulation techniques (ICMS), while recording electromyographic (EMG) activity of forelimb muscles in sedated rats. A subthreshold conditioning stimulation was applied in either CFA or RFA in one hemisphere simultaneously or before a suprathreshold test stimulation in either CFA or RFA in the opposite hemisphere. Both CFA and RFA tended to facilitate motor outputs with short (0–2.5 ms) or long (20–35 ms) delays between the conditioning and test stimuli. In contrast, they tended to inhibit motor outputs with intermediate delays, in particular 10 ms. When comparing the two areas, we found that facilitatory effects from RFA were more frequent and powerful than the ones from CFA. In contrast, inhibitory effects from CFA on its homolog were more frequent and powerful than the ones from RFA. Our results demonstrate that interhemispheric modulations from CFA and RFA share some similarities but also have clear differences that could sustain specific functions these cortical areas carry for the generation of forelimb movements. NEW & NOTEWORTHY We show that caudal and rostral forelimb areas (CFA and RFA) have distinct effects on motor outputs from the opposite hemisphere, supporting that they are distinct nodes in the motor network of rats. However, the pattern of interhemispheric modulations from RFA has no clear equivalent among premotor areas in nonhuman primates, suggesting they contribute differently to the generation of ipsilateral hand movements. Understanding these interspecies differences is important given the common use of rodent models in motor control and recovery studies.

The somatosensory cortex receives information about motor output

During voluntary movement, the somatosensory system not only passively receives signals from the external world but also actively processes them via interactions with the motor system. However, it is still unclear how and what information the somatosensory system receives during movement. Using simultaneous recordings of activities of the primary somatosensory cortex (S1), the motor cortex (MCx), and an ensemble of afferent neurons in behaving monkeys combined with a decoding algorithm, we reveal the temporal profiles of signal integration in S1. While S1 activity before movement initiation is accounted for by MCx activity alone, activity during movement is accounted for by both MCx and afferent activities. Furthermore, premovement S1 activity encodes information about imminent activity of forelimb muscles slightly after MCx does. Thus, S1 receives information about motor output before the arrival of sensory feedback signals, suggesting that S1 executes online processing of somatosensory signals via interactions with the anticipatory information.

Anatomical Description of the Forelimb Muscles of the Brown-Throated Sloth (Bradypus variegatus)

Background: Brown-throated sloths are mammals of the order Xenarthra, suborder Pilosa, family Bradypodidae. These folivorous and arboreal animals, which possess a peculiar type of arboreal quadrupedalism, move through the forest canopy by means of suspensory locomotion. On the ground, their extremely slow movements make them easy targets for road accidents, often leading to serious injury or even death. This paper describes the forelimb muscles of the brown-throated sloth (Bradypus variegatus), updating the literature on the subject to help veterinarians in clinical and surgical interventions on this species, and to provide data for comparative animal anatomy.Materials, Methods & Results: Five brown-throated sloths (Bradypus variegatus Schinz, 1825), two adults and three babies were dissected. The animals were donated by the Arruda Câmara Zoo and Botanical Park in João Pessoa, state of Paraíba, Brazil, where they were thawed and fixed in 10% formalin. The sloths’ forelimbs were dissected by lifting and folding over a skin flap to expose, identify and describe the underlying musculature. The dissection revealed the following muscles: supraspinatus, infraspinatus, deltoideus, teres major, subscapularis, coracobrachialis, brachialis, biceps brachii, triceps brachii, anconeus epitrochlearis, dorsoepitrochlearis, brachioradialis, supinator, pronator teres, pronator quadratus, extensor carpi radialis, extensor carpi ulnaris, extensor carpi obliquus, flexor carpi radialis, flexor carpi ulnaris, extensor digitorum communis, extensor digitorum lateralis, palmaris longus, flexor digitorum superficialis, flexor digitorum profundus, extensor indicis longus second finger, extensor indicis brevis second finger, extensor digitorum third finger, abductor digitorum second finger, abductor digitorum third finger, palmaris brevis, and interosseous muscles. Characteristics found in this species revealed differences in the muscular development of the upper forelimb, whose muscles are less developed than those of the lower forelimb, which are visibly more developed with greater muscle density. An interesting feature of this musculature is the presence of three flexor tendons, short and thick, originating from the flexor muscles, which give the hand of the brown-throated sloth a hook-like aspect.Discussion: Our observations indicate that some of the muscles are very similar to those of other animals of this order and also of domestic mammals. However, the most relevant characteristics resemble those of arboreal animals and humans, since the forearm and hand pronation and supination muscles are essential for their arboreal habits. Therefore, all the pronator and supinator muscles of the brown-throated sloth are well developed. Due to the functional adaptations of the species of the family Bradypodidae, the shoulder muscles, especially the deltoideus, are more developed, providing greater support to the shoulder joint, and their origin and insertion assist in faster and easier movements, albeit exerting less force. The group of flexor muscles in this species gives the forearm stronger and more concentrated action than the extensor muscles. This study enabled us not only to produce a more precise description of the muscles of the forelimb of this species but also to update the literature, since there are few relevant studies on the subject and the terms in the literature are outdated and no longer in use. Keywords: sloth, Bradypodidae, morph