Action of neck accessory muscles on rib cage in dogs

Effects of stretching the scalene muscles on slow vital capacity

[Purpose] The purpose of this study was to examine whether stretching of the scalene muscles would improve slow vital capacity (SVC). [Subjects and Methods] The subjects of this study were 20 healthy female students to whom the study’s methods and purpose were explained and their agreement for participation was obtained. The SVC was measured using spirometry (Pony FX, COSMED Inc., Italy). The intervention used was stretching of the scalene muscles. Stretching was carried out for 15 min, 10 times at per each portion of scalene muscles: the anterior, middle, and posterior parts. [Results] Expiratory vital capacity (EVC) and tidal volume (Vt) noticeably increased after stretching. However, there were no changes in any of the SVC items in the control group. [Conclusion] This study demonstrated that stretching of the scalene muscles can effectively improve SVC. In particular, we confirmed that stretching of the scalene muscles was effective in increasing EVC and Vt, which are items of SVC.

Comparative functional anatomy of the epaxial musculature of dogs (Canis familiaris) bred for sprinting vs. fighting

The axial musculoskeletal system of quadrupedal mammals is not currently well understood despite its functional importance in terms of facilitating postural stability and locomotion. Here we examined the detailed architecture of the muscles of the vertebral column of two breeds of dog, the Staffordshire bull terrier (SBT) and the racing greyhound, which have been selectively bred for physical combat and high speed sprint performance, respectively. Dissections of the epaxial musculature of nine racing greyhounds and six SBTs were carried out; muscle mass, length, and fascicle lengths were measured and used to calculate muscle physiological cross-sectional area (PCSA), and to estimate maximum muscle potential for force, work and power production. The longissimus dorsi muscle was found to have a high propensity for force production in both breeds of dog; however, when considered in combination with the iliocostalis lumborum muscle it showed enhanced potential for production of power and facilitating spinal extension during galloping gaits. This was particularly the case in the greyhound, where the m. longissimus dorsi and the m. iliocostalis lumborum were estimated to have the potential to augment hindlimb muscle power by ca. 12%. Breed differences were found within various other muscles of the axial musculoskeletal system, particularly in the cranial cervical muscles and also the deep muscles of the thorax which insert on the ribs. These may also highlight key functional adaptations between the two breeds of dog, which have been selectively bred for particular purposes. Additionally, in both breeds of dog, we illustrate specialisation of muscle function by spinal region, with differences in both mass and PCSA found between muscles at varying levels of the axial musculoskeletal system, and between muscle functional groups.

Intercostal muscle pacing with high frequency spinal cord stimulation in dogs

High frequency spinal cord stimulation (HF-SCS) is a novel and more physiologic method of inspiratory muscle activation which involves stimulation of spinal cord pathways. In the present study, we determined if activation of the inspiratory intercostal muscles alone by this technique could be utilized to maintain artificial ventilation. In 7 anesthetized dogs, following C2 spinal cord section and bilateral phrenicotomy, trains of electrical stimulation (12 times/min) were applied at the T2 level. Eucapnea was maintained during an initial 5.5h period of continuous stimulation. During a subsequent 0.5h period, stimulus parameters were increased to induce hyperventilation resulting in a sustained fall in end-tidal P(CO(2)) to 29.3 + or - 0.4 mmHg. Single motor unit peak firing frequencies of the intercostal muscles during HF-SCS were similar to those occurring during spontaneous breathing. This technique holds promise as a method to restore ventilation in ventilator-dependent tetraplegics who do not have adequate phrenic nerve function for diaphragm pacing.

Neuromechanical matching of drive in the scalene muscle of the anesthetized rabbit

The scalene is a primary respiratory muscle in humans; however, in dogs, EMG activity recorded from this muscle during inspiration was reported to derive from underlying muscles. In the present studies, origin of the activity in the medial scalene was tested in rabbits, and its distribution was compared with the muscle mechanical advantage. We assessed in anesthetized rabbits the presence of EMG activity in the scalene, sternomastoid, and parasternal intercostal muscles during quiet breathing and under resistive loading, before and after denervation of the scalene and after its additional insulation. At rest, activity was always recorded in the parasternal muscle and in the scalene bundle inserting on the third rib (medial scalene). The majority of this activity disappeared after denervation. In the bundle inserting on the fifth rib (lateral scalene), the activity was inconsistent, and a high percentage of this activity persisted after denervation but disappeared after insulation from underlying muscle layers. The sternomastoid was always silent. The fractional change in muscle length during passive inflation was then measured. The mean shortening obtained for medial and lateral scalene and parasternal intercostal was 8.0 +/- 0.7%, 5.5 +/- 0.5%, and 9.6 +/- 0.1%, respectively, of the length at functional residual capacity. Sternomastoid muscle length did not change significantly with lung inflation. We conclude that, similar to that shown in humans, respiratory activity arises from scalene muscles in rabbits. This activity is however not uniformly distributed, and a neuromechanical matching of drive is observed, so that the most effective part is also the most active.

Respiratory Dysfunction in Chronic Neck Pain Patients. A Pilot Study

The aim of this pilot study was to add weight to a hypothesis according to which patients presenting with chronic neck pain could have a predisposition towards respiratory dysfunction. Twelve patients with chronic neck pain and 12 matched controls participated in this study. Spirometric values, maximal static pressures, forward head posture and functional tests were examined in all subjects. According to the results, chronic neck patients presented with a statistically significant decreased maximal voluntary ventilation ( P = 0.042) and respiratory muscle strength (Pimax and Pemax), ( P = 0.001 and P = 0.002, respectively). Furthermore, the current study demonstrated a strong association between an increased forward head posture and decreased respiratory muscle strength in neck pateits. The connection of neck pain and respiratory function could be an important consideration in relation to patient assessment, rehabilitation and consumption of pharmacological agents.

The gross morphology and histochemistry of respiratory muscles in bottlenose dolphins, Tursiops truncatus

Most mammals possess stamina because their locomotor and respiratory (i.e., ventilatory) systems are mechanically coupled. These systems are decoupled, however, in bottlenose dolphins (Tursiops truncatus) as they swim on a breath hold. Locomotion and ventilation are coupled only during their brief surfacing event, when they respire explosively (up to 90% of total lung volume in approximately 0.3 s) (Ridgway et al. 1969 Science 166:1651-1654). The predominantly slow-twitch fiber profile of their diaphragm (Dearolf 2003 J Morphol 256:79-88) suggests that this muscle does not likely power their rapid ventilatory event. Based on Bramble's (1989 Amer Zool 29:171-186) biomechanical model of locomotor-respiratory coupling in galloping mammals, it was hypothesized that locomotor muscles function to power ventilation in bottlenose dolphins. It was further hypothesized that these muscles would be composed predominantly of fast-twitch fibers to facilitate the bottlenose dolphin's rapid ventilation. The gross morphology of craniocervical (scalenus, sternocephalicus, sternohyoid), thoracic (intercostals, transverse thoracis), and lumbopelvic (hypaxialis, rectus abdominis, abdominal obliques) muscles (n = 7) and the fiber-type profiles (n = 6) of selected muscles (scalenus, sternocephalicus, sternohyoid, rectus abdominis) of bottlenose dolphins were investigated. Physical manipulations of excised thoracic units were carried out to investigate potential actions of these muscles. Results suggest that the craniocervical muscles act to draw the sternum and associated ribs craniodorsally, which flares the ribs laterally, and increases the thoracic cavity volume required for inspiration. The lumbopelvic muscles act to draw the sternum and caudal ribs caudally, which decreases the volumes of the thoracic and abdominal cavities required for expiration. All muscles investigated were composed predominantly of fast-twitch fibers (range 61-88% by area) and appear histochemically poised for rapid contraction. These combined results suggest that dolphins utilize muscles, similar to those used by galloping mammals, to power their explosive ventilation.

Optimal arrangement of magnetic coils for functional magnetic stimulation of the inspiratory muscles in dogs

In an attempt to maximize inspiratory pressure and volume, the optimal position of a single or of dual magnetic coils during functional magnetic stimulation (FMS) of the inspiratory muscles was evaluated in twenty-three dogs. Unilateral phrenic magnetic stimulation (UPMS) or bilateral phrenic magnetic stimulation (BPMS), posterior cervical magnetic stimulation (PCMS), anterior cervical magnetic stimulation (ACMS) as well as a combination of PCMS and ACMS were performed. Trans-diaphragmatic pressure (Pdi), flow, and lung volume changes with an open airway were measured. Transdiaphragmatic pressure was also measured with an occluded airway. Changes in inspiratory parameters during FMS were compared with 1) electrical stimulation of surgically exposed bilateral phrenic nerves (BPES) and 2) ventral root electrical stimulation at C5-C7 (VRES C5-C7). Relative to the Pdi generated by BPES of 36.3 +/- 4.5 cm H2O (Mean +/- SEM), occluded Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and a combined PCMS + ACMS were 51.7%, 61.5%, 22.4%, 100.3%, and 104.5% of the maximal Pdi, respectively. Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and combined ACMS + PCMS were 38.0%, 45.2%, 16.5%, 73.8%, and 76.8%, respectively, of the Pdi induced by VRES (C5-C7) (48.0 +/- 3.9 cm H2O). The maximal Pdi(s) generated during ACMS and combined PCMS + ACMS were higher than the maximal Pdi(s) generated during UPMS, BPMS, or PCMS (p < 0.05). ACMS alone induced 129.8% of the inspiratory flow (73.0 +/- 9.4 L/ min) and 77.5% of the volume (626 +/- 556 ml) induced by BPES. ACMS and combined PCMS + ACMS produce a greater inspiratory pressure than UPMS, BPMS or PCMS. ACMS can be used to generate sufficient inspiratory pressure, flow, and volume for activation of the inspiratory muscles.

Respiratory action of the intercostal muscles

The mechanical advantages of the external and internal intercostals depend partly on the orientation of the muscle but mostly on interspace number and the position of the muscle within each interspace. Thus the external intercostals in the dorsal portion of the rostral interspaces have a large inspiratory mechanical advantage, but this advantage decreases ventrally and caudally such that in the ventral portion of the caudal interspaces, it is reversed into an expiratory mechanical advantage. The internal interosseous intercostals in the caudal interspaces also have a large expiratory mechanical advantage, but this advantage decreases cranially and, for the upper interspaces, ventrally as well. The intercartilaginous portion of the internal intercostals (the so-called parasternal intercostals), therefore, has an inspiratory mechanical advantage, whereas the triangularis sterni has a large expiratory mechanical advantage. These rostrocaudal gradients result from the nonuniform coupling between rib displacement and lung expansion, and the dorsoventral gradients result from the three-dimensional configuration of the rib cage. Such topographic differences in mechanical advantage imply that the functions of the muscles during breathing are largely determined by the topographic distributions of neural drive. The distributions of inspiratory and expiratory activity among the muscles are strikingly similar to the distributions of inspiratory and expiratory mechanical advantages, respectively. As a result, the external intercostals and the parasternal intercostals have an inspiratory function during breathing, whereas the internal interosseous intercostals and the triangularis sterni have an expiratory function.

Respiratory effects of the scalene and sternomastoid muscles in humans

Previous studies have shown that in normal humans the change in airway opening pressure (DeltaPao) produced by all the parasternal and external intercostal muscles during a maximal contraction is approximately -18 cmH(2)O. This value is substantially less negative than DeltaPao values recorded during maximal static inspiratory efforts in subjects with complete diaphragmatic paralysis. In the present study, therefore, the respiratory effects of the two prominent inspiratory muscles of the neck, the sternomastoids and the scalenes, were evaluated by application of the Maxwell reciprocity theorem. Seven healthy subjects were placed in a computed tomographic scanner to determine the fractional changes in muscle length during inflation from functional residual capacity to total lung capacity and the masses of the muscles. Inflation induced greater shortening of the scalenes than the sternomastoids in every subject. The inspiratory mechanical advantage of the scalenes thus averaged (mean +/- SE) 3.4 +/- 0.4%/l, whereas that of the sternomastoids was 2.0 +/- 0.3%/l (P < 0.001). However, sternomastoid muscle mass was much larger than scalene muscle mass. As a result, DeltaPao generated by a maximal contraction of either muscle would be 3-4 cmH(2)O, which is about the same as DeltaPao generated by the parasternal intercostals in all interspaces.

Functional, cellular, and biochemical adaptations to elastase-induced emphysema in hamster medial scalene

The scalene has been reported to be an accessory inspiratory muscle in the hamster. We hypothesize that with the chronic loads and/or dynamic hyperinflation associated with emphysema (Emp), the scalene will be actively recruited, resulting in functional, cellular, and biochemical adaptations. Emp was induced in adult hamsters. Inspiratory electromyogram (EMG) activity was recorded from the medial scalene and costal diaphragm. Isometric contractile and fatigue properties were evaluated in vitro. Muscle fibers were classified histochemically and immunohistochemically. Individual fiber cross-sectional areas (CSA) and succinate dehydrogenase (SDH) activities were determined quantitatively. Myosin heavy chain (MHC) isoforms were identified by SDS-PAGE, and their proportions were determined by scanning densitometry. All Emp animals exhibited spontaneous scalene inspiratory EMG activity during quiet breathing, whereas the scalene muscles of controls (Ctl) were silent. There were no differences in contractile and fatigue properties of the scalene between Ctl and Emp. In Emp, the relative amount of MHC(2A) was 15% higher whereas that of MHC(2X) was 14% lower compared with Ctl. Similarly, the proportion of type IIa fibers increased significantly in Emp animals with a concomitant decrease in IIx fibers. CSA of type IIx fibers were significantly smaller in Emp compared with Ctl. SDH activities of all fiber types were significantly increased by 53 to 63% in Emp. We conclude that with Emp the actively recruited scalene exhibits primary-like inspiratory activity in the hamster. Adaptations of the scalene with Emp likely relate both to increased loads and to factors intrinsic to muscle architecture and chest mechanics.

Interaction between left and right intercostal muscles in airway pressure generation

The interactions between the different rib cage inspiratory muscles in the generation of pleural pressure remain largely unknown. In the present study, we have assessed in dogs the interactions between the parasternal intercostals and the interosseous intercostals situated on the right and left sides of the sternum. For each set of muscles, the changes in airway opening pressure (DeltaPao) obtained during separate right and left activation were added, and the calculated values (predicted DeltaPao) were then compared with the DeltaPao values obtained during symmetric, bilateral activation (measured DeltaPao). When the parasternal intercostals in one or two interspaces were activated, the measured DeltaPao was commonly greater than the predicted value. The difference, however, was only 10%. When the interosseous intercostals were activated, the measured DeltaPao was nearly equal to the predicted value. These observations strengthen our previous conclusion that the pressure changes produced by the rib cage inspiratory muscles are essentially additive. As a corollary, the rib cage can be considered as a linear elastic structure over a wide range of distortion.

Rib cage muscle interaction in airway pressure generation

We have previously demonstrated in dogs that the change in airway opening pressure (DeltaPao) produced by isolated maximum activation of the parasternal intercostal or triangularis sterni muscle in a single interspace, the sternomastoids, and the scalenes is proportional to the product of muscle mass and the fractional change in muscle length per unit volume increase of the relaxed chest wall. In the present study, we have assessed the interactions between these muscles by comparing the DeltaPao obtained during simultaneous activation of a pair of muscles (measured DeltaPao) to the sum of the DeltaPao values obtained during their separate activation (predicted DeltaPao). Measured and predicted DeltaPao values were compared for the following pairs of muscles: the parasternal intercostals in two interspaces, the parasternal intercostals in one interspace and either the sternomastoids or the scalenes, two segments of the triangularis sterni, and the interosseous intercostals in two contiguous interspaces. For all these pairs, the measured DeltaPao was within approximately 10% of the predicted value. We therefore conclude that 1) the pressure changes generated by the rib cage muscles are essentially additive; and 2) measurements of the mass of a particular muscle and of its fractional change in length during passive inflation can be used to estimate the potential pressure-generating ability of the muscle during coordinated activity as well as during isolated activation.

Mechanical advantage of sternomastoid and scalene muscles in dogs

Theoretical studies have led to the prediction that the maximal effect of a given respiratory muscle on airway opening pressure (Pao) is the product of muscle mass, the maximal active muscle tension per unit cross-sectional area, and the fractional change in muscle length per unit volume increase of the relaxed chest wall. It has previously been shown that the parasternal intercostals behave in agreement with this prediction (A. De Troyer, A. Legrand, and T. A. Wilson. J. Physiol. (Lond) 495: 239-246, 1996; A. Legrand, T. A. Wilson, and A. De Troyer. J. Appl. Physiol. 80: 2097-2101, 1996). In the present study, we have tested the prediction further by measuring the response to passive inflation and the pressure-generating ability of the sternomastoid and scalene muscles in eight anesthetized dogs. With 1-liter passive inflation, the sternomastoids and scalenes shortened by 2.03 +/- 0.17 and 5.98 +/- 0.43%, respectively, of their relaxation length (P < 0.001). During maximal stimulation, the two muscles caused similar falls in Pao. However, the sternomastoids had greater mass such that the change in Pao (delta Pao) per unit muscle mass was -0.19 +/- 0.02 cmH2O/g for the scalenes and only -0.07 +/- 0.01 cmH2O/g for the sternomastoids (P < 0.001). After extension of the neck, there was a reduction in both the muscle shortening during passive inflation and the fall in Pao during stimulation. The delta Pao per unit muscle mass was thus closely related to the change in length; the slope of the relationship was 3.1. These observations further support the concept that the fractional changes in length of the respiratory muscles during passive inflation can be used to predict their pressure-generating ability.

Spike trains from single motor units in human parasternal intercostal muscles

Recordings of single motor unit activity were obtained from parasternal intercostal muscles of normal humans during quiet breathing. Spike trains from 52 individual motor units were analyzed. All these units were low threshold ones, recruited at low inspired volumes and therefore at low tension thresholds. Mean frequency of firing at onset was 7.8 Hz and mean increase in frequency through the breath was 3.6 Hz. Onset and peak frequencies were positively correlated with inspiratory flow rate. Alternation of interspike intervals between long and short was found in the spike trains of 6 of 13 units tested and this occurred at frequencies of 6-12/s. Doublet discharges at the beginnings of spike trains were seen during voluntary neck flexion but never in quiet breathing or voluntary deep breaths. The pattern of activity in these human intercostal motor units was similar to that reported for low threshold, slow twitch units in other mammalian skeletal muscles, including respiratory muscles.

Respiratory roles of genioglossus, sternothyroid, and sternohyoid muscles during sleep

We examined the respiratory activity of the genioglossus, sternothyroid, and sternohyoid muscles of the rat during nonrapid eye movement (non-REM) and REM sleep. Each animal carried implanted electrodes for recording the integrated EMG activity of respiratory muscles, the postural tone (EMG), and electrocortical activity (polygraphic identification of sleep-waking states). The three upper airway muscles exhibited inspiratory activity during non-REM sleep while rats breathed ambient air. Curled up postures promoted inspiratory activity of genioglossus and sternothyroid muscles, an effect enhanced by CO2 breathing but reduced by hypoxic breathing. During REM sleep, genioglossus and sternothyroid muscles lost their activity but the sternohyoid muscles retained their inspiratory activity. We conclude that the genioglossus and sternothyroid muscles contribute to upper airway patency during non-REM sleep, an effect CO2 augments but hypoxia reduces. The sternohyoid muscles have at least two functions during both sleep states: they contribute to maintenance of upper airway patency and to rib cage fixation, thereby optimizing the ventilatory action of the diaphragm.

Mechanical coupling between the ribs and sternum in the dog

We measured the axial (cranio-caudal) displacements of the sternum and the second and seventh bony ribs using linear displacement transducers in five supine anesthetized dogs during passive inflation and deflation, during quiet breathing and static inspiratory efforts before and after bilateral phrenicotomy, and during tetanic stimulation of either the sternocleidomastoids or the sternal fibers of the rectus abdominis. Quiet inspiration before and after phrenicotomy was always associated with a caudal displacement of the sternum and a cranial displacement of the seventh rib; the second rib, however, was either motionless or also showed an inspiratory caudal displacement. During static inspiratory efforts, the second rib was always moving in concert with the sternum in the caudal direction, while the seventh rib, in particular after phrenicotomy, usually moved in the cranial direction. Finally, for any given axial (cranial or caudal) displacement of the sternum, stimulation of the sternocleidomastoid or rectus abdominis muscles invariably caused the second rib to move disproportionately more than the seventh. These results indicate that the upper ribs are more tightly linked to the sternum than the lower ribs. This presumably results from the fact that the costal cartilages increase in length from above downwards, and it implies that the upper portion of the rib cage behaves more as a unit with the sternum than the lower portion.