External Levator Research Articles

The electro-mechanical response of canine inspiratory intercostal muscles to increased resistance: the cranial rib-cage.

1. The effect of graded increases in inspiratory airflow resistance on the electrical activity and the mechanical behaviour of the three groups of inspiratory intercostal muscles (parasternal intercostal, external intercostal, levator costae) situated in the cranial portion of the rib-cage has been studied in ten anaesthetized, spontaneously breathing dogs. The mechanical behaviour of the muscles was determined by measuring the respiratory changes in muscle length and the displacements of the rib. 2. During unloaded inspiration, the three muscles were active, the rib moved in the cranial direction, and the parasternal intercostal and levator costae muscles shortened; in most animals, the external intercostals shortened as well. 3. Graded increases in inspiratory airflow resistance elicited a progressive inhibition of parasternal intercostal activity and a gradual facilitation of external intercostal and levator costae activities. Concomitantly, the parasternal intercostals continued to shorten during inspiration. However, both the external intercostals and the levator costae progressively lengthened, and the rib was gradually displaced in the caudal direction. This pattern persisted after increases in chemical respiratory drive had developed. 4. Sectioning the phrenic nerve roots did not alter the electrical or the mechanical response of the parasternal intercostal muscles to loading, but it markedly affected the response of the external intercostals and levator costae. After phrenicotomy, the external intercostals and levator costae continued to shorten during loaded breaths, the rib continued to be displaced in the cranial direction, and although the rate of inspiratory muscle shortening and of rib motion decreased, the facilitation of external intercostal and levator costae activities was markedly reduced or abolished. 5. Lengthening of the external intercostals and caudal displacement of the rib was reproduced by isolated stimulation of the phrenic nerves. 6. The reflex facilitation of external intercostal and levator costae activities that takes place during inspiratory resistive loading thus results primarily from the collapsing action of the diaphragm on the cranial portion of the rib-cage and the consequent lengthening of these muscles. The mechanical effectiveness of this reflex facilitation, however, appears to be relatively small.

Involuntary action of the external anal sphincter. Manometric and electromyographic studies.

The changes in the rectal neck (anal canal) pressure and electromyography (EMG) of the external anal sphincter and levator ani muscle were studied in 20 dogs before and after internal anal sphincter excision by 2 weeks and monthly up to 10 months. The rectal neck pressure dropped to 41% of the preoperative level. It was then gradually elevated from the beginning of the 1st month following excision till it reached 88% of the level before excision at the 10th month. The external sphincter EMG basal activity after internal sphincterectomy by 2 weeks showed a decrease in the amplitude of the slow and high potential waves. From the 1st to the 7th month after excision, changes in the frequency and amplitude of the waves occurred, and by the 10th month, the external sphincter showed the characteristics of the EMG of both the internal and external anal sphincters. The basal activity of the levator ani muscle increased at the 10th month. As reported previously, the changes in the rectal neck pressure and in the EMG of the external anal sphincter after internal sphincterectomy seem to be due to the histological changes in the muscle and include striated and smooth muscle fiber hypertrophy and proliferation. These changes adapt the external sphincter to serve the function of not only voluntary but also involuntary continence.

Inspiratory elevation of the ribs in the dog: primary role of the parasternals [published errata appear in J Appl Physiol 1991 Aug;71(2):following Table of Contents and 1991 Dec;71(6):following Author Index

To assess the relative contributions of the different groups of inspiratory intercostal muscles to the cranial motion of the ribs in the dog, we have measured the axial displacement of the fourth rib and recorded the electromyograms of the parasternal intercostal, external intercostal, and levator costae in the third interspace in 15 anesthetized animals breathing at rest. In eight animals, the parasternal intercostals were denervated in interspaces 1-5. This procedure caused a marked increase in the amount of external intercostal and levator costae inspiratory activity, and yet the inspiratory cranial motion of the rib was reduced by 55%. On the other hand, the external intercostals in interspaces 1-5 were sectioned in seven animals, and the reduction in the cranial rib motion was only 22%; the amount of parasternal and levator costae activity, however, was unchanged. When the parasternals in these animals were subsequently denervated, the levator costae inspiratory activity increased markedly, but the inspiratory cranial motion of the rib was abolished or reversed into an inspiratory caudal motion. These studies thus confirm that, in the dog breathing at rest, the parasternal intercostals have a larger role than the external intercostals and levator costae in causing the cranial motion of the ribs during inspiration. A quantitative analysis suggests that the parasternal contribution is approximately 80%.

Results of Blepharoptosis Surgery with Early Postoperative Adjustment

The authors reviewed 157 cases (207 eyes) of blepharoptosis corrected by external levator resection. Twenty-eight patients (29 eyelids) had unsatisfactory results. The authors adjusted 13 eyelids within 1 week of surgery. Eleven eyelids were reoperated 6 or more weeks after surgery. The mean delay between the initial and final surgery in the early group was 2.0 weeks; and, for the late group, 33.5 weeks. For those having reoperations, there was no difference in the number of procedures required to achieve satisfactory outcomes. Four patients (5 eyelids) with unsatisfactory results were offered, but declined, late repair. Early or late reoperation is effective in correcting unsatisfactory results after external levator resection. The benefits of early surgery are a reduction in time to final result, the ease with which it is performed, potential cost savings, and the opportunity one has to correct unsatisfactory results.

Blepharochalasis.

Blepharochalasis is an uncommon disorder distinguished by recurrent episodes of eyelid oedema in young patients. A hypertrophic form, manifested as fat herniation, and an atrophic form, manifested as fat atrophy, have been described. Ptosis with excellent levator function, laxity of the lateral canthal structures with rounding of the lateral canthal angle, nasal fat pad atrophy, and redundant eyelid skin develop after many episodes of eyelid swelling. Fine wrinkling, atrophy, and telangiectasias characterise the excess eyelid skin. We describe four cases of this syndrome in which external levator aponeurosis tuck, blepharoplasty, lateral canthoplasty, and dermis fat grafts were used to correct atrophic blepharochalasis after the syndrome had run its course.

Ptosis associated with fatty infiltration of Müller's muscle and levator muscle.

Prominent fatty infiltration of Müller's muscle and the anterior levator muscle was noted at the time of surgery in nine of 115 patients undergoing external levator resection for ptosis. This distribution of adipose tissue was confirmed by light microscopy. Fatty infiltration appeared to be a degenerative change found in adults with congenital and acquired ptosis. Preoperatively these patients exhibited moderate to severe ptosis, fair to good levator function, and no elevation of the lid crease. The desired postoperative results were achieved with standard external levator resection and advancement techniques. The fatty appearance may lead to difficulty in identifying customary anatomical landmarks during surgery. With the exception of Horner's syndrome, abnormalities of Müller's muscle have not previously been described in association with ptosis, to these authors' knowledge.

Development of the musculus levator externus IV and the musculus obliquus posterior in Haplochromis elegans trewavas, 1933 (teleostei:Cichlidae): A discussion on the shift hypothesis.

An ontogenetic study of several postembryonic stages of Haplochromis elegans shows that the muscle referred to as "musculus levator externus IV" by previous authors in the adult stage of cichlid fishes is a compound muscle composed of the M. levator externus IV and the central portion of the M. obliquus posterior. Thus the supposed shift of the insertion area of the fourth external levator muscle from the fourth epibranchial to the fifth ceratobranchial, as deduced from adult morphology only, did not occur. Although this ontogenetic discovery does not change the functional specializations ascribed to the muscle by previous authors, it could have very important implications for the phylogenetic placement of the Cichlidae relative to such major taxa as Labroidei and Embiotocidae.