Question

A patient with Duchenne muscular dystrophy presents with fibrosis, where some of the contractile muscle fibers are replaced with scar tissue, although the nervous system and neuromuscular junction are not altered. In this patient the sarcomere length, muscle belly length, and fiber type distribution are equivalent to a control subject.

A. What would be the predicted change in maximum isometric force and maximum unloaded shortening velocity compared to a control subject. Describe using terms of muscle architecture (sarcomere arrangement- parallel and series).

B. this patient the muscle is only able to produce 200 N of maximum isometric force. The bicep tendon inserts 5 cm from the elbow axis of rotation and the wrist is 50 cm from the axis of rotation of the elbow. When the elbow is bent at 90˚, what is the maximum force that the patient can withstand at the wrist? (assume negligible weight of the forearm).

C. In DMD, the diaphragm cannot produce the required inspiration force. How is airflow altered during inspiration? Describe in terms of pulmonary pressures and volume.

D. A control subject is able to maintain a systemic arterial PO₂ of 100 mmHg and a venous PO₂ of 40 mmHg. A dystrophic patient is only able to reach an arterial PO₂ of 60 mmHg and venous PO₂ of 35 mmHg. How is O₂ delivery altered? How would arterial PO₂ be altered if this patient was strenuously exercising?

E. Treatments for DMD include gene therapies to restore dystrophin expression. However, this requires that the patients maintain immunosuppression for their lifetime because the dystrophin produced is not recognized as self. What type of adaptive immune cell produces an opsonizing agent? How are the muscle fibers affected by opsonization?

Answer 1

Isometric contraction, refers to one in which the muscle is activated, but instead of being allowed to lengthen or shorten, it is held at a constant length. The force generated during isometric contraction is the Isometric force.

The force generated during an isometric contraction is wholly dependant on the length of the muscle while contracting. As in this patient the sarcomere length, muscle belly length, and fiber type distribution are equivalent to a control subject and we know that Maximal isometric force is produced at the muscle's optimum length, where the length of the muscle's sarcomeres are on the plateau of the length-tension curve. So in this case , there will be no predicted change in maximum isometric force.

Maximum velocity of unloaded shortening and specific tension in single muscle cells is primarily characterized according to expression of myosin heavy chains (types I, Ha, and IIb) or combinations of myosin isoforms (type I-IIa and IIab).Single fibres of older muscles containing myosin heavy chains of both type I and II show lower tension and shortening velocity with respect to the fibres of young muscles. In addition some of the contractile muscle fibers are replaced with scar tissue in this patient,which leads to early fatique and due to which the maximum velocity of unloaded shortening decreases.