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Skeletal Muscle relaxants II

Due to shape of the force-frequency relationship, one would expect to see the greatest change in force between frequencies of:

Explanationa The shape of the force-frequency relationship is initially steep and then flattens out as frequency increases, indicating that at lower the frequencies there is a greater force increase for a given increase in frequency.

Skeletal Muscle relaxants II

The force of a tetanic contraction is greater than that of a twitch contraction because:

Explanationb Compared to a twitch, the greater force of a tetanus is due to greater Ca2+ release and greater take-up of the series elastic component. The greater force is not related to the size or speed of muscle action potentials

Skeletal Muscle relaxants II

The sarcoplasmic reticulum does not:

Explanationd The sarcoplasmic reticulum (SR) makes up only about a few percent of the contents of a muscle fibre. The SR does surround each myofibril, release Ca2+ in response to a muscle action potential and has a Ca2+ "pump" in its membrane.

Skeletal Muscle relaxants II

In excitation-contraction coupling:

Explanationa The muscle action potential causes the sarcoplasmic reticulum to release Ca2+, which binds to troponin, which causes tropomyosin to be pulled aside, allowing cross-bridges (CBs) to bind to actin to initiate CB cycling. Relaxation occurs when Ca2+ is taken back up into the sarcoplasmic reticulum.

Skeletal Muscle relaxants II

During one cross-bridge (CB) cycle:

Explanationc In each CB cycle, the CB binds to an actin binding site, one power stroke is performed and one molecule of ATP is used.

Skeletal Muscle relaxants II

A 36-year-old woman presents to the clinic with a new complaint of fatigue of several months’ duration. She also reports stiffness in both hands that is worse in the morning and decreases after soaking them in her warm morning bath each day. Physical examination reveals a low-grade fever. Subcutaneous nodules are palpated along her forearm bilaterally. What type of hypersensitivity reaction is causing this patient’s arthritis?

ExplanationC. This patient has RA, which is characterized by systemic symptoms of fever, fatigue, pleuritis, and pericarditis. Women are affected by RA more frequently than men. Patients with RA classically experience symmetric morning stiffness of joints that improves with use. Patients may also have subcutaneous rheumatoid nodules, ulnar deviation of the fingers, and joint subluxation. RA is mediated by a type III hypersensitivity reaction in which immune complexes form and activate complement. In RA, rheumatoid factor is an IgM autoantibody that is directed against the Fc region of the patient’s IgG antibody, leading to immune complex formation and deposition. Rheumatoid factor antibodies are present in a number of asymptomatic patients; more recently, anti-CCP (citrulline-containing protein) antibodies have become more popular as sensitive and specific serological diagnostic indicators of RA.

Skeletal Muscle relaxants II

A 25-year-old man develops acute onset of fever, malaise, muscle pain, hypertension, abdominal pain, bloody stool, and prerenal failure six months after recovering from an acute hepatitis B infection. Which of the following disease processes is most likely responsible for the patient’s findings?

ExplanationD. Polyarteritis nodosa (PAN) is a vasculitis (ie, inflammation of a blood vessel) characterized by inflammation affecting small to medium-sized arteries, particularly the renal, cardiac, and GI-tract vessels (usually not the pulmonary vasculature). As many as 30% of patients have had prior hepatitis-B infections.

Skeletal Muscle relaxants II

A 74-year-old man presents to his physician with a bulge in his scrotum. He is diagnosed with an inguinal hernia on his left side, and undergoes surgery two days later to repair the hernia. On postoperative day two he complains of numbness and tingling of his scrotum. Which nerve root contributes to the affected nerve?

ExplanationA. The genitofemoral nerve arises from the L1 and L2 nerve roots. In male subjects its genital branch travels through the superficial inguinal ring along with the spermatic cord, and supplies the cremaster and the scrotal skin. Severing the genitofemoral nerve during a hernia repair leads to numbness of the scrotum and/or inner thigh. The genito femoral nerve and the ilioinguinal nerve have overlapping territory. Thus, severing one only leads to transient anesthesia.

Skeletal Muscle relaxants II

A 20-year-old man presents to the physician with a nontender indurated mass over his mandible. He has had this mass for four months after undergoing oral surgery and decided to come to the physician because the mass started to ooze a thick yellow exudate. Yellow granules are seen on microscopic examination of the discharge and an antibiotic is prescribed. Which of the following best describes the mechanism of action of the antibiotic most likely prescribed?

ExplanationThe correct answer is B. The infection caused by Actinomyces israelii typically presents as a chronic, slowly progressing mass that eventually evolves into a draining sinus tract. Characteristic sulfur granules are seen in the thick yellow exudate. Penicillin G is the first-line treatment.

Skeletal Muscle relaxants II

In regard to the cross-bridge (CB) power stroke, it is true that:

Explanationa CB power strokes always act to cause sarcomere shortening. Whether shortening, lengthening or no change in sarcomere length occurs depends on the load on the sarcomere (muscle).

Skeletal Muscle relaxants II

When a muscle fibre shortens, the following also shortens:

Explanationa The sarcomere shortens but myosin and actin filaments do not. The thickness of the Z-line remains unchanged.

Skeletal Muscle relaxants II

The ends of the actin filaments are anchored (attached) to the:

Explanationb The ends of the actin filaments are anchored to the Z-line or disk, whereas the M-line is in the centre of each sarcomere. The perimysium envelops a bundle of muscle fibres. The sarcoplasmic reticulum surrounds each myofibril.

Skeletal Muscle relaxants II

Which of the following is true about muscle structure?

Explanationc Six actin filaments surround each myosin filament, producing a hexagonal array. Muscle fibres can contain up to a few thousand myofibrils, which make up about 85% of the content of a muscle fibre. Myosin filaments are close to 3 times as thick as actin filaments

Skeletal Muscle relaxants II

Which is most likely to extend the entire length of a muscle fibre?

Explanationb Myofibrils extend the length of a muscle fibre, whereas many sarcomeres in series make up the length of a myofibril. Myosin filaments are contained within each sarcomere. The M-line is part of the cytoskeleton that stabilizes each sarcomere.

Skeletal Muscle relaxants II

A 79-year-old m an underwent surgery to remove a stomach cancer. Pertinent laboratory results before surgery were creatinine 3.5 m g/dL (norm al 0.6−1.2 m g/dL), alanine aminotransferase 25 U/L (norm al 8−20 U/L), urea nitrogen 65 mg/dL (norm al 7−18 m g/dL). Diazepam was given the night before surgery. The general anesthesia was induced by propofol, maintained by sevoflurane and nitrous oxide, and supplemented by cisatracurium . Which of the following was most likely the primary reason for the use of cisatracurium instead of tubocurarine in this patient?

ExplanationD The patient’s creatinine and urea nitrogen levels indicate that he was most likely suffering from renal insufficiency. Tubocurarine is mainly eliminated as such by the kidney and is therefore contraindicated in patients with impaired renal function. Cisatracurium is a neuromuscular blocking drug that is inactivated primarily by a form of spontaneous break- down and does not exhibit an increase in half-life in patients with renal insufficiency.

Skeletal Muscle relaxants II

A 62-year-old woman underwent surgery to remove an ovarian cancer. General anesthesia was induced by thiopental, maintained by sevoflurane and nitrous oxide, and supplemented by vecuronium , which is currently one of the most commonly used skeletal muscle relaxants during surgery. Which of the following is a primary advantage of vecuronium over tubocurarine?

ExplanationB Vecuronium , a steroid-derivative muscle relaxant, is currently com m only used during surgery. Its main advantages over tubocurarine are negligible effects on histamine release and no action on nicotinic neuronal receptors of autonomic ganglia. Moreover, unlike tubocurarine, the drug is mainly eliminated by liver metabolism , which can allow its use in patients with kidney disease.

Skeletal Muscle relaxants II

A 48-year-old m an presented to his physician complaining of intermittent limb muscle spasm s. The patient was referred to the neurologic clinic, w here the diagnosis of stiffman syndrome was made. A pharmacotherapy was prescribed to improve his muscle spasms. Which of the following drugs would be most appropriate for this patient?

ExplanationB Stiffman syndrome is a disorder of neuromuscular transmission characterized by the insidious onset of progressive stiffness in the trunk, abdomen, legs, and arm s. The cause of the syndrome is unknow n, but an autoimmune pathogenesis is suspected. Only symptomatic therapy is available, and central spasmolytic drugs (benzodiazepines and baclofen) consistently relieve the muscle stiffness.

Skeletal Muscle relaxants II

A 41-year-old m an suffering from amyotrophic lateral sclerosis presented to his physician with muscle fasciculations, limb spasticity, hyperactive deep tendon re exes, and extensor plantar re exes. Baclofen was prescribed to reduce spasticity and cram ps. Which of the following actions most likely mediated the therapeutic effect of the drug in the patient’s disease?

ExplanationA Baclofen is an agonist at GABAB receptors. These are metabotropic transmembrane receptors for GABA that are linked via Gi proteins to potassium channels. Activation of these receptors by baclofen results in increased K+ conductance, which in turn causes hyperpolarization of presynaptic terminals. This hyperpolarization reduces the release of excitatory transmitters in both the brain and the spinal cord. Baclofen is at least as effective as diazepam in reducing spasticity and is frequently used for this purpose.

Skeletal Muscle relaxants II

A 64-year-old woman complained to her physician of involuntary blinking and closing of the eyes. She noticed that the eyelid spasm was made worse by fatigue and anxiety. Further exam s led to the diagnosis of benign essential blepharospasm , and a treatment with local injections of botulinum toxin was prescribed. Which of the following adverse effects was most likely to occur in this patient?

ExplanationD Blepharospasm is spasm of muscles around the eye that causes involuntary blinking and eye closing. It affects women more than men and tends to occur within families. The cause is most often unknow n. Injection of botulinum toxin into the eyelid muscle is often the preferred treatment (the effects of each treatment last about 3 months). The most com m on adverse effect is eyelid ptosis (up to 20% of cases), which represents an unwanted extension of the pharmacological effect.

Skeletal Muscle relaxants II

A 34-year-old woman suffering from hemifacial spasm s started treatment with botulinum toxin injected directly into the abnormally contracting muscles. Which of the following molecular actions most likely mediated the therapeutic effect of the drug in the patient’s disorder?

ExplanationD Botulinum toxin must get inside the axon terminals to cause paralysis. Following the attachment of the toxin to proteins on the surface of axon terminals, the toxin can be taken into neurons by endocytosis. There it is able to cleave endocytotic vesicles and reach the cytoplasm . The light chain of the toxin has protease activity and proteolytically degrades a specific protein that is required for vesicle fusion that releases acetylcholine from the axon endings. In this way it prevents neurosecretory vesicles from fusing with the nerve synapse plasm a membrane and releasing the neuro- transmitter.

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