Boy with periodic paralysisA 6-year-old boy is brought to the family physician after his parents noticed that he had difficulty moving his arms and legs after a soccer game. About 10 minutes after leaving the field, the boy became so weak that he could not stand for about 30 minutes. Questioning revealed that he had complained of weakness after eating bananas, had frequent muscle spasms, and occasionally Hand myotonia, which was expressed as difficulty in releasing his grip or difficultyopening his eyes after squinting into the sun. After a thorough physicalexamination, the boy was diagnosed with…………………………………..The family was advised to feed the boy carbohydrate-rich, low-potassium foods, give him glucose-containing drinks during attacks, and have him avoid strenuous exercise and fasting.Answer the following questions•What is the effect of hyperkalemia on cell membrane potential?•What is responsible for the repolarizing phase of an action potential?•What is the effect of prolonged depolarization on the skeletalmuscle Na+ channel?

Woman with jaw weaknessJill Rothman, a 26 year-old gymnastics instructor, presents with complaints of muscle weakness in her face that comes and goes, but has been getting worse over the past two months. Most notably, she complains that her “jaw gets tired” as she chews and that swallowing has become difficult. She also notes diplopia (“double vision”) which seems to come on late in the evening, particularly after reading for a few minutes. At work, it has become increasingly difficult to “spot” her gymnasts during acrobatic moves because of upper arm weakness.On physical examination, she has notable ptosis (“drooping”) of both eyelids after repeated blinking exercises. When smiling, she appears to be snarling. Electromyographic testing revealed progressive weakness and decreased amplitude of contraction of the distal arm muscles upon repeated mild shocks (5 shocks per second) of the ulnar and median nerves. (These nerves stimulate the flexor muscles of the hand and fingers.) Both her symptoms and electromyographic findings were reversed within 40 seconds of intravenous administration of endrophonium (Tensilon), an acetylcholinesterase inhibitor (i.e. an “anticholinesterase”). Blood testing revealed high levels of an anti-acetylcholine receptor antibody in her plasma, and a diagnosis of myasthenia gravis was made.Answer the following questions• What effect would an AChE inhibitor have at the neuromuscularjunction?2. How would a large reduction in extracellular [Ca2+] affect synaptictransmission at the neuromuscular junction?3.What is the ionic mechanism that underlies the endplate potential(EPP) produced by acetylcholine (ACh) release?

A 21-year-old man presents to a rural emergency center with a 1-day history of progressive stiffness of the neck and jaw, difficulty swallowing, stiff shoulders and back, and a rigid abdomen. Upon further questioning, the patient reports that the stiff jaw was the first symptom, followed by the stiff neck and dysphagia. On examination he is noted to have stiffness in the neck, shoulder, and arm muscles. He has a grimace on his face that he cannot stop voluntarily and an arched back from contracted back muscles. The physician concludes that the patient has “tetanic” skeletal muscle contractions. A 3-cm laceration is noted on his left foot. The patient reports sustaining the laceration about 7 days ago while he was plowing the fields on his farm. He has not had a tetanus booster. He is diagnosed with a tetanus infection, and an injection of the tetanus antitoxin is given.

◆ On which skeletal muscle filament is troponin located?

◆ What is the function of the sarcoplasmic reticulum (SR)?

◆ What is the molecular basis for initiation of contraction in skeletalmuscle?

An 8-year-old boy is brought to the emergency room after being stung by a bee. His mother noticed that he was playing in the backyard when he suffered the sting, and within minutes he began having trouble in breathing. She also noticed that he had a “hive” rash over most of his body, along with increased difficulty breathing. When the emergency medical service (EMS) arrived, they administered epinephrine subcutaneously, which seemed to relieve most of the symptoms. In the emergency center, the boy was diagnosed with an anaphylacticreaction from the bee sting.

◆ What type of smooth muscle (unitary versus multiunit) is present in the bronchi of the lungs?

◆ Why does smooth muscle not appear striated?

◆ What is the molecular basis for contraction in a smooth muscle?