Question

A 30-year-old woman, Isabel Yang, was brought into the emegency room. she had been dining in a restaurant, and while eating dessert she noticed the following symptoms. initially there was a tingling sensation that affected her mouth and lips, but then spread to the faced neck. then the tingling spread down her arms and legs, to the fingers and toes.

at the hospital Isabel reported numbness of the areas that previously tingled and had difficulty walking in a coordinated fashion. she was asked to describe the meal she had just eaten and stated that she had shrimp cocktail as and appetizer, followed b salad, steak, and baked potatoe and green beans, and apple pie and coffee for dessert. Isabel had no history of allergic response to shellfish. her superficial reflexes were almost absent, and her deep reflexes were markedly hypoactive. an extracellular electrode was placed in Isabel's ulnar nerve. then the palmer surface of her little finger was scraped with the physician's fingernail in a way that would normally be painful to the patient. the patient could not feel this stimulus, and no action potentials were detected in the ulnar nerve. when an intracellular microelectrode was placed on a sensory nerve fiber in the ulnar nerve, the resting membrane potential was found to be near -70 mV (normal). when an action potential was evoked by repeated vigorous scraping of the skin of the little finger as described earlier, the action potential was slower to rise and of shorter amplitude than that expected from measurements in normal individuals. the duration of the action potential was normal.

1. what can be concluded from the failure to elicit action potential in the ulnar nerve by vigorous scraping of the skin of the little finger

2. what can be concluded from the gross neurologic symptoms and findings taken together

3. what can be concluded from the findings that the resting membrane potential in the sensory fibers in the ulnar nerve is near normal.

4. what might explain the finding that the action potential in the sensory fiber in the ulnar nerve is slow to rise and of smaller amplitude than normal.

5. why is the action potential in the sensory fiber of normal duration

6. what may be the cause of Isabel's difficulties

7. how should Isabel's care be managed, and what is her prognosis

Answer 1

1.Isabel seems to have encountered a saxitoxin poisoning along with a certain malfunction of the sensory system of the skin. The compound saxitoxin is present in prawns. The skin sensory receptors which are involved in the in the conduction of action potentials in the sensory nerve fibers has been affected.

2. Tingling and numbness has occured because of a aberration in the functioning of the cutaneous sensory system. The absence of superficial reflexes and the hypoactivity of deep reflexes indicate a larger issue of motor output and input failure from the central nervous system. Difficulty in walking is owing to the depressed conduction of action potential in the nervous system.

3.Normal resting potential shows that the resting K+ conductance of the sensory nerve fiber which determines the resting potential is normal. An aberration in the K+ conductance will also chane the membrane potential and also affect the conductance of Na+.

4. This condition of Isabel indicates a defect in the sodium ion channel. If a lesser number of sodium ions are involved in conducting the action potential then it would lead to a slower rate of rise and a smaller amplitude of the action potential.

5.The delayed opening of K+ channels and the voltage-inactivation of the Na+ channels are the major determinants of the duration of the action potential. The finding that the action potential is of approximately normal duration suggests that there is nothing abnormal about the kinetics of opening of the K+ channels and that the voltage dependence and time dependence of the inactivation of Na+ channels are not abnormal. This, together with the reduced rate of rise and amplitude of the action potential, suggests that those Na+ channels that are activated are functioning normally, but that a fraction of the Na+ channels fails to open in response to membrane depolarization.

6. Isabel showed symptoms within half an hour. Other than that she also showed some neurologic defects. These combined together reek of paralytic shellfish poisoning. The toxin saxitoxin blocks the opening of voltage-gated Na+ channels leading to the current situation.

7. Although paralytic shellfish poisoning is often fatal, it might also cause respiratory muscle paralysis. If Isabel doesn't experience it within half a day, she should be monitored closely and would require to be on a ventilator in case of respiratory distress. Her recovery might taken a day or two in case she is lucky. As saxitoxin strongly binds to the sodium ion chanel and disengages it original function of conductance, it takes a day to be ecreted from the system.

 it takes a day to be ecreted from the system.