Scientists have long known that the soft surface of the bill of the platypus is perforated with openings that contain sensitive nerve endings. Only recently, however, have biologists concluded on the basis of new evidence that the animal uses its bill to locate its prey while underwater, a conclusion suggested by the fact that the animal's eyes, ears, and nostrils are sealed when it is submerged. The new evidence comes from neurophysiological studies, which have recently revealed that within the pores on the bill there are two kinds of sensory receptors: mechanoreceptors, which are tiny pushrods that respond to tactile pressure, and electroreceptors, which respond to weak electrical fields. Having discovered that tactile stimulation of the pushrods sends nerve impulses to the brain, where they evoke an electric potential over an area of the neocortex much larger than the one stimulated by input from the limbs, eyes, and ears, Bohringer concluded that the bill must be the primary sensory organ for the platypus. Her finding-was supported by studies showing that the bill is extraordinarily sensitive to tactile stimulation: stimulation with a fine glass stylus sent a signal by way of the fifth cranial nerve to the neocortex and from there to the motor cortex. Presumably nerve impulses from the motor cortex then induced a snapping movement of the bill. But Bohringer's investigations did not explain how the animal locates its prey at a distance.
Scheich's neurophysiological studies contribute to solving this mystery. His initial work showed that when a platypus feeds, it swims along, steadily wagging its bill from side to side until prey is encountered. It thereupon switches to searching behavior, characterized by erratic movements of the bill over a small area at the bottom of a body of water, which is followed by homing in on the object and seizing it. In order to determine how the animal senses prey and then distinguishes it from other objects on the bottom, Scheich hypothesized that a sensory system based on electroreception similar to that found in sharks might exist in the platypus.
In further experiments he found he could trigger the switch from patrolling to searching behavior in the platypus by creating a dipole electric field in the water with the aid of a small 1.5-volt battery. The platypus, sensitive to the weak electric current that was created, rapidly oriented toward the battery at a distance of 10 centimeters and sometimes as much as 30 centimeters. Once the battery was detected, the platypus would inevitably attack it as if it were food. Scheich then discovered that the tail flicks of freshwater shrimp, a common prey of the platypus, also produce weak electric fields and elicit an identical response. Scheich and his colleagues believe that it is reasonable to assume that all the invertebrates on which the platypus feed must produce electric fields.
During the studies supporting Bohringer's finding, as they are described in the passage, which one of the following occurred before a nerve impulse reached the motor cortex of the platypus?
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