see if the worm still stopped feeding and started
spitting in response to light. Killing four different
classes of neurons caused a defect in the behavioral response to light. These neurons ended up defining three distinct neural circuits that can modulate
and alter the function of the pharynx.
Q:Could you briefly explain how you set up your experiment, such as how you recorded the
A: We studied the worm’s feeding behavior by watching the worm under a dissecting microscope, which magnified the worm 120x. Since
the worm is transparent, you can see right into the pharynx and observe a
rhythmic “chewing” behavior as the pharynx grinds up bacteria, which is
the worm’s food. Many researchers have studied the worm’s feeding in just
this way. We threw in a new twist, though, as we also exposed the worm
to bright violet light while it was feeding. Throughout the experiment, we
scored the “chewing” or “pumping” behavior by eye and recorded the data
on a computer.
Since the worm feeds so quickly–five times per second–we suspected
that our eyes might miss a crucial detail that happened too fast for us to
see. So, we increased the magnification to 200x and attached a very fast
camera to the microscope. Standard videos, like those on You Tube, are recorded at 30 frames per second, but our camera could record much faster,
at 1,000 frames per second. This meant that we could record videos and
replay them at a slower speed to see whether we had missed anything. This
technical advance in recording turned out to be very auspicious.
Q:What information did these recordings reveal?
A: Although the C. elegans worm lacks eyes, the worms show a striking response to light. The worm immediately stops feeding when exposed
to light, and then follows up with a “burst” of pumping that we initially interpreted to be a paradoxical increase in feeding. What was extra interesting
was that occasionally the worm blew bubbles during the burst. When we
looked more closely using the camera, it turned out that the worm was actually spitting instead of swallowing.
Neither blowing bubbles nor spitting had ever been reported before, so
this was a delightful surprise. How could light cause such a dramatic reversal in function of the pharyngeal pump? And what might this mean for other
muscular pumps, such as the heart?
Q:How do these results translate to useful information about the human heart?
A: The C. elegans pharynx is strikingly similar to the human heart. Both organs function primarily as nutritive pumps, with one pumping food itself while the other pumps blood, which is essentially food for cells distributed throughout the body. Both organs also share common molecular features
during their development.
Very little is known about the nervous system of the heart, so we looked to
the worm pharynx to learn something about the nervous systems of muscular organs more generally. Since the worm pharynx is like the heart, neural
circuits similar to those we identified might also function to control the heart.
For example, we found three separate neural circuits that control the pharynx. One neural circuit is the simplest that has ever been described, where
a single pair of sensorimotor neurons both detect the light and directly block
muscle contractions, with no other neurons in between. A second neural
circuit likely transmits a signal received outside the pharynx into the pharynx
to stop feeding. This polysynaptic circuit is interesting because it is structurally reminiscent of how the autonomic nervous system controls target organ
function, such as heart rate. How the autonomic system works also remains
cryptic in mammals, so studies of this simplified neural circuit in worms
might provide key insights into the autonomic system.
Finally, the last neural circuit that we discovered transforms swallowing