Trying to get to sleep on a sweltering Mediterranean summer's night is difficult enough at the best of times, especially when you are being serenaded by a horde of noisy insects. Male crickets court females by rubbing their forewings together to produce a song composed of a series of chirps. Each chirp is made up of syllables that are produced when the cricket closes his wings, producing a racket as loud as a pneumatic drill!
The cricket therefore faces a challenging problem - how can it prevent itself going deaf from singing so loudly, yet still be able to hear what is going on around it? James Poulet and Berthold Hedwig at the University of Cambridge have discovered the clever mechanism the crickets use to stop deafening themselves; unfortunately, it is not as simple as sticking fingers in their ears! They found that a male cricket's nervous system dampens down the response to self-generated sounds, so he can concentrate on listening to his rivals whilst remaining alert to the presence of hungry predators.
To find out what happens when male crickets sing, Poulet and Hedwig recorded the activity of auditory neurons. The first type of neuron is called an auditory afferent; there are about 60 of these that fire action potentials every time a sound reaches the ear. The second type of neuron is called the omega 1 neuron (ON1). This acts as a link between the auditory afferents and the rest of the nervous system. Both types of neuron also form connections to other neurons about which very little is known.
In the first set of experiments, the team found that both types of neuron responded to self-generated sounds. When they recorded from one-winged silently singing crickets, the auditory neurons did not respond. This showed that the auditory neurons respond to the sound and are not being influenced by something else, such as wing movements.
However, during these experiments the team made an exciting discovery. Inhibitory signals were seen in the auditory neurons that were in phase with the silent syllables in each chirp and coincided with the part of the song that should have been the noisiest. These signals usually coincide with sound-generated signals, and the team were surprised that these signals are sent to both types of auditory neuron, acting on them in subtle but different ways. The signal from the auditory afferent to ON1 and the rest of the nervous system is dampened, whilst the number of spikes produced in ON1 is reduced,softening the cricket's voice in its ear.
Finally, Poulet and Hedwig were able to show that the inhibitory signals come from neurons somewhere else in the nervous system. They are very keen to track down these inhibitory neurons and find out how they work, so the cricket can serenade his mates and keep half an ear on what his rivals are up to without listening too much to the sound of his own voice.
