After a busy day with posters and organizing a social yesterday, I now have finally found some time to blog from the 9th German Neuroscience Meeting in Göttingen.
This plenary lecture was about insect acoustic communication, i.e. crickets, grasshoppers, cicadas and so on. Berthold first briefly recapitulated the anatomy and physiology about sound production and perception in the different insect groups. He then showed typical examples of how this communication is used by the animals, e.g., during courtship. There, he focused on crickets, showing how male calling song attracts females which approach the males which then, in turn, switches from calling song to courtship song.
The calling song in these crickets can be elicited by extracellularly stimulating the mushroom-body alpha-lobes. In this region sits a command neuron which controls singing behavior: it is sufficient and necessary for singing in crickets, which has been shown by intrecellular electrophysiology of this neuron.
After this command neuron was found, the researchers set out to find the central pattern generatr (CPG) controlling the singing movements. Initially, the CPG was localized to the last two thoracic and the first abdominal ganglion of the cricket ventral nerve chord. Intracellular electrophysiology revealed that only very few neurons in these ganglia are related to singing. For instance, there is the Metathoricic Opener interneuron, which fires in synch with the singing movements of the wings. The Abdominal Opener neuron can reset the singing pattern, meaning that it is critical for the singing rhythm.One of the next research areas planned to investigte is to use these neurons comparatively in different singing species to investigate their role in the evolution of song.
In the next section of the talk, Berthold Hedwig told us about impact of self-generated sound on the singing cricket itself. This was probably the work for which he is most well known, at least from my perspective. He showed that singing crickets protect their own hearing system from desensitisation during their own song by inhibiting sound-sensitive neurons during song using a corollary discharge (or eference copy). Some of this work involved the famous 'omega' neuron, which is named after the peculiar shape it has in the auditory neuropil. This neuron is inhibited precisely at the times when the cricket is producing the sound. This inhibition is mediated by the 'corollary discharge neuron' which connects the CPG with the auditory interneurons in the prothoracic ganglion by making direct inhibitory monosynaptic connections with the Omega neuron. The result of this corollary discharge is that the sensitivity of the auditory system is maintained even during very loud self-generated song.
Next was a section on female crickets, i.e., how they recognize the male song and then show positive phonotaxis towards the source of the song. He described how already the anatomy of the tympanal system is organized to make the animal highly direction-sensitive with regard to sound, i.e., a sound from one side of the animal results in large ipsilateral/contralateral volume differences within the tympanal canals. This leads to bilateral differences in tympanic membrane oscillations which are proportional to the direction from which the sound is coming. Manipulating different physical parameters of the male song and then recording the female response to these manipulated song, the researchers showed that females are tuned to the pulse period of the male song. How is the pulse period detected ? The scientists have found neurons in the female brain which fire only when the 'correct' pulse width is being played back to the femae. They sit in the 'ring-ike' neurpil, bilaterlly in the brain. The tuning curve of these local interneurons fits the tuning curve of the phonotactic behavior very well.