Electrophysiology
The mechanoreceptive hair cells of the lateral line structure are integrated into more complex circuits through their afferent and efferent connections. The synapses that directly participate in the transduction of mechanical information are excitatory afferent connections that utilize glutamate. However, a variety of different neuromast and afferent connections are possible, resulting in variation in mechanoreceptive properties. For instance, a series of experiments on the superficial neuromasts of Porichthys notatus revealed that neuromasts can exhibit a receptive specificity for particular frequencies of stimulation. Using an immobilized fish to prevent extraneous stimulation, a metal ball was vibrated at different frequencies. Utilizing single cell measurements with a microelectrode, responses were recorded and used to construct tuning curves, which revealed frequency preferences and two main afferent nerve types. One variety is attuned to collect mechanoreceptive information about acceleration, responding to stimulation frequencies between 30–200 Hz. The other type is sensitive to velocity information and is most receptive to stimulation below <30 Hz. This suggests a more intricate model of reception than was previously considered.
The efferent synapses to hair cells are inhibitory and utilize acetylcholine. They are crucial participants in a corollary discharge system designed to limit self-generated interference. When a fish moves, it creates disturbances in the water that could be detected by the lateral line system, potentially interfering with the detection of other biologically relevant signals. To prevent this, an afferent signal is sent to the hair cell upon motor action, resulting in inhibition which counteracts the excitation resulting from reception of the self-generated stimulation. This allows the fish to retain perception of motion stimuli without interference created by its own movements.
After signals transduced from the hair cells are transmitted along lateral neurons, they eventually reach the brain. Visualization methods have revealed that the area where these signals most often terminate is the medial octavolateralis nucleus (MON). It is likely that the MON plays an important role in the processing and integrating mechanoreceptive information. This has been supported through other experiments, such as the use of Golgi staining and microscopy by New & Coombs to demonstrate the presence of distinct cell layers within the MON. Distinct layers of basilar and non-basilar crest cells were identified within the deep MON. Drawing a comparison to similar cells in the closely related etectrosensory lateral line lobe of electric fish, it seems to suggest possible computational pathways of the MON. The MON is likely involved in the integration of sophisticated excitatory and inhibitory parallel circuits in order to interpret mechanoreceptive information.
Read more about this topic: Lateral Line