Q1. What are the contributions mentioned in the paper "Anatomical and electrophysiological analysis of the trigeminal nerve in a teleost fish, oncorhynchus mykiss" ?
The trigeminal nerve in the rainbow trout, Oncorhynchus mykiss, was examined for the presence of Adelta and C fibres. Trigeminal sensory afferents convey tactile and thermal information from the skin but also provide information about tissue damaging ( noxious ) stimuli. The trigeminal somatosensory system has not been systematically studied in fish especially in relation to nociception. The present study was carried out on the trigeminal nerve of the rainbow trout, Oncorhynchus mykiss, to determine if A-delta and C fibres are present that innervate the skin of the head and could potentially act as nociceptors. The objectives of this study are firstly to examine the trigeminal afferent nerves to determine whether A-delta and C fibres are present in each of the trigeminal branches of the fish, secondly to determine the size of the cell bodies of these nerves which are present in the trigeminal ( Gasserian ) ganglion, and thirdly to confirm electrophysiologically A-delta and C fibre presence. Few C fibres were present and therefore it was decided that a full electron microscopy study would not be necessary. A-beta fibres were the most common ( 53 % ) followed by A-delta ( 33 % ) and Aalpha ( 9 % ) fibres. Previous studies in fish have shown that the fish have free nerve endings beneath the epithelium [ 3 ] and the present study on the trout has demonstrated that the trigeminal nerve has both C and A-delta fibres. The trigeminal nerve projects to the relevant brain areas, the thalamus, cerebellum and medulla, which are involved in pain or nociceptive processing in higher vertebrates. Nociceptors are slowly adapting polymodal mechanoreceptors [ 8 ] and these receptor types were found on the head and face of the rainbow trout in this study. L. U. S. is grateful to Mike Gentle for his comments on this manuscript, Louise Dunn, Graeme Robertson, Sandra Wilson, Laura Dick and Linton Brown ( Institute of Aquaculture, Stirling ) for technical assistance, and to BBSRC ( Grant No. 215/S11042 ) for funding. [ 1 ] The evolutionary significance of these results is discussed as well as the potential for nociceptive capability in a lower vertebrate. The ability to detect and react to potentially noxious stimuli is crucial to survival. Studies in elasmobranchs have suggested a general lack of C fibres although A-delta fibres have been found as well as neurotransmitters in the dorsal horn [ 5,10 ] which are known to be involved in pain mechanisms in higher vertebrates. They were placed into the glutaraldehyde fixative for a further hour and left overnight in 0. Glass insulated tungsten microelectrodes with a tip diameter of 10 mm were used to record extracellular action potentials from the cell bodies in the ganglion. Spontaneous and evoked activity were recorded and the durations of action potentials were calculated to determine what kind of fibres were present. The number of action potentials per fish ranged from one to 12 different units as characterized by Spike 2 waveform analysis ( Fig. 3 ). The duration of the action potentials ranged from 0. 23 to 2. 27 ms with the majority in the range of 0. 67–1. 90 ms. There were fewer fast action potentials ( < 1 ms ) and only three types were greater than 2 ms. Fig. 3. Action potentials recorded from four different units of the trigeminal nerve of the rainbow trout. ( A ) Fast adapting mechanoreceptor with an action potential duration of 0. 8 ms. ( B ) Slowly adapting mechanoreceptor with an action potential duration of 1. 9 ms. The action potential duration was 1. 9 ms. The action potential duration was 2. 0 ms. In the stingray only 0. 7–1. 2 % of fibres were C fibres [ 10 ] and other evidence has suggested a lack of C fibres in other elasmobranch species [ 5 ]. Leonard [ 5 ] suggested that nociception was not important in the elasmobranchs, however, they do posses the relevant neuromodulators associated with nociception ( e. g. enkephalin activity in the spinal cord [ 11 ] ; substance P in the brain [ 13 ] ) and they also possess A-delta fibres. There were a large number of A-delta fibres in the trigeminal nerve of the rainbow trout, therefore, they have the potential for nociceptive capabilities and in the rainbow trout enkephalin activity has been found in regions of the central nervous system particularly associated with pain processing [ 13 ]. It is conceivable that it would be adaptive for the animal to be able to perceive these potentially injurious stimuli and react appropriately. The physiological recordings demonstrate that there were action potentials of slow duration that could potentially be A-delta and C fibres in the rainbow trout trigeminal nerve. Therefore, the physiological recordings mirror the anatomical findings and show that there are fibres that could potentially act as nociceptive nerves. Further work should involve single unit recording to further characterize the type of receptors present on the face and mouth of the fish and to determine if they preferentially respond to noxious, potentially painful stimuli.