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This chapter summarizes what is known about the role of lateral line flow sensors in different types of fish orienting behaviors and the spatiotemporal characteristics of flow patterns that guide fish. Where possible, fundamental and shared principles of flow guidance are identified for behaviors as diverse as rheotaxis, prey orientation, and predator avoidance. Multisensory, egocentric direction maps in the midbrain optic tectum are thought to underlie oriented movements toward discrete targets of interest (e.g., prey), whereas differential biasing of bilaterally paired Mauthner cells by multisensory inputs are involved in fast-start oriented behaviors such as predator avoidance. Wide-field integration of global flow fields inspired by optic flow studies in flies is suggested as a central mechanism by which cells are tuned to expected flow patterns over large body regions during common motion states, such as forward translational movement. Disruptions in those patterns caused by, e.g., the destabilizing influences of currents could then be used as part of closed-loop feedback control for corrective actions. Flow guidance of behaviors is likely to be best understood when viewed from a multisensory, evolutionary perspective in which nonvisual and visual systems work in tandem to guide behavior.

The first unambiguous demonstration of enhanced spinal cord nerve regeneration in the living animal was carried out in a primitive fish, the ammocoete larvae of the lamprey (Borgens et al. 1981). This investigation utilized the special characteristics of the lamprey brain and spinal cord. In this animal, individually identifiable giant nerve cells in the brain (Muller and Mauthner cells) give rise to descending nerve fibers that are also identifiable in the spinal cord as part of the descending reticulospinal system (Rovainen 1967, 1974). Since the lamprey is an Agnathan, a primitive cartilaginous fish, there is no bony vertebral column surrounding and protecting the spinal cord: the cord and brain is easy for the investigator to surgically expose. Therefore, the fish system provides some of the same advantages as nerve fibers in culture where the investigator can describe the regeneration or growth of specific individual and identifiable nerve cells. The cell bodies of lamprey reticulospinal neurons are giant (~100 µm in diameter) and so are their spinal cord axons (~30 µm), making them easy to fill with intracellular dyes from a micropipette and to identify at any location within the brain and spinal cord (Wood and Cohen 1979; Borgens 1981) (Fig. 20).

It has been already noted in Chapter 12 that recording of UPSPs is preferable for a more precise quantal analysis. However, the recording of UPSPs is not practical for studies of long-lasting events in neurones of vertebrate CNS. In fact, simultaneous recordings from two hippocampal neurones in our in vivo experiments [90] revealed UPSPs in only few cases (Sect. 12.3). Even under more stable conditions of recordings from hippocampal slices, the success rate of UPSPs from CA3-CA1 pairs was found to be very low [351, 864, 865]. In addition, several other problems are not resolved for CNS neurones (Chaps. 7, 11). I mention here the high S„ and also small sample sizes (N) because of time limitations of intracellular recordings from healthy cells and necessity of “stationarity” for reliable statistical analysis. The limitation of N might be illustrated by our in vivo studies (Chaps. 11 and 12), as well as by studies of other authors. An example is the publication on unitary IPSPs in the Mauthner cell [528] with the average number of counts equal to 161 and with

Publisher Summary This chapter focuses on inhibitory synapses operating by electrical transmission. In most species of fish, the two large Mauthner cells (M-cells) are the most conspicuous neurones in the brain. The superimposed traces show that the brief anelectrotonus of the axon hillock blocks its invasion by the four concurrent antidromic impulses but not the antidromic impulses immediately before or after. The EHP has almost the same amplitude throughout the axon cap, so the hyperpolarizing current applied to the initial segment of the M-axon must be generated by nerve fibers in the axon cap, which are presumably activated by the axon collaterals of the M-cells. Stimulation of the ipsilateral or contralateral eighth nerve also produces a brief extracellular positive wave that resembles the EHP in that it is of uniform size throughout the axon cap. Structures resembling the helicoidal feltwork of the axon cap also exist around axon hillocks of other nerve cells, so it seems likely that there may be other examples of this electrical inhibitory mechanism.