The exterolateral nucleus occupies an extremely-lateral position in the midbrain. Left view: normal view of the brain of Brienomyrus brachyistius with the cerebellum intact. Right view: with the valvula of the cerebellum removed to show the underlying midbrain structures.
The axon from NELL ascends the lateral lemniscus, divides and sends fibers to the ipsilateral side and to the contrallateral side, and then terminates on two types of cells in ELa.
The diagram below shows the synaptic terminals of a single afferent fiber. Green terminals are on LARGE cell bodies that are intrinsic to the ELa. RED terminals are on small cells, which are the output cells of ELa. Each fiber terminates first on one or two large cells, then travels a long distance before terminating on many small cells.
The following is a diagram of one pattern of one large cell and one NELL axon. The large black spheres are the large cells in ELa. These cells are known to be loaded with GABA, an inhibitory transmitter. All of the large cell terminals are on small cell cell bodies.
The NELL afferent terminates on one or two large cells as it first enters ELa, then it travels a distance of 4 to 6 mm before terminating on many small cells. The terminals (green) are excitatory.
A stimulus such as an EOD or a square-wave mimick of the EOD, when applied in a transverse geometry, excites one side of the body on the rising phase, while exciting the opposite side of the body on the falling phase. In this example, the right side of the fish is stimulated when the right outside of the fish goes positive. For a given "small cell" in ELa, which is inhibited by large cell input which derives from stimulation of the right side, while excited by delayed NELL input from the left side, the small cell may be expected to be silent for most stimulus durations except those for which the delay line is the same duration as the duration of the pulse (see timing diagram below).
Friedman, M. A. and Hopkins, C. D. (in press). Neural substrates for species recognition in the time-coding electrosensory patheway of mormyrid electric fish. J. Neuroscience .
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