Single axon terminal from topographic nucleus isthmi

Below is a drawing of an axon terminal in the turtle tectum that arises from a cholinergic neuron in the topographic nucleus isthmi (=parabigeminal nucleus in mammals) in a turtle. The over 3500 terminal boutons shown are distributed in the superficial and intermediate layers of the ipsilateral tectum. The boutons are all restricted to a very small portion of the area of the tectum (see inset).

By contrast, the non-topographic rostral nucleus isthmi synapses diffusely over a much larger area of the tectum (not shown). See Sereno and Ulinski (1987) for (many more!) details.

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