1 Introduction -- 2 Materials and Methods -- 2.1. Animals -- 2.2 Electron Microscopy -- 2.3 Immunocytochemical Experiments -- 2.4 In Situ Hybridization Histochemistry -- 2.5. Peripheral Nerve Axotomy -- 2.6 Data and Image Analysis -- 3 Results -- 3.1 Normal Morphology of the Mesencephalic Trigeminal Nucleus -- 3.2 Neurochemistry of the Mesencephalic Trigeminal Nucleus -- 4 Discussion -- 4.1 Structural and Synaptic Organization of the Mesencephalic Trigeminal Nucleus -- 4.2 Connections of the Mesencephalic Trigeminal Nucleus -- 4.3 Neurochemistry of the Mesencephalic Trigeminal Nucleus -- 4.4 Neurochemical Plasticity of Mesencephalic Trigeminal Nucleus Neurons -- 4.5 Functional Considerations -- 4.6 Conclusions and Future Directions -- 5 Summary -- References