Ultra structure of pre‐synaptic input to motor neurons in Onuf's nucleus: controls and motor neuron disease

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Abstract
Ultrastructural analyses of sphincter motoneurons in Onuf's nucleus at S2 were undertaken in human spinal cord obtained 3–6 h post‐mortem from three subjects with no neurological disease (‘controls’) and five in which death was due to motor neuron disease (MND). Neurons in specified locations within Onuf's nucleus of control subjects ranged between 17.8 and 71.7 pm diameter (mean 38.6 pm). Analyses of synaptology revealed five ultrastructural classes of pre‐synaptic terminal synapsing with the neuronal surface membrane. When classified by size, vesicle morphology, and synaptic site structure these conformed to the S, F, T, M and C‐terminals defining somatic motoneurons. No terminals characteristic of autonomic motoneurons were found. In MND subjects, neurons in Onuf's nucleus at S2 were preserved despite a paucity of neurons in medial and lateral motor nuclei and were of similar size range to those in control subjects. The morphological classes of pre‐synaptic terminal found in controls, also characterized sphincteric motoneurons in MND subjects, including the C‐type terminal. The presence of C‐terminals indicates (i) that sphincteric motoneurons are somatic alpha‐motoneurons, and (ii) that hypotheses explaining the survival of sphincteric motoneurons in MND on the basis of Onuf's nucleus being an extension of the pre‐ganglionic parasympathetic nucleus, or having intrinsic autonomic properties are incorrect.