Quantitative synaptology of feline motoneurones to external anal sphincter muscle

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Abstract
Motoneurones innervating the cat external anal sphincter muscle were labelled retrogradely following intramuscular injections with horseradish peroxidase (HRP). Labelled motoneurones were examined by correlative light and electron microscopy (LM and EM) with special regard to a qualitative and morphometric analysis of the axon terminals resident on the neuronal membrane. By LM, labelled motoneurones were (1) ipsilateral to the injections; (2) all in S1‐S2; (3) found only in the superior dorsomedial region of Onuf's nucleus; and (4) exhibited a broad spectrum of diameters (25–72 μm, mean 47.4 ± 11.3 μm). By EM, axon terminals on the neuronal membrane when classified according to size, vesicle shape, and synaptic complex ultrastructure conformed to the S‐, F‐, T‐, M‐, and C‐type terminals previously described for cat lumbosacral motoneurones. C‐terminals confirmed these sphincteric motoneurones to be skeletomotor. Pooled data from midnuclear sections through 15 random labelled motoneurones (20–64‐μm diameter) revealed that S‐ and F‐type terminals predominated, with numerically few M and C types. Notwithstanding their low frequency (0.3/100 μm membrane) C‐terminals contributed 1% of the mean areal coverage by terminals, which implies a potentially larger synaptic influence relative to other terminal types. Linear relationships occurred between terminal frequency (or cover) and motoneurone diameter. While motoneurones >40μm in diameter exhibited all five terminal types, labelled motoneurones ≤30 μ generally possessed only S‐, F‐, and occasional T‐type terminals, and in this respect resembled gamma motoneurones.