Motoneuronal and motor axonal innervation in the rat hindlimb: a comparative study using horseradish peroxidase

In order to validate the horseradish peroxidase (HRP) technique as a quantitative method for assessing neuronal pools, we compared counts of labeled motoneurons to numbers of corresponding motor axons. Rat spinal motoneurons were labeled by immersing in HRP either the L4 radicular nerve or the normal or deafferented nerve to the tibialis anterior muscle (TA). The technique slightly under-valued the L4 motoneuronal population estimated by counting ventral root motor fibres, while in deafferented TA, counts of labeled neurons closely reflected numbers of motor axons. Measurements of neuron and nerve fibre diameters revealed good correlations of alpha and gamma motor estimates at spinal cord and nerve levels. Values were considered representative because of the approximation obtained in HRP-estimates between control and deafferented animals, indicating no significant damage from the surgical deafferentation.