Localization and neurophysiological properties of motoneurones of the M. triceps surae of the rat after retrograde labelling with Evans blue.

Evans blue was injected into either the medial and lateral gastrocnemius or the soleus muscle of adult albino rats to mark retrogradely the corresponding motoneurones in the spinal cord. The labelled motoneurones were identified by the red fluorescence of their perikarya in the ventral horn of segments L4--L6. In addition, a monosynaptic reflex action potential was recorded only in the ventral roots L4--L6 after stimulation of the nerves to the medial and lateral gastrocnemius and the soleus muscle. Excitability and reflex latencies of labelled and unlabelled motoneurones of segment L4--L6 as well as the conduction velocities of their axons were measured. Apart from a small but statistically nonsignificant increase in excitability, no functional differences were found between labelled and control neurones. Thus, retrograde labelling of motoneurones with Evans blue prior to performing electrophysiological experiments has the advantage that the marked motoneurones can be identified under the fluorescence microscope without the need of additional staining or fixation.