Localization, morphology and immunohistochemistry of the efferent and afferent neurons innervating the gastrocnemius and the flexor digitorum superficialis muscles in the cattle

Objective—To determine the location, morphology, and neurochemical code of spinal cord and dorsal root ganglion neurons that innervate the gastrocnemius muscle (GM) and superficial digital flexor muscle (FDSM) in cattle. Animals—5 healthy Friesian male calves. Procedure—2 different types of neuronal retrograde fluorescent tracers (fast blue and diamidino yellow) were injected into the GM and FDSM, respectively. The neurochemical code (substance P, calcitonin gene-related peptide, galanin, and neuronal nuclear protein) of labeled neurons was investigated by immunohistochemistry. Results—Neurons innervating the GM and FDSM were located along the L6-S2 spinal cord segments and ganglionic levels. A cranial-to-caudal topographic distribution for each muscle was found, indicating that the motor nuclei of the 2 muscles are organized by a somatotopic pattern. The GM and FDSM motoneurons were immunoreactive only for calcitonin gene-related peptide, whereas the afferent neurons were immunoreactive for all of the neurochemical markers considered. Conclusion and Clinical Relevance—In our study, location and the extent of neurons that supply the GM and FDSM of cattle were characterized completely. Because the GM and FDSM are involved in spastic paresis of calves and it is thought that spastic paresis results from an excessive activity of the neuromuscular spindle reflex arc, findings in our study may be useful for further electrophysiologic and clinical studies. Knowledge of the neurochemical code of neurons that supply the GM and FDSM in healthy calves could be used to compare chemical alterations in the same neuronal population of affected calves.