Somatotopic Changes of the Stylohyoid Muscle Subnucleus After Section and Repair of the Facial Nerve

Abstract

Peripheral nerve injury induces a series of changes along the circuit levels of the involved nerve, from the cortex to the periphery. Functional regeneration should be influenced by these changes. In the present work, the rat facial nerve was sectioned and repaired at the stylomastoid for-amen. From 3 to 21 monts after surgery, the somatotopic rearrangement of the motor facial nucleus was studied. Horseradish peroxidase (HRP) was injected in the isolated stylohyoid muscle to retrogradely label the parent motoneuron pool in the brain stem. The most important findings can be summarized as follows: (1) the volume of the subnucleus was markedly increased after regeneration because of the more dispersed motoneuron pattern as compared to the intact controls; (2) more than 80% of the HRP-labeled motoneurons were located outside the borders of the control stylohyoid subnucleus overlying the other subnucleus either in the main nucleus of the facial nerve and dorsally to the main nucleus or outside the borders of the accessory nucleus of the facial nerve; (3) the stylohyoid muscle subnucleus showed a bilateral somatotopic representation in 50% of the operated animals because of HRP-labeled motoneurons in the contralateral facial nerve nucleus; (4) the number of HRP-labeled motoneurons was increased twofold with respect to the intact controls; (5) the mean soma diameter of motoneurons was similar in both the operated and intact control rats. This study provides original information on the plastic changes which occur in the stylohyoid muscle subucleus following regeneration of the facial nerve. These data provide the basis for comparative anatomofunctional studies.