Neuronal loss after transsection of the facial nerve: a morphological and neurophysiological study in monkeys.

Functional recovery after nerve lesions seems to depend on peripheral as well as central factors. To investigate the central neuronal loss after transsection of a pure motor nerve, the middle branch of the facial nerve on one side was transsected and immediately repaired microsurgically by epineural suturing. After a period of 6-15 months, a quantitative neurophysiological recording was made to estimate muscle response. A nerve tracer was injected into the mimic muscles innervated by the nerve to label the surviving motor neurons within the facial nucleus. The opposite side was used as the control in all cases. After the regenerative period, a mean loss of 15% of the total cell number was observed within the facial nucleus compared with the opposite side. The cell loss comprised all types of neurons. This amount of neuronal loss was followed by an even greater loss of muscle response when a quantitative neurophysiological recording was made after nerve regeneration. The results are discussed in relation to loss of nerve elements after nerve lesions and its effect on functional recovery.