Nerve regeneration in the peripheral nervous system versus the central nervous system and the relevance to speech and hearing after nerve injuries

Schwann cells normally form myelin sheaths around axons in the peripheral nervous system (PNS) and support nerve regeneration after nerve injury. In contrast, nerve regeneration in the central nervous system (CNS) is not supported by the myelinating cells known as oligodendrocytes. We have found that: 1) low frequency electrical stimulation can be used to elevate cAMP thereby promoting regeneration of CNS axons and 2) a conditioning lesion, created by a crush of the peripheral branch of the dorsal root ganglion sensory neurons along with a simultaneous cut of these axons in the CNS, promotes even greater neural outgrowth than electrical stimulation. The effectiveness of the lesion results from both an acceleration of axon outgrowth and an increase in the rate of axon growth. However, electrical stimulation remains a more viable treatment of nerve injuries to stimulate regeneration and has been successfully used to promote development of the auditory pathways in children with severe to profound deafness who use cochlear implants. Without nerve regeneration, there is only a random reinnervation of affected muscles. An example occurs when the laryngeal nerve attempts to reinnervate the vocal cords after injury, causing deficits in speech. Synkinesis occurs when reinnervation of antagonistic muscles effectively paralyze the vocal cords and, in turn, severely compromises speech. The misdirection of laryngeal nerve reinnervation can be alleviated surgically by strategies favoring inspiratory abduction. Readers of this article will gain an understanding of (1) the potential for axon regeneration in the central nervous system and (2) problems and possible solutions for random reinnervation of laryngeal muscles for speech.