Demonstration of functionally ineffective synapses in the guinea pig spinal cord

Abstract

Listen

Translate article

Hemisection of the spinal cord rostral to the nucleus of the phrenic nerve results in a permanent paralysis of the ipsilateral hemidiaphragm. In several mammalian species, functional recovery of the paretic hemidiaphragm can be achieved by transecting the contralateral phrenic nerve. The phrenicotomy results in an increased medullary respiratory output which activates a functionally latent, crossed medullospinal respiratory pathway. The nerve fibers of the crossed pathway trasmit respiratory impulses across the spinal cord at the level of the phrenic nuclei, thereby generating action potetentials in the phrenic neurons ipsilateral to the paretic hemidiaphragm. This response has been termed the “crossed phrenic phenomenon.” Previous studies in guinea pigs showed that the crossed phrenic phenomenon did not occur if the spinal hemisection was followed immediately by contralateral phrenicotomy, but it did occur if several months elapsed between the two operations. In the present experiment we sought to determine the shortest time interval that would permit development of a crossed phrenic phenomenon in order to decide whether functional recovery in guinea pigs results from the establishment of a new synaptic pathway by collateral sprouting or from the activation of a previously ineffective pathway. The former would require several weeks to occur, whereas the latter might be expected to occur within hours or days. In the present study the crossed phrenic phenomenon occurred within 3.5 h after spinal hemisection. We therefore conclude that, within several hours after spinal hemisection, functionally ineffective synapses become converted to functionally latent ones. The stress of contralateral phrenicotomy is sufficient to activate this functionally latent pathway. The mechanism by which the ineffective pathway is transformed to a latent one is unknown.