The role of neural connections crossed at the cervical level in determining rhythm and amplitude of respiration in cats and rabbits.

The part played by crossed cervical pathways in determining respiratory rhythm and phrenic nerve amplitude was studied in rabbits and cats. The animals were anesthetized with halothane, paralyzed, vagotomized and mechanically ventilated. All decussating brain stem pathways were surgically interrupted. This resulted in asynchronous firing of the respiratory nerves on the left and right side of the body. However, both frequency and amplitude of the respiratory output depended on the mutual phase relationships due to neural connections crossed at the cervical level. Analysis of the mutual phase and amplitude relationships lead to the following conclusions. a. Phrenic motor neurons receive during inspiration a considerable amount of excitation via diffused pathways crossing the midline at C4-C6 level. b. Phrenic motor neurons are actively inhibited via crossed cervical pathways. c. The level of tonic bias transmitted via descending bulbo-spinal pathways significantly influences the magnitude of PhN output. d. Activity of phrenic motor neurons is transmitted back to the bulbar respiratory centers via an ascending spino-bulbar pathway.