Mid-cervical interneuron networks following high cervical spinal cord injury.

Spinal interneuron (IN) networks can facilitate respiratory motor recovery after spinal cord injury (SCI). We hypothesized that excitatory synaptic connectivity between INs located immediately caudal to unilateral cervical SCI would be most prevalent in a contra- to ipsilateral direction. Adult rats were studied following chronic C2 spinal cord hemisection (C2Hx) injury. Rats were anesthetized and ventilated and a multi-electrode array was used to simultaneously record INs on both sides of the C4-5 spinal cord. The temporal firing relationship between IN pairs was evaluated using cross-correlation with directionality of synaptic connections inferred based on electrode location. During baseline recordings, the majority of detectable excitatory IN connections occurred in a contra- to- ipsilateral direction. However, acute respiratory stimulation with hypoxia abolished this directionality, while simultaneously increasing the detectable inhibitory connections within the ipsilateral cord. We conclude that propriospinal networks caudal to SCI can display a contralateral-to-ipsilateral directionality of synaptic connections and that these connections are modulated by acute exposure to hypoxia.