Recruitment of Polysynaptic Connections Underlies Functional Recovery of a Neural Circuit after Lesion.

The recruitment of additional neurons to neural circuits often occurs in accordance with changing functional demands. Here we found that synaptic recruitment plays a key role in functional recovery after neural injury. Disconnection of a brain commissure in the nudibranch mollusc, Tritonia diomedea, impairs swimming behavior by eliminating particular synapses in the central pattern generator (CPG) underlying the rhythmic swim motor pattern. However, the CPG functionally recovers within a day after the lesion. The strength of a spared inhibitory synapse within the CPG from Cerebral Neuron 2 (C2) to Ventral Swim Interneuron B (VSI) determines the level of impairment caused by the lesion, which varies among individuals. In addition to this direct synaptic connection, there are polysynaptic connections from C2 and Dorsal Swim Interneurons to VSI that provide indirect excitatory drive but play only minor roles under normal conditions. After disconnecting the pedal commissure (Pedal Nerve 6), the recruitment of polysynaptic excitation became a major source of the excitatory drive to VSI. Moreover, the amount of polysynaptic recruitment, which changed over time, differed among individuals and correlated with the degree of recovery of the swim motor pattern. Thus, functional recovery was mediated by an increase in the magnitude of polysynaptic excitatory drive, compensating for the loss of direct excitation. Since the degree of susceptibility to injury corresponds to existing individual variation in the C2 to VSI synapse, the recovery relied upon the extent to which the network reorganized to incorporate additional synapses.

Encephalopathy and death in infants with abusive head trauma is due to hypoxic-ischemic injury following local brain trauma to vital brainstem centers.

BACKGROUND: Infants with abusive head trauma (AHT) have diffuse brain damage with potentially fatal brain swelling. The pathogenesis of the brain damage remains unclear. We hypothesize that brain damage in AHT is due to hypoxic-ischemic injury with hypoxic-ischemic encephalopathy (HIE) rather than primary traumatic brain injury (TBI) with traumatic diffuse axonal injury (tDAI). METHODS: We studied brain tissue of AHT victims. Primary outcome measure was the presence of primary traumatic versus hypoxic-ischemic brain injury. The diagnosis of tDAI followed a standardized semiquantitative diagnostic approach yielding a 4-tiered grading scheme (definite, possible, improbable, and none). In addition, results of quantitative immunohistochemical analysis in a subgroup of AHT victims with instant death were compared with matched SIDS controls. RESULTS: In our cohort of 50 AHT victims, none had definite tDAI (no tDAI in 30, tDAI possible in 2, and tDAI improbable in 18). Instead, all AHT victims showed morphological findings indicative of HIE. Furthermore, the subgroup with instant death showed significantly higher counts of damaged axons with accumulation of amyloid precursor protein (APP) in the brainstem adjacent to the central pattern generator of respiratory activity (CPG) (odds ratio adjusted for age, sex, brain weight, and APP-count in other regions = 3.1; 95 % confidence interval = 1.2 to 7.7; p = 0.015). CONCLUSIONS: AHT victims in our cohort do not have diffuse TBI or tDAI. Instead, our findings indicate that the encephalopathy in AHT is the due to hypoxic-ischemic injury probably as the result of respiratory arrest due to local damage to parts of the CPG in the brainstem.