In vivo imaging of the neuronal response to spinal cord injury: a narrative review

Spinal cord injury (SCI) often causes permanent disability with motor, sensory, and autonomic dysfunction. The neuronal response is key to neural regeneration failure and locomotor dysfunction following SCI. Recent progress in molecular labeling and in vivo imaging approaches has enabled the observation of neuronal dynamics in the healthy and injured CNS. MRI provides macroscopic information about the spatial location of structures, it cannot provide structural information at the cellular or subcellular level to directly observe cell bodies or axons, nor can it evaluate calcium signaling. Two-photon microscopy is a high-resolution imaging technique that uses a focused laser beam to stimulate fluorescent molecules in biological tissues. The functions and locations of spinal sensory neurons and motor neurons are different, and therefore different in vivo imaging approaches are used to study them. Sensory neurons detect and transmit sensory information from the periphery to the spinal cord and brain, and in vivo imaging of sensory neurons often uses genetically encoded calcium indicators (GECIs).

• 1
  In vivo imaging (especially two-photon microscopy) have provided direct optical access to both the superficial and deep layers of the spinal cord, to explore neuronal and non-neuronal activities of deeper structures in response to behaviors or specific diseases such as SCI.
• 2
  Spinal cord sensory neuron activity in response to SCI occurs in two places—the neurons in the spinal cord and in the DRG. In SCI, the superficial spinal cord sensory neurons would be directly damaged and then experience swelling, disintegration, and necrosis, whereas the DRG sensory neurons would develop axonal dieback in the spinal cord but their deeper cell bodies could survive.
• 3
  Within a few weeks of SCI, anterior horn motor neurons may be damaged to varying degrees, including the ventral root. The loss of motor neurons is associated with permanent motor impairment after SCI, and research on anterior horn motor neurons is crucial for understanding the underlying mechanisms of SCI and developing effective therapeutic interventions.