## 1 Introduction  
The central nervous system (CNS), consisting of the brain and spinal cord, has a limited capacity to regenerate following injury or neurodegenerative disease [1]. This lack of regenerative capability represents a huge financial burden and heavy strain on the quality of life for patients and caregivers. Each year in the US, more than 80,000 people are afflicted with severe traumatic brain injury (TBI), over 12,000 experience spinal cord injury (SCI), over 500,000 people suffer from stroke, and millions of people are diagnosed with chronic neurodegenerative diseases [2,3]. Following brain injury or neurodegenerative disease, successful regeneration often requires the reformation of long-distance communication fibers, referred to as axons. Long-distance axonal regeneration does not occur in the mature brain, and the formation of a glial scar, occurring after a severe injury, inhibits axon regeneration and cell migration [4]. Strategies to augment axonal regeneration and modulate the post-injury microenvironment are actively being investigated to promote healing and facilitate functional regeneration. Non-thermal plasma (NTP), sometimes referred to as cold atmospheric plasma, has been investigated to facilitate wound healing and promote tissue regeneration. It is a partially ionized gas that consists of several physical (e.g., electric fields, ultraviolet light) and chemical (e.g., radical species and charged and neutral molecules) constituents [5]. NTP has previously been shown to directly affect biological processes, including the promotion of differentiation of osteoblasts and chondrocytes in vitro and enhancing limb autopod development ex vivo [6,7]. NTP has also been reported to enhance cell proliferation, migration, and tube formation in endothelial cells in vitro [8, 9, 10, 11]. This has been associated with plasma-generated reactive oxygen species (ROS) and stimulation of growth factors including vascular endothelial growth factor (VEGF) and fibroblast growth factor-2 (FGF-2)--factors that stimulate angiogenesis and are crucial for tissue growth and wound healing [12, 10, 13, 14]. Our group has also previously studied the effect of NTP on peripheral artery disease, where stimulation of vasculogenesis and angiogenesis are critical steps for treatment [15]. Following the plasma treatment of excised mice aortic rings, microvessel outgrowth was significantly enhanced. Furthermore, the total RNA isolated from NTP-treated rings showed that VEGF, matrix metalloproteinase-9 (MMP-9), and chemokine (C-X-C motif) ligand 1 (CXCL) 1) were upregulated after treatment over the control aortic rings. Not only are these factors critical for angiogenesis, but VEGF and MMP-9 have also been linked to the stimulation of neuron outgrowth [16,17]. Though the utility of NTP for neuronal regeneration has not been as widely studied, there are reports that NTP can direct in vitro differentiation of neural stem cells and protect against oxidative stress [18, 19, 20]. These effects have been attributed to NTP-generated nitric oxide (NO) and NTP-stimulated intracellular NO generation via the nitric oxide synthase. Therefore, taken together, we postulate that the NTP treatment of neural cells may stimulate the upregulation of similar factors and enhance neurite growth [20]. Another moderator of neuronal outgrowth and regeneration following injury are localized glial cells in the microenvironment. Glial cells consist of several different cell types, including astrocytes, and promote neuron health and viability by maintaining homeostasis and providing support and protection for neurons. In the healthy CNS, astrocytes maintain neuron health through the secretion of proteins and neurotrophic factors [21]. Post-injury, astrocytes assume a reactive state and form a glial scar--a physical and chemical barrier to sequester the injured area. This barrier also impedes the penetration of regenerating axons.