The neuronal soma represents a critical decision point between regeneration and degeneration following BPAI. Immediately after injury, affected neurons-including SMNs, dorsal root ganglion (DRG) sensory neurons, and sympathetic neurons-undergo rapid morphological and metabolic remodeling known as chromatolysis (He and Jin, 2016).

Histologically, chromatolysis is marked by somatic swelling, nuclear eccentricity, and dispersion of Nissl substance. Rather than a purely degenerative change, chromatolysis represents an active adaptive response that supports neuronal survival and axonal regeneration. Following injury, neurons undergo marked metabolic reprogramming, with RNA synthesis increasing by 300 ~ 500% and the rough endoplasmic reticulum reorganizing into dispersed ribosomal granules to enhance translational capacity. Within a critical window of approximately 4–20 days post-injury, neurons display a robust biosynthetic surge, characterized by increased rRNA/tRNA ratios and ribosome assembly into “protein synthesis factories,” enabling efficient production of cytoskeletal proteins such as tubulin and neurofilaments required for axonal growth and transport (Lieberman, 1971; Fawcett and Verhaagen, 2018).

However, the distinctive mechanical severity of BPAI-caused by violent traction forces that forcibly detach ventral roots from the spinal corddrives neurons toward a degenerative tipping point. Abrupt interruption of retrograde neurotrophic signaling (e.g., NGF, BDNF) initiates a cascade of cellular dysfunction marked by mitochondrial failure, ATP depletion, calcium overload, and activation of both caspase-dependent apoptotic pathways and RIPK1-MLKL-mediated necroptosis (Fricker et al., 2018). As these degenerative signals intensify, they irreversibly shift neurons from a regenerative trajectory toward retrograde degeneration, ultimately leading to soma death and permanent loss of regenerative potential. Disruption of the delicate balance between anabolic repair and catabolic degeneration thus represents a defining event in the pathophysiological progression of BPAI.

Axonal regeneration is a tightly coordinated molecular process governed by the spatiotemporal regulation of immediate-early gene (IEG) expression. These genes function as molecular switches that determine whether injured neurons successfully engage in regenerative programs or instead enter maladaptive states that promote degeneration.

Within 72 h after injury, the distal axonal segment undergoes classical Wallerian degeneration, marked by rapid axonal fragmentation, cytoskeletal collapse, and disintegration of axonal structure. Concurrently, Schwann cells rapidly transition to a repair phenotype, characterized by dedifferentiation, enhanced phagocytic activity, and efficient clearance of axonal and myelin debris. This coordinated response transiently creates a growth-permissive microenvironment that normally supports axonal regeneration. However, in brachial plexus avulsion, regeneration from the proximal compartment is largely unsuccessful due to irreversible loss of axonal continuity at the nerve root and progressive spinal motor neuron degeneration, ultimately preventing effective reinnervation (Jessen and Mirsky, 2016; Coleman and Höke, 2020).

After distal axonal degeneration, loss of neural input drives target tissues-particularly skeletal muscle and skin-into chronic denervation. In skeletal muscle, sustained denervation leads to progressive myofiber atrophy, fibrotic infiltration, and extensive extracellular matrix remodeling. At the neuromuscular junction (NMJ), deprivation of motor neuron-derived trophic and activity-dependent signals causes destabilization of acetylcholine receptor (AChR) clusters, followed by receptor dispersion, internalization, and postsynaptic structural collapse. When denervation persists for 12 ~ 18 months, these molecular and architectural alterations become largely irreversible, resulting in permanent NMJ dysfunction. Consequently, even delayed axonal reinnervation fails to restore effective neuromuscular transmission, severely limiting functional recovery (Tintignac et al., 2015; Li et al., 2018).

Despite advances in microsurgical repair, functional recovery after BPAI remains primarily limited by early SMN loss. Clinical and experimental studies indicate that the first ~4 weeks post-injury constitute a critical prognostic window: when SMN survival falls below ~40%, regenerating axons that eventually reach target muscles predominantly form structurally immature and electrophysiologically silent contacts-termed “ghost synapses.” These pseudo-synaptic structures lack effective signal transmission, rendering anatomically reconstructed pathways functionally ineffectively. Together with Wallerian degeneration, irreversible target organ denervation, and early SMN depletion, ghost synapse formation represents a major biological barrier to meaningful functional recovery after BPAI (Gordon, 2020). These findings highlight the need for early, mechanism-driven neuroprotective interventions that preserve SMNs, shorten denervation duration, and promote formation of functional neuromuscular synapses rather than purely anatomical reconnection.

After BPAI, irreversible SMN loss is predominantly driven by activation of convergent apoptotic pathways. Axonal disconnection abruptly interrupts retrograde neurotrophic signaling, destabilizes mitochondrial homeostasis, and induces oxidative stress, thereby engaging intrinsic apoptosis programs. In parallel, severe mechanical trauma and sustained neuroinflammation activate extrinsic death signaling, collectively lowering the apoptotic threshold of SMNs. Importantly, SMN apoptosis is not restricted to the acute phase but persists chronically: both clinical observations and animal studies demonstrate significantly higher apoptotic indices in proximal motor neurons months after injury, indicating a self-perpetuating degenerative process. This prolonged vulnerability highlights apoptosis as a central barrier to functional recovery and a critical target for early neuroprotective intervention.

In addition to apoptosis, regulated necrosis (necroptosis) is a major contributor to SMN loss following BPAI, particularly during the early post-injury period. Necroptosis is a genetically controlled, pro-inflammatory form of cell death mediated by the RIPK1-RIPK3-MLKL signaling axis, and is preferentially activated when caspase-dependent apoptosis is impaired or overwhelmed (Vandenabeele et al., 2010).

Necroptosis is initiated by extrinsic inflammatory cues, including TNF-α/TNFR1 signaling, Toll-like receptor activation, and interferon pathways. Upon receptor engagement, RIPK1 associates with RIPK3 to form the necrosome, leading to phosphorylation and oligomerization of MLKL, which translocates to the plasma membrane and induces membrane permeabilization, ionic imbalance, and cell lysis. In SMNs after BPAI, necroptotic signaling emerges within hours to days after injury and temporally overlaps with apoptotic cascades, collectively accelerating early neuronal attrition (Ito et al., 2016; Yuan et al., 2019). A critical regulatory checkpoint between apoptosis and necroptosis is caspase-8. Active caspase-8 suppresses necroptosis by cleaving RIPK1 and RIPK3, whereas caspase-8 inhibition-resulting from severe cellular stress, mitochondrial dysfunction, or inflammatory signaling-unleashes necroptotic execution (Oberst et al., 2011). This crosstalk explains why apoptosis-targeted neuroprotection alone is often insufficient to prevent SMN loss after severe root avulsion (Figure 2).

Necroptotic SMN death is intrinsically pro-inflammatory. Membrane rupture leads to the release of damage-associated molecular patterns (DAMPs), such as HMGB1 and ATP, which activate microglia and infiltrating macrophages, establishing a feed-forward inflammatory loop that further amplifies neuronal loss and distal denervation. Consequently, necroptosis critically contributes to the rapid collapse of the motor neuron pool, defining a narrow therapeutic window for early intervention. Targeting necroptosis therefore represents a promising adjunct to apoptosis-focused strategies. RIPK1 inhibitors, such as Necrostatin-1 and its optimized derivatives, have demonstrated efficacy in reducing MLKL activation, limiting neuronal loss, and improving functional outcomes in experimental models of neural trauma (Degterev et al., 2005; Nishijima et al., 2023). Combined modulation of necroptosis, apoptosis, and neurotrophic signaling may offer a more effective, multimodal neuroprotective approach for preserving SMN viability after BPAI. The involvement of necroptotic signaling highlights the therapeutic potential of targeting RIPK1-RIPK3-MLKL-mediated pathways to limit inflammatory cell death and secondary tissue damage after BPAI.

Pyroptosis is a highly inflammatory form of PCD characterized by gasdermin-mediated plasma membrane pore formation, rapid ionic imbalance, and release of potent proinflammatory cytokines such as IL-1β and IL-18. Unlike apoptosis, pyroptotic neurons actively release DAMPs, thereby amplifying neuroinflammation and propagating secondary tissue injury (Shi et al., 2017). Increasing evidence indicates that pyroptosis contributes significantly to SMN loss following BPAI and SCI.

Pyroptotic signaling is primarily driven by inflammasome activation, most notably the NLRP3 inflammasome, which promotes caspase-1 activation and subsequent cleavage of gasdermin D (GSDMD). The N-terminal fragment of GSDMD oligomerizes to form membrane pores, leading to cell swelling, membrane rupture, and inflammatory mediator release (Broz and Dixit, 2016). Notably, neuronal pyroptosis often persists longer and extends beyond the immediate lesion site compared with apoptosis, indicating a sustained role in secondary neurodegeneration. Microglia act as critical upstream regulators of pyroptosis in the injured spinal cord. Excessive production of reactive ROS by activated microglia promotes NLRP3 inflammasome assembly. Genetic or pharmacological inhibition of microglial ROS generation-such as deletion of the voltage-gated proton channel Hv1-significantly attenuates NLRP3 activation, reduces neuronal pyroptosis, and preserves spinal cord tissue integrity (Al Mamun et al., 2021; Zheng et al., 2022). These findings position microglial ROS as a key mechanistic link between oxidative stress, neuroinflammation, and pyroptotic SMN injury.

Pyroptosis is not restricted to neurons but also occurs in peripheral glial cells. In injured peripheral nerves, Schwann cells undergo caspase-1-dependent pyroptosis, releasing inflammatory mediators that impair dorsal root ganglion neuron (DRGN) function. Pharmacological inhibition of pyroptosis in Schwann cells reduces local inflammation, enhances axonal regeneration, and improves functional recovery, highlighting the broader contribution of glial pyroptosis to nerve repair failure (Wang et al., 2023; Wang et al., 2024). Therapeutically, targeting pyroptotic signaling has emerged as a promising neuroprotective strategy. Inhibition of inflammasome components or gasdermin activation, as well as metabolic or anti-inflammatory interventions (e.g., N-acetylglucosamine), has been shown to reduce pyroptotic cell death, promote remyelination, and improve motor outcomes in experimental BPAI and SCI models. Collectively, these findings suggest that modulation of pyroptosis, particularly at the level of microglial inflammasome activation, may effectively limit secondary neuroinflammation and preserve SMN viability after severe nerve root injury (Amo-Aparicio et al., 2022; Wang et al., 2022; Broz, 2025). These findings implicate inflammasome-mediated pyroptosis as a potential therapeutic target, particularly through modulation of microglial activation and cytokine release to restrain inflammation-driven neuronal injury.

Ferroptosis is an iron-dependent, lipid peroxidation-driven form of regulated cell death that has recently been implicated in motor neuron vulnerability after severe nerve injury (Dixon et al., 2012; Stockwell et al., 2017). Accumulating evidence from BPAI, SCI, and peripheral nerve trauma models indicate that proximal SMNs exhibit key molecular hallmarks of ferroptosis, including iron dysregulation, glutathione depletion, GPX4 inactivation, and excessive lipid peroxidation.

Following root avulsion, axonal disconnection and mechanical stress disrupt mitochondrial homeostasis and intracellular iron handling, leading to reactive ROS accumulation and peroxidation of polyunsaturated fatty acids within neuronal membranes. Loss of glutathione-dependent antioxidant capacity-together with functional impairment of glutathione peroxidase 4 (GPX4)-renders SMNs highly susceptible to ferroptotic execution. These events occur early after injury and may persist, contributing to both neuronal loss and chronic neuropathic pain (Huang L. et al., 2022; Huang and Bai, 2024; Liu et al., 2025). Experimental evidence supports a causal role for ferroptosis in nerve injury-induced neurodegeneration. In rodent models of peripheral nerve injury and SCI, genetic ablation or pharmacological inhibition of GPX4 precipitates rapid motor neuron degeneration, whereas ferroptosis inhibitors such as ferrostatin-1, liproxstatin-1, iron chelators, or antioxidant supplementation significantly attenuate neuronal loss and improve functional recovery (Bai et al., 2023; Li W. et al., 2023; Liu et al., 2024). Recent studies further suggest that modulation of calcium-related signaling pathways can influence ferroptotic susceptibility in the injured spinal cord and alleviate neuropathic pain-associated behaviors, highlighting functional relevance beyond cell survival alone (Guo et al., 2021; Li L. et al., 2023).

Autophagy is a conserved lysosome-dependent degradation pathway essential for neuronal homeostasis, enabling the clearance of damaged organelles, misfolded proteins, and toxic aggregates (Mizushima and Levine, 2010). After BPAI, SMNs exhibit a robust but dysregulated autophagic response, which can exert either neuroprotective or neurotoxic effects depending on timing and flux integrity (Hara et al., 2006).

Early after axonal root avulsion, mitochondrial dysfunction, ATP depletion, reactive ROS accumulation, and intracellular Ca²⁺ overload activate the AMPK-ULK1-Beclin-1 signaling axis, triggering autophagosome formation. Increased expression of autophagy markers such as LC3-II and Beclin-1 has been detected in the spinal anterior horn within 24–48 h post-injury, consistent with an adaptive attempt to restore cellular homeostasis (Ribas et al., 2015). However, accumulating evidence indicates that autophagic flux is frequently impaired during the subacute and chronic phases after BPAI. Pathological upregulation of aldose reductase (AR) in the spinal ventral horn has been shown to suppress the SIRT1-AMPK-mTOR pathway, leading to defective autophagosome clearance, persistent oxidative stress, neuroinflammation, and progressive SMN loss. Genetic deletion of AR or pharmacological inhibition using epalrestat restores autophagic flux, reduces inflammatory cytokine production, preserves motor neuron survival, and significantly improves forelimb motor function in BPAI models (Zhong et al., 2022; Li et al., 2025).

Together, these findings support a dual-role model of autophagy in BPAI: transient activation of autophagy is initially protective, whereas sustained or dysregulated autophagy accelerates neurodegeneration. Therapeutic strategies aimed at normalizing autophagic flux, rather than globally enhancing or suppressing autophagy, may therefore represent an effective approach to preserve SMNs and promote functional recovery after BPAI.

Beyond its cell-autonomous role in neuronal homeostasis, autophagy dysregulation may sensitize SMNs to inflammatory and oxidative cues, providing a mechanistic bridge between intracellular stress responses and microenvironment-driven death amplification.

Following brachial BPAI, SMN degeneration extends far beyond the initial mechanical insult, driven by a self-amplifying inflammatory microenvironment formed by interactions among glial cells, infiltrating immune cells, and neurons (Bellver-Landete et al., 2019; Zhong et al., 2021). SMN degeneration evolves through a temporally orchestrated inflammatory cascade. Within the acute phase (≤72 h), resident microglia rapidly acquire a pro-inflammatory phenotype and release TNF-α, IL-1β, IL-6, and ROS, initiating early apoptotic and pyroptotic signaling that accounts for the first wave of SMN loss. During the subacute phase (3–14 days), disruption of the blood-spinal cord barrier permits infiltration of monocyte-derived macrophages and T lymphocytes, which further amplify cytokine release, complement activation, and oxidative stress, thereby accelerating secondary motor neuron degeneration. In the chronic phase (>14 days), sustained astrocyte activation and excessive extracellular matrix deposition culminate in glial scar formation, chemically and physically isolating surviving SMNs, inhibiting axonal regeneration, and stabilizing a persistently neurotoxic microenvironment (Table 2) (Kigerl et al., 2009; Brennan et al., 2012; Komine and Yamanaka, 2015; Lacagnina et al., 2025; Saad et al., 2025).

At the molecular level, this immune-glia–neuron triangle is maintained by interconnected feedback loops, including TNF-α-NF-κB-ROS signaling, IL-1β-NLRP3 inflammasome activation, complement C5a-C5aR1-mediated necroptotic bias, and astrocyte-derived CXCL10-dependent immune recruitment (Figure 3). Together, these autonomous (neuronal) and non-autonomous (glial and immune) mechanisms create a self-sustaining inflammatory microenvironment that dictates irreversible proximal SMN loss after BPAI. These microenvironment-driven mechanisms underscore the therapeutic value of modulating immune-glia interactions to disrupt death-amplifying feedback loops and preserve spinal motor neuron viability after BPAI (Figure 4).

Recent advances in single-cell and single-nucleus RNA sequencing have provided valuable insights into the cellular heterogeneity and injury-induced transcriptional programs underlying spinal cord neurodegeneration. Although BPAI-specific single-cell datasets are currently limited, evidence from spinal cord injury models offers an important framework for understanding BPAI-associated SMN loss.

Single-cell analyses indicate that neurons in ventral spinal regions undergo dynamic and heterogeneous transcriptional changes after injury. Vulnerable neuronal populations exhibit downregulation of genes related to synaptic transmission and membrane excitability, alongside upregulation of stress-response, RNA-processing, autophagy-related, and metabolic pathways during chronic phases. These findings suggest that motor neurons may transition from an early regenerative or compensatory state toward maladaptive stress and catabolic programs, increasing susceptibility to programmed cell death (Fan et al., 2023).

In parallel, glial cells show robust and spatially heterogeneous activation. Microglia progressively acquire disease-associated phenotypes enriched for lipid metabolism, phagocytosis, and inflammatory signaling, while astrocytes and oligodendrocyte lineage cells display region-specific transcriptional remodeling. Notably, distal spinal segments below the injury develop a sustained degenerative microenvironment characterized by persistent microglial activation and altered neuronal composition, supporting the concept of long-range, non-cell-autonomous amplification of neurodegeneration (Floriddia et al., 2020).

Together, these single-cell transcriptomic findings support a network-based model in which BPAI-related SMN loss reflects the convergence of neuron-intrinsic stress responses and cell-extrinsic inflammatory and metabolic pressures imposed by a heterogeneous immune–glia microenvironment. Future BPAI-focused single-cell and spatial transcriptomic studies will be essential to delineate vulnerable SMN subpopulations and define stage-specific therapeutic windows.

PCD-including apoptosis, pyroptosis, ferroptosis, and necroptosis-is a major driver of SMN loss following BPAI. Recent studies indicate that metabolic stress, autophagy failure, oxidative injury, and inflammatory amplification converge to lower neuronal survival thresholds, making pathway-specific inhibition a rational strategy for precision neuroprotection.

Beyond neuron-intrinsic death pathways, a pathological immune-glia–neuron microenvironment critically amplifies SMN loss after BPAI. Activated glial cells, infiltrating immune populations, and inflammatory mediators form self-reinforcing feedback loops that propagate apoptosis, necroptosis, and pyroptosis. Targeting this microenvironment therefore represents a complementary and clinically relevant neuroprotective strategy.

While inhibition of PCD and immunomodulation preserve proximal SMNs, durable functional recovery after BPAI ultimately requires structural, synaptic, and circuit-level reconstruction. Cell- and biomaterial-based strategies provide integrated platforms for neuroprotection, axonal guidance, and reinnervation.