Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels (pLGICs) that mediate rapid cholinergic synaptic transmission throughout the central and peripheral nervous systems. These receptors are integral to neuromuscular junction signaling, autonomic regulation, cognitive learning, reward-related behavior, and neuroplasticity. The nAChR family exhibits remarkable functional diversity through combinatorial assembly of seventeen identified subunits (α1-α10, β1-β4, γ, δ, and ε). Among these, the α9 subunit occupies a distinctive phylogenetic position, exhibiting greater sequence divergence from other neuronal α subunits and possessing unique biochemical properties that distinguish it from all other family members. α9 can form homopentamers and also co-assemble with the α10 subunit to form functional heteropentamers, primarily in (α9)₂(α10)₃ and (α9)₃(α10)₂ stoichiometries. This dual assembly strategy generates functional heterogeneity, as distinct subunit compositions confer differential ion permeation, ligand sensitivity, and desensitization kinetics. The structural determinants governing this stoichiometric variability remain incompletely resolved, representing a critical gap in our mechanistic understanding. Unlike most other nAChR subtypes, traditional agonists of nAChRs (such as nicotine) elicit virtually no agonistic effect on α9* nAChRs (α9-containing receptors, including α9 homopentamers and α9α10 heteropentamers). This pharmacological divergence reflects structural differences within the orthosteric site, particularly in regions at complementary subunit interfaces. As cation-selective channels, α9* nAChRs exhibit high permeability to Ca²⁺. In cochlear outer hair cells, α9α10 nAChRs mediate cholinergic efferent modulation by the medial olivocochlear (MOC) bundle. Acetylcholine-evoked Ca²⁺ influx activates functionally coupled SK2 potassium channels, generating net hyperpolarization that dampens electromechanical amplification through a tightly constrained signaling microdomain. This Ca²⁺-dependent excitatory-to-inhibitory conversion exemplifies sophisticated sensory gain control, and its disruption contributes to noise-induced hearing loss, age-related hearing loss, and tinnitus. Beyond canonical ionotropic signaling, α9* nAChR engages metabotropic transduction pathways. In macrophages, receptor activation modulates cytokine production and inflammatory responses. In keratinocytes, it regulates wound healing by modulating cell migration and differentiation. In dorsal root ganglion sensory neurons, α9* signaling contributes to nociceptive processing and inflammatory hyperalgesia. This functional pleiotropy, spanning both ionotropic signaling and metabotropic transduction, positions α9* nAChR as an exemplary model for investigating signal polymorphism within the pLGIC superfamily. Pathophysiologically, α9* nAChR dysfunction is implicated across multiple organ systems. In the auditory system, disruption of α9α10 nAChR-mediated MOC efferent feedback impairs cochlear gain control, predisposing to noise-induced synaptopathy and age-related hearing loss. In immune cells, dysfunctional α9* nAChR signaling disrupts cholinergic anti-inflammatory pathway activity, exacerbating pro-inflammatory responses. In the peripheral sensory system, aberrant α9* nAChR signaling has been implicated in neuropathic and inflammatory pain states. In skin, impaired receptor function compromises keratinocyte migration and re-epithelialization through disrupted signaling pathways, leading to chronic wound healing defects and inflammation. These pathophysiological associations have catalyzed pharmacological interest, yielding promising chemical entities including α-conotoxins, small-molecule antagonists/agonists, and allosteric modulators. α-Conotoxin peptides, in particular, demonstrate remarkable subunit selectivity and potent antinociceptive effects in preclinical pain models. However, therapeutic translation faces substantial challenges: the broad tissue distribution of α9* nAChR risks on-target adverse effects in non-target organs; pronounced species differences between rodent and human receptors complicate preclinical validation; and the complexity of ionotropic-metabotropic signaling crosstalk demands pharmacological strategies that extend beyond conventional orthosteric agonism or antagonism. Looking forward, integrating cryo-electron microscopy of full-length receptors in distinct conformational states with single-channel electrophysiology and systems-level circuit analysis promises to illuminate the molecular mechanisms governing α9* nAChR function and regulation. The development of signal pathway-biased ligands and tissue-selective delivery strategies may ultimately harness the therapeutic potential of this receptor while mitigating safety liabilities. As a paradigm for understanding pLGIC signaling diversification, α9* nAChR research will continue to inform broader questions regarding ion channel evolution, allosteric regulation, and pathophysiological mechanisms of chronic pain and autoimmune inflammatory diseases.