Degenerative cervical myelopathy (DCM) represents the most common cause of spinal cord dysfunction in adults, frequently leading to progressive motor impairment and loss of functional independence. While conventional imaging modalities are essential for identifying structural spinal cord compression, they provide limited insight into the functional integrity and adaptive capacity of the motor network—critical factors influencing clinical presentation and prognosis. In this chapter, we present the concept of corticospinal reserve capacity as a functional framework for understanding disease progression in DCM. Using navigated transcranial magnetic stimulation (nTMS), specific neurophysiological parameters—including resting motor threshold (RMT), motor evoked potential (MEP) latency, recruitment curve (RC), cortical silent period (CSP), and motor area size—allow for a detailed assessment of corticocortical and corticospinal excitability. We describe a three-stage model of DCM progression based on these functional markers: This chapter highlights how the assessment of corticospinal reserve capacity via nTMS offers a novel and clinically meaningful approach for staging disease severity, monitoring progression, and potentially guiding therapeutic decision-making in DCM. Integrating functional assessment into routine clinical practice may enhance individualized patient care and inform future research directions in spinal cord disorders.