The GON follows a predictable course through the posterior vertebral musculature which has been described as comprising three portions and two bends 1, giving ample opportunity for entrapment and compression.
It arises from the medial branch of the dorsal ramus of C2 with a small contribution from C3. The first portion (P1) runs a short oblique course laterally towards the lower border of the obliquus capitis inferior muscle; the first bend (A1) occurs around this muscle.
The second longer portion (P2) travels postero-medially and upwards between two muscular layers. The GON successively crosses over the obliquus capitis inferior and rectus capitis posterior major muscles which comprise the deep layer. The semispinalis capitis makes up the superficial layer of this portion. The second bend (A2) occurs as the GON crosses the semispinalis capitis, piercing through its fibers.
The third and last portion (P3) travels superiorly between the dorsal aspect of the semispinalis capitis and the deep aspect of the trapezius. The GON perforates the trapezius aponeurosis to reach the subcutaneous layer near the nuchal line. It predominantly divides into its terminal branches after traversing the trapezius.
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