The tendinous septum of the semispinalis capitis muscle spatially separates the dorsal ramus between C3 and C4.

Local anesthetic injection into the medial head of the semispinalis capitis muscle can anesthetize the greater occipital nerve (GON) and third occipital nerve (TON) simultaneously (greater and third occipital nerve block: GTO block). Alternatively, inter-semispinal plane (ISP) block can anesthetize the dorsal rami of the cervical spinal nerves from C4 to T4. The GON, TON, and the dorsal rami of the inferior level cannot be blocked with a single injection. To elucidate this phenomenon from an anatomical standpoint, we performed an ISP block either alone or with a GTO block using water-based acrylic dye in three thiel-embalmed cadavers. Both dyes were clearly separated by the tendinous septum running obliquely inside the semispinalis capitis muscle (SCA). The tendinous septum of the SCA may have a relatively strong connection with the dorsal edge of the semispinalis cervicis muscle, and this structure may stem the injectate spread. Therefore, the GON and TON, running through the medial head of the SCA, and the dorsal rami of the inferior level are spatially separated by the tendinous septum, and cannot be blocked with a single injection.