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The Occipital Nerves are a group of nerves that arise from the C2 and C3 spinal nerves. They innervate the posterior scalp up as far as the vertex and other structures as well, such as the ear.[rx] There are three major occipital nerves in the human body: the greater occipital nerve (GON), the lesser (or small) occipital nerve (LON), and the third (or least) occipital nerve (TON).
Structure and Function
Greater occipital nerve
The GON is the biggest purely afferent nerve that arises from the medial division of the dorsal ramus of the C2 spinal nerve. It goes backward between the C1 and C2 vertebrae and traverses between the inferior capitis oblique and semispinalis capitis muscles from underneath the suboccipital triangle.[rx] Rarely does the GON travel within the inferior oblique. While traveling to the subcutaneous layer, the GON is found to pierce the semispinalis capitis muscle in most cases, and in some cases, the trapezius and the inferior oblique.[rx] This complex involvement with the nearby musculature may make the GON a potential source of nerve compression, entrapment, or irritation. The GON then perforates the aponeurotic fibrous layer of the trapezius and the sternocleidomastoid to travel to the scalp and the superior nuchal line.[rx] The GON also traverses along the occipital artery after passing through the semispinalis capitis. The GON innervates the skin of the back of the scalp up to the vertex of the skull, the ear, and the skin just above the parotid gland.[rx]
Lesser occipital nerve
The LON originates from the ventral rami of the C2 and C3 spinal nerves and goes to the occipital region along the posterior margin of the sternocleidomastoid muscle.[rx] It pierces the deep cervical fascia close to the cranium and travels upward. Near the cranium, it penetrates the deep cervical fascia and goes superiorly above the occiput to innervate the skin and communicate with the GON. [rx] The LON has three branches: the auricular, mastoid, and occipital branches. The LON divides into medial and lateral segments between the inion and intermastoid line.[rx] The LON innervates the scalp in the lateral region of the head behind the ear and the cranial surface of the ear.[rx]
Third occipital nerve
The TON is a superficial medial branch of the dorsal ramus of the C3 spinal nerve and is thicker compared to other medial branches.[rx] The dorsal ramus of the C3 spinal nerve divides into lateral and medial branches. The medial division further divides into superficial and deep branches, of which the superficial division is named the TON. The TON travels through the dorsolateral surface of the C2-C3 facet joint.[rx] Based on a study by Tubbs et al., the TON was found to send out small branches that travel across the midline and interact with the contralateral TON in 66.7% of patients.[rx] The TON also perforates the splenius capitis, trapezius, and semispinalis capitis. It then communicates with the GON and innervates the region of the skin below the superior nuchal line after innervating the semispinalis capitis. The TON also innervates the facet joint between the C2 and C3 spinal nerves and a portion of the semispinalis capitis.[rx]
Blood Supply and Lymphatics
The scalp is highly vascularized and is characterized by having many arterial anastomoses. Most of the blood supply comes from the external carotid arteries.[rx] With regards to the occipital region of the scalp, the vascularization is via the occipital artery and the posterior auricular arteries.[rx] Within the auriculomastoid sulcus, the posterior auricular artery travels superficially and separates into three branches: the mastoid, auricular, and transverse nuchal arteries.[rx] The LON is found to be close to the occipital artery. According to Kemp et al., the LON was found to be situated 2.5cm lateral to the occipital artery above the occiput.[rx] Also, according to Lee et al., who studied the topography of the LON in 20 sides of 10 heads from fresh cadavers, branches from the occipital artery communicated with the LON in 55% of samples.[rx] Among these samples, 45% of samples had the occipital artery crossing the LON at a single location while 10% of samples had the occipital artery communicating with the LON via a helical intertwining relationship. The researchers also found a fascial band as a compression point in 20% of samples.[rx]
The GON is also closely associated with the occipital artery in that after the GON perforates the semispinalis capitis, it travels with the occipital artery that is medial to the nerve.[rx] The GON may have a much more intimate relationship than previously thought. According to a study conducted by Janis et al., in which the researchers analyzed the topographic relationship between the GON and occipital artery in fifty samples of 25 posterior necks and scalps from cadavers, the GON, and occipital artery were found to cross each other in 54% of samples.[rx] Among samples where there was an intersection between the GON and the occipital artery, these crossings could differ from intersecting each other at a single point (29.6%) to intertwining with each other in a helical fashion (70.4%). These crossings were usually discovered in the tunnel of the trapezius caudal to the occipital protuberance but were also present above the occipitalis.[rx] These findings may be useful for migraine patients, as many of these patients report having pulsatile symptoms, and their headaches may contain a vascular component. Many researchers have proposed that the intersections between the GON and occipital artery may be responsible for these symptoms.[rx] Furthermore, another study by Shimizu et al. discovered the occipital artery and GON intersected in the nuchal subcutaneous layer, and the GON was always more superficial to the occipital artery at the point of intersection.[rx] They postulated the intimate relationship between the GON and occipital artery might be a contributing factor for occipital neuralgia (ON).[rx]
As mentioned previously, the GON arises from the medial branch of the dorsal ramus of the C2 spinal nerve and innervates the skin of the back of the scalp up to the vertex of the skull, the ear, and the skin just above the parotid gland.[rx] When the GON is over the occiput, it communicates with the LON laterally and the TON. The LON comes from the ventral rami of the C2 and C3 spinal nerves and provides innervation to the scalp in the lateral region of the head behind the ear.[rx] The LON also transmits a branch to the GON as it goes above the occiput near the cranium.[rx] It also communicates with the mastoid division of the greater auricular nerve. The TON originates from the medial branch of the dorsal ramus of the C3 spinal nerve and innervates the facet joint between the C2 and C3 spinal nerves and a portion of the semispinalis capitis.[rx] Its cutaneous division also innervates the skin below the occiput. The TON also communicates with the GON and innervates the region of the skin below the superior nuchal line.[rx]
Greater occipital nerve
As stated previously, the GON traverses between the inferior capitis oblique and semispinalis capitis muscles from underneath the suboccipital triangle.[rx] Rarely does the GON travel within the inferior oblique. While traveling to the subcutaneous layer, the GON is found to pierce the semispinalis capitis muscle in most cases, and in some cases, the trapezius and the inferior oblique.[rx] For the treatment of GON entrapment neuropathy, the regions where the GON traverses between the atlas and the axis, the GON courses between the obliquus capitis inferior and semispinalis capitis, or the GON perforates the semispinalis capitis and the trapezius, which are potential areas of GON irritation and entrapment.[rx] These zones could be affected by other medical issues, such as whiplash injuries and posture imbalances, and could serve as possible origins of ON.[rx] However, there are many physiological variants of the GON, which will be a topic in the following section.
Lesser occipital nerve
With regards to the LON, the area where the LON traverses from behind the sternocleidomastoid, the area where the LON ascends along the posterior margin of the sternocleidomastoid, and the area where the LON intersects with the nuchal line have been found to serve as potential compression points.[rx] This article will cover the physiological variants of the LON in the following section.