Spinal nerve C1

The first spinal nerve C1, also known as the suboccipital nerve, emerges from the vertebral canal above the posterior arch of the atlas. Unlike most other spinal nerves, C1 is purely motor and lacks sensory fibers. Typically, spinal nerves arise from both anterior (motor) and posterior (sensory) roots that converge to form mixed spinal nerves. However, the organization of C1 is atypical: its anterior (motor) root is well-developed, while the posterior (sensory) root is either absent, rudimentary, or non-functional, making C1 exclusively motor.

a) Anterior root of C1:

The anterior root of C1 contains motor fibers that contribute to the innervation of the neck and suboccipital muscles.

b) Posterior root of C1:

In contrast, the posterior root of C1 is minimally developed or absent, meaning it does not serve any sensory function. As a result, C1 does not contribute to cutaneous (skin) innervation or sensory pathways, and there is no C1 dermatome.

A. ANTERIOR RAMUS OF SPINAL NERVE C1:

1. Motor innervation (somatic efferent fibers):

Association with the Hypoglossal Nerve: Some fibers from the anterior ramus of C1 partially join with the hypoglossal nerve (cranial nerve XII) during their course. Through this connection, C1 contributes motor fibers that innervate muscles such as the geniohyoid and thyrohyoid via branches stemming from the hypoglossal nerve rather than directly from C1 itself.

Formation of the Ansa Cervicalis: The superior root of the ansa cervicalis arises from C1 fibers and travels alongside the hypoglossal nerve. After contributing to the hypoglossal nerve, the remaining fibers continue as the superior root, which later joins with the inferior root (originating from C2 and C3) to form the completed loop of the ansa cervicalis. The nerve loop is responsible for motor innervation of infrahyoid muscles, including sternohyoid, omohyoid, and sternothyroid (all C1 to C3).

Prevertebral and lateral muscles: The anterior ramus of C1 also supplies the prevertebral muscles such as the longus capitis (C1-C3) and lateral muscles like the rectus capitis anterior and rectus capitis lateralis (C1, C2).

2. Sensory innervation:

None.

B. POSTERIOR RAMUS OF SPINAL NERVE C1 (the suboccipital nerve):

The posterior ramus of the first cervical (suboccipital) nerve is larger than its anterior ramus. It exits the vertebral canal superior to the posterior arch of the atlas and inferior to the vertebral artery.

1. Motor innervation (somatic efferent fibers):

The posterior ramus of C1 targets the muscles around the suboccipital triangle, including the rectus capitis posterior major and minor, as well as the obliquus capitis superior and inferior. A filament from the Obliquus inferior branch connects to the posterior ramus of the second cervical nerve.

2. Sensory innervation:

None.

C. ADDITIONAL STRUCTURES ASSOCIATED WITH SPINAL NERVE C1:

a) Recurrent Meningeal Nerve (Meningeal branch):

The recurrent meningeal nerve (or meningeal branch) originates from either the C1 spinal nerve or one of its rami. This small nerve re-enters the vertebral canal and provides sensory innervation to:

• The meninges (protective coverings of the spinal cord),

• The vertebral bodies and periosteum,

• Adjacent blood vessels.

b) Sympathetic connections:

The spinal nerve C1 connects to the sympathetic trunk through the gray ramus communicant. The gray ramus communicant enables the postganglionic sympathetic fibers to reach peripheral targets such as sweat glands and blood vessels.

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SUMMARY TABLE