Spinal nerve S1

The first sacral spinal nerve S1 is a mixed nerve, containing both somatic efferent (motor) and somatic afferent (sensory) fibers. It originates from the spinal cord at the S1 spinal segment.

a) Roots

Like other nerves, the spinal nerve S1 is formed by the union of two roots:

1. Posterior (dorsal) root:

• This root contains sensory fibers that carry signals from peripheral structures (innervated by the anterior and posterior rami) to the spinal cord.

• These fibers enter the dorsal gray horn within the spinal cord.

2. Anterior (ventral) root:

• This root consists of motor fibers originating from the anterior gray horn within the spinal cord.

• These fibers transmit motor commands to peripheral muscles via the spinal nerve and its branches.

b) Mixed spinal nerve and rami

The anterior and posterior roots merge within the sacral vertebral canal to form the spinal nerve S1. This mixed nerve, containing both sensory and motor fibers, divides into two primary rami:

1. Anterior (ventral) ramus:

• This ramus emerges via the anterior sacral foramen and contributes to the sacral plexus, which provides motor and sensory innervation to structures, including the lower limbs and gluteal regions. The anterior ramus further subdivides into ventral and dorsal divisions.

• It also forms connections with the sympathetic trunk via the gray rami communicantes, which carry postganglionic sympathetic fibers (white rami communicantes are absent at this level).

2. Posterior (dorsal) ramus:

• This ramus provides motor innervation to the deep muscles of the back.

• It also provides sensory innervation to the skin overlying the back.

A. ANTERIOR RAMUS OF SPINAL NERVE S1:

The anterior ramus of spinal nerve S1 is the larger division of the nerve and functions as a mixed nerve, carrying both motor (somatic efferent) and sensory (somatic afferent) fibers. After exiting through the anterior sacral foramen, it merges with the lumbosacral trunk and anterior rami of sacral nerves S2, S3, and part of S4 to form the sacral plexus. This plexus is located on the posterolateral wall of the pelvic cavity, lying on the anterior surface of the piriformis muscle and deep to the internal iliac vessels.

Key functions and innervations

B. POSTERIOR RAMUS OF SPINAL NERVE S1:

The posterior ramus of the spinal nerve S1 emerges through the dorsal sacral foramen, lying deep to the multifidus, an intrinsic deep muscle of the pelvic back region. It then divides into two main branches:

a) Medial branch:

This branch innervates the multifidus muscle.

b) Lateral branch:

The lateral branch provides sensory innervation to the posterior gluteal skin. After its division from the posterior ramus of S1, it unites with branches from the posterior rami of other sacral nerves and receives contributions from the L5 spinal nerve. Together, these branches form neuronal loops (a.k.a. the posterior sacrococcygeal plexus) on the dorsal surface of the sacrum, beneath the gluteus maximus muscle. From these loops, gluteal branches emerge, piercing the gluteus maximus to supply sensory innervation to the overlying posterior gluteal skin.

Some studies also suggest that the posterior ramus provides innervation to the posterior sacroiliac joint.

C. ADDITIONAL STRUCTURES ASSOCIATED WITH SPINAL NERVE S1:

a) Recurrent Meningeal Nerve (Meningeal branch):

The recurrent meningeal nerve (or meningeal branch) originates from either the S1 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:

Through the gray rami communicantes, the sympathetic trunk connects to the spinal nerve S1, enabling postganglionic sympathetic fibers to reach peripheral targets such as sweat glands and blood vessels.