Spinal nerve L4

The fourth lumbar spinal nerve L4 is a mixed nerve, containing both somatic efferent (motor) and somatic afferent (sensory) fibers. It originates from the spinal cord at the L4 spinal segment.

a) Roots

Like other nerves, the spinal nerve L4 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

At the level of the intervertebral foramen, the anterior and posterior roots merge to form the spinal nerve L4. This mixed nerve, containing both sensory and motor fibers, exits the intervertebral foramen between the L4 and L5 vertebrae. Shortly after its emergence, the nerve divides into two primary rami:

1. Anterior (ventral) ramus:

• This ramus further subdivides into ventral and dorsal divisions that contribute to the lumbosacral plexus, providing motor and sensory innervation to structures, including the lower limbs and gluteal regions.

• 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 L4:

The anterior ramus is the larger division of spinal nerve L4. As a mixed nerve, it carries both somatic efferent (motor) and somatic afferent (sensory) fibers. After leaving the vertebral column, it contributes significantly to the lumbar plexus, a network of nerves located within the psoas major muscle. The anterior ramus of L4 collaborates with the anterior rami of L1, L2, L3, and occasionally T12 to form this plexus.

A portion of the anterior ramus of L4 combines with the anterior ramus of L5 to form the lumbosacral trunk. This trunk descends vertically from the abdomen into the pelvic cavity, passing just anterior to the sacroiliac joint. Here, it contributes to the formation of the sacral plexus, which lies on the posterolateral wall of the pelvic cavity, positioned 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 L4:

The posterior ramus of the spinal nerve L4–similar to posterior rami of other lumbar spinal nerves–primarily innervates the deep intrinsic muscles of the back and the overlying skin, and thus plays a critical role in the stabilization and movement of the spine. After L4 spinal nerve exits the intervertebral foramen (between L4 and L5 vertebrae), its posterior ramus traverses the intertransverse ligament and the overlying intertransverse muscle, and ends up branching into lateral and medial branches:

C. ADDITIONAL STRUCTURES ASSOCIATED WITH SPINAL NERVE l4:

a) Recurrent Meningeal Nerve (Meningeal branch):

The recurrent meningeal nerve (or meningeal branch) originates from either the L4 spinal nerve or one of its rami. This small nerve re-enters the vertebral canal through the intervertebral foramen and provides sensory innervation to::

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

• The annulus fibrosus of the intervertebral discs,

• The vertebral bodies and periosteum,

• Adjacent blood vessels.

b) Sympathetic connections:

Through the gray rami communicantes, the sympathetic trunk connects to the spinal nerve L4, enabling postganglionic sympathetic fibers to reach peripheral targets such as sweat glands and blood vessels.