Spinal nerve T12
Nervus spinalis T12
- Related terms: Twelfth thoracic nerve; T12
Definition
The twelfth thoracic spinal nerve T12 is a mixed nerve, containing both somatic efferent (motor) and somatic afferent (sensory) fibers. It originates from the spinal cord at the T12 spinal segment.
a) Roots
Like other nerves, the spinal nerve T12 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 T12. This mixed nerve, containing sensory and motor fibers, exits the intervertebral foramen between the T12 and L1 vertebrae. Shortly after its emergence, the nerve divides into two primary rami:
1. Anterior (ventral) ramus:
This ramus continues anteriorly along the abdominal wall as the subcostal nerve, providing motor and sensory innervation to the abdominal wall.
It also connects with the sympathetic trunk via the white and gray rami communicantes, which carry preganglionic and postganglionic sympathetic fibers.
2. Posterior (dorsal) ramus:
This ramus provides motor innervation to the deep muscles of the lower back.
It also provides sensory innervation to the skin overlying the lower back.
A. ANTERIOR RAMUS OF SPINAL NERVE T12 (a.k.a. the subcostal nerve):
The anterior ramus of the spinal nerve T12, also known as the subcostal nerve, represents the larger branch of the T12 spinal nerve. As a mixed nerve, it contains both motor (somatic efferent) and sensory (somatic afferent) fibers. Upon exiting the intervertebral foramen, the nerve travels anteriorly along the abdominal wall, running within the costal groove of the twelfth rib. Here, it is accompanied by the intercostal arteries and veins, forming a neurovascular bundle. This bundle lies in a plane between the internal oblique muscle externally and the transversus abdominis muscle internally.
Occasionally, the spinal nerve T12 sends fibers to the L1 spinal nerve, thus contributing to the lumbar plexus.
Key functions and innervations
Motor Innervation (Somatic Efferent Fibers): | Sensory Innervation (Somatic Afferent Fibers): | ||
As the anterior ramus passes beneath the twelfth rib—and continues as the subcostal nerve—it provides motor innervation to several abdominal wall muscles, including: Internal oblique and transversus abdominis mscles (T7-T12 & L1). The nerve pierces the internal oblique muscle to supply the overlying external oblique muscle (T7-T12) and the pyramidalis muscle (T12). The latter helps to tense the linea alba. After penetrating the rectus sheath at the lateral edge of the rectus abdominis, the intercostal nerve extends medially, running behind the rectus abdominis to innervate this muscle (T7-T12). Lastly, T12 fibers reach the posterior back via the lumbar plexus, innervating muscles such as the quadratus lumborum (T12 - L4). | The sensory fibers of the subcostal nerve extend to the skin via two primary cutaneous branches: 1- Lateral abdominal cutaneous branch: The lateral branch arises from the subcostal nerve as it runs inferomedially along the body wall within the subcostal groove. Here, it subdivides into anterior and posterior branches. 2- Anterior abdominal cutaneous branch: This terminal branch forms as the nerve courses medially behind the rectus abdominis muscle. It pierces the rectus abdominis and the anterior rectus sheath near the midline, emerging as the anterior cutaneous nerve. Together, the lateral and anterior cutaneous branches provide cutaneous innervation to the skin along the T12 dermatome. Beyond its cutaneous distribution, the subcostal nerve supplies sensory innervation to structures beneath the abdominal wall muscles, such as the parietal peritoneum. | ||
B. POSTERIOR RAMUS OF SPINAL NERVE T12:
The posterior ramus of the T12 spinal nerve is the smaller, posterior branch of the spinal nerve T12. As with other posterior rami of thoracic spinal nerves, it innervates the deep intrinsic muscles of the back and the overlying skin, playing a vital role in spine stabilization and movement. Once the T12 spinal nerve exits the intervertebral foramen (between the T12 and L1 vertebrae), its posterior ramus passes through the intertransverse ligament and the overlying intertransverse muscle. It divides into lateral and medial branches:
Lateral branch: | Medial branch: | ||
Muscular branches: These innervate: - The erector spinae muscles, specifically the iliocostalis and longissimus portions. - Segmental muscles: These include the levtores costarum muscles. Cutaneous branch: After piercing the iliocostalis muscle and the thoracolumbar fascia, the lateral branch extends to supply the skin of the back. | The medial branch travels posterolaterally before curving medially along the edge of the multifidus muscle. Along its course, it gives rise to the following branches: Articular branches: These branches innervate the zygapophyseal joints. Muscular branches: These branches supply the intrinsic muscles of the back, including: - Erector spinae: primarily the spinalis potion, with a minor contribution to the longissimus. - Transversospinalis group: This includes the multifidus, semispinalis (specifically the semispinalis thoracis), and the rotatores (rotatores thoracics). - Segmental muscles: These include the interspinales and medial slips of the intertransversarii muscles. Cutaneous branch: After innervating the multifidus muscle, the medial branch pierces the thoracolumbar fascia and the overlying extrinsic back muscles, to become a cutaneous branch. |
C. ADDITIONAL STRUCTURES ASSOCIATED WITH SPINAL NERVE T12:
a) Recurrent Meningeal Nerve (Meningeal branch):
The recurrent meningeal nerve (or meningeal branch) originates from either the T12 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 white and gray rami communicantes, the spinal nerve T12 connects to the sympathetic trunk. The white rami communicantes enable preganglionic sympathetic fibers to reach the sympathetic trunk, while the gray rami communicantes enable the postganglionic sympathetic fibers to reach peripheral targets such as sweat glands and blood vessels.
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SUMMARY TABLE
STRUCTURES INNERVATED BY THE SPINAL NERVE T12 | |||
ANTERIOR RAMUS | POSTERIOR RAMUS | ||
MOTOR STRUCTURES | SENSORY STRUCTURES | MOTOR STRUCTURES | SENSORY STRUCTURES |
External, internal, and innermost intercostal muscles, subcostal, and transversus thoracis muscles. Internal oblique, transversus abdominis, external oblique muscles, and the quadratus lumborum muscle. | The T12 dermatome (encompassing the skin above the inguinal crease and includes the pubic region). | Erector spinae muscles (iliocostalis, longissimus, spinalis), transversospinales muscles (multifidus, semispinalis, rotatores), segmental muscles (interspinales, intertransversarii, levtores costarum). | Zygapophyseal joints between T12 & L1 vertebrae, and the skin of the back corresponding to the T12 dermatome level. |
References
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