Spinal nerve T8
Nervus spinalis T8
- Related terms: Eighth thoracic nerve; T8
Definition
The eighth thoracic spinal nerve T8 is a mixed nerve, containing both somatic efferent (motor) and somatic afferent (sensory) fibers. It originates from the spinal cord at the T8 spinal segment.
a) Roots
Like other nerves, the spinal nerve T8 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 T8. This mixed nerve, containing sensory and motor fibers, exits the intervertebral foramen between the T8 and T9 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 eighth intercostal nerve, within the intercostal space, 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 back.
It also provides sensory innervation to the skin overlying the back.
A. ANTERIOR RAMUS OF SPINAL NERVE T8 (the eighth intercostal nerve):
The anterior ramus of the spinal nerve T8, commonly known as the eighth intercostal nerve, is the larger branch of the T8 spinal nerve. As a mixed nerve, it carries both motor (somatic efferent) and sensory (somatic afferent) fibers. Upon exiting the intervertebral foramen, the nerve travels anteriorly, coursing through the lower thoracic and abdominal wall.
Initially, the nerve runs within the costal groove of the eighth rib, located within the eighth intercostal space. Here, it forms a neurovascular bundle together with the intercostal arteries and veins. This bundle is strategically positioned between the internal intercostal muscle on its superficial side and the innermost intercostal muscle on its deeper aspect, as it traverses the chest wall.
As the nerve continues into the abdominal wall, it progresses between the internal oblique muscle, located on its superficial side, and the transversus abdominis muscle, found on its deep side.
Key functions and innervations
Motor Innervation (Somatic Efferent Fibers): | Sensory Innervation (Somatic Afferent Fibers): | ||
As the anterior ramus of T8 passes beneath the eighth rib—and continues as the eighth intercostal nerve—it provides motor innervation to several thoracic wall muscles, including: 1. Thoracic wall muscles: External, internal, and innermost intercostal muscles (T1-T11) Subcostal, and transversus thoracis muscles 2. Abdominal wall muscles: The intercostal nerve also provides motor innervation to the following abdominal wall muscles: 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). 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). | The sensory fibers of the eighth intercostal nerve innervate the skin through two primary cutaneous branches: 1- Lateral cutaneous branch: The lateral branch arises from the eighth intercostal nerve as it courses anteriorly along the chest wall, positioned within the costal groove of the eighth rib. As the lateral branch progresses through the intercostal space, it subdivides into anterior and posterior branches. 2- Anterior cutaneous branch: The anterior branch continues medially, emerging near the sternum between adjacent costal cartilages as the terminal anterior cutaneous nerve. It subsequently divides into medial and lateral branches. Together, the lateral and anterior cutaneous branches provide sensory innervation to the skin corresponding to the T8 dermatome. This dermatome includes the area below the level of the xiphoid process (but still above the umbilicus level). Beyond its cutaneous distribution, the eighth intercostal nerve also supplies sensory innervation to deeper structures, including the parietal layers of pleura and peritoneum. | ||
B. POSTERIOR RAMUS OF SPINAL NERVE T8:
The posterior ramus of the T8 spinal nerve is the smaller, posterior branch of the spinal nerve T8. 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 T8 spinal nerve exits the intervertebral foramen (between the T8 and T9 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 T8:
a) Recurrent Meningeal Nerve (Meningeal branch):
The recurrent meningeal nerve (or meningeal branch) originates from either the T8 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 T8 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 T8 | |||
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, and external oblique muscles. | The T8 dermatome (encompassing the skin below the level of the xiphoid process and above umbilicus). | Erector spinae muscles (iliocostalis, longissimus, spinalis), transversospinales muscles (multifidus, semispinalis, rotatores), segmental muscles (interspinales, intertransversarii, levtores costarum). | Zygapophyseal joints between T8 & T9 vertebrae, and the skin of the back corresponding to the T8 dermatome level. |
References
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