Spinal nerve T4
Nervus spinalis T4
- Related terms: Fourth thoracic nerve; T4
Definition
The fourth thoracic spinal nerve T4 is a mixed nerve, containing both somatic efferent (motor) and somatic afferent (sensory) fibers. It originates from the spinal cord at the T4 spinal segment.
a) Roots
Like other nerves, the spinal nerve T4 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 T4. This mixed nerve, containing both sensory and motor fibers, exits the intervertebral foramen between the T4 and T5 vertebrae. Shortly after its emergence, the nerve divides into two primary rami:
1. Anterior (ventral) ramus:
This ramus continues anteriorly along the chest wall as the fourth intercostal nerve, within the intercostal space, providing motor and sensory innervation to the chest wall.
It also forms connections 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 T4 (the fourth intercostal nerve):
The anterior ramus of the spinal nerve T4, commonly known as the fourth intercostal nerve, constitutes the larger branch of the T4 spinal nerve. As a mixed nerve, it carries both motor (somatic efferent) and sensory (somatic afferent) fibers. Emerging from the intervertebral foramen, it travels anteriorly through the chest wall within the fourth intercostal space. Here, it lies in the costal groove of the fourth rib, accompanied by the intercostal arteries and veins to form a neurovascular bundle. This bundle is positioned between the internal intercostal muscle on its superficial side and the innermost intercostal muscle on its deep side as it courses along the chest wall.
Key functions and innervations
Motor Innervation (Somatic Efferent Fibers): | Sensory Innervation (Somatic Afferent Fibers): | ||
As the anterior ramus of T4 passes beneath the fourth rib—and continues as the fourth intercostal nerve—it provides motor innervation to several thoracic wall muscles, including: External, internal, and innermost intercostal muscles (T1-T11) Subcostal, and transversus thoracis muscles | The sensory fibers of the fourth intercostal nerve innervate the skin through two primary cutaneous branches: 1- Lateral cutaneous branch: This branch arises from the fourth intercostal nerve as it courses anteriorly along the chest wall within the costal groove of the fourth rib. Within the intercostal space, the lateral branch bifurcates 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 T4 dermatome. This dermatome includes the nipple region and the surrounding breast tissue. Beyond its cutaneous distribution, the fourth intercostal nerve also supplies sensory innervation to deeper structures, including the parietal pleura, located beneath the chest wall muscles. | ||
B. POSTERIOR RAMUS OF SPINAL NERVE T4:
The posterior ramus of the T4 spinal nerve is the smaller, posterior branch of the spinal nerve T4. 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 T4 spinal nerve exits the intervertebral foramen (between the T4 and T5 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, 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 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 cervicis), and the rotatores (rotatores thoracics and cervicis). - 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 T4:
a) Recurrent Meningeal Nerve (Meningeal branch):
The recurrent meningeal nerve (or meningeal branch) originates from either the T4 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 T4 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 T4 | |||
ANTERIOR RAMUS | POSTERIOR RAMUS | ||
MOTOR STRUCTURES | SENSORY STRUCTURES | MOTOR STRUCTURES | SENSORY STRUCTURES |
External, internal, and innermost intercostal muscles, subcostal, and transversus thoracis muscles | T4 dermatome skin (upper portion of the anterior chest wall, encompassing the nipple and areola). | Erector spinae muscles (iliocostalis, longissimus, spinalis), transversospinales muscles (multifidus, semispinalis, rotatores), segmental muscles (interspinales, intertransversarii, levtores costarum). | Zygapophyseal joints between T4 & T5 vertebrae, skin of the upper back corresponding to the T4 dermatome level. |
References
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Drake, R.L., Vogl, A.W., and Mitchell, A.W.M. (2010). ‘Chapter 3: Thorax’, in Gray’s anatomy for students. (2nd ed.) Churchill Livingstone Elsevier, Philadelphia PA 19103, pp.149-155.
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Ishizuka K, Sakai H, Tsuzuki N, Nagashima M. Topographic anatomy of the posterior ramus of thoracic spinal nerve and surrounding structures. Spine (Phila Pa 1976). 2012 Jun 15;37(14):E817-22.
