Spinal nerve T1
Nervus spinalis T1
- Related terms: First thoracic nerve; T1
Definition
The first thoracic spinal nerve T1 is a mixed nerve, containing both somatic efferent (motor) and somatic afferent (sensory) fibers. It originates from the spinal cord at the T1 spinal segment.
a) Roots
Like other nerves, the spinal nerve T1 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 T1. This mixed nerve, containing both sensory and motor fibers, exits the intervertebral foramen between the T1 and T2 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 first 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 upper back.
It also provides sensory innervation to the skin overlying the upper back.
A. ANTERIOR RAMUS OF SPINAL NERVE T1 (the first intercostal nerve):
The anterior ramus of the spinal nerve T1 exits the vertebral canal through the intervertebral foramen between the T1 and T2 vertebrae. As a mixed nerve, it contains both motor (somatic efferent) and sensory (somatic afferent) fibers. A significant portion of this ramus contributes to the brachial plexus by forming a prominent communicating branch that joins with the anterior ramus of the C8 spinal nerve to create the inferior trunk of the brachial plexus.
The remaining portion of the T1 anterior ramus becomes the first intercostal nerve, which traverses the first intercostal space of the chest wall. In this region, the nerve is part of a neurovascular bundle that includes the intercostal artery and vein. This bundle is positioned between the internal intercostal muscle on its superficial side and the innermost intercostal muscle on its deep side, as the nerve courses along the chest wall.
Key functions and innervations
Motor Innervation (Somatic Efferent Fibers): | Sensory Innervation (Somatic Afferent Fibers): | ||
1- Upper limb muscles: The anterior ramus of spinal nerve T1, a critical contributor to the inferior trunk of the brachial plexus, provides motor innervation to several upper limb muscles. These include: - Pectoral muscles: Pectoralis minor muscle (C8, T1) and the sternocostal head of the pectoralis major muscle (C6-T1), both supplied via the medial pectoral branch of the medial cord of the brachial plexus. - Forearm muscles (flexor side): Flexor carpi ulnaris (C7, C8, T1) via the ulnar nerve, flexor digitorum superficialis (C8, T1), via the median nerve, and flexor digitorum profundus (C8, T1); its medial half via the ulnar and lateral half via the median (anterior interosseus) nerve. - Hand muscles: T1 contributes to the innervation of intrinsic hand muscles, supplied via: a)- The superficial branch of the ulnar nerve (C8, T1) to palmaris brevis muscle. b)- The deep branch of the ulnar nerve (C8, T1) to hypothenar muscles (abductor, opponents, & flexor digiti minimi muscles), adductor pollicis, 3rd & 4th lumbricals, palmar interossei, dorsal interossei muscles. c)- The recurrent branch of the median nerve (C8, T1) to the thenar (abductor, opponens, & flexor pollicis brevis) muscles, d)- The digital branches of median nerve (C8, T1) to the 1st & 2nd lumbricals. 2- Thoracic wall muscles: The remaining portion of the anterior ramus of spinal nerve T1, which continues beneath the first rib as the first intercostal nerve, provides motor innervation to important thoracic wall muscles, including: External, internal, and innermost intercostal muscles (T1-T11) Subcostal, and transversus thoracis muscles | The sensory fibers of the first intercostal nerve (T1) innervate the skin corresponding to the T1 dermatome, which is primarily located on the medial aspect of the arm. These sensory fibers reach their target region via a cutaneous branch of the ulnar nerve, known as the medial cutaneous nerve of the arm. | ||
B. POSTERIOR RAMUS OF SPINAL NERVE T1:
The posterior ramus of the T1 is the smaller, posterior branch of the spinal nerve T1. 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 T1 spinal nerve exits the intervertebral foramen (between the T1 and T2 vertebrae), its posterior ramus passes through the intertransverse ligament and the overlying intertransverse (intertransversarii) 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 extrinsic back muscles, to become a cutaneous branch. |
C. ADDITIONAL STRUCTURES ASSOCIATED WITH SPINAL NERVE T1:
a) Recurrent Meningeal Nerve (Meningeal branch):
The recurrent meningeal nerve (or meningeal branch) originates from either the T1 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 T1 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 T1 | |||
ANTERIOR RAMUS | POSTERIOR RAMUS | ||
MOTOR STRUCTURES | SENSORY STRUCTURES | MOTOR STRUCTURES | SENSORY STRUCTURES |
External, internal, and innermost intercostal muscles, subcostal, and transversus thoracis muscles. Pectoralis minor muscle (C8, T1) and the sternocostal head of the pectoralis major muscle (C6-T1). Flexor carpi ulnaris (C7, C8, T1), flexor digitorum superficialis (C8, T1), and flexor digitorum profundus (C8, T1) muscles. Palmaris brevis, hypothenar (abductor, opponens & flexor digiti minimi) muscles, thenar (abductor, opponens, & flexor pollicis brevis) muscles, adductor pollicis, lumbricals, palmar interossei, dorsal interossei. | T1 dermatome of skin encompassing the medial aspects of the arm and forearm, above and below the elbow). | Erector spinae muscles (iliocostalis, longissimus, spinalis), transversospinales muscles (multifidus, semispinalis, rotatores), segmental muscles (interspinales, intertransversarii, levtores costarum). | Zygapophyseal joints between T1 & T2 vertebrae, skin of the upper back corresponding to the T1 dermatome level. |
References
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