Comprehensive Anatomy of Spinal Nerves and Nerve Plexuses

External Description and Segmentation of the Spinal Cord

The spinal cord is structurally divided into distinct segments, with a total of $31$ segments identified in the human body. A segment is specifically defined as the portion of the spinal cord that corresponds to one pair of spinal nerves. These segments are categorized based on their location within the vertebral column, mirroring the various regions of the spine itself. Each part consists of a specific number of segments: the cervical part ($Pars \text{ } cervicalis$) contains $8$ cervical segments ($segmenta \text{ } cervicalia$); the thoracic part ($Pars \text{ } thoracica$) contains $12$ thoracic segments ($segmenta \text{ } thoracica$); the lumbar part ($Pars \text{ } lumbalis$) contains $5$ lumbar segments ($segmenta \text{ } lumbalia$); the sacral part ($Pars \text{ } sacralis$) contains $5$ sacral segments ($segmenta \text{ } sacralia$); and the coccygeal part ($Pars \text{ } coccygea$) contains $1$ coccygeal segment ($segmentum \text{ } coccygeum$).

Morphological Structure and Formation of Spinal Nerves ($Nn. \text{ } Spinales$)

Spinal nerves ($Nn. \text{ } Spinales$) are symmetrical and metameric structures. As established, there are $31$ pairs in total: $8$ pairs of cervical nerves ($Nn. \text{ } Cervicales$), $12$ pairs of thoracic nerves ($Nn. \text{ } Thoracici$), $5$ pairs of lumbar nerves ($Nn. \text{ } Lumbales$), $5$ pairs of sacral nerves ($Nn. \text{ } Sacrales$), and $1$ pair of coccygeal nerve ($N. \text{ } Coccygeus$). Each spinal nerve is formed by the fusion of two roots: the dorsal root ($Radix \text{ } dorsalis \text{ } s. \text{ } sensoria$) and the ventral root ($Radix \text{ } ventralis \text{ } s. \text{ } motoria$).

The dorsal root ($Radix \text{ } dorsalis$) enters the spinal cord through the $sulcus \text{ } posterolateralis$. Located on this root is the spinal nerve ganglion ($ganglion \text{ } spinale$), which contains pseudounipolar sensory cells. The peripheral processes of these cells (dendrites) originate in the periphery with receptors and travel through the spinal nerve to the ganglion, where the cell bodies (pericaryon) are located. The central processes (axons) enter the spinal cord via the posterior root and form synapses with interneurons, whose axons subsequently synapse with motor neurons in the motor nuclei. The ventral root ($Radix \text{ } ventralis$) exits the spinal cord via the $sulcus \text{ } anterolateralis$ and is composed of axons from cells in the motor nuclei that travel toward the periphery. Additionally, specific segments carry vegetative fibers: sympathetic cell axons from the $nucleus \text{ } intermediolateralis$ travel via the ventral roots from segments $C8$ to $L2$, while parasympathetic cell axons from the $nucleus \text{ } parasympathicus \text{ } lateralis \text{ } s. \text{ } sacralis$ travel via the ventral roots in segments $S2$ to $S4$.

Topography and Primary Branching of Spinal Nerves

Lateral to the $ganglion \text{ } spinale$ near the $foramen \text{ } intervertebrale$, the two roots join to form the spinal nerve ($N. \text{ } Spinalis$). The length of these roots varies; they are relatively short for cervical and thoracic nerves but significantly longer for lumbar, sacral, and coccygeal nerves. Because the roots merge at the level of their respective exit foramina—which is far from their origin in the lower cord—the descending roots form a bundle parallel to the $filum \text{ } terminale$ known as the $cauda \text{ } equina$ (horse\'s tail). In an adult, the spinal nerve itself is approximately $1\,cm$ long. It exits the $canalis \text{ } vertebralis$ through the $foramen \text{ } intervertebrale$ and immediately divides into four primary branches: the anterior branch ($Ramus \text{ } ventralis$), the posterior branch ($Ramus \text{ } dorsalis$), the white communicating branch ($Ramus \text{ } communicans \text{ } albus$), and the meningeal branch ($Ramus \text{ } meningeus$).

Spinal nerves are mixed nerves, containing sensory, motor, and vegetative (sympathetic and/or parasympathetic) fibers. The posterior branches ($rr. \text{ } dorsales$) innervate the deep muscles of the back and provide sensory innervation to the skin of the occiput, neck, and back. The white communicating branches ($rr. \text{ } communicantes \text{ } albi$) contain sympathetic fibers that travel from the spinal cord to the ganglia of the sympathetic system. The meningeal branches ($rr. \text{ } meningei$) return to the vertebral canal through the $foramina \text{ } intervertebralia$ to innervate the spinal meninges. The anterior branches ($rami \text{ } ventrales$) of the cervical, lumbar, sacral, and coccygeal nerves form four plexuses: $Plexus \text{ } cervicalis$, $Plexus \text{ } brachialis$, $Plexus \text{ } lumbalis$, and $Plexus \text{ } sacralis$. Crucially, the anterior branches of thoracic nerves do not form plexuses but instead become the intercostal nerves ($nn. \text{ } intercostales$).

Cervical Plexus ($Plexus \text{ } cervicalis$)

The cervical plexus is formed by the anterior branches of the first four cervical nerves ($C1-C4$). It is located at the base of the $trigonum \text{ } colli \text{ } laterale$, resting against the deep neck muscles and covered anteriorly by the $m. \text{ } sternocleidomastoideus$. The plexus gives off sensory nerves, motor nerves, and one mixed nerve. The sensory nerves (4) emerge around the posterior margin of the $m. \text{ } sternocleidomastoideus$: the lesser occipital nerve ($N. \text{ } occipitalis \text{ } minor$), which innervates the skin of the posterior ear surface and lateral occipital region; the great auricular nerve ($N. \text{ } auricularis \text{ } magnus$), which innervates the skin near the angle of the mandible, the ear lobe, and surrounding areas; the transverse cervical nerve ($N. \text{ } transversus \text{ } colli$), which innervates the skin of the $trigonum \text{ } colli \text{ } mediale$; and the supraclavicular nerves ($Nn. \text{ } supraclaviculares$), which innervate the skin of the $trigonum \text{ } colli \text{ } laterale$, the anterior chest wall down to the level of the $2$nd rib, and the shoulder region.

The motor branches ($rr. \text{ } musculares$) innervate the deep neck muscles ($Mm. \text{ } Profundi \text{ } colli$), lateral neck muscles ($Mm. \text{ } Laterales \text{ } colli$), and craniothoracic muscles ($Mm. \text{ } Craniothoracales$) in conjunction with the accessory nerve. A key motor structure is the $ansa \text{ } cervicalis$, formed by two roots: the $Radix \text{ } superior$ (from $C1$ and $C2$, travelling briefly with the $n. \text{ } hypoglossus$) and the $Radix \text{ } inferior$ (from $C2-C3$). This loop lies on the anterior surface of the neurovascular bundle of the neck. The mixed nerve of the plexus is the phrenic nerve ($n. \text{ } phrenicus$), which contains motor, sensory, and postganglionic sympathetic fibers. It descends on the $m. \text{ } scalenus \text{ } anterior$, enters the thorax between the subclavian artery and vein, and travels through the mediastinum to the diaphragm. It provides motor innervation to the diaphragm and sensory innervation to the pericardium, mediastinal pleura, diaphragmatic pleura, and the peritoneum covering the inferior surface of the diaphragm.

Brachial Plexus ($Plexus \text{ } brachialis$)

The brachial plexus is formed by the anterior branches of the lower four cervical nerves ($C5-C8$), with contributions from $C4$ and $Th1$. It is located in the $spatium \text{ } interscalenum$ between the $m. \text{ } scalenus \text{ } anterior$ and $m. \text{ } scalenus \text{ } medius$ alongside the $a. \text{ } subclavia$. It then enters the axillary fossa through the $apertura \text{ } superior \text{ } cavi \text{ } axillaris$. Topographically, it is divided into the supraclavicular part ($Pars \text{ } supraclavicularis$) in the neck and the infraclavicular part ($Pars \text{ } infraclavicularis$) in the axilla.

The supraclavicular part gives off seven motor nerves: $N. \text{ } subclavius$ (to $m. \text{ } subclavius$); $Nn. \text{ } pectorales$ (to $m. \text{ } pectoralis \text{ } major \text{ } et \text{ } minor$); $N. \text{ } thoracicus \text{ } longus$ (to $m. \text{ } serratus \text{ } anterior$); $Nn. \text{ } subscapulares$ (to $m. \text{ } subscapularis$, $m. \text{ } teres \text{ } major$, and the shoulder joint); $N. \text{ } suprascapularis$ (to $m. \text{ } supraspinatus \text{ } et \text{ } m. \text{ } infraspinatus$); $N. \text{ } thoracodorsalis$ (to $m. \text{ } latissimus \text{ } dorsi$); and $N. \text{ } dorsalis \text{ } scapulae$ (to $mm. \text{ } rhomboidei$ and $m. \text{ } levator \text{ } scapulae$).

The infraclavicular part consists of three cords—lateral, medial, and posterior—surrounding the $a. \text{ } axillaris$. The lateral cord ($fasciculus \text{ } lateralis$) yields the musculocutaneous nerve ($N. \text{ } musculocutaneus$), which innervates the anterior arm muscles ($m. \text{ } biceps \text{ } brachii$, $m. \text{ } brachialis$, $m. \text{ } coracobrachialis$) and continuous as the lateral cutaneous nerve of the forearm. The medial cord ($fasciculus \text{ } medialis$) gives off the medial cutaneous nerves of the arm and forearm, and the ulnar nerve ($N. \text{ } ulnar_is$). The median nerve ($N. \text{ } medianus$) is formed by roots from both the lateral and medial cords. The posterior cord ($fasciculus \text{ } posterior$) divides into the axillary nerve ($N. \text{ } axillaris$), innervating the deltoid and $m. \text{ } teres \text{ } minor$, and the radial nerve ($N. \text{ } radialis$), which innervates the triceps and the posterior groups of the forearm.

Intercostal Nerves ($Nn. \text{ } intercostales$)

There are $12$ pairs of thoracic spinal nerve anterior branches, termed intercostal nerves ($Nn. \text{ } intercostales$). The twelfth pair is specifically called the subcostal nerve ($n. \text{ } subcostalis$). These mixed nerves travel along the $sulcus \text{ } costae$ below the corresponding vein and artery. The upper six nerves reach the sternum, while the lower six traverse the ribs to the lateral and anterior abdominal muscles, reaching the $m. \text{ } rectus \text{ } abdominis$. They innervate the deep chest muscles, the anterior/lateral abdominal muscles down to the hypogastrium, and the deep layers of superficial back muscles ($m. \text{ } serratus \text{ } posterior \text{ } superior \text{ } et \text{ } inferior$). Sensory innervation is provided to the costal pleura and the parietal peritoneum. Skin innervation covers the anterior and lateral surfaces of the thorax (below the second rib) and abdomen down to the hypogastrium.

Lumbar Plexus ($Plexus \text{ } lumbalis$)

The lumbar plexus is formed by the anterior branches of $L1-L3$, partially $L4$, and sometimes $Th12$. It is situated within the mass of the $m. \text{ } psoas \text{ } major$. It gives off short muscular branches to the $m. \text{ } iliopsoas$, $m. \text{ } psoas \text{ } minor$, and $m. \text{ } quadratus \text{ } lumborum$, and six long nerves. Emerging from the lateral border of the $m. \text{ } psoas \text{ } major$ are: the iliohypogastric nerve ($N. \text{ } iliohypogastricus$), providing motor/sensory supply to the hypogastrium and lateral gluteal skin; the ilioinguinal nerve ($N. \text{ } ilioinguinalis$), supplying abdominal muscles and skin of the scrotum or labia majora; the lateral cutaneous nerve of the thigh ($N. \text{ } cutaneus \text{ } femoris \text{ } lateralis$); and the femoral nerve ($N. \text{ } femoralis$). The femoral nerve is the largest, innervating the anterior thigh muscles ($m. \text{ } quadriceps \text{ } femoris$, $m. \text{ } sartorius$) and providing the saphenous nerve ($n. \text{ } saphenus$) which reaches the medial foot. The obturator nerve ($N. \text{ } obturatorius$) emerges from the medial border of the psoas, innervating the medial thigh muscles, while the genitofemoral nerve ($N. \text{ } genitofemoralis$) emerges from the anterior surface of the psoas.

Sacral Plexus ($Plexus \text{ } sacralis$)

The sacral plexus is the largest plexus, formed by the $truncus \text{ } lumbosacralis$ ($L4-L5$) and the anterior branches of all sacral and the coccygeal nerve. It lies on the $m. \text{ } piriformis$ in the lesser pelvis and contains parasympathetic fibers from $S2-S4$. Short branches include muscular nerves to the pelvic girdle, the superior gluteal nerve ($N. \text{ } gluteus \text{ } superior$), the inferior gluteal nerve ($N. \text{ } gluteus \text{ } inferior$), and the mixed pudendal nerve ($N. \text{ } pudendus$), which supplies the perineum and external genitalia.

Long branches consist of the posterior cutaneous nerve of the thigh ($N. \text{ } cutaneus \text{ } femoris \text{ } posterior$) and the sciatic nerve ($N. \text{ } ischiadicus$). The sciatic nerve is the largest peripheral nerve in the body. It descends the posterior thigh, innervating its muscles, and divides above the popliteal fossa into the tibial nerve ($n. \text{ } tibialis$) and the common fibular (peroneal) nerve ($n. \text{ } fibularis \text{ } communis$). The tibial nerve innervates the posterior leg and plantar foot. The common fibular nerve divides into the deep fibular nerve (anterior leg muscles and first interdigital space) and the superficial fibular nerve (lateral leg muscles and dorsal foot skin).