Neuroanatomy, Spine

Structure and Function

The vertebral column serves multiple essential functions, including protecting the spinal cord and nerves, providing structural support, and enabling movement. The spinal cord is housed within the spinal canal, a central lumen within each vertebral body, while spinal nerves emerge at each vertebral level to form the sympathetic trunk and splanchnic nerves. The diameter of the spinal canal varies along the vertebral column, being wider in the cervical and lumbar regions and narrower in the thoracic region.

Structurally, the spine acts as the body's central axis for weight bearing, supporting the head and transferring the weight of the trunk and abdomen to the lower limbs.[3] This bony framework facilitates dynamic movement by serving as the attachment site for multiple muscle groups in the neck, thorax, abdomen, and pelvis. The spine's unique jointed structure allows rotation and bending while maintaining its natural curvatures: cervical lordosis, thoracic kyphosis, and lumbar lordosis.[4][5] Intervertebral discs cushion the vertebrae and consist of an annulus fibrosus and a nucleus pulposus. These cartilaginous structures support the spine's anterior and posterior longitudinal ligaments.[6]

As mentioned, the adult spine has a natural curvature characterized by cervical lordosis, thoracic kyphosis, and lumbar lordosis.[7] These curvatures originate during embryogenesis and continue developing throughout childhood. Thoracic kyphosis and sacral curvature form during fetal development, creating the characteristic “fetal position.” Cervical lordosis emerges as infants begin supporting their heads, while lumbar lordosis develops when children start standing and bearing weight.[8] Abnormal exaggeration or alteration of these curvatures can result from various pathological conditions.

Embryology

The human skeleton begins developing shortly after conception. The notochordal process forms between days 15 and 30.[9] The floor of this structure fuses with the roof of the yolk sac, derived from the underlying endoderm, to create the notochordal plate. This plate then folds inward, giving rise to the notochord, which provides structural support to the embryo and serves as the foundation for vertebral column development. In adults, remnants of the notochord persist as the nucleus pulposus of intervertebral discs.[10] Additionally, the notochord induces the overlying ectoderm to form neuroectoderm, leading to the development of the neural plate. The neural plate subsequently forms the neural tube, with its rostral portion developing into the brain and its caudal end giving rise to the spinal cord.[11]

Around the 3rd week of embryogenesis, during gastrulation, the sclerotome—a mesodermal derivative of the somites—grows around the neural tube and notochord, forming the vertebral bodies, arches, and ribs. Each vertebral body arises through resegmentation, a process in which the caudal and cranial halves of adjacent sclerotomes fuse.[12]

Vertebral development occurs through endochondral ossification. Cartilage forms around the 6th week of gestation and begins to be replaced by bone between weeks 7 and 8. Complete ossification is typically achieved in early adulthood by age 26, although bone remodeling, involving both degeneration and regeneration, continues throughout life.

Blood Supply and Lymphatics

Spinal arteries arise from branches of larger arteries that supply internal organs. In the neck, the subclavian arteries give rise to the vertebral and ascending cervical arteries, which supply the spine. The posterior intercostal arteries originate from the thoracic aorta, while the lumbar arteries arise from the abdominal aorta (see Image. Artery of Adamkiewicz).[13] In the pelvis, the lateral sacral arteries branch from the internal iliac arteries. Each of these blood vessels divides into anterior and posterior branches.[14] The anterior branch supplies the vertebral body, and its posterior counterpart supplies the vertebral arch. Radicular arteries travel along the middle of the vertebral column, supplying the interior of the vertebral canal and spinal structures.

Venous drainage of the spine occurs through the radicular veins, which collect blood from smaller veins of the spinal cord and the internal and external vertebral veins.[15] These veins empty into segmental and intervertebral veins, which subsequently drain into 1 of 3 major vessels depending on location. The superior vena cava drains the cervical spine, while the azygos and hemiazygos veins drain the thoracic region. The inferior vena cava is responsible for draining the lumbar and sacral spine.[16]

Nerves

The central nervous system consists of the brain and spinal cord, while the peripheral nervous system includes the cranial nerves, spinal nerves, and their associated ganglia. The spinal cord is enclosed within the vertebral column and extends from the base of the brain to the lower spine, terminating at the conus medullaris and filum terminale. The conus medullaris is the tapered, cone-shaped end of the spinal cord, typically ending at the L1-L2 level in adults, though anatomical variations may extend it further.[17]

In newborns, the spinal cord reaches as low as L3-L4. This termination point is clinically significant for procedures such as lumbar puncture to minimize the risk of spinal cord injury (see Image. Lumbar Puncture Landmarks). The filum terminale is a delicate fibrous extension from the conus medullaris that anchors the spinal cord to the dorsal surface of the coccyx. Below the conus medullaris, the cauda equina (“horse’s tail”) consists of nerve roots that continue downward (see Image. Human Caudal Spinal Cord).[18]

Thirty-one pairs of spinal nerves emerge from the spinal cord and exit through the intervertebral foramina to innervate peripheral structures. Meningeal branches of these spinal nerves provide innervation to the vertebrae.

Muscles

As the central weight-bearing structure, the spine endures significant mechanical stress. Muscles attached to the spine help maintain posture and distribute uneven forces from body weight. These muscles are classified into extrinsic and intrinsic back muscle groups.[19]

The extrinsic muscles are further divided into superficial and intermediate groups. The superficial extrinsic muscles, including the trapezius, latissimus dorsi, levator scapulae, and rhomboid major and minor, facilitate upper limb movement by controlling the scapula and humerus. The intermediate extrinsic muscles, comprising the serratus posterior superior and serratus posterior inferior, assist in rib movement to aid respiration.

The intrinsic back muscles are organized into superficial, intermediate, and deep layers.[20] These muscles aid in spinal movement and postural maintenance. The superficial layer consists of the splenius capitis and splenius cervicis, which contribute to neck flexion, rotation, and extension. The intermediate layer is primarily composed of the erector spinae muscles—iliocostalis, longissimus, and spinalis—which are essential for spinal extension and maintenance of its natural curvature.

The deep layer of intrinsic back muscles includes those situated between the transverse and spinous processes of the vertebrae, often referred to as the "paravertebral muscles." These muscles are categorized into 3 groups. The semispinalis is most superficial and prominent in the thoracic and cervical regions. The multifidus lies beneath the semispinalis and is most developed in the lumbar region. The rotatores, the deepest group, are most pronounced in the thoracic region.[21]

The suboccipital muscles, located deep in the posterior neck, attach to the skull and control head movement. This group includes the muscles forming the suboccipital triangle: the rectus capitis posterior major, obliquus capitis superior, and obliquus capitis inferior.[22] The suboccipital triangle is clinically significant as it houses the vertebral artery’s loop from the transverse foramen of the atlas before it enters the foramen magnum to supply the brainstem.

Surgical Considerations

Spinal surgical procedures vary based on the underlying pathology and may be performed in either acute or elective settings. Indications for surgery include fractures, dislocations, tumors, scoliosis, and other mechanical abnormalities. The posterior approach, with the patient positioned prone, is the most commonly used surgical technique. However, certain conditions may warrant an anterior approach, such as in anterior lumbar interbody fusion.[23] Potential complications of spinal surgery include neurological injury, persistent pain, vertebral fracture, and infection.[24]

Clinical Significance

Spinal conditions can significantly impact structural integrity, neurological function, and overall quality of life. These disorders may be inborn or acquired. Congenital anomalies arise during embryonic development, whereas noncongenital pathologies result from degenerative, inflammatory, infectious, or traumatic processes.

Congenital Spinal Anomalies

Congenital anomalies of the spine arise from developmental defects during embryogenesis, affecting vertebral formation and neural structures (see Image. Os Odontoideum). One such condition is VACTERL association, which presents with vertebral defects, anal atresia, cardiac anomalies, tracheoesophageal fistula, renal malformations, and limb abnormalities. Spinal defects in VACTERL may present as fused, missing, supernumerary, or malformed vertebrae. Diagnosis requires at least 3 of these features, with vertebral anomalies occurring in 60% to 80% of affected individuals.[25]

Spina bifida is a neural tube defect characterized by incomplete closure of the vertebral column and meninges. The condition presents in 3 forms of increasing severity: spina bifida occulta, meningocele, and myelomeningocele. Myelomeningocele, the most severe type, is frequently associated with Arnold-Chiari malformation and tethered spinal cord, which can lead to significant neurological impairment.

Acquired Spinal Pathology

Noncongenital spinal disorders encompass a range of conditions affecting the structural integrity and function of the spine. These disorders may result from degenerative changes, inflammatory processes, trauma, or infections, often leading to pain, neurological deficits, and impaired mobility.

Osteoarthritis, the most common form of arthritis, involves progressive erosion of joint cartilage and frequently affects the spine. In advanced cases, this degenerative condition can lead to hyperextension of the cervical spine and compression of spinal nerves, resulting in pain. Osteoporosis is characterized by bone demineralization. The United States Preventive Services Task Force (USPSTF) recommends screening for osteoporosis in all women aged 65 and older, as well as younger women with risk factors, which can be assessed using the Fracture Risk Assessment (FRAX) tool.[26] Severe osteoporosis increases the risk of developing a vertebral compression fracture.[27]

Seronegative spondyloarthropathies are inflammatory diseases of the vertebral column that lack the serum markers antinuclear antibody (ANA) and rheumatoid factor but have an increased incidence in individuals with the human leukocyte antigen B27 (HLA-B27) allele. This group includes ankylosing spondylitis, reactive arthritis, psoriatic arthritis, juvenile idiopathic arthritis, and spondylitis associated with inflammatory bowel disease.[28] Spinal cord injuries, often resulting from trauma, can cause severe neurological deficits such as paraplegia, significantly impacting quality of life and functional independence.[29]

Spinal disc herniation occurs when the annulus fibrosus of an intervertebral disc tears, allowing the nucleus pulposus to herniate outward (see Image. Cauda Equina Syndrome). This condition can lead to symptoms such as pain, numbness, weakness, and decreased reflexes.[30] Spinal stenosis, or narrowing of the central spinal canal, may cause similar symptoms in the upper or lower extremities. This condition can result from osteoarthritis, rheumatoid arthritis, Paget disease, or trauma, with symptoms classically improving when bending forward.[31]

Spondylolisthesis refers to the anterior displacement of one vertebra over another, which may compress spinal nerves and cause pain along their distribution.[32] Tuberculous spondylitis, also known as Pott disease, is an infection of the vertebral bodies and intervertebral discs caused by Mycobacterium tuberculosis. This condition can present with back pain, kyphosis, lower limb weakness, fever, and weight loss.[33]