Back and Spine Overview

The back extends from the base of the skull to the coccyx, comprising the vertebral column, the surrounding back muscles (superficial/extrinsic and deep/intrinsic layers), the thoracolumbar fascia, and (within the vertebral canal) the spinal cord, meninges, and nerve roots.

• A flexible, segmented column of 33 vertebrae (though the number of mobile, individually articulating vertebrae is fewer, given sacral and coccygeal fusion), providing structural support for the trunk, protection for the spinal cord, and attachment points for the ribs, pelvis, and musculature.

Cervical region: 7 vertebrae (C1-C7), supporting the head, the most mobile region of the spine.

Thoracic region: 12 vertebrae (T1-T12), articulating with the ribs, the least mobile region given this rib articulation and the more vertically oriented facet joints.

Lumbar region: 5 vertebrae (L1-L5), the largest individual vertebrae, bearing the greatest axial load.

Sacral region: 5 fused vertebrae forming the sacrum, a single wedge-shaped bone forming the posterior pelvic wall.

Coccygeal region: typically 4 (variably 3-5) rudimentary, usually fused vertebrae forming the coccyx.

• Primary curvatures (thoracic and sacral), concave anteriorly, present from fetal development, reflecting the fetus's flexed intrauterine posture, retained into adulthood.
• Secondary curvatures (cervical and lumbar), convex anteriorly (lordotic), developing postnatally in response to functional demands: the cervical curvature develops as the infant begins to hold up its head; the lumbar curvature develops as the infant begins to stand and walk, shifting the body's weight-bearing axis.
• This alternating curve pattern (concave-convex-concave-convex, cervical to sacral) provides the spine with greater resilience to axial load than a straight column would offer, functioning similarly to a spring.

• Interbody joints (secondary cartilaginous, via the intervertebral discs), zygapophyseal (facet) joints (synovial, paired, posterior), the specialised craniovertebral joints (atlanto-occipital, atlanto-axial), and the sacroiliac joints.

• Anterior and posterior longitudinal ligaments (running the column's full length), ligamentum flavum, interspinous and supraspinous ligaments, intertransverse ligaments, and the specialised craniovertebral junction ligaments (transverse ligament of the atlas, alar ligaments, and others).

• The extrinsic back muscles (trapezius, latissimus dorsi, levator scapulae, rhomboids, and the serratus posterior muscles), developmentally migrated from the limb bud/upper body wall, acting predominantly on the shoulder girdle/upper limb or ribs rather than the spine itself, innervated by nerves other than the posterior rami.

• The true (intrinsic) back muscles, developmentally derived from the paraxial mesoderm alongside the vertebral column itself, acting directly on the spine (extension, rotation, lateral flexion, postural control), uniformly innervated by the posterior rami of the spinal nerves, organised into superficial (splenius), intermediate (erector spinae), and deep (transversospinalis and minor segmental muscles) layers.

• The continuous canal formed by the successive vertebral foramina of the articulated vertebral column, housing the spinal cord, its meninges, nerve roots, and the internal vertebral venous plexus; full detail in Section 8.

• The spinal cord occupies the vertebral canal from the foramen magnum to approximately L1-L2 in adults, shorter than the canal itself given differential growth; full detail in Section 9 (and see Neuroanatomy content, Section 9, for the cord's own detailed internal anatomy).

• 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal), each formed by the union of a dorsal (sensory) and ventral (motor) root, exiting the vertebral canal via the intervertebral foramina; full detail in Section 9-10.

• Segmental arteries (vertebral, posterior intercostal, lumbar, lateral sacral arteries, depending on level) supply the vertebral column itself, while the spinal cord receives its own dedicated supply via the anterior and posterior spinal arteries, reinforced by segmental radicular/medullary arteries.

• External occipital protuberance, C7 spinous process (vertebra prominens), scapular spine (T3 level), inferior angle of scapula (T7 level), iliac crest (L4 level), posterior superior iliac spine, sacral hiatus, coccyx.

• Flexion/extension (predominantly cervical and lumbar, given their greater mobility and lordotic curvature), lateral flexion, and rotation (predominantly cervical and thoracic, given lumbar facet orientation's relative restriction of rotation), the specific combination and range of movement available at each spinal level is directly determined by the local facet joint orientation and disc characteristics.