Spinal Cord Anatomy and Neuroanatomy
SCI · EP 01 · NEUROANATOMY
Before You Listen
- Prerequisites: basic central nervous system organization (gray matter neurons, white matter tracts, primary sensory and motor cortex); the four meningeal layers in cross-section; awareness that the corticospinal tract carries voluntary motor signals from cortex to muscle; the difference between upper motor neuron (UMN) and lower motor neuron (LMN) signs.
- Runtime: 43 minutes 55 seconds.
- Topic in one line: the bony container (33 vertebrae, the four physiologic curves, the C1–C2 dens articulation, the posterior ligamentous complex) versus its neural contents (cord segments C1 through coccygeal, conus medullaris at L1–L2, cauda equina, denticulate ligaments), the Rule of 1, 2, 3 vertebral-cord level discrepancy that decouples imaging level from neurologic level, the three ascending sensory systems (dorsal column-medial lemniscus, lateral spinothalamic, spinocerebellar) and the descending corticospinal tract with their crossing points and somatotopic maps, the Rexed laminar organization of gray matter with Clarke’s nucleus at C8–L2, the intermediolateral cell column at T1–L2 (origin of all sympathetic outflow and the anatomic substrate for autonomic dysreflexia), the sacral parasympathetic nucleus at S2–S4, the Onuf nucleus controlling continence, the anterior spinal artery and its segmental reinforcement by the artery of Adamkiewicz between T9 and T12, the T4–T8 mid-thoracic watershed zone, and the valveless Batson plexus that explains hematogenous vertebral metastasis.
Vignette. A 68-year-old man with long-standing cervical spondylosis catches his toe on a rug, instinctively throws his head backward as he falls, and strikes the floor. Imaging shows no fracture, no dislocation, and no canal disruption on plain films. On bedside examination he has profound weakness and impaired pinprick sensation in the hands, with much milder leg weakness; proprioception and vibration are preserved throughout.
Which incomplete cord syndrome did he sustain, what is the anatomic explanation for the upper-extremity-greater-than-lower-extremity weakness pattern, which two ascending tracts account for his preserved versus impaired sensation, and how does the somatotopy of the lateral corticospinal tract drive that selective hand weakness?
Section 1: The Bony Container — Vertebral Column, Curves, and the C1–C2 Pivot
Bottom line: 33 vertebrae stack into a coiled spring with two primary kyphotic curves (thoracic, sacral) and two secondary lordotic curves (cervical, lumbar); only the upper 24 are mobile; the C1 atlas lacks a body and the C2 axis carries the dens, which generates 50% of total cervical rotation.
The vertebral column is a 33-vertebra axial skeleton organized into seven cervical, twelve thoracic, five lumbar, five sacral (fused into the sacrum), and four coccygeal segments. Only the upper 24 are mobile in the adult; the sacral five are wedged into the pelvis and the coccygeal four are variably fused. The structure is not a straight rod. It is a coiled spring with four physiologic curvatures: the thoracic and sacral curves are primary, present at birth and concave anteriorly (kyphotic), while the cervical and lumbar curves are secondary, lordotic, and develop in response to gravity. The cervical lordosis appears as the infant gains head control; the lumbar lordosis appears as the child begins to walk upright. Boards reward the primary-versus-secondary distinction directly.
A typical vertebra has two functional parts. The anterior vertebral body (centrum) is the weight-bearing core, made of cancellous bone wrapped in cortical bone. The posterior vertebral arch is built from two pedicles and two laminae, and together with the back of the body it encloses the vertebral foramen. Stack the foramina and you get the vertebral canal that houses the spinal cord, meninges, and segmental vessels. Seven processes project from the arch: one spinous process, two transverse processes, and four articular processes. The orientation of the articular processes (the facet joints) varies by region and dictates the dominant plane of motion at each level. Thoracic facets are coronal (favoring rotation), and lumbar facets are sagittal (favoring flexion-extension while limiting rotation).
Cervical vertebrae from C1 through C7 carry small bodies, bifid spinous processes from C2 through C6, and transverse foramina that transmit the vertebral arteries from C1 through C6. Two vertebrae are exceptional. The atlas (C1) lacks both a body and a spinous process; it consists of anterior and posterior arches with two lateral masses, and it cradles the skull. The axis (C2) bears the dens (odontoid process) projecting upward from its body. The dens articulates with the anterior arch of the atlas at the median atlantoaxial joint, and that joint alone provides approximately 50% of total cervical rotation. The other 50% of flexion-extension belongs to the atlanto-occipital joint above. Memorize the 50-50 split. Thoracic vertebrae carry costal facets on the body and transverse process for rib articulation, with long downward-angled spinous processes. Lumbar vertebrae have the largest bodies of any segment, reflecting cumulative axial load.
High Yield — Bony container essentials
- 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused), 4 coccygeal. Only the upper 24 are mobile.
- Primary curves (kyphotic): thoracic and sacral, present at birth.
- Secondary curves (lordotic): cervical (head control) and lumbar (upright gait).
- C1 atlas has no body, no spinous process; it is a ring with lateral masses cradling the skull.
- C2 axis carries the dens; the median atlantoaxial joint provides approximately 50% of cervical rotation.
- Vertebral arteries pass through the transverse foramina of C1 through C6.
- Facet orientation: thoracic coronal (rotation), lumbar sagittal (flexion-extension).