The Five Regions of the Spine
The human spine (vertebral column) consists of 33 individual bones that transition from high mobility at the top to stability and weight-bearing at the bottom.
| Region | Number of Vertebrae | Primary Function |
|---|---|---|
| Cervical (C1–C7) | 7 | Supports the head; allows for the widest range of motion (rotation/tilting). |
| Thoracic (T1–T12) | 12 | Anchors the rib cage; protects vital organs (heart/lungs); limited mobility. |
| Lumbar (L1–L5) | 5 | Supports the bulk of body weight; common site for disc issues. |
| Sacral (S1–S5) | 5 (fused) | Connects the spine to the hip bones (ilium) via the sacroiliac joints. |
| Coccygeal | 4 (fused) | The "tailbone"; provides an attachment point for ligaments and muscles. |
Anatomy of a Single Vertebra
While vertebrae vary in size, most share a common structure designed to protect the spinal cord and allow for movement.
- Vertebral Body: The large, drum-shaped front portion that bears weight.
- Vertebral Arch: The posterior (back) ring of bone that creates the Vertebral Foramen, the hollow "tunnel" through which the spinal cord passes.
- Processes: Bony protrusions (Spinous and Transverse) that serve as levers for muscles and ligaments.
- Facet Joints: Paired joints at the back of each vertebra that link them together, allowing for flexibility while preventing over-rotation.
Intervertebral Discs: The Shock Absorbers
Between each vertebra (except C1 and C2) lies a fibrocartilaginous disc.
- Annulus Fibrosus: The tough, circular exterior made of concentric collagen fibers.
- Nucleus Pulposus: The jelly-like center ($80\%$ water) that absorbs impact and distributes pressure evenly.
The Central Nervous System Pathway
The spine is essentially a "hard case" for the nervous system:
- Spinal Cord: Travels from the brainstem down to approximately the L1-L2 level.
- Cauda Equina: A bundle of nerve roots (resembling a horse's tail) that continues below the end of the spinal cord.
- Foramen: Small openings on the sides of the vertebrae where individual nerve roots exit the spine to travel to the limbs.
Natural Spinal Curves
A healthy spine is not a straight line; it has four natural curves that act like a coiled spring to absorb shock:
- Lordosis: The inward curve of the neck (Cervical) and lower back (Lumbar).
- Kyphosis: The outward curve of the mid-back (Thoracic) and the sacrum.