Overview
The spine provides support for our body, allowing us to sit and stand upright, bend, and twist, while protecting the spinal cord from injury. It is made up of 33 bones, multiple muscles, joints, ligaments, the spinal cord and the nerves. Any injury or disease affecting any of these structures can cause pain or dysfunction of the spine.
"The spine is not just a structural column — it is the central highway of the nervous system. At Stavya Spine Hospital, understanding spinal anatomy is the first step toward accurate diagnosis and effective treatment."
Spinal Curves
When a child is born, his spine has a single 'C' shaped curve. As the child starts lifting his head and gains neck control, concavity in the cervical (neck) region develops. When the child starts walking, another concavity develops in the lumbar (low back) region.
Thus, the adult spine has a natural 'S' shaped curve when viewed from the side. The curves work like a coiled spring to absorb shock, maintain balance, and allow range of motion throughout the spinal column.
The spinal curves are maintained by back and abdominal muscles. Excessive body weight or weak muscles can disturb the spine alignment and lead to increased stress on the spine.
Vertebrae
There are 33 vertebrae which are interconnected like the bogies of a train. These can be divided into five regions from neck down to the pelvis:
- Cervical (C1–C7): Supports the head (~10 lbs). C1 (atlas) is ring-shaped and connects to the skull, allowing the "yes" nodding motion. C2 (axis) is peg-shaped with an odontoid projection that the atlas pivots around, allowing the "no" side-to-side motion.
- Thoracic (T1–T12): Holds the rib cage and protects the heart and lungs. Range of motion is limited to stabilize the upper body.
- Lumbar (L1–L5): Bears the weight of the body. These large vertebrae absorb the stress of lifting and carrying heavy objects.
- Sacrum: Five fused vertebrae that connect the spine to the hip bones (iliac), forming the pelvic girdle.
- Coccyx: Four fused bones (tailbone) providing attachment for ligaments and muscles of the pelvic floor.
Intervertebral Discs
Vertebral bodies are stacked on each other joined by intervertebral discs, which act like a cushion. Discs are made up like car tyres — the outer ring (annulus) is formed by fibrous bands, while the inner part (nucleus) is filled with gel-like fluid.
The nucleus of the disc absorbs water during the night when we lie down and releases water when we are active in the day time. It is imperative to be active for proper functioning of the disc.
Ageing of the intervertebral discs starts as early as 25 years of age. With ageing, the disc becomes more prone to damage when subjected to physical stress, thus leading to disc prolapse or stenosis.
Vertebral Arch & Spinal Canal
On the back of each vertebra are bony projections that form the vertebral arch. The arch is made of two supporting pedicles and two laminae. The hollow spinal canal contains the spinal cord, fat, ligaments, and blood vessels.
Under each pedicle, a pair of spinal nerves exits the spinal cord and passes through the intervertebral foramen to branch out to your body. Surgeons often remove the lamina of the vertebral arch (laminectomy) to access the spinal cord and nerves to treat stenosis, tumors, or herniated discs.
Seven processes arise from the vertebral arch: the spinous process, two transverse processes, two superior facets, and two inferior facets.
Facet Joints & Ligaments
Facet Joints
The facet joints connect one vertebral body with another and allow movement of the back. Each vertebra has four facet joints — one pair connecting to the vertebra above (superior facets) and one pair connecting to the vertebra below (inferior facets).
Ligaments
The ligaments are strong fibrous bands that hold the vertebrae together, stabilize the spine, and protect the discs. The three major ligaments are the ligamentum flavum (which attaches between the lamina of each vertebra), the anterior longitudinal ligament (ALL), and the posterior longitudinal ligament (PLL). They prevent excessive movement and injury.
Spinal Cord
The spinal cord is a bundle of nerves carrying signals from the brain to all four limbs as well as to control bowel and bladder function. The spinal cord is protected in a bag of water, covered with meninges. Any damage to the spinal cord can result in a loss of sensory and motor function below the level of injury.
Spinal Nerves
Thirty-one pairs of spinal nerves branch off the spinal cord. The spinal nerves act as "telephone lines," carrying messages back and forth between your body and spinal cord to control sensation and movement. The spinal nerves are numbered according to the vertebrae above which they exit the spinal canal:
- Cervical: 8 pairs (C1–C8) innervating the neck and arms
- Thoracic: 12 pairs (T1–T12) innervating the chest and trunk
- Lumbar: 5 pairs (L1–L5) innervating the lower back and legs
- Sacral: 5 pairs (S1–S5) innervating the pelvis and lower limbs
- Coccygeal: 1 pair at the base of the spine
The spinal nerves innervate specific areas forming a striped pattern across the body called dermatomes. This is why a disc prolapse at the low back can result in pain at the calf (sciatica) — the compressed nerve root refers pain along its entire distribution.