The vertebral column

1. The Spine (Vertebral Column)

  • Composed of 33 bones called vertebrae.

  • Provides body support and protects the spinal cord.

2. Groups of Vertebrae

Vertebrae are divided into five groups, each with unique features:

  1. Cervical Vertebrae (C1-C7):

    • Seven vertebrae in the neck.

    • Support the head (weighs around 10 to 13 pounds).

    • C1 (Atlas):

      • Ring-shaped, attaches directly to the skull.

      • Allows nodding motion.

      • Named after the Greek mythological figure who held up the Earth.

    • C2 (Axis):

      • Serves as an axis for C1 to pivot.

      • Allows head shaking motion (saying "no").

      • Has a specialized process called the dens (or odontoid process) that C1 pivots on.

  2. Thoracic Vertebrae (T1-T12):

    • Twelve vertebrae in the mid and upper back.

    • Hold the ribcage and protect the heart and lungs.

    • Have a limited range of motion.

  3. Lumbar Vertebrae (L1-L5):

    • Five vertebrae in the lower back.

    • Bear the weight of the upper body.

    • Larger to support stress, especially when lifting heavy objects.

  4. Sacral Vertebrae (S1-S5):

    • Five fused vertebrae.

    • Connect the spine to the hip bones.

    • Together with hip bones, they form the pelvic girdle.

  5. Coccyx (Tailbone):

    • Made up of four fused bones.

    • Provides attachment points for ligaments and muscles of the pelvic floor.

3. Vertebrae Articulation and Fusion

  • The upper 24 vertebrae (cervical, thoracic, lumbar) are articulating, separated by intervertebral discs.

  • The sacral vertebrae and the bones of the coccyx are fused.

4. Parts of a Vertebra

Each typical vertebra has three main parts:

  1. Body:

    • Bears weight.

  2. Vertebral Arch:

    • Houses the spinal cord.

    • Made up of two supporting pedicles and two laminae.

    • The space inside where the spinal cord is housed is called the vertebral foramen.

    • Spinal nerves exit under each pedicle, passing through the intervertebral foramina.

  3. Processes:

    • Allow for muscle attachment.

    • Seven bony processes arise from each vertebral arch, forming:

      • Facet Joints: Two pairs (one connecting to the vertebra above, one to the vertebra below) allow for back motion.

      • Transverse Processes: A pair of processes extending laterally.

      • Spinous Process: A single process extending posteriorly.

5. Intervertebral Discs

  • Provide cushioning between vertebrae and prevent bones from rubbing together.

  • Note: There is no intervertebral disc between C1 and C2.

  • Composition:

    • Annulus (Annulus Fibrosus):

      • Made up of several layers (lamina) of fibrocartilage.

      • Criss-crossing fibrous bands that attach between the bodies of the vertebrae above and below.

    • Nucleus (Nucleus Pulposus):

      • A gel-filled center.

      • Distributes pressure evenly within each disc during compression.

      • Full of proteoglycans (large hydrophilic molecules with sugar subunits) that swell with water.

  • Fluid Dynamics and Aging:

    • The nucleus absorbs fluid at night or when lying down.

    • Fluid is pushed out during the day when upright and pressure is applied.

    • Discs shorten as we age because they lose the ability to reabsorb fluid during rest, leading to a decrease in height.