Spinal Column: An Integral Part of the Human Body
The purpose of this chapter is to introduce you to the human spine. As part of the body, the spine is a unique and complex structure. The spine is composed of living bone, cartilaginous elements (car-t-lay-gin-us), joints, a spinal cord, nerve roots, ligaments, tendons, muscles, and a vascular system. The whole body, as well as the spine, is dependent on the function of each element. Knowledge about these spinal elements may help you to better understand how lifestyle, aging, injury, and disease may adversely affect the spine.
The Human Skeleton is the body's framework or scaffolding system. Skeletal bones are classified as long, short, flat, or irregular and vary in length, width, and depth. The bones in the spine are irregular in shape and provide places to connect to other bones. The function of the skeletal system is to support the body against the force of gravity, protect soft body parts, produce red blood cells, store inorganic calcium, and phosphorus salts, and to provide sites for muscle attachment to enable body movement.
Bone is a living tissue. During prenatal development bones are cartilaginous. A newborn's body may contain more than 300 cartilaginous bones that gradually fuse to form approximately 206 permanent bones by adulthood. Osteoblasts (os-t-o-blasts) help to form bone and ossification (os-e-fik-kay-shun) hardens bone. Bone tissue resembles reinforced concrete. Collagen (call-ah-gin), threads of fibrous protein reinforce the hard cement created by calcium and phosphorus compounds.
Further, concentric rings of bone fibers called Haversian spaces (hav-er-sh-on) surround canals that contain nerve fibers and blood vessels. Osteocytes (os-t-o-sites) are cells that help maintain bone structure. During adulthood bone continually rejuvenates itself by breaking down and rebuilding. Osteoclasts (os-t-o-klasts) break bone down and osteoblasts return to build new. Calcium is very important to the action of the osteoblasts.
The Spinal Column is also called the vertebral column. The bones in the spine are called vertebrae (ver-ta-bray). The column starts at the base of the skull and continues to the pelvis. Alternate layers of bone (vertebrae) and cartilage (car-til-ledge, the intervertebral discs) stack vertically one on top of the other in the spinal column. The lattice-like structure of the cancellous bone (cancel-lus, the spongy interior) in a vertebra absorbs external pressure.
The cartilaginous discs between vertebrae absorb and distribute shock and keep the vertebrae from grinding together during movement.
The spine has four natural curves. Two are lordotic (lor-dot-ick) and two are kyphotic (kye-fah-tick). The cervical and lumbar curves are lordotic. The thoracic (thor-as-ick) and sacral (say-kral) curves are kyphotic. The curves help to distribute mechanical stress as the body moves.
A Vertebra (ver-ta-bra) is one spinal segment or level. The term vertebrae refers to more than one spinal segment or level. The atlas, axis, cervical spine, thoracic spine, lumbar spine (lum-bar), sacrum (say-krum), and coccyx (cock-six) are the bony elements of the spinal column.
The Atlas and Axis are the first two cervical (sir-ve-kal) vertebrae below the skull. These structures do not look like typical vertebrae. The atlas is ring shaped. It balances and supports the head. The axis has a tooth-like projection (called the odontoid process, oh-don-toyed) that fits up into the atlas. The combination of these two structures allows the head to turn from side to side. The atlas pivots around the axis.
Five Cervical vertebrae follow below the atlas and axis. The Thoracic spine is located in the chest area and contains 12 vertebrae. The ribs connect to the thoracic spine and protect many vital organs. Next is the Lumbar spine. Most people have five lumbar vertebrae although it is not unusual to have six. The lumbar vertebrae are larger than the cervical or thoracic as this spinal region carries most of the body's weight. The sacrum and coccyx are uniquely shaped.
Medical professionals often abbreviate the levels (vertebrae) of the spinal column. For example, the seven cervical vertebrae are C1, C2, C3, C4, C5, C6 and C7. The thoracic levels are T1, T2, and T3 through T12. Similarly the lumbar levels are L1 through L5 (or L6). The sacrum is simply S1. The coccyx is not abbreviated or numbered. With the exception of the atlas, axis, Sacrum and Coccyx, each cervical, thoracic, and lumbar vertebra is similarly shaped. The following figure illustrates the individual components of one vertebral segment.
Intervertebral Discs separate each vertebra. A disc is made of fibrocartilage (fybro-car-til-ledge). Fibrocartilage is a type of cartilage consisting of a dense matrix of collagen fibers giving discs great tensile strength. Discs function to absorb and distribute shock from movement (e.g. walking) and prevent the vertebrae from grinding against one another.
A disc is similar to a donut with jelly filling. The outer donut part is called the anulus fibrosus (an-you-lus fye-bro-sis) and the inner filling is the nucleus pulposus (nu-klee-us pul-poe-sis). The term "anulus" is also correctly spelled "annulus". The anulus fibrosus is made up of strong circular layers of fibrocartilage that encase the nucleus pulposus, an elastic gel-like substance. Endplates hold each intervertebral disc between the upper and lower vertebrae.
Facet Joints (fah-set) are also called Zygapophyseal Joints (zye-gap-o-fiz-e-al). These joints are located in the posterior spine and help to enable spinal movement. The cervical, thoracic and lumbar vertebrae each have a pair of facet joints. The facets from the upper and lower vertebrae join together (like entwined fingers) to form a facetjoint. Like other joints in the body, the articulating surfaces (ar-tick-you-late-ing) are coated with smooth cartilage to facilitate movement.
This article is an excerpt from the book Save Your Aching Back and Neck, A Patient's Guide, edited by Dr. Stewart Eidelson.