The Anatomy Of Our Back

The back is the structure that supports your upper trunk, houses and protects the spinal cord, and helps you move.

The spine or vertebral column may be thought of as a segmented rod of limited flexibility. The normal anatomy of the spine is usually described by dividing up the spine into 3 major sections: the cervical (sir-vike-al) spine; the thoracic (thor-ass-ik) spine; and the lumbar (lumm-barr) spine. The three sections the spine form three large curves which form the spine into an "S" shape. The low back normally curves forward or inward, which is called lumbar lordosis. Below the lumbar spine is the sacrum, a large triangular shaped bone at the base of the spine, which is part of the pelvis. The coccyx is the small bone at the end of the spine, and is also called the tailbone.

Each section is made up of individual bones called vertebrae. The discs connect the vertebral segments in the front while facet joints link them in the back. The purpose of the vertebrae is to house and protect the spinal cord and its nerve roots and to also give support to the body. The body of the vertebra is the primary area of weight bearing and provides a resting place for the discs which separate each of the vertebrae. The lamina covers the spinal canal, the large hole in the centre of the vertebra through which the spinal nerves pass. There are four facet joints associated with each vertebra. A pair that face upward (superior articular facet) and another pair that face downward (inferior articular facet). These interlock with the adjacent vertebrae and provide stability to the spine. They allow the vertebrae to be linked like a chain and provide mobile connections between each vertebra. Like other joints in the body, the facet joints can be affected by arthritis. The spinous process is the bone you can feel when running your hands down your back. The paired transverse processes are oriented 90 degrees to the spinous process and provide attachment for back muscles.

Though humans are born with 33 separate vertebrae, by adulthood most only have 24. Physicians use a code to identify the vertebrae. The seven in the neck are referred to as the cervical vertebrae numbered C1 to C7. They support and provide movement for the head. The thoracic vertebrae numbered T1 to T12, join with and are supported by the ribs, which protect the heart and lungs. Because they are fairly rigid, thoracic vertebrae don't permit much movement and consequently, aren't injured as often as the other vertebrae. The lumbar vertebrae, numbered L1 to L5 are below the thoracic vertebrae and above the sacrum and are most frequently involved in back pain because they carry most of the body's stress.

The intervertebral discs are the cushiony material sandwiched between each of the adjacent vertebrae, and are constructed somewhat like a jelly doughnut. The centre of the disc is jelly like and is called the nucleus pulposus. The outer part is made up of slightly elastic woven bands and is referred to as the annulus. Discs are flat, round structures about a quarter of an inch thick. They compress when weight is put on them and spring back when weight is removed. The discs keep the vertebrae apart, and permit movement between the vertebrae which allowes our spine to bend. The discs act as shock absorbers, cushioning impact on the spine.

Discs do not have a supply of blood vessels to nourish them like other organs in the body. They depend on fluids rich with oxygen and nutrients passing back and forth from the bones above and below them. The fluid exchange depends on the difference of pressure between the inside of the discs and blood vessels in the bones around them. When the pressure drops inside the discs, fluids and nutrients rush in. This is why all disc nutrition and regeneration happens when we lie down and the pressure is taken off our discs. The mechanism is not very efficient, and as we age, the disc is exposed to wear and tear greater than its ability to heal and regenerate. The annulus layers become weaker and injuries, including repeated scarring, can occur.

The spinal cord is an extension of the brain and extends as far as L1, where it ends in a sheaf of nerves. Throughout the length of the spine, 31 pairs of nerves branch off from the spinal cord and serve all parts of the body, transmitting sensory information to the brain, and information from the brain to the muscles. Movement of the body, bladder functions and sensory functions are all dependent on this information travelling up and down the spinal cord. Each individual nerve root exits the spinal canal through a space called the neural foramen (or intervertebral foramen).

The 24 vertebrae, 23 discs, 31 pairs of spinal nerves, 140 muscles that hook on to the vertebrae, plus ligaments, tendons and cartilage are all very complicated and are potential sources of back trouble. The low back area bears the most stress of movement and, therefore is usually the site of most problems.