How does the spine work?
The spine is our body’s central support structure. It keeps us upright and connects the different parts of our skeleton to each other, such as the head, chest, pelvis, shoulders, arms and legs. Although the spine is made up of a chain of bones, it is flexible due to elastic ligaments and spinal disks.
The length of someone’s spine depends on their height. The average length is 71 cm in men and 61 cm in women. Your spine has many functions: It carries the weight of your head, torso and arms, and allows your body to move in any direction. Some sections of the spine are more flexible than others. The neck is the most flexible part. The spine also surrounds and protects the spinal cord. This important part of the nervous system runs through the middle of the spinal canal.
Viewed from the side, there are normally four slight natural curves in an adult spine: The cervical (neck) and lumbar (lower back) sections of the spine curve inward, and the thoracic (upper back) and sacral (lower spine) sections curve outward.
These curves make the spine stable: They help you keep your balance when you are in an upright position, act as shock absorbers when you walk, and protect the individual bones in the spine (the vertebrae) from fractures.
Adults normally have 26 vertebrae, from top to bottom:
- 7 cervical vertebrae
- 12 thoracic vertebrae
- 5 lumbar vertebrae
- 1 sacrum (made up of 5 fused sacral vertebrae)
- 1 tailbone (made up of 4 fused coccygeal vertebrae)
The lower down the vertebrae are in the spine, the more weight they have to carry. That is why the lower vertebrae are larger and more stable than the top vertebrae.
Vertebrae and spinal disks
The front part of the vertebra is called the vertebral body. There are 23 elastic spinal disks between each of the vertebrae – except for between the skull and the first cervical vertebra, and between the first and second cervical vertebrae.
The disks have a solid, multi-layered casing of cartilage fiber and a gel-like core. They keep the spine flexible so that we can lean over and rotate our upper body. They also absorb shocks that are transferred to the spine when we run or jump.
When we put pressure on our spine, the spinal disks release fluid and become thinner (“compress”); when the pressure is relieved they absorb fluid again and become thicker (“decompress”). Because we usually put more pressure on our spine during the day and relieve the pressure at night, we are around 1.5 to 2 centimeters shorter by the end of the day. Over many years our spine starts to wear, meaning that as we age our spinal disks become thinner, the vertebrae become compressed and the spine curves more. That is why we are usually lose a few centimeters in height as we age.
Except the first two vertebrae in the neck, all vertebrae are made up of a vertebral body, which leans forwards, and the “spinous process,” which points backwards. Both parts are connected by the vertebral arch which forms a cavity in the middle – the spinal foramen. These cavities align to form the spinal canal, which surrounds the spinal cord.
Spinal cord and nerves
The vertebral arches have a slight recess at the top and bottom of each edge. Each individual vertebral arch has two small indentations along the top and bottom edges. Where two vertebrae come together these indentations form two gaps – one on the left and one on the right side of the spine – through which spinal nerves can leave the spinal canal. Because the spinal nerves branch off as they go down the spine, the spinal cord thins out at the lower end of the spine.
The spinal nerves carry electrical signals from the brain to the skeletal muscles and internal organs through the spinal cord. They also carry sensory information like touch, pressure, cold, warmth, pain and other sensations from the skin, muscles, joints and internal organs to the brain via the spinal cord.
Together, the spinal cord and the brain make up the central nervous system.