By Elizabeth A. Murray, Mount St. Joseph University
When the vertebrae are stacked, most with intervertebral discs between them, the series is linked into what we call the vertebral column, or spinal column. Notches at the pedicles of adjacent vertebrae leave a space between the bones, called the intervertebral foramen. So, while each vertebra has a vertebral foramen posterior to its body, you can’t find an intervertebral foramen on a vertebra; they are formed only when the vertebrae are stacked.
What Makes the Spine Stable
The vertebral foramina extend down the spinal column, forming a spinal cavity through which the spinal cord passes. Off that spinal cord come the spinal nerves that exit the vertebral column by passing laterally through the intervertebral foramina. Remember, the anterior and posterior roots of a spinal nerve are within the spinal canal, and they fuse to form a spinal nerve just before it exits its intervertebral foramen. Immediately after that, lateral to the intervertebral foramina, the spinal nerves divide into their dorsal and ventral rami.
The dorsal rami are the distributional branches that supply the true back muscles and skin down the middle third of the back, while the ventral rami become intercostal nerves or braid into one of the plexuses—cervical, brachial, lumbar, or sacral.
How Ligaments Help
But wait, there’s more! A series of ligaments connect adjacent vertebrae to help aid the stability of the spine. There’s an anterior longitudinal ligament that is wide and strong and runs down the length of the stacked bodies of the vertebrae on their anterior side. And there’s a narrow posterior longitudinal ligament that runs down the posterior aspect of the stacked vertebral bodies.
These ligaments reinforce the cartilaginous joints formed by the intervertebral discs and attach to the discs as well as the vertebrae they join. Other ligaments unite adjacent spinous processes, transverse processes, and laminae. Because the posterior longitudinal ligament is narrow, disc herniations often occur just lateral to it, precisely in the location where spinal nerves exit their intervertebral foramina. This is why a herniated disc often pinches a nerve, causing pain.
The Development of the Spine
The assembled spine is held together well, but that doesn’t mean it is held straight. Developmentally, when a fetus is growing, the body is in the so-called fetal position. This results in a spine that is anteriorly concave, with the head, neck, and back bent toward the lower limbs. This is what’s known as the primary curvature of the spine. When an infant is born, that’s really the first time the newborn has to support his or her own weight.
As you likely know, the caregivers of the child must support the baby’s neck, because the muscles that support the large head we humans are born with have not developed their ability to carry that large noggin. But as the infant’s neck muscles gain control, the spine changes its curvature in the neck—to better balance the head on the cervical spine.
The neck develops a curvature that is posteriorly concave—in other words, a curve in the opposite direction of the primary curvature of the fetal position. This cervical curvature is known as a secondary curvature.
Later, as the baby begins to sit up, and later pulls up on the coffee table, the spine changes again. We develop a lumbar curvature in the low back that—like the cervical curvature—is posteriorly concave.
So, the adult spine has a slight S shape when viewed from the side. The thoracic and sacral curvatures retain the directionality of the fetal spine, so are called our primary curvatures, and the cervical and lumbar curvatures are in the opposite direction and are called secondary curvatures. The net result helps maintain the proper center of gravity for upright posture and balance.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Abnormalities of the Vertebral Column
But some individuals develop abnormal curvatures—we’ve likely all heard of scoliosis, which is a lateral bend in the spine. I’ve seen cases of scoliosis in our cadavers that are so severe that they cause changes to the internal organs. For example, one elderly female had such serious lateral curvature in her thoracic spine, that the lung on the side most affected was compressed into a space that wasn’t more than a few inches in height. Clearly that would have affected her breathing and been so uncomfortable.
There are other abnormal curvatures, as well. An exaggerated lumbar curvature is known as lordosis, a secondary lumbar curvature that is much greater than normal. You may have heard the term swayback used for this.
But a certain degree of lordosis is normal during pregnancy, because the increased amount of weight anterior to the vertebral column in a pregnant woman is somewhat offset by a temporary lordosis that helps readjust her center of gravity during her pregnancy. Some medical conditions, including obesity, can cause lordosis, as well.
There are also exaggerated thoracic curvatures, among them is kyphosis which is sometimes called a humpback or hunchback. Like scoliosis, this disorder can also result in compression of thoracic organs, reducing lung capacity and leading to difficulty breathing.
Most often kyphosis is seen in the elderly, especially older females—that’s why it has been called a dowager’s hump. Kyphosis can be caused by conditions such as polio or muscular dystrophy,
Another condition which is also more prevalent in older females is osteoporosis. Kyphosis is often due to osteoporosis, causing demineralization and weakening of bone. The body of a vertebra is much less dense in its bony makeup than its more posterior regions, such as the articular facets, and transverse and spinous processes.
As these vertebral bodies weaken from osteoporosis, their anterior aspects collapse, leaving a vertebral body that is wedge-shaped, rather than being of symmetrical heights both anteriorly and posteriorly. The stacking of these wedge-shaped vertebrae results in an exaggerated anteriorly concave curvature.
When we see a person with this condition, it looks so uncomfortable, and we may wonder why the person doesn’t just straighten up—but they simply can’t, the bony support of the vertebral column just isn’t what it once was. The good news is that in mild and early cases, physical therapy to strengthen back muscles can help, as can wearing a brace. But in severe cases, surgery—including spinal fusion or the insertion of hardware to support the spine—may be needed.
Common Questions about the Vertebral Column and Its Possible Abnormalities
There are a series of ligaments connecting adjacent vertebrae: a wide and strong anterior longitudinal ligament, accompanied by a narrow posterior longitudinal ligament. These help the stability of the vertebral column.
Scoliosis and lordosis are some of the abnormalities.
It happens because the bony support of the vertebral column is simply not what it once was anymore.