The human head is supported in space by the action of numerous muscles. Many of these muscles are attached at one end to the vertebral or spinal column, a remarkable structure that defines the mid-line of the human body and largely defines the boundary between the central and peripheral nervous system.
Humans, like a majority of other species are bilaterally symmetric. That is, starting from a defined center axis or mid-line along the length of the body; most structures are duplicated to either side in mirrored fashion. The spinal column serves to define the center of the human body. It also serves to protect the spinal cord from injury and is the point of exit for many nerves to the periphery of the body and the point of entry for many other nerves into the central nervous system.
The Different Divisions of the Spinal Column
The vertebrae of the spinal column are anatomically divided into different regions, the cervical, thoracic, and lumbar vertebrae and the sacrum and coccyx (or tailbone). Structurally speaking the individual vertebrae are actually quite distinct from each other; in fact, a cervical vertebra can be easily differentiated from a lumbar or thoracic vertebra on appearance alone. The number of vertebrae in each region is also different with 7 cervical vertebrae (also referred to as C1 to C7), 12 thoracic vertebrae (T1 to T12), and 5 lumbar vertebrae (L1 to L5). The sacrum and coccyx originally contain distinct bones during early development but each set of bones fuse to become one structure although the sacral region is still sometimes referred to as 5 segments (S1 to S5).
Between each of the vertebrae are specialized shock absorbers known as intervertebral discs. These are the discs that are referred to when a person says that they are having back problems, such as a slipped disc or a ruptured disc. These discs support the weight of the individual vertebral bodies and allow for the flexibility that is present between each vertebra.
Protecting the Spinal Cord
Each vertebra has a solid bony body with several bony protruberances that project away from it. Immediately in back of the bony vertebral body (closer to the back or posterior) is an opening formed from the bone that surrounds it, known as the vertebral foramen. When all of the vertebrae are stacked one upon the other, these foramina align to create the passageway through which the spinal cord travels. Alongside this pathway are various small openings that allow both nerves and blood vessels to traverse the territory of the spinal column.
The spinal cord is not as long as is the spinal column. In an adult human the spinal cord actually ends at the level of the L1/L2 lumbar vertebrae. Below this point the nerve bundles at the end of the spinal cord form a structure known as the "conus medullaris". And as other nerves exit this structure, the remaining nerve fibers finally form what is known as the “cauda equina” (tail of the horse). Most people recognize intuitively that the spinal cord must end before the end of the vertebral column because they know that lumbar punctures can be performed for diagnostic purposes without damaging the spinal cord.
The spinal column is a remarkable structure. Almost every sensory and motor nerve that acts below the level of the neck exits from the spinal column. It is a stanchion of strength with places for attachment of the ribs, attachment of muscles, attachment of numerous ligaments and tendons and substantially aids humans in standing upright. Learn more about the spinal column at the US National Institutes of Health (NIH).
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