Anatomy of the Spine
In this section of our site, we will explore the anatomy of the spine. We will look at the structure of the individual elements of the spine, their individual function, and how these parts function as part of a complex system of movements and protection of the spinal cord. The spine, also known as the backbone or vertebral column, consists of the vertebral bones (vertebrae) which exist in an adult male and female as individual and fused units. The spine is bordered by the base of the skull at its apex, and is anchored to the Ilium of the pelvis at its base. The human spine consists of 33 bones if you consider the elements of the sacrum and coccyx as individual parts, or 26 bones if you consider the fused sacrum as one bone and the fused coccyx as one bone. When we are born, there are 7 cervical bones, 12 thoracic bones, 5 lumbar bones, 5 sacral bones, and 4 bones of the coccyx. The bones of the cervical, thoracic, and lumbar spine remain separated throughout life. The elements of the sacrum and coccyx become fused after birth. These bones create stability for the spine, as well as the attachment for the ribs in the thoracic cavity, as well as attachment for the spinal ligaments and muscles.
The primary purpose of the spine is protection of the spinal cord that travels through and is housed within sections of the vertebral bones known as the vertebral foramen. The secondary purpose of the spine is movement. Movement of the spine is accomplished by the action of the various muscles and ligaments that attach to the processes of the vertebral bones. The spinal cord is approximately 17 inches long, and extends from the base of the skull until about the level of L1, which is located towards the top of our lower back.
The spinal cord itself is composed of spinal nerves and nerve fibers, which generate electrical signals that enable us to see, feel, send, and move. The inner layer of the spinal cord consists of grey matter, and is composed of nerve cell bodies. The myelin-sheathed nerve fibers make up the outer layer of the spinal cord, and form the white matter section. These nerve fibers exit through the sides of the spine, through openings known as the intervertebral foramina. These nerve fibers are known as nerve roots where they exit the spine, and branch so many times therafter leaving the spine that nearly every cell in our body is close to a nerve fiber. These nerve fibers are bundled into specialized tracts that conduct the impulses of the nervous system. These nerve pulses activate glands and muscles, and they allow us to have the sensations of pain, heat, and pressure, so that we may be able to react to the environment we are in.
The autonomic functions these nerve fibers (i.e. bowel and bladder control, respiration) as well as the ability to conduct motor impulses that enable us to consciously move our bodies may be disrupted by spinal cord injuries. This disruption of the nerve signals may cause muscle weakness and our ability to control our limbs and other bodily functions (e.g. sexual reproduction, bowel and bladder control). Disruptions in the transmission of these nerve signals may occur in the event of spinal cord Injuries (SCI). In the event of a spinal cord injury, the transmission of nerve signals may be disrupted below that section of the spinal cord where the SCI took place. The bodily effects of spinal cord injuries may include paralysis of the limbs, loss of bowel and bladder control, and loss of sensation in the sections of the Integumentary System (skin) in that region of the body. Other negative effects of spinal cord injuries may include the inability of the body to regulate blood pressure, low blood pressure, decreased core body temperature, chronic pain, and the inability to seat below the level of the SCI.
Labeling and Classifying the Spinal Structures: The primary elements of the spine are labeled according to the section of the spine they are located in as well as the structures they are connected to. The spine is also recognized for its curvatures. The main sections of the spine are the cervical spine, thoracic spine, lumbar spine, and sacral spine. In each section of the spine, the individual bone is given a capital letter and a number. The letter stands for the section of the spine, and the number stands the number of vertebral bones down it is from the top of that section. The letters that are used to label the spine are C (Cervical C1-C7), T (T1-T12), Lumbar (L1-L5), and Sacral (S1-S5). For example, the first - or top - lumbar vertebra is labeled L1, and the bottom - or last - lumbar vertebra - is labeled L5.
Conversely, the intervertebral discs of the spine are labeled according to which vertebral bones that they lay between. For example, the disc between the 4th and fifth lumbar vertebrae is labeled L4-L5. The disc that is located between the last lumbar vertebra (L5) and top of the sacrum (S1) is labeled the L5-S1 disc. The discs mentioned in this article are the most vulnerable to disc disease, due to their presence in the bottom of our spine, which takes on the most weight due to gravity.