The spine is divided into four segments:
Cervical spine (C1 to C7) forms the neck displaying the most flexible section of the spine;
Thoracic spine (T 1 to 12) connects to the rib cage thus having limited flexibility;
Lumbar spine (L1 to L5) including the largest vertebrae suited to support most of the body weight;
Sacrum (S1 to S5) consisting of fused vertebrae that attach laterally to the hip ring, and joining underneath to the tailbone or coccycx formed by five fused bones.
The curvature of the spine
Spinal curves are important to balance and stand upright. If any one of the curves becomes too large or small, it is difficult to stand up straight and our posture appears abnormal.
Changes in the curvature of the spine are referred to as spinal deformities. They include: kyphosis of the thoracic spine ("hunchback"), and lordosis of the lumbar spine ("swayback"). Scoliosis is a sideways curvature of the spine forming a shape like an "S" or a "C" rather than straight from the front or back view.
The vertebrae are the main constituents of the spine. They are round-shaped bones piled up to form the spinal column. The cavity in the centre, or foramen, is only present in the cervical, thoracic and lumbar vertebrae and is occupied by the spinal cord and its nerves. The anatomy of the vertebrae includes a body with a large oval shape made by the strongest bone structure. The rim of the body is thicker to provide a concave form.
The pedicles are short lateral processes and the laminae are flat bones extending from the pedicles and joining at the midline. The processes divided into articular, transverse and spinous are thin bones required for connecting the spine to ligaments and tendons. Four articular processes connect each vertebra with the next one to create the facet joints, which facilitate the spine movement.
The vertebrae of the upper cervical spine have a different shape and function. Their structures are the atlas (C1), the axis (C2) and the dens (also C2). They connect the spine to the cranium and allow the rotation, extension and flexion movements of the head.
Facet joints are small hinges located in the back of the vertebrae. They contribute to the spine movement by allowing spinal rotation. Similarly to other joints, they have a cartilage surface that may degenerate into an arthritic condition and become a source for low back or neck pain.
The ligaments of the spine
Ligaments are fibrous elastic tissues, which connects bones with each other to stabilise the joints. By limiting spine flexibility, they prevent damage caused by hyperextension and flexion.
The anterior longitudinal ligament and the posterior longitudinal ligament confer the principal spinal support. They extend through the entire length of the spine (skull to sacrum) in the anterior/ventral side and posterior/back side, respectively. These ligaments attach directly to the annulus fibrosis rings.
The supraspinatus ligament links the spinous processes with one another.
The interspinous ligament connects the rest of the spinous process to each other.
The ligamentum flavum is located in the deep spine. This is the strongest ligament and runs from the skull base to the pelvis on the anterior side between the laminae and the facet joint capsules.
Function of the ligament during movement
A. In upright position, the ligaments are at normal length. However, when bending the body forwards, the ligaments are stretched to their maximal length to hold the vertebrae together and support the body.
B. The facet joints prevent excessive extension when the spine is bent backwards (full bridge pose).
The muscular system of the spine is very complex. All these muscles essentially stabilise the spine by keeping the alignment of the vertebrae. Strong back muscles support the spine while keeping it in a good straight position. If the muscles are not trained, the spine can adopt a pathological posture which over time can cause injury or deformity.
Specific muscles control movements of specific sections of the spine. For instance the sternocleidomastoid muscle at the cervical spine permits the movement of the head, while the psoas major (sacral area) is associated with the flexion of the thighs. If well trained, the muscles of the spine can reduce the load on the bones, facet joints, disks, and ligaments. In contrast, when they are weakened from the lack of use or injury, they lose their size, strength, endurance, and flexibility.
Table describing the main muscles of the spine and their function:
The spinal cord
The spinal cord is a bundle of nerves originating from the brain reaching out to the limbs and internal organs. It runs from the skull, through the spinal canal up to the first lumbar vertebra. It has a length of 40-50 cm and a diameter of approximately 1-1.5 cm. The spinal canal plays the important role to direct and protect the spinal cord. Similar to the brain, the spinal cord is enclosed by a membrane, the meninges, and is embedded in a liquid, the cerebrospinal fluid. This fluid is produced in brain cavities (the ventricles) to cushion both the brain and spinal cord. The gray matter, with a butterfly shape located in the center of the cord, contains the interneurons and motor neurons. The lateral projections of the gray matter are called horns. Motor neurons of the ventral horn project their axons to innervate skeletal and smooth muscles that control voluntary and involuntary reflexes. It also contains neurons projecting axons to mediating autonomic control of the visceral functions. The white matter is located outside the gray matter and is made of myelinated motor and sensory axons. These axons transmit information along the spinal cord. Each level of the spinal cord is associated with a pair of dorsal root ganglia, located outside the cord. The ganglia contain the sensory neurons whose axons travel into the spinal cord via the dorsal roots. Ventral roots contain axons from motor brain and spinal cord to the periphery. Dorsal and ventral roots merge, exit the intervertebral foramina to become spinal nerves.
At each vertebral body the nerve roots branch out laterally in a total of 31 pairs through the intravertebral foramen and from there they become peripheral nerves. Spinal nerves connect the brain with other nerves of the body. This complex network of nerves is part of the central nervous system and transmits signals between the brain and the rest of the body. There are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves. A group of nerves at the conus medullaris located in the cauda equina, resemble a horsetail. The nerves control movement and function of all organs but also transmit signals from the external environment back to the brain including pressure, heat, cold, and pain. Any interruption of this bidirectional communication severely affects the nervous system and the complex functions of the organism. The nerve roots exit from the spaces between vertebrae to transmit signals originating from the brain via the nerves throughout the body.
The cauda equina
The spinal cord ends around the first and second lumbar vertebra in the lower back and continues as nerve roots. This bundle of nerve roots is called the cauda equina. They exit the spinal canal through openings in the vertebrae (foramen), just like other nerve roots.