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SCIATICA: ANATOMY To better understand all the terms related to the spine and to sciatica, this section expands on the basic anatomy and gives you more in depth details. The spine is made up of a combination of bones and associated joints, discs, ligaments, the spinal cord, and spinal nerves and their associated coverings. Bones The spine is made up of 26 bones; 24 vertebrae, a sacrum and a coccyx. (Image 1) The upper most level is the cervical region and has 7 vertebrae. The mid back is known as the thoracic spine and is comprised of 12 vertebrae. The low back, is known as the lumbar spine and is comprised of 5 vertebrae. The sacrum is comprised of 5 vertebrae that are fused together, S1 through S5. The normal abbreviation for each vertebra corresponds to the location and the order. For instance, the cervical spine is made up of C1 thru C7. The thoracic spine is T1 thru T12. And the lumbar spine is L1 thru L5. The vertebrae interlock to form the spinal column. The primary role of the cervical spine is to support the weight of the head. The primary role of the thoracic spine is to hold the rib cage which in turn protects the heart and the lungs and to a lesser degree the other major organs. The lumbar spine’s primary function is to support the weight of the body, which is why the vertebrae are much larger in size to absorb the stress of lifting and carrying large objects. Image 1 The vertebrae have 3 main parts (image 2): • body, • vertebral arch, and • processes. The body of the vertebrae is designed to bear weight and withstand compressive forces whereas the vertebral arch surrounds the spinal canal and is designed to protect the spinal cord. The processes, including the spinous, transverse, and superior articular are where muscles attach to the bone and where each vertebral bone joins the next level (AKA-facet joints). Each vertebrae has 2 facet joints that connect the vertebrae above called the superior facets, and 2 facet joints that connect the vertebrae below called the inferior facets. Additional facet joints are for the articulations (joints) between the vertebrae and the ribs. (Image 3) Image 2 Image 3 Discs Between each vertebral body of the spine there is an intervertebral disc, which is essentially a liquid or gel filled cushion which keeps the bones from rubbing together and provides space between the bones through which the spinal nerves pass to innervate different parts of the body. There are two primary components of the disc, the outer layers called the annulus and the inner gel-filled center called the nucleus. (Images 4 & 5) The annulus is made up of 15-25 layers of a fibrous bands that alternate in a crisscross like pattern to hold the vertebral bodies together and prevent the gel like nucleus from escaping. The nucleus, as mentioned previously is primarily fluid Image 4 and gel like, and is designed to move around as you move to protect the vertebral structures including the joints and nerve roots. During the ages of birth to 20, the majority of the weight through the spine is borne through the posterior or back 1/3 of the disk. As we age beyond that point, weight bearing is shifted even further posteriorly (backward) to the facet joints which connect each vertebrae or a neighboring structure together. Image 5 Ligaments There are 3 primary ligaments (Image 6) which are strong fibrous bands that are the major structures that hold the vertebrae together as well as offer stability and protection to the discs. They are the: • • • anterior longitudinal ligament (ALL) posterior longitudinal ligament and the (PLL) ligamentum flavum. Both the ALL and PLL are continuous bands that run the length of the entire spinal column along the front (anterior) and back (posterior) of the vertebral bodies. The ligamentum flavum is not continuous, but instead attaches between the lamina of each vertebra. All 3 ligaments work together to prevent excessive movement of the bones. A 4th ligament runs between each spinous process and is called the interspinous ligament. Image 6 Another interesting fact regarding the size and shape of the ALL and the PLL concerns the evolution of man from being on all fours to upright in stature. (Image 7) It is because of our start on all fours, that the ALL is much broader to support the downward forces of gravity on what would have been considered the bottom of the spine versus the front or anterior portion of the spinal column today. The common opinion is that the broadness of the ALL compared to the narrower PLL, creates a weak point on either side of the PLL and is the major reason why herniations typically occur Image 7 posteriorly rather than anteriorly. Spinal Cord Averaging about 18 inches long in men and 17 inches in women, the spinal cord is about ½ inch thick. It runs inside the spinal canal from the brainstem to the 1st of the 5 lumbar vertebra. At the lower most end of the cord, the fibers separate into the Cauda equina (which means horses tail) and continues down to the tail bone (coccyx). The primary function of the spinal cord is to relay information from the brain to different areas of the body so that movement can occur. In turn, our arms, legs, and body, send signals to the brain through the spinal cord as to what we touch and feel. Our spinal reflexes also occur at the spinal cord but they do so without sending signals to the brain. The spinal reflexes are designed to protect our body from harm. The spinal cord is made up of nerve cells called upper motor neurons. The lower motor neurons branch off of the spinal cord to each side of the spine between the vertebrae and go to different parts of the body dependent upon what level of the spine they exit from. Any damage to the cord or upper motor neurons can cause loss of either sensory or motor function below the level of the damage or injury. Spinal nerves There are 31 pairs of spinal nerves that branch off of the spinal cord. A “pair” is made up of a left which controls sensation and motor function on the left side of the body and one on the right side which controls sensation and motor function on the right side. Each spinal nerve has two “nerve roots”. One which carries impulses to the cord and brain called the dorsal root, and one which carries impulses away from the brain and cord called the ventral root. The dorsal and ventral roots fuse together and form the spinal nerve. (Image 8) The spinal nerves exit through the intervertebral foramen. (Image 9) At that point, the nerve branches out to include the anterior and posterior primary rami each of which includes motor (muscle function) and sensory (sensation) fibers. The smaller is the posterior primary ramus and supplies the skin and muscles of the back of the body. The larger anterior primary ramus supplies the skin and muscles of the front of the body as well as forms most of the major nerves of the body. Image 10 Image 8 Image 9 Because each spinal nerve “innervates” a specific part of the body, a system of identification is necessary to keep track of the nerves and where they go. (Image 10) Each spinal nerve is labeled according to the vertebrae either above or below where they emerge dependent upon which section they are in. In the case of the cervical spine, they are named for the vertebra below, with exception to C8 which emerges below C7 and above T1. So even though there are only 7 cervical vertebrae, there are 8 pairs of cervical spinal nerves. For the thoracic and lumbar spine they are labeled according to the vertebrae above. There are therefore 12 pairs of thoracic spinal nerves, 5 pairs of lumbar spinal nerves and 5 pairs of sacral nerves. The final, or 31st spinal nerve is the coccygeal nerve which only supplies the skin over the back of the coccyx (tail bone). The area of the skin supplied by a single spinal nerve is called a dermatome, (Images 11 & 12) whereas the group of muscles that are controlled by a single spinal nerve is called a myotome. (Chart 1) By using the chart of the dermatomes and myotomes, one can often times identify the location of the spinal problem based upon the area of pain or muscle weakness. That is why it is so important to pay close attention to where your symptoms are and convey that information to the medical practitioner. Where you feel the pain is not necessarily the source of the pain. For instance, pain in your lower leg may not indicate an injury to your leg, but an injury to the lower part of your spine (L4-S2). Myotomes of upper and lower extremities C1/C2: neck flexion/extention C3: neck lateral flexion C4: shoulder elevation C5: shoulder abduction C6: elbow flexion/wrist extension C7: elbow extension/wrist flexion C8: finger flexion T1: finger abduction L2: hip flexion L3: knee extension L4: ankle dorsi-flexion L5: great toe extension S1: ankle plantar-flexion/ankle eversion/hip extension S2: knee flexion S3-S4: anal wink Chart 1 Image 11 Image 12 Coverings and spaces The meninges are membranes that cover the spinal cord (as well as the brain). (Image 13) There are 3 distinct layers. The inner, the mid, and the outer. The inner membrane, called the pia mater, is actually attached to the cord. Between the pia mater the arachnoid mater (mid membrane) is the subarachnoid space which surrounds the spinal cord and contains the cerebrospinal fluid. The tough outer membrane is called the dura mater. The space between the dura mater and the bone is the epidural space. It is the epidural space that is most often accessed to deliver an epidural numbing agent (anesthetic) or steroid injection (to help fight the disease process such as swelling and pain), whereas the subarachnoid space is most often accessed to perform a lumbar puncture or myelogram. Image 13