Understanding The Anatomy Of The Spine
The spine is made of 33 individual bones stacked one on top of one another. Ligaments and muscles connect the bones together and keep them aligned. The spinal column provides the main support for your body, allowing you to stand upright, bend and twist. Protected deep inside the bones is where the spinal cord connects your body to the brain, allowing movement of your arms and legs. Strong muscles and bones, flexible tendons and ligaments and sensitive nerves contribute to a healthy spine. Understanding the anatomy of the spine will help keep your spine healthy, which is extremely important if you want to live an active life without back pain.
When viewed from the side, an adult spine has a natural S-shaped curve. The neck (cervical) and lower back (lumbar) regions have a slight concave curve and the thoracic and sacral regions have a gentle convex curve. The curves work like a coiled spring to absorb shock, maintain balance and allow range of motion throughout the spinal column. The muscles and the correct posture maintain the natural spinal curves. Good posture involves training your body to stand, walk, sit and lie so that the least amount of strain is placed on the spine during movement or weight-bearing activities. Excess body weight, weak muscles and other forces can pull at the spine’s alignment.
- An abnormal curve of the lumbar spine is lordosis – which is called sway back.
- An abnormal curve of the thoracic spine is kyphosis – which is called hunchback.
- An abnormal curve from side-to-side is called scoliosis.
The two main muscle groups that affect the spine are extensors and flexors. The extensor muscles are attached to the back of the spine which enables us to be able to stand up and lift objects. The flexor muscles are in the front and include the abdominal muscles, which helps us to flex or bend forward and are important in lifting and controlling the arch in the lower back. The back muscles stabilize your spine. Poor muscle tone or a large belly can pull your entire body out of alignment and that can put incredible strain on the spine.
The Vertebrae is the 33 individual bones that interlock with each other to form the spinal column. The vertebrae is numbered and divided into regions, cervical, thoracic, lumbar, sacrum and coccyx. Only the top 24 bones are moveable, the vertebrae of the sacrum and coccyx are fused. The vertebrae in each region have unique features that help them perform their main functions.
Cervical (Neck) – Cervical spine’s main function is to support the weight of the head (about 10 pounds). The seven cervical vertebrae are numbered C1 to C7. Because of two specialized vertebrae that connect to the skull, the neck has the greatest range of motion. The first vertebra (C1) is the ring-shaped atlas that connects directly to the skull. This joint allows for the nodding or “yes” motion of the head. The second vertebra (C2) is the peg-shaped axis, which has a projection called the odontoid, that the atlas pivots around. This joint allows for the side-to-side or “no” motion of the head.
Thoracic (mid back) – The thoracic spine main function is to hold the rib cage and protect the heart and lungs. The twelve thoracic vertebrae are numbered T1 to T12. The range of motion in the thoracic spine is limited.
Lumbar (low back) – The lumbar spines main function is to bear the weight of the body. The five lumbar vertebrae are numbered L1 to L5. These vertebrae are much larger in size to absorb the stress of lifting and carrying heavy objects.
Sacrum – The main function of the sacrum is to connect the spine to the hip bones (iliac). There are five sacral vertebrae, which are fused together. Together with the iliac bones, they form a ring called the pelvic girdle.
Coccyx region – The four fused bones of the coccyx or tailbone provide attachment for ligaments and muscles of the pelvic floor.
Intervertebral discs sit in between the vertebrae. They are flat and round, and about a half inch thick.
Intervertebral discs are made up of two components:
- Nucleus pulposus. The nucleus pulposus is gel-filled and makes up the center of the disc. The gel-filled nucleus is composed mostly of fluid and gives the disc flexibility and strength.This fluid is absorbed during the night as you lie down and is pushed out during the day as you move upright.
- Annulus fibrosus. This is the flexible outer ring of the disc. It’s made up of several layers, similar to elastic bands.
When you are standing or moving, weight is put onto the nucleus. In response, the nucleus expands. The annulus holds the nucleus in place. This allows movement to take place, yet maintains the strength of the spine. In effect, discs act as shock absorbers for the spine.The disc, like other cartilage, consists mainly of a matrix of collagen fibres (which are embedded in a gel of proteoglycan) and of water. These together make up 90 to 95% of the total tissue mass, although the proportions vary with location within the disc and with age and degeneration. There are cells interspersed throughout the matrix that are responsible for synthesizing and maintaining the different components within it.
With age, our discs increasingly lose the ability to reabsorb fluid and become brittle and flatter, this is why we get shorter as we grow older. The intervertebral disc is a very important structure. Many nerve endings supply the annulus and as a result, an injured annulus can cause severe pain.
Vertebral arch & spinal canal
On the back of each vertebra are bony projections that form the vertebral arch. The arch is made of two supporting pedicles and two laminae. The hollow spinal canal contains the spinal cord, fat, ligaments and blood vessels. Under each pedicle, a pair of spinal nerves exits the spinal cord and pass through the intervertebral foramen to branch out to your body.
Between the back of the vertebrae are small joints that also help your spine move. These facet joints have a cartilage surface, very much like a hip or a knee joint does. The facet joints are important for allowing rotation of the spine but may develop arthritis and become a source for lower back or even neck pain.
Ligaments and Tendons
Ligaments and tendons are fibrous bands of connective tissue that attach to bone. Ligaments connect two or more bones together and help stabilize joints. Tendons attach muscle to bone. Tendons vary in size and are somewhat elastic and attach bones to muscles. The system of ligaments in the vertebral column, combined with the tendons and muscles, provides a natural brace to help protect the spine from injury. Ligaments aid in joint stability during rest and movement and help prevent injury from hyperextension and hyperflexion (excessive movements).
The spinal cord is the most important structure between the body and the brain. The Spinal Cord is connected to the brain and is about the diameter of a human finger. From the brain the spinal cord descends down the middle of the back and is surrounded and protected by the bony vertebral column. The spinal cord is surrounded by a clear fluid called Cerebral Spinal Fluid (CSF), that acts as a cushion to protect the delicate nerve tissues against damage from banging against the inside of the vertebrae. The anatomy of the spinal cord itself, consists of millions of nerve fibres which transmit electrical information to and from the limbs, trunk and organs of the body, back to and from the brain. The nerves which exit the spinal cord in the upper section, the neck, control breathing and the arms. The nerves which exit the spinal cord in the mid and lower section of the back, control the trunk and legs, as well as the bladder, bowel and sexual function. The nerves which carry information from the brain to muscles are called Motor Neurones. The nerves which carry information from the body back to the brain are called Sensory Neurones. Sensory Neurones carry information to the brain about skin temperature, touch, pain and joint position. The brain and spinal cord are referred to as the Central Nervous System, whilst the nerves connecting the spinal cord to the body are referred to as the Peripheral Nervous System. Any damage to the spinal cord can result in a loss of sensory and motor function below the level of injury. For example, an injury to the thoracic or lumbar area may cause motor and sensory loss of the legs and trunk (called paraplegia). An injury to the cervical (neck) area may cause sensory and motor loss of the arms and legs (called tetraplegia, formerly known as quadriplegia).
The spinal cord is made up of neuronal cell bodies and axons. The cell bodies are located in the horns (grey matter) and ganglia, and the axons comprise the white matter (tracts) of the cord. Thirty-one pairs of spinal nerves branch off the spinal cord. The spinal nerves act as telephone lines carrying messages back and forth between your body and spinal cord to control sensation and movement. Each spinal nerve has two roots. The ventral (front) root carries motor impulses from the brain and the dorsal (back) root carries sensory impulses to the brain. The ventral and dorsal roots fuse together to form a spinal nerve. The spinal nerve travels down the spinal canal alongside the cord, until it reaches its exit hole, the intervertebral foramen. Once the nerve passes through the intervertebral foramen, it branches, each branch has both motor and sensory fibers. The smaller branch, called the posterior primary ramus, turns posteriorly to supply the skin and muscles of the back of the body. The larger branch, called the anterior primary ramus turns anteriorly to supply the skin and muscles of the front of the body and forms most of the major nerves.
The spinal nerves innervate specific areas and form a striped pattern across the body called dermatomes. Doctors use this pattern to diagnose the location of a spinal problem based on the area of pain or muscle weakness. Like leg pain (sciatica) usually indicates a problem near the L4-S3 nerves.
Spinal Meninges and Cerebrospinal Fluid
Collectively, the Dura mater, arachnoid mater, and pia mater surrounding the spinal cord form the spinal meninges. These membranes and CSF surround, support, and protect the spinal cord and the spinal nerve roots, including those in the cauda equina. The spinal Dura mater, composed of tough, fibrous, and some elastic tissue, is the outermost covering membrane of the spinal cord. The spinal Dura mater is separated from the vertebrae by the extradural (epidural) space. The Dura forms the spinal Dural sac, a long tubular sheath within the vertebral canal. The spinal Dural sac adheres to the margin of the foramen magnum of the cranium, where it is continuous with the cranial Dura mater. The spinal Dural sac is pierced by the spinal nerves and is anchored inferiorly to the coccyx by the filum terminale. The spinal Dura extends into the IV foramina and along the posterior and anterior nerve roots distal to the spinal ganglia to form Dural root sheaths, or sleeves. These sheaths blend with the epineurium, outer connective tissue covering of spinal nerves, that adheres to the periosteum lining the IV foramina.
The spinal arachnoid mater is a delicate, avascular membrane composed of fibrous and elastic tissue that lines the Dural sac and the Dural root sheaths. It encloses the CSF-filled subarachnoid space containing the spinal cord, spinal nerve roots, and spinal ganglia. The arachnoid mater is not attached to the Dura but is held against the inner surface of the Dura by the pressure of the CSF. In a lumbar spinal puncture, the needle traverses the Dura and arachnoid mater simultaneously. Their apposition is the Dura–arachnoid interface, often erroneously referred to as the subdural space. No actual space occurs naturally at this site, it’s rather a weak cell layer. Bleeding into this layer creates a pathological space at the Dura–arachnoid junction in which a subdural hematoma is formed. In the cadaver, because of the absence of CSF, the arachnoid falls away from the internal surface of the Dura and lies loosely on the spinal cord. The arachnoid mater is separated from the pia mater on the surface of the spinal cord by the subarachnoid space containing CSF. Delicate strands of connective tissue, the arachnoid trabeculae, span the subarachnoid space connecting the arachnoid and pia.
The spinal pia mater, the innermost covering membrane of the spinal cord, consists of flattened cells with long, equally flattened processes that closely follow all the surface features of the spinal cord. The pia mater also directly covers the roots of the spinal nerves and spinal blood vessels. Inferior to the conus medullaris, the pia continues as the filum terminale.
The spinal cord is suspended in the Dural sac by the filum terminale and especially by the right and left saw tooth denticulate ligaments (L. denticulus, a small tooth), which run longitudinally along each side of the spinal cord. These ligaments consist of a fibrous sheet of pia mater extending midway between the posterior and the anterior nerve roots. Between 20 and 22 of these processes, shaped much like sharks’ teeth, attach to the internal surface of the arachnoid-lined Dural sac. The superior processes (uppermost part) of the right and left denticulate ligament attach to the cranial Dura mater immediately superior to the foramen magnum. The inferior process extends from the conus medullaris passing between the T12 and the L1 nerve roots.
The subarachnoid space lies between the arachnoid mater and the pia mater and is filled with CSF. The enlargement of the subarachnoid space in the Dural sac, caudal to the conus medullaris, and containing CSF and the cauda equina is the lumbar cistern.
Your spine is one of the most important parts of your body. It gives your body structure and support. Without it you could not stand up or keep yourself upright. It allows you to move about freely and to bend with flexibility. The spine is also designed to protect your spinal cord. The spinal cord is the most important, it has nerves that connects your brain to the rest of your body, allowing you to control your movements. Without a spinal cord you could not move any part of your body, and your organs could not function. Keeping your spine healthy is vital if you want to live an active life. Be sure to browse through our website we have plenty of valuable information for your health, spine and even how to help with back pain, the causes and treatment.