Spine Course Anatomy

Anatomy of the vertebral column ©2010 Copyright insight Medical Academy, LLC All rights reserved.

Studying the anatomy of the spine will leave you in awe of its function and structure. We will learn the anatomy and function of

Vertebral bone structure

Functional spinal unit

Spinal disks and ligaments

Articulating (moving) joints

The four regions of the spine

Anterior and posterior longitudinal ligaments

Major muscles of the back

Central and peripheral nervous system

Functional spinal unit

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The functional spinal unit, or FSU, simply refers to two vertebrae, the disk between them, and the ligaments connecting them. If you understand the FSU, it will be much easier to understand the more advanced topics ahead.

External vertebral structure

All the vertebrae have the same general structure, with modifications in the different regions of the spine. This animation shows a representative vertebra so we can examine the basic structures. The vertebral body is the roughly cylindrical portion of the  vertebra. It carries most of the weight (about 80%) of the structures above it. Its superior and inferior surfaces are adjacent to the intervertebral disks, but do not form a true joint with the disks.

The articulating facets form true joints (synovial joints) with the corresponding facets on the adjacent vertebrae. The surface of the facet is articular cartilage, forming a sliding joint and giving the spine flexibility and stability.

The spinous process protrudes from the posterior aspect of the spine in a slightly downward direction. It serves as the attachment for several muscles and is tied to other spinous processes by a strong ligament. Touch one of yours!

The transverse processes protrude from the lateral aspects of the spine. They serve as an attachment for  muscles and are tied to other processes by ligaments. The spinal and transverse processes act as levers, allowing muscles to more easily stabilize and move the vertebral column. The lamina connect the pedicles to complete a protective ring of bone surrounding the spinal cord. Now we see the facets from the opposite (contralateral) side of the body. The facets of each vertebra articulate with the  complimentary facets on the adjacent vertebrae like hands pressing against each other.

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The facet's surface is articular cartilage that forms a sliding joint, giving the spine flexibility and stability.

The lamina connect the pedicles to complete a protective ring of bone surrounding the spinal cord. The spinous process and inferior facets connect to the lamina.

In the inferior view we are introduced to two new key sructures, the vertebral foramen and the pedicles. The pedicles are important because they connect the posterior structures to the vertebral body. They also provide the mass important to many interventional techniques. The vertebral foramen contains the spinal cord. Its integrity is key for the normal functioning of the nervous system.

The lamina connect the pedicles to complete a protective ring of bone surrounding the spinal cord. The spinous process and inferior facets connect to the lamina. In the superior view we again see the vertebral foramen and the pedicles. The pedicles are important because they connect the posterior structures to the vertebral body. They also provide the mass important to many interventional techniques. The vertebral foramen contains the spinal cord. Its integrioty is key to the normal nural functioning of the body.

Great! You have now seen the key basic structures of the vertebra. Remember this lesson, and you will be ahead of the game.

Internal vertebral structure

The vertebra is composed of a shell of cortical bone (compact, hard, strong) and an interior of

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cancellous bone (porous, mostly liquid). The same structure is found in many bones and provides a balance of strength and weight. Cancellous bone has a trabecular (sponge-like) structure. This mesh is constantly restructuring in response to stress  (Wolf's Law) and the body's need for calcium.  (Bone restructuring is the subject of a different course in this series.) Osteoporosis is a breakdown of the trabeculae.

Intervertebral disks The disks of the spine have two basic components: the annulus fibrosis and the nucleus pulposus. Together they form a system providing strength, shock absorption, and flexibility.

In this image, you can see the outer zone of the annulus fibrosis. Its crossing collagen fibers are of high tensile strength and provide most of the structural strength to the disk.  The crisscross pattern increases the composite's flexibility and strength.

At the center is the nucleus pulposa, a gelatinous body which contains about 80% water.

It is incompressible and serves to distribute the forces placed upon it.

The annulus fibrosis consists of two zones. The outer zone contains the strong collagen fibers.

The outer zone blends into the softer fibrous tissues of the inner zone. The inner zone assists with absorbing shocks and distributing loads.

The annulus fibrosis consists of two zones. The outer zone contains the strong collagen fibers.

The outer zone blends into the softer fibrous tissues of the inner zone. The inner zone assists

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with absorbing shocks and distributing loads.

Facet joints Each vertebra (except the uppermost) has two pairs of facet (little face) joints. One pair orients upward, toward the superior vertebra, and the other pair faces downward toward the inferior vertebra. Each facet surface is covered with articular cartilage that acts as a bearing surface.

Side note: the facet joints are more formally called zygapophyseal, or simply, apophyseal joints.

In this image, you can see the superior articular facets. The white surface is the articular cartilage.

As we assemble the spinal motion unit, the respective facets connect to form a facet joint.

The facet joint is enclosed by connective tissue and filled with lubricating synovial fluid, making it a true synovial joint.

The facet joints help to stablize and strengthen the spinal column.

The vertebral body carries roughly 80% of the load on the spine, with the facets carrying the remaining 20%.

Let's look at motions of the spine (these are exagerated for clarity). You can see the role of the facet joints.

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Compression and decompression.

Rotating right and left.

FSU ligaments There are several ligaments that are local to the functional spinal unit. The most significant are the interspinous ligament and the pairs of intertransverse ligaments and the ligamenta flava.

Again these ligaments are part of the complex system that contributes to the stability and flexibility of the vertebral column.

Motion unit muscles There are several pairs of ligaments that are local to the spinal mition unit. The most signifiant are the two intertransverse ligaments, interspinous ligaments, and the two ligamenta flava. Again these ligaments are part of the complex sytem that contributes to the stability and flexibility of the vertebral column.

Side note: the facet joints are more formally called zygapophyseal or, more simply, apophyseal Joints.

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Regions of the vertebral column Here we will examine the four regions of the spine in more detail.

Each or these regions has unique characteristics that distinguish it from the other regions.

Each of the four regions of a normal spine has a saggital curvature. These curves allow greater flexibility and resistance to shock.

Each region has its own anatomy and function.

The cervical region is very flexible, allowing the head a great range of motion. Its seven vertebrae are relatively small.

The thoracic region has relatively llittle flexibility. The ribs articulate with the thoracic vertebrae to form a protective cage for the heart and lungs.

The lumbar region supprts the weight of the upper body. The vertebrae are relatively large, and the disks subject to disease.

The adult sacral region is made up of two bones: the sacrum and the coccyx. The sacrum joins the pelvis to transfer weight to the legs and forms a portion of the pelvic floor.

Male (left) and female (right) pelvis.  The many differences are a significant example of human sexual dimorphism. Why?

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Cervical region The cervical region has seven vertebrae: C1 through C7.

C1 and C2 differ significantly from the other vertebrae. They are similar to a ball and socket joint, giving the head its great flexibility.

C2 through C6 have bifid (split in two) spinal processes, permitting more muscle and ligament attachment for finer control of the head.

The spinal process of C7 is larger and longer than the others. It is the first palpable spinal process and is called the vertebra prominens.

Go ahead and check yours.

Only the cervical vertebrae have "holes" through their pedicles called transverse foramina. The transverse foramina protect the vertebral arteries passing through them.

Thoracic region The thoracic region has 12 vertebrae: T1 through T12. The most striking feature of the thoracic region of the spine is that the vertebrae articulate with the ribs. This forms a protective cage around the vital organs and structures in the chest.

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Another feature that distinguishes the thoracic region is its relative inflexibility compared to the cervical and lumber regions. This provides additional protection to the chest cavity. The twelve ribs are connected to the vertebrae by ligaments and joints.

T1 through T12 also have additional facets, called costal facets, to interface with the ribs.

Lumbar region The lumbar region contains five vertebrae, L1 through L5. Since the lumbar is at the lower (caudal) end of the spine, it endures the greatest wear and tear. The vertebgral bodies are large, as are the transverse and spinal processes.

Interventions in the lumbar region often involve fixation of the vertebrae and disks with rigid rods secured by metal screws passing through the vertebral pedicles.

Sacral region The sacral region of an adult contains two bones: the large sacrum and the coccyx, or tail bone.  The two bones are connected at the sacrococcygeal joint. The curvature of the sacral region is kyphotic (forming a "hump" on the back). The sacrum forms an articulating joint with the two iliac bones of the pelvis: the sacroiliac  joint.

*The name, sacrum, comes from ancient Latin "os sacrum" (holy bone). Some think this is because it is home to the female reroductive system. The coccyx took a less noble route, being named after its resemblamce to the beak of a cuckoo; refering to the sound made in its vicinity.

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The eight sacral foramina (holes) allow the lower spinal nerves to pass from interior to the spinal column to the lower body.

Vertebral comparison The vertebrae of the cervical, thoracic, and lumbar regions vary in size and structure. In general vertebrae become larger and stronger, progressing cephalid to caudal, reflecting the increasing axial load.

The cervical region is very flexible and vertebrae contain transverse foramina.

Thoracic vertebrae have overlapping spinus processes and the region is relatively inflexible. They have costal facets that interface with the ribs.

Lumbar vertebrae are strong. Their transverse processes are so large that they are also called costal processes because they resemble primitive ribs.

Major spinal ligaments

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The anterior and posterior longitudinal ligaments are the major stabilizers for flexion and extension motions of the spine. The posterior longitudinal ligament (PLL) runs along the posterior surface of the vertebral body. It extends from the skull base into the sacral canal.

The PLL lies within the vertebral foramen. This ligament bonds securly to the posterior face of the vertebral bodies, but only loosely to the interevetebral disks. The anterior logintudinal ligament (ALL)  runs along the anterior surface of the vertebral body. It extends from the skull base into the sacral canal. This ligament also bonds securly to the vertebral bodies, but only loosely to the interevetebral disks.

Back muscles

This is a quick review of some of the muscles of the back. It is not meant to be a comprehensive lesson, but just provide an overview of the complex musculature of the back. The most superficial muscle is the trapezious. It connects medially to the spinal processes. Caudal to the trapezius are the latissimus dorsi. Both of these large muscles are superficial to what are called the intrinsic muscles of the back. The "long" muscle is the longissimus. It connects to the transverse processes in the lumbar region and the thoracic region. The quadratus lumborum stretches from the superior edge of the pelvis (illium) and connects to the lowest rib and the tips of the lumbar transverse processes.The multifidus muscles fill the space between the spinal and transverse processes and work to stabilize each joint.The lateral intertraversarious muscles help stabilize adjacent vertebrae. The semispinalis thoracis muscles help stabilize the thoracic region. The semispinalis capitus muscles help to tilt the head from side to side. The capitis muscles help to extend and flex the head.

Vertebral arteries The paired vertebral arteries, like the carotid arteries, supply oxygenated blood to the brain. They enter the spine at the transverse foramina of C6 and proceed through the transverse foramina of C6 to C1 and then into the cranium.

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Spinal cord and nerve branches Below the head, all the nerves of the human body are connected through the spinal cord to the brain. Although the  details can be extremely complex, a basic understanding of the system is well within our scope.

Central & peripheral nervous systems The nervous system can be physically divided into the central nervous system and the peripheral nervous system.

The central nervous system is made up of the brain (the body's CPU) and the spinal cord that resides within the vertebral foramen. The central nervous system is encapsulated in a tough membrane called the dura mater (or just the dura). The spinal cord is a signal highway that transmits all information to and from the brain and the body.

Pairs of nerve groups branch out of the spinal cord at each spinal joint to join the peripheral nervous system.

Spinal cord and nerve roots The cross section of the spinal cord shows "grey matter" surrounded by "white matter." The spinal cord is covered by the dura mater (not shown).

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The peripheral nervous system can again be divided into motor and sensory (or more properly somatosensory) nerves. The sensory nerves carry information to the brain about pressure, touch, temperature, and pain. In turn the brain excites the motor neurons that control the muscles.

All the motor nerve roots exit the sipnal cord from the anterior (or ventral) aspect.

All the sensory nerve roots enter the spinal cord through the posterior (or dorsal) surface. All roots pass through the intervertebral foramen and join to form the spinal nerve.

Another critical component of the spinal cord is the autonomic nervous system. As you can guess by the name, these nerves function to regulate the organs of the body without our conscious control.

Motor and sensory nerves The spinal nerves continue to separate and branch out until the final connections to the millions of sensors and actuators are complete.

The sensory nerves are more delicate than the motor nerves.  They manifest symptoms of neurological problems sooner than the more robust motor neurons.

The pressure of a ruptured disk may first induce numbness in a limb. As the disease progresses, weakness in the related muscles may develop.

Spinal nerves and function

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The C1 spinal nerve exits ABOVE the C1 vertebra. This nomenclature continues until nerve C8 exits BELOW the C7 vertebra.

From then on the spinal nerves take their names from the vertebrae above them. (Here "above" refers to cephalid, but you knew that.)

Breathing Head & neck movement. Heart rate

Shoulder movement

Wrist and elbow movement

Hand and finger movement

Abdominal muscles, trunk stability

Temperature stability

Sexual function

Hip motion

Knee Extension

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Foot motion

Knee flexion

Sexual organs

Anal control

Spine anatomy recap The vertebrae have a dense, strong shell of cortical bone, enclosing cancellous bone, having a more porous structure

Disks have strong, fibrous outer bands, the annulus fibrosis, enclosing the jelly-like, but incompressible, nucleus pulposus.

Articulating facet joints, together with the disks, allow motion of the spine.

The posterior longitudinal ligament (PLL) and the anterior longitudinal  ligament (ALL) link the vertebral bodies for more stability.

Vertebral processes are linked by the interspinous ligaments, pairs of intertransverse ligaments, and the pairs of ligamenta flava connecting the pedicles.

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The spine has four anatomical regions: cervical, thoracic, lumbar, and sacral. Each region has a distinct curvature. The physical structure of individual vertebrae varies for each region.

The brain and spinal cord make up the central nervous system. All other nerves make up the peripheral nervous system. The spinal cord branches out into nerve trunks at each joint. These trunks connect to many specific regions of the body.

QUIZ

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