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General Anatomy of the Spine

Functions of the Spine

The spine consists of vertebrae that support the body’s trunk while also enclosing the spinal cord. Understanding the general anatomy and functions of the spine (S.S.) is extremely important for patients with issues in this area.

This article provides an overview of the complex anatomy of the spine, covering the following topics:

A general overview of spinal functions, including foramina and primary curves.
A detailed description of individual anatomical components, including vertebrae, discs, the spinal cord, nerves, joints, muscles, and ligaments.

Regions of the Spine

The spine is typically divided into five main regions:

Cervical
Thoracic
Lumbar
Sacral
Coccygeal

Each spinal region has distinct characteristics and functions.

Cervical Spine

The cervical spine is part of the human neck. It consists of seven vertebrae that protect the spinal cord, support the head, and allow for the neck’s significant flexibility.

The first cervical vertebra is called the atlas, shaped like a ring.
The second cervical vertebra is called the axis, which, as the name suggests, resembles an axis. It extends upward and fits into a specific position within the atlas.
The atlas and axis allow the head to rotate, flex, and extend, accounting for about 50% of neck movement.
The remaining cervical vertebrae (C3 to C7) do not have distinct anatomical features but are responsible for the other 50% of cervical mobility.

Thoracic Spine

The thoracic spine consists of 12 vertebrae, with the first located below the last cervical vertebra.

As you move downward, the thoracic vertebrae increase in size: T1 is the smallest, while T12 is the largest.
Each thoracic vertebra connects to a rib on the left and right, forming the rib cagewhen joined at the front with the sternum.
The thoracic spine, along with the ribcage and sternum, protects the lungs and heart.

Lumbar Spine

The lumbar spine consists of five vertebrae (L1 to L5), increasing in size from top to bottom.

The lumbar spine has greater mobilitythan the thoracic spine.
Due to the orientation of its small joints, the lumbar spine primarily allows forward and lateral flexion, whereas rotational movement is limited compared to the thoracic spine.

Sacral Spine

The sacral spine (or sacrum) is commonly referred to as the tailbone in everyday language.

It consists of five fused vertebrae (S1 to S5), forming a single bone.
The sacrum connects the pelvis to the spine, acting as a link between the two.
Below the sacrum is the coccyx.

Spinal Curvature

When viewed from the front (frontal plane), the spine appears straight.

When viewed from the side (sagittal plane), it has four curvesclassified as either kyphotic or lordotic.

Kyphosisoccurs naturally in the thoracic and sacral regions.

Lordosisoccurs naturally in the cervical and lumbar regions.

Vertebrae – Spinal Bodies

All vertebrae, except the first and second cervical ones, share the same fundamental structure:

Outer layer:
Cortical bone, which is hard, compact, and highly durable.
Inner layer:
Trabecular bone, which is softer and has a honeycomb-like structure.
Bone marrow:
located inside, produces red and some white blood cells.

Components of the Vertebrae

Each vertebra consists of the following elements:

Vertebral body:
The largest part, shaped like an hourglass, with a narrower middle section.
Pedicles:
Two small cylindrical structures extending from the vertebral body.
Lamina:
A bony plate extending between the pedicles, forming part of the vertebral canal.

Vertebrae – Processes and Facet Joints

Each vertebra has seven processesextending from the lamina.

Two articular processesper side (one facing up, one facing down) form the facet joints, which articulate with adjacent vertebrae.

The facet joints are enclosed by a capsule filled with synovial fluid, acting as a lubricant.

These joints:

1. Facilitate movementand guide spinal motion.

2. Limit excessive flexion and extensionof the spine.

Additional spinous and transverse processesprovide attachment points for tendons and ligaments.

Vertebrae – Endplates

Other spinal structures include endplates, which are cartilage layers covering the top and bottom of each vertebral body.

These plates surround and nourishthe intervertebral discs by facilitating hydration and nutrient exchange.

Intervertebral Foramina

The intervertebral foramenis the space between two adjacent vertebrae.

This opening allows for the exit of spinal nerve roots, which then form nerve plexusesextending to the limbs.

Intervertebral Discs

Between the vertebral bodies are intervertebral discs, acting as shock absorbers. Their functions include:

Absorbing movement-related loads.
Holding vertebrae together.
Receiving nutrients through osmosisfrom the endplates.

Each disc consists of:

Annulus fibrosus(outer ring).
Nucleus pulposus(inner core).

Annulus Fibrosus

Structurally resembles a car tire:

The outer ringis the annulus fibrosus.
The soft centeris the nucleus pulposus.

It provides:

Rotational stability.
Compression resistance.
Composed of collagen fibers, arranged in a woven pattern, similar to tire reinforcements, ensuring durability and stability.

Nucleus Pulposus

The center of the disc contains a gel-likesubstance known as the nucleus pulposus.

It distributes loadsacross the vertebrae.

It is composed of:

Water.
Collagen.
Proteoglycans.

Spinal Cord and Nerve Roots

The spinal cord is an extension of the brain within the spinal canal:

The brainstem, the lower part of the brain, transitions into the spinal cord at the upper cervical spine.
The spinal cord extends to the L1 vertebra, after which it becomes the cauda equina(a bundle of nerve roots resembling a horse’s tail).
Spinal nerves exit the spinal canal via intervertebral foraminaand extend to the limbs.

The central nervous system consists of the brain and spinal cord, while the peripheral nervous system includes nerve roots, plexuses, and peripheral nerves.

Spinal Canal Stenosis

The spinal canal is the space between the vertebral body and lamina, housing the spinal cord. It may become narrowed (stenosis) due to:

Disc herniation(bulging into the canal).
Facet joint arthritis, leading to swollen joints occupying spacein the spinal canal.

In such cases, the condition is referred to as spinal stenosis.