Vertebral Column

The vertebral column, also known as spine or backbone, is a flexible column that forms a part of the axial skeleton. The vertebral column is a characteristic of the vertebrates that has evolved from the notochord of chordates by segmented bones and intervertebral discs. The vertebral column is placed dorsally. It is composed of 26 segmented units, called vertebrae.

This article focuses on the vertebral column, its function, anatomy, diagram and structure. Read further for a detailed description of the backbone.

Location

The vertebral column, which is also known as the spinal column, forms the central axis of the body skeleton. It emerges from the neck at the base of the skull and extends upto the tail region.

Structure of Vertebral Column

A human vertebral column consists of 33 bones at birth. With time, as the person grows into an adult, the lower 9 bones fuse into two separate bones and the upper 24 bones are separated and articulated by intervertebral discs. The articulating vertebrae are named according to their locations. Let us discuss them one by one briefly.

The number of bones in the vertebral column in humans is 26. Generally, there are 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar, 1 sacral (fused) and 1 coccygeal (fused) vertebrae. The length of the backbone in humans ranges from 61cm to 71cm.

Each vertebrae is made up of the vertebral body and vertebral arch. The vertebral arch is composed of a pair of pedicles and a pair of laminae. A vertebral foramen (opening) is formed by the body and arch that contains the spinal cord. Each vertebra consists of seven processes (projection or outgrowth), two superior, two inferior, two transverse and one spinous process.

Cervical Vertebrae:

• The cervical vertebrae are located right below the skull. It is composed of 7 segments (C1 – C7) that can be distinguished from other vertebrae by the presence of a hole in the spinous process. The spinous process is a bony projection that allows the attachment of muscles and ligaments.
• Atlas or C1 is the topmost bone of the vertebral column that supports the skull. It connects the skull to the spine. It is a ring-like structure that consists of an anterior and posterior arch.
• Axis or C2 is the second bone that lies just below the C1. It forms the axis upon which the atlas rotates. The characteristic feature of the axis bone is the presence of dens. It is a slight constriction that rises perpendicularly from the body of the bone. The dens help to restrict the movement of the atlas.
• The C3, C4, C5 and C6 are small vertebrae that are broad on the sides.
• The C7 or vertebra prominens can be identified by the presence of the longest spinous process. The projection is visible as a neck furrow. The C7 vertebra serves as the site of major muscle attachments.

Thoracic Vertebrae:

• There are twelve thoracic vertebrae that are denoted as T1 – T12. The thoracic vertebrae can be distinguished from the vertebral bones by the presence of facets that act as the point of attachment with the associated tubercle of the rib.
• The thoracic vertebrae increase in size as we go down, the lower ones are a lot bigger than the upper ones.
• The thoracic bones are almost triangular in shape and consist of longer pedicles and laminae as compared to the cervical vertebrae.
• T1-T4 bones are similar to cervical vertebrae and T9-T12 possess some of the lumbar characteristics. All the bones together give a curvature to the body, called kyphotic curve.
• T1, or the first thoracic vertebrae, has whole facets that articulate with the first rib. It possesses a long spinous process, same as C7.
• T11 and T12 are other unique bones in this region that form a transition to the lumbar vertebrae.

Lumbar Vertebrae:

• The lumbar region is composed of 5 segments that increase in size as we go down. These vertebrae are the largest among all the vertebral bones.
• The lumbar vertebrae lies between the chest region and pelvic area.
• The large size of these bones capacitates this region to support the entire upper body.
• These vertebrae have distinctly short and thick spinous processes with the facets projecting vertically. The spinal cord ends at the lumbar region.
• L1 or the first lumbar vertebrae, lies in level with the ninth rib. The gallbladder, pylorus and celiac trunk are also located at this level.
• L5 or the last lumbar vertebrae has the largest body and transverse processes.

Sacrum:

• The sacrum is an inverted triangular bone that is concave in the front and convex in the back.
• It is composed of five bones (S1-S5) fused together at the age of 18 to 30.
• The sacral vertebrae is articulated at four different angles, with the last lumbar vertebrae, with the coccyx and with the hip bones on either side.
• The sacrum is connected to the pelvis and hence stabilises the pelvic region. It functions to support the weight of our entire body.

Coccyx:

• The coccyx, commonly referred to as the tailbone, is composed of four (sometimes, five) fused segments.
• It is the remnant present in tailless primates after the evolution of tailed ones. It is a vestigial structure.
• The fused segments are rudimentary. The first tailbone is articulated with the sacral bone with a merely visible transverse process.
• Pedicle, laminae and spinous processes are absent in the coccygeal region.
• The coccyx serves as the region for the attachment of various muscles and ligaments.

Anatomy of Vertebral Column

The vertebrae vary in size and shape in all the five regions of the vertebral column. But they share a common anatomy such as they have a body, a vertebral arch and seven processes. The body is usually known as a ventral body or a centrum, and the arch is Y-shaped.

From the arch extends a spinous process both backwards and downwards that feel like bumps on the body. Two transverse processes emerge from either side which provide a site for the attachment of muscles and ligaments.

The arch and centrum together surround an opening called vertebral foramen, the space through which the spinal cord passes. Intervertebral discs are present between each centrum that provide cushioning and protection against shock while locomotion.

Function

• It forms the central axis for weight bearing and supports the body by transferring the weight of abdomen and trunk to the legs.
• It protects the spinal cord. Spinal nerves emerge from the vertebral column that make up the sympathetic nervous system.
• It helps in movement and allows bending and rotation.
• It also serves as the site of attachment of ribs in the thoracic region.
• Multiple muscles and ligaments are also attached by the transverse processes.
• Last but not the least, the intervertebral discs provide cushioning between each vertebrae.

Embryology

The development of the human spine begins soon after conception. It starts around day 17. The vertebral column arises from the somites during embryogenesis. Somites are primary segments of the paraxial mesoderm.

Vertebral Column in Lower Vertebrates

Lower vertebrates have a more complex vertebral column as compared to humans and other higher vertebrates. In primitive chordates, a notochord (rod like structure) is found that gives support and stiffens the body.

The notochord arises in all vertebrates which is later replaced by the vertebral column in adults. However, in some fishes the notochord stays for life. While in some vertebrates, the spine gets lost in development.

In turtles, some of the vertebrae are fused to its shell. The cervical vertebrae in birds are free while the caudal and sacral vertebrae are fused to form a rigid structure.The whales lack a sacrum.

This sums up the details of a vertebral column. Keep visiting BYJU’S for more important updates on NEET.

Also See:

• Difference Between Backbone And Spinal Cord
• Flashcards Of Biology For NEET Locomotion And Movement
• What Are Floating Ribs