Understanding the Human Spine: Structure and Function
The Human Spine: Structure and Function
The basic element of the spine is the vertebra. This bone structure is repeated up to 33 times to form a column. It is classified into five different regions according to location:
- Cervical: Composed of 7 cervical vertebrae, extending from the occipital bone of the head to the thoracic vertebrae, forming the neck.
- Thoracic: Consists of 12 vertebrae and supports the thorax.
- Lumbar: Contains 5 vertebrae and corresponds to the abdomen.
- Sacral: 5 fused vertebrae forming the sacrum, which is part of the pelvis.
- Coccygeal: 4 (sometimes 3 or 5) very small, fused vertebrae forming the coccyx.
Each vertebra has two main parts:
- Vertebral Body: Cylinder-shaped, providing strong support for the joint.
- Vertebral Foramen: The opening through which the spinal cord runs. Its walls are formed by a bony arch from which prominences, or processes, arise. These include the transverse processes on the side walls and the spinous process on the posterior wall. These processes connect to the vertebral body through interapophyseal ligaments.
Between each vertebra lies a fibrous cylinder called an intervertebral disc. At the center of this disc is a harder core known as the nucleus pulposus. The presence of the nucleus pulposus defines the cartilaginous joints (amphiarthrosis) of the spine. The sum of all these amphiarthroses along the spine allows for a wide range of motion.
The spine is the vertical axis of the body and a continuation of the axis between the legs. It is fundamental for the balance of the head, thoracic structures, and upper extremities. The spine’s stability is maintained by numerous muscles arranged in several layers. Here’s a breakdown of the key muscle groups by region:
Muscles of the Spine
Cervical Spine Region
The muscles that stabilize the cervical spine are anchored at one end to a structure of the cervical spine and at the other end at a distance, especially in the shoulder region. Examples include:
- Trapezius (especially its upper fibers)
- Sternocleidomastoid (involved in neck turning)
Image Representation:
C – Cervical, D – Dorsal, L- Lumbar, Sternocleidomastoid, Trapezius, Scalene
Thoracic Spine Region
The muscles stabilizing the thoracic segment of the spine are inserted at one end into the thoracic vertebrae and at the other end into the shoulder girdle, arm, and ribs. The muscle fibers generally run from the bottom up and from the inside out. Examples include:
- Trapezius (lower part; contraction lowers the shoulder)
- Latissimus Dorsi (an adductor of the upper limb)
- Serratus
- Iliocostalis
Lumbar Spine Region
The lumbar spine lacks costovertebral joints and is an area with few bones, so the muscles in this region must provide significant stability. These muscles can be classified into two groups:
- Anterior Muscles: Abdominals
- Posterior Muscles: Anchored at one end in the lumbar spine and at the other in the pelvis and even in the lower extremities.
Examples include:
- Quadratus Lumborum (trunk flexor)
- Iliopsoas (anteversor and adductor of the leg)
There are two main groups of intervertebral muscles:
- Short Intervertebral Muscles (Thick): Very short muscles connecting the processes of adjacent vertebrae, acting as local reinforcements. They play a small role in large spinal movements; their primary function is local stabilization.
- Long Intervertebral Muscles: Very long muscles running longitudinally and parallel to the spine, connecting multiple vertebral structures. Their main action is to stabilize the spine. They are also involved in dorsal hyperextension of the body and head elevation.