Spinalis

The spinalis muscle is a deep muscle of the back, belonging to the erector spinae group. It is the most medial of the three columns of the erector spinae, the other two being the longissimus and iliocostalis. The spinalis muscle is further subdivided into three parts, often named according to the region of the vertebral column they are associated with: spinalis thoracis, spinalis cervicis, and spinalis capitis. However, the spinalis capitis is not always considered a distinct part and may be included within the semispinalis capitis, another deep back muscle.

Origin and Insertion:

The origin and insertion points vary depending on the specific part of the spinalis muscle being considered. Generally:

• Spinalis Thoracis: Originates from the spinous processes of the upper lumbar and lower thoracic vertebrae (typically L1-L2 and T11-T12) and inserts onto the spinous processes of the upper thoracic vertebrae (typically T2-T8).
• Spinalis Cervicis: Originates from the spinous processes of the upper thoracic vertebrae (typically T1-T2) and/or the ligamentum nuchae and inserts onto the spinous processes of the lower cervical vertebrae (typically C2-C4). It is often a poorly developed or absent muscle.
• Spinalis Capitis: Originates from the spinous processes of the upper thoracic and lower cervical vertebrae (typically C7 and T1-T3) and inserts onto the occipital bone. Its existence as a distinct muscle is debated.

Function:

The primary function of the spinalis muscle is to extend the vertebral column and assist in lateral flexion. Working bilaterally, it extends the spine. Unilaterally, it assists in lateral bending of the spine to the same side. It also plays a role in maintaining posture.

Innervation:

The spinalis muscle is innervated by the dorsal rami of the spinal nerves corresponding to the vertebral levels at which it is located.

Clinical Significance:

The spinalis muscle, along with other back muscles, can be a source of back pain due to strain, injury, or poor posture. Trigger points in the spinalis muscle can refer pain locally.

Variations:

The size and development of the spinalis muscle can vary considerably between individuals. The spinalis cervicis, in particular, is often quite small or even absent. The existence and distinct nature of the spinalis capitis are also subject to anatomical variation and debate.