The semispinalis muscles are a complex group of three distinct muscles—semispinalis capitis, semispinalis cervicis, and semispinalis thoracis—that form part of the transversospinales system. These muscles are critical for the intricate movements and stability of the head and vertebral column. Their effectiveness stems directly from their precise anatomical origins and insertions, which dictate their mechanical actions. Understanding these attachment
points provides a clearer picture of how these deep muscles contribute to overall posture and movement, particularly in the neck and upper back regions.
Each semispinalis muscle has a unique set of origins and insertions, allowing them to act on different segments of the spine and head. Despite their individual characteristics, they share a common innervation pattern, receiving signals from the dorsal rami of the cervical spinal nerves. This shared neurological supply underscores their coordinated function within the larger muscular system of the back and neck.
The Semispinalis Capitis: From Vertebrae to Occipital Bone
The semispinalis capitis, also known as the complexus, is strategically located at the upper and back part of the neck. Its origins are quite extensive, arising from a series of tendons. These tendons emerge from the tips of the transverse processes of the upper six thoracic vertebrae. In addition to these thoracic origins, the muscle also takes root from the articular and transverse processes of the lower four cervical vertebrae. This broad base of origin allows the muscle to gather significant force for its actions.
These numerous tendons then unite to form a broad muscle belly that ascends towards the skull. The insertion point for the semispinalis capitis is between the superior and inferior nuchal lines of the occipital bone, which is the bone forming the back and base of the skull. This direct connection to the skull highlights its primary role in head movement and positioning. The muscle lies deep to the more superficial splenius muscles and the trapezius, yet it can sometimes be felt as a firm mass lateral to the cervical spinous processes.
The Semispinalis Cervicis: Connecting Thoracic to Cervical Spines
The semispinalis cervicis, also known as the semispinalis colli, plays a vital role in the cervical spine. Its origins are found in a series of tendinous and fleshy fibers that arise from the transverse processes of the upper six thoracic vertebrae. This connection between the thoracic and cervical regions is crucial for coordinated spinal movement.
From these origins, the muscle fibers extend upwards to insert into the cervical spinous processes. Specifically, it attaches to the spinous processes from the axis (the second cervical vertebra) up to the fifth cervical vertebra, inclusive. The semispinalis cervicis is noted for being thicker than the semispinalis thoracis, suggesting a more substantial contribution to neck stability and movement. The fasciculus, or bundle of muscle fibers, that connects with the axis is particularly large and predominantly muscular, indicating its importance in controlling the movements of the uppermost cervical spine.
The Semispinalis Thoracis: Segmental Control of the Thoracic Spine
Finally, the semispinalis thoracis, or semispinalis dorsi, focuses its actions on the thoracic spine. This muscle is characterized by its thin, narrow, and fleshy fasciculi, which are interspersed with relatively long tendons. This structure allows for precise, segmental control over the vertebral column.
The origins of the semispinalis thoracis are from a series of small tendons that arise from the transverse processes of the sixth to the tenth thoracic vertebrae. From these points, the muscle ascends to insert into the spinous processes of the upper four thoracic vertebrae and the lower two cervical vertebrae. This dual insertion across both thoracic and cervical segments enables it to contribute to the extension and rotation of these parts of the spine, working in conjunction with the other semispinalis muscles to maintain spinal integrity and facilitate movement.
