The semispinalis muscles is the longest and most superficial set of muscles in the transversospinal group. They lie in the deep layer of the deep back muscles, and they are located on either side of the spine, extending from the skull to the tenth thoracic vertebrae.

The semispinalis muscles are subdivided based on their location into semispinalis capitis muscle, semispinalis cervicis muscle and semispinalis thoracis muscle.

Semispinalis capitis

The semispinalis capitis muscle is the most superior and longest part of the semispinalis muscles, and it overlies the semispinalis cervicis.

Origin: articular processes of the fourth to seventh cervical vertebrae (C4 - C7), transverse processes of the first to sixth thoracic vertebrae (T1 - T6).

Insertion: occipital bone between the superior nuchal line and inferior nuchal line.

Action: bilateral contractions - extension of the head and neck; unilateral contractions - rotation of the head and neck to the opposite side (contralateral), lateral flexion of the head and neck to the same side (ipsilateral).

Innervation: descending branches of the greater occipital nerve (C2), third cervical spinal nerve (C3).

Blood supply: occipital artery - a branch of the external carotid artery.

Semispinalis cervicis (semispinalis colli)

The semispinalis cervicis muscle, also known as the semispinalis colli, is the middle portion of the semispinalis muscles. It lies over the cervical and thoracic parts of the multifidus muscles.

Origin: transverse processes of the first to the sixth thoracic vertebrae (T1 - T6).

Insertion: spinous processes of the second to fifth cervical vertebrae (C2 - C5).

Action: bilateral contractions - extension of the head and neck; unilateral contractions - rotation of the head and neck to the opposite side (contralateral), lateral flexion of the head and neck to the same side (ipsilateral).

Innervation: medial branches of the dorsal rami of the spinal nerves.

Blood supply: occipital artery, deep cervical artery and vertebral artery.

Semispinalis thoracis

The semispinalis thoracis muscle is the lowest portion of the sempispinalis muscles that overlie the multifidus thoracis muscle.

Origin: transverse processes of the sixth to tenth thoracic vertebrae (T6 - T10).

Insertion: spinous processes of the sixth cervical to fourth thoracic vertebrae (C6 - T4).

Action: bilateral contractions - extension of the thoracic spine; unilateral contractions - lateral flexion of the thoracic spine to the same side (ipsilateral), rotation of the thoracic spine to the opposite side (contralateral).

Innervation: medial branches of the dorsal rami of the spinal nerves.

Blood supply: dorsal branches of the posterior intercostal arteries.

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Semispinalis muscles