Semispinalis Thoracis

Back · Trunk

The Semispinalis Thoracis is a Trunk muscle within the Back group. Originating at Transverse processes of T6–T10, it inserts at Spinous processes of C6–T4. Its chief action is that it extends the thoracic and cervical spine and rotates it to the opposite side. Innervation is provided by the Posterior (dorsal) rami of thoracic spinal nerves.

Origin, Insertion, Action & Nerve

Origin

Transverse processes of T6–T10

Insertion

Spinous processes of C6–T4

Action

Extends the thoracic and cervical spine and rotates it to the opposite side

Nerve

Posterior (dorsal) rami of thoracic spinal nerves

Attachments explained

The Semispinalis Thoracis is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.

Origin

Transverse processes of T6–T10

Insertion

Spinous processes of C6–T4

Action & function

When the Semispinalis Thoracis contracts, it produces the following movement: Extends the thoracic and cervical spine and rotates it to the opposite side.

As part of the Back group of the Trunk, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.

Nerve supply & clinical relevance

The Semispinalis Thoracis receives its nerve supply from the Posterior (dorsal) rami of thoracic spinal nerves.

Because a muscle can only contract when its nerve is intact, injury to the Posterior can weaken or paralyse the Semispinalis Thoracis, impairing the movements it normally produces (extends the thoracic and cervical spine and rotates it to the opposite side). This is why knowing the innervation is central to localising nerve lesions in clinical practice.

How to study the Semispinalis Thoracis (the OIANS method)

OIANS stands for Origin, Insertion, Action and Nerve, the four facts that uniquely define every skeletal muscle. To learn the Semispinalis Thoracis, work through them in order: picture its origin, trace the muscle to its insertion, reason out the action that shortening between those two points must create, then add the nerve that drives it.

Most students remember the Semispinalis Thoracis fastest by linking its action back to its attachments rather than memorising each fact in isolation. Once the origin and insertion make sense, the action usually follows logically.