Attachment points and topography
The muscle is located in the deep layer of the posterior compartment of the forearm.
- Origin (Origo): muscle fascicles arise from the distal third of the posterior surface of the ulna. An additional point of origin for the muscle is the adjacent interosseous membrane.
- Course of the tendon: running distally, the tendon of m. extensor indicis passes beneath the extensor retinaculum. It lies in the fourth osteofibrous compartment together with the tendons of the extensor digitorum (m. extensor digitorum).
- Insertion (Insertio): upon reaching the dorsum of the hand, the tendon proceeds toward the index finger and securely blends into its dorsal tendinous (aponeurotic) expansion.
Biomechanics and functions
The main task of this muscle is directly reflected in its name.
- Isolated extension: contraction of the muscle fibers pulls the tendinous expansion, which allows for the selective extension of the index finger even when the other fingers are clenched into a fist.
- Synergy in joints: in addition to working with the phalanges of the second digit, the muscle crosses the wrist joint. Due to this, it acts as a synergist and assists in extension of the hand as a whole.
Blood supply and innervation
Life support and control of the muscle are provided by structures typical of the posterior forearm:
- Innervation is entirely supplied by the radial nerve — n. radialis. The fibers controlling the muscle originate from spinal segments C7–C8.
- Blood supply is provided by branches of the posterior interosseous artery (a. interossea posterior), which runs in the deep layer of the forearm.
Lateral group of forearm muscles
When studying the extensors, it is important to remember the adjacent lateral muscle group, which topographically intersects with them. All these muscles are also innervated by the radial nerve (n. radialis):
- Brachioradialis (m. brachioradialis): flexes the forearm and places the hand in a neutral position. Supplied by the a. radialis.
- Extensor carpi radialis longus and brevis (m. extensor carpi radialis longus et brevis): their function is to extend the wrist and abduct it radially (radial deviation). Supplied by the a. radialis and a. recurrens radialis.
Briefly: answers to topical questions
Which muscles comprise the deep layer of the posterior compartment of the forearm?
The deep layer of the posterior compartment of the forearm includes the supinator, extensor indicis, and the muscles that control the thumb. These include:
- Supinator (m. supinator) — located in the superior lateral region of the forearm.
- Abductor pollicis longus (m. abductor pollicis longus) — originates from the posterior surface of both forearm bones and the interosseous membrane.
- Extensor pollicis brevis (m. extensor pollicis brevis) — inserts at the base of the proximal phalanx of the thumb.
- Extensor pollicis longus (m. extensor pollicis longus) — inserts at the base of the distal phalanx of the thumb.
- Extensor indicis (m. extensor indicis) — originates from the posterior surface of the ulna and the interosseous membrane.
To easily remember the function of m. extensor indicis, recall the gesture of pointing at something: this very muscle allows you to keep the index finger straight (extends it) while the other fingers may remain bent.
Class and exam questions
Through which compartment does the extensor indicis tendon pass?
Its tendon passes through the fourth osteofibrous compartment beneath the extensor retinaculum.
With the tendon of which muscle does it share this compartment?
In the fourth compartment, the tendon of m. extensor indicis passes together with the tendons of the extensor digitorum (m. extensor digitorum).
Where does the extensor indicis originate?
The muscle originates from the posterior surface of the ulna (in its distal third) and the interosseous membrane.
Which nerve controls the function of this muscle?
Like most forearm extensors, it is innervated by the radial nerve (n. radialis, segments C7–C8).
What else to study on this topic
- Topography of the osteofibrous compartments of the dorsal wrist (compartments 1–6)
- Anatomy and functions of the extensor digitorum
- Branches of the radial nerve in the forearm
- Structure of the dorsal digital aponeurotic expansion of the hand
- Mechanism of radial deviation of the hand (action of the radial wrist extensors)
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