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Deep Layer (AC) – Anatomy of the Forearm

by James Pickering, PhD
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    00:01 It helps us to form a fist. If we then look at the deep layer, we can see we’ve got some muscles which are lying deep to flexor digitorum superficialis. We have flexor digitorum profundus. We can see that this is coming from the ulna. It’s also coming from the interosseous membrane that’s between the two bones of the forearm, the radius and the ulna. And this again gives rise to four long tendons, that this time, go towards the distal phalanx of each digit 2 to 5, not including the thumb. The thumb is different because it has its own specific flexor, and this is known as flexor pollicis longus, pollicis meaning the thumb, flexor pollicis longus. The fact that we call it flexor pollicis longus indicates it’s going to be a flexor pollicis brevis, and this muscle is within the hand.

    00:56 So we’ll look at it later. The final muscle, the deep muscle in the anterior compartment is pronator quadratus, and that runs between the two distal ends of the radius and the ulna. So let’s have a look at the attachments of these muscles. Flexor digitorum profundus is running from the proximal surfaces of the medial and anterior surfaces of ulna and the interosseous membrane. So it’s coming from the ulna. It then passes via its four long tendons to the distal phalanges, the medial four distal phalanges of digits 2, 3, 4, and 5. The nerve supply, it has a dual nerve supply. The lateral muscle bellies, these are the ones that are going to digits 2 and 3 are supplied by the median nerve, and the tendons that are going to digits 4 and 5 are supplied by the ulnar nerve.

    01:56 So, flexor digitorum profundus has two nerves supplying it, the median nerve and the ulnar nerve. Its function is to flex the hand at the wrist joint, and it also flexes the distal interphalangeal joint. So while flexor digitorum superficialis flex the proximal interphalangeal joint, this now flexes the very distal interphalangeal joint, the joint between the distal phalanx and the middle phalanx. If we go back and look at flexor pollicis longus, we can see this is really coming from the radius, and this gives a long tendon that goes and attaches to the distal phalanx of the thumb. So we can see that flexor pollicis longus running from the anterior surface of the radius, and also the interosseous membrane passes to the distal phalanx of the first digit, the first digit being your thumb. The pronator quadratus, as we saw here, is running between the two distal ends of the radius and the ulna.

    03:00 It runs from the distal quarter of the ulna, so the last quarter of the ulna, and it actually passes towards the radius. So, when pronator quadratus contracts, it is actually going to move the radius. It’s going to move the radius into a more pronated position. So from this supine position, contraction of pronator quadratus is going to pronate the radius, pronate the forearm. It pulls the radius over the ulnar bone. Both flexor pollicis longus and pronator quadratus are innervated via the anterior interosseous nerve. And this is a branch of the median nerve. So the median nerve is supplying the majority of these muscles within the anterior compartment, but a specific branch, the anterior interosseous nerve, is supplying the flexor pollicis longus and pronator quadratus. Flexor pollicis longus does similar functions to flexor digitorum muscles, except it acts at the first digit. It continued the contraction, will flex the wrist, it flexes the interphalangeal and metacarpophalangeal joints associated with the first digit. Pronator quadratus pronates the forearm. So it acts on the radio-ulnar joints. Here, we can look at the anatomy with all of these muscles put


    About the Lecture

    The lecture Deep Layer (AC) – Anatomy of the Forearm by James Pickering, PhD is from the course Upper Limb Anatomy.


    Included Quiz Questions

    1. Median nerve.
    2. Ulnar nerve.
    3. Anterior interosseous nerve.
    4. Radial nerve.
    5. Musculocutaneous nerve.
    1. Flexion of the distal interphalangeal joint.
    2. Extension of hand at the wrist joint.
    3. Flexion of the proximal interphalangeal joint.
    4. Extension of the metacarpophalangeal joint.
    5. Flexion of the metacarpophalangeal joint.

    Author of lecture Deep Layer (AC) – Anatomy of the Forearm

     James Pickering, PhD

    James Pickering, PhD


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