Deep Layer (AC) – Anatomy of the Forearm

00:00 If we then look at the deep layer, we can see we've got some muscles which are lying deep to flexor digitorum superficialis.

00:09 We have flexor digitorum profundus.

00:13 We can see that this is coming from the ulna.

00:16 It's also coming from the interosseous membrane that's between the two bones of the forearm, the radius and the ulna.

00:24 And this again gives rise to four long tendons that this time go towards the distal phalanx of each digit, two to five, not including the thumb.

00:36 The thumb is different because it has its own specific flexure and this is known as flexor pollicis longus.

00:43 Pollicis meaning the thumb.

00:45 Flexor pollicis longus.

00:47 The fact that we call it flexor pollicis longus indicates is going to be a flexor pollicis brevis.

00:54 And this muscle is within the hands. So we'll look at it later.

00:57 The final muscle, the deep muscle in the anterior compartment is pronator quadratus.

01:03 And that runs between the two distal ends of the radius and the ulna.

01:08 So let's have a look at the attachment of these muscles.

01:11 Flexor digitorum profundus is running from the proximal surfaces of the medial and anterior surfaces of ulna, and the interosseous membrane.

01:22 So it's coming from the ulna.

01:25 It then passes via its four long tendons to the distal phalanges, the medial four distal phalanges of digits 2, 3, 4, and 5.

01:38 The nerve supply, it has a jewel nerve supply.

01:42 The lateral muscle bellies, these the ones that are going to digits two, and three are supplied by the anterior interosseous nerve.

01:50 And the tendons that are going to digits four and five are supplied by the ulna nerve.

01:57 So flexor digitorum profundus has two nerves supplying it, the median nerve and the ulna nerve.

02:06 Its function is to flex the hand at the wrist joints, and it also flexes the distal interphalangeal joint.

02:13 So where flexor digitorum superficialis flex the proximal interphalangeal joint.

02:19 This knife flexes the very distal interphalangeal joint.

02:23 The joint between the distal phalanx and the middle phalanx.

02:28 If we go back and look at flexor pollicis longus, we can see this is really coming from the radius.

02:34 And this gives a long tendon that goes and attaches to the distal phalanx of the thumb.

02:41 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.

02:51 First digit be in your thumb.

02:54 The pronator quadratus, as we saw here, is running between the two distal ends of the radius and the ulna.

03:01 It runs from the distal quarter of the ulna.

03:04 So the last quarter of the ulna and it actually passes towards the radius.

03:10 So when pronator quadratus contracts, it is actually going to move the radius.

03:15 It is going to move the radius into a more pronated position.

03:18 So from this supine position, contraction of pronator quadratus is going to protonate the radius, pronate the forearm.

03:28 It pulls the radius over the ulna bone.

03:31 Both flexor pollicis longus and pronator quadratus are innervated by the anterior interosseous nerve.

03:40 And this is a branch of the median nerve.

03:42 So the median nerve is supplying the majority of these muscles within the anterior compartment.

03:48 But a specific branch the anterior interosseous nerve is supplying the flexor pollicis longus and pronator quadratus.

03:58 Flexor pollicis longus does similar functions to flexor digitorum muscles except it acts at the first digit.

04:06 Continued contraction will flex the wrist, it flexes the interphalangeal, and metacarpophalangeal joints associated with the first digit.

04:17 Pronator quadratus pronates the forearm.

04:20 So it acts on the radioulnar joints.