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Human Anatomy Lesson 23

Updated on September 22, 2016

Forearm, Wrist, Hand

In this lesson, we will cover the bones and joints of the forearm, wrist, and hand, and the muscles, nerves, arteries, and veins in the forearm and wrist. You may have noticed to this point through 23 lessons that each region of anatomy has its own particular "something" that is the "story" of that region - for example, when we discussed the abdomen and pelvis, we focused a lot of attention on arterial supply, but when we covered the head we focused more time on cranial nerves and their branches than anything else. The "story" of the limbs (both upper and lower) is muscles. You got a preview of that in the last lesson, but this lesson is really the first where there are lots of muscles. On the other hand, the distribution of nerves, arteries, and veins is very straightforward, and can be understood since they are arranged into two compartments. Even though there are a large number of muscles, they are logically arranged and, along with the nerves, arteries, and veins, are largely confined to either an anterior or posterior compartment. The median nerve innervates most of the muscles in the anterior compartment (with the ulnar nerve picking up the remainining 1 1/2 muscles on the medial side of the forearm/hand), whereas the radial nerve or one of its branches provides motor innervation to the muscles in the posterior compartment. Another thing that makes memorization of forearm muscles easier is how logically they are named - flexor digiti minimi, for example, is the flexor of the pinkie, whereas abductor pollicis longus is the long AB-ductor of the thumb (pollex is thumb in Latin). Third, all of the flexors are in the anterior compartment, and they have a common origin on the medial epicondyle of the humerus. The extensors are all in the posterior compartment and they have a common origin around the lateral epicondyle of the humerus. So, as you can see, even though there are lots of muscles to memorize there are some helpful methods of organization.

Learning Objectives - By the end of this lesson, you should be able to ...

  1. List the bones and joints of the forearm, wrist, and hand, and understand how they articulate with each other
  2. Understand how nerves, arteries, and veins pass into the forearm from the arm through the cubital fossa, and through the wrist into the hand through the carpal tunnel
  3. List and describe the origin, insertion, innervation, and function of forearm muscles in the anterior and posterior compartments of the forearm, and understand how these muscles are organized into several layers
  4. List and describe the course of nerves, arteries, and veins in the anterior and posterior compartments

Bones and Joints

There are two bones in the forearm: the radius, which is on the lateral side and connects the humerus to the bones of the wrist and hand, and the ulna, which is on the medial side and participates more in the elbow joint than it does at the wrist joint. Nerves, arteries, and veins from the arm pass into the forearm through the cubital fossa, except for the ulnar nerve, which passes posterior to the medial epicondyle of the humerus. Structures pass into the hand through the wrist, and pass either through the carpal tunnel or just anterior to it - except for the radial artery, which passes posterior to the wrist.

  1. Radius: in the last lesson, we covered the anatomy of the proximal end of the radius. Here we will consider relevant anatomy on the shaft and distal part of the radius. The oblique line extends distally from the radial tuberosity, and forms the origin site of one of the muscles that we will cover in this lesson, flexor digitorum superficialis. The origin site of a second muscle, pronator teres, can be seen on the lateral side of the radius about halfway down its length. The anterior border of the radius is relatively flat, and the posterior border is somewhat rounded, with a dorsal tubercle at the distal end, which forms a pulley for the extensor pollicis longus muscle. The styloid process projects distally from the lateral side of the radius, and the distal radius articulates with two bones of the wrist, the scaphoid and lunate (see below). The sharp medial border of the radius is called the interosseous crest, and it forms the attachment site of the interosseous membrane which attaches the radius to the ulna.
  2. Ulna: as noted above, the ulna is more substantial proximally than it is distally, but it does participate, however minimally, in the wrist joint. Just like the radius, the ulna has an interosseous border and styloid process.
  3. Distal radio-ulnar joint: the radius and ulna form a joint that is bound with fibrocartilage and which, like the other joints we've discussed in this course, has an articular disc and synovial membrane.
  4. Interosseous membrane: this thin fibrous sheet tightly ties the radius to the ulna. There is a space proximally, just inferior to the radial tuberosity, that forms an opening where nerves, arteries and veins pass between the anterior and posterior compartments on either side of the interosseous membrane, and an aperture distally just superior to the wrist for the anterior interosseous artery to pass between compartments. Otherwise, the interosseous membrane separates the anterior and posterior compartments of the forearm.
  5. Carpal bones: There are two rows of carpals in the wrist - a proximal row which includes the pisiform, triquetrum, lunate, and scaphoid, which articulate with the radius and ulna; and a distal row which includes the hamate, capitate, trapezoid, and trapezium, and articulates with the metacarpals in the palm. There are two mnemonics that you can use to remember the names of the hand carpals. The classic mnemonic is Scared Lovers Try Positions That They Can't Handle, which lists the carpals from proximal to distal and lateral to medial. I have to admit that that mnemonic is very catchy, but, myself, I have a problem remembering the carpals listed lateral to medial, and so I've come up with a mnemonic that lists them from proximal to distal and medial to lateral: Please Try & Learn Some Hand Carpals This Time! Feel free to use either one.
  6. Metacarpals and phalanges: as noted above, the distal row of carpals articulate with five metacarpals. The five metacarpals articulate with five phalanges. The digits are numbered 1 to 5 - digit 1 is the thumb, 2 is the index finger, 3 is the middle finger, 4 is the ring finger, and 5 is the pinkie. The thumb has two phalanges (a proximal and a distal) and digits 2 through 5 have three phalanges each (proximal, middle, distal). The joint between the carpals and each metacarpal is called a carpometacarpal joint. The joint between each metacarpal and phalanx (the singular of "phalanges") is called a metacarpophalangeal joint. The joint between phalanges is called an interphalangeal joint, and each of the fingers has a proximal interphalangeal joint between the proximal and middle phalanges and a distal interphalangeal joint between the middle and distal phalanges.

Anterior Compartment Muscles

The muscles in the anterior compartment of the forearm all originate on the medial epicondyle of the humerus or adjacent surface of the proximal ulna around the coronoid process and run distally through the carpal tunnel, or just anterior to it. These muscles act on the wrist joint, flex the thumb and fingers, and pronate the forearm. The anterior compartment is traditionally split into superficial, intermediate, and deep layers. Flexor carpi ulnaris, palmaris longus, flexor carpi radialis, and pronator teres belong to the superficial group; flexor digitorum superficialis belongs to the intermediate group; and flexor digitorum profundus, flexor pollicis longus, and pronator quadratus belong to the deep group. Flexor carpi ulnaris and 1/2 of flexor digitorum profundus are innervated by the ulnar nerve, and all the others are innervated by the median nerve.

  1. Flexor carpi ulnaris: runs from the medial epicondyle of the humerus, olecranon process, and posterior border of the ulna to the pisiform and via ligaments onto the hamate and base of the 5th metacarpal. Innervated by the ulnar nerve. Flexes and AD-ducts the wrist joint. It should be noted that the ulnar nerve passes deep to the medial epicondyle and the humeral head of flexor carpi ulnaris.
  2. Palmaris longus: runs from the medial epicondyle of the humerus to the palmar aponeurosis. Innervated by the median nerve. Flexes the wrist joint, but primarily opposes shearing forces on the skin of the palm during gripping, since it attaches to the skin through the palmar aponeurosis. This muscle is absent in about 15% of the population.
  3. Flexor carpi radialis: runs from the medial epicondyle of the humerus to the base of the 2nd and 3rd metacarpals. Innervated by the median nerve. Flexes and AB-ducts the wrist. The tendon for this muscle is the most distinct one on the lateral side of the distal 1/2 of the forearm and can be easily palpated by making a fist and palpating the forearm. The radial artery lies immediately lateral to this tendon, so the radial pulse can be located in this way.
  4. Pronator teres: runs from the medial epicondyle and supraepicondylar ridge of the humerus and a small part of the coronoid process of the ulna to the lateral surface of the radius midway along its length. Innervated by the median nerve. Rotates the radius over the ulna during pronation. The four muscles listed above - flexor carpi ulnaris, palmaris longus, flexor carpi radialis, and pronator teres, in that order - run from medial to lateral across the proximal part of the forearm, and form the superficial layer of forearm muscles in the anterior compartment.
  5. Flexor digitorum superficialis: runs from a broad insertion on the medial epicondyle, coronoid process, and oblique line of the radius to terminate in four tendons which attach to the middle phalanges of digits 2-5. Innervated by the median nerve. Flexes the proximal metacarpophalangeal and interphalangeal joints of the four fingers, as well as the wrist joint. FDS forms the intermediate layer of the anterior compartment of the forearm. The median nerve and ulnar artery pass deep to FDS between the parts of it that originate on the humerus/ulna and radius.
  6. Flexor digitorum profundus: runs from a broad origin on the anterior and medial surfaces of the ulna and interosseous membrane to insert onto four tendons that attach to the palmar surfaces of the distal phalanges of digits 2-5. Up to the middle phalanx of each finger, the tendon of FDS lies superficial to the tendon of FDP, and it splits so that the FDP tendon passes through to its insertion on the distal phalanx. The lateral half of FDP is innervated by the median nerve, but the medial half is innervated by the ulnar nerve. Flexes the metacarpophalangeal and the proximal and distal interphalangeal joints of digits 2-5, in addition to the wrist joint.
  7. Flexor pollicis longus: runs from the anterior surface of the radius and interosseous membrane to palmar surface of the base of the distal phalanx of the first digit (the thumb). Innervated by the anterior interosseous nerve, a branch of the median nerve. Flexes the metacarpophalangeal and interphalangeal joints of the thumb.
  8. Pronator quadratus: unlike the other forearm muscles, pronator quadratus does not run in a primarily proximal-to-distal direction, but instead runs transversely, from the distal-most end of the anterior ulna across to the anterior surface of the radius. Innervated by the anterior interosseous nerve, a branch of the median nerve. Pulls the anterior end of the radius over the ulna during pronation, as advertised in its name. Flexor digitorum profundus, flexor pollicis longus, and pronator quadratus all make up the deep layer of muscles in the anterior compartment of the forearm.

Anterior Compartment Arteries

Remember that the brachial artery enters the forearm through the cubital fossa. It quickly divides into the two main arteries of the forearm, the radial artery and ulnar artery.

  1. Radial artery: begins at about the neck of the radius, and continues on an inferior and lateral course through the forearm. In the proximal 1/2 of the forearm, the radial artery lies just deep to the brachioradialis muscle, which is the most superficial muscle in the lateral side of the posterior compartment of the forearm (see below). In the distal part of the forearm, the radial artery is just medial to the tendon of brachioradialis and lateral to the tendon of flexor carpi radialis, where it is covered only by deep fascia, superficial fascia, and skin. In the wrist, the radial artery gives off a superficial palmar branch which runs anteriorly into the palm, then passes laterally and posteriorly to loop around the first metacarpal and form the deep palmar arch.
  2. Ulnar artery: leaves the cubital fossa by passing deep to pronator teres, and passes through the forearm in the fascial plane between flexor carpi ulnaris and flexor digitorum profundus. The ulnar artery is immediately lateral to the ulnar nerve in the distal forearm, but it is not easily palpable, since it is tucked under the flexor carpi ulnaris tendon. The ulnar artery gives off several important branches: the common interosseous artery, which splits into the anterior interosseous artery and posterior interosseous artery just superior to the proximal attachment of the interosseous membrane; in the wrist, a branch which curves around the FDP tendon for the 5th digit to form the medial half of the deep palmar arch and anastomoses with the contribution from the radial artery; and the superficial palmar arch, which is the distal termination of the ulnar artery and anastomoses with the superficial palmar branch of the radial artery. Although the anastomoses in the palm are complex, it is generally considered that the radial artery terminates in the deep palmar arch and the ulnar artery terminates in the superficial palmar arch.

Nerves in the Anterior Compartment

  1. Median nerve: innervates all the muscles in the anterior compartment of the forearm except for flexor carpi ulnaris and the medial part of flexor digitorum profundus. It passes between the two heads of pronator teres and the two heads of flexor digitorum superficialis, continuing distally in the fascia on the deep surface of FDS. Proximal to the wrist, the median nerve is fairly superficial, lying between the tendons of palmaris longus and flexor carpi radialis. It passes through the carpal tunnel. The largest, most important branch is the anterior interosseous nerve, which originates between the two heads of pronator teres, and innervates muscles in the deep layer (except for 1/2 of flexor digitorum profundus) before ending as articular branches to joints in the distal forearm and wrist. A small palmar branch runs superficial to the flexor retinaculum to supply skin in the palm.
  2. Ulnar nerve: in the forearm, this nerve innervates flexor carpi ulnaris and the medial 1/2 of flexor digitorum profundus. The ulnar nerve enters the forearm by passing posteriorly around the medial epicondyle of the humerus and between the two heads of flexor carpi ulnaris. It passes distally in the plane between flexor carpi ulnaris and FDP and enters the wrist superficial to the flexor retinaculum lateral to the pisiform bone. The ulnar nerve gives off a small palmar branch which provides sensory innervation to skin on the medial side of the palm and a larger dorsal branch which innervates skin on the posterior and medial side of the hand and the posterior surfaces of digit 5 and the medial half of digit 4.
  3. Radial nerve: splits into deep and superficial branches under the brachioradialis muscle. The deep branch supplies muscles in the posterior compartment, forming the posterior interosseous nerve between the two heads of supinator in the posterior compartment of the forearm. The superficial branch provides sensory innervation to skin on the anterolateral aspect of the arm and posterolateral surface of the hand.

Posterior Compartment Muscles

There are two layers in the posterior compartment: a superficial layer, which consists of brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and anconeus; and a deep layer, which consists of supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicus. All of these muscles are innervated either by the radial nerve or one of its branches, the posterior interosseous nerve.

  1. Brachioradialis: runs from the proximal part of the lateral supraepicondylar ridge of the humerus and adjacent intermuscular septum to the lateral surface of the distal end of the radius. Innervated by the radial nerve. Flexes the elbow when the forearm is in mid-pronation.
  2. Extensor carpi radialis longus: runs from the distal part of the lateral supraepicondylar ridge of humerus and adjacent intermuscular septum to the posterior surface of the base of the 2nd metacarpal. Innervated by the radial nerve. Extends and AB-ducts the wrist.
  3. Extensor carpi radialis brevis: runs from the lateral epicondyle of the humerus and adjacent intermuscular septum to the posterior surface of the base of the 2nd and 3rd metacarpals. Innervated by the radial nerve. Like extensor carpi radialis longus, extends and AB-ducts the wrist.
  4. Extensor digitorum: runs from the lateral epicondyle of humerus and adjacent intermuscular septum to the posterior aspects of the bases of the middle and distal phalanges of digits 2-5. Innervated by the posterior interosseous nerve. Extends the four fingers, and weakly extends the wrist.
  5. Extensor digiti minimi: runs from the lateral epicondyle of humerus and adjacent intermuscular septum (along with extensor digitorum) to the extensor hood of digit 5. Innervated by the posterior interosseous nerve. Extends digit 5.
  6. Extensor carpi ulnaris: runs from the lateral epicondyle of humerus and posterior border of ulna to the base of the medial side of the 5th metacarpal. Innervated by the posterior interosseous nerve. Extends and AD-ducts the wrist.
  7. Anconeus: runs from lateral epicondyle of humerus to olecranon process and proximal surface of ulna. Innervated by the radial nerve. AB-ducts the ulna during pronation and weakly extends the elbow joint. The extensor muscles listed above are all in the superficial layers of the posterior compartment, and run from lateral to medial. Brachioradialis and extensor carpi radialis longus both originate from the lateral supraepicondylar ridge, and the rest originate from the lateral epicondyle.
  8. Supinator: runs from the lateral epicondyle of humerus, the radial collateral and anular ligaments, and the supinator crest of ulna, to the lateral surface of radius superior to the anterior oblique line. Innervated by posterior interosseous nerve. Along with biceps brachii, supinates the forearm.
  9. Abductor pollicis longus: runs from the posterior surfaces of ulna and radius and intervening interosseous membrane to the lateral side of the base of the first metacarpal. Innervated by the posterior interosseous nerve. AB-ducts carpometacarpal joint of the thumb, and weakly extends the thumb.
  10. Extensor pollicis brevis: runs from the posterior surface of radius and adjacent interosseous membrane to the posterior surface of the base of the proximal phalanx of the thumb. Innervated by the posterior interosseous nerve. Extends the metacarpophalangeal joint of the thumb, and can also extend the carpometacarpal joint.
  11. Extensor pollicis longus: runs from the posterior surface of the ulna and adjacent interosseous membrane to the posterior surface of the base of the distal phalanx of the thumb. Innervated by the posterior interosseous nerve. Extends the interphalangeal joint of the thumb, although it can also extend the carpometacarpal and metacarpophalangeal joints.
  12. Extensor indicis: runs from the posterior surface of the ulna and adjacent interosseous membrane to the extensor hood of digit 2. Innervated by the posterior interosseous nerve. Extends digit 2.

    Supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis are all part of the deep layer of the posterior compartment, and, as such, they are innervated by the posterior interosseous nerve, a branch of the radial nerve. Supinator originates, at least partially, on the lateral epicondyle, whereas the other muscles in the deep part of the posterior compartment originate on the posterior surfaces of the radius, ulna, and interosseous membrane. They originate more distally as you move down the list (i.e., move from laterally-positioned muscles to those positioned more medially).

Posterior Compartment Arteries and Veins

The blood supply to the posterior compartment is via branches of the radial, posterior interosseous, and anterior interosseous arteries.

  1. Radial artery: muscular branches supply blood to extensor muscles.
  2. Posterior interosseous artery: the common interosseous artery in the anterior compartment splits into an anterior interosseous artery and a posterior interosseous artery. The posterior interosseous artery passes posteriorly over the interosseous membrane into the posterior compartment of the forearm to supply the superficial layer of extensor muscles.
  3. Anterior interosseous artery: this artery joins the posterior interosseous artery by passing through an aperture in the interosseous membrane just superior to the wrist. In addition, numerous perforating arteries pass through the interosseous membrane to supply structures in the posterior compartment.

Upcoming Lessons

This lesson wraps up the upper limb - in lesson 24, we move on to the lower limb.

© 2015 Robert McCarthy

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