Illustrated Encyclopedia of Human Anatomic Variation: Opus III: Nervous System: Plexuses

Ulnar Nerve

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed

There are a number of variations in the formation, course, and distribution of this nerve. It may have a root from the seventh cervical nerve by way of the lateral cord.

It may be derived from the eighth cervical nerve only or from both the seventh and eighth cervical nerves.

The ulnar may pass in front of the medial epicondyle of the humerus or lie behind the condyle.

Connecting branches with the median nerve have been reported in the arm, forearm and hand. Other connecting branches with the medial brachial cutaneous and radial nerve have also been described.

Muscular branches to the medial head of the triceps, anconeus, flexor digitorum superficialis, first and second lumbricals, and the superficial head of flexor pollicis brevis have been reported. When an epitrochleoanconeus muscle is present, it receives a branch from the ulnar nerve.

The branch to the dorsum of the hand may be deficient; this is compensated for by the radial, the dorsal lateral antebrachial cutaneous branch of the radial, or the medial brachial cutaneous nerves. The ulnar may supply the all four fingers when the radial nerve is deficient. The dorsal terminal branches may encroach on the radial side of the hand, often reaching the radial side of the second metacarpal bone and even the dorsum of the first phalanx of the thumb.

The all-ulnar motor hand is without forearm anastomoses. Digital sensory fibers were normally distributed in median and ulnar nerves. The unusual motor innervation probably occurs from palmar communication between ulnar and median branches (Riche-Cannieu anastomoses). Riche-Cannieu anastomoses occur in cases where motor neurons normally destined for the median nerve continue from the medial cord into the ulnar nerve. Harness and Sekeles found Riche-Cannieu anastomoses in 27 out of 35 (77%) dissected hands. They also reported that Paturet said the anastomosis of Cannieu and Riche is of the simple type, i.e. the anastomotic fibers pass transversely or ansiformly to become enclosed in a common sheath. In addition, they indicated that Hovelacque reported the following variations: A) Chevrier said that it can be a separate branch of the median nerve to the superficial head of the flexor pollicis brevis which sends a ramus to the anastomosis. In this case, the anastomosis can be located either on the surface of, or deep in, the flexor brevis. B) The anastomosis can be with one of the two digital branches of the thumb coming from the median nerve. In this case, the anastomosis is located medial to the tendon of the flexor pollicis longus and sometimes double or triple anastomoses can be found. C) Riche (quoted by Hovelacque) provides two additional possibilities. Anastomosis may occur between one of the branches of the digital nerve of the thumb and branch to the adductor pollicis, coming from the deep branch of the ulnar nerve. The anastomosis is also medial to the tendon of flexor pollicis longus and is located deep in the adductor pollicis, in which case there is no ulnar innervation to the deep head of flexor pollicis brevis. D) The anastomosis passes through the first lumbrical, which then receives its innervation from the deep branch of the ulnar nerve. Opponens pollicis is supplied by the median and ulnar nerves in about 30% of cases.

The superficial head of the short flexor of the thumb is supplied by the median and ulnar in about 75% of cases and by the ulnar alone in 25%.

Much variation in innervation is due to the interchange of axons between the ulnar and median nerves, either in the forearm (most commonly) or in the hand.

Communicating branches between the ulnar and median nerves of digital sensory nerves are variable in occurrence as well as in length. Some have been found which arise above the wrist others in mid palm.

In one study, a double motor innervation (median and ulnar) was found bilaterally in 54 out of the 70 examinees, while the remaining 16 had only median innervation to opponens pollicis of both hands.

The ulnar nerve has been found split by an anomalous tendon of insertion of flexor carpi ulnaris. The nerve recombined also beyond the tendon and was subject to compression, proximal to the pisiform bone.

Riche-Cannieu anastomoses (between the median and ulnar nerves in the palm of the hand) may exist in 70% of individuals. Martin-Gruber (median-ulnar nerves) anastomoses in the forearm have a reported frequency of 15%. It should be remembered that the "anastomoses" occur between the median, ulnar, radial, and musculocutaneous nerves, as well as between other nerves. These are errors in the pathway (course) of some, inappropriately placed nerve fibers. In order for these nerve fibers to get to their proper end-point, the bundle of nerves fibers leave the inappropriate trunk and join the proper nerve trunk. Both Martin-Gruber and Riche-Cannieu "anastomoses" are examples of nature's correction of nerve fibers located in inappropriate nerve pathways. Certainly a mystery, these "anastomoses". On a broader and larger scale, the brachial plexus and sacral plexuses are other examples of neuronal sorting.

Oyedun, Onatola, Kanu, et al. (2014) reported a rare bifurcation of the ulnar nerve with dorsal and palmar cutaneous branches having high origins above the medial epicondyle of the ulna forming posteromedial and anterolateral branches. Hence traditional ulnar nerve blocks would fail in such cases.

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