Ulnar Nerve: What is it? Structure, Function, Clinical Significance and Ulnar Neuropathy

In human anatomy, it is a nerve that runs close to the ulna bone.

The ulnar collateral ligament of the elbow joint is in relation to the ulnar nerve.

It is the largest unprotected nerve in the human body (meaning it is not protected by muscle or bone), so injuries are common.

This nerve is directly connected to the little finger and the adjacent half of the ring finger, innervating the palmar aspect of these fingers, including the front and back of the tips, perhaps as far back as the nail beds.

This nerve can cause an electric shock-like sensation by striking the medial epicondyle of the humerus from the back or bottom with the elbow flexed. The ulnar nerve is trapped between the bone and the overlying skin at this point.

This is commonly known as bumping into the ‘ funny bone ‘. This name is believed to be a play on words, based on the sound similarity between the name of the upper arm bone, the “humerus,” and the word “humorous.”

Alternatively, according to the Dictionary of the Spanish Language, it may refer to “the peculiar sensation experienced when struck.”

Structure

Arm

The ulnar nerve originates from the C8-T1 nerve roots (and occasionally carries C7 fibers) that are part of the medial cord of the brachial plexus, and descends medial to the brachial artery, to the insertion point of the coracobrachialis muscle (center 5 cm above the medial border of the humerus).

Then, it pierces the medial intermuscular septum and enters the posterior compartment of the arm, accompanied by superior ulnar collateral vessels.

It runs on the posteromedial aspects of the humerus, passing behind the medial epicondyle (in the cubital tunnel) at the elbow, where it can be palpated by hand.

Forearm

The ulnar nerve is not a content of the ulnar fossa. It enters the anterior compartment (flexor) of the forearm, between the two heads of the flexor carpi ulnaris, and runs along the lateral border of the flexor carpi ulnaris.

The ulnar nerve runs between the superficial flexor of the fingers (laterally) and the deep flexor of the fingers inward. Near the wrist, it runs superficially to the flexor retinaculum of the hand, but covered by volar palpal ligament to enter the hand.

In the forearm it emits the following branches:

Muscular branches of the ulnar nerve : it provides a muscle and a half (flexor carpi ulnaris and the medial half of the deep flexor of the fingers).

Palmar branch of the ulnar nerve : arises from the middle part of the forearm and supplies the skin over the hypothenar eminence.

Dorsal branch of the ulnar nerve : Arises 7.5 cm above the wrist, travels backward to irrigate the skin of the proximal ulnar and middle finger and the adjacent area between the fingers.

The articular branches are detached at the elbow joint.

Mano

The ulnar nerve enters the palm of the hand through Guyon’s canal, is superficial to the flexor retinaculum and lateral to the pisiform bone. Here it issues the following branches:

Superficial branch of the ulnar nerve : supplies the short palmar muscle and gives digital branches to the middle and middle fingers.

Deep branch of the ulnar nerve : accompanies the deep branch of the ulnar artery. It passes backward between the abductor, digiti minimi (extensor digiti minimi muscle), digiti minimi flexor, and opposing digiti minimi, supplying all three muscles and resting on the hook of the hamate bone.

It then rotates laterally, supplying the third and fourth lumbrical muscles and all the palmar and dorsal interosseous muscles of the hand.

It ends by supplying the adductor pollicis (adductor pollicis). Articular branches in the wrist.

Function

The ulnar nerve is also known as the “musician’s nerve” as it controls fine movements of the fingers.

Sensorial

The ulnar nerve also provides sensory innervation to the fifth digit and the medial half of the fourth digit, and the corresponding part of the palm:

Palmar branch of the ulnar nerve : supplies cutaneous innervation to the anterior skin and nails

Dorsal cutaneous branch of the ulnar nerve : provides cutaneous innervation to the dorsal medial hand and the back of the medial fingers.

Motor

The ulnar nerve and its branches innervate the following muscles in the forearm and hand:

An articular branch that passes to the elbow joint while the ulnar nerve is passing between the olecranon and the medial epicondyle of the humerus.

In the forearm, through the muscular branches of the ulnar nerve:

Flexor carpi ulnaris : the flexor carpi ulnaris muscle, is a muscle of the human forearm that acts by flexing and adducting (medial deviation) the hand.

Deep flexor digitorum (medial half) : The deep common flexor muscle of the fingers of the hand is a muscle in the forearm of humans that flexes the fingers (also known as digits).

It is considered an extrinsic muscle of the hand because it acts on the hand while its muscle belly is in the forearm.

In the hand, through the deep branch of the ulnar nerve:

Hypothenary muscles : The hypothenary muscles are a group of three muscles in the palm that control the movement of the little finger.

Opposite digiti minimi : the opponent of the pinky (opponent of the pinky quinti in older texts) is a muscle in the hand.

It is triangular in shape, and is placed immediately below the palmaris brevis / short palmar muscle, digital abductor minimi, and flexor digiti minimi brevis / short flexor muscle of the fifth finger. It is one of the three hypothenary muscles that control the little finger.

Adductor muscle pinky : in human anatomy, abductor digiti minimi the / adductor muscle of the little finger, is a skeletal muscle located at the ulnar edge of the palm.

It forms the ulnar border of the palm, and its fusiform shape defines the hypothaar eminence of the palm along with the surrounding skin, connective tissue, and fat. Its main function is to move the little finger away from the other fingers (i.e. abduction).

Flexor digiti minimi brevis : The flexor digiti minimi brevis / flexor brevis muscle of the fifth finger, is a hypothenar muscle in the hand that flexes the little finger (digit V) at the metacarpophalangeal joint.

It is located lateral to the abductor abductor digiti minimi muscle when the hand is in anatomical position.

The third and fourth lumbric muscles : The lumbrical muscles are intrinsic muscles of the hand that flex the metacarpophalangeal joints and extend the interphalangeal joints.

There are also lumbrical muscles of the foot that have a similar action, although these are of less clinical concern.

Dorsal interossei : These are four muscles in the back of the hand that act to separate the index, middle, and ring fingers from the midline of the hand (middle finger ray).

It helps flexion in the metacarpophalangeal joints and extension in the interphalangeal joints of the index, middle and ring fingers.

Palmar interossei : In human anatomy, the volar or volar interossei (volar interossei in older literature) are three small, unipended muscles in the hand that lie between the metacarpal bones and are attached to the index, ring, and fingers. They are smaller than the dorsal interossei of the hand.

Adductor Pollicis : In human anatomy, the adductor pollicis muscle is a muscle in the hand that functions to adduct the thumb. It has two heads: transverse and oblique.

It is a fleshy, flat, triangular, fan-shaped muscle deep in the thenar compartment below the long flexor tendons and the lumbrical muscles in the center of the palm. It overlaps the metacarpal bones and the interosseous muscles.

Flexor pollicis brevis (deep head) : The flexor pollicis brevis is a muscle in the hand that flexes the thumb. It is one of the three thenar muscles. It has a superficial part and a deep part.

In the hand, through the superficial branch of the ulnar nerve : The superficial branch of the ulnar nerve is a terminal branch of the ulnar nerve. It supplies the palmar brevis muscle and skin on the ulnar side of the hand, and divides into a common palmar digital nerve and a suitable palmar digital nerve.

The appropriate digital branches are distributed to the fingers in the same way as those of the median nerve.

Short palmar muscle : it is a thin, quadrilateral muscle that is placed under the integument on the ulnar side of the hand. It acts to transversely fold the skin of the hypothenar eminence.

Clinical significance

The ulnar nerve can be injured anywhere from its origin proximal to the brachial plexus to the distal branches of the hand. It is the most commonly damaged nerve around the elbow.

Although it can be damaged under various circumstances, it is commonly damaged by local trauma or physical shock (“pinched nerve”). Ulnar nerve injury at different levels causes a specific motor and sensory deficit:

On the elbow

Common mechanisms of injury : cubital tunnel syndrome, fracture of the medial epicondyle of the humerus (causing direct injury to the ulnar nerve), fracture of the lateral epicondyle of the humerus (causing the ulna valgus with late ulnar nerve palsy).

Motor deficit : weakness in flexion of the hand at the wrist, loss of flexion of the ulnar half of the digits, or the 4th and 5th digits, loss of the ability to cross the fingers of the hand.

Note : motor deficit is absent or very mild in cubital tunnel syndrome as the ulnar nerve is compressed in the cubital tunnel, rather than transected.

Presence of a deformity in the claw hand when the hand is at rest, due to hyperextension of the 4th and 5th digits in the metacarpophalangeal joints, and flexion in the interphalangeal joints.

Thumb adduction weakness, which can be assessed by the presence of Froment’s sign.

Sensory deficit : loss of sensation or paresthesia in the ulnar half of the palm and the back of the hand, and the middle 1½ fingers on both palmar and dorsal sides of the hand.

On the wrist

Common mechanism : penetrating wounds, Guyon duct cyst.

Motor deficits : loss of flexion of the ulnar half of the digits, or the 4th and 5th digits, loss of the ability to cross the digits of the hand.

Presence of a deformity in the claw hand when the hand is at rest, due to hyperextension of the 4th and 5th digits in the metacarpophalangeal joints, and flexion in the interphalangeal joints.

Claw hand deformity is more prominent with a wrist injury as opposed to an injury higher up the arm, for example at the elbow, as the ulnar half of the deep flexor digits is not affected .

This brings the distal 4th and 5th digit interphalangeal joints into a more flexed position, producing a more deformed “claw.” This is known as the ulnar paradox.

Thumb adduction weakness, which can be assessed by the presence of Froment’s sign.

Sensory deficit : loss of sensation or paresthesias in the ulnar half of the palm, and the medial 1½ digits on the palmar aspect of the hand, with dorsal preservation.

The dorsal aspect of the hand is not affected as the posterior cutaneous branch of the ulnar nerve is released higher up in the forearm and does not reach the wrist.

In severe cases, surgery may be done to relocate or “release” the nerve to prevent further injury.

Ulnar neuropathy

The ulnar nerve is an extension of the medial cord of the brachial plexus.

It is a mixed nerve that supplies innervation to the muscles of the forearm and hand and provides sensation over the medial half of the fourth digit and the fifth complete digit (the ulnar aspect of the palm) and the ulnar portion of the posterior aspect of the hand ( dorsal ulnar cutaneous distribution).

Ulnar nerve entrapment is the second most common entrapment neuropathy in the upper limb (after median nerve entrapment).

The most common site of ulnar nerve entrapment is in or near the elbow region, especially in the region of the cubital tunnel or in the epicondylar (ulnar) groove; the second most likely site is on or near the wrist, especially in the area of ​​the anatomical structure called Guyon’s canal.

However, entrapment can also occur in the forearm between these two regions, below the wrist within the hand, or above the elbow.

Pressure or injury to the ulnar nerve can cause denervation and paralysis of the muscles that supply the nerve. Affected patients often experience numbness and tingling along the little finger and the ulnar half of the ring finger.

This discomfort is often accompanied by weak grip and, rarely, intrinsic loss. One of the most serious consequences is the loss of intrinsic muscle function in the hand.

When the ulnar nerve splits at the wrist, only the opponens pollicis, the superficial head of the short flexor of the thumb, and the 2 lateral lumbricals function.

Nonsurgical conservative treatment can play a useful role in treatment. If such treatment fails or if the patient develops progressive weakness or weakness or loss of function, surgical treatment is warranted.

Various surgical approaches have been employed, each of which has its advocates; the results for all of them appear to be satisfactory.

A careful medical history is essential. Both the onset and the progress of symptoms can be variable.

The presentation of symptoms of ulnar nerve entrapment can range from mild transient paresthesias in the ring and little fingers to puncture of these fingers and severe intrinsic muscle atrophy.

It is important to determine when symptoms began, how long they last, whether they are transient or continuous, and whether they are related to work, sleep, or recreation.

Also, although the answer will often be negative, one should ask specifically about trauma and pressure to the arm and wrist, especially the elbow, the medial side of the wrist, and other sites near the course of the ulnar nerve.

Many patients complain of sensory changes in the fourth and fifth digits.

On rare occasions, a patient notices that the unusual sensations are mainly on the medial side of the ring finger (fourth digit) rather than the lateral side, which corresponds to the textbook sensory distribution.

Sometimes the third digit is also involved, especially on the ulnar (i.e. medial) side. Sensory changes can include numbness, tingling, or burning. If the patient rests on his elbows at work, he may notice numbness and paresthesia throughout the day.

Pain rarely occurs in the hand. Pain complaints tend to be most common in the arm, including the elbow area. In fact, the elbow is probably the most common site of pain in ulnar neuropathy.

Occasionally, patients specifically say “I have pain in my elbow,” “I have pain in my funny bone,” or even “I have pain in this little groove in my elbow,” but they are usually not so explicit unless instructed to do so.

Sometimes severe pain in the elbow or wrist can radiate to the hand or to the shoulder and neck. Patients rarely notice specific muscle atrophy, but when they do, they often complain that their hands “look older.”

Weakness can also be a filing complaint. For example, patients may report difficulty opening jars or turning doorknobs, or they may experience early fatigue or weakness with jobs that require repetitive hand movements.

The complaint of weakness can also be expressed in more subtle ways. For example, a traditional sign of ulnar neuropathy, the Wartenberg sign, is actually a complaint of weakness.

In this scenario, the patient complains that the little finger is caught on the edge of the trouser pocket when trying to place the hand in the pocket.

At first, this complaint may surprise, because most doctors, remembering that the abduction of a finger is governed by the ulnar nerve

They are probably willing to assume that a patient who has ulnar neuropathy would be less, rather than more, likely to have the little finger abducted and thus caught at the edge of the pocket.

However, adduction is also mediated by the ulnar nerve. In essence, the patient cannot firmly abduct the fifth digit against the fourth due to weak interosseous muscles.

In addition, the muscle that extends the fifth digit at the metacarpal joint of the phalanx (the extensor digiti quinti) is radially innervated and inserts on the ulnar side of the joint.

Normally, this muscle is opposed by the muscles innervated by the ulnar that flex the joints.

In the setting of an ulnar neuropathy, however, the muscle is relatively unopposed and therefore raises the finger up and to the ulnar side. This is the perfect position to reach the edge of the pocket.

The patient may also express the complaint of weakness by saying, “My grip is weak.” Many of the grip muscles are ulnar.

Also, when someone tries to grip hard, the hand usually deviates in the ulnar direction under the influence of the flexor carpi ulnaris.

If this ulnar deviation is affected, the grip mechanism does not work optimally, even for the muscles that are not affected.

Sometimes a patient notices that the grip of the forceps with the index finger of the thumb is weak. Two of the key muscles involved in this movement are the adductor pollicis (thumb adduction) and the first dorsal interosseous muscle (index finger adduction).

In addition to weak clamp impingement, the partially innervated flexor pollicis longus partially compensates for the weakened adductor pollicis and the thumb flexes at the distal joint.

This flexion is generally unnoticed by the patient, but when demonstrated by the examiner, it constitutes Froment’s sign.