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The ulnar nerve is the largest nerve in the human body that is unprotected by muscle or bone. It runs near the ulna, one of the two long bones in the forearm. The ulnar nerve innervates several muscles in the forearm and hand, including the flexor carpi ulnaris, the medial half of the flexor digitorum profundus, and the abductor digiti minimi, flexor digiti minimi, and opponens digiti minimi. There is also some evidence that the ulnar nerve may innervate the triceps brachii muscle, although this is controversial.
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What You'll Learn
Ulnar nerve branches
The ulnar nerve is a mixed nerve that provides motor and sensory innervation to various muscles in the forearm and hand. It is the largest nerve in the human body that is unprotected by muscle or bone, making it susceptible to injury, especially at the elbow and wrist.
The ulnar nerve has two main branches: the superficial branch and the deep branch. The superficial branch supplies sensory innervation to the anterior aspect of the ulnar digits, including the little finger and the medial half of the ring finger, as well as the medial palmar skin. It also provides motor innervation to the palmaris brevis muscle in the hypothenar region of the hand. This branch arises from the ulnar nerve just distal to the pisiform bone and often divides into two palmar digital nerves.
The deep branch of the ulnar nerve emerges from the same region as the superficial branch and courses between the muscles of the hypothenar region. This branch provides motor innervation to the majority of hand muscles, including the hypothenar muscles (abductor, flexor, and opponens digiti minimi), the interossei, the medial two lumbricals, the deep head of flexor pollicis brevis, and the adductor pollicis.
In the forearm, the ulnar nerve gives rise to two muscular branches. The first branch supplies the flexor carpi ulnaris, which flexes and adducts the hand at the wrist. The second branch supplies the ulnar (medial) part of the flexor digitorum profundus, which is responsible for flexing the ring and little fingers at the distal interphalangeal joint. These two muscles are also innervated by the ulnar nerve as it descends into the forearm.
Additionally, the ulnar nerve gives rise to two cutaneous branches in the forearm: the palmar cutaneous nerve and the dorsal cutaneous nerve. These nerves pass into the hand and provide sensory innervation to the skin of the hand, including the palmar and dorsal aspects of the medial digits and the adjacent palm.
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Triceps brachii muscle
The triceps brachii is a large, thick muscle on the dorsal part of the upper arm. It often appears in the shape of a horseshoe on the posterior aspect of the arm. The triceps brachii is the only muscle in the posterior compartment of the arm. The primary function of the triceps is the extension of the forearm at the elbow joint. It is composed of three heads: a long head, a lateral head, and a medial head. The tendons all have different origins, but the three heads converge to form a single tendon distally. The long head originates from the infraglenoid tubercle of the scapula, while the lateral and medial heads originate from the humerus. The three heads converge into a single tendon, which attaches to the proximal portion of the olecranon process (the bony prominence of the elbow) located on the upper portion of the ulna. The long head also assists with the extension and adduction of the arm at the shoulder joint. The lateral head is also active during the forearm extension at the elbow joint when the forearm is supinated or pronated.
The triceps brachii begins to develop during the fifth week of embryogenesis. The muscle originates from the dorsal muscle mass of the upper limb bud and is derived from the paraxial mesodermal leaflet. The arterial supply to the triceps is provided by the deep brachial artery, which is a branch of the brachial artery and is supplied by ulnar collateral arteries. The venous drainage is the brachial vein that runs with the deep brachial artery. The provision of nerve supply to the triceps is by the radial nerve (root C6, C7, and C8). The C6 root value of the radial nerve innervates the lateral head, root value C7 innervates the long head, and root value C8 supplies the medial head. The triceps brachii muscle is mainly supplied by the deep brachial artery and the superior ulnar collateral artery, which arises from the brachial artery.
The ulnar nerve is closely situated to the triceps muscle and rarely examined above the elbow. The ulnar nerve contributes to the innervation of the triceps brachii muscle, constituting an important anatomical variation. The ulnar nerve and the ulnar collateral branch of the radial nerve are previously unrecognized sources of innervation of the medial head of the triceps brachii. The ulnar innervated part of the medial head of the triceps muscle may be used like an independent motor unit.
Axillary nerve damage can have an impact on the long head of the triceps brachii (LTB). Therefore, people with axillary nerve damage should undergo an assessment for the function of the LTB. If they demonstrate lost function, this shows a poor prognosis, and early repair at three months is recommended. The triceps muscle can undergo reinnervation through a distal nerve transfer. Commonly used nerves for reinnervation include the flexor carpi ulnaris fascicle of the ulnar nerve and the posterior branch of the axillary nerve.
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Innervation of the medial head
The ulnar nerve is closely situated to the triceps muscle and rarely examined above the elbow. A cadaveric study was conducted to explore the contribution of the ulnar nerve to motor innervation of the medial head of the triceps. The ulnar nerve and the ulnar collateral branch of the radial nerve are previously unrecognized sources of innervation of the medial head.
The ulnar nerve contributes to the innervation of the triceps brachii muscle, which constitutes an important anatomical variation. The triceps brachii muscle has three heads: the lateral, medial, and long heads. The medial head is located deep to the lateral and long heads, and all three heads converge to form a common tendon that inserts into the olecranon and the deep antebrachial fascia.
The medial head of the triceps brachii muscle originates from the posterior surface of the humerus, specifically along the posterior humerus between the insertion of the teres major muscle and the olecranon fossa. It also has fibres originating from the intermuscular septum. The medial head is active in the presence or absence of resistance, and it enables more precise, low-force movements.
While the radial nerve is typically associated with innervating all three heads of the triceps brachii, the ulnar nerve has been found to contribute to the innervation of the medial head. This knowledge is important in elbow surgery, particularly when accessing the posterior side of the joint, and can be used for motor reconstructions.
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Ulnar nerve injury
The ulnar nerve is a major peripheral nerve of the upper limb that provides motor and sensory innervation to various muscles of the forearm and hand. Ulnar nerve injuries can occur due to various causes, including accidents, overuse, and chronic conditions. These injuries can result in pain, numbness, and weakness in the affected arm.
Causes and Symptoms
Ulnar nerve injuries can be caused by a variety of factors, including acute trauma, repetitive elbow and wrist motions, and chronic conditions. People with ulnar nerve injuries may experience pain, numbness, and weakness in the arm. There might also be a burning sensation that travels along the path of the nerve, described as shooting or like an electric shock.
Diagnosis
Ulnar nerve injuries are evaluated based on their location above (proximal) or below (distal) the elbow, which can affect the symptoms experienced. Numbness and tingling (paresthesia) are signs of interrupted ulnar nerve signals and can occur when there is pressure or inflammation around the nerve. In cubital tunnel syndrome, the most common location for paresthesia, the ring and small fingers are affected.
Treatment
Treatment options for ulnar nerve injuries typically involve avoiding aggravating activities, taking pain medication, and sometimes surgery. However, chronic nerve compression can lead to irreversible muscle atrophy, resulting in a loss of function and grip in the hand.
Anatomical Course
The ulnar nerve is the distal continuation of the medial cord of the brachial plexus, formed by nerve fibers around the shoulder blade. It courses through the axilla, arm, and forearm into the hand. Along its path, the nerve gives rise to muscular and cutaneous branches, supplying muscles such as the flexor carpi ulnaris and the medial half of the flexor digitorum profundus.
The ulnar nerve is most susceptible to injury at the elbow and wrist, where it passes through the cubital tunnel and Guyon's canal, respectively. Compression or pathology at these sites can result in motor and sensory deficits, with varying presentations depending on the exact location of the compression.
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Muscular branches
The ulnar nerve is a major peripheral nerve of the upper limb. It is the largest nerve in the human body that is not protected by muscle or bone, and is therefore susceptible to injury. The nerve runs down the forearm and hand, passing behind the medial epicondyle of the humerus at the elbow.
The muscular branches of the ulnar nerve supply two muscles in the anterior forearm: the flexor carpi ulnaris and the flexor digitorum profundus. The former allows for flexion and adduction of the hand at the wrist, while the latter flexes the ring and little fingers at the distal interphalangeal joint. The ulnar nerve also gives rise to two cutaneous branches in the forearm: the palmar cutaneous nerve and the dorsal cutaneous nerve. These nerves pass into the hand to provide sensory innervation.
The ulnar nerve can be entrapped as a result of the thickening of the humeroulnar aponeurosis or bulging of the medial collateral ligament of the elbow joint. It is also susceptible to injury at the elbow and wrist, which can result in varying degrees of motor and sensory loss. Cubital tunnel syndrome, for example, can cause direct ulnar nerve injury.
The superficial branch of the ulnar nerve supplies sensory innervation to the anterior aspect of the ulnar digits (the little finger and half of the ring finger) and the medial palmar skin. It also provides motor innervation to the palmaris brevis muscle in the hypothenar region of the hand. The deep branch of the ulnar nerve emerges from the same region as the superficial branch and provides motor innervation to the vast majority of hand muscles, including the hypothenar muscles, the interossei, the medial two lumbricals, the deep head of flexor pollicis brevis, and the adductor pollicis.
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Frequently asked questions
The ulnar nerve is a major peripheral nerve of the upper limb. It is the largest nerve in the human body that is unprotected by muscle or bone, and it runs near the ulna—one of the two long bones in the forearm.
The ulnar nerve innervates muscles in the anterior compartment of the forearm and in the hand. In the forearm, the muscular branch of the ulnar nerve supplies two muscles: flexor carpi ulnaris and flexor digitorum profundus. In the hand, the nerve supplies the intrinsic hand muscles, including the hypothenar muscles, the interossei, and the adductor pollicis.
The ulnar nerve provides both sensory and motor innervation to the hand. It supplies sensory innervation to the skin of the hand, including the palmar and dorsal aspects of the medial digits and adjacent palm. It also provides motor innervation to muscles involved in wrist and finger flexion.
Injury to the ulnar nerve can cause specific motor and sensory deficits, such as weakness in wrist flexion and abduction, and impaired movement of the fourth and fifth digits. It can also lead to a loss of dexterity and decreased grip strength.
One test for ulnar nerve injury is Froment's sign, which specifically tests for paralysis of the adductor pollicis. The patient is asked to hold a piece of paper between the thumb and index finger, and a positive test is indicated by the patient's inability to adduct the thumb.
