Dominic Stone
Nerves are present throughout the human body and are one of the foundational parts of the nervous system. They send electrical signals that help us feel sensations and move our muscles. The neuromuscular system connects muscles and nerves, which control body movements and functions. Nerves called motor neurons send messages from the brain to muscles, making them contract and move. For example, the brachial plexus is a group of nerves that branch from the neck and travel under the clavicle. At this point, the axillary nerve travels behind the humerus bone to supply muscles around the shoulder. The nerve also provides function to three muscles: the deltoid, the teres minor, and the long head of the triceps muscle.
| Characteristics | Values |
|---|---|
| Nerves that travel under the clavicle | Brachial plexus, Axillary nerve |
| Nerves that travel through the armpit | Brachial plexus, Median nerve |
| Nerves that travel down the arm | Ulnar nerve, Median nerve |
| Nerve that wraps around the humerus bone | Radial nerve |
| Nerve that provides sensation to the thumb side of the hand | Radial nerve |
| Nerve that provides sensation to the small finger side of the hand | Ulnar nerve |
| Nerve that provides sensation to the dorsal aspect of the hand | Median nerve |
| Nerve that provides function to the triceps muscles | Radial nerve |
| Nerve that provides function to the muscles that straighten the wrist and fingers | Radial nerve |
| Nerve that provides function to the muscles that bend the wrist | Ulnar nerve |
| Nerve that provides function to the muscles that bend the tips of the small and ring fingers | Ulnar nerve |
| Nerve that provides function to the deltoid muscle | Axillary nerve |
| Nerve that provides function to the teres minor muscle | Axillary nerve |
| Nerve that provides function to the long head of the triceps muscle | Axillary nerve |
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What You'll Learn
The radial nerve and its role in the upper arm
The radial nerve is a major peripheral nerve in the upper limb that plays a crucial role in the upper arm. It is one of five nerve branches that extend from the brachial plexus, a network of nerves originating from the spinal cord in the lower neck and upper chest.
In the upper arm, the radial nerve wraps around the back of the humerus bone, the long bone that runs from the shoulder to the elbow. This nerve provides motor function to the triceps muscles, enabling elbow extension or straightening. The radial nerve also provides sensory information to the skin on the inner upper arm, allowing for sensations like touch, pain, and temperature perception.
The radial nerve then continues its path down the arm, travelling through the radial groove, a shallow depression on the surface of the humerus. As it descends, it supplies a branch to the medial head of the triceps brachii, contributing to the movement of the arm.
The radial nerve has both motor and sensory functions. The motor branches stimulate the triceps muscles, allowing for elbow extension, and the sensory branches provide information from the skin on the anterolateral arm, distal posterior arm, and posterior forearm.
Injuries to the radial nerve can occur through compression, ischemia, arm fractures, penetrating wounds, or surgical complications. Radial nerve palsy, characterised by weakness or paralysis, can result from sleeping on an outstretched arm or compression in the armpit from crutches. Wartenberg syndrome, a type of mononeuropathy, affects the radial nerve when it is trapped or compressed by muscles in the wrist.
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The median nerve and its four layers of muscles
Nerves are indeed located under muscles. For example, the ulnar nerve travels under the ulnar head of the pronator teres. The median nerve is one of five nerve branches of the brachial plexus, a complex network of nerves that help us move our shoulders, arms, and hands. The brachial plexus is a group of nerves that branch from the cervical spine (neck).
The median nerve provides motor (movement) functions to the forearm, wrist, and hand. It also sends touch, pain, and temperature sensations from the lower arm and hand to the brain. The median nerve stimulates muscles in the forearm, allowing us to bend and straighten our wrists, thumbs, and first three fingers, as well as rotate our forearm and hand to turn our palm downward.
The muscles innervated by the median nerve can be divided into four layers of the anterior compartment of the forearm:
- First layer: Arises from the medial epicondyle and contains three muscles: the pronator teres, flexor radialis longus, and palmaris longus.
- Second layer: Contains the flexor digitorum superficialis, whose tendons insert into the middle phalanx of the four fingers. Thus, it is a flexor of the fingers at the proximal interphalangeal joint. The first and second layers are innervated by the median nerve proper.
- Third layer: Contains the lateral half of the flexor digitorum profundus that innervates the index and ring fingers, the flexor pollicis longus, and the pronator quadratus. These are innervated by the anterior interosseous nerve, which has no sensory branches.
- Fourth layer: The median nerve gives rise to the anterior interosseous nerve, which supplies the deep flexors: flexor pollicis longus, pronator quadratus, and the lateral half of the flexor digitorum profundus (the medial half of the muscle is innervated by the ulnar nerve).
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The ulnar nerve and its function in the hand
The ulnar nerve is a major peripheral nerve of the upper limb. It is a terminal branch of the brachial plexus that supplies the forearm and hand. The ulnar nerve is the distal continuation of the medial cord of the brachial plexus, from the C8 and T1 nerve roots. It often carries fibres from C7 via a communicating branch from the lateral cord.
The ulnar nerve runs down the hand, passing behind the medial epicondyle of the humerus at the elbow. It gives rise to an articular branch that supplies the elbow joint. In the forearm, the ulnar nerve pierces the two heads of the flexor carpi ulnaris and travels deep into the muscle, alongside the ulna. The nerve gives branches to the flexor carpi ulnaris and the flexor digitorum profundus. As the nerve descends into the forearm, it stays medially above the flexor digitorum profundus and under the flexor carpi ulnaris, giving branches to these muscles.
The ulnar nerve enters the hand via the ulnar canal (Guyon's canal). In the hand, the nerve terminates by giving rise to superficial and deep branches. The superficial branch supplies and passes under the palmaris brevis muscles and divides into palmar digital nerves. The deep branch innervates the hypothenar muscles, the medial lumbricals, the interossei, the adductor pollicis, and the deep head of the flexor pollicis brevis. The ulnar nerve provides sensation to the small finger side of the palm, the small finger, and the side of the ring finger next to the small finger. It also provides sensation to the back of the small finger side of the hand.
Ulnar nerve injuries are common and can occur at various sites along its course through the upper limb. The characteristic presentation of an ulnar nerve injury is the "claw hand". This deformity causes hyperextension of the metacarpophalangeal joints and flexion of the interphalangeal joints of the fourth and fifth fingers.
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The brachial plexus and its branches
The brachial plexus is a network of nerve fibres that supplies the skin and musculature of the upper limb. It is a group of nerves that branches from the cervical spine (neck). The brachial plexus is divided into five parts: roots, trunks, divisions, cords, and branches. The roots refer to the anterior rami of the spinal nerves that comprise the brachial plexus, which are the anterior rami of spinal nerves C5, C6, C7, C8, and T1. The lateral pectoral nerve arises from the lateral cord of the brachial plexus, carrying fibres from C5, C6, and C7 spinal nerves. The medial pectoral nerve arises from the medial cord, carrying fibres from the eighth cervical and first thoracic nerves.
The brachial plexus travels under the clavicle, through the armpit, and down the inside of the arm to the inner elbow. The axillary nerve travels behind the humerus bone to supply muscles around the shoulder, including the deltoid, the teres minor, and the long head of the triceps muscle. The radial nerve wraps around the back side of the humerus bone and gives function to the triceps muscles on the back of the arm, allowing the elbow to straighten. The median nerve provides motor innervation to the flexor muscles of the forearm and hand, as well as sensory innervation to the dorsal aspect of the distal first two digits of the hand, the palmar aspect of the thumb, index, middle, and half of the ring finger, the palm, and the medial aspect of the forearm.
The ulnar nerve travels down the arm and over the medial epicondyle, providing motor innervation to the flexor carpi ulnaris and the ulnar half of the flexor digitorum profundus. It also provides sensation to the small finger side of the palm, the small finger, and the side of the ring finger next to the small finger. The musculocutaneous nerve is one of the terminal branches of the brachial plexus.
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Neuromuscular diseases and their impact
Nerves are indeed located under muscles, and neuromuscular disorders affect the nerves that control voluntary muscles and those that communicate sensory information back to the brain. When neurons become unhealthy or die, the communication between the nervous system and muscles breaks down, leading to muscle weakness and atrophy.
Neuromuscular diseases or disorders encompass a wide range of conditions that involve the dysfunction of peripheral nerves, muscles, or their communication. These disorders can be caused by issues with the neuromuscular junction, where motor nerves communicate with muscles through the release of acetylcholine. Examples of such disorders include Lambert-Eaton myasthenic syndrome (LEMS) and Myasthenia gravis.
Myopathies are another group of neuromuscular disorders that directly affect skeletal muscles, causing weakness by attacking muscle fibers. Metabolic myopathies are caused by issues with genes that provide instructions for enzymes needed by muscles, including glycogen storage diseases and lipid storage disorders. Mitochondrial myopathies occur when there is a defect in muscle mitochondria, the energy-producing component of cells. Toxic myopathies, on the other hand, are the result of toxins or medications interfering with muscle structure or function.
Anterior horn cell diseases are progressive, degenerative disorders of motor neurons, which are essential for skeletal muscle movement. These diseases lead to a lack of nerve supply to muscles, resulting in weakness. Examples include amyotrophic lateral sclerosis (ALS), spinal muscular atrophy, and spinal-bulbar muscular atrophy (SBMA) or Kennedy's disease.
Peripheral neuropathy, the most common type of neuromuscular disorder, encompasses various types and causes. It can manifest as radial nerve injury, leading to wrist drop and the inability to straighten the wrist, or ulnar nerve injury, resulting in clawing of the small and ring fingers and weakness in the hand muscles.
Currently, there is no cure for neuromuscular disorders. Treatment focuses on managing symptoms, delaying disease progression, and improving patients' quality of life through medications, physical therapy, occupational therapy, and, when necessary, surgery.
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Frequently asked questions
The neuromuscular system connects muscles and nerves, which work together to control body movements and functions such as breathing.
The brachial plexus is a group of nerves that branch from the neck (cervical spine). It travels under the clavicle and through the armpit.
The radial nerve wraps around the back of the upper arm bone (humerus). It gives function to the triceps muscles, allowing the elbow to straighten. It also provides sensation to the thumb side of the hand.
The ulnar nerve provides sensation to the small finger side of the palm, the small finger, and the side of the ring finger next to it. It also powers the forearm muscles that bend the tips of the small and ring fingers.
