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Innervation refers to the process by which nerves supply and control muscles. This process is essential for muscle function, and knowledge of innervation is crucial for medical professionals such as physical therapists, physicians, and occupational therapists. The nervous system and muscular system are intricately linked, with nerves transmitting electrical signals to muscles, resulting in contractions and relaxation. The complexity of innervation is evident in invertebrate muscles, where a single muscle fibre can be influenced by multiple motor neurons, leading to a range of responses. In the human body, various nerves innervate specific muscles, such as the ulnar nerve, which powers forearm muscles and provides sensation to parts of the hand. Innervation is a vast subject, encompassing the intricate connections between nerves and muscles, and understanding these connections is vital for both physiological and pathological insights.
| Characteristics | Values |
|---|---|
| Nerve | Muscle |
| Ulnar nerve | Forearm muscles that bend the tips of the small and ring fingers (flexor digitorum profundus), flexor carpi ulnaris, flexor carpi minimi, lumbricals (3 and 4), interossei, hypothenar muscles, lumbricals to the ring and small finger, palmar and dorsal interossei muscles, adductor pollicus, deep head of flexor pollicus brevis |
| Median nerve | Flexor muscles, pronator muscles of the forearm, lateral two lumbricals of the hand, thenar muscles |
| Radial nerve | Triceps muscles, extensor muscles in the forearm, brachialis, brachioradialis, supinator, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, extensor indicis, abductor pollicis |
| Musculocutaneous nerve | Muscles of the anterior compartment of the upper arm |
| Axillary nerve | Deltoid, teres minor |
| Spinal accessory nerve | Upper trapezius, middle trapezius, lower trapezius |
| Suprascapular nerve | Supraspinatus, infraspinatus |
| Dorsal scapular nerve | Levator scapulae, rhomboideus minor, rhomboideus major |
| Long thoracic nerve | Serratus anterior, thoracodorsal (middle subscapular), latissimus dorsi |
| Abdominal muscles | Innervated by the anterior rami of the thoracic and lumbar spinal nerves |
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What You'll Learn
The radial nerve and the triceps
The radial nerve is a peripheral nerve that supplies movement and sensory function to specific parts of the arm, forearm, wrist, and hand. It is formed from the C5, C6, C7, C8, and T1 nerve roots of the brachial plexus, which is a group of nerves branching from the cervical spine (neck). The radial nerve takes a winding path down the arm, starting from the lower armpit and wrapping around the back side of the humerus bone.
The radial nerve has both motor and sensory branches. The motor branch stimulates the triceps muscles on the back of the upper arm, allowing for elbow straightening. Specifically, it innervates the medial and lateral heads of the triceps brachii muscle. It also stimulates muscles in the forearm that rotate the forearm and straighten the wrist and fingers. The sensory branch provides touch, pain, and temperature sensations to the skin on the inner upper arm, outer forearm, wrist, and the back of the hand near the thumb.
The radial nerve can be susceptible to injury, particularly when the humerus bone is broken, as it wraps around the bone. Radial nerve injury can lead to wrist drop, or the inability to straighten the wrist. It is important to understand the anatomy of the radial nerve and its functions to manage any injuries or conditions associated with it effectively.
The triceps muscle, innervated by the radial nerve, is responsible for extending or straightening the elbow. It is located at the back of the upper arm and works in conjunction with other muscles and tendons to facilitate movements involving the elbow joint. The triceps muscle is essential for various activities and movements, such as pushing, throwing, and maintaining stability during standing and walking.
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Ulnar nerve and the hand
The ulnar nerve is one of the five nerve branches of the brachial plexus. The brachial plexus is a group of nerves that branch off from the cervical spine (neck). The ulnar nerve starts at the brachial plexus in the armpit and runs down the arm, through the cubital tunnel (a tissue opening located under a bony bump in the elbow), and into the forearm.
The ulnar nerve is responsible for controlling movement and sensation in the hand, ring finger, and little finger. It powers the forearm muscles that bend the tips of the ring and little fingers, as well as one of the muscles that bends the wrist. It also provides sensation to the small finger side of the palm, the small finger, and the side of the ring finger next to it. This nerve controls almost all of the small muscles in the hand, including the hypothenar muscles, the lumbricals to the ring and small finger, the palmar and dorsal interossei muscles, the adductor pollicus, and the deep head of the flexor pollicus brevis.
Ulnar nerve entrapment occurs when the nerve is compressed or irritated as it passes through the wrist or elbow, causing pain, numbness, and tingling in the forearm and the fourth and fifth fingers. In severe cases, ulnar nerve entrapment can lead to weakness in the hand and loss of muscle mass, as well as difficulty gripping items. Treatment options include occupational therapy, medications, splints, and in severe cases, surgery.
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Median nerve and the forearm
The median nerve is one of five nerve branches of the brachial plexus. It controls movement and feeling in the forearm, wrist, hand, thumb, and fingers. The nerve starts at the armpit, but its functions are all located in the forearm or hand.
The median nerve travels along the inside of the arm near the brachial artery. It does not provide any function until after it crosses the elbow. In the forearm, the nerve passes between the two heads of the pronator teres, and then travels in a plane between the flexor digitorum profundus and flexor digitorum superficialis muscles. The median nerve supplies almost all the flexor muscles and all the pronator muscles of the forearm with nerves. It allows the wrist and fingers to bend and pronates the forearm (rotating the hand to face palm down).
The median nerve gives rise to two major branches in the forearm: the anterior interosseous nerve and the palmar cutaneous nerve. The anterior interosseous nerve supplies the deep muscles in the anterior forearm, while the palmar cutaneous nerve innervates the skin of the lateral palm. The median nerve also gives rise to the recurrent branch, which innervates the thenar muscles (muscles associated with movements of the thumb), and the palmar digital branches, which innervate the lateral two lumbricals (muscles that perform flexion at the metacarpophalangeal joints and extension at the interphalangeal joints).
The median nerve is susceptible to compression at many points along its course, which can lead to carpal tunnel syndrome (CTS). CTS is a common condition that causes tingling, numbness, and sometimes pain in the hand and fingers, particularly the thumb, index finger, and middle finger. Symptoms typically worsen at night and can interfere with sleep. Treatment for CTS may involve splinting, corticosteroid injections, or surgical decompression of the carpal tunnel.
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Abdominal muscles and the lumbar spinal nerves
The lumbar spine, or lower back, is a five-vertebral bone section of the spine. The abdominal muscles are innervated by the anterior rami of the thoracic and lumbar spinal nerves. Early descriptions of the innervation of the abdominal muscles indicate that nerves T5–T12 and L1 are involved. More recent anatomical texts describe the innervation of the abdominal muscles by nerves T7–L1. A cadaveric study found evidence of innervation by nerves T5–L2, T10–L1, and T6–L1.
The activation of abdominal muscles has been found to increase lumbar spinal stability. This has been observed in extension, lateral bending, and axial rotation. The activation of all abdominal wall muscles is associated with intra-abdominal pressurization, which is thought to reduce spinal loading and stabilize the spine. This has been supported by multiple studies.
The relationship between abdominal muscle activity and lumbar muscle morphology has been investigated in patients with chronic non-specific low back pain (CNSLBP). Significant correlations were found between abdominal muscle activity during the posterior pelvic tilt movement and the cross-sectional area (CSA) of the erector spinae muscle at the L4 and L5 levels, and the psoas muscle at the L2–L5 levels. These findings contribute to the functional assessment of patients with CNSLBP.
The lumbar spine is susceptible to various problems, including lower back pain, degenerative disc disease, lumbar lordosis, and adult scoliosis. Sciatica, or lumbar radiculopathy, is nerve pain caused by injury or irritation to the sciatic nerve, which runs through the hips, buttocks, and down each leg. It can be caused by a herniated disc, spondylolisthesis, osteoarthritis, trauma to the spine or nerve, or a tumor in the spinal canal. Sciatica can result in back pain, tingling, numbness, and muscle weakness.
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Axillary nerve and the upper arm
The axillary nerve plays a crucial role in the functioning of the upper limbs, particularly near the shoulder. It originates in the neck, travelling beneath the collar bone and through the armpit (axilla), and provides both motor and sensory functions to the upper arm.
One of its key roles is innervating the deltoid muscle, which is responsible for abducting the arm, or moving it away from the body. This abduction occurs at the glenohumeral joint, allowing the arm to be lifted. The axillary nerve has anterior and posterior branches that specifically innervate the anterior and posterior portions of the deltoid muscle, respectively.
In addition to the deltoid muscle, the axillary nerve also innervates the teres minor muscle, which is responsible for lateral rotation of the arm and maintaining the stability of the glenohumeral joint. The posterior branch of the axillary nerve contributes to this lateral rotation by giving rise to the upper lateral cutaneous nerve after innervating the teres minor.
Furthermore, the axillary nerve provides sensation to the area near the shoulder. This includes the ability to sense temperature and the position of the arm. Damage to the axillary nerve can result in a loss of sensation and movement in the upper limb, specifically impairing the ability to abduct the arm beyond 15 degrees.
The axillary nerve is susceptible to injury through trauma to the proximal humerus or shoulder girdle, often associated with fractures of the humeral surgical neck, shoulder dislocations, or iatrogenic injuries during shoulder surgery. Symptoms of an axillary nerve injury include difficulty moving the shoulder or arm, pain when lifting the arms, and numbness or paraesthesia (pins and needles sensation) in the affected area.
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Frequently asked questions
The ulnar nerve is the main nerve that powers the muscles in the hand. It powers the forearm muscles that bend the tips of the small and ring fingers and one of the muscles that bends the wrist. The median nerve also plays a role in the hand, supplying almost all the flexor muscles and all the pronator muscles of the forearm.
The radial nerve, formed from the C5, C6, C7, C8 and T1 nerve roots, wraps around the humerus bone and gives function to the triceps muscles.
The abdominal muscles are innervated by the anterior rami of the thoracic and lumbar spinal nerves.
