Dominic Stone
The thenar muscles are a group of four intrinsic muscles of the thumb: abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, and adductor pollicis. The innervation of these muscles is an unusual process, as most intrinsic muscles on the palm of the hand are supplied by the ulnar nerve. The thenar muscles, however, are primarily innervated by the median nerve, also known as the thenar nerve, which arises from the medial and lateral cords of the brachial plexus.
| Characteristics | Values |
|---|---|
| Number of thenar muscles | 4 |
| Names of the thenar muscles | Abductor pollicis, adductor pollicis, opponens pollicis, and flexor pollicis brevis |
| Muscle controlling thumb movement | Adductor pollicis |
| Muscle with two heads | Flexor pollicis brevis |
| Innervation of the opponens pollicis and abductor pollicis brevis | Recurrent branch of the median nerve |
| Innervation of the flexor pollicis brevis | Deep branch of the ulnar nerve (C8 and T1 roots) |
| Innervation of the thenar eminence | Median nerve (T1) and ulnar nerve (C8, T1) |
| Nerve arising from medial and lateral cords of the brachial plexus | Median nerve |
| Nerve affected in carpal tunnel syndrome | Recurrent branch of the median nerve |
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What You'll Learn
The role of the thenar muscles
The thenar muscles are the four intrinsic muscles of the thumb: abductor pollicis brevis (APB), opponens pollicis (OPP), flexor pollicis brevis (FPB), and adductor pollicis (ADD). The word 'thenar' comes from the Ancient Greek word 'θέναρ' for the palm of the hand. The thenar muscles form the thenar eminence, a mound at the base of the thumb on the palm of the hand.
The thenar muscles facilitate thumb motion and allow for precision pinching and power gripping. The APB abducts the thumb, the FPB flexes the thumb, and the OPP opposes the thumb, bringing it against the fingers. The ADD is responsible for rotation and opposition.
The thenar muscles receive their blood supply from the superficial palmar branch of the radial artery. The thenar eminence is primarily innervated by the median nerve, specifically the recurrent branch of the median nerve, also known as the thenar nerve. However, the two heads of the FPB have different innervations. The superficial head is usually innervated by the recurrent branch of the median nerve, while the deep head is often innervated by the deep branch of the ulnar nerve. The nerves that reach the thenar muscles arise from the C8 and T1 roots, passing through the lower trunk of the brachial plexus.
Several anatomical variations have been observed in the innervation of the thenar muscles. In most cases, the abductor pollicis brevis, opponens pollicis, and flexor pollicis brevis each receive innervation from one of the three terminal branches of the main thenar trunk. However, some specimens exhibit different patterns, with two, three, or four branches arising from the main trunk. In rare cases, seven thenar branches have been observed supplying the thenar musculature.
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The innervation process
The thenar muscles are the group of intrinsic muscles of the thumb, which allow for thumb motion and facilitate precision pinching and power gripping. These muscles form the thenar eminence, a mound at the base of the thumb on the palm of the hand. The thenar muscles include the abductor pollicis brevis (APB), opponens pollicis (OPP), flexor pollicis brevis (FPB), and adductor pollicis (ADD).
The innervation of the thenar muscles is an unusual process, as most of the intrinsic muscles on the palm of the hand are supplied by the ulnar nerve. The thenar muscles, however, are primarily innervated by the median nerve, also known as the thenar nerve. The median nerve arises from the medial and lateral cords of the brachial plexus and travels distally down the arm between the two heads of the pronator teres. It then passes within the carpal tunnel, deep to the flexor retinaculum, and branches further. One of these branches is the recurrent branch, which innervates the thenar eminence. The recurrent branch usually passes distal to the transverse carpal ligament and ends in the opponens pollicis.
The opponens pollicis and abductor pollicis brevis are normally innervated by the recurrent branch of the median nerve. The flexor pollicis brevis, which has two heads, is also innervated by the recurrent branch of the median nerve, but the deep part is often innervated by the deep branch of the ulnar nerve. In some cases, the opponens pollicis and abductor pollicis brevis may be innervated by fibres from the deep branch of the ulnar nerve, known as a Cannieu-Riche anastomosis.
The thenar eminence can be affected by carpal tunnel syndrome, a compressive neuropathy of the median nerve that causes pain, paresthesia, and muscle wasting. The recurrent branch of the median nerve may also be affected by carpal tunnel syndrome, as well as its own separate peripheral neuropathies.
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The muscles involved
The thenar muscles are the four intrinsic muscles of the thumb. These muscles include the abductor pollicis, adductor pollicis, opponens pollicis, and flexor pollicis brevis. The thumb's motion is facilitated through the coordination of these intrinsic muscles, allowing for precision pinching and power gripping.
The abductor pollicis brevis abducts the thumb. This muscle is the most superficial of the thenar group. The abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis receive their blood supply from the superficial palmar branch of the radial artery. The abductor pollicis brevis is located superficially and laterally to the other two muscles, with origins mainly from the flexor retinaculum, as well as the tubercles of the scaphoid and trapezium.
The flexor pollicis brevis lies next to the abductor pollicis brevis and is responsible for flexing the thumb, curling it up in the palm. The flexor pollicis brevis has two heads: a superficial head and a deep head. The superficial head is usually innervated by the recurrent branch of the median nerve, while the deep head is often innervated by the deep branch of the ulnar nerve (C8 and T1 roots).
Opponens pollicis lies deep to the abductor pollicis brevis and functions to oppose the thumb, bringing it against the fingers. The opponens pollicis originates from the transverse carpal ligament, the trapezium, and the thumb carpometacarpal joint capsule, inserting into the volar-radial distal thumb metacarpal.
The adductor pollicis lies deeper and more distal to the flexor pollicis brevis and is responsible for rotation and opposition. This muscle is not part of the thenar group and is supplied by the ulnar nerve.
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Variations in innervation
The thenar muscles are innervated by the motor branch or recurrent branch of the median nerve, also known as the thenar nerve. This branch is usually given off at the level of the distal margin of the flexor retinaculum. The thenar eminence is composed of three muscles that control the movements of the thumb: abductor pollicis brevis (APB), flexor pollicis brevis (FPB), and opponens pollicis (OPP). The opponens pollicis and abductor pollicis brevis are normally innervated by the recurrent branch of the median nerve. The flexor pollicis brevis has two heads: a superficial and a deep head. The superficial head is usually innervated by the recurrent branch of the median nerve, while the deep part is often innervated by the deep branch of the ulnar nerve (C8 and T1 roots).
There are several anatomical variations in the origin, course, and number of thenar branches. One unusual observation is the presence of seven thenar branches supplying the thenar musculature. The traditionally described pattern of one main thenar trunk with three terminal branches, one each to the abductor pollicis brevis, opponens pollicis, and flexor pollicis brevis muscles, was observed in nine specimens (45%). One main trunk with two terminal branches (one branch to the abductor and one to the opponens) was seen in six specimens (30%). The remaining five specimens (25%) exhibited four other terminal patterns with two, three, or four branches off the main trunk.
In a Cannieu-Riche anastomosis, fibres from the deep branch of the ulnar nerve innervate the opponens pollicis and/or abductor pollicis brevis. The thenar eminence is primarily innervated by the median nerve (T1) but also receives some contribution from the ulnar nerve (C8, T1). The two heads of the flexor pollicis brevis have different innervations. The median nerve has multiple branches, and the palmar cutaneous branch is not affected by carpal tunnel syndrome. This branch is responsible for skin sensation over the thenar eminence. The recurrent branch travels through the carpal tunnel and proximally innervates the muscles of the thenar eminence.
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Medical conditions affecting the thenar eminence
The thenar eminence is the mound formed at the base of the thumb on the palm of the hand by the intrinsic group of muscles of the thumb. The thenar eminence helps to control the fine movements of the thumb, including the ability to grab, grip, and pinch objects. The three muscles that make up the thenar eminence are the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis. The opponens pollicis is the largest muscle found in the thenar eminence and enables the thumb to be opposable.
There are several medical conditions that can affect the thenar eminence, leading to a decrease in function or even atrophy of the muscles. Some of these conditions include:
Carpal Tunnel Syndrome
Carpal tunnel syndrome is caused by a compression or pinching of the median nerve as it runs through the wrist. Common symptoms include numbness, tingling, and weakness in the thumb. This can lead to a decrease in sensation in the area of the thumb.
Basal Thumb Arthritis
Basal thumb arthritis is caused by the breakdown of cartilage around the lower thumb joint. While this condition affects the joints around the thenar eminence and not the muscles themselves, it can still result in a loss of motion or weakness of the thumb.
Trauma to the Forearm, Wrist, or Thumb
Injuries to the lower arm can predispose individuals to nervous or arthritic conditions that may affect the thenar eminence. For example, a forearm fracture can damage the median nerve, leading to decreased sensation and weakness in the thumb.
Nerve Issues
The thenar muscles are innervated by the median nerve, also known as the thenar nerve. Issues with this nerve can lead to numbness or "pins and needles" sensations in the thumb.
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Frequently asked questions
The thenar muscles are the four intrinsic muscles of the thumb: abductor pollicis brevis (APB), opponens pollicis (OPP), flexor pollicis brevis (FPB), and adductor pollicis (ADD).
The thenar muscles facilitate thumb movement and allow for precision pinching and power gripping.
The thenar muscles are primarily innervated by the median nerve (T1) but also receive some contribution from the ulnar nerve (C8, T1).
In a unique observation, seven thenar branches were found to supply the thenar musculature, with origins from the main trunk of the median nerve and its lateral and medial aspects.
The median nerve arises from the medial and lateral cords of the brachial plexus and travels distally down the arm. One of its branches, the recurrent branch, innervates the thenar eminence, a mound formed at the base of the thumb on the palm.
