Nerve Root Innervation of Flexor Carpi Ulnaris and Flexor Carpi Radialis
The flexor carpi ulnaris (FCU) is primarily innervated by C8-T1 nerve roots (via the ulnar nerve), while the flexor carpi radialis (FCR) is innervated by C6-C7 nerve roots (via the median nerve).
Flexor Carpi Ulnaris (FCU)
Primary Innervation
- The FCU receives its nerve supply from the C8 and T1 nerve roots through the ulnar nerve, which arises from the medial cord of the brachial plexus 1.
- The ulnar nerve is formed from the ventral rami of C8-T1 spinal segments 2.
Important Anatomical Variation
- A lateral root contribution from C7 exists in approximately 13% of individuals, creating a connecting branch between the lateral cord and the ulnar nerve 3.
- When this C7 contribution is present, abnormal electromyographic findings in the FCU occur in 46.2% of C7 radiculopathy cases, compared to 76.5% in C8 radiculopathy 3.
- C6 radiculopathy shows 0% abnormal FCU findings, confirming no C6 contribution 3.
Clinical Implications
- Needle EMG of the FCU is most useful for detecting C8 radiculopathy, but may also show abnormalities in C7 radiculopathy due to anatomical variations 3.
- The FCU is typically paralyzed in complete brachial plexus injuries except when only T1 remains intact, confirming its primary T1 innervation 4.
Flexor Carpi Radialis (FCR)
Primary Innervation
- The FCR is innervated by both C6 and C7 nerve roots via the median nerve 5.
- The median nerve arises from contributions of both the lateral and medial cords of the brachial plexus 2.
Specific Root Contribution
- The FCR H-reflex is specifically and selectively related to C7 nerve root function, not C6 5.
- Abnormal FCR H-reflexes occur in 88.2% of C7 radiculopathy patients but only 3.7% of C6 radiculopathy patients 5.
- This selective C7 sensitivity makes the FCR H-reflex a highly specific diagnostic tool for differentiating C7 from C6 radiculopathy 5.
Clinical Testing Strategy
- Combining FCR H-reflex testing with needle electromyography yields the highest diagnostic accuracy for C7 radiculopathy 5.
- The FCR H-reflex provides 88.2% sensitivity for C7 radiculopathy, superior to provocative tests (29.4% sensitivity) 5.
Key Clinical Pitfalls
- Do not assume FCU is purely C8-T1: The lateral root variation from C7 occurs in 13% of cases and can cause diagnostic confusion in cervical radiculopathy 3.
- Do not assume FCR responds equally to C6 and C7 lesions: Despite dual innervation, the FCR H-reflex is selectively sensitive to C7, not C6 5.
- Rare anomalous innervation exists: Accessory FCU muscles with aberrant median nerve innervation have been reported, though extremely rare 6.