Recurrent Laryngeal Nerve course - Why is it different on both sides?
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Why Does the Recurrent Laryngeal Nerve Have a Different Course on Each Side?
The recurrent laryngeal nerves (RLNs) — branches of the vagus nerve (cranial nerve X) — are essential for motor and sensory innervation of the larynx. Interestingly, they follow asymmetrical paths on the right and left sides. Let's understand why.
📍 Origin of the Recurrent Laryngeal Nerve
-
Right RLN:
- Arises from the right vagus nerve at the root of the neck, near the T1 vertebral level.
- Loops around the first part of the right subclavian artery.
-
Left RLN:
- Arises from the left vagus nerve in the thorax, at the T3 vertebral level.
- Loops around the arch of the aorta, posterior to the ligamentum arteriosum.
🛣 Course of the Nerve
After looping around their respective arteries:
- Both nerves ascend in the tracheoesophageal groove.
- Pass deep to the inferior constrictor muscle.
- Enter the larynx behind the cricothyroid joint.
- Divide into motor and sensory branches.
🧬 Embryological Explanation
The asymmetry is explained by the development of aortic arches:
Left Side:
- The dorsal part of the 6th aortic arch persists as the ductus arteriosus in fetal life and becomes the ligamentum arteriosum after birth.
- The left RLN gets "hooked" beneath the arch of the aorta due to this persistent ligament.
- The 4th aortic arch contributes to the arch of the aorta.
Right Side:
- The 5th and 6th aortic arches regress completely.
- The 4th aortic arch forms the proximal part of the right subclavian artery.
- The right RLN loops around this artery instead of a structure in the thorax.
As a result:
- The left RLN has a longer and deeper course in the thorax.
- The right RLN takes a shorter course in the neck.
🧠 Summary Table
| Feature | Right RLN | Left RLN |
|---|---|---|
| Origin | Root of neck (T1) from right vagus | Thorax (T3) from left vagus |
| Loops Around | Right subclavian artery (from 4th arch) | Arch of aorta (near ligamentum arteriosum) |
| Embryological Arch | 4th arch only | 4th and persistent 6th arch |
| Relative Course | Shorter, in neck | Longer, in thorax |
| Enters Larynx | Behind cricothyroid joint | Behind cricothyroid joint |
📝 Viva Tip
When asked "Why is the RLN course different on both sides?" — always link your answer to embryological arch derivatives and anatomical development for full marks.
📝 All the topics and questions mentioned in this post are explained in detail in my ENT notes - built for exam success and clinical understanding. Get full access by purchasing the notes.