[Anesthesia and laryngeal muscle, especially intrinsic laryngeal muscles]

The intrinsic laryngeal muscles are highly specialized for phonation, respiration and sphincter activity. The muscles are disposed between three unpaired cartilages (the thyroid, cricothyroid and epiglottic) and the paired arytenoid cartilages. These laryngeal muscles, which are striated in character, are bilaterally disposed and can be grouped as adductors and abductors of the glottis and tensors of the vocal cords. The motor neurons for the intrinsic laryngeal muscles are located in the caudal half of the nucleus ambiguous. The intrinsic laryngeal muscles receive motor nerve supply from the external branches of the superior laryngeal and recurrent nerves on each side. The external branch of the superior laryngeal nerve serves in motor innervation of the cricothyroid muscle, while the other laryngeal muscles receive a motor innervation from the recurrent nerve. Compared with limb skeletal muscles, the intrinsic laryngeal muscle have unique morphologic and histochemical characteristics. In general, the intrinsic laryngeal muscle fibers contain greater percentages of histochemically type 1 fibers than limb skeletal muscles. In addition, morphometrical studies of the subneural apparatus at the neuromuscular junctions of the intrinsic laryngeal muscles demonstrate the difference between the morphological features of the nerve endings and the distribution pattern of the motor end-plates of the laryngeal muscles and those of the limb skeletal muscles. Finally, the influence of anesthetic drugs including neuromuscular blocking agents on the intrinsic laryngeal muscles is discussed. The gross anatomical features such as the motor innervation, localization of the neurons for the motor nerve fibers, and arrangement and characteristics of the motor end-plates of the intrinsic laryngeal muscles are described based on a review of literature and our current investigations.