Innervation and Functional Anatomy of Laryngeal Muscles

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As a matter of basic orientation, a conventional view of laryngeal anatomy is briefly presented. The intrinsic laryngeal muscles have both their origin and insertion on the cricoid, thyroid, or arytenoid cartilage. In concert, they orchestrate the motion of the vocal folds during respiration, vocalization, and airway protection. The paired posterior cricoarytenoid (PCA) muscles are situated on the posterior larynx (Fig. 2.1A). When the PCA contracts, it rocks the arytenoid cartilage in a posteromedial direction to swing open the vocal process and fold (Fig. 2.1B). The articular facets of the cricoarytenoid joint are contoured to also permit a sliding motion along the mediolateral axis. The PCA is the sole abductor of the vocal folds during inspiration and acts as a co-contractor antagonist to the adductors during phonation. The thyroarytenoid (TA) muscle is the principal adductor and intrinsic tensor of the vocal fold (Fig. 2.1C). In synergy with the lateral cricoarytenoid (LCA) and interarytenoid (IA) muscles, it acts to close the glottic airway during vocalization and airway protection. The cricothyroid (CT) does not insert on the arytenoid, and therefore affects vocal fold motion indirectly. It tilts the thyroid cartilage toward the cricoid ring ventrally in a visor-like action, effectively stretching the vocal folds. It is known to have a dual role: it is active in both adduction and abduction. With the exception of the CT, the abductor and adductor muscles are supplied by motor fibers in the recurrent laryngeal nerve (RLN). The CT, on the other hand, receives its innervation from the external branch of the superior laryngeal nerve (SLN). Sensory information from the larynx is carried in both the RLN and SLN. For the most part, sensory fibers in the RLN originate from receptors below the level of the vocal folds, while those in the internal branch of the SLN originate at the glottal level and above; however, some sensory fibers in the internal SLN or RLN originate remotely and cross over via Galen's anastomosis.

Recent reports provide a radical departure from this conventional view of anatomy and suggest an anatomical complexity not previ ously appreciated. Careful dissection and analysis has revealed that each laryngeal muscle is not a single entity but rather an assembly of anatomically distinct compartments potentially adapted for different functions. While this notion can be traced back as far as the seven teenth century, in the past decade Sanders et al. have published a series of articles providing strong evidence for the existence of functional compartments [1-4]. Based on the presence of fascial barriers, differences in fiber direction and site of insertion, the PCA, TA, and CT

Laryngeal MusclesLarynx Anatomy
Cricoid Cartilage c Cartilage

Fig. 2.1. a-c Laryngeal anatomy and muscle actions. For the normally innervated larynx, stimulation of afferents in the superior laryngeal nerve (internal branch) reflexly activate reflex glottic closure motor units in the recurrent laryngeal nerve and thyroary tenoid (TA) muscle to adduct the vocal fold and close the airway (arrows, c). Inspiratory motor units in the recurrent laryngeal nerve and posterior cricoarytenoid (PCA) muscle are recruited during hypercapnia to abduct the vocal fold and open the airway (arrows, b).

Vocal Fold

Thyroid Cartilage

Motor Unit Anatomy

Fig. 2.1. a-c Laryngeal anatomy and muscle actions. For the normally innervated larynx, stimulation of afferents in the superior laryngeal nerve (internal branch) reflexly activate reflex glottic closure motor units in the recurrent laryngeal nerve and thyroary

Arytenoid Cartilage

PCA Muscles

Arytenoid Cartilage

PCA Muscles

Vocal Fold

Thyroid Cartilage

Laryngeal Nerve Stimulator

Fig. 2.3. Innervation of the human posterior cricoarytenoid (PCA) muscle. The initial branch of the recurrent laryngeal nerve (RLN) is to this muscle. On entering the larynx, the RLN passes superiorly along the lateral edge of the PCA. In half of the specimens, two branches came off separately from the RLN to innervate two different areas of the muscle. The first branch innervates the vertical-oblique compartment of the muscle (2), while the second branch innervates the horizontal compartment (2). The arrow points to one of the communicating nerve branches that connect the nerves to the PCA and the interarytenoid muscles. LB indicates the nerve branch to the interarytenoid muscle. (From Sanders et al. [8])

muscles have been found to comprise two to three distinct compartments (Fig. 2.2). Muscle biochemistry may lend support for the theory of functional compartmentalization. For example, the horizontal division of the PCA has a greater percentage of slow-twitch type-I fibers than the vertical or oblique compartments. Likewise, the superior medial division of the thyroarytenoid muscle, known as the vocalis, contains a highly specialized slow-twitch fiber that is multiply innervated and fatigue resistant [5]. In general, these slow contracting fibers are optimally adapted for tonic functions such as vocalization or quiet respiration. A high density of muscle spindles in these two compartments suggests that they are engaged in a motor behavior requiring fine control with feedback [6], In contrast, the vertical and oblique divisions of the PCA and the lateral division of the TA have a higher concentration of fast-twitch type-II fibers, which likely reflects their specialization for rapid breathing and airway protection, respectively.

Fig. 2.2. Compartments of the canine PCA muscle. Dissection reveals three discrete anatomical compartments, each with a distinctive origin and angle of insertion on the muscular process of the arytenoids: A vertical compartment; B oblique compartment; C horizontal compartment. In the human PCA muscle, the vertical and oblique compartments are combined. (From Sanders et al. [81])

The theory of compartmentalization is further strengthened by recent studies of the innervation pattern of laryngeal muscles. Although the branching pattern is quite variable among individuals and even between the two sides of the larynx, the compartments of a muscle often receive innervation by separate nerve branches. There is inference from work by Maranillo et al. [7] and Sanders et al. [8] that the horizontal and vertical-oblique divisions of the human PCA muscle are innervated independently, most commonly by two different branches off the RLN (Fig. 2.3). In some cases, it

Fig. 2.3. Innervation of the human posterior cricoarytenoid (PCA) muscle. The initial branch of the recurrent laryngeal nerve (RLN) is to this muscle. On entering the larynx, the RLN passes superiorly along the lateral edge of the PCA. In half of the specimens, two branches came off separately from the RLN to innervate two different areas of the muscle. The first branch innervates the vertical-oblique compartment of the muscle (2), while the second branch innervates the horizontal compartment (2). The arrow points to one of the communicating nerve branches that connect the nerves to the PCA and the interarytenoid muscles. LB indicates the nerve branch to the interarytenoid muscle. (From Sanders et al. [8])

appears that the vocalis may receive additional motor innervation from the external branch of the SLN in addition to its normal source, the RLN [9].

The laryngeal muscles engage in a great diversity of motor behaviors from ballistic-type phasic movements during coughing to finely graded more tonic motor acts such as singing a scale. Presented with functions of such a range of complexity, it is tempting to postulate that a muscle may have become compartmentalized with specialization of its muscle fiber contraction properties and organization. Certainly, the vocalis with its slow twitch fiber composition, multilobar construction, and muscle spindle density is most suited for the accurate generation of sound used in communication; however, there is no direct evidence that compartments are actually involved in different laryngeal behaviors. Furthermore, the variation in fiber type between compartments is apparently no greater than regional differences within a compartment. In general, slow contracting type-I fibers tend to be located deep within a muscle in apposition to the bone or cartilage and nearest the midline for axial muscles. This is true for the CT and PCA muscles and is simply an adaptation of nature to conserve heat loss since these fibers are more dependent upon a circulatory supply than type-II fibers. Teleological-ly, type-II fibers may have also been reserved a more peripheral location in a muscle to give them a greater mechanical advantage in generating high levels of torque across a joint. High levels of torque are not required in the maintenance of tonus by type-I fibers.

Although it is uncertain whether laryngeal muscles are adapted to perform different behaviors through compartmentalization, there is direct evidence that the motor units that compose them are specialized. In 1979, Zealear performed a study of single motor units of the TA muscle in the cat [10]. Two types of motor units were discovered; of these, 85% were fast-twitch fatigue-resistant motor units. They had faster conducting axons, had a greater range in the number of muscle fibers innervated, and could generate larger tension. The remaining motor units were slow-twitch fatigue-resistant. They had slower conducting axons and tended to be smaller in size. The fiber type composition of the muscle, as revealed histochemical-ly, was found to correlate with its motor unit makeup. Approximately 80% of the fibers were type IIA and 20% type I. Differences in the muscle fiber characteristics of these two types of motor units appeared to make them suited for different functions: the fast type for gagging and the slow type for vocalization. Recordings from their axons confirmed this hypothesis. During arousal in light anesthesia, only slow motor units were recruited for tonic firing during expiration as a prelude to vocalization. This finding was not unexpected since slow motor units have smaller motoneuron cell bodies and are recruited first by synaptic activity for any motor behavior, according to Henneman's size principle [11]; however, fast motor units appeared to be recruited preferentially for airway protection, counter to the size principle. Although gag activity from slow motoneurons could be recorded with stimulation of the internal branch of the SLN, fast motor units exhibited a greater number of spikes at any stimulus level. It can be concluded from this study that fast and slow motor units that comprise each laryngeal muscle have special adaptations and may be preferentially recruited for phasic and tonic functions irrespective of the particular motor behavior performed. Coordination of the participating muscles or their subdivisions would then provide a higher level of control in mediating a particular function.

The idea that fast and slow motor unit adaptations are fundamental to laryngeal behavior is further supported by the report of Hinrich-sen and Ryan [12]. Using a retrograde labeling technique, they found that the motoneurons projecting to each muscle were grouped into large and small neurons, presumably providing the source for dual innervation of muscles by fast and slow motoneurons.

The new anatomical reports confirm Dil-worth's early description of laryngeal innervation as being complex, composed of many branches with anastomotic channels, and terminating in plexus formations [13]. There is even speculation that some laryngeal muscles may be innervated by both the SLN and RLN. Using the Sihler's stain technique, the interary-

tenoid muscle has been shown to have innervation from the internal branch of the SLN as well as the RLN [8], Apparently, some of these fibers may terminate in the PCA muscle as well, providing it innervation by two different pathways. Unfortunately, Sihler's stain cannot differentiate motor from sensory fibers. Until physiological proof is provided, these secondary pathways of innervation must be viewed as strictly sensory, likely representing a more complicated version of Galen's anastomosis. With regard to this, there has been no report that stimulation of the internal branch of the SLN will cause a direct, non-reflexive response of any laryngeal muscle. Furthermore, injury of the RLN in both animal and patient subjects results in paralysis of all laryngeal muscles, with the exception of the CT. There is no indication of secondary innervation that contributes to vocal fold movement.

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  • JAMES
    Where does pca muscle originate?
    6 years ago

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