Slow tonic muscle fibers in the thyroarytenoid muscles of human vocal folds; a possible specialization for speech

Abstract

Most of the sounds of human speech are produced by vibration of the vocal folds, yet the biomechanics and control of these vibrations are poorly understood. In this study the muscle within the vocal fold, the thyroarytenoid muscle (TA), was examined for the presence and distribution of slow tonic muscle fibers (STF), a rare muscle fiber type with unique contraction properties. Nine human TAs were frozen and serially sectioned in the frontal plane. The presence and distribution pattern of STF in each TA were examined by immunofluorescence microscopy using the monoclonal antibodies (mAb) ALD-19 and ALD-58 which react with the slow tonic myosin heavy chain (MyHC) isoform. In addition, TA muscle samples from adjacent frozen sections were also examined for slow tonic MyHC isoform by electrophoretic immunoblotting. STF were detected in all nine TAs and the presence of slow tonic MyHC isoform was confirmed in the immunoblots. The STF were distributed predominantly in the medial aspect of the TA, a distinct muscle compartment called the vocalis which is the vibrating part of the vocal fold. STF do not contract with a twitch like most muscle fibers, instead, their contractions are prolonged, stable, precisely controlled, and fatigue resistant. The human voice is characterized by a stable sound with a wide frequency spectrum that can be precisely modulated and the STF may contribute to this ability. At present, the evidence suggests that STF are not presented in the vocal folds of other mammals (including other primates), therefore STF may be a unique human specialization for speech.

Intrinsic Properties of the Adult Human Mylohyoid Muscle: Neural Organization, Fiber-Type Distribution, and Myosin Heavy Chain Expression

“…The specificity and characterization of these mAbs against MHC isoforms have been previously established. The mAbs NOQ7-5-4D is specific to type I MHC [7,23], MY-32 to all fasttype MHCs [7,39,40], SC-71 to type IIa MHC [7,25,40,41], ALD-58 to MHC-ton [7,33,42], BA-G5 to MHC-a [7,35], F1.652 to MHCemb [7,35], and N2.261 to MHC-neo [35].…”

“…The presence of the unusual MHC isoforms in some cranial muscles is most likely associated with their unique functional requirements. For instance, MHC-ton isoform exists only in the muscles that perform fine movements [17,32,33]. The high proportions of the m/u hybrid fibers found in the adult human MH muscle may be suitable for postural and endurance activities of the muscle that are needed to displace and stabilize the mandible and hyoid bones during various upper airway functions.…”

“…However, some adult cranial muscles express some unusual MHC isoforms (i.e., slow-tonic, a-cardiac, and developmental) that are not found in normal adult limb and trunk muscles. For instance, these unusual MHC isoforms have been identified in the extraocular [17][18][19][20][21][22][23], masseter [21,[24][25][26][27][28][29][30][31], middle ear [32], laryngeal [33,34], and digastric [7] muscles. These MHC isoforms are also expressed in the intrafusal fibers of muscle spindles [28,35,36].…”

“…The reported observations suggest that some cranial muscles have intrinsic specializations that appear to be associated with embryonic origin, innervation pattern, and unique functional requirements [30,34]. For example, slow-tonic MHC isoform is present only in the muscles that perform fine movements as required by specific physiologic tasks [17,32,33].…”

The preferential involvement of extraocular muscles by myasthenia gravis

“…Existence of these fibers was thought to be limited largely to EOM, tensor tympani, and stapedius; however, studies of humans indicate that MIFs are also found in laryngeal muscles and the tongue. 20 In mammalian EOM, two MIFs are present, one in each of the anatomic regions.The global MIF has en-grappe endplates along its length and contracts in a graded or 'tonic' fashion. The en-grappe endplates compose a significantly smaller surface area of the muscle and have very shallow synaptic folds compared to the en-plaque endplates, which are similar in morphology to the endplates of other skeletal muscles.…”