OBJECTIVES: To analyze characteristic features and details on motor-evoked potentials (MEPs) of the cricothyroid and vocalis muscles from single-pulse cortical transcranial magnetic stimulation (TMS) in normal subjects to characterize cortical motor representation of laryngeal muscles. STUDY DESIGN: Prospective, experimental investigation on healthy volunteers. METHOD: MEPs of the cricothyroid and vocalis muscles elicited by cortical TMS with a figure-8-shaped coil were investigated in two groups of six healthy subjects each, with special regard to MEP amplitude as a function of the coil position on the head surface along the interaural line. RESULTS: Bilateral reproducible responses of the cricothyroid and the vocalis muscles could be observed in all subjects. For the cricothyroid muscle, maximal responses were obtained at mean stimulus positions of 7.5 +/- 1.4 cm (contralateral) and of 7.3 +/- 1.3 cm (ipsilateral), respectively. For the vocalis muscle, we found maximal responses at mean stimulus positions of 10.3 +/- 1.9 cm (contralateral) and of 9.6 +/- 1.6 cm (ipsilateral), respectively. Despite a considerable overlap of these coil positions, from which reproducible MEPs could be elicited in both groups of the laryngeal muscles, statistically significant separation of the cricothyroid-and vocalis-associated cortical representation areas was possible. CONCLUSIONS: Our observations point to two different cortical motor representation areas, with the cricothyroid muscle-related area being located more medially.
In order to better understand the descending voluntary vocal control pathway, the efferent subcortical projections of the laryngeal motorcortex were studied in the rhesus monkey (Macaca mulatta). For this purpose, the left motorcortex was exposed in three animals under narcosis. By electrical brain stimulation, sites were identified yielding vocal fold adduction. Effective sites were injected with the anterograde tracer biotin dextran amine. Subcortical projections could be traced within the forebrain to the putamen, caudate nucleus, claustrum, zona incerta, field H of Forel and a number of thalamic nuclei, with the heaviest projections to the nuclei ventralis lateralis, ventralis posteromedialis, including its parvocellular part, medialis dorsalis, centralis medialis, centrum medianum and reuniens. In the midbrain, labeling was found in the deep mesencephalic nucleus. In the lower brainstem, fibers terminated in the pontine and medullary reticular formation, locus coeruleus, nucleus subcoeruleus, medial parabrachial nucleus, nucleus of the spinal trigeminal tract, solitary tract nucleus and facial nucleus. No projections were found to the nucl. ambiguus. The fact that monkeys, in contrast to humans, lack a direct connection of the motorcortex with the laryngeal motoneurons suggests that this connection has evolved in the last few million years and might represent one of the factors that made speech evolution possible.
The efferent cortico-cortical projections of the motorcortical larynx area were studied in three rhesus monkeys (Macaca mulatta), using biotin dextranamine as anterograde tracer. Identification of the larynx area was made with the help of electrical brain stimulation and indirect laryngoscopy. Heavy projections were found into the surrounding ventral and dorsal premotor cortex (areas 6V and D), primary motor cortex (area 4), the homolog of Broca's area (mainly area 44), fronto- and parieto-opercular cortex (including secondary somatosensory cortex), agranular, dysgranular and granular insula, rostral-most primary somatosensory cortex (area 3a), supplementary motor area (area 6M), anterior cingulate gyrus (area 24c) and dorsal postarcuate cortex (area 8A). Medium projections could be traced to the ventrolateral prefrontal and lateral orbital cortex (areas 47L and O), the primary somatosensory areas 3b and 2, the agranular and dysgranular insula, and the posteroinferior parietal cortex (area 7; PFG, PG). Minor projections ended in the lateral and dorsolateral prefrontal cortex (areas 46V and 8B), primary somatosensory area 1 and cortex within the intraparietal sulcus (PEa) and posterior sulcus temporalis superior (TPO). Due to its close spatial relationship to the insula on the one hand and the premotor cortex on the other, the larynx area shows projections which, in some respects, are not typical for classical primary motor cortex.