Muscle receptors: content of some of the extrinsic and intrinsic muscles of the larynx in the bonnet monkey (Macaca radiata)

Density of muscle spindles in prosimian shoulder muscles reflects locomotor adaptation

We examined the correlation between the density of muscle spindles in shoulder muscles and the locomotor mode in three species of prosimian primates: the slow loris (Nycticebus coucang), Garnett's galago (Otolemur garnettii), and the ring-tailed lemur (Lemur catta). The shoulder muscles (supraspinatus, infraspinatus, teres major, teres minor, and subscapularis) were embedded in celloidin and cut into transverse serial thin sections (40 microm); then, every tenth section was stained using the Azan staining technique. The relative muscle weights and the density of the muscle spindles were determined. The slow loris muscles were heavier and had sparser muscle spindles, as compared to Garnett's galago. These features suggest that the shoulder muscles of the slow loris are more adapted to generating propulsive force and stabilizing the shoulder joint during locomotion and play a less controlling role in forelimb movements. In contrast, Garnett's galago possessed smaller shoulder muscles with denser spindles that are suitable for the control of more rapid locomotor movements. The mean relative weight and the mean spindle density in the shoulder muscles of the ring-tailed lemur were between those of the other primates, suggesting that the spindle density is not simply a consequence of taxonomic status.

Laryngeal muscle responses to mechanical displacement of the thyroid cartilage in humans

Speakers may use laryngeal sensory feedback to adjust vocal fold tension and length before initiating voice. The mechanism for accurately initiating voice at an intended pitch is unknown, given the absence of laryngeal muscle spindles in animals and conflicting findings regarding their existence in humans. Previous reports of rapid changes in voice fundamental frequency following thyroid cartilage displacement suggest that changes in vocal fold length modulate laryngeal muscle contraction in humans. We tested the hypothesis that voice changes resulting from mechanical perturbation are due to rapid responses in the intrinsic laryngeal muscles. Hooked wire electrodes were used to record from the thyroarytenoid, cricothyroid, and sternothyroid muscles along with surface electrodes on the skin overlying the thyroid cartilage in 10 normal adults. Servomotor displacements produced consistent changes in the subjects' vocal fundamental frequency at 70-80 ms, demonstrating changes in vocal fold length and tension. No simultaneous electromyographic responses occurred in the thyroarytenoid or cricothyroid muscles in any subjects. Instead, short-latency responses at 25-40 ms following stimulus onset occurred in the sternothyroid muscles, simultaneous with responses in the surface recordings. The sternothyroid responses may modulate long-latency changes in voice fundamental frequency (approximately 150 ms). The absence of intrinsic laryngeal muscle responses is consistent with a lack of spindles in these muscles. Our results suggest that other sensory receptors, such as mucosal mechanoreceptors, provide feedback for voice control.

Monosynaptic circuitry of trigeminal proprioceptive afferents coordinating jaw movement with visceral and laryngeal activities in rats

Jaw movement is intimately related to various oromotor and visceral functions such as feeding, swallowing, vocalization, respiration and cardiac function. Neuronal circuitry that links jaw movement and these visceral and oromotor functions is largely unknown. The purpose of this study is to determine whether the trigeminal proprioceptive and jaw-muscle spindle afferents send axons to and synapse with the motoneurons innervating visceral organs and laryngeal muscles utilizing multiple double-labeling and physiological approaches. Double labeling of anterograde tracing combined to retrograde transport was performed by injection of biotinylated dextran amine into the mesencephalic trigeminal nucleus and cholera toxin B subunit or horseradish peroxidase into the vagus nerve and the recurrent laryngeal nerve. Mesencephalic trigeminal nucleus neuronal terminals contacted with visceral and laryngeal muscle motoneurons in the ambiguus nucleus and the nearby intermediate reticular zone. By electron microscopic observation, we confirmed that mesencephalic trigeminal nucleus terminals made asymmetric axodendritic synapses with these motoneurons. Double labeling of physiologically characterized jaw muscle spindle afferent neurons combined with anti-choline acetyltransferase immunohistochemistry showed that jaw-muscle spindle afferent boutons closely contacted choline acetyltransferase-immunoreactive motoneurons in the ambiguus nucleus and intermediate reticular zone. This report is the first to demonstrate that the trigeminal proprioceptive afferents synapsed upon visceral and laryngeal muscle motoneurons and provide neuronal networks for the jaw-visceral and jaw-laryngeal coordination.

Synaptic organization of monosynaptic connections from mesencephalic trigeminal nucleus neurons to hypoglossal motoneurons in the rat

Synaptological characteristics of synapses between axonal boutons of the trigeminal mesencephalic nucleus (Vme) neurons and the hypoglossal nucleus (XII) motoneurons (MNs) were studied using biotinylated dextran amine (BDA) anterograde labeling combined with horseradish peroxidase (HRP) retrograde transport in the rat. BDA was initially iontophoresed into Vme unilaterally and 7 days later HRP was injected into the anterior two-thirds of the ipsilateral tongue. After histochemical reactions, BDA anterogradely labeled boutons were seen to appose closely to somata and dendrites of HRP retrogradely labeled MNs in XII by light microscopy. A total of 212 BDA-labeled Vme boutons were examined ultrastructurally, which had an average diameter of 1.3 +/- 0.4 microm and contain small clear spherical vesicles. Eighty-eight percent of Vme boutons (187/212) synapsed on dendrites of HRP-labeled XII MNs. Twenty-five Vme boutons (25/212, 12%) made synapses with somata of HRP-labeled XII MNs. Thirty-five percent (74/212) of BDA-labeled Vme boutons were also contacted by unlabeled P-type terminals. Presynaptic P-type terminals contained spherical (47%, 35/74), pleomorphic (43%, 32/74), and flattened (10%, 7/74) synaptic vesicles. Thus, P-type terminals (as a presynaptic element), BDA-labeled Vme boutons, and XII MNs constitute axoaxodendritic and axoaxosomatic synaptic triads. There are four types of synaptic microcircuits in XII neuropil: synaptic convergence, synaptic divergence, presynaptic inhibition synaptic circuits, and feedforward regulation circuits. This detailed ultrastructure examination of the synaptic organization between Vme neurons and XII MNs provides insights into the synaptic mechanisms of the trigeminal proprioceptive afferents involved in the jaw-tongue reflex and coordination during oral motor behaviors.

Light and electron microscopic observations of a direct projection from mesencephalic trigeminal nucleus neurons to hypoglossal motoneurons in the rat

A direct projection from rat mesencephalic trigeminal nucleus (Vme) neurons to the hypoglossal nucleus (XII) motoneurons was studied using a double labeling method of anterogradely biotinylated dextran amine (BDA) tracing combined with retrogradely horseradish peroxidase (HRP) transport at both light and electron microscopic levels. BDA was iontophoresed unilaterally into the caudal Vme, and 7 days later HRP was injected into the ipsilateral tongue to label hypoglossal motoneurons. The BDA-labeled fibers were seen descended along Probst' tract and were traced to the caudal medulla. In this course, the fibers gave off axon collaterals bearing varicosities in the trigeminal motor nucleus (Vmo), the parvicellular reticular formation (PCRt), the dorsomedial portions of the subnuclei of oralis (Vodm) and interpolaris (Vidm) and in the XII ipsilaterally. The labeling of terminals was most dense in the PCRt at the levels of caudal pons and rostral medulla, which displayed a "dumbbell-shaped" form in the transverse planes. In the XII, labeled terminals were distributed mainly in the dorsal compartment of the nucleus. One hundred sixty-eight appositions made by BDA-labeled terminals on HRP-labeled motoneurons were seen in the dorsal compartment (71%) and in the lateral subcompartment (24%) of the ventral XII. Under electron microscopy BDA-labeled boutons containing clear, spherical synaptic vesicles were found to form synaptic contacts with the somata and dendrites of hypoglossal motoneurons with asymmetric specializations. The present study provides new evidence that the trigeminal proprioceptive afferent neurons terminate in the XII and make synaptic contacts with their motoneurons.

Short-latency changes in voice F0 and neck surface EMG induced by mechanical perturbations of the larynx during sustained vowel phonation

Nineteen healthy young adult males with normal voice and speech attempted to sustain the vowel /u/ at a constant pitch (target: 180 Hz) and a constant and comfortable loudness level while receiving a sudden mechanical perturbation to the larynx (thyroid prominence) via a servo-controlled probe. The probe moved toward or away from the larynx in a ramp-and-hold fashion (3.3-mm displacement, 0.7 N force, 20-ms rise time, 250-ms duration) as the subjects attempted to maintain a constant probe-larynx pressure. Eighty stimuli were applied in each direction, one stimulus per phonation. Pairs of surface electromyography (EMG) electrodes were attached to the skin of the anterior neck over laryngeal, infralaryngeal, and supralaryngeal areas. The rectified EMG signals, the voltage analog of the voice fundamental frequency (VAF0), and the voltage analog of the probe displacement were digitized and signal-averaged relative to the onset of the stimulus. Sudden perturbation of the larynx induced an instantaneous decrease or increase in VAF0, depending on the direction of the probe's movement, and a short-latency increase in the EMG (30-35 ms) and VAF0 (55-65 ms). We argue that the instantaneous VAF0 change was related to a mechanical effect, and the short-latency VAF0 and EMG changes to reflexogenic effects-the latter most likely associated with both intrinsic and extrinsic laryngeal sensorimotor mechanisms. Further physiological studies are needed to elucidate the sources of the VAF0 and EMG responses. Once elucidated, the present method may provide a powerful noninvasive tool for studying laryngeal neurophysiology. The theoretical and clinical implications of the present findings are addressed.

Motor neurons of the laryngeal nerves

The present study was undertaken to determine the relationship between the motor neurons of the superior and recurrent laryngeal nerves within the nucleus ambiguus. The retrograde transport of horseradish peroxidase was utilized to identify the motor neurons subsequent to its application to the proximal transected end of the superior and recurrent laryngeal nerves. Labeled superior laryngeal motor neurons were distributed ventrolaterally in the rostral portion the nucleus. The recurrent laryngeal motor neurons were distributed throughout the nucleus with two distinct populations: a rostral group and a caudal group. The rostral group overlaps the motor neurons of the superior laryngeal nerve. The caudal group occupies that portion of the nucleus that is classically described for the recurrent laryngeal nerve. Additional superior laryngeal nerve labeled perikarya were found in the dorsal motor nucleus of the vagus. This study defines the rostral distribution of the recurrent laryngeal nerve motor neurons and suggests that this rostral group is a component of the neuroanatomical substrate that is involved in the co-activation of the laryngeal abductors controlling the laryngeal aperture.