Ludlow and PolettoCentral Conduction Time Asymmetries
Central Accommodation for Differences in the Length of Recurrent Laryngeal Nerves
Christy L. Ludlow, Ph.D
Christopher Poletto, Ph.D.
Laryngeal and Speech Section, NINDS-NIH, Bethesda MD
The difference in length of the left and right recurrent laryngeal nerves results in different latencies of evoked muscle responses with transcranial magnetic stimulation (TMS) of the vagus where it emits from the skull (1). There may be central asymmetries in conduction time (2) to allow for precise bilateral control of vocal fold opening and closing during voice onset and offset for speech sounds despite these length differences. In 1998, Roubeau (3) noted a difference in the central conduction times in the medulla between the right and left sides when evoking the laryngeal adductor reflex (LAR) in the thyroarytenoid (TA) muscles by electrical stimulation of the internal branch of the superior laryngeal nerve (iSLN) in the pig. The right to left central conduction time was shorter than left to right conduction time (Figure 1). A similar study in the pig (4) also reported asymmetrical mean latencies for LAR with iSLN (Table 1).
The purpose of this study was to examine data from 3 studies to address this issue in both cats and human normal volunteers to determine whether there are central conduction time asymmetries a) at the level of the medulla in both humans and cats, and b) in the corticobulbar tracts in the human.
First, in the cat, electrical iSLN stimulation on the right and on the left were used to measure ipsilateral and contralateral TA LAR latencies in two groups from a previous study (5) to confirm differences in central conduction times between the two sides in the medulla (Table 2).
Second, in 5 humans we used TMS over the M1 region in the cortex to evoke direct TA muscle responses on both right and left sides and TMS at the right and left mastoid to stimulate the vagus as it emits from the skull. To estimate the central conduction time, the latency of the TA response from TMS mastoid stimulation was subtracted from the TA latency during cortical M1 TMS stimulation for the same muscle (Table 3).
Using electrical stimulation of the iSLN in humans, we measured latencies of ipsilateral and contralateral responses when one group was stimulated on the right and others were stimulated on the left using both previous (6, 7) and unpublished data (Table 4).
Preliminary Results
We confirmed asymmetries of LAR response latencies from the right and left medulla in the cat. The right to left central conduction time was less than the left to right conduction time. In the human however, preliminary findings do not indicate the same asymmetries in central conduction time at the level of the medulla during iSLN stimulation to evoke TA LARs. The TMS central conduction times were asymmetric, however, with a shorter right cortex to left medulla than left cortex to right medulla. This resulted in less difference in cortical to evoked muscle latencies between the left and right sides than during mastoid stimulation. The results suggest that during development as the left recurrent nerve lengthens with descent of the aortic arch, more rapidly conducting fibers are preferentially retained in the right to left than in the left to right corticobulbar tract (8, 9).
References
1.Sims S, Yamashita T, Rhew k, Ludlow CL. An evaluation of the use of magnetic stimulation to measure laryngeal muscle response latencies in normal subjects Otolaryngology-Head and Neck Surgery 1996;114:761-767.
2.Peters M. Cerebral asymmetry for speech and the asymmetry in path lengths for the right and left recurrent nerves. Brain Lang 1992;43:349-352.
3.Roubeau B, Lefaucheur JP, Moine A, Lacau St Guily J. Asymmetry of the laryngeal reflex responses to superior laryngeal nerve stimulation unrelated to the length of the recurrent nerves in the porcine model. Acta Otolaryngol 1998;118(6):882-886.
4.Sasaki CT, Ho S, Kim YH. Critical role of central facilitation in the glottic closure reflex. Ann Otol Rhinol Laryngol 2001;110(5 Pt 1):401-405.
5.Ambalavanar R, Purcell L, Miranda M, Evans F, Ludlow CL. Selective suppression of late laryngeal adductor responses by N-Methyl-D-Asparate receptor blockade in the cat. J Neurophysiol 2002;in press.
6.Ludlow CL, Schulz GM, Yamashita T, Deleyiannis FW. Abnormalities in long latency responses to superior laryngeal nerve stimulation in adductor spasmodic dysphonia. Ann Otol Rhinol.Laryngol. 1995;104:928-935.
7.Ludlow CL, VanPelt F, Koda J. Characteristics of late responses to superior laryngeal nerve stimulation in humans. Ann Otol Rhinol Laryngol 1992;101:127-134.
8.Wolpaw JR, Kaas JH. Taking sides: corticospinal tract plasticity during development. Neurology 2001;57(9):1530-1531.
9.Eyre JA, Taylor JP, Villagra F, Smith M, Miller S. Evidence of activity-dependent withdrawal of corticospinal projections during human development. Neurology 2001;57(9):1543-1554.
Figure 1. RightLeft
Table 1. Central asymmetries in the medulla in previous studies in the pig
Central Conduction time
13.95 / 13.1 / 0.85
Right iSLN to left TA / Left iSLN to left TA / Right to Left
Central Conduction time
15.85 / 15.7 / 0.15
Sasaki et al., 2001 / Left iSLN to right TA / Right iSLN to right TA / Left to Right
Central Conduction time
14.7 / 13.3 / 1.4
Right iSLN to left TA / Left iSLN to left TA / Right to Left
Central Conduction time
14.9 / 14.7 / 0.20
Table 2. Central asymmetries in the medulla in current study in the cat
Left iSLN to right TA / Right iSLN to right TA / Left to RightCentral Conduction time
11.37 / 9.19 / 2.18
Right iSLN to left TA / Left iSLN to left TA / Right to Left
Central Conduction time
12.38 / 11.00 / 1.38
Table 3. Central corticobulbar asymmetries in the human
Left cortex to right TA / Right mastoid to right TA / Left to right corticobulbarCentral Conduction time
14.242 / 9.011 / 5.231
Right cortex to left TA / Left mastoid to left TA / Right to left corticobulbar
Central Conduction time
13.278 / 10.911 / 2.367
Table 4. Central conductions times in the medulla in the human (preliminary data)
Left iSLN to right TA / Right iSLN to right TA / Left to RightCentral Conduction time
20.7 / 17.34 / 3.36
Right iSLN to left TA / Left iSLN to left TA / Right to Left
Central Conduction time
26.16 / 21.94 / 4.22
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