Activity of genioglossus motor units over sleep onset and arousal from sleep

The transitions to and from sleep are critical periods for the genioglossus (GG) muscle and for general respiratory activity. Multiunit recordings suggest that the GG is important for patency of the upper airway (UA) as well as being a model for the activity of all muscles in the UA region. Multiunit EMG recordings show phasic GG activity falling at sleep onset, before increasing as nREM sleep progresses. Airway patency, which is attributable to UA muscles, is compromised during sleep and airway occlusion can occur in susceptible individuals. Arousal from sleep leads to an increase in UA muscle activity with inspiratory multiunit recruitment of the GG known to occur. It is thought that arousals are involved in maintaining patency of the UA, with arousals hypothesised to be involved in reopening the airway following an apneic event in sleep disordered breathing (SDB) syndromes such as Obstructive Sleep Apnea (OSA), although an opposing view suggests that arousals are incidental, rather than essential, events. Recent single motor unit (SMU) studies of human GG reveal a complex pattern of muscle activity made up of motor units (MU) whose individual patterns of activities differ in their relation to the respiratory cycle or remain tonically active without respiratory modulation. These MUs have been categorised into six distinct discharge patterns; inspiratory phasic (IP, active only during inspiration), inspiratory tonic (IT, active throughout the respiratory cycle but with peak activity during inspiration), expiratory phasic (EP, active only during expiration), expiratory tonic (ET, active throughout the respiratory cycle but with peak activity during expiration), tonic (TT, active throughout the respiratory cycle without respiratory or other modulation) and tonic other (TO, active throughout the respiratory cycle with variation in their discharge rate, but not in phase with the respiratory cycle). ii Experiment One investigated the individual activity of GG MUs immediately prior to and following the transition from wake to sleep. The hypothesis for Experiment One was that MUs of the GG which are highly correlated with the respiratory cycle (eg. IP, IT, EP, ET) will not be affected by sleep onset, whereas those GG MUs classified as TT will decrease or cease activity at sleep onset. The hypotheses for the experiment derived from the findings of Orem and colleagues, which stated that the rate of respiratory neuron activity in the medulla is greater during nREM sleep for strong respiratory neurons than weak, and that respiratory neurons receiving greater afference from non-respiratory inputs have greater reduction in their activity during sleep (Orem, Osorio et al. 1985). 127 MUs from 61 wake to sleep transitions and 8 subjects were analysed. The most common MUs were those classified as IT, followed by IP, with also a sizeable number of TT spiratory MUs (IT and IP) dramatically reduced their activity at sleep onset, while the activity of MUs without respiratory modulation (TT units), or those with expiratory modulation, were not affected by the EEG transition. Unexpectedly, Experiment One showed that the reductions in inspiratory modulated units were due to individual MUs ceasing activity, rather than reductions in MU firing frequencies. Thus, MUs discharge rates when they were still active was not affected. The overall activity of the GG was decreased due to approximately 50% of IP and IT units ceasing activity at, or in the breaths immediately after, sleep onset, and 20% of IT units becoming IP. The major finding of Experiment One was that GG MU behaviour during a sleep onset transition depends on the MUs discharge pattern during prior wakefulness. However, it remains unclear if this effect is due to differences in MUs, such as in their threshold, or to differences in synaptic input at the level of the hypoglossal motor nucleus. del of the behaviour of respiratory neurons during sleep was discarded as a basis for predicting the behaviour of GG MUs due to the unexpected results of Experiment iii One. Experiment One suggested that changes in inspiratory modulated MUs were responsible for sleep-wake state changes in GG EMG activity. Thus, for Experiment Two, it was hypothesised that GG MUs activity at an arousal from sleep would show that; 1) The increase in the proportion of a breath that IP MUs are active will account for part of the overall increase in GG activity at an arousal, 2) Inspiratory modulated MUs will be recruited at an arousal, 3) All MUs which are active pre-arousal will maintain their pre-arousal discharge rates post-arousal. In Experiment Two three separate approaches were used to analyse the activity of GG MUs at arousals from sleep. First, activity in the three breaths preceding and following each arousal was analysed for the total number of action potentials. Next, SMUs active in the three pre-arousal sleep breaths and the three breaths following the arousal were individually analysed over the arousal using the same methods as Experiment One to assess 1) Unit discharge pattern distribution, and 2) Quantification of these discharge patterns. Finally, units recruited during the three breaths following the arousal were analysed using the same two methods. Using the first approach, the overall activity of the GG was explored by assessing the total number of action potentials per second over the arousal period to allow for comparisons to the conventional multiunit analysis method of GG activity. A large increase in activity following the arousal was seen, with the inspiratory portions of the breath consistently higher than the expiratory portions. This verifies that the overall activity of individual MUs used in this analysis jointly follow a similar pattern described in multiunit GG recordings over the arousal period. Both inspiratory and expiratory GG activity was observed to increase significantly following the arousal. In multiunit analysis, the increases in GG activity following an arousal are attributed to inspiratory recruitment of the muscle (Carlson, Carley et al. 1994), whereas this analysis of the total number of action potentials demonstrates both iv inspiratory and expiratory increases in GG activity following arousals. 138 individual MUs from 92 separate arousals were classified according to their prearousal activity. They were found to be mainly TT or IT, with a considerable proportion of IP, but very few expiratory MUs. 80 recruited MUs were also identified, making up 37.6% of post-arousal GG activity. IP MUs consisted of close to half of the analysed recruited units, and were present in all subjects displaying recruited units. IT units were also common, with lower numbers of TT and, again, very few expiratory units. Following the arousal, most units (67%) active prior to the arousal retained the same classification. However, almost 50% of IP MUs converted to a different MU discharge pattern, usually to IT, but also some to TT MUs, at or immediately following the arousal. TT MUs were again largely unaffected by the transition other than a slight decrease in TT activity. The conversion of IP MUs to IT MUs increased the total activity output of these units. This conversion can account for some of the large increase in GG activity observed in multiunit GG EMG recordings, but is unlikely to be a significant enough to account for the entire increase. Further, conversions to IT units primarily affected the tonic component of GG activity, rather than the phasic component which has been observed to increase in multiunit studies. However, this increase in tonic activity explains why there is an increase seen in the expiratory section of the respiratory cycle, as well as the inspiratory section, in the analysis of total number of action potentials completed in the first analysis of Experiment Two. Many recruited MUs were only briefly active for one or two breaths, nevertheless it is likely that a large portion of the post-arousal increase in activity was due to MUs that were only transiently active. This brief post-arousal activity possibly indicates that there is an influence of a reflex activation pathway on the MUs. Of these briefly active "burst" recruited units, the majority (75%) were phasic, whereas of the recruited units which remained active v for longer than two breaths ("constant" units), 78% were TT or respiratory modulated tonic. Just as the increases in tonically active units following the arousal explains the overall increase in GG activity (particularly why expiratory activity as well as inspiratory activity increases), this large recruitment of IP MUs can explain the larger increase in the inspiratory portion of the breath (Experiment Two, Analysis of Total Number of Action Potentials). In addition, approximately 20% of MUs active pre-arousal became silent in the immediate postarousal period, which combined with the decrease in TT MU firing rates, suggests a reorganisation of muscle activity. In summary, the GG muscle is composed of MUs that have a range of different discharge patterns. Sleep onset and arousal transitions profoundly alter their inspiratory modulation. Inspiratory modulated units were found to make up approximately 60% of the units active during quiet wakefulness. Sleep onset reduced the firing duration of these units resulting in many ceasing activity entirely, but without affecting their discharge rate when still active. Conversely, TT units and those with an expiratory pattern were unaffected by sleep onset. Arousal from sleep resulted in IP units increasing their peak frequency rate slightly and regularly converting to an IT pattern, thus increasing their overall activity. At the same time, TT units were found to have a small decrease in activity at, and in the breaths following, the arousal. Units recruited at the arousal made up 37.6% of total post-arousal activty and were largely of the IP type. Finally, many units, 15.9% of total pre-arousal activity als