Our study systematically evaluated the role of posterior fossa activation with TMS on pharyngeal motor pathway excitability. Although there have been several studies that have demonstrated activation of limb muscles with cerebellar TMS, this study was the first to explore the physiologic role of cerebellar activity with muscles involved in swallowing. Studies of hand muscle responses have indicated that when the cerebellum is conditioned by magnetic stimuli, cortico-thenar MEPs are inhibited, suggesting a suppression of excitability.19,26,27 In contrast, our study has demonstrated that stimulation of the cerebellum seems to facilitate the excitability of the motor output from the pharyngeal area of the motor cortex. These findings merit further consideration.
Direct effects of cerebellar stimulation
We found that the optimal distance to stimulate either cerebellar hemisphere was ∼4 cm lateral to the inion. Moreover, co-registration of cerebellar stimulation sites with magnetic resonance imaging in two subjects confirmed close proximity of the magnetic coils to the cerebellum. As there is no comparable cerebellar TMS study of the swallowing muscles, and indeed, there remains controversy concerning the precise role of the cerebellum in controlling swallowing, this information will be helpful in guiding future studies of cerebellar stimulation in health and disease. Of relevance, although cerebellar stimulation of vermis or hemisphere evoked similar pharyngeal MEPs responses, they were consistently smaller when compared with cortical stimulation, despite no inter-brain site difference in latency. One explanation for the relatively long latency seen to cerebellar stimulation might lie in the pauci-synaptic pathway that a cerebellar action potential may have to take to reach swallowing muscular, possibly via brainstem structures and hind-brain nuclei.28 Our data do not support the contention that a very fast and direct pathway exists between the cerebellum and the pharynx, which may be important in considering how the cerebellum and swallowing control mechanisms interact.
Comparison of cerebellar and cranial nerve responses
Stimulation of the scalp overlying the posterior fossa has become an established method of activating the cerebellum. However, several authors have raised the probability of extra-cerebellar activation that may be responsible for the resulting MEP. Of relevance, there is no evidence from hand muscle studies that it is possible to stimulate the cortico-spinal fibers directly, so activating cortico-bulbar fibers in the brainstem seems unlikely. Moreover, direct brainstem stimulation to TMS seems unlikely for the following reasons: Firstly, the TMS magnetic field is unlikely to penetrate deeply into brain tissue. Most studies have suggested that TMS can only penetrate to between 25 and 30 mm beneath the scalp. Given that the first structure beneath the coil would be the cerebellum, it seems likely that most of the field would be dissipated by this organ. Secondly, the figure of eight coil used in our study does not generate a highly focused, high field stimulus that one might need for brainstem stimulation. Nonetheless, stimulating afferent inputs to swallowing centers could provoke a short latency reflex response, although this appears to be very different in morphology and latency from direct trigeminal reflexes. However, Meyer et al., showed that stimulation of the posterior fossa in patients with complete hemi-cerebellar agenesis and cerebellar infarction was still able to modulate cortical motor responses originating contralaterally to the site of stimulation.29 In addition, Meyer et al. showed that scalp muscles at a distance from the stimulation site can also be concomitantly activated with TMS over the posterior fossa, thus persuading the author to propose the possibility of concurrent stimulation of the brainstem or cranial nerves such as the trigeminal nerve. Taking this into account, we decided to carefully evaluate the contribution of the trigeminal input in resulting PMEPs.
Magnetic stimulation of a trigeminal nerve branch produced PMEP morphology and latency similar to that described earlier by Hamdy et al.20 These responses are different from the short latency or elementary reflexes described in animal studies to afferent stimulation.30 The MEP amplitude from late pharyngeal responses to cranial nerve stimulation was lower in this study compared with that found by Hamdy et al., but this is probably explained by the lower stimulation intensity used during our protocol (110% in this study vs 120% stimulator output by Hamdy et al.). In view of the proximity of the brainstem and cranial nerve pathways to the cerebellum, it might be expected that TMS over the posterior fossa could also stimulate some extra-cerebellar structures. It is thus reassuring that pharyngeal MEP amplitude, latency, and morphology elicited from stimulation of the cerebellum was clearly different from that produced by the stimulation of the trigeminal nerve.
There are of course, limitations on using trigeminal nerve stimulation to reflect brainstem involvement during cerebellar stimulation, namely the probability of different circuitries and mechanisms. However, even after considering these limitations, this study appears to suggest that TMS stimulation of the posterior fossa is not likely to primarily induce brainstem stimulation and hence effects on pharyngeal excitation. Indeed, only invasive monitoring of brainstem activity can confidently exclude the direct involvement of the brainstem in modulating the swallow motor pathways during TMS of the cerebellum.
Preconditioning with cerebellar stimulation facilitates pharyngeal responses
We found that cerebellar stimulation was facilitatory to the pharyngeal motor evoked responses and that the maximal facilitation of pharyngeal motor cortex following preconditioning with cerebellar stimulation occurred at ISIs of between 50 and 200 ms. Moreover, there was concomitant shortening of latency by cerebellar stimulation at equivalent ISIs. Added to this, when the effects of trigeminal stimulation were subtracted from that of cerebellar stimulation, we still observed a potent effect of cerebellar stimulation in facilitating pharyngeal motor responses. Contrary to other studies 19,26,27 of the hand motor cortex, our study showed that cerebellar stimulation was facilitatory to the pharyngeal motor cortex. Therefore, why should cerebellar stimulation be excitatory to the swallowing system, but inhibitory to the hand system?
One possible reason may reflect methodological differences. For example, in the hand studies, only a relatively short range of ISIs (3–20 ms) was tested. Hence, it is conceivable that investigation of longer ISIs may reveal a different outcome. In fact, this was supported by Daskalakis et al. who showed that cerebellar stimulation can both excite and inhibit neurons in the human motor cortex.31 However, a more plausible explanation may relate to the likelihood of the different neural network connecting the cerebellum to the swallow muscles in comparison with hand muscle. In hand muscle, for example, studies of cortical activity of areas representative of hand in primates 32 and retrograde transneuronal viral studies in primates 33 both suggest that deep cerebellar nuclei such as the dentate nuclei have disynaptic excitatory pathways to the motor cortex via the ventral thalamus.28 In addition, stimulation of the cerebellum activates Purkinje cells in the cerebellar cortex, which in turn exert an inhibitory effect on deep cerebellar nuclei that are the source of cerebellar outflow neurons.17 Pinto et al. also showed that conditioning stimulation of the cerebellum had no influence on hand MEP latency, while inhibiting the motor cortex,28 thus suggesting that the cerebellum-hand neutral network is unlikely to receive significant modulation by brainstem nuclei. This is contrary to findings of our study, which found significant shortening of latency of pharyngeal MEP, thus highlighting the likelihood of the cerebellar-pharynx network, possibly involving brainstem interneurons. Taken together, the above evidence suggests that the involvement of the brainstem and its pattern generator in the swallowing neural network may explain the contrasting effects of cerebellar stimulation when comparing the swallow and hand systems.
Hence, a key question is, at what level do the conditioning pulses of the cerebellum facilitate cortico-pharyngeal MEPs (cortex, brainstem or both)? Intriguingly, apart from animal studies,32,34,35 there have been no studies in humans that have investigated the motor output tracts from the cerebellum to help answer this question. Thus, one explanation for this question may come from some key principles in neurophysiology.36 If a brainstem motor neuron is activated by TMS, its discharge threshold will fall, with consequent shortening of the MEP latency to cortical stimulation by temporal summation of the excitatory postsynaptic potentials. Conversely, if the cortex is excited while the brainstem motor nucleus is at resting levels of activation, this will result in an increase in the amplitude of cortically evoked response without a large effect on latency.
Our study demonstrated that the application of cerebellar preconditioning increased the amplitude of PMEPs, while greatly shortening its latency, suggesting that modulation was occurring within brainstem circuitry. However, in the absence of direct recordings to confirm brainstem and cortical activity, these assumptions must remain speculative, and do not exclude a direct or indirect cortical effect.
It is interesting to note that with cerebellar preconditioning, significant reduction in latency of cortically induced pharyngeal MEP begins to occur at 20 ms ISI, just prior to the increase in amplitude that begin at 50 ms ISI. This may reflect the evolving sequence of events triggered by cerebellar stimulation as indicated by a fall in threshold of the brainstem motor neuron (reduction in latency) followed by excitation of the pharyngeal motor cortex (increase in amplitude). Alternatively, the facilitation effect may be somehow linked to the time taken for peripheral inputs to reach the cortex. For example, evidence from the swallow literature suggests that pharyngeal motor cortex is critically responsive to stimulation applied at a frequency of 5 Hz, which translates to trains of stimuli with 200 ms ISIs.37 Moreover, electrical stimuli to the pharynx seem optimally effective for inducing excitability in the pharyngeal motor cortex at 5 Hz.38 Further evaluation by the same group with paired pulse techniques also confirmed that maximal facilitation of TMS evoked PMEPs occurred when pharyngeal electrical stimulation was followed by cortical stimulation at ISIs between 50 and 100 ms (in a test range of 10–100 ms).39 Additional support of this significant ISI comes from the application of repetitive TMS at 5 Hz, which was found to be optimal at inducing long-term facilitation of the swallow motor cortex.37,40 This current paired pulse study may also help us postulate that using high frequency rTMS (between 5 and 20 Hz) to stimulate the cerebellum may be effective at inducing longer term excitation in the swallowing network. However, future studies will be required to confirm this assertion.
In conclusion, our findings suggest that magnetic stimulation of the cerebellum can evoke motor responses within the pharynx. Secondly, the application of stimulation to the cerebellum as a conditioning stimulus is facilitatory to the swallow motor pathway, this process being time dependent. In particular, these observations provide parameters suitable for future studies designed to explore manipulations to the swallow motor network following cerebellar stimulation. These findings may contribute to the application of neurostimulation to the cerebellum for the purpose of exploiting recovery mechanisms in dysphagia following brain injury.