The Tongue – A Portal for Neuromodulationby Ajay Rajaram, MBBS, MTech | April 7, 2013
The human tongue does more than just taste food. Of course, one needs the tongue for better pronunciation of words and effective communication. Surprisingly the tongue is also a valuable channel for communication with one’s brain. An electrolyte-rich saliva and a high density of sensory nerve receptors on the tongue enable this communication. With the nerve-endings of the tongue being superficial, this communication is completely non-invasive.
But what is the need for this communication and why is it done? The answer here is a recent and rather lengthy term: Cranial Nerve Non-Invasive Neuromodulation (CN-NINM). Neuromodulation, in general, implies the use of certain techniques that help stimulate nerves or certain brain centers for probable reorganization and functional restoration (neuroplasticity) of neurons. It could involve implantation of pulse generators in the brain, electrical stimulation of nerves, or maybe, even stimulation of the tongue. Branches of the fifth and seventh cranial nerves have sensory input in the anterior tongue. The brainstem centers of both these cranial nerves (trigeminal and solitary nuclei, respectively) are in close proximity to the vestibular nuclei, which regulates the function of balance. Thus, the targeted electric stimulation of the tongue, and thereby the fifth and seventh cranial nerves, induces the modulation of the vestibular nuclei in the brain. To put it simply, stimulation of the tongue could influence the person’s balance.
The neuromodulation of these cranial nerves paves way for a novel and non-invasive therapeutic modality for motor-related symptoms of diseases such as Parkinson’s disease, traumatic brain injury (TBI), and even multiple sclerosis. The anterior aspect of the tongue is stimulated using an electrode array that contains many gold-plated electrodes. The stimulation consists of low voltage square-pulse bursts, which produces a sensation similar to the feeling of drinking a carbonated beverage. In a clinical study, nine CN-NINM stimulations over a period of one week were delivered to patients with chronic balance dysfunction. An “optic flow” produced by creating relative motion of the surfaces and edges on a checkerboard image can induce the sensation of egomotion, increasing one’s postural responses. These patients were put through an optic flow stimuli designed to induce postural sway for the study, which in turn activates the brain centers related to balance processing. Functional MRI (fMRI) imaging was also performed during the visual stimuli test to ascertain the region of stimulation in the brain. A comparison of their pre- and post-stimulation postural sway tests showed that all patients had reductions in their overall sway to the optic flow after just a week of CN-NINM intervention. The images acquired through fMRI showed that the stimulated regions were superimposed over the anatomical location of centers for balance processing.
Yet another exciting observation with CN-NINM is that its neuromodulatory effect is sustained and persists even after turning off the low-voltage electrical neurostimulation. This contrasts with most other electric stimulation techniques, such as deep brain stimulation (DBS) or vagal nerve stimulation, where the effects disappear on switching off the stimulator. It is thus certain that CN-NINM induces neuroplastic changes in the brain, enabling better integration of neuronal signals. In another study it was found that similar electrical stimulation could also activate embryonal stem cells to differentiate into neuronal fates.
Non-invasive neuromodulation with long-lasting beneficial effects can be translated into home-based therapies for neurological disorders with minimal medical supervision. With funding from the US Army, Advanced Neurorehabilitation LLC (Madison, WI) has developed a Portable Neuromodulation Stimulator (PoNS) intended for patients with TBI. With ongoing clinical studies, the US Army is hoping to get an approval for this handheld device from the US Food and Drug Administration (FDA) soon. With neurostimulator devices like the PoNS, the tongue becomes yet another portal of entry to the brain!
Wildenberg JC, Tyler ME, Danilov YP, Kaczmarek KA, & Meyerand ME (2010). Sustained cortical and subcortical neuromodulation induced by electrical tongue stimulation. Brain imaging and behavior, 4 (3-4), 199-211 PMID: 20614202
Yamada M, Tanemura K, Okada S, Iwanami A, Nakamura M, Mizuno H, Ozawa M, Ohyama-Goto R, Kitamura N, Kawano M, Tan-Takeuchi K, Ohtsuka C, Miyawaki A, Takashima A, Ogawa M, Toyama Y, Okano H, & Kondo T (2007). Electrical stimulation modulates fate determination of differentiating embryonic stem cells. Stem cells (Dayton, Ohio), 25 (3), 562-70 PMID: 17110622
Ellen CJ (Feb 14 2013). U.S. Army Medical Research and Materiel Command.
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