Neurobiology of stimulating

the Vagus Nerve


Vagus Nerve Stimulation


Vagus nerve stimulation is a new approach to treating neurological diseases that uses a device sometimes referred to as a ‘neuromodulator’, due to its ability to modulate neural functions. With the typical vagus nerve stimulation procedure, a battery-powered device is surgically implanted in the chest wall, connecting to a wire wrapped around the vagus nerve in the neck, sending micro pulses of electricity along the nerve directly into the brain.



Non-invasive Vagus Nerve Stimulation


Parasym Health have developed a device which is able to non-invasively stimulate the Vagus Nerve using an external electrode which attaches to the tragus of the left ear. This method has been shown to activate the same brain regions and activate vagal fibres in the same way as with invasive stimulation, now making this treatment very accessible and affordable.



What Stage is this Treatment


Vagus nerve stimulation therapy is US FDA approved for the adjunctive treatment of epilepsy and FDA approved for the treatment of medication-resistant depression. Hundreds of thousands of people have benefited from this treatment to date even though there have been high barriers such as surgery, cost (upwards of $40,000 USD), and limited geographical access.  


Treatment Mechanisms and Advantages


This treatment mechanism takes advantage of the natural role of the vagus nerve in communicating information to the brain via electrical signals. This method (tVNS) avoids drug–drug interactions or systemic side effects, making it a promising new choice for accompanying or replacing pharmacological interventions. This treatment has also shown high long-term tolerability in patients.


Neurobiological Findings


One of the most consistent findings of stimulating the vagus nerve  is a diminished activity of the limbic system (the part of the brain that deals with emotional processing) (Henry et al. 1998; Devous 2001) and a decrease of limbic cerebral blood flow (Henry et al. 2004; Zobel et al. 2005). Further, this method shows a marked decrease in activation of the amygdala, hippocampus and parahippocampal gyrus. Through projections to the amygdala, the nucleus of the tractus solitarius (NTS), which receives greatest afferent vagal input, gains access to the amygdala-hippocampus entorhinal cortex of the limbic system (Henry 2002). With the strong association between hyperactivated limbic brain areas and affective disorders like depression or bipolar disorder (Ebert et al. 1994; Malhi et al. 2004), non-invasive VNS can offer a novel therapeutic option by suppression of this hyperactivity.


Suppression of limbic activation also correlates with clinical improvement in depressed patients treated with selective serotonin reuptake inhibitors (Mayberg et al. 2000) and might be a major target of antidepressive neuronal pathways. Furthermore, the state of activation of the neuronal network in the posterior cingulate cortex and precuneus seems to be decisive in conscious awareness and self-reflection (Vogt and Laureys 2005). Non-invasive VNS showed decreased activation of this network, a phenomenon also observed during information processing of complex cognitive tasks (Vogt and Laureys 2005; Gould et al. 2006). Increased activation of the insula during VNS reflects projection from the NTS-nucleus parabrachialis pathway (Henry 2002) and may be a correlate of modulation in cerebral cortical activity (Narayanan et al. 2002).


Improved Cognitive Functions with Vagus Nerve Stimulation


Further research has also shown Improvement in cognitive functions like recognition memory (Clark et al. 1999; Schachter 2004) and enhanced alertness (Clark et al. 1999; Malow et al. 2001) in patients with an implanted VNS device.