Phrenic motor neuron activation using temporal interference stimulation
Authors: Sunshine Michael D. , Cassarà Antonino M. , Neufeld Esra M. , Grossman Nir , Otto Kevin J. , Boyden Edward S. , Fuller David D. 
Published: April 2020

Based on deep brain stimulation data, we are using a method called temporal interference (T‐I) stimulation to activate phrenic motor neurons in adult rats. The premise is to target the deep ventral motor pools of the spinal cord with two high frequency but low amplitude electrical waveforms. The waveforms are delivered at kilohertz frequencies that are well above values that directly stimulate neurons, and amplitudes insufficient to activate neurons until the electric field combine in the tissue. The frequency of the two waveforms is offset by a small amount (e.g. 1–5 Hz), and where the two electrical fields combine to form a difference‐frequency time‐modulated signal in the tissue, neuronal populations are recruited. This work tested the hypotheses that 1) T‐I stimulation using epidural electrodes can activate phrenic motoneurons via direct depolarization, and 2) T‐I stimulation using percutaneously electrodes in the neck can activate the diaphragm and sustain breathing after opioid overdose. For epidural stimulation, electrodes were placed at C3 and experiments were conducted in ventilated, urethane anesthetized adult rats. Initial experiments showed that a pair of electrodes, one medial and one lateral, over the left hemicord was sufficient to enable diaphragm activation via T‐I. Intraspinal delivery of CNQX and AP5 was used to block excitatory synaptic inputs to phrenic motoneurons, confirmed by a complete cessation of endgoenous diaphragm activity. Under these conditions, epidural T‐I (5000Hz and 5001Hz 360–1800μA) could activate the diaphragm, but the response was attenuated by 50–75% as compared to pre‐drug conditions. Blocking inhibitory synaptic inputs using strychnine and bicuculline increased the impact of epidural T‐I by 100–170%. Opioid overdose (fentanyl, 30mcg/kg) in spontaneously breathing rats produced severe hypoventilation and mortality in 4 of 4 cases. However, when T‐I stimulation was given via intramuscular wires placed in the neck (5000Hz and 5001Hz 8–10mA), 4 of 4 rats survived the fentanyl overdose and were able to eventually resume spontaneous breathing. Ongoing efforts by our group are aimed at understanding the flow of electrical current in the spinal cord during epidural and percutaneous delivery to T‐I. To this end, in silico models of spinal cord stimulation are being developed using the Sim4Life software (image‐based electromagnetic and neural simulations in a rat anatomical model with detailed spinal cord structures). The modeling data are intended to help refine the electrode design, placement, and stimulation parameters, to minimize off‐target muscle activation. At this time, we conclude that 1) epidural T‐I of the cervical spinal cord can activate phrenic motoneurons by a combination of pre‐ and post‐synaptic inputs, and 2) percutaneous T‐I is capable of producing airflow which can sustain life through opioid overdose.Support or Funding InformationF31 HL145831‐01 (MDS), R21‐NS109571 (DDF)