Dependence of cortical neuronal strength-duration properties on TMS pulse shape

Parvathi Menon; Nathan Pavey; Aman S Aberra; Mehdi A J van den Bos; Ruochen Wang; Matthew C Kiernan; Angel V Peterchev; Steve Vucic

doi:10.1016/j.clinph.2023.03.012

Dependence of cortical neuronal strength-duration properties on TMS pulse shape

Clin Neurophysiol. 2023 Jun:150:106-118. doi: 10.1016/j.clinph.2023.03.012. Epub 2023 Mar 30.

Authors

Parvathi Menon¹, Nathan Pavey¹, Aman S Aberra², Mehdi A J van den Bos¹, Ruochen Wang³, Matthew C Kiernan⁴, Angel V Peterchev⁵, Steve Vucic⁶

Affiliations

¹ Brain and Nerve Research Centre, Concord Clinical School, University of Sydney, Concord Hospital, Sydney, Australia.
² Department of Biological Sciences, Dartmouth College, Hanover, NH, USA.
³ Department of Biomedical Engineering, Duke University, Durham, NC, USA.
⁴ Brain and Mind Centre, University of Sydney, Australia.
⁵ Department of Biomedical Engineering, Duke University, Durham, NC, USA; Department of Psychiatry and Behavioural Sciences, Duke University, Durham, NC, USA; Department of Electrical and Computer Engineering, Duke University, Durham, NC, USA; Department of Neurosurgery, Duke University, Durham, NC, USA. Electronic address: angel.peterchev@duke.edu.
⁶ Brain and Nerve Research Centre, Concord Clinical School, University of Sydney, Concord Hospital, Sydney, Australia. Electronic address: steve.vucic@sydney.edu.au.

PMID: 37060842
PMCID: PMC10280814 (available on 2024-06-01)
DOI: 10.1016/j.clinph.2023.03.012

Abstract

Objective: The aim of present study was to explore the effects of different combinations of transcranial magnetic stimulation (TMS) pulse width and pulse shape on cortical strength-duration time constant (SDTC) and rheobase measurements.

Methods: Resting motor thresholds (RMT) at pulse widths (PW) of 30, 45, 60, 90 and 120 µs and M-ratios of 0.2, 0.1 and 0.025 were determined using figure-of-eight coil with initial posterior-to-anterior induced current. The M-ratio indicates the relative phases of the induced current with lower values signifying a more unidirectional stimulus. Strength-duration time constant (SDTC) and rheobase were estimated for each M-ratio and various PW combinations. Simulations of biophysically realistic cortical neuron models assessed underlying neuronal populations and physiological mechanisms mediating pulse shape effects on strength-duration properties.

Results: The M-ratio exerted significant effect on SDTC (F_(2,44) = 4.386, P = 0.021), which was longer for M-ratio of 0.2 (243.4 ± 61.2 µs) compared to 0.025 (186.7 ± 52.5 µs, P = 0.034). Rheobase was significantly smaller when assessed with M-ratio 0.2 compared to 0.025 (P = 0.026). SDTC and rheobase values were most consistent with pulse width sets of 30/45/60/90/120 µs, 30/60/90/120 µs, and 30/60/120 µs. Simulation studies indicated that isolated pyramidal neurons in layers 2/3, 5, and large basket-cells in layer 4 exhibited SDTCs comparable to experimental results. Further, simulation studies indicated that reducing transient Na⁺ channel conductance increased SDTC with larger increases for higher M-ratios.

Conclusions: Cortical strength-duration curve properties vary with pulse shape, and the modulating effect of the hyperpolarising pulse phase on cortical axonal transient Na⁺ conductances could account for these changes, although a shift in the recruited neuronal populations may contribute as well.

Significance: The dependence of the cortical strength-duration curve properties on the TMS pulse shape and pulse width selection underscores the need for consistent measurement methods across studies and the potential to extract information about pathophysiological processes.

Keywords: Rheobase; SDTC; Sodium channel; Strength-duration time constant; cTMS.

Publication types

Research Support, N.I.H., Extramural
Research Support, Non-U.S. Gov't

MeSH terms

Axons
Evoked Potentials, Motor / physiology
Heart Rate
Humans
Neurons*
Transcranial Magnetic Stimulation* / methods

Substances

sarcosine dithiocarbamate

Abstract

Publication types

MeSH terms

Substances

Grants and funding