The Weiss–Lapicque and the Lapicque–Blair strength—duration curves revisited

What is the best pulse shape for pacing purposes?

Introduction

Cell pacing is a fundamental procedure for generating action potentials (AP) in excitable tissues. Various pulse shapes have been proposed for this purpose, with the aim of either facilitating the achievement of the excitation threshold or minimizing energy delivery to the patient. This study seeks to identify the optimal pulse shape for each of these objectives.

Methods

To determine the most effective pulse forms, we employed a mathematical model simulating nonlinear tissue responses to a range of pulse shapes.

Results

Our results demonstrate that the rectangular pulse is optimal for reaching the excitation threshold, while the Gaussian pulse is superior in minimizing energy delivery. Other pulse shapes examined, including ramp-up, ramp-down, half-sine, and triangular (tent-like), fall between these two in terms of performance.

Discussion

From a clinical perspective, the appropriate pulse shape should be selected based on the specific goal. For minimizing the pulse amplitude required to cross the excitation threshold, the rectangular pulse is recommended. In contrast, if reducing energy delivery to the patient is paramount, the Gaussian pulse is the preferred choice. In other scenarios, a judicious selection can be made based on the outcomes of our model and the clinical requirements.

Closed-Loop Estimation Method of Neurostimulation Strength-Duration Curve Using Fisher Information Optimization

BACKGROUND

The existing estimation methods of strength-duration (SD) curve are based on open-loop uniform and/or random pulse durations, which are chosen without feedback from neuronal data.

OBJECTIVE

To develop a closed-loop estimation method of the SD curve, where the pulse durations are adjusted iteratively using the neuronal data.

METHOD

In the proposed method, after the selection of each pulse duration through Fisher information matrix (FIM) optimization, the corresponding motor threshold (MT) is computed, the SD curve estimation is updated, and the process continues until satisfaction of a stopping rule.

RESULTS

250 simulation cases were run, and the results were compared with the iterative random and uniform sampling methods. The FIM method satisfied the stopping rule in 90% runs and estimated the rheobase (chronaxie in parenthesis) with an average absolute relative error (ARE) of 1.57% (2.15%), with an average of 85 samples. At the FIM termination sample, methods with two and all random pulse durations, and uniform methods with descending, ascending and random orders led to 5.69% (20.09%), 2.22% (3.93%), 7.34% (40.90%), 3.10% (4.44%), and 2.05% (3.45%) AREs.

CONCLUSIONS

The FIM method proposes the SD identification by fitting to the data of the minimum and maximum pulse durations. The range of pulse duration should cover the vertical and horizontal parts of the SD curve. Iterative random or uniform samples from only the vertical or horizontal areas of the curve might not result in satisfactory estimation.

SIGNIFICANCE

This paper provides insights about pulse durations selection for SD curve estimation.

Comments on "Identifiability Analysis and Noninvasive Online Estimation of the First-Order Neural Activation Dynamics in the Brain With Closed-Loop Transcranial Magnetic Stimulation"

Presents corrections to the paper, (Identifiability Analysis and Noninvasive Online Estimation of the First-Order Neural Activation Dynamics in the Brain With Closed-Loop Transcranial Magnetic Stimulation).

Harnessing the 2D Structure-Enabled Viscoelasticity of Graphene-Based Hydrogel Membranes for Chronic Neural Interfacing

Stiffness and viscoelasticity of neural implants regulate the foreign body response. Recent studies have suggested the use of elastic or viscoelastic materials with tissue-like stiffness for long-term neural electrical interfacing. Herein, the authors find that a viscoelastic multilayered graphene hydrogel (MGH) membrane, despite exhibiting a much higher Young's modulus than nerve tissues, shows little inflammatory response after 8-week implantation in rat sciatic nerves. The MGH membrane shows significant viscoelasticity due to the slippage between graphene nanosheets, facilitating its seamless yet minimally compressive interfacing with nerves to reduce the inflammation caused by the stiffness mismatch. When used as neural stimulation electrodes, the MGH membrane can offer abundant ion-accessible surfaces to bring a charge injection capacity 1-2 orders of magnitude higher than its traditional Pt counterpart, and further demonstrates chronic neural therapy potential in low-voltage modulation of rat blood pressure. This work suggests that the emergence of 2D nanomaterials and particularly their unique structural attributes can be harnessed to enable new bio-interfacing design strategies.

Activation of the sciatic nerve evoked during epidural spinal cord stimulation in rodents

Aim

To validate the use of motor activation thresholds (MoT) to titrate stimulation amplitudes for spinal cord stimulation in rodent models.

Methods

We recorded thresholds for MoT and sciatic compound action potentials in ten Sprague-Dawley rats implanted with epidural electrodes. Strength duration curves were fitted to the threshold values.

Results

Activation thresholds were in the same order for both MoT and sciatic compound action potentials.

Conclusion

Many of the large, myelinated fibers traversing the dorsal columns in the rodent spine are activated at similar current levels to MoT. Epidural stimulation in rodents needs to be applied at amplitudes close to MoT to activate these axons.

Intraoperative direct cortical stimulation motor evoked potentials: Stimulus parameter recommendations based on rheobase and chronaxie

OBJECTIVE

To determine optimal interstimulus interval (ISI) and pulse duration (D) for direct cortical stimulation (DCS) motor evoked potentials (MEPs) based on rheobase and chronaxie derived with two techniques.

METHODS

In 20 patients under propofol/remifentanil anesthesia, 5-pulse DCS thenar MEP rheobase and chronaxie with 2, 3, 4 and 5ms ISI were measured by linear regression of five charge thresholds at 0.05, 0.1, 0.2, 0.5 and 1msD, and estimated from two charge thresholds at 0.1 and 1msD using simple arithmetic. Optimal parameters were defined by minimum threshold energy: the ISI with lowest rheobase²×chronaxie, and D at its chronaxie. Near-optimal was defined as threshold energy <25% above minimum.

RESULTS

The optimal ISI was 3 or 4 (n=7 each), 2 (n=4), or 5ms (n=2), but only 4ms was always either optimal or near-optimal. The optimal D was ∼0.2 (n=12), ∼0.1 (n=7) or ∼0.3ms (n=1). Two-point estimates closely approximated five-point measurements.

CONCLUSIONS

Optimal ISI/D varies, with 4ms/0.2ms being most consistently optimal or near-optimal. Two-point estimation is sufficiently accurate.

SIGNIFICANCE

The results endorse 4ms ISI and 0.2msD for general use. Two-point estimation could enable quick individual optimization.