Method and apparatus for magnetically stimulating neurons

• US 4,940,453 A
• Filed: 01/28/1987
• Issued: 07/10/1990
• Est. Priority Date: 01/28/1987
• Status: Expired due to Term

• Current Assignee: Cadwell Industries Incorporated
• Original Assignee: Cadwell Industries Incorporated
• Inventors: Cadwell, John A.
• Primary Examiner(s): Jaworski, Francis
• Assistant Examiner(s): Hanley, John C.
• Application Number:
  US07/008,210
• Time in Patent Office: 1,259 Days
• Field of Search: 600/13-15, 128/741
• US Class Current: 600/13
• CPC Class Codes: A61B 5/296 for electromyography [EMG]
• CPC Class Codes: A61B 5/377 using evoked responses
• CPC Class Codes: A61N 1/40 Applying electric fields by inductive or capacitive coupling microwave apparatus A61N5/00; Applying radio-frequency signals
• CPC Class Codes: A61N 2/006 for magnetic stimulation of nerve tissue
• CPC Class Codes: A61N 2/02 using magnetic fields produced by coils, including single turn loops or electromagnets A61N2/12 takes precedence

Abstract

An efficient method and apparatus for magnetically stimulating the neural pathways of a higher level organism, namely the human body, is disclosed. The method includes selectively applying sinusoidally fluctuating electric power to a stimulator coil that overlies the neurons to be stimulated. The frequency of the power and, thus, the period of magnetic field produced by the coil is chosen to correspond to the time constant of the neurons to be stimulated. Realizable values fall in the range of 1.25 to 1.43 times the time constant of the neurons to be stimulated. The current and voltage of the applied power are in phase quadrature with the current lagging the voltage. During the first polarity (e.g., positive) excursion of the applied voltage, the magnetic field produced by the coil is insufficient to stimulate the underlying neurons, i.e., create a neuron depolarizing electric field. Rather, stimulation occurs during the second polarity (e.g., negative) excursion of the applied voltage. Preferably, coil current flow terminates at the end of the first current cycle. Alternatively, if restimulation during the third and subsequent polarity excursions of the applied voltage is desired, the coil current can be allowed to decay. The apparatus of the invention includes a series circuit comprising the stimulator coil 59 and a high voltage capacitor bank 57 connected in parallel with a power switch 55, across the output of a power supply 51. Closure of the power switch 55 results in the capacitor bank 57 being discharged through the coil 59 and the creation of the magnetic field that stimulates the underlying neurons. Efficiency is high because the resistance of both the capacitor discharge circuit and the power supply output are low. Further, because the resistance of the capacitor discharge circuit is low, discharge current flow is high, whereby an intense magnetic field is produced. Interlock circuits are provided to prevent the inadvertent operation of the apparatus. If desired, magnetic neuron stimulation can be enhanced simultaneously and/or sequentially by applying electric power to a pair of spaced-apart electrodes located in the vicinity of the coil.

Claims

1. A method of stimulating the neural pathways of an organism, such as the human body, to create evoked potentials by depolarizing neurons forming part of the neural pathway to be stimulated, said method comprising the steps of:

placing a stimulator coil on the surface of the organism in the region where a neural pathway is to be stimulated; and

,

energizing the stimulator coil by applying sinusoidal voltage to the coil that causes a high sinusoidal current to flow in the coil that causes the coil to generate a magnetic field that creates an electric field in the body of said organism in the region where a neural pathway is to be stimulated, said sinusoidal current flow being in phase quadrature with and lagging the sinusoidal voltage, the magnitude of said sinusoidal voltage being inadequate to create a current flow sufficient to generate a magnetic field that creates a neuron depolarizing electric field in the body of said organism during the first quarter of the first sinusoidal cycle of said applied voltage and adequate to generate a magnetic field that creates a neuron depolarizing electric field in the body of said organism during the second and third quarters of the first sinusoidal cycle of said applied voltage.

2. A method of stimulating the neural pathways of an organism as claimed in claim 1 including the further steps of:

placing a pair of electrodes on the surface of said organism in the region where a neural pathway is to be stimulated; and

,

energizing said pair of electrodes by applying power to said electrodes that causes a current to flow through said organism from one electrode to the other electrode, said current flow creating an electric field that assists the electric field created by the magnetic field generated by said coil in creating a neuron depolarizing electric field in the body of said organism.

3. A method of stimulating the neural pathways of an organism as claimed in claim 2, wherein the electric field created by current flow between said electrodes is generated simultaneously with the electric field created by the magnetic field generated by said coil.

4. A method of stimulating the neural pathways of an organism as claimed in claim 3 wherein said electrodes and said coil are positioned on the surface of said organism such that one of said electrodes is surrounded by said coil.

5. The method of stimulating the neural pathways of an organism as claimed in claim 1 wherein said voltage is applied for a single sinusoidal cycle.

6. The method of stimulating the neural pathways of an organism as claimed in claim 5 wherein the frequency of said applied voltage is chosen such that the period of said single cycle falls in the range of 1.25 to 1.43 times the time constant of the neurons that form the neural pathway to be stimulated.

7. A method of stimulating the neural pathways of an organism as claimed in claim 6 including the further steps of:

placing a pair of electrodes on the surface of said organism in the region where a neural pathway is to be stimulated; and

,

energizing said pair of electrodes by applying power to said electrodes that causes a current to flow through said organism from one electrode to the other electrode, said current flow creating an electric field that assists the electric field created by the magnetic field generated by said coil in creating a neuron depolarizing electric field in the body of said organism.

8. A method of stimulating the neural pathways of an organism as claimed in claim 7 wherein the electric field created by current flow between said electrodes is generated simultaneously with the electric field created by the magnetic field generated by said coil.

9. A method of stimulating the neural pathways of an organism as claimed in claim 8 wherein said electrodes and said coil are positioned on the surface of said organism such that one of said electrodes is surrounded by said coil.

10. A method of stimulating the neural pathways of an organism as claimed in claim 1 wherein the frequency of said applied voltage is chosen such that the period of the magnetic field that creates the depolarizing electric field falls in the range of 1.25 to 1.43 times the time constant of the neurons that form the neural pathway to be stimulated.

11. A method of stimulating the neural pathways of an organism as claimed in claim 10 including the further steps of:

placing a pair of electrodes on the surface of said organism in the region where a neural pathway is to be stimulated; and

,

energizing said pair of electrodes by applying power to said electrodes that causes a current to flow through said organism from one electrode to the other electrode, said current flow creating an electric field that assists the electric field created by the magnetic field generated by said coil in creating a neuron depolarizing electric field in the body of said organism.

12. A method of stimulating the neural pathways of an organism as claimed in claim 11 wherein the electric field created by current flow between said electrodes is generated simultaneously with the electric field created by the magnetic field generated by said coil.

13. A method of stimulating the neural pathways of an organism as claimed in claim 12 wherein said electrodes and said coil are positioned on the surface of said organism such that one of said electrodes is surrounded by said coil.

14. A method of stimulating the neural pathways of an organism, such as the human body, to create evoked potentials by depolarizing neurons forming part of the neural pathway to be stimulated, said method comprising the steps of:

placing a stimulator coil on the surface of the organism in the region where a neural pathway is to be stimulated; and

,

energizing the stimulator coil by applying at least one full wave of lightly damped sinusoidal voltage to the coil that causes a high sinusoidal current to flow in the coil, said sinusoidal current flow causing the coil to generate a magnetic field that creates a neuron depolarizing electric field in the body of said organism in the region where a neural pathway is to be stimulated, said sinusoidal current flow being in phase quadrature with and lagging, said sinusoidal voltage, the frequency of said applied voltage being chosen such that the period of said magnetic field that creates a neuron depolarizing electric field falls in the range of 1.25 to 1.43 times the time constant of the neurons that form the neural pathway to be stimulated.

15. A method of stimulating the neural pathways of an organism as claimed in claim 14 including the further steps of:

placing a pair of electrodes on the surface of said organism in the region where a neural pathway is to be stimulated; and

,

energizing said pair of electrodes by applying power to said electrodes that causes a current to flow through said organism from one electrode to the other electrode, said current flow creating an electric field that assists the electric field created by the magnetic field generated by said coil in creating a neuron depolarizing electric field in the body of said organism.

16. A method of stimulating the neural pathways of an organism as claimed in claim 15 wherein the electric field created by current flow between said electrodes is generated simultaneously with the electric field created by the magnetic field generated by said coil.

17. A method of stimulating the neural pathways of an organism as claimed in claim 16 wherein said electrodes and said coil are positioned on the surface of said organism such that one of said electrodes is surrounded by said coil.

18. A method of stimulating the neural pathways of an organism as claimed in claim 14 wherein said voltage is applied for a single sinusoidal cycle.

19. A method of stimulating the neural pathways of an organism as claimed in claim 18 including the further steps of:

placing a pair of electrodes on the surface of said organism in the region where a neural pathway is to be stimulated; and

,

energizing said pair of electrodes by applying power to said electrodes that causes a current to flow through said organism from one electrode to the other electrode, said current flow creating an electric field that assists the electric field created by the magnetic field generated by said coil in creating a neuron depolarizing electric field in the body of said organism.

20. A method of stimulating the neural pathways of an organism as claimed in claim 19 wherein the electric field created by current flow between said electrodes is generated simultaneously with the electric field created by the magnetic field generated by said coil.

21. A method of stimulating the neural pathways of an organism as claimed in claim 20 wherein said electrodes and said coil are positioned on the surface of said organism such that one of said electrodes is surrounded by said coil.