this presentation is about the effect of electric current in neural tissue

2. INTRODUCTION
DEFINITION
NEURAL CONTROL OF MUSCLES
ACTION POTENTIAL
BATTERY
CATHODAL STIMULATION
ANODAL STIMULATION
REFRENCES

3. INTRODUCTION
• In 1791,Luigi galvani documented his
observations of frog muscles contracting under
influence of ‘electricity’.
• In 1833,Duchenne found that he could stimulate
muscles without peircing the skin and devised
cloth covered electrodes for the same. He called
that ‘localised current’ (1st to use frardism).
• Duchenne observed that there were certain
points where contractions were the strongest(
motor points).

4. • Differences in galvanic (unidirectional pulses
lasting for more than 1 sec & named after
Galvani) and faradic currents(shorter pulses
usually b/w 0.1 & 1ms and applied b/w 30 and
100hz) were recognised with denervated
muscle responding to galvanic rather than
faradic.

5. DEFINITION
• Muscle and nerve stimuating currents are
electrical currents which are capable of
causing he generation of action potential.

6. Neural control of muscles
• A brain uses stereotypical electrical signals –
nerve action potentials –to process
information received in CNS and analyse
information at various levels.

8. NERVE

9. ACTION POTENTIAL
• Conduction of action potential occours b/w membranes because there is
a potential difference b/w intracellular and extracellular fluid.
• The resting potential of skeletal muscle is -90mV and -70mV for lower
motorneurons (the minus sign indicating that the inside of cells has a
negitive potential relative to the exterior;this potential difference can be
altered by passage of ions)
• The cell membranes of nerve and muscle have protein channels as the
passage of ions (selectively permeable to ions)
• Higher concentrations of potassium ions are present in the intracellular
fluid and higher concentrationsof both sodium and chloride ions in the
extracellular fluid.
• Ions at high concentrations tend to diffuse to lower concentration in
presence of voltage gradients .

10. • the equillibrium potential of any ion is propotional to the differnce between the
logarithms of intracellular concentration and extracellular concentration and is
defined by the Nernst equqtion.
• Bernstein (1902), the potassium ions could diffuse across resting cell membrane.
• Hodgkin and kyenes (1955), showed that cell membrane is permeable to other
ions ,including sodium ions and that sodium ions are in a continuous state of flux
across the membrane against both the concentration and electrical gradients .
• Evidence support that the expulsion of sodium ions to the influx of potassium ions
is of the order 3:2.

11. CHANGES IN CELL MEMBRANE

14. How a battery works?
• A battery is a charge-separating device. It stores electric energy by
separating cations and anions into two separate compartments, or
terminals (Figure 1).
• Cations are positively-charged ions (+).
• Anions are negatively-charged ions (-).
• The take-home message is that, in a battery, current flows from anode to
cathode. To learn more about batteries,

15. If you refer to the illustration in Figure 1, you will see that one terminal of the battery contains
an excess of cations (+), and this is the positive terminal (+). Because it contains cations (+), the
positive (+) terminal of the battery is called the cathode (+). The other terminal of the battery
contains an excess of anions (-), and this is the negative terminal (-). Because it
contains anions (-), the negative (-) terminal of the battery is called the anode (-).
When the battery is connected to a load, in this case a lightbulb, the device is powered by the
flow of current. Electrical Current is what actually happens, as electrons (-) flow out of the
negative terminal (anode), through the circuit and into the positive terminal (cathode).
The take-home message is that, in a battery, current flows from anode to cathode.

16. How an elecrical stimulator works
• In an electrical stimulator, the flow of anions (-) and cations (+) is controlled by
the mechanics of the circuitry within the stimulator. The stimulator is unique in
that the cathode is the negative pole (-) because it discharges anions (-), and
the anode is the positive pole (+) because it discharges cations (+). At the end of
the day, that's the fundamental difference between a battery and a stimulator.
• Depending on how we configure the polarity, the stimulator will discharge either
cations or anions into the body part being stimulated.
• In cathodal stimulation, anions (-) are discharged into the body as current flows
from the cathode (-), through the tissue, and back to the anode (+).
• In anodal stimulation, cations (+) are discharged into the body as current flows
from the anode (+), through the tissue, and back to the cathode (-).
• Now, let's imagine that we place an electrical stimulator on the surface of the
skin with a nerve bundle running underneath (Figure 2). Within the nerve bundle
is a single nerve fibre (axon) upon which we will focus.
• At rest, the inside of a cell is more negative than the outside of a cell. This occurs
because there is a slightly greater number of negative charges than positive
charges inside of the cell (intracellular space), and a slightly greater number of
positive charges than negative charge outside of the cell (extracellular
space). Because of the electrical difference, the cell is said to be polarized - just
like a magnet, one side is more positive and the other side is more negative. If
the electrical gradient were suddenly reversed, the cell would be depolarized,
and we might see an action potential.

17. Figure 2

18. Cathodal Stimulation of Peripheral
Nerves
As the electrical current flows from cathode to anode, negative charges (anions)
tend to accumulate on the outer surface of the nerve membrane as they will be
repelled by the negatively-charged cathode. This makes the outside of the
membrane more negative. Consequently, the inside of the membrane becomes
more positive due to accumulation of positive ions on the inside. This will result
in depolarization, which, if sufficient in magnitude, will result in an action
potential (nerve impulse or muscle activation).
• Figure 3 illustrates activation of the axon under the cathode. As a result of
stimulation, an action potential is sent in both directions along the length of the
nerve, starting at the cathode. Something interesting happens underneath the
anode, though! All of the negative charge from the extracellular space is
attracted to the anode, leaving the outside of the cell excessively electrically
positive relative to the inside of the cell. The cell is thus hyperpolarized under the
anode, meaning that it is very, very difficult to activate.
• If you apply the information above to the median nerve SSEP (Figure 4), then you
can see why the anode is always distal, and the cathode is always proximal.

19. Cathodal stimulation

20. • What happens when you accidentally reverse your stimulating
electrodes when performing an SSEP test? The difficulty that you
may experience in attempting to acquire an SSEP is explained by
the phenomenon of anodal blocking (Figure 3). Thus, when bipolar
electrodes have tips in the same orientation as a fiber, a fiber
will be depolarized under the cathode, and hyperpolarized under
the anode. If the hyperpolarization is large enough, an action
potential initiated under the cathode may not be able to
propagate through the region of hyperpolarization. If this is the
case, the action potential will propagate in only one direction.
While we often talk about the phenomenon of anodal blocking,
you won't see this in the clinical scenario if you use appropriate
stimulation parameters. For intraoperative monitoring of SSEPs,
you should be using supramaximal stimulation. The high intensity
stimulus will overcome any issues that may be experience as a
result of anodal blocking.

21. Figure 3

22. Anodal stimulation of peripheral nerve
When applied to the surface of a nerve, anodal current will increase the
concentration of cations (+) in the extracellular space under the anode. This will
result in hyperpolarization, which, as I just mentioned, puts the cell in a
heightened state of rest. So, what we see in Figure 5 is that the nerve axon
becomes deactivated (hyperpolarized) under the anode.
The Importance of Cell Orientation
• In all of the examples described thus far, the orientation of the cell under the
stimulator has been horizontal with respect to the orientation of the anode and
cathode (Figures 2-5). This is usually the case when stimulating nerves in the
arms and legs.
• What happens when the orientation of the cell is vertical with respect to the
orientation of the anode and cathode? The answer is that things usually work
exactly opposite to what we just discussed regarding horizontally-oriented
cells.This becomes particularly important in the brain where pyramidal cells of
the cerebral cortex are vertically-oriented with respect to the surface where we
stimulate.

23. Anodal stimulation

24. Refrences
• Fritsch GT, Hitzig E. 1870. Über die elektrische Erregbarkeit des Grosshirns. Arch Anat Physiol
Med Wiss 300–32. Translation in Von Bonin G. 1960. Some papers on the cerebral
cortex. Springfield (IL): Charles C Thomas.
• Merrill DR, Bikson M, Jefferys JGR. Electrical stimulation of excitable tissue: Design of
efficacious and safe protocols. J Neurosci Methods. 2005 Feb 15; 141(2):171-198.
• Nair DR, Burgess R, McIntyre CC, Lüders H. Chronic subdural electrodes in the management
of epilepsy. Clin Neurophysiol. 2008 Jan;119(1):11-28. Epub 2007 Nov 26. Review.
• Ranck JB Jr. Which elements are excited in electrical stimulation of mammalian central
nervous system: a review. Brain Res. 1975 Nov 21;98(3):417-40. Review.
• Stephani C, Luders HO. Electrical Stimulation of Invasive Electrodes in Extratemporal Lobe
Epilepsy. In: Koubeissi MZ, Maciunas RJ, eds. Extratemporal Lobe Epilepsy Surgery.
Montrouge, France: John Libby Eurotext; 2011. 261-313. Print.
• Note: This article was originally published by Richard Vogel in 2015 and is being republished
here for the benefit of ASNM members.
• Sheila kitchen
• Basanta kumar nanda
• Sembulingham book of physiology
• Human neurophysiology