Analog to digital conversion, information, electronics troubleshooting techniques; half-stepping, swapping equipment, being prepared. efficient use of time; the importance of understanding quirks and nuances of computers to successfully minimize time spent on troubleshooting. backups are a key thing, extra computer, extra cables, extra supplies, everything. how to establish communication, the nature of electronic devices. charge and ohm's law, resistance, capacitance, impedance, amplifiers, properties of an oscillocope.

Neurophysiological Instrumentation

Surgical Neurophysiology requires an understanding of electrical phenomena, terminology and equipment

Charge - Q - electric charge, like mass, is a fundamental property of matter and there are two kinds of charge; positive (+) and negative (-). Like charges repel and unlike charges attract one another.

Current - I - Current is the measure of how much electric charge flows past a point per unit time. The direction of current flow is the direction that a positive charge would move, i.e., from regions of positive potential (the source) to regions of negative potential (the sink). In a metal conductor, current is carried by negatively charged electrons which move from the negative cathode to the positive anode. The practical unit of current is the ampere

Electric Potential - Voltage - V - Because of the force of attraction between + and - charges, work must be done to separate them. Work is done when a force acts through a distance and is defined as the product of the force and distance in the direction of the force. The potential or voltage difference between two points is one volt when the work required to move a unit of charge from the point of lower to the point of higher potential is one joule

Resistance - R - As electrons or ions move through a medium encounters with atoms generate a resistance to flow The magnitude of resistance is conveniently considered as the constant of proportionality between voltae applied and current flowing. Its unit is the ohm, defined as the resistance that will limit the flow of current across a potential difference of 1 volt to a magnitude of 1 ampere.

Ohm's Law - V= IR or I = V/R or R = V/I - states that the current is proportional to voltage and inversely proportional to resistance.
*The reciprocal to resistance is conductance - G - G = 1/R

Impedance - Z - A wire carrying a current produces a magnetic field. If the current is changing, the varying magnetic field generates a voltage which opposes the change in current. This effect is known as self-inductance and it impedes the flow of any varying current, such as 60Hz alternating current. This effect is analougous to the effect of resistance on a steady or direct current and is termed incutive reactance. A related phenomenon, capacitive reactance, results from the passage of alternating current through a capacitor, because the build up in charge and potential across the capacitor oppose the flow of current. These three effects, resistance, inductive reactance and capacitive reactance are grouped toghter under the term impedance. There is a fundamental difference between resistance and reactance, and generate heat; but no energy is lost because of reactance. It is rather stored in the magnetic field (inductive reactance) or the electrostatic field (capacitive reactance.) In a series circuit where Impedance = Z, resistance = R & reactance = X; Z = sqrt X2 + R2

Electrical Problems Associated With Living Tissue

Signal Size and Amplification - The first problem is that bioelectric events are small, varying from about 120 mV (transmembrane action potentials) to less than 100 microvolts (unfavorable conditions of extracellular recording). This problem is solved by diverting an insignificant amount of energy from the preparation and using it to control a much larger energy source; this process is electrical amplification.

Noise - An inevitable consequence of high amplification is that undesired and biologically meaningless extraneous electrical events are also picked up. these events are termed noise and they come from a number of sources. Thermal or shot noise, contact resistance,

It should be kept in mind that potential measurements involve the potential difference between two points. In most practical measurements, this potential difference must cause a current to flow in a closed loop or circuit: out one wire to the measuring equipment, through the input resistance of the measuring equipment, back to the preparation in the other wire, and through some path in the preparation to the first wire.

The potential difference which is to be measured between two points of the tissue can be considered to be produced by a generator within the tissue, the generator representing an electrochemical process taking place there. This generator produces a current if its two terminals are joined electrically by the above indicated closed conducting loop. Associated with the generator is its internal resistance. This, together with the contact resistance between the electrodes and the preparation, constitutes the source resistance. Assuming good electrode contact, this source resistance is typically 1000 ohms for extracellular electrodes. For intracellular electrodes, it is 3 to 5 orders of magnitude higher.

The input resistance of the measuring equipment must be high relative to the source resistance for two reasons. First, the source potential to be measured will be distributed across the source and input resistances according to their relative magnitudes. Measurement accuracy dictates that the input resistance should be 10 to 100 times as large as the source resistance for approximately 10% and 1% accuracies, respectively (remember, only the potential drop across the input resistance will be measured by the instrument) For example, if the input resistance equals the source resistance, only half the potential developed by the tissue will be measured, corresponding to a 50% error. A second consideration involves minimizing the power (or energy per unit time) extracted from the preparation by the measuring equipment. The larger the input resistance, the smaller the amount of power extracted. If too much power is extracted for measurement, the abnormal stress on the living system causes the information extracted to be severely distorted and practically worthless.

It should be emphasized that the contact resistance will add to the source resistance as may be seen from the diagram. Unless a good contact is made between the electrodes and the preparation, the large contact resistance will be the resistor which generates the most significant noise.

We live in an electrically noisy world. A.C. transmission lines, outlets, and power cords all emit electromagnetic radiation (EMR) at 60Hz (cycles per second), due to the currents flowing in these wires (Ampere's Law). Alternating electromagnetic fields induce AC currents in any circuit consisting of conductors in the form of a closed loop immersed in the field (Faraday's law). Such currents are induced in the circuit connecting an instrument to a source, and these currents generate voltages across the resistances in the circuit. Thus amplifiers recording intra- or extracellular signals from nerves also "pick up" 60Hz interference from the environment. This can be minimized by blocking the EMR at its source by enclosing radiating wires in a grounded conductive conduit or "shield" that absorbs the electromagnetic energy when currents induced in it flow to ground. Interference is further minimized by similarly blocking the residual EMR at the point of recording by enclosing the preparation in a grounded conductive cage (Faraday Cage) and enclosing the wires to the recording instrument in a shielded cable whose shield is connected to ground

Operating Room Instrumentation

Successful work in the neurosurgery suite/operating room/theater does not generally require a sophisticated understanding of electronics or instrumentation design. It does require a good working knowledge of the controls and limitations of the instruments, and it is very helpful to have a good idea of how the machines perform their operations.

This familiarity is best gained by working with the instruments and with their instruction manuals. do not be alarmed by their apparent complexity; they are not as overwhelmingly sophisticated as the may seem

Volts/DIV, stimulation rate, stimulation frequency, pulse duration, twin pulses, intensity, storing

Many are unduly concerned if their results do not exactly duplicate those described in textbooks. It should be evident, however, that the object of the laboratory work is to learn as much as possible of the phenomena and their causes; this learning is certainly not confined to the exact reproduction of a classical experiment.

To succeed in approaching neurophysiological instrumentation, every effort must be made to work consistently, observe carefully, record accurately and interpret intelligently. The importance of arriving at the hospital/operating room with a clear concept of what is to be done cannot be overemphasized.

Essential understanding before the operation is to be performed is paramount!

Situations often arise where success is dependent on smooth teamwork. The laboratory offers an opportunity to develop the habit of functioning efficiently as a member of a team.

The habit of obtaining results with a minimum of delay and false motion should be cultivated. organize the job in advance and proceed through the surgery smoothly with neither wasted time nor careless haste. Be sure to test all apparatus and have it running properly before preparing the patient for surgery

Generation of Electrical Activity in the Nervous System and Muscles
basic principles of potentials that can be evoked from the nervous system
how waveforms change as a result of injury
elicited response from surgical manipulation
evoked potential generators for anatomical localization
three categories: unit, near-field, far field
unit recorded from single nerve; action potentials from individual nerve fibers, information is coded in the rate and time pattern
near field larger recording from nerve, nucleus or muscle, sum of activity of many
far field recorded from electrodes placed at a long distance from structures; smaller amplitudes, more difficult to interpret