Download MBBS Physiology Presentations 38 Experimental Methods Nerve Lecture Notes

Experimental methods in nerve

muscle physiology

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? Voltage clamp, patch clamp
? Recording from mixed nerves: Compound

action potential, Strength duration curve

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? Nerve conduction studies
? Electromyography

Methods in single neuron

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preparations


Voltage clamp

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Voltage clamp

? Most bioelectric events , voltage and current

flowing through the membrane change

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simultaneously. Difficult to study experimentally

? The voltage clamp holds the voltage down(

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"clamps" the voltage) at one predetermined

value and studies the current flowing through the

membrane at this level.

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? Uses electronic feedback to maintain the voltage

balancing the current carried by flowing ions so

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the potential does not change.


Voltage clamp

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Voltage clamp

? The voltage clamp is a current generator with two electrodes, both of which

are placed inside a cell.

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? Transmembrane voltage is recorded through a "voltage electrode", relative

to ground, and a "current electrode" passes current into the cell.

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? The experimenter sets a "holding voltage", or "command potential", and the

voltage clamp uses negative feedback to maintain the cell at this voltage.

? The electrodes are connected to an amplifier, which measures membrane

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potential and feeds the signal into a feedback amplifier. This amplifier also

gets an input from the signal generator that determines the command

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potential, and it subtracts the membrane potential from the command

potential (Vcommand - Vm), magnifies any difference, and sends an output

to the current electrode.

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? Whenever the cell deviates from the holding voltage, the operational

amplifier generates an "error signal", that is the difference between the

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command potential and the actual voltage of the cell.

? The feedback circuit passes current into the cell to reduce the error signal to

zero. Thus, the clamp circuit produces a current equal and opposite to the

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ionic current. This can be measured, giving an accurate reproduction of the

currents flowing across the membrane.

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3.2 Bernstein's hypothesis

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The squid giant axon ? up to 1mm in diameter

Rediscovered in 1930s by JZ Young

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Patch clamp

(Neher and Sakmann 1976)

? For studying current flows

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through single channels

? uses a single electrode

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both to control membrane

potential and to measure

current

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? amplifier is highly

sensitive and is able to

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resolve the tiny currents

(pA) flowing thru single

channels

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Studies on mixed nerves


Or...

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Why Frogs Hate Scientists

Biphasic action potential

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Compound action potential

Compound action potential

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Susceptibility of Different Types of Fibers to

Conduction Block by Various Agents

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Effect

Most susceptible

Intermediate

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Least susceptible

Block by hypoxia

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B

A

C

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Block by pressure

A

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B

C

Block by local

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C

B

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A

anesthetics

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Nerve excitability: Strength duration

curve

? Rheobase: the minimum stimulus strength

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which when allowed to pass through the

excitable tissue for an infinite period of

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time , excites the tissue

? Chronaxie: minimum time required to

stimulate excitable tissue when the current

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used is twice the rheobase
Nerve conduction velocity

Nerve conduction velocity

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? Nerve conduction velocity (NCV) is a test of the

speed of conduction of impulses through a nerve.

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? The nerve is stimulated surface electrodes, placed

on the skin over the nerve at various locations. One

electrode stimulates the nerve with a very mild

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electrical impulse.

? The resulting electrical activity is recorded by the

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other electrodes. The distance between electrodes

and the time it takes for electrical impulses to travel

between electrodes are used to calculate the nerve

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conduction velocity.


Nerve conduction velocity

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Utility of nerve conduction studies

Most often, abnormal results are caused by some sort of

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neuropathy (nerve damage or destruction) including:

? Demyelination (destruction of the myelin sheath)

? Conduction block (the impulse is blocked

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somewhere along the nerve pathway)

? Axonopathy (damage to the nerve axon)

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Some of the associated diseases or conditions include:

? Alcoholic neuropathy

? Diabetic neuropathy

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? Nerve effects of uremia (from kidney failure)

? Traumatic injury to a nerve
Electromyography

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? Study of muscle function through the

examination of the muscle's electric

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signals

? `Electro' ? electric
? `Myo' ? muscle
? `Graphy' ? to graph

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? Why EMG?

? Determine in vivo muscle forces for various

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activities

? Quantify muscle pathology

EMG

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? Skeletal muscle performs mechanical work. It is

stimulated to contract when the brain or spinal

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cord activates motor units.

? An action potential in the motoneuron causes

activation of muscle fibers.

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? The activation of motor units by action potentials

generates a voltage signal in the muscle.

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? EMG is a high gain amplifier to which electrodes

are connected which records these voltages

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EMG

Mixture of signals from different

motor units

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EMG types

? Surface EMG (SEMG) ? Electrodes are

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applied to the surface of the skin.

? Used to measure muscle signals in large

muscles that lie close to the surface of the

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skin

? Indwelling EMG ? Electrodes are inserted

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into the muscle (usually via a needle)

? Used to measure muscle signals in small or

deep muscles, which can not be adequately

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monitored using SEMG.


Noise

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? EMG signals are very small
? External noise

? Electronics noise

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? Recording/measuring equipment

? Ambient noise

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? TV, radio, overhead lights

? Motion artifact

? Movement of electrodes or wires

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EMG with surface electrodes


EMG

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? Electromyograms are obtained at

? Rest : no muscle activity
? During slight muscle contration to asses the

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size and duration of ativiy of motor units

? During maximal contraction to study

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recruitment

? Abnormal patterns at rest

-fibril ation

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-fasciculation