? 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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preparationsVoltage 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 predeterminedvalue 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 thevoltage 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 commandpotential (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 diameterRediscovered 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 membranepotential 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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EffectMost susceptible
Intermediate
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Least susceptible
Block by hypoxia
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BA
C
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Block by pressure
A
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BC
Block by local
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C
B
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Aanesthetics
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Nerve excitability: Strength durationcurve
? 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, placedon 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 electrodesand 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 electrodesare connected which records these voltages
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EMGMixture 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