Download MBBS (Bachelor of Medicine, Bachelor of Surgery) 1st Year, 2nd Year, 3rd Year and Final year Physiology 38 Experimental Methods Nerve PPT-Powerpoint Presentations and lecture notes
Experimental methods in nerve
muscle physiology
? Voltage clamp, patch clamp
? Recording from mixed nerves: Compound
action potential, Strength duration curve
? Nerve conduction studies
? Electromyography
Methods in single neuron
preparations
Voltage clamp
Voltage clamp
? Most bioelectric events , voltage and current
flowing through the membrane change
simultaneously. Difficult to study experimentally
? The voltage clamp holds the voltage down(
"clamps" the voltage) at one predetermined
value and studies the current flowing through the
membrane at this level.
? Uses electronic feedback to maintain the voltage
balancing the current carried by flowing ions so
the potential does not change.
Voltage clamp
Voltage clamp
? The voltage clamp is a current generator with two electrodes, both of which
are placed inside a cell.
? Transmembrane voltage is recorded through a "voltage electrode", relative
to ground, and a "current electrode" passes current into the cell.
? 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
potential and feeds the signal into a feedback amplifier. This amplifier also
gets an input from the signal generator that determines the command
potential, and it subtracts the membrane potential from the command
potential (Vcommand - Vm), magnifies any difference, and sends an output
to the current electrode.
? Whenever the cell deviates from the holding voltage, the operational
amplifier generates an "error signal", that is the difference between the
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
ionic current. This can be measured, giving an accurate reproduction of the
currents flowing across the membrane.
3.2 Bernstein's hypothesis
The squid giant axon ? up to 1mm in diameter
Rediscovered in 1930s by JZ Young
Patch clamp
(Neher and Sakmann 1976)
? For studying current flows
through single channels
? uses a single electrode
both to control membrane
potential and to measure
current
? amplifier is highly
sensitive and is able to
resolve the tiny currents
(pA) flowing thru single
channels
Studies on mixed nerves
Or...
Why Frogs Hate Scientists
Biphasic action potential
Compound action potential
Compound action potential
Susceptibility of Different Types of Fibers to
Conduction Block by Various Agents
Effect
Most susceptible
Intermediate
Least susceptible
Block by hypoxia
B
A
C
Block by pressure
A
B
C
Block by local
C
B
A
anesthetics
Nerve excitability: Strength duration
curve
? Rheobase: the minimum stimulus strength
which when allowed to pass through the
excitable tissue for an infinite period of
time , excites the tissue
? Chronaxie: minimum time required to
stimulate excitable tissue when the current
used is twice the rheobase
Nerve conduction velocity
Nerve conduction velocity
? Nerve conduction velocity (NCV) is a test of the
speed of conduction of impulses through a nerve.
? 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
electrical impulse.
? The resulting electrical activity is recorded by the
other electrodes. The distance between electrodes
and the time it takes for electrical impulses to travel
between electrodes are used to calculate the nerve
conduction velocity.
Nerve conduction velocity
Utility of nerve conduction studies
Most often, abnormal results are caused by some sort of
neuropathy (nerve damage or destruction) including:
? Demyelination (destruction of the myelin sheath)
? Conduction block (the impulse is blocked
somewhere along the nerve pathway)
? Axonopathy (damage to the nerve axon)
Some of the associated diseases or conditions include:
? Alcoholic neuropathy
? Diabetic neuropathy
? Nerve effects of uremia (from kidney failure)
? Traumatic injury to a nerve
Electromyography
? Study of muscle function through the
examination of the muscle's electric
signals
? `Electro' ? electric
? `Myo' ? muscle
? `Graphy' ? to graph
? Why EMG?
? Determine in vivo muscle forces for various
activities
? Quantify muscle pathology
EMG
? Skeletal muscle performs mechanical work. It is
stimulated to contract when the brain or spinal
cord activates motor units.
? An action potential in the motoneuron causes
activation of muscle fibers.
? The activation of motor units by action potentials
generates a voltage signal in the muscle.
? EMG is a high gain amplifier to which electrodes
are connected which records these voltages
EMG
Mixture of signals from different
motor units
EMG types
? Surface EMG (SEMG) ? Electrodes are
applied to the surface of the skin.
? Used to measure muscle signals in large
muscles that lie close to the surface of the
skin
? Indwelling EMG ? Electrodes are inserted
into the muscle (usually via a needle)
? Used to measure muscle signals in small or
deep muscles, which can not be adequately
monitored using SEMG.
Noise
? EMG signals are very small
? External noise
? Electronics noise
? Recording/measuring equipment
? Ambient noise
? TV, radio, overhead lights
? Motion artifact
? Movement of electrodes or wires
EMG with surface electrodes
EMG
? Electromyograms are obtained at
? Rest : no muscle activity
? During slight muscle contration to asses the
size and duration of ativiy of motor units
? During maximal contraction to study
recruitment
? Abnormal patterns at rest
-fibril ation
-fasciculation
This post was last modified on 08 April 2022