Download MBBS Physiology Presentations 38 Experimental Methods Nerve Lecture Notes

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