Measurement of Electrical PotentialsRef. Essential Medical Physiology (2nd Edn 1997) L.R. Johnson (ed.) pp

89-92. Lipincott Raven Philadelphia

• Measuring membrane potentials

• Why extracellular potentials happen around cells during signalling

• What sorts of things extracellular potentials can tell us

• How we will measure the nerve compound action potential in prac class

• Electromyography

Measuring membrane potentials

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Measurements of membrane potential

• Require electrodes to be placed both inside the cell (cytoplasm) and in the extracellular fluid

• The potential differences are small (~0.005- 0.1V) must be amplified (preamplifier)

• Changes recorded over periods of 0.001 -0.1 seconds (oscilloscope or ‘Maclab’)

• Absolute measurements of membrane potential difference can be made

Why extracellular potentials happen around cells during signalling

• Usually voltage- or ligand-gated ion channels on the surface of a cell do not all open at the same time

• A localised membrane current will give rise to local circuit currents that change the distribution of charge on both the inside and outside of the membrane

Why extracellular potentials happen around cells during signalling

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Extracellular fluid

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Recording extracellular potentials• If electrodes are be placed at two positions

along the length of an axon or dendrite extracellular potentials can be detected as changes in membrane potential spread between them.

• Extracellular potentials do not measure membrane potential but rather they detect local circuit currents

What sorts of things extracellular potentials can tell us?

• Arrays of electrodes spread over the surface of the scalp can detect the summed activity of large numbers of synapse on pyramidal neurons in the underlying cerebral cortex (electroencephalogram, EEG)

• Electrodes placed on the skin one either side of the heart can detect the sequential depolarisation of the cardiac muscle cells in the chambers of the heart (electrocardiograph, ECG).

How we will measure the nerve compound action potential in prac class

• Place electrodes at two points along the nerve in contact with extracellular fluid

• Trigger action potential at one end of nerve with a stimulating pulse

• Record the passage of the action potential down the nerve fibres

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Biphasic extracellular Compound action potential recording/trace

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Biphasic compound(extracellular) action potential

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Recordings after nerve crush between eletrodes.

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Uniphasic extracellular compound action potential

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Compound Extracellular Potentials• Peripheral nerves like the sciatic contain many fibres of

manydifferent diameters

• The COMPOUND action potential refers the fact that it represents the summed extracellular local circuits of many fibres

Applying an extracellular potential initiates depolarising and hyperpolarising currents within the

nerve fibres

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Stimulating current

Electromyography

• Records muscle compound action potential after the synaptic transmission at the neuromuscular synapse.

• Stimulate action potentials in the motor nerve• Record extracellular compound action potential

over the muscle (even through the skin.

• Provides information about ‘synaptic delay’,number of functioning ‘motor units’

Clinical Electromyography

Needle Electromyography

• If fine needle electrodes are placed close on either side of an individual muscle fibre, we can measure the responsiveness of that single fibre to nerve activity.

• In disorders of the neuro-muscular synapse the muscle fibre action potential may not promptly and reliably after the motor nerve is activated.

8 credit point additional Friday lecture #3

Control of Voluntary Muscles by Motor Neurons

• Structure of the Neuromuscular junction• Acetylcholine transmitter release zones and Ca2+

mediated exocytosis from motor nerve terminals• Synaptic vesicle recycling• Synaptic acetylcholinesterase and acetylcholine

recycling• The endplate potential and miniature endplate

potentials• Postsynaptic voltage-gated Na+ channels

8 credit point additional Friday lecture 4

Disorders that affect signalling and nerve-muscle control

• Disorders of action potential propagation in nerves- eg nerve damage and demyelination

• Disorders that impair the function of the neuromuscular synapse- eg Myasthenia Gravis and Lambert Eaton Syndrome

• Disorders of skeletal muscle- eg Muscular Dystrophy