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Measurement of Electrical Potentials
Ref. 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
+30mV
0mV
-6 0 m V
Stimulate
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
Extracellular fluid
cytoplasm
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
Biphasic extracellular Compound
action potential recording/trace
-10mV
Recordings after nerve crush between eletrodes
.
0 mV
+ + + + + + + + + + + + + + + ++ +
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Reference
elect rode
nerve crush
- - +++
- - - - - - - - - - + + +
+++++ + + + - - - - - - - -
0 mV
+ + + + + + + + + + - - - - - - - - - - - - - - - - - - - - + + + + + + +
+0mV
+++++++++++++++++ +
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0 mV
+++++++++++++++++++
- - - - - - - - - - - - - - - - - - -
Time (milliseconds)
-1 0 mV
Uniphasic extracellular
compound action potential
-10mV
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
+
-
+
-
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