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
19
Embed
Measurement of Electrical Potentials Ref. Essential Medical Physiology (2nd Edn 1997) L.R. Johnson (ed.) pp 89-92. Lipincott Raven Philadelphia Measuring.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
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
+3 0 m V
0 m V
- 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
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
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