ionic basis and recording of acton potential
DESCRIPTION
PHYSIOLOGY OF ACTION POTENTIAL IN NERVE WITH HISTORY AND RECORDINGTRANSCRIPT
IONIC BASIS AND RECORDING OF ACTION POTENTIAL
Dr.Anu Priya.J.
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Introduction
History
Resting membrane potential
Graded potential
Action potential
Ionic basis
Types
Recording
Applied aspects
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Introduction
• Nerve and muscle are excitable tissues
• Can undergo rapid changes in their membrane potentials
• Change their resting potentials into electrical signals that aid in cellular communication
• These signaling events are mediated by ion channels
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Since the 18th century, when Galvani introduced the concept of "animal electricity", electric potentials have been observed and recorded in different nerves and muscles.
History
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Illustration of Italian physician Luigi Galvani's experiments, in which he applied electricity to frogs legs; from his book De Viribus Electricitatis in Motu Musculari (1792).
History
1963- A. L. Hodgkin and A. F. Huxley - Nobel prize in Physiology or Medicine- study of sodium and potassium channels – voltage clamp method
Sir John Carew Eccles-shared-work on synapse
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The patch clamp technique - Erwin Neher and Bert Sakmann - Nobel Prize in Physiology or Medicine in 1991
Record the currents of single ion channels for the first time, proving their involvement in fundamental cell processes such as action potential conduction.
History
Sir A. F. Huxley passed away on 30 May 2012 – age 94 years
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History
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It is the potential difference existing across the cell membrane at rest
Interior of the cell is negatively charged in relation to the exterior
State of polarisation
Resting Membrane Potential
Resting Membrane Potential
RMP is maintained by:
1. Natural concentration gradient
2. Selective permeability of cell membrane
3. Impermeable anions
4. Sodium-potassium ATPase pump
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Neurons have a selectively permeable membrane
During resting conditions membrane is:
permeable to potassium (K+) (channels are open)
impermeable to sodium (Na+) (channels are closed)
Diffusion force pushes K+ out (concentration gradient)
This creates a positively charged extra-cellular space.
Electrostatic force pushes K+ in
Thus, there is a ‘dynamic equilibrium’ with zero net movement of ions.
The resting membrane potential is negative
Resting Membrane Potential
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Graded potential
Subthreshold stimuli cause sensory receptors to depolarize and
produce a voltage called a generator potential(Receptor
Potential)
Does not obey all or none law
Graded response
it is not propagated
Summation
No refractory period
Duration(5-10 ms)
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Graded potential & Action potential
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Action potential
An action potential is a rapid change in the membrane potential in response to a threshold stimulus followed by a return to the resting membrane potential.
The size and shape of action potentials differ considerably from one excitable tissue to another.
An action potential is propagated with the same shape and size along the whole length of a cell.
The action potential is the basis of the signal-carrying ability of nerve cells.
Voltage-dependent ion channel proteins in the plasma membrane are responsible for action potentials.
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Hodgkin cycle
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Graded potential & Action potential
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Role of other ions
Impermeable Anions
Calcium ions
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Properties
Voltage inactivation
Refractory period
All or none law
Propagative
depolarization and repolarization
No summation
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Mammalian axons less than 20 μm diameter
Squid-giant cells-largest axon in neck region-about 1 mm diameter
Recording of action potential
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Requirements of instrument used :
a) It should be capable of responding extremely rapidly
b) The potential changes which are in millivolts has to be amplified before being recorded
Recording of action potential
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The instruments used are:
1. Microelectrodes
2. Electronic amplifiers
3. Cathode ray oscilloscope (CRO)
Recording of action potential
Microelectrodes
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Micropipette – tip size less than 1 mm diameter
Filled with strong electrolyte solution- KCl
Resistance – 1 billion Ω
The tip of the micropipette is passed through the cell membrane of the nerve fibre
Indifferent electrode – in extracellular fluid
Connected to cathode ray oscilloscope through amplifier
Electronic amplifier
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magnify the potential changes of the tissue to be recorded on the oscilloscope screen
Cathode ray oscilloscope
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Rapid and instantaneous recording of electrical events of living tissues
Parts
i. Glass tube
ii. Cathode
iii. Fluorescent screen
iv. Two sets ( horizontal and vertical ) electrically charged plates
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Cathode ray oscilloscope
Recording of action potential
Patch clamp method
Voltage clamp method
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Patch clamp method
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Voltage clamp method
Types
Monophasic
Biphasic
Compound
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Biphasic action potential
Biphasic action potential
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Peripheral nerves in mammals are made up of many axons bound together in a fibrous envelope called the epineurium.
Potential changes recorded extracellularly from such nerves therefore represent an algebraic summation of the all-or-none action potentials of many axons.
The thresholds of the individual axons in the nerve and their distance from the stimulating electrodes vary.
With subthreshold stimuli, none of the axons are stimulated and no response occurs.
Compound action potential
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When the stimuli are of threshold intensity, axons with low thresholds fire and a small potential change is observed.
As the intensity of the stimulating current is increased, the axons with higher thresholds are also discharged.
The electrical response increases proportionately until the stimulus is strong enough to excite all of the axons in the nerve.
The stimulus that produces excitation of all the axons is the maximal stimulus, and application of greater, supramaximal stimuli produces no further increase in the size of the observed potential.
Compound action potential
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Applied aspects
Hereditary spherocytosis (HS)
Plasma membrane of red cells three times more
permeable to Na+
The level of Na+,K+-ATPase elevated.
When HS red blood cells have sufficient glucose to
maintain normal ATP levels, they extrude Na+ as
rapidly as it diffuses into the cell cytosol. Hence
the red blood cell volume is maintained.
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When HS erythrocytes are delayed in the venous sinuses of the spleen, where glucose and ATP are present at low levels, the intracellular ATP concentration falls.
Therefore, Na+ cannot be pumped out by the Na+,K+-ATPase as rapidly as it enters.
The red blood cells swell - osmotic effect of elevated intracellular Na+ concentration.
Spleen targets these swollen erythrocytes for destruction - anemia.
Applied aspects
Tetrodotoxin (TTX)- a potent poison - specifically blocks the Na+ channel- binds to the extracellular side of the sodium channel.
Tetraethylammonium (TEA+), another poison, blocks the K+ channel when it is applied to the interior of the nerve fiber.
Applied aspects
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The ovaries of certain species of puffer fish, also known as blowfish, contain TTX. Raw puffer fish - Japan.
Applied aspects
Saxitoxin is another blocker of Na+ channels that is produced by reddish-colored dinoflagellates that are responsible for so-called red tides.
Applied aspects
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Shellfish eat the dinoflagellates and concentrate saxitoxin in their tissues.
A person who eats these shellfish may experience life-threatening paralysis within 30 minutes after the meal
Applied aspects
In an inherited disorder, called primary hyperkalemicparalysis, patients have episodes of painful spontaneous muscle contractions, followed by periods of paralysis of the affected muscles.
Elevated levels of K+ in the plasma and extracellular fluid.
Some patients with this disorder have mutations of voltage-gated Na+ channels that result in a decreased rate of voltage inactivation.
This results in longer-lasting action potentials in skeletal muscle cells and increased K+ efflux during each action potential. This can raise the extracellular levels of K+.
Applied aspects
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The elevation of extracellular K+ causes depolarization of skeletal muscle cells.
Initially, the depolarization brings muscle cells closer to threshold, so that spontaneous action potentials and contractions are more likely.
As depolarization of the cells becomes more marked, the cells accommodate because of the voltage-inactivated Na+
channels.
Consequently, the cells become unable to fire action potentials and are unable to contract in response to action potentials in their motor axons.
Applied aspects
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Low potentials recorded in neuropathy and spinal cord compression
INJURY POTENTIAL
The difference in electrical potential between the injured and uninjured parts of a nerve or muscle – also called demarcation potential
Applied aspects
Applied aspects
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TETANY
Hypocalcemia – sodium channels activated by very little increase of membrane potential from resting state
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References
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Guyton and Hall Textbook of Medical Physiology 12th edition
Ganong's Review of Medical Physiology 23rd edition
Berne & Levy Physiology 6th edition
Boron and Boulpaep Medical physiology 2nd edition
Basics of Medical physiology by Dr.Venkatesh.D 3rd edition
Textbook Of Medical Physiology by Indu Khurana 1st edition
Internet references