The document discusses action potentials and how neurons conduct impulses. It explains that action potentials are rapid changes in membrane potential that allow neurons and muscles to communicate. Action potentials are initiated when stimuli open sodium channels, allowing sodium to rush into the cell and depolarize the membrane. Potassium channels then open, repolarizing the membrane back to its resting potential. This process allows impulses to propagate down axons via changes in membrane potential driven by ion fluxes.
Radiation Dosimetry Parameters and Isodose Curves.pptx
Action Potential Generation and Propagation
1. 24 Oct. 2012 Ashok Solanki 1
Action Potentials-
the language of excitable tissue
How neurons conduct impulses
How the muscle contract?
How the heart pump?
2. Resting membrane potential created
by semi-permeable membrane and
ions
• Intracellular
– Na 50
– K 400
– Cl 52
• Resting membrane
potential created by semi-
permeable membrane
and ions
• Intracellular
– Na 50
– K 400
– Cl 52
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3. Cell – the functional unit.
• 100 trillion cells organize systems of the body.
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6. AP OBEYS ALL OR NONE LAW
ALL-OR-NONE RESPONSE
A stimulus below the
threshold also will not
stimulate the neuron
once a threshold limit is
reached any stronger
stimulus will not increase
the cell's response
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What starts an Action Potential??
• STIMULATION (chemical, electrical, mechanical)
opens Na+ channels
– low intensity stimulation opens few channels,
– local, graded potential
– resting potential restored without action potential
9. THE NEURON MEMBRANE AT REST
• Neuron maintains a resting membrane
potential of about -70 millivolts across the
cell membrane
• Sodium(Na+) and potassium(K+) are the main
ions involved
• Na+ and K+ cannot pass through the lipid
bilayer membrane
• move through the membrane by using
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10. ACTION POTENTIAL
• What is it?
• Excitable tissue.
• All or none law of A.P.
• Change in RMP.
• Role of ions?
• Propagation of.
• Propreties of A.P.
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11. Distribution of important ions in ECF & ICF
Na+ K+ Cl-
INSIDE 14 mEq / L 120 mEq / L 8 mEq / L
OUTSIDE 142 mEq / L 4.5 mEq / L 107 mEq / L
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Action Potential
• ALL-OR-NONE phenomenon
• All Action Potentials are the same intensity.
– stronger sensations result from more
impulses, not stronger impulses.
– more impulses from same neuron
– more impulses from many neurons
15. Na+ / K+ PUMP
Membrane proteins actively transport sodium
out of the cell
potassium in Three Na+ are pumped out for
every two K+ pumped in
result is the cell has more Na+ on the outside
and more K+ on the inside
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How neurons conduct impulses:
• Membrane potential (as
seen in muscle cells)
• K+ diffuses out of neurons
faster than Na+ diffuses
in,
• Na-K pump moves 3Na+
back out for 2K+ back in
• Cl-, phosphate, protein
anions balance cations
• “Resting potential”
= - 70 mV
22. The Lipid Barrier of the Cell Membrane, and Cell Membrane Transport
Proteins
• Active Transport" of Substances Through Membranes
• Primary Active Transport and Secondary Active Transport
• Co-Transport of Glucose and Amino Acids Along with Sodium Ions
• Sodium Counter-Transport of Calcium and Hydrogen Ions
• Na+-K+ pump performs a continual surveillance role in maintaining
normal cell volume.
• A Positive-Feedback Cycle Opens the Sodium Channels
• Threshold for Initiation of the Action Potential
• A major function of the voltage-gated calcium ion channels is to
contribute to the depolarizing phase on the action potential in
some cells
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26. STIMULATED NEURON
Nerve cells are unique in their ability to carry a signal
using membrane potential changes
Stimulation of a neuron opens some of the membrane
proteins (a.k.a. Na+gates)
allows Na+ to pass freely into the cells
free flow of Na+ into the cell causes a reversal of
membrane polarity
polarity reversal is called the action potential
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27. Resting Potential
• At rest, the inside of the cell is at -70 microvolts
• With inputs from dendrites inside becomes more positive
• If resting potential rises above threshold, an action potential
starts to travel from cell body down the axon
• Figure shows resting axon being approached by an AP
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28. How Neurons Communicate
• Action Potential is the electrical process
that neurons use to communicate with
each other
• Action Potentials are based on movements
of ions (charged particles) between the
outside and inside of the axon
• Action Potential is an All or Nothing Process
(like a gun firing)
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Conduction of an Action Potential
• Propagation of A.P. along neuron membrane
• Na+ diffuses, attracted to negative charges in
front of impulse
• A.P. at "A"
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What Keeps Impulse Going
the Same Way ?
• Limits to stimulation of neuron/membrane
• Absolute Refractory Period
• Relative Refractory Period
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35. Regarding the ionic basis of action
potential
in cardiac muscle cells, which one of the
following is incorrect?
A. Phase 0: Na influx
B. Phase 1: K influx
C. Phase 2: Ca influx
D. Phase 3: K efflux
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How neurons conduct impulses:
– 3: K+ channels open,
K+ diffuses out,
Potential returns to
zero
– 4: All channels
closed, Na-K pump
moves Na+ back out
& K+ back in
– Hyperpolarization
– Resting potential
restored
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How neurons conduct impulses:
• Action potential
– describes events at one
point of nerve fiber
– 1: stimulus to
threshold potential
– 2: Na+ channels open,
Na+ diffuses in
• Polarity briefly
reversed, to +30 mV
– 3: Na+ channels close
35. Two Ionic Equilibria and Resting Membrane
Potentials
• The resting membrane potential plays a
central role in the excitability of nerve and
muscle
• An action potential is a rapid change in the
membrane potential followed by a return to
the resting membrane potential
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36. Generation and Conduction of Action Potentials
• An action potential is propagated with the same
shape and size along the whole length of a nerve
or muscle cell
• The action potential is the basis of the signal-
carrying ability of nerve cells
• In muscle cells, an action potential allows the
entire length of these long cells to contract
almost simultaneously.
• Voltage-dependent ion channel proteins in the
plasma membrane are responsible for action
potentials.
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40. Voltage dependent ion channels
• Extracellular Na activation gate with
intracellular inactivation gate and slow K
activation gait
• Conformational changes due to membrane
potential changes influence ion permeability
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42. Nomenclature
• Polarized membrane: Intracellular potential is
negative relative to extracellular space
• Depolarization = less polarization of the
membrane -80mV -> +20mV
• Hyperpolarization = more polarization of
membrane -80mV -> -100mV
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43. Cell membrane
• Necessary for life as we know it
• Border role for cell
– Separates intracellular from extracellular milleau
• Allows ion and protein concentration
gradients to exist
– Creates electric charge gradients
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