Download lecture notes #4 membrane potentials

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Physiology 1
LECTURE 4 NOTES
By Dr. Tom Madayag
Membrane Potentials and Action Potentials
Basic Physics of Membrane Potentials
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Membrane Potentials Caused by Ion Concentration
Differences across a Selectively Permeable Membrane
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A concentration difference of ions across a selectively permeable membrane can, under
appropriate conditions, create a membrane potential
Resting Membrane Potential of Neurons
https://www.youtube.com/watch?v=fAGfUlUNNxs
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The resting membrane potential of large nerve fibers when they are not transmitting nerve
signals is about -90 mV (negative inside the fiber)
Why is there negativity inside the membrane
1. The Na+/K+ pump continually transports sodium ions to the outside of the cell
and potassium ions to the inside (3Na+/2 K+), leaving a net deficit of positive
ions on the inside
2. The Na+/K+ pump causes large concentrations gradient for sodium and
potassium across the membrane
Outside
Inside
Na+
142 mEq/L
14 mEq/L
K+
4 mEq/L
140 mEq/L
3. Leakage of potassium through the nerve cell membrane-- there are potassium
leak channels through which K+ can leak even in a resting cell
Neuron Action Potential
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Each action potential begins with a sudden change from the normal resting negative membrane
potential to a positive potential
To conduct a nerve signal, the action potential moves along the nerve fiber until it comes to the
fiber’s end
Stages of the Action Potential
o Resting stage
 Membrane is polarized (-90 mV negative potential is present)
o Depolarization stage
 Membrane suddenly becomes permeable to sodium ions
 Potential rises in the positive direction
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In large fibers, the influx of sodium causes the positive rise to overshoot the
zero level
 In some smaller fibers, as well as in many central nervous system neurons, the
potential merely approaches the zero level and does not overshoot to the
positive state
Repolarization Stage
 Sodium channels begin to close
 Potassium channels open to a greater degree than normal
 Potassium diffuses outside rapidly, re-establishes negative potential
How else does sodium and potassium leave the cell?
1. Voltage-gated channels (for Na+ and K+)
 A voltage gated channel has two gates- one near the outside of the channel
called activation gates and another inside called the inactivation gate
 Activation of the Na+ channel
 When the membrane potential becomes less negative (toward zero), it
usually reaches somewhere between -70 and -50 mV, the gate changes
shape and OPENS
 Inactivation of the Sodium channel
 Once the activation gate opens, in a few seconds the inactivation gate
closes
 It will not reopen until the membrane potential returns to or near the
original resting membrane potential*
 Activation of the Potassium channel
 When the membrane potential rises toward zero, gate opens
 They open just as the sodium channels are about to close
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Initiation of the Action Potential. What initiates the action potential?
https://www.youtube.com/watch?v=7EyhsOewnH4
1. A positive feedback cycle opens the sodium channels
2. An action potential will not occur until the initial rise in membrane potential is great enough to
create the positive feedback (THRESHOLD)
a. A sudden rise in membrane potential of 15-30 mV is usually required (thus up to about 65 mV)
b. This (-65 mV) is known as the threshold for stimulation
Propagation of the Action Potential
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An action potential elicited at any one point on an excitable membrane usually excites adjacent
portions of the membrane
The depolarization process travels along the entire length of the fiber
The transmission of depolarization along a nerve or muscle fiber is called a nerve or muscle
impulse
An action potential has no single direction; it travels all branches of a nerve fiber until the entire
membrane has become depolarized
All-or-nothing principle:
o Occasionally, the action potential reaches a point in the membrane at which it does not
generate sufficient voltage to the next area of the membrane
o When this happens the depolarization wave stops
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Special Characteristics of Signal Transmission in Nerve Trunks: Myelinated and Unmyelinated
Nerve Fibers
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The average trunk contains about twice as many unmyelinated fibers as myelinated fibers
Myelinated fibers
o Central core is the axon
o The axon is filled with axoplasm
o The myelin sheath surrounds the axon
o The sheaths are interrupted by areas with no myelin sheaths and they are called the
nodes of Ranvier
o The myelin sheath is deposited by Shwann cells. They deposit sphingomyelin around
the nerve. This is an insulator
o Action potentials occur only at the nodes of Ranvier yet the action potential s are
conducted from node to node. This is called saltatory conduction.
o Saltatory conduction importance of
 Increases the velocity of impulses in myelinated fibers
 Conserves energy for the axons
Excitation—the Process of Eliciting the Action Potential
o Any factor that causes of diffusion in the cell
 Mechanical disturbance
 Chemical effects (chemical neurotransmitters)
 Electricity
Refractory Period
o A new action potential cannot occur in an excitable fiber as long as the fiber is still
depolarized
 This is because the sodium channels (or calcium channels or both ) become
inactivated
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https://www.youtube.com/watch?v=DJe3_3XsBOg