Resting membrane potential?
-70mV
¾ Excitable tissues – able to produce electric signals
• Nerve & muscle cells
¾ Electric signals brought about by changes in polarization
1.
Depolarization
2.
Repolarization
3.
Hyperpolarization
Upward deflection = Decrease in potential
Downward deflection = Increase in potential
Repolarization
Hyperpolarization
Depolarization
Resting potential
Figure 4.1 Page 100
Altering membrane potential
• Leak channels
• Gated channels (ions)
Graded & Action potentials:
1.
2.
Graded potential
• Depolarization of varying magnitude
• Generally short distances
Action potentials
• Do not diminish in strength (nondecremental)
• Initial stimulus (depolarization) must be strong enough to reach threshold potential
9 -50 to -55mV
1

Graded potential
(change in membrane potential relative to resting potential)
Magnitude of stimulus
Time
Resting potential
Stimuli applied Figure 4.2 Page 101
Unbalanced charges distributed across the plasma membrane that are responsible for membrane potential
Closed Na+ channel
Extracellular fluid
Intracellular fluid
Portion of an excitable cell
Entire membrane at resting potential
Figure 4.3 (1) Page 101
2

Triggering event opens Na+ channels
Inactive area at resting potential
Active area depolarized
(a graded potential)
Inactive area at resting potential
Figure 4.3 (2) Page 101
Local current flow occurs between the active and adjacent inactive areas
Inactive area
Previously inactive area being depolarized
Original active area
Spread of depolarization
Previously inactive area being depolarized
Inactive area
Figure 4.3 (3) Page 101
Loss of charge
Initial site of potential change
Loss of charge
Direction of current flow from initial site
* Numbers refer to the local potential in mV at various points along the membrane.
Direction of current flow from initial site
Figure 4.4 Page 102
3

= Action potential = After hyperpolarization
Na + equilibrium potential
Triggering event
Threshold potential
Resting potential
K + equilibrium potential
Figure 4.6
Page 103
2 specific channels responsible for AP:
1.
Voltage-gated Na +
Extracellular fluid (ECF)
Plasma membrane
Inactivation gate
Closed but capable of opening
At resting potential
(–70 mV)
Activation gate
Intracellular fluid (ICF)
Rapid opening triggered at threshold
Slow closing triggered at threshold
Open (activated)
From threshold to peak potential
(–50 mV to +30 mV)
Closed and not capable of opening (inactivated)
From peak to resting potential
(+30 mV to –70 mV)
4

2 specific channels responsible for AP:
2.
Voltage-gated K +
Extracellular fluid (ECF)
Plasma membrane
Intracellular fluid (ICF)
Delayed opening triggered at threshold
Closed
At resting potential; delayed opening triggered at threshold; remains closed to peak potential
(–70 mV to +30 mV)
Open
From peak potential through after hyperpolarization
(+30 mV to –80 mV)
At rest (-70mV):
1.
Na + channels closed (but capable of opening)
2.
K + channels closed
• Leak channels more prevalent
Initial depolarization:
1.
Na + activation gates open
• Favorable for Na + to move into cell
• Develops into positive feedback cycle
Figure 4.8
Page 105
Triggering event
Depolarization
(decreased membrane potential)
Positive-feedback cycle
Influx of Na +
(which further decreases membrane potential)
Opening of some voltage-gated
Na+ channels
5

Threshold potential:
¾ Explosive increase in Na + permeability
• 600x greater than K +
• Causes cell to become much more positive (+30mV)
9
Slows when reaching equilibrium potential (+60mV)
¾ Slowing of Na + into cell:
1.
Rapid opening (threshold) causes the inactivation gate to begin closing
9 Process takes time
2.
At peak entry of Na + , K + channels begin to open
9 Slowly
Figure 4.9 (1)
Page 106
At resting potential
Figure 4.9 (2)
Page 106
Na + activation gate opens
Threshold reached
Depolarizing triggering event
6

Figure 4.9 (3)
Page 106
Action potential begins
Figure 4.9 (4)
Page 107 depolarization; potential reaches 0 mV
Figure 4.9 (5)
Page 106
Na + inactivation gate begins to close
K + gate opens
Peak of action potential; potential reversed
7

Figure 4.9 (6)
Page 106
Repolarization begins
Figure 4.9 (7)
Page 106
Na +
Na + inactivation gate opens; activation gate closes
Action potential complete; after hyperpolarization begins
After hyperpolarization is complete; return to resting potential
8

a
C s u d e y
b
N
+ a x u fl in is
R in
p lli
p y
b d e
K s u a
C x lu ff e
Threshold potential
Resting potential
Figure 4.10 Page 107
Nucleus
Arrows indicate the direction in which nerve signals are conveyed.
Input Zone
Dendrites and
Cell body
Figure 4.11
Page 109
Trigger Zone
Axon hillock
Conducting Zone
Axon (may be from 1mm to more than 1m long)
Output Zone
Axon terminals
9

Active area at peak of action potential
Adjacent inactive area into which depolarization is spreading; will soon reach threshold
Remainder of axon still at resting potential
Direction of propagation of action potential
Figure 4.12 (1) Page 110
Previous active area returned to resting potential
Adjacent area that was brought to threshold by local current flow; now active at peak of action potential
New adjacent inactive area into which depolarization is spreading; will soon reach threshold
Remainder of axon still at resting potential
Saltatory Conduction
10

1mm
Nodes of Ranvier
Myelin
Axon
Figure 4.15 (1)
Page 114
Peripheral Nervous System
Axon
Cytoplasm
Schwann cell
Nucleus
Node of Ranvier
Node of Ranvier
Schwann cell
11

Active node at peak of action potential
Adjacent inactive node into which depolarization is spreading; will soon reach threshold
Remainder of nodes still at resting potential
Myelin
Myelinated axon
Direction of propagation of action potential returned to resting
Page 115
Adjacent node that was Figure 4.16 (2) local current flow; now active at peak of action potential
New adjacent inactive node into which depolarization is spreading; will soon reach threshold
12