Resting membrane potential?
-70mV
Excitable tissues – able to produce electric
signals
¾
•
Nerve & muscle cells
Electric signals brought about by changes in
polarization
¾
1. Depolarization
2. Repolarization
3. Hyperpolarization
Upward deflection = Decrease in potential
Downward deflection = Increase in potential
Repolarization
Hyperpolarization
Resting potential
Depolarization
Figure 4.1 Page 100
Altering membrane potential
•
•
Leak channels
Gated channels (ions)
Graded & Action potentials:
Graded potential
1.
•
•
Depolarization of varying magnitude
Generally short distances
Action potentials
2.
•
•
Do not diminish in strength (nondecremental)
Initial stimulus (depolarization) must be strong
enough to reach threshold potential
9 -50 to -55mV
1
Graded Potentials
Graded
potential
(change in
membrane
potential
relative to
resting
potential)
Resting
potential
Time
Magnitude
of stimulus
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 Inactive area
(a graded potential)
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
Previously
inactive area
being depolarized
Original
active area
Inactive
area
Spread of depolarization
Figure 4.3 (3) Page 101
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.
Loss of charge
Direction of current
flow from initial site
Figure 4.4 Page 102
3
Action Potentials
= Action potential
= After hyperpolarization
Na+ equilibrium
potential
Figure 4.6
Page 103
Threshold
potential
Resting
potential
K+ equilibrium
potential
Triggering event
2 specific channels responsible for AP:
1. Voltage-gated Na+
Extracellular
fluid (ECF)
Plasma
membrane
Inactivation
gate
Activation
gate
Closed but capable
of opening
At resting potential
(–70 mV)
Intracellular
fluid (ICF)
Rapid
opening
triggered
at threshold
Open (activated)
Slow
closing
triggered
at threshold
Closed and not capable
of opening (inactivated)
From threshold to peak potential From peak to resting potential
(+30 mV to –70 mV)
(–50 mV to +30 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
Open
At resting potential; delayed
opening triggered at threshold;
remains closed to peak potential
(–70 mV to +30 mV)
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:
Na+ activation gates open
1.
•
•
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:
Explosive increase in Na+ permeability
¾
•
•
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
Explosive
Action potential
begins
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+ inactivation gate opens;
Na+ activation gate closes
Action
potential
complete;
after
hyperpolarization
begins
After
hyperpolarization
is complete;
return to
resting
potential
8
influx
by Na +
Rising phas
e
x
+
by K efflu
Cause
d
Caused
Falling phase
Threshold potential
Resting potential
Figure 4.10 Page 107
AP propagation
Figure 4.11
Page 109
Input Zone
Dendrites
and
Cell body
Nucleus
Trigger Zone
Axon hillock
Conducting Zone
Axon (may be from 1mm
to more than 1m long)
Arrows indicate the
direction in which nerve
signals are conveyed.
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 Remainder of axon
still at resting potential
reach threshold
Saltatory Conduction
Speed of conduction
Myelinated or Fiber Size
10
Figure 4.15 (1)
Page 114
Nodes of Ranvier
1mm
Myelin
Axon
Peripheral Nervous System
Axon
Nucleus
Cytoplasm
Schwann cell
Node of Ranvier
Schwann cell
Node of Ranvier
11
Active node at peak
of action potential
Adjacent inactive node
into which depolarization Remainder of nodes
is spreading; will soon
still at resting potential
reach threshold
Myelin
Myelinated
axon
Direction of propagation
of action potential
Adjacent node that was
brought to threshold by
local current flow; now
active at peak of action
potential
Figure 4.16 (2)
Page 115
Previous active node
returned to resting
potential
New adjacent inactive
node into which
depolarization is
spreading; will soon
reach threshold
Questions?
12