2. Graded Potentials
Action Potential
Objectives: Student should know
1. Graded potential
2. Types of graded potentials
3. Action potential
4. Three stages of action potential
5. Types of action potential
6. Generation of action potential
7. Properties of action potentials
8. Differences – graded vs action potential
3. Graded Potentials
A. Subthreshold electrical stimuli that do not produce a
true action potential but do generate electrical signals
B. Stimuli may be electrical, chemical, or mechanical
C. Stimuli produce two types of physiochemical
disturbances
4. Graded Potentials
1. Local, graded, non propagated potentials called
receptor or generator potentials, synaptic potentials or
electrotonic potentials
2. Action potentials (complete depolarization) or nerve
impulses which are propagated down the axon to cause
the release of neurotransmitters
8. Graded Potentials Local Response
A. Subthreshold response
B. Characteristics of graded potentials
1. It is Local - changes in membrane potential are confined
to relatively small regions of the plasma membrane
2. It is graded - Refers to the magnitude of the potential
change and that the signal can be reinforced.
9. Graded Potentials Local Response
A. Magnitude can
vary (is graded) with
the magnitude of the
stimulus
B. Graded events can
be hypopolarizing
(depolarizing -
decrease in potential
difference) or
hyperpolarizing
10. Graded Potentials Local Response
3. Graded potentials
are conducted with
decrement.
(conduction
magnitude falls off
the further you get
from the point of
origin)
11. Graded Potentials Local Response
A. Charge is lost across the membrane because of “leaky” channels and
the magnitude of the potential decreases with distance from the site of
origin (charge density falls).
B. Graded potentials and the local current they generate can function
as signals over very short distances
C. Graded potentials die out in 1 - 2 mm of the origin
12. Types of Graded Potentials
1. Characteristics of Graded Potentials
a. Only type of communication by some neurons
b. Play an important role in the initiation and integration of
long distance signals by neurons and other cells
13. Types of Graded Potentials
2. Specific types of graded potentials
a. Receptor (Generator) potentials
1) Sensory receptors respond to stimuli from mechanoreceptors,
thermoreceptors, nociceptors (pain), chemoreceptors, and
electromagnetic receptors (vision)
a) Graded potential from stimuli is
called receptor potential
b) If graded potential reached
threshold an action potential is
generated and sensory information is
sent to the spinal cord and brain
14. Types of Graded Potentials
b. Pacemaker potential - heart
1) Specialized coronary muscle
cells in the cardiac pacemaker
region (SA node) have “leaky” ion
channels graded potentials can
potentially induce a true cardiac
action potential
2) Graded potential is responsible
for cardiac automaticity
15. Types of Graded Potentials
c. Postsynaptic membrane potentials
1) Graded potentials that develop
on the postsynaptic membrane
during synaptic transmission
(stimuli from other nerves - can be
stimulatory or inhibitory)
2) If graded potentials reach
threshold action potential develops
16. Types of Graded Potentials
D. EPP End Plate Potential
Post synaptic graded potential that develops at the
neuromuscular junction (always stimulatory and always reach
threshold if generated by an action potential in the
innervating alpha motor neuron). Postsynaptic membrane
potentials are important in AP generation in nerve to nerve
and nerve to muscle communication.
17. Action Potential
Generation
Graded Potentials which reach threshold generate action
potentials
1. Much larger response - Membrane polarity reverses
(complete depolarization)
2. AP are propagated without decrement
a. Size and shape of AP are constant along nerve fiber
18. Action Potential
Generation
All or None Response - Size
and shape of AP are not
influenced by the size of the
stimulus
Action Potential - Rapid but transient change in a
membrane potential - Change in local membrane polarity
-
Polarized___Depolarized___Polarized
20. Action Potential
Generation
The Action Potential
Characteristics of action potentials
1. Requires specific voltage- gated ion channels
2, AP are the result of rapid changes in ion conductance
3. AP occur only on regions of cell membranes that are electrically excitable
4. AP generally are a standard size and shape for a specific cell type
5. All or none - when membrane reaches threshold an AP is generated (Not-
Graded)
21. Action Potential Generation
The Action Potential
6. Time - AP not only have a specific size and shape but also
exists within a specific time frame , ave. 1 to 5 msec.- (ie time
duration of the action potential is always the same for a
specific tissue)
Specific to transport protein cycle times
22. Action Potential
Generation
Importance of Action Potentials
Nerve traffic, muscle contraction,
hormone release, G.I. secretions,
Cognitive thought, etc.
Action Potentials are required for the
senses - Sight, hearing, and touch are
all dependent on action potentials for
transmission of information to the brain
Threshold stimuli (Graded Potential)
cause the.generation of an action
potential
23. Action Potential
Generation
Three Stages of the Action Potential
1. Resting stage - Polarized stage - This is the normal resting
membrane potential and varies with the cell type nerve = -
90 mV, heart pacemaker = -60 mV, and skeletal muscle = -83
mV
2. Depolarization stage - Sodium ions (Na+) flow into the
cell as the threshold for voltage gated Na+ channels are
exceeded.
24. Action Potential
Generation
3. Repolarization stage - Potassium (K+) ions flow out of the
cell as voltage gated K+ channels are opened and the cell
membrane potential moves back toward the resting
membrane potential.
30. Action Potential
Generation
Components of an Action Potential
1. Threshold - Membrane potential at which voltage gated
channels will open
2. Rising phase - as Na+ channels open membrane potential
begins to shift toward the equilibrium potential for Na+
(Nernst Potential for Na+)
3. Overshoot - The point at which the membrane potential
becomes positive. The greater the overshoot
potential the further the membrane will
stay above threshold
31. Action Potential
Generation
4. Peak - At the peak of the action potential the sodium
conductance begins to fall (Closure of the slow gate)
5. Repolarization - Inactivation of sodium channels and
opening of the K+ channels (Opening of the K+ voltage
channel slow gate) causes repolarization
6. Threshold - As the membrane potential passes back
through threshold the voltage gated channels reset (both the
Na+ and K+ channels)
32. Action Potential
Generation
7. After - hyperpolarization - The Na+ voltage gated
channels have a fast gate and a slow gate passage of the
membrane potential back through threshold causes the fast
gate to close too rapidly for any Na+ ions to pass while the
slow gate opens. The K+ voltage gate with it’s single slow
gate begins to close slowly so for a period of time K+ still
flows out of the cell hyperpolarizing the cell. Return to
resting membrane voltage is due to Na+K+ATPase
35. Action Potential Generation
Properties
Properties of Action
Potentials -
Refractory periods
are times when it is
either impossible or
more difficult than
normal to generate
a second action
potential.
36. Action Potential Generation
Properties
Absolute Refractory
During this period the
voltage gated
channels responsible
for the action
potential have not
reset and therefore,
do not respond to
stimulation.
37. Action Potential Generation
Properties
Relative Refractory
This period
corresponds to the
positive after
potential period and
due to the
hyperpolarization of
the cell it is more
difficult to generate a
second action
potential.
38. Action Potential
Generation
Properties
Voltage Inactivation - If a cell membrane is
maintained at a voltage potential above
threshold than the voltage gated channels
are not reset and, hence, inactivated and
no action potentials can be generated.
Accommodation to Slow Depolarization -
If a slow depolarization occurs the voltage
gated channels do not respond and no
action potential occurs.
40. Definition
Hypoxia is the commonest pathological
process，which is defined as that cells can
not obtain enough oxygen or can not fully
utilize oxygen，thus leading to changes in
their metabolism，function，or structure.
Hypoxia
42. Partial pressure of oxygen（PO2）
1.Definition：the tension produced by oxygen
molecules physically dissolved in plasma.
2.Normal value： PaO2=13.3kPa(100mmHg)
PvO2=5.33kPa(40mmHg)
Hypoxia
43. Oxygen binding capacity （CO2max）
1. Definition：It is the maximum amount of
oxygen that can be combined to hemoglobin in
100 ml of blood.
2.Normal value： 20ml/dl
Hypoxia
45. Arteriovenous oxygen content (Ca-vO2)
19ml/dl 14ml/dl
5ml/dl
O2 O2 O2 O2 O2
A V
Definition: It is the difference of oxygen
content between arterial blood and mixed
venous blood.
Normal value： 5ml/dl
46. Definition: It is the percentage of
hemoglobin present as oxyhemoglobin.
Oxygen saturation (SO2)
Normal value：SaO2=95%; SvO2=70%
CO2-dissolved O2
CO2max
100%
=
SO2
Hypoxia
47. P50
Definition: P50 is a parameter reflecting
the affinity of Hb to oxygen; it is the O2
pressure required to sturate 50% of Hb.
Normal value：26~28 mmHg
