Action Potential Generation

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PHYSIOLOGY 1
LECTURE 12
Graded Potentials
Action Potential
Generation
Graded Potentials
Action Potential

Objectives: Student should know
– 1.
– 2.
– 3.
– 4.
– 5.
– 6.
– 7.
– 8.
Graded potential
Types of graded potentials
Action potential
Three stages of action potential
Types of action potential
Generation of action potential
Properties of action potentials
Differences – graded vs action potential
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

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

ACTION POTENTIAL
Graded Potentials
Graded Potentials
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.
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
Graded Potentials Local
Response

3. Graded
potentials are
conducted with
decrement.
(conduction
magnitude falls off
the further you get
from the point of
origin)
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

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
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
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
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
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.
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
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
Action Potential Generation
The Action Potential
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)
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
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
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.
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.
Action Potential Generation
The Action Potential





Three Stages
1. Resting Stage
(Polarized State)
2. Depolarization
3. Repolarization
1
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
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)
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
Action Potential Generation
The Action Potential Types
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.
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.
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.
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.
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