The document discusses the resting membrane potential and action potential in neurons. It explains that: 1) The resting membrane potential is around -70mV, created by different ion concentrations inside and outside the neuron. 2) Graded potentials are local, short-lived changes in membrane potential caused by ion channels opening briefly in response to stimuli. 3) Action potentials are brief, large changes in potential from -70mV to +30mV caused by voltage-gated sodium and potassium channels. The influx of sodium ions depolarizes the membrane before potassium channels repolarize it. 4) Action potentials propagate along the axon as one area repolarizes and depolarizes the next, allowing nerve

1. Ambreen Umar
RESTING MEMBRANE POTENTIAL AND
ACTION POTENTIAL

2. Electrical Current and the
Body
 Potential energy generated by separated
charges is called voltage.
 Reflects the flow of ions rather than electrons
 There is a potential on either side of
membranes when the number of ions is
different across the membrane

3. Role of Ion Channels
 Types of plasma membrane ion channels:
 Passive, or leakage, channels – always open
 Chemically gated channels – open with
binding of a specific neurotransmitter
 Voltage-gated channels – open and close in
response to membrane potential (change in
charge)
 Mechanically gated channels – open and
close in response to physical deformation of
receptors

4. Operation of chemical Gated
Channel

5. Operation of a Voltage-Gated
Channel

6. Gated Channels
 When gated channels are open:
 Ions move along chemical gradients, diffusion
from high concentration to low concentration.
 Ions move along electrical gradients, towards
the opposite charge.
 Together they are called the Electrochemical
Gradient
 An electrical current and Voltage changes are
created across the membrane

7. Electrochemical Gradient
 The EG is the foundation of all electrical
phenomena in neurons.
 It is also what starts the Action Potential.

8. Resting Membrane Potential
(Vr)
 The potential difference (–70 mV) across the
membrane of a resting neuron
 It is generated by different concentrations of Na+,
K+, Cl , and protein anions (A )
 The cytoplam inside a cell is negative and the
outside of the cell is positive. (Polarized)

9. Membrane Potentials: Signals
 Used to integrate, send, and receive
information
 Membrane potential changes are produced
by:
 Changes in membrane permeability to ions
 Alterations of ion concentrations across the
membrane
 Types of signals – graded potentials and
action potentials

10. Changes in Membrane
Potential
 Changes are caused by three events
 Depolarization – the inside of the membrane
becomes less negative
 Repolarization – the membrane returns to its
resting membrane potential
 Hyperpolarization – the inside of the
membrane becomes more negative than the
resting potential

11. Changes in Membrane
Potential

12. Graded Potentials
 Short-lived, local changes in membrane
potential
 Decrease in intensity with distance
 Their magnitude varies directly with the
strength of the stimulus
 Sufficiently strong graded potentials can
initiate action potentials

13. Graded Potentials
 A stimuli from sensory input causes the gated
ion channels to open for a short period of
time.
 Positive Cations flow into the cell and move
towards negative locations around the stimuli.
 Alternately the now negative area on the
outside of the cell will flow towards the
positive areas.
 However, this spread of depolarization is short
lived because the lipid membrane is not a
good conductor and is very leaky, so charges
quickly balance out.

14. Graded Potentials

15. Graded Potentials

16. Action Potentials (APs)
 A brief change in membrane potential from -
70mV(resting) to +30mV (hyperpolarization)
 Action potentials are only generated by
muscle cells and neurons
 They do not decrease in strength over
distance
 An action potential in the axon of a neuron is
a nerve impulse

17. Action Potential: Step 1
Resting State
 Na+ and K+ channels are closed
 Leakage accounts for small movements of
Na+ and K+
 Each Na+ channel has two voltage-regulated
gates
 Activation gates –
closed in the resting
state
 Inactivation gates –
open in the resting
state

18. Action Potential: Step 2
Depolarization Phase
 The local depolarization current flips open the
sodium gate and Na+ rushes in.
 Threshold: when enough Na+ is inside to
reach a critical level of depolarization (-55 to -
50 mV) threshold, depolarization becomes
self-generating

19. Action Potential: Step 2 Cont.
 Na + will continue to rush in making the inside
less and less negative and actually
overshoots the 0mV (balanced) mark to about
+30mV

20. Action Potential: Step 3
Repolarization Phase
 After 1 ms enough Na+ has entered that positive
charges resist entering the cell.
 Sodium inactivation gates close and membrane
permeability to Na+ declines to resting levels
 As sodium gates close, voltage-sensitive K+
gates open
 K+ exits the cell and
internal negativity
of the resting neuron
is restored

21. Action Potential: Step 3
Repolarization Phase
 After 1 ms enough Na+ has entered that positive
charges resist entering the cell.
 Sodium inactivation gates close and membrane
permeability to Na+ declines to resting levels
 As sodium gates close, voltage-sensitive K+
gates open
 K+ exits the cell and
internal negativity
of the resting neuron
is restored

22. Action Potential: Step 4
Hyperpolarization
 Potassium gates are slow and remain open,
causing an excessive efflux of K+
 This efflux causes hyperpolarization of the
membrane (undershoot).
 The neuron is
insensitive to
stimulus and
depolarization
during this time

23. Action Potential:
Role of the Sodium-Potassium
Pump
 Repolarization
 Restores the resting electrical conditions of
the neuron
 Does not restore the resting ionic conditions
 Ionic redistribution back to resting conditions
is restored by the sodium-potassium pump

24. Phases of the Action Potential
 1 – resting state
 2 – depolarization phase
 3 – repolarization phase
 4 – hyperpolarization

25. Propagation of an Action
Potential
 When one area of the cell membrane has
begun to return to resting the positivity has
opened the Na+ gates of the next area of the
neuron and the whole process starts over.
 A current is created that depolarizes the
adjacent membrane in a forward direction
 The impulse propagates away from its point of
origin

26. Propagation of an Action
Potential (Time = 0ms)

27. Propagation of an Action
Potential (Time = 1ms)

28. Propagation of an Action
Potential (Time = 2ms)

29. Coding for Stimulus Intensity
 All action potentials are alike and are
independent of stimulus intensity
 Strong stimuli can generate an action
potential more often than weaker stimuli
 The CNS determines stimulus intensity by the
frequency of impulse transmission

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