The document discusses cell membrane potential and action potentials. It explains that cell membranes maintain a voltage gradient called the membrane potential due to different ion concentrations inside and outside the cell. Ion channels, including passive, chemically-gated, and voltage-gated channels, control the flow of ions across the membrane and influence the membrane potential. An action potential occurs when the membrane potential rapidly changes from its normal resting potential of -70mV to +30mV before repolarizing back. This wave of depolarization propagates along the membrane via opening and closing of sodium and potassium ion channels.

1. Cell Membrane Potential
GIDEON ROBERT UNIVERSITY
ANATOMY AND PHYSIOLOGY
NSWANA CHING’AMBU

2. Cell Membrane Potential
• What is cell membrane potential
• Types of ion channels
• Operations of ion channels
• Stages of action potential

3. Electric 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

4. Role of Ion Channels
• Types:
1. Passive or leakage channels – always open
2. Chemically gated channels – open w/
binding of specific neurotransmitter/chemical
3. Voltage-gated channels – open & close in
response to membrane potential

5. Operation of a Chemically-gated
Channels
• Example: Na+-K+ gated channel
• Closed when a neurotransmitter
is not bound to the extracellular
receptor
• Na+ cannot enter the cell and K+
cannot exit the cell
• Open when a neurotransmitter is
attached to the receptor
• Na+ enters the cell and K+ exits
the cell

6. Operation of a Voltage-gated Channels
• Example: Na+ channel
• Closed when the intracellular
environment is negative
• Na+ cannot enter the cell
• Open when the intracellular
environment is positive
• Na+ can enter the cell

7. 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

8. Resting Membrane Potential (Vf)
• 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

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. Action Potential (AP)
• 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

13. Threshold & Action Potential
• Threshold – membrane is depolarized by 15 to 20 mV
• Established by the total amount of current flowing
through the membrane
• Weak (subthreshold) stimuli are not relayed into action
potentials
• Strong (threshold) stimuli are relayed into action
potentials
• All-or-none phenomenon – action potentials either
happen completely, or not at all

14. AP: Stage 1 Resting Stage
• Cell has resting membrane potential (-70mV)
• Na+ and K+ channels are closed
• Leakage accounts for small movements of Na+ and K+

15. AP: Stage 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.

16. AP: Stage 2 Continues
• Na+ will continue to rush
in making the inside less
and less negative and
actually overshoots the
0mV (balanced) mark to
about +30mV.

17. AP: Stage 3 Repolarization Phase
• After 1 ms enough Na+ has entered that positive
charges resist entering the cell.
• Sodium channel closes 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

18. AP: Stage 4 Hyperpolarization Phase
• 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

19. Phases of Action Potential
1. Resting state
2. Depolarization phase
3. Repolarization phase
4. Hyperpolarization

20. Propagation of 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

21. Propagation of Action Potential