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
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
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
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.
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
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
