Cell Membrane Potential
Chapter 9: Nervous System
Unit 3: Integration and Coordination
Neurons communicate with one another
through lots of nerve action potentials
Generation of action potentials depends
on two features of the cell membrane
1. A resting membrane potential
2. Existence of ion channels
Cell Membrane Potential
A Membrane Potential is when body cells
exhibit a difference in the amount of electrical
charge on the inside and outside of the
Cells with a membrane potential = polarized
Like voltage stored in a battery!
Polarization occurs due to an unequal distribution of
positive and negative ions between sides of the
Resting membrane potential = neurons at “rest”,
not conducting action potentials
Flow of Ions
Connecting the terminals of a battery with a
piece of metal to a radio an electrical current
flows from the battery allowing you to listen to
In living tissues, a flow of ions generates this
Ions can flow across the membrane through
Some channels are always open, and others can be
opened and closed.
Channels can also be selective.
What are Ion Channels?
When open, ion channels allow specific
ions to diffuse across the membrane (high
Also, + charged ions will move towards a -
charged area and vice versa
There are 2 Types of ion channels
1. Leakage Channels
2. Gated Channels
Types of Ion Channels
Leakage channels - allow a slow but steady
stream of ions to leak across the
Example - Sodium (Na +), Potassium (K+), Cl-
Gated channels - open and close on
Example - Voltage-gated channels open in
response to a change in membrane potential
Two main ions are involved in potentials,
K+ and Na+.
Resting potential arises from the
unequal distributions of various ions in
cytosol and interstitial fluid.
In a resting neuron, the outside of the
membrane has a + charge and inside
the membrane has a - charge.
This creates potential energy, -70mV
Nerve cells exhibit electrical
Changes (stimuli) usually affect the resting
potential in a particular region of a nerve
When a membrane’s resting potential
decreases (as the inside of the membrane
becomes less negative when compared to the
outside), the membrane is said to be
Changes in Membrane Potential
Changes in the resting potential of a
membrane have two main phases:
1. Depolarizing Phase - reversal of charge inside to
2. Repolarizing Phase - membrane polarization is
restored to resting state
Changes are directly proportional to the intensity of
If additional stimulation arrives before the effect of
previous stimulation subsides, summation takes
As a result of summated potentials, a level called
Threshold Potential may be reached.
An action potential (AP) is a sequence of
rapidly occurring events that decrease and
reverse the membrane potential before
eventually restoring it to the resting state
Generation of Action Potential
Many subthreshold potential changes must
combine to reach threshold and create an
At the threshold potential, permeability suddenly
changes at the region of the cell membrane being
Channels highly selective for sodium ions open
and allow sodium to diffuse freely inward.
As sodium ions diffuse inward, the membrane loses its
negative electrical charge and becomes Depolarized.
Action Potential Continued
As sodium ions diffuse inward, the membrane
loses its negative electrical charge and becomes
At almost the same time, potassium ions diffuse
outward, and the inside of the membrane becomes
negatively charged once more.
The membrane become Repolarized, and it remains
in this state until stimulated again.
Major Events of Action Potential
When the membrane reaches threshold,
sodium channels open, some sodium
diffuses in, and the membrane is
Soon afterward, potassium channels open.
Potassium ions diffuse out, and the membrane
Axons are capable of AP’s, dendrites and the
cell body are not.
AP’s in one region of a nerve cell membrane
cause a bioelectric current to flow to adjacent
portions of the membrane.
This Local Current stimulates the adjacent
membrane to its threshold level and triggers
another action potential.
A wave of action potentials move down the axon to
This propagation of action potentials along a nerve
axon constitutes a Nerve Impulse.
Events of a Nerve Impulse
1. Neuron membrane maintains resting
2. Threshold stimulus is received.
3. Sodium channels in a local region of the
4. Sodium ions diffuse inward,
depolarizing the membrane.
5. Potassium channels in the membrane
6. Potassium ions diffuse outward,
repolarizing the membrane.
7. The resulting action potential causes a
local bioelectric current that stimulates
adjacent portions of the membrane.
8. Wave of action potentials travels the
length of the axon as a nerve impulse.
A myelinated axon functions as an
insulator and prevents almost all ion flow
through the membrane it encloses.
Nodes of Ranvier between adjacent
Schwann cells interrupt the sheath.
Action potentials occur at these nodes, and
jump from node to node.
This is called saltatory conduction.
Speed of Nerve Impulses
The speed of nerve impulse conduction is
proportional to the diameter of the axon.
The greater the diameter, the faster the
An impulse on a thick, myelinated motor neuron of
skeletal muscle and travel 120 meters/second!
A thin, unmyelinated skin neuron might be 0.5
Nerve impulse conduction is an all-or-
If a neuron responds at all, it responds
A nerve impulse is conducted whenever a
stimulus of threshold intensity or above is
applied to an axon, and all impulses carried on
that axon are of the same strength.
A greater intensity of stimulation does not produce
a stronger impulse, but more impulses per