Download Membrane and Action Potentials

Membrane and Action Potentials
Proteins Involved in Establishing Membrane Potentials and Action Potentials
K+ and Na+ Leak
K+ and Na+ VoltageNa+/K+ ATPase pump
Channels
gated Channels
Always open
Allow ions to travels w/ their
concentration gradient
open/close in response to
change in membrane potential
3 Na+ out
2 K+ in
^ this creates a net negative
charge establishing the
electrical potential and it
establishes the concentration
gradients of each ion
Uses 1 ATP
Membrane Potentials




Established by ion concentration gradients and electrical potential
o K+ = inside the cell. Na+ = outside the cell.
Our neuron cells have a normal negative membrane potential (resting
potential = -70mV) due to the Na+/K+ ATPase pump
Equilibrium potential basically is the membrane potential
o It is the equilibrium b/w the strength of the concentration gradient
(diffusion potential) vs. the strength of the electrical gradient
o K+ wants to leave the cell through the K+ leak channels due to its
concentration gradient; however, it stays inside the cell b/c the
strength of the electrical gradient for K+ is stronger than the strength
of the concentration gradient.
Action Potentials


In simple terms, moving Na+ into the neuron creates an action potential.
1. Resting State (membrane potential negative): both the Na+ voltage
gated channel and K+ voltage gated channel are closed
a. Na+ outside cell
b. K+ inside cell
2. Depolarization (membrane potential positive): The membrane gets
depolarized which causes only the activation gate of the Na+ voltage
gated channel to open
a. So, Na+ goes INTO the cell
b. K+ stays inside cell
3. Repolarization (membrane potential returning to negative) happens
b/c the inactivation gate of the Na+ voltage gated channel closes while
the activation gate of the K+ voltage gated channel opens
a. Na+ stays outside cell
b. K+ goes INTO the cell
4. Hyperpolarization (membrane potential more negative than resting)
occurs b/c some K+ voltage gated channels take longer to close than
Na+ voltage gated channels
5. K+ leak channels and Na+/K+ ATPase pump reestablish the resting
membrane potential


Absolute refractory period occurs b/c the inactivation gates of the Na+
voltage gated channel are closed. Therefore, no matter what you cannot fire
off another action potential at this time.
During the relative refractory period you can fire off another action
potential but only if it is a very strong depolarizer in order to make the
activation gate of the Na+ voltage gated channel to reopen.
Propagation of the AP
 It is an all or nothing response  once the threshold is met the AP will fire
from the axon hillock down the axon with the same strength along the entire
axon.
o Myelination ensures that the strength of the AP is the same
throughout and flows continuously
o Also, there are Na+ voltage gated channels located in the nodes of
Ranvier that continue to propagate the signal. This is known as
salutatory conduction.
o


Note that larger axons are faster than smaller axons b/c they face less
resistance.
If the neuron is unmyelinated, then it only relies on Na+ voltage-gated
channels to keep renewing the AP… therefore, the impulse is choppy. This is
known as continuous conduction.

Graded potential: the stronger the stimulus the stronger the cell gets depolarized.
 Basically any change in the membrane potential is considered a graded
potential… some changes aren’t strong enough to elicit an AP while others
are. It just depends on whether or not they reach the threshold.
o If they do reach the threshold then the graded potential turns into an
action potential and the same strength is carried throughout the axon.
 Unlike AP’s, graded potentials can lose strength as they travel. This is
because it travels from the dendrite to the axon hillock and there is no
myelination and less Na+ voltage gated channels.

Synaptic Terminal
The AP is transmitted to another cell by either electrical synapses or
chemical synapses (thoroughly explained later in this document) by the
release of neurotransmitters (NTs) from the pre-synaptic cell.
AP  depolarization of presynaptic terminal membrane causes Ca2+ to rush in 
vesicles filled w/ NT’s fuse w/ presynaptic terminal membrane  NT’s released into
synaptic cleft  NT’s bind to receptor on postsynaptic cell  either cations or
anions rush in and transmit or inhibit the AP
o Inhibitory NTs  hyperpolarize the postsynaptic membrane by
causing Cl- to rush in  inhibits transmitting the AP
o Excitatory NTs  depolarize the postsynaptic membrane by causing
Na+ to rush in  transmitting the AP

Clinical Relevance:Tetrodotoxin (toxin from pufferfish) and lidocaine
(anesthetic) block Na+ voltage gated channels