Lecture 2 NS Physio / APs Flashcards
Describe the concentrations of sodium and potassium inside and outside the neuronal cell membrane
Na+ is higher outside the cell 140mM
K+ is higher inside the cell 140mM
Na+ 15 mM inside
K+ is 5mM outside
All neurons have different electrical properties resulting from
A)
B)
A) differences in ion concentration across the cell membrane
B)special permeability characteristics of the plasma membrane to ions
How are the concentration differences MAINTAINED across the neuronal cell membrane?
Na+ K+ ATPase pump located on the cell membrane
What is the permeability of the neuronal plasma membrane to ions due to?
The presence of ion channels located in the membrane.
The ions follow the rules of diffusion aka; Na+ will move IN and K+ will move out
Describe non-gated ion channels in relation to the neuronal membrane.
Always in the open position
Higher number of K* channels than Na+
At rest is the neuronal membrane more permeable to k+ or Na+ ?
K+
There is more non-gated channels
What are gated ion channels?
And the 3 types ?
Not involved at rest.
Require stimulation to open. Opening to stimulus defines the type of gate:
Voltage gated
Chemical gated - binding of nT of a hormone
Mechanical gated - temperature changes
What is potential difference ?
The DIFFERENCE in charge across a cell membrane. If the inside is -70. That means the inside of the cell is 70 mV more negative than the outside of the cell.
Resting membrane potential is
-70mV potential difference
Inside is 70 times more negative than out
Describe the Na+ / K+ ATPase pump
Uses 1 ATP to pump 3 Na+ out and 2K+ in. It maintains RMP.
Moves ions against gradient.
Only slightly contributes to RMP
3 reasons the resting membrane potential is maintained
1) Na+ k+ ATPase pump
2) the high amount of nongated K+ channels
3) the negatively charged organic molecules inside a cell
How does the higher preponderance of non gated K+ channels help maintain RMP?
The membrane is more permeable to K+ than Na+ at rest.
K+ diffuses out I the cell down it’s gradient making the inside more negative.
How does large negatively charged molecules in the cell help maintain RMP?
Too large to leave cell and carry a negative charge
So when K+ leaves the inside is more negative
When the cell becomes more negative toward RMP what begins to happen?
Diffusion of K out slows
Na+ diffusion in increases
At some point they stabilize so that the net movement of charge is 0 and the RMP is -70mv
When is the net movement of charge 0 in a neuron?
When resting membrane potential is reached.
What is a polarized membrane ?
An unequal distribution of charge
Just means not balanced
This unequal distribution of charge is also known as A POTENTIAL DIFFERENCE
There are only 2 types of cells that are electrically excitable, what are they?
Muscle and nerve cells
What does an electrically excitable cell entail?
They are able to produce departures from the resting membrane potential. Called an ACTION POTENTIAL.
What is an action potential ?
an impulse
A large change in membrane potential that propagates along an axon with no change in intensity.
Action potentials are produced in response to stimuli which are
Changes in internal or external environment of the body
What Is a graded potential ?
Small change in membrane potential usually on the dendrite or cell body
Causes ions to move - travel passively a short distance.
Short lived UNLESS the grades potential is large enough to reach threshold potential .. Then an action potential will occur.
True or false the larger the stimulus the larger the graded potential?
True
What kind of transport is involved in the ions when a graded potential occurs?
Passive
Creating a flow of electrical current
Graded potentials can occur two different ways what are they?
Depolarization.
And
hyperpolarization
What is depolarization
Give an example
The resting membrane potential is becoming less polar . In a cell whose RMP is -70mV .. A depolarization would be if a stimulus occurred that changed it’s mV to 65mV
Describe hyperpolarization and give an example
This occurs when the stimulus makes the cell more polar. In the case of a cell with RMP of -70 mV a hyperpolarization would cause the cell to become more negative ie, -80mV
Depolarization and hyperpolarization make use of which chanels?
gated ion channels
Graded potentials are given different names for different areas, what are they?
Receptor potential or generator potential - sensory receptor excited
Post synaptic potential - when stimulus is a nT released by another neuron
Repolarization is
Return to resting membrane potential
Summation (of graded potential)
If the first graded potential is still present when the second stimulus happens the resulting graded potential will be larger than the first
Why are graded potentials important ?
Because if the GP is large enough than AP will occur
What potential difference must be reached at axon hillock for an AP to occur ?
-55mV @ axon hillock
Where does an action potential begin?
Along what does it propagate ?
Axon hillock (trigger zone) or dendrites of UNIPOLAR neurons
Axon
Why are graded potentials short distance signals ?
Current is lost through leaky plasma membrane, the voltage declines with distance from the stimulus ie voltage is decremental
Sequence of events of action potential
1) -70mv - graded potential occurs, if reaches -55mv then the action potential takes place which is steps 2, 3, and 4.
2) depolarization - +30mv
3) repolarization
4) hyperpolarization , below -70mv
Describe the depolarization of action potential
MUST BE ABOVE THRESHOLD (-55)
Voltage gated Na+ channels open
Na+ rushes in cell depolarizing it
The inside of the cell becomes positively charged to +30mV
Describe the repolarization phase of action potential
The Na+ channels become inactivated as the cell becomes more polar again
The voltage gated channels for k+ also open and that allows K+ to exit the cell.- making cell more negative.
Approaches RMP again
Describe what happens during hyperpolarization and after
As K+ rapidly leaves the cell via the voltage gated channels there is a period where the cell is more negative than RMP = hyperpolarization
The gated channels then close their gates and RMP is re-established via diffusion / Na+ K+ ATPase pump
During what stage of action potential is K+ most permeable?
Repolarization
The Na+ gates have become inactivated once +30 has been reached and the cell wants to become more negative quickly so K+ gates open and it rushes out.
What is the absolute refractory period ?
The period in which an action potential is not possible even by the largest stimuli
Why does an absolute refractory period occur ?
1) because the Na+ channels are still open (depolarization)
2) Na+ channels inactivated (cannot open until membrane potential passes resting membrane potential)
What is the relative refractory period ?
The period when an action potential can be generate but requires a stimulus larger than normal.
After the membrane potential passes resting membrane potential on the way to hyperpolarization the Na+ gates are able to be reopened. The k+ channels are open and therefore leaving the cell ; hyperpolarized, so now a greater than normal stimulus is needed to reach threshold.
What is the all or none principal ?
Action potentials are all or nothing.
-55 must be reached or AP will not occur
All APs look the same.
If a stimulus is very strong the AP will not be different
A weak stimulus will not generate AP because it’s below threshold
What types of gated channels ate associated with graded potentials… With action potentials ?
Chemical and mechanical for gated
Voltage for AP
Describe what happens when an action potential propagates:
Depolarization during the action potential:
Positive charge moves toward more negative charge on adjacent membrane. The depolarization is large enough and gets another action potential .
Why do action potentials move in only one direction ?
Because the preceding membrane is still in refractory period.
The rate of action potential is dependant on a couple things, what are they ?
1) fiber diameter
2) myelination
If a fiber undergoing action potential is large, do you think the speed of propagation would be faster or slower ? Why?
Faster
There is less resistance to current
Aka; less stuff in the way
Would the speed of propagation be Slower in a myelinated fiber or an unmyelinated one?
Slower in the unmyelinated
Where does the action potential occur on myelinated fibers?
What is a name for myelinated fiber conduction ?
Nodes of Ranvier
Saltatory conduction
Where does action potential occur in an unmyelinated fiber?
What’s a name for this kind of conduction ?
All along the length of the fiber
This is called continuous conduction
Slower
