Week 3 Flashcards
(19 cards)
Threshold of Excitation
The voltage level required for an action potential to occur
- -55mv
What must happen to bring about the necessary change in RMP for an action potential to occur
- One nerve cell needs to receive enough excitatory stimulation to bring RMP up to the threshold of excitation
Inhibitory Stimulation
- Inhibitory signals hyperpolarise the cell i.e. they increase the negative charge inside f the cell relative to outside of the cell
- This makes action potential less likely to occur
Excitatory Stimulation
- Excitatory signals depolarise the cell i.e. they decrease the negative charge inside of the cell relative to outside of the cell
- This makes action potential more likely to happen
Change in voltage for Action Potential to occur
-70mv to -55mv
Action Potential
-A brief reversal in electrical charge of an axon
- Starts at the end of the axon nearest the soma and travels towards the terminal buttons
- If RMP moves past threshold membrane potential, it quickly moves to +40 mv and then returns to resting
Stage 1 of Action Potential
- Na ion channels open up allowing Na+ ions to enter the cell due to diffusion and forces of electrostatic pressure
Stage 2 of Action Potential
-When Na ions enter the cell, the membrane potential is reversed
- The reversal in membrane potential causes nearby Na channels to open down the axon
Step 3 of Action Potential
-Na gates close and K+ voltage sensitive gates open
- K+ ions leave the cell as the nerve cell membrane is now positive and so are the K+ ions
- Diffusion and electrostatic pressure causes the K+ to move outside, causing the cell to become negative again and returning RMP to normal
Step 4 of Action Potential
The ion transporters pump the Na and K ions back to their original positions, fully restoring RMP
Myelin Sheath
A fatty material made up of glial cells that insulates some axons to allow faster movement of electrical impulses along the axon
Ionic Basis for Action Potential
- Na+ enters on the upswing of the spike, when the voltage sensitive gates opened
- K+ leaves on downswing of the spike due to diffusion and electrostatic pressure
Relative Refractory Period
- The brief period of time following action potential when a neurons membrane potential is more negative, making it harder to fire again
- Typically lasts 2 milliseconds
- Means that a stronger stimulus is required to trigger a second action potential
Absolute Refractory Period
- Just after a neuron has fired an action potential, it cannot generate another
- Many Na channels cannot open i.e. an action potential cannot fire when one is already firing
Hyperpolarisation Phase
- As the RMP returns to -70mv, the previously opened ion channels require a period to revert to initial conformation
- There is a brief dip in RMP below normal resting voltage; about -80mv
- A stronger stimulus required to achieve action potential
All or none Principle
-The principle that when a neuron fires, it fires with the same potency each time; a neuron either fires or does not, although the frequency can vary
- Intensity of the stimulus is independent to the strength of the action potential
Other properties of Action Potential
- Notion of successive patches of membrane in a non-myelinated axon
- Has a fixed amplitude
- Has a conduction velocity in meters/ second that is impacted by factors such as age, health condition
Non- Myelinated Axons
- The myelin sheath increases the speed/ velocity of action potential
- Myelinated axons have nodes of Ranvier which means the axon only needs to polarise the nodes and can skip huge parts of the axon
Nodes of Ranvier
Small gaps of exposed axon between the segments of the myelin sheath where action potentials take place
