Unit 2.3: Graded Potentials & Action Potentials Flashcards
Where do graded potentials occur?
In the dendrites or cell body of the neurons`
What are graded potentials triggered by?
The opening or closing of ion channels
Why are graded potentials called “graded”?
Because the amplitude of the potential is proportional to the strength of the triggering event
Why do graded potentials lose strength?
- Current leak
2. Cytoplasmic resistance
What causes ions to enter the cell?
Initiated by neurotransmitters binding to the membrane receptors and opening ion channels
What happens after the neurotransmitter binds?
- ion channels open
- Ions move in (Na+) or out (K+) of the neuron along their electrochemical gradient
- A wave of depolarization spreads through the cell
What is signal strength determined by?
Strength is determined by number of ions entering the cell
How do action potentials differ from graded potentials?
- Action potentials are all identical (no volume control, they are on or off)
- Action potentials do not diminish in strength as they travel long distance through the neuron, signal is strong all the way
Where do action potentials start?
At the trigger zone, also called axon hillock
What is the axon hillock known to the neuron?
The integrating center of the neuron
Where is the trigger zone in afferent and efferent neurons/interneurons?
Afferent (sensory) neurons: adjacent to the receptor
Efferent (motor) neurons/interneurons: axon hillock and initial segment (very first part of the axon)
What is the minimum depolarization necessary to trigger an action potential called? What is its value?
Threshold potential, -55 mV
What are the 2 ways graded potentials can sum over?
- Spatial summation
2. Temporal summation
What are depolarizing graded potentials?
Make the membrane potential less negative
Brings the membrane potential closer to the threshold potential
What are depolarizing graded potentials also called? Why?
Excitatory Post Synaptic Potentials (EPSP)
Because they increase the chance of exciting the axon to “fire”
What are hyperpolarizing graded potentials?
Make the membrane potential more negative
Brings the membrane potential away to the threshold potential
What are hyperpolarizing graded potentials also called? Why?
Inhibitory Post Synaptic Potentials (EPSP)
Because they decrease the chance of exciting the axon to “fire”
What are the phases of the action potential?
- Resting membrane potential (-70 mV)
- Depolarizing stimulus -> reaches threshold potential
- Rising (depolarization) phase -> max +30 mV
- Repolarization phase -> membrane potential travels towards resting potential
- Hyperpolarization phease -> membrane potential overshoots resting potential
- resting membrane potential -> repolarization back to resting
What is sodium activation?
Action potentials cannot fire unless the voltage gated Na+ channels open
What sets the threshold potential?
Sodium channels properties
They open at -55 mV
What are the steps to the rising phase?
- Resting membrane potential
- Depolarizing stimulus
- Voltage-gated Na+ channels open
- Na+ enters the cell
- Causes further depolarization
- More voltage-gated Na+ channels opens
What stops the positive feedback loops of the Na+?
After a short delay, the inactivation gate closes, preventing further Na+ entry to the cell
How does the membrane potential return to the resting level?
K+ leave the cells -> falling phase of AP
What is the gating potential of K+?
+30 mV
What are the 2 parts of the refractory period?
- Absolute refractory period (NO action potentials can be triggered at all)
- Relative refractory period (really large suprethreshold stimulus is required)
Why can’t an action potential be generated during the absolute refractory period?
Because the Na+ channel is in the inactive state
Why does it take a suprathreshold stimulus to generate an AP during the relative refractory period?
Because the K+ channels are still open to more Na+ is needed to reach threshold potential
Why is the refractory period important?
It sets the direction of the current flow, prevents temporal summation and it prevents the AP from going backwards
How can action potentials travel long distances along neurons without decreasing in strength?
Because of the Na+ channel positive feedback loop
What determines how fast an action potential can travel along the neuron?
The resistance of the membrane
How can membrane resistance be altered?
By changing the diameter of the neuron or by reducing the amount of leakiness
How are axons in invertebrates?
They have GIANT axons
This lowers the surface area to volume ratio
Reduces leakiness and increases conduction
How are axons in vertebrates?
Vertebrates have insulated nerves
Reduces resistance and increases conduction
What is a break in the myelin sheath called?
Node of Ranvier
What is saltatory conduction>
Na+ channels are concentrated in nodes of Ranvier
Action potentials jump from one node to the other
Conduction is faster
