Graded and Action Potentials

Core functional difference between Graded and Action potentials?

Graded Potentials: Transmit information across short distances (local rumor)

Action Potentials: Transmit information over long distances (broadcast signal)

5 key properties of a Graded potential

Local: first electrical signal generated

Variable Magnitude: They are graded( bigger/Smaller stimulus=bigger/smaller potential)

Decremental: They fade (decrease) as they travel from the source

Can be + or -: Depolarizing (toward threshold) or Hyperpolarizing (away from threshold)

Initiated by ligand-gated channels (Chemical keys)

Two main types of graded potentials and where are they found

Receptor Potential: At the peripheral endings of afferent (sensory neurons)

Synaptic Potential: at the postsynaptic neuron after a neurotransmitter binds

How to remember that graded potentials fade?

GRADED HILL. A ball (the signal) rolls down but loses energy and stops after a short distance. It doesn’t reach the bottom unless pushed really hard (to threshold)

Describe the shape and requiremnt of an action Potential

A rapid rise and fall in membrane voltage with a characteristic pattern. They require voltage-gated ion channels

What kind of cells are excitable and can generate APs

Neurons, Muscle cells, some Endocrine cells. They have excitable membranes (change membrane voltage in response to stimuli)

What two channels underlie the permeability changes during AP

Voltage gated NA+ and K+ channels

How do the speeds and features of V-G Na+ and K+ channels differ

Na+ channels open fast/close quickly. Have an inactivation gate that acts like a timer to stop the flow

K+ channels: Slower to open and close. No inactivation gate

Name the 7 phases of the membrane potential during an AP

1. Resting Membrane Potential

2. Depolarizing Stimulant

3. Rapid Depolarization

4. Overshoot (Peak, inside becomes +)

5. Repolarization

6. Afterhyperpolarization (Dip below RMP)

7. Return to RMP

What is happening to permeability during each AP phase

1. RMP: PK>PNA

2. Stimulus: PNA increases

3. Depolarization: PNA»PK (Massive Na+ influx)

4. Overshoot: PNA at max

5. Depolarization: PNA decreases, PK increases (K+ efflux)

6. afterhyperpolarization: PK»PNA

7. Recovery: PK decreases back to PK>PNA

Describe the positive and negative feedback loops of an AP

POSITIVE (STARTs it):
Depolarization → Opens V-G Na⁺ channels → More Na⁺ in → More depolarization (RUNS AWAY until channels inactivate).

NEGATIVE Feedback (STOPs it):
Depolarization → Opens slow V-G K⁺ channels → More K⁺ out → Repolarization.

What is the difference between a subthreshold and a threshold stimulus

Subthreshold: Too weak to open enough V-G NA+ channels. Only creates a graded potential

Threshold: Just strong enough o open sufficient V-G NA+ channels to trigger the positive feedback loop and an AP

What does All or None mean for APS?How do we sense stimulus strength then?

An AP either fully happens or does not at all. A stronger stimulus doesn’t make a bigger AP. INstead, the nervous system codes for strength by the frequency

How do local anesthetics like Novocaine work?

They block voltage-gated Na+ channels, preventing them from opening. This stops Ap propagation, so the graded pain signals never reach the brain

Why is it impossible to fire a 2nd AP during the absolute Refractory period

Because the V-G Na+ channels are either already wide open or inactivated. They can’t be re opened

During the relative refractory period, you can fire another AP, but it is harder. Why?

1. The membrane is hyperpolarized (further from threshold)

2. Some Na+ channels are still inactivated. You need a stronger than normal stimulus to overcome this

Why do Aps travel in one direction along an axon?

The membrane behind the Ap is in its refractory period (channels inactivated), so the AP can only move forward into the rested membrane ahead of it

Do Aps travel faster in large or small diameter axons. Why?

FASTER IN LARGER DIAMETERS. Its like water in a pipe: a wider axon offers less resistance to the flow of ions. I=V/R

Why are myelinated axons faster than unmyelinated axons

Myelin acts as an insulator. It prevents charge ions from leaking out across the membrane, so the signal can travel farther down the axon neeeding to be regenerated

How does an AP jump in a myelinated axon

V-G NA+ channels are concentrated at the Nodes of Ranvier(gaps in myelin). The Ap regenerates only at these nodes, leaping from node to node. SALTATORY CONDUCTION

what happens if myelin is damaged

APS fail or slow down dramatically. Charge leaks out between nodes, the signal can’t jump efficiently, and the axon may exhaust itself trying to depolarize the entire length

During the relative refractory period, why might a second action potential have a smaller amplitude?

Because not all voltage-gated channels have recovered from inactivation. with fewer channels available, the maximum Na+ influx is reduced, potentially leading to a slightly lower peak voltage

Imagine an Ap at one point in an axon. Why does it propagate only forawrd?

The membrane behind is refractory (inactivated). The membrane ahead is at Resting Membrane Potential with all channels ready. THe signal can only move forward into the ready tissue