W7 - POTENTIALS

RMP

• at rest, inside the neuron has a negative electrical charge (relative to outside)

  • relative more negatively charged ions internally

• RMP - difference in charge

• cell is said to be polarised (inside/outside different polarity)

• -70 mv = avg. RMP of neuron

• maintained by movement of ions

• potenial to communicate electrically

generation of RMP

1. more negative protein intracellular

2. difference in ion extracellular and intracellular

   3 K+ channel out, 1 Na+ channel in = more +ve ions out

3. transport of +ve ions across membrane

4. use of Na/K pump = 3 Na⁺ out of the cell and 2 K⁺ into the cell

graded potential

• small changes in membrane potential that are proportional to the strength of the signal

  • generated in dendrites and travels to axon hillock (determines whether stimulus great enough to lead to further communicated → AP)

Action potential

• all-or-none response to fluctuations to RMP

  • dependendent on cell reaching “threshold”

  • produces outgoing signal that travels down axon to be transmitted to another neuron/effector organ

Graded potential (neurotransmitters)

• Neurotransmitter is released from the axon terminal of the presynaptic neuron.

• It binds to chemically gated Na⁺ channels on the postsynaptic membrane (usually dendrites).

• The channels open, allowing Na⁺ to enter the neuron.

• This makes the inside of the neuron less negative → depolarisation.

• If enough Na⁺ enters and the threshold is reached, it may trigger an action potential

GP (stimuli)

• small stimuli = small change in MP (few Na+ enering, -70mV → -60 mV)

• large stimuli = large change in MP (more Na+ entering, -70mV → -50mV)

• strength of GP determines whether the neuron will fire (generate AP) at the axon hillock

• AP occurs we=hen threshold is reached (-55mV)

Excited AP

Excitatory postsynaptic potential (EPSP)

• GP depolarises cell (less -ve, more +ve)

• closer to the threshold (-55 mV)

• more likely to cause an AP

Inhibit AP

Inhibitory postsynaptic potential (EPSP)

• GP hyperolerises cell (more -ve)

  • Neurotransmitters bind to chemically gated chloride (Cl⁻) channels, allowing Cl⁻ ions to enter the neuron. This makes the inside more negative (hyperpolarizes), inhibiting the neuron from firing.

• further from threshold (-55mV)

• less likely to achieve AP

Action potential

1. threshold - When the membrane potential reaches around -55 mV, voltage-gated Na⁺ channels open.

2. depolarisation - Na⁺ rushes into the cell through open voltage-gated channels, making the inside more positive.

3. peak of action potential - Voltage-gated Na⁺ channels close, and voltage-gated K⁺ channels begin to open.

4. repolarization - K⁺ rushes out of the cell, causing the membrane potential to become negative again.

5. hyperpolarization - K⁺ channels stay open a bit too long, causing the cell to become more negative than resting potential (below -70 mV).

6. return to RMP - The Na⁺/K⁺ pump restores normal ion concentrations (Na⁺ out, K⁺ in) and brings the membrane potential back to -70 mV.

7. refractory period - tA short period when voltage-gated Na⁺ channels are inactivated, and the neuron cannot fire another action potential yet.

Action potential down the axon

• Signal starts at axon hillock when threshold is reached.

• Sodium (Na⁺) channels open → sodium enters → depolarization occurs.

• Depolarization opens sodium channels in the next region → signal moves forward.

• Behind the wave, potassium (K⁺) channels open → potassium exits → repolarization occurs.

• Refractory period prevents backward movement → ensures one-way travel.

• In myelinated axons, signal jumps between nodes (Nodes of Ranvier) → faster transmission.

• Action potential reaches axon terminal → triggers neurotransmitter release

they are seld popagating

Transmitting info

1. AP arrives at axon terminal in presynaptic neuron

2. AP causes voltage gated Ca2+ channels to open

3. Ca2+ rushes into axon terminal

4. ↑ Ca2+ inside cell = synaptic vesicles to fuse w/ membrane

5. exocytosis occurs, releasing neurotransmitter

6. neurotransmitter crosses synaptic cleft

7. neurotransmitter binds to dendrites of postsynaptic neuron

8. chemically gated ion channels open

9. ions enter/exit the cell, making the postsynaptic cell close/further away from threshold