BIOL 1002- Chapter 42.3-42.4 Nervous System Pt. 2

What starts an action potential?

A depolarization.

What happens when depolarization is large enough?

Voltage-activated Na⁺ channels open.

What happens when voltage-activated Na⁺ channels open?

Na⁺ causes rapid further depolarization.

Why is Na⁺ channel activation positive feedback?

Depolarization opens Na⁺ channels → more depolarization → more Na⁺ channels activate.

What happens after the rapid depolarization?

Na⁺ channels inactivate and voltage-activated K⁺ channels open.

What do voltage-activated K⁺ channels do?

Bring the membrane potential back toward resting potential.

What does "all-or-none" mean for an action potential?

Threshold reached = action potential; threshold not reached = no action potential.

What is threshold?

The level of depolarization needed to trigger an action potential.

Where does an action potential travel?

Down the axon toward the axon terminals.

What is a synapse?

Communication point between a presynaptic neuron and postsynaptic cell.

What is the presynaptic neuron?

The neuron sending the signal from its axon terminal.

What is the postsynaptic neuron?

The neuron receiving the signal.

What is the synaptic cleft?

The space between the presynaptic and postsynaptic cells.

Where are neurotransmitters stored before release?

In synaptic vesicles.

What is Step 1 of synaptic transmission?

Action potential arrives → presynaptic axon terminal depolarizes.

What is Step 2?

Depolarization opens voltage-activated Ca²⁺ channels → Ca²⁺ enters.

What is Step 3?

Ca²⁺ causes vesicles to release neurotransmitter into the synaptic cleft.

What is Step 4?

Neurotransmitter crosses the cleft and binds postsynaptic receptors.

What determines the postsynaptic response?

The neurotransmitter and receptor involved.

What does an excitatory signal do?

Depolarizes the postsynaptic neuron.

What does an inhibitory signal do?

Hyperpolarizes the postsynaptic neuron.

What neurotransmitter is an example of excitatory signaling?

Glutamate.

What neurotransmitter is an example of inhibitory signaling?

GABA.

What are examples of modulatory neurotransmitters?

Serotonin and dopamine.

What do modulators do?

Change how a neuron responds to other synaptic inputs.

Can one firing of one synapse usually trigger an action potential?

No.

How does a postsynaptic neuron reach threshold?

By summation of multiple synaptic inputs.

What happens if excitatory inputs bring the neuron to threshold?

It fires an action potential.

What would increased inhibitory/GABA signaling generally do?

Make reaching threshold less likely.