What causes an axon potential to occur at the axon hillock?

Depolarization to threshold

What happens at threshold?

Sodium channels open

What happens if there is a weak stimulus at the axon hillock and threshold is not reached?

The signal just decays

Do action potentials always have the same amplitude and the same duration?

Yes

What happens to sodium voltage-gated channels at threshold?

They open

Explain how the positive feedback loop maintains the rising phase of the action potential.

Depolarization causes some sodium channels to open which results in more depolarization which opens more sodium channels , etc

The rising phase of the action potential ends when the positive feedback loop is interrupted. What two processes break the loop?

Sodium channels inactivate , potassium channels open

Describe the two gates on the voltage-gated sodium channels?

1. Voltage gate opens when threshold is reached

2. Inactivation gate which closes at +30 mV

When does the voltage-sensitive gate open

IN response to depolarization

What is the function of the time-sensitive inactivation gate?

To stop the depolarization phase and to determine the duration of the absolute refractory period

What happens to the voltage gated sodium channels at the peak of the action potential?

They inactivate

When do the voltage-gated potassium channels open

They begin to open as soon as the membrane reaches threshold, but they are slow to open and do not fully open until the peak of the depolarization phase

What happens when the voltage-gated potassium channels open and the potassium moves out of the cell?

The membrane repolarizes

What is hyperpolarization?

Membrane potential greater than ( more negative than ) -70

During the action potential, when does sodium permeability increase rapidly?

during the rising phase of the action potential

Why does hyperpolarization occur?

The potassium channels are slow to close and they remain open allowing potassium to approach its equilibrium potential of -90 mV

During the action potential, when does sodium permeability decrease rapidly?

during repolarization

During the action potential, when does potassium permeability decrease slowly?

during hyperpolarization

The rapid increase in sodium permeability is responsible for _____.

the rising phase of the action potential

The rapid decrease in sodium permeability and simultaneous increase in potassium permeability is responsible for _____.

the repolarization of the cell

The slow decline in potassium permeability is responsible for _____.

the hyperpolarization

What is the absolute refractory period?

Time when another action potential cannot be initiated regardless of the intensity of stimuli

Why can't a neuron generate another action potential during the absolute refractory period?

The sodium channels are inactive

What is the relative refractory period?

Time when another action potential can only be initiated if the initiating stimulus is suprathreshold ( ie larger than normal )

What two factors does conduction velocity depend on?

Axon diameter and myelination

What is the effect of axon diameter on conduction velocity?

Direct, as diameter increases , so does conduction velocity

What is the effect of myelin on conduction velocity?

Direct, as the degree of myelination increases , so does conduction velocity

Why do myelinated axons conduct action potentials faster than non myelinated axons?

The action potential is reinitiated less frequently

If potassium channels were suddenly opened , in which direction would potassium ions move ?

Into the cell

What prevents the “peak” of the action potential from reaching E_Na?

Sodium channel inactivation

Curare binds to and blocks the acetylcholine receptor at the neuromuscular junction. An isolated nerve muscle preparation treated with a less-than-saturating dose of curare would be associated with a decrease in the:

amplitude of EPSPS.

Jim observes a cell that has wrapped itself around the axons of several neurons.

Oligodendrocyte (CNS)

Jim observes a cell that has wrapped itself around the axon of a single neuron. He remembers reading a paper that suggests that some axons can regrow in the presence of this glial cell.

Schwann cell (PNS)

Jim observes a cell that is close to a synaptic cleft. He observes the same cell type at a blood-brain barrier.

Astrocyte

Jim observes a cell that is engulfing debris.

Microglia

Jim observes many of these cells surrounding the central canal of the spinal cord.

Ependymal cell