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?
What happens to sodium voltage-gated channels at threshold?
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?
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 the action potential, when does potassium permeability decrease slowly?
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 _____.
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 ENa?
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.
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.
Jim observes a cell that is engulfing debris.
Jim observes many of these cells surrounding the central canal of the spinal cord.