A and P - chp 12 pt 2 - Development of Action Potentials (nerve impulses)

definition of action potentials (nerve impulses)

an abrupt change in the electrical potential difference across the cell membrane which occurs after a stimulus

Where does an action potential begin?

axon hillock

what other set of channel proteins are involved?

• voltage-gated sodium channels and voltage-gated potassium channels

• they open and close in response to a change in transmembrane potential

what is the 1 step?

1. resting neuron

   • INSIDE = low Na+

   • OUTSIDE = high K+

   • -70 mV

what is 2nd step?

2. application of stimulus

   • localized change in resting membrane potential causes depolarization to threshold

   • -70mV → -60 mV

what is 3rd step?

3. Depolarization

   • voltage-gated sodium channel opens when threshold is reached

   • plasma membrane becomes permeable to Na+

   • Na+ quickly flows down its concentration gradiet

   • -60 mV → +10mV

what is the 4th step?

4. repolarization

   • voltage-gated sodium channels close at +30 mV = sodium channel inactivation

   • sodium permeability and sodium movement stops

   • voltage-gated potassium channels OPEN

   • plasma membrane becomes permeable to K+

   • K+ flows out of the cell down its concentration gradient +30 mV to -90mV ( hyperpolarization because K+ channels are slow to close) and then to resting -70 mV

PIC of graph for generation of action potential

what is the absolute refractory period?

period when membrane cant respond to new stimulus

what is the relative refractory period?

Na+ channels are returning to normal resting levels - membrane can respond to normal than larger stimulus

What is the propagation of an action potential: “The wave” mean?

• action potentials are relayed from one location to another in a series of steps

• initiated at axon hillock

• occurs in only one direction in unmyelinated axons - can only go forward

• wave goes from resting → depolarizing → absolute refractory period