nerve impulses

nerve impulses

• resting neuron

• depolarization

• action potential

• propagation of the action potential

• repolarization

resting neuron

• the plasma membrane at rest is polarized (has + or - charge)

• fewer positive ions are inside the cell than outside the cell

  • Na+ is located on the outside of the cell membrane, and K+ (with a slightly negative charge) is located on the inside of the cell; causes concentration gradient

  • when at rest, outside is positive, inside is negative

depolarization

• a stimulus depolarizes the neuron’s membrane

• the membrane is now permeable to Na+ as Na+ channels open

• a depolarized membrane allows Na+ to flow inside the membrane

• essentially, Na+ can now go inside cell, so its positive charge goes inside with it, leaving the outside of the cell without a charge

• causes membrane to become unstable, making nerve fire (depolarization)

• sodium rushes in first, K+ doesn’t leave at the same time; does it in a linear fashion; Na+, then K+

• as this is occurring, electrons goes down that nerve

action potential

• the movement of ions initiates an action potential (electrical stimulus; gaining potential energy; when you get to the top of an action potential [think hill]; muscle will contract) in the neuron due to a stimulus

• a graded potential (localization depolarization) exists where the inside of the membrane is more positive and the outside is less positive (opposite than resting)

propagation of the action potential

• if enough sodium enters the cell, the action potential (nerve impulss) starts and is propagated over the entire axon

• impulses travel faster when fibers have a myelin sheath

repolarization

• K ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane

• repolarization involves restoring the inside of the membrane to a negative charge and the out surface to a positive charge (going back to resting state)

• initial ionic conditions are restored using the sodium-potassium pump

• this pump, using ATP, restores the original configuration

• three Na ions are ejected from the cell while two K ions are returned to the cell