Nerve Impulse
Resting Neuron
- The plasma membrane at rest is polarized, as long as inside the cell is more negative than outside, the cell stays at rest
- Fewer positive ions are inside (K+) the cell than outside (Na+)
- A stimulus depolarizes the neuron's membrane, it is now permeable to sodium as sodium channels open (a depolarized membrane allows Na+ to flow inside the membrane)
- a stimulus leads the movement of ions, which in turn becomes a graded potential (localized depolarization - inside of the membrane becomes more positive than the outside)
- if stimulus is strong enough and enough sodium flows into the cell, the neuron is able to conduct an action potential (nerve impulse)
- the nerve impulse is either propagated or not, there is no partial nerve impulse
- K+ ions rush out of the neuron after sodium rushes in, depolarizing the membrane
- the inside of the membrane is restored to a negative charge, compared to the positive charge outside the cell
- until depolarization is complete, a neuron cannot conduct another nerve impulse
- Initial conditions are restored using the sodium-potassium pump, which uses ATP to pump 3 sodium ions out for every 2 potassium ions into the cell
- When the action potential reaches the axon terminal, the electrical charge opens calcium channels
- the released calcium causes tiny neurotransmitter filled vesicles to fuse with the axon terminal for release into the synapse
- the neurotransmitter molecules diffuse across the synapse and bind to receptors on the membrane of the next neuron
- once the signal leaves the axon terminal it changes from being electrical to chemical