Transmission of Nerve Impulses

Nerve Impulse• An electrical current, travels along

dendrites or axons

• Ions move through voltage-gated channels

• Channels open/close in response to changes in electrical charge around them

Resting Membrane Potential• Polarized = charge difference

between inside and outside of cell membrane

• At rest, net negative charge on inside of cell

Resting Membrane Potential• Selectively permeable ion

channels• Maintained by sodium potassium

pumps (active transport)– let Potassium (K+) in easily– weak permeability to Sodium (Na+)

Resting Membrane Potential• Net Result:

– more Potassium (K+) inside neuron

– more Sodium (Na+) outside neuron

Stimulated Neuron• Stimulated neuron:

(Example: light, sound, pressure)• Nerve Impulse begins when

stimulus disturbs dendrite• Result: Membrane permeability

changes

Stimulated Neuron• Sodium channels open and

sodium ions flow into neuron• Lessens charge difference• Net charge inside neuron

more positive• Called Depolarization

Depolarization• Local region inside the neuron has

net positive charge and outside is net negative

• Neighboring voltage-gated sodium channels open

• If depolarization above threshold, there is complete reversal of membrane potential

Action Potential• Depolarization moves along

the membrane = action potential

• Called Action potential or nerve impulse

Action Potential• “All-or-none” response –

either conducted over entire axon, or doesn’t happen at all

Repolarization• Sodium channels close &

potassium channels open• Potassium (K+) floods out of

the cell• Result: restores internal

negative charge

Hyperpolarization• If K+ channels stay open after

reaching resting potential• More Potassium (K+) outside

cell than necessary• Result: Leaves greater negative

charge inside cell

Refractory period• Potassium and Sodium on

wrong sides of membrane• ‘Recovery period’• Action: Sodium and

Potassium channels restore them