MEMBRANE POTENTIAL AND NERVE IMPULSE TRANSMISSION

Resting neurons maintain a difference in electrical charge across their cell membranes
The inside of the resting neuron is negatively charged, the outside is positively charged
When a neuron is stimulated this polarity is reversed, these reversals are called action potentials

ACTION POTENTIAL

Nerve impulses are conducted along the neuron by a wave of membrane polarity reversals (action potentials)
Chemical messengers (neurotransmitters) carry nervous impulses from one neuron to another across the synapse

THE NEURON MEMBRANE AT REST

Neuron maintains a resting membrane potential of about -70 millivolts across the cell membrane
Sodium(Na+) and potassium(K+) are the main ions involved

Na+ and K+ cannot pass through the lipid bilayer membrane
move through the membrane by using membrane proteins (pumps)

MEMBRANE PROTEINS

Membrane proteins do several things:

Some "leak" ions all the time
Some "leak" ions only when the cell has been stimulated (ion "gates")
Some "pump" ions by active transport

Na⁺ / K⁺ PUMP

Membrane proteins actively transport

sodium out of the cell
potassium in

Three Na+ are pumped out for every two K+ pumped in

result is the cell has more Na+ on the outside and more K+ on the inside

Na⁺ / K⁺ PUMP

The pumping of Na+ out makes the outside more positive and the inside of the cell more negative

STIMULATED NEURON

Nerve cells are unique in their ability to carry a signal using membrane potential changes
Stimulation of a neuron opens some of the membrane proteins (a.k.a. Na+ gates)

allows Na+ to pass freely into the cells

free flow of Na+ into the cell causes a reversal of membrane polarity

polarity reversal is called the action potential

ACTION POTENTIAL

reversal of polarity(action potential) moves along the cell like a wave
the membrane restores the resting potential very quickly

less than 7 milliseconds
The cell can be stimulated again

ALL-OR-NONE RESPONSE

once a threshold limit is reached any stronger stimulus will not increase the cell's response
A stimulus below the threshold also will not stimulate the neuron

SYNAPSE

neurons communicate across the synapse by using chemical messengers called neurotransmitters neurotransmitters may act to inhibit neurons or to excite neurons attachment of the neurotransmitters to presynaptic membrane receptores causes ion channels to open generate an action potential