2. A NERVE IS USUALLY COMPOSED OF A BUNDLE OF NEURONS
WITH GLIAL CELLS AND BLOOD VESSELS TO SUPPLY NEEDED
MATERIALS.
NEURONS ARE NOT CONNECTED DIRECTLY TO ONE ANOTHER –
THERE ARE GAPS (SYNAPSES) BETWEEN THE NEURONS.
ACTION POTENTIALS TRAVEL ALONG THE AXONS OF NEURONS

3. THE CELL MEMBRANE OF NEURONS HAVE AN UNEVEN
DISTRUBTION OF CHARGES, WITH THE INSIDE MORE NEGATIVE
THAN THE OUTSIDE (RESTING POTENTIAL -70mV)
A CHANGE IN THIS BALANCE BY MOVING IONS CAUSES AN
ELECTRICAL CHARGE ALONG THE LENGTH OF THE CELL  THIS IS
AN ACTION POTENTIAL.
ACTION POTENTIALS START AT ONE SPECIFIC AREA AND THEN
SPREAD OF PROPAGATE ALONG THE LENGTH OF THE AXON.

4. IF A STIMULUS OCCURS THAT IS STRONG ENOUGH TO REACH
THE THRESHOLD LEVEL AN ACTION POTENTIAL RESULTS.
THEY ARE “ALL OR NOTHING” EVENTS

6. A STIMULUS CAUSES Na+ LEAK CHANNELS TO
OPEN IN THE CELL MEMBRANE IN A SMALL
AREA.
THIS DEPOLARIZES A SMALL AREA OF THE
AXON AS Na+ RUSH INTO THE CELL (GRADED
POTENTIAL). IF THIS REACHES THRESHOLD
THE VOLTAGE GATED Na+ CHANNELS OPEN
CAUSING MORE Na+ TO FLOOD INTO THE
CELL.
POSITIVE FEEDBACK MECHANISMS CAUSE
MORE AND MORE Na+ GATED CHANNELS TO
OPEN CAUSING THE MEMBRANE POTENTIAL
TO BECOME MORE AND MORE POSITIVE.
THE AMOUNT OF Na+ IN EXCEEDS THE
AMOUNT OF K+ GOING OUT

7. ONCE THE MEMBRANE POTENTIAL INSIDE THE
CELL BECOMES POSITIVE THE VOLTAGE GATED
Na+ CHANNELS SNAP SHUT AND THERE IS NO
INFLUX OF Na+ INTO THE CELL.
THIS IS THE „PEAK‟ OF THE ACTION POTENTIAL
GATES FOR K+ IONS OPEN THAT ALLOW K+ TO
LEAK ACROSS THE CELL MEMBRANE.
THIS IS A NEGATIVE FEEDBACK MECHANISM
EVENTUALLY THE MEMBRANE RETURNS TO
RESTING POTENTIAL (-70mV)

8. HOWEVER… THE CONCENTRATIONS OF Na+ AND
K+ AREN‟T WHERE THEY WERE BEFORE THE
ACTION POTENTIAL.
THE CONCENTRATIONS HAVE BEEN REVERSED!
WE NOW HAVE MORE Na+ IN THE CELL AND MORE
K+ OUTSIDE THE CELL.
THE Na/K PUMP KICKS IN AND RESTORES THE
ORIGINAL CONCENTRATIONS.
UNTIL THIS OCCURS THE CELL IS
HYPERPOLARIZED AND CANNOT FIRE ANOTHER
ACTION POTENTIAL  REFRACTORY PERIOD

9. THE MYELIN SHEATHS SECRETED BY SCHWANN CELLS
INSULATE PORTIONS OF THE AXON, ALLOWING THE WAVE OF
DEPOLARIZATION TO “JUMP” TO THE NEXT GAP (NODE OF
RANVIER”
THIS INCREASES THE RATE TO AROUND 120 m/s