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

THE CELL MEMBRANE OF NEURONS HAVE AN UNEVEN DISTRUBTION OF CHARGES, WITH THE INSIDE MORE NEGATIVE THAN THE OUTSIDE (RESTING POTENTIAL -70mV)

THE BALANCE IS MAINTAINED BY THE Na/K PUMP – WHERE Na+ IS MORE CONCENTRATED OUTSIDE THE CELL, AND K+ IS MORE CONCENTRATED INSIDE THE CELL. THE CELL MEMBRANE ALLOWS K+ TO PASS MORE EASILY.

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 ARE VERY QUICK “FLIPS” OF THE RESTING POTENTIAL. Na & K GATES OPEN TO ALLOW THE IONS TO CROSS AND REVERSE POSITIONS (Na+ INSIDE … K+ OUTSIDE). THE REASON +VE IONS MOVE IS BECAUSE THE –VE IONS ARE TOO LARGE.

ACTION POTENTIALS START AT ONE SPECIFIC AREA AND THEN SPREAD OF PROPAGATE ALONG THE LENGTH OF THE AXON.

ACTION POTENTIALS ARE THE NERVE IMPULSES THAT OCCUR ALONG AXONS

IF A STIMULUS (SOUND, LIGHT, TOUCH, etc) OCCURS THAT IS STRONG ENOUGH TO REACH THE THRESHOLD LEVEL … AN ACTION POTENTIAL RESULTS. THERE IS NO DIFFERENCE IN THE STRENGTH OF THE ACTION POTENTIAL. THEY ARE “ALL OR NOTHING” EVENTS

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

CONFORMATION SHIFTS OCCUR IN THE Na+ CHANNELS THAT ALLOW Na+ IONS TO ENTER THE CELL

A. AT REST: ACTIVATION GATE CLOSED (AG) … INACTIVATION GATE OPEN (IG)

B. ACTIVATION: BOTH GATES OPEN

C. INACTIVATION: INACTIVATION GATE CLOSED (IG) … ACTIVATION GATE OPEN (AG)

D. REFRACTORY PERIOD: BOTH GATES CLOSED

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)

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

QUESTIONHOW FAST DOES AN ACTION POTENTIAL TRAVEL?

ROUGHLY A RATE OF 2 m/s

HOW CAN WE INCREASE THE SPEED?

1. THE DIAMETER OF THE AXON IS PROPORTIONAL TO THE SPEED OF THE ACTION POTENTIAL

THE GIANT SQUID HAS SOME AXONS ABOUT 1mm IN DIAMETER. IF OUR AXONS WERE AS LARGE HUMANS WOULD HAVE LARGE HEADS AND WE WOULD HAVE TO KEEP THOSE MUSCLES CLOSE TO THE DENDRITE OF THE NEURON.

2. USE MYELIN

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