Cardiac Muscle 5- Electrical conduction system

Cardiac Muscle 5- Electrical conduction system


– SO THE ELECTRICAL
CONDUCTION SYSTEM OF YOUR HEART IS HOW
THE HEART HAS DISTRIBUTED THE 1% OF ITS CELLS
THAT ARE AUTORHYTHMIC. THE AUTORHYTHMIC CELLS CAN GENERATE THEIR
OWN ACTION POTENTIALS. SO HERE’S THE SCOOP. I’M GOING TO TELL YOU
ABOUT SOME ANATOMICAL STRUCTURES THAT IT’S NOT REALLY
GROSS ANATOMY. LIKE IF WE CUT OPEN A HEART, CAN WE REALLY FIND
THESE STRUCTURES? THEY’RE EMBEDDED
IN THE HEART TISSUE, AND I GUARANTEE THAT WE COULD
EVALUATE THE FUNCTION OF THE HEART TISSUE
AND FIND THEM. BUT CAN WE ACTUALLY SEE THEM? NOT SO MUCH. THE FIRST
STRUCTURE IS CONSIDERED THE HEART’S PACEMAKER, AND THIS IS THE SA NODE,
THE SINOATRIAL NODE, AND IT’S THE PACEMAKER
OF THE HEART. ALL AUTORHYTHMIC
CELLS HAVE PACEMAKER POTENTIALS. ALL OF THEM COULD INITIATE
THE HEART’S CONTRACTION IF THEY ARE THE FIRST
ONES TO FIRE. AND HERE’S THE COOLEST PART,
THE SA NODE FIRES THE FASTEST. OUT OF ALL OF OUR
AUTORHYTHMIC CELL STRUCTURES THAT WE’RE GOING TO TALK ABOUT
IN THIS PART OF THE LECTURE, THE SA NODE FIRES THE FASTEST. WHEN IT FIRES,
EVERYBODY’S GOT TO LISTEN. WHY DO THEY HAVE TO LISTEN? BECAUSE THEY’RE ALL CONNECTED
BY TUNNELS, AND THE ACTION POTENTIAL,
WHICH IS THE, “OKAY, MAKE SOMETHING HAPPEN NOW,” THE ACTION POTENTIAL,
IT’S NOT LIKE A CHOICE. IT’S NOT LIKE THE OTHER
CONTRACTILE CELLS CAN BE LIKE, “YOU KNOW WHAT? I’M SICK OF THIS BOSSY SA NODE. I’M JUST GOING
TO SHUT MY GAP JUNCTIONS FOR A LITTLE WHILE
AND NOT LISTEN.” THEY GAP JUNCTIONS ARE TUNNELS, AND IF THE SA NODE SAYS,
“FIRE,” THAT ACTION POTENTIAL IS GOING
TO TRAVEL THROUGH THE WHOLE HEART. SO IF THE SA NODE SAYS,
“FIRE AT,” OH, LET’S SAY, “90 BEATS
PER MINUTE,” THEN THE HEART IS GOING
TO RESPOND, EVERYBODY’S GOING TO RESPOND,
AND BEAT 90 BEATS PER MINUTE. THE SA NODE IS NOT ACTING ALONE, AND IF IT WAS ACTING ALONE,
YOUR HEART WOULD BEAT IN AN EXTREMELY INEFFICIENT
AND UNORGANIZED MANNER. SO THERE ARE STRUCTURES CALLED
INTERNODAL PATHWAYS, AND I’M DRAWING ALL OF
THIS IN THE RIGHT ATRIUM. AND THAT’S BECAUSE MY TWO NODES, YOU’RE JUST GOING
TO HAVE TO BEAR WITH ME HERE, THIS WAS MY SA NODE UP
AT THE TOP OF THE RIGHT ATRIUM. TOWARD THE BOTTOM OF THE RIGHT
ATRIUM IS AN AV NODE. I GUESS I’M GOING TO GO
LIKE THIS, AV NODE. I DIDN’T EVEN NEED TO WRITE IT. THE AV NODE IS CONNECTED
TO THE SA NODE BY LITTLE STRUCTURES CALLED
INTERNODAL PATHWAYS. SO, OKAY, IT MAKES PERFECT
SENSE, RIGHT? YOU HAVE ONE NODE,
THE SA NODE, YOU HAVE ANOTHER NODE,
THE AV NODE, AND CONNECTING MY TWO
NODES ARE INTERNODAL PATHWAYS, BETWEEN THE NODE PATHWAYS. HERE’S THE PART
THAT I CAN’T DRAW FOR YOU. THE INTERNODAL PATHWAYS DO
CATCH THE LEFT ATRIUM, AS WELL. SO REMEMBER
THAT MY HEART IS JUST THIS BASICALLY EXPANDED BALL OF BLOOD VESSEL WITH ALL
THIS MUSCLE AROUND IT, AND SO THERE ARE PATHWAYS
BETWEEN THE ATRIA. WE COULD ACTUALLY
CONNECT THOSE TWO ATRIA, SO WE HAVE INTERNODAL PATHWAYS. AND TAKE A DEEP BREATH BECAUSE
THIS JUST ISN’T GOING TO WORK WITH THIS DIAGRAM, BUT THERE ARE PATHWAYS
THROUGH THE MYOCARDIUM THAT COULD GET US OVER HERE
INTO THE RIGHT ATRIUM. INTERNODAL PATHWAYS ARE
GOING TO GO AROUND AND CATCH ALL THE ATRIAL MUSCLE. AND NOW THINK ABOUT THIS. SA NODE FIRES
ITS ACTION POTENTIAL. THAT ACTION POTENTIAL SPREADS
ALL THROUGH BOTH ATRIA. MY PICTURE DOESN’T
DO IT JUSTICE, AND MOST PICTURES
DON’T DO IT JUSTICE, BUT THAT’S A REALLY
IMPORTANT CONCEPT TO UNDERSTAND. BECAUSE ONCE YOU HAVE
AN ACTION POTENTIAL THAT FIRES, CONTRACTILE CELLS WILL FOLLOW. SA NODE SAYS, “DUDE,
LET’S GO, 20 BEATS PER MINUTE,
BOOM, BOOM, BOOM.” CONTRACTILE CELLS SAY,
“THEY TOLD ME WHAT TO DO. I ALWAYS FOLLOW DIRECTIONS. I’M CONTRACTING,
TOLD ME TO DO IT AGAIN, 90 BEATS PER MINUTE.
I’M COOL. I’M CONTRACTING EVERY SINGLE
TIME, WITHOUT FAIL.” UNLESS YOU ARE NOT DOING WELL,
IN WHICH CASE, THEN WE MIGHT HAVE SOME ISSUES.
OKAY. SO NOW YOU HAVE THE MESSAGE,
THIS IS WILD, THE MESSAGE IS GATHERED
AT THE AV NODE. SO ACTION POTENTIAL TRAVELED
ALL THROUGH THE ATRIA, AND THEN ALL OF THAT CONVERGES
BACK ON THE AV NODE. DOES THAT SEEM WEIRD TO YOU? THERE IS A FUNCTIONAL VALUE
IN HAVING SOMEONE ELSE DELIVER THE MESSAGE
FROM THE ATRIA TO THE VENTRICLES AND CONTROLLING THE PATHWAY
THAT WE MUST FOLLOW TO GET AN ACTION POTENTIAL
INTO THE VENTRICLES. FROM THE AV NODE, I’M GOING
TO A NEW COLOR LIKE BLUE, FROM THE AV NODE, WE PASS
INTO THE VENTRICULAR SEPTUM, THE INTERVENTRICULAR SEPTUM. AND AGAIN
LIKE THIS IS A GARBAGE IMAGE, BECAUSE I’M PRETENDING
LIKE I’M GOING OVER THE VALVES IN THE PULMONARY TRUNK,
AND, DOGGIES, THAT AIN’T HOW IT ROLLS. SO ALL OF MY DRAWINGS ARE
ACTUALLY IN MYOCARDIUM, I’M JUST DRAWING IT
ON TOP OF A PICTURE THAT DOESN’T WANT
TO BE DRAWN ON TOP OF. THE AV NODE CONNECTS
TO THE INTERVENTRICULAR SEPTUM, WHICH IS THIS STRUCTURE,
THIS MYOCARDIUM, THIS MUSCLE THAT SEPARATES MY
TWO VENTRICLES. AND THAT STRUCTURE RIGHT
THERE IS CALLED THE AV BUNDLE, AND IT KIND OF MAKES SENSE. WE’VE GOT THE AV NODE THAT DELIVERS INFORMATION
TO THE AV BUNDLE. NOW THE SA NODE IS FIRING. THE AV NODE SAYS, “LET’S HANG
ON TO THIS FOR JUST A MINUTE. LET’S GO AHEAD, YEAH,
YOU TOLD ME WHAT TO DO. YOU TOLD ME TO FIRE
AN ACTION POTENTIAL. I WILL. I DON’T HAVE A CHOICE
ABOUT THE MATTER, BUT I’M GOING TO SLOW IT
DOWN JUST A TAD.” AND THE ACTION POTENTIAL, IF IT’S GOING TO GET
INTO THE VENTRICULAR MYOCARDIUM, THE ACTION POTENTIAL HAS TO GO
THROUGH THE AV NODE. SO AT ABOUT 1/20 OF THE PACE
THAT THE SA NODE SET, THE AV NODE SAYS,
“OKAY, HERE COMES YOUR ACTION
POTENTIAL, AV BUNDLE.” THE AV BUNDLE PASSES
THE ACTION POTENTIAL ON, AND THESE GUYS ARE ALL INNATELY
SLOWER THAN THE SA NODE. LIKE IF THEY WE
IN CHARGE OF THINGS, YOUR HEART RATE WOULD
DROP DRAMATICALLY. THE AV BUNDLE SPLITS INTO LEFT
AND RIGHT BUNDLE BRANCHES, AND THESE GUYS SPLIT INTO LITTLE
FIBERS CALLED PURKINJE FIBERS. AND REALLY THE WHOLE
THING IS PURKINJE FIBERS, BUT WE’VE GOT LEFT AND RIGHT
BUNDLE BRANCHES MADE OF STRINGS OF AUTORHYTHMIC CELLS. ALL OF THESE ARE THE SAME THING, AND THEN WE’VE GOT
THESE PURKINJE FIBERS THAT BRANCH FROM THE BOTTOM. HERE COME YOUR TWO
BUNDLE BRANCHES DOWN THE INTERVENTRICULAR
SEPTUM, AND THEN WE HAVE THIS NET
FROM THE BOTTOM OF THE HEART UP OF THESE PURKINJE FIBERS. AND THEY’RE AUTORHYTHMIC,
AND SO THEY’RE GOING TO BE ABLE TO GENERATE THEIR OWN
ACTION POTENTIALS. WHEN THEY GET THE MESSAGE
FROM THE BUNDLE BRANCHES, THEY’RE GOING TO FIRE.
NOW LOOK AT THIS. FIRST, WE GET THE MESSAGE
IN FROM SA NODE. SA NODE SENDS IT
ALL AROUND THE ATRIA AND COMES BACK
TO CONVERGE ON AV NODE. WHAT DOES THAT DO? MESSAGE TO THE ATRIA,
“DOGS, LET’S GO, CONTRACT.” ATRIA ARE GOING TO CONTRACT. AV NODE HANGS ON TO IT
FOR A SECOND, MEANWHILE YOUR ATRIA ARE
CONTRACTING. THE AV NODE FINALLY SAYS,
“OKAY, I THINK I’M COOL,” AND SENDS THE MESSAGE
THROUGH A VERY SMALL, RELATIVELY SPEAKING,
STRUCTURE, THE AV BUNDLE, AND THE LEFT
AND RIGHT BUNDLE BRANCHES. THESE ARE NOT NETS. THESE ARE GOING SPECIFICALLY BETWEEN LEFT
AND RIGHT VENTRICLES. AND SURE, WE HAVE A LITTLE BIT OF CONTRACTION THAT HAPPENS WITH
THE CONTRACTILE MUSCLE TISSUE IN THAT ZONE,
BUT WE’RE NOT HAVING A VENTRICULAR CONTRACTION YET,
NOT EVEN CLOSE. AS SOON AS
THE MESSAGE TRAVELS SUPER FAST INTO THOSE PURKINJE FIBERS,
NOW WE’VE NETTED. NOW, YOU CAN IMAGINE, IT’S LIKE SPREADING OUT
THE MESSAGE FROM THE BOTTOM UP. WHY?
WHY FROM THE BOTTOM UP? DUDE, IT’S FREAKING BRILLIANT. IF WE DID NOT– OKAY, I GOT TO DO IT BLACK. THAT’S COOL.
LOOK, THIS IS THE OUTLET. BLOOD HANGING OUT
IN THE VENTRICLE, IN ORDER TO GET
OUT OF THE VENTRICLE, IT HAS TO GO OUT THE TOP. SO IF YOU CONTRACTED
FROM THE TOP DOWN, YOU’RE GOING
TO SQUISH ALL THE BLOOD DOWN INTO THE BOTTOM
OF YOUR VENTRICLE, AND YOU’RE NOT GOING
TO BE ABLE TO GET IT OUT. SO WHAT A FANTASTIC SET UP. LET’S CONTRACT
FROM THE BOTTOM UP. THANK YOU PURKINJE FIBERS,
WOOF, WOOF, FOR SPREADING OUT, AND THANK YOU AV NODE
FOR SLOWING DOWN THE MESSAGE. THANK YOU BUNDLE BRANCHES FOR
KEEPING THE MESSAGE VERY TIGHT, JUST IN THAT INTERVENTRICULAR
SEPTUM, AND THEN SPREAD IT OUT,
CONTRACT FROM THE BOTTOM UP, SQUISH THE BLOOD OF VENTRICLES,
ALL THE BLOOD. OKAY, NOT ALL OF IT, BUT A LOT OF BLOOD,
MOST OF THE BLOOD. IF YOU END UP WITH BLOOD POOLING
IN YOUR HEART, POOLING BLOOD CLOTS. THAT AIN’T COOL. IF YOU HAVE CLOTTING BLOOD
IN YOUR HEART, YOU’RE GOING TO THROW A CLOT, AND YOU’RE PROBABLY NOT
GOING TO LAST TOO LONG. THAT’S GOING TO BE A SAD STORY. SO LET’S GET AS MUCH
OF THAT BLOOD OUT AND SEND IT TO THE BODY
OR TO THE LUNGS, BECAUSE THAT’S HOW WE FUNCTION. WHOA, ISN’T THAT COOL? NOW YOUR HEART CAN DO THIS WITHOUT ANY INPUT
FROM THE NERVOUS SYSTEM. HOWEVER WE KNOW
THAT IF WE SEE A BEAR, AS LONG AS IT’S NOT
A TEDDY BEAR, HEART RATE’S GOING TO INCREASE. HOW DOES THAT HAPPEN? SYMPATHETIC NERVOUS SYSTEM
PROBABLY PLAYS A ROLE, RIGHT, SO HOW? HOW CAN WE INCREASE
OUR HEART RATE? OR WHEN YOU’RE SLEEPING, IF OUR PACEMAKER POTENTIAL IS 90
BEATS PER MINUTE, WHICH IT IS, I KNOW RIGHT NOW MY HEART IS NOT
BEATING 90 BEATS PER MINUTE, SO THERE HAS TO BE SOME KIND
OF PARASYMPATHETIC INPUT SAYING, “DOGS, JUST SLOW IT DOWN. THERE’S NO NEED TO GO THAT FAST. YOU’RE JUST HANGING OUT
IN YOUR OFFICE BY YOURSELF, JUST TALKING TO YOUR VIDEO
CAMERA ‘CAUSE, YOU KNOW, THAT’S A FUN THING
TO DO ON A SUNDAY AFTERNOON.” ALL RIGHT. SO LET’S FIGURE OUT HOW THE NERVOUS SYSTEM GETS
INVOLVED IN THIS WHOLE THING