Cardiac Muscle 3- Contractile cells

Cardiac Muscle 3- Contractile cells


– TWO KINDS OF CARDIAC
MUSCLE CELLS, WE HAVE CONTRACTILE CELLS
AND AUTORHYTHMIC CELLS. 99% OF ALL CARDIAC
MUSCLE CELLS ARE CONTRACTILE. AND HERE’S THE DEAL
WITH CONTRACTILE CELLS, THEY RECEIVE ACTION POTENTIALS
FROM SOMEONE ELSE. SO THE ACTION POTENTIAL ACTUALLY PASSES
INTO THE CONTRACTILE CELL, ENTERS VIA,
DOES ANYBODY WANT TO GUESS? I BET I CAN HEAR YOU
IF YOU YELL REALLY LOUD. THE ACTION POTENTIAL ENTERS VIA THE GAP JUNCTIONS
IN THE INTERCALATED DISCS. YEAH, SO THERE IS NO THRESHOLD WHEN YOU THINK ABOUT A
CONTRACTILE CARDIAC MUSCLE CELL. I AM GOING TO DRAW YOU
THE ACTION POTENTIAL GRAPH. BUT WHAT I WANT YOU
TO REMEMBER IS THAT DOWN HERE, WHILE WE’RE WAITING
FOR AN ACTION POTENTIAL TO ARRIVE OR THE MESSAGE TO,
LET’S GO, WE’RE HANGING OUT
AT RESTING MEMBRANE POTENTIAL, WHICH IN A CONTRACTILE
CELL IS -90 MILLIVOLTS. THAT’S DIFFERENT THAN MY
CHILLING SOMATIC MOTOR NEURON, WHICH IS, YOU KNOW, -70. SO THERE ARE SOME THINGS
THAT ARE DIFFERENT, BUT THERE’S NO STIMULUS. THE ACTION POTENTIAL IS ALREADY
HAPPENING, AND IT ENTERS
THROUGH THE GAP JUNCTIONS IN THE INTERCALATED DISCS.
OKAY. AS SOON AS THE ACTION
POTENTIAL ENTERS THE CYTOPLASM OF MY CONTRACTILE CELL,
AUTOMATICALLY, WE’LL SEE WHAT I
CAN DO DOWN HERE, ACTION POTENTIAL ENTERS. AND WHO’S GOING TO OPEN? MAKES PERFECT SENSE, VOLTAGE GATED SODIUM
CHANNELS OPEN. VOLTAGE GATED SODIUM
CHANNELS OPEN, SODIUM RUSHES OUT,
ZOOMORAMA, WE BUST A MOVE AND DEPOLARIZE
UP TO ABOUT 20 MILLIVOLTS. AND DOING
AN ACTION POTENTIAL GRAPH, DON’T BE DISTURBED OR CONFUSED
BY THE FACT THAT WE DID, WE JUST DID GRAPHS
OF SKELETAL MUSCLE CONTRACTION. WE DID TENSION GRAPHS. WE DID SOME ACTION POTENTIAL
GRAPHS OF OUR SKELETAL MUSCLE. THIS IS DIFFERENT. WHAT WOULD YOU EXPECT
TO HAPPEN NEXT? I MEAN, EVERYTHING THAT WE’VE DEALT WITH SO FAR
SAYS THAT AS SOON AS THE VOLTAGE GATED SODIUM CHANNELS OPEN,
THEN WE’RE GOING TO GET VOLTAGE GATED POTASSIUM
CHANNELS OPENING. AND THAT’S ACTUALLY TRUE, VOLTAGE GATED POTASSIUM
CHANNELS OPEN. AND THEN WHAT ARE WE GOING
TO EXPECT TO HAPPEN WHEN VOLTAGE GATED
POTASSIUM CHANNELS OPEN? WE’RE TOTALLY GOING
TO EXPECT A REPOLARIZATION AS POTASSIUM RUSHES
OUT OF THE CELL. BUT THIS IS WHERE
THE PLOT GETS EVEN CRAZIER, BECAUSE GUESS WHAT? SOMETHING HAPPENS
IN THE CONTRACTILE MUSCLE CELL THAT IS UNUSUAL,
CARDIAC MUSCLE CELL. STAGE 3, WE
HAVE CALCIUM CHANNELS THAT OPEN. WHAT?
SERIOUSLY? NOW THINK ABOUT THIS. CALCIUM CHANNELS OPEN. CALCIUM RUSHES IN. POSITIVE CHARGES ARE RUSHING IN. WE WOULD’VE THOUGHT,
HERE, WE WOULD’VE THOUGHT POTASSIUM IS
GOING TO LEAVE. BUT NOW WE HAVE, SURE,
POTASSIUM IS LEAVING, BUT CALCIUM IS ALSO COMING IN AND WHAT’S THAT GOING TO DO
TO MY ACTION POTENTIAL? IT’S GOING TO CREATE A PLATEAU, BECAUSE THE POTASSIUM THAT’S
LEAVING IS BEING BALANCED
BY THE CALCIUM THAT’S COMING IN. THE CALCIUM COMING
IN PLAYS A ROLE IN MAINTAINING THIS PLATEAU. AND THINK ABOUT THIS,
WHAT DOES THIS PLATEAU DO? MY FRIEND, THE CONTRACTILE CELL CANNOT BE RESTIMULATED
DURING THIS TIME. THIS IS A REFRACTORY
PERIOD, WHY, BECAUSE WE’RE ALREADY
FULLY DEPOLARIZED. HOW CAN WE DEPOLARIZE MORE? IT DOESN’T MATTER IF ANOTHER
ACTION POTENTIAL COMES IN, I AM NOT GOING TO CONTRACT
DURING THIS TIME. EVENTUALLY THE CALCIUM CAME IN,
EVENTUALLY. NOW THIS IS REALLY COOL. WE HAVE FAST POTASSIUM CHANNELS. THESE GUYS OPENED SLOWER
THAN THE SODIUM CHANNELS, BUT STILL FASTER THAN THESE GUYS’ SLOW
POTASSIUM CHANNELS OPENED. NOW OUR CALCIUM HAS
EQUILIBRATED, OUR CALCIUM CHANNELS CLOSE, AND THESE SLOW
POTASSIUM CHANNELS OPEN, AND NOW WE ARE
GOING TO REPOLARIZE AND GO BACK TO WHERE WE STARTED. ONCE WE’VE REPOLARIZED, THAT MEANS THAT WE’RE READY
TO FIRE AGAIN. THAT MEANS THAT THE MESSAGE CAN
COME IN THROUGH THE GAP JUNCTION AND SAY, “OKAY, YOU CAN GO AHEAD
AND DO IT AGAIN NOW.” DURING THIS REFRACTORY PERIOD, THIS IS 0.25
TO 0.3 SECONDS LONG. NOW THINK ABOUT THAT. SO HOW LONG WAS THE,
I MEAN, EVEN OUR SKELETAL MUSCLE CONTRACTION
WAS 100 MILLISECONDS. THIS IS 300 MILLISECONDS, SO IT WAS 0.1 SECONDS
FOR SKELETAL MUSCLE. AND IT’S 0.3 SECONDS,
LIKE, FROM START TO FINISH,
YOU CAN’T BEAT YOUR HEART FASTER THAN 1 CONTRACTION
EVERY 0.3 SECONDS. AND IF YOU DO THE MATH THAT IF
WE GO WITH THIS ONE RIGHT HERE, WE COULD GET OUR HEART RATE
UP TO 240 BEATS PER MINUTE. AND I WENT ONLINE TO SEE,
OKAY, WHAT IS THE HIGHEST HEART RATE
THAT YOU COULD POSSIBLY HAVE, AND EVERYTHING
THAT I SAW WENT TO 220. THAT IF YOU GO HIGHER
THAN 220, NOW THINK ABOUT THIS, AND GO BACK TO YOUR
LITTLE HEART, THE PUMP, IF YOUR PUMPING FASTER THAN 240 BEATS PER MINUTE
OR 220 BEATS PER MINUTE, YOU’RE PUMPING SO
FAST THAT IS THERE ANY ROOM FOR BLOOD TO COME IN? SO YOU’RE PUMPING NOTHING. YOUR HEART IS BEATING SO FAST THAT IT CAN NO
LONGER DO ITS JOB. IT’S ALMOST LIKE IT’S
IN A GIANT CRAMP, WHICH IS YOU CAN IMAGINE
TERRIBLY UNPRODUCTIVE. AND ANY CRITTER THAT HAD A HEART THAT BEAT LIKE THAT IS PROBABLY
GOING TO BE A DONE DEAL, SORRY FOR YOUR LUCK,
MAN, AND NOT PASS ON THEIR GENES,
SO IT MAKES SENSE THAT WE WOULDN’T
HAVE THAT PHENOMENON HAPPENING. OKAY, SO CONTRACTILE
CELLS ARE AWESOME. YOU CAN THINK ABOUT THIS. IF WE INCREASE THE PERMEABILITY
TO CALCIUM, WE MAKE MORE CALCIUM COME IN. WE CAN ACTUALLY CHANGE THE FORCE THAT WE CAN GENERATE
WITH THIS MUSCLE TISSUE. THIS IS JUST
THE ACTION POTENTIAL. THIS IS JUST
THE ACTION POTENTIAL THAT SENDS THE MESSAGE
TO THE REST OF THE MUSCLE CELL TO GO AHEAD AND ACTIVATE
OUR SARCOMERES AND SHORTEN, YOU KNOW, MYOSIN
HEADS GRABBING OUR ACTIN AND SHORTENING
THE MUSCLE TISSUE, SO THAT WHOLE
PROCESS IS HAPPENING DURING THIS PERIOD OF TIME. DURING THE REFRACTORY PERIOD,
THAT WHOLE PROCESS IS HAPPENING. WHAT DOESN’T HAPPEN,
NOW THIS IS WILD, TOO, WHAT DOESN’T HAPPEN IS
OUR CONTRACTILE CELLS CAN’T SAY, “HEY, FIRE AGAIN FOLKS.” WE MUST RELAX
THE CARDIAC MUSCLE TISSUE BEFORE FIRING
ANOTHER ACTION POTENTIAL. WHAT HAPPENS IN SKELETAL MUSCLE WHEN WE FIRE A WHOLE
BUNCH OF TIMES? WE GET TETANUS. WE GET A SUSTAINED,
CONSTANT, SMOOTH CONTRACTION. AND JUST GO AHEAD AND THINK
ABOUT THAT FOR A SECOND. WHAT WOULD HAPPEN
IF YOU ENDED UP WITH TETANUS
IN YOUR HEART MUSCLE? YEAH, THAT’S NOT GOING
TO BE EVOLUTIONARILY SOUND. HOW DOES
A CONTRACTILE MUSCLE GET THE ACTION POTENTIAL
IN THE FIRST PLACE? BECAUSE WE’VE ALREADY
ESTABLISHED THAT IT DOESN’T COME
FROM THE NERVOUS SYSTEM. SO WHERE DOES IT COME FROM? WHERE IS THIS RIGHT HERE? WHERE IS THIS? OH, I’M GOING
TO MARK IT IN ORANGE, WHERE DOES THAT COME FROM? WHERE DOES THE ACTION POTENTIAL
COME FROM? SERIOUSLY, COULD IT POSSIBLY BE
THE AMAZING AUTORHYTHMIC CELLS?

25 Replies to “Cardiac Muscle 3- Contractile cells”

  1. Hi!, I can't believe I just found your videos yesterday, but I need to say THANK YOU!!. I love what I've seen so far, they're great and so are you!. Please don't stop. Hope you are doing great!

  2. Love your video! Β Just found this and it is the perfect refresher for me. Β I need to go back to the basics on some topics as my undergraduate courses were too long ago. Β Thanks!!

  3. So at 2 a voltage gated potassium channel lets potassium out of the cell, but then at 4 another voltage gated potassium channel lets potassium in?

  4. Hi Wendy. I know your videos are more focused for AP Bio for high school and intro level college classes, but I am a dental student at an ivy league institution and you explained better in 20 minutes what my teachers (couldn't) in a few hours. Thank you!

  5. I think you made a mistake, actually two other than the one you corrected already. It is Fast Na1+ channels and slow Ca2+. channels.

  6. I love this woman! "Where is that action potential coming from? Huh? Could it be, the amazing autorhythmic cells?! Stay tuned." I could learn anatomy all day from this lady πŸ™‚

  7. You are a great help mam . Thank you for your lecture . When you say action potential enters via gap junction does that mean cytoplasm will move from one cell to another or is there any specific charge particle ( like Na ) will transfer between cells via gap junction

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