• Skip to primary navigation
  • Skip to main content
  • Skip to footer

How Muscle Contracts

Transcript of Part 1: How Muscle Contracts

00:00:17;09		I began the work that ultimately led to the
00:00:19;28		sliding filament theory of muscle contraction in 1950.
00:00:25;01		I was a research student in a small MRC unit with John Kendrew as my supervisor
00:00:36;01		and Max Perutz being the head of the unit and that was it.
00:00:40;26		It was just three people.
00:00:43;14		I'd been a student in physics, and my main interest had been in experimental particle physics.
00:00:53;07		But, after graduation I decided to change fields completely, joined this unit,
00:01:00;09		and was in fact in complete freedom to choose my own particular subject of research.
00:01:07;06		Now during the first couple of years I spent doing some work on hemoglobin
00:01:12;09		and learning a bit of biology of which I was totally ignorant originally.
00:01:17;15		And then I chose muscle as a possible topic because
00:01:24;23		I'd found that although an awful lot was known about the
00:01:28;23		mechanics and energetics and biochemistry of muscle,
00:01:33;10		virtually nothing was known about its submicroscopic structure
00:01:37;26		nor about what actually happened during contraction.
00:01:41;23		Now, one way of getting that information might be
00:01:46;28		that if muscle was composed of regular, repeating or semi-crystalline units of these
00:01:55;20		two structural proteins then one might get evidence about what was happening from X-ray diffraction.
00:02:04;12		However, people had looked at wide-angle X-ray diffraction,
00:02:09;03		which showed you any changes in the polypeptide chain configuration,
00:02:14;13		and essentially it didn't show anything.
00:02:17;02		And they'd also looked at dried muscle by low angle diffraction, which showed you
00:02:23;20		structural regularities and the size of protein molecules, but that hadn't been informative either.
00:02:31;04		However, I'd learnt that one had to keep protein crystals fully hydrated
00:02:37;18		if you were going to get anything useful and an X-ray diagram from them.
00:02:41;27		So I thought, "Well, maybe that would be true of muscle and
00:02:44;09		maybe muscle really did contain lots of regular structures if only one could attack it the right way."
00:02:51;21		And so for that purpose I built a miniaturized X-ray camera,
00:02:58;00		which would optimize the sensitivity of images collected on film.
00:03:08;12		And I was also very fortunate that my supervisor John Kendrew knew
00:03:14;05		a man called Bernal in London in whose X-ray group two people were
00:03:20;03		actually building a very fine-focus, high-intensity X-ray generator (X-ray tube) for another purpose,
00:03:30;09		and they were kind enough to give me an early prototype of this,
00:03:34;02		which formed an essential part of the X-ray set-up that I built.
00:03:39;08		I was very thrilled and relieved to see that I did in fact get quite a well defined X-ray diagram.
00:03:50;13		Not very detailed, but it consisted basically of two reflections.
00:03:56;29		One here and then another one a bit further out.
00:04:03;01		And, the spacing of these told one they came from a hexagonal arrangement of objects,
00:04:11;05		and I concluded that this must be a hexagonal arrangement of myosin filaments.
00:04:17;21		But, what was particularly interesting was that in a muscle in rigor,
00:04:22;08		the pattern had the same spacing, approximately,
00:04:26;07		but the relative intensity of the two spots had changed,
00:04:31;11		and this told one that there was some alteration in the character of the lattice.
00:04:39;15		And that in fact you can get a projection of the electron density from those diagrams.
00:04:47;16		This is a myosin filament here on my picture, and then this is another myosin filament there
00:04:54;03		and another one in the lattice over here.
00:04:56;27		In between the myosin filaments is what I took to be actin filaments,
00:05:03;28		which appeared at low density in the relaxed muscle, but in rigor muscle,
00:05:10;08		their density was much increased.
00:05:13;13		Now I knew that actin and myosin tended to combine with others in the absence of ATP, that's in a rigor muscle.
00:05:21;14		So I concluded, and it turned out I was correct, that
00:05:25;25		was in fact what I was seeing: a double array of actin and myosin filaments interpenetrating with each other.
00:05:36;19		I could also see some reflections, axial reflections, which give you information
00:05:43;03		about the longitudinal regularities along the length of the muscle.
00:05:48;00		This is a spacing of about 145 angstroms, and there are another series of reflections not shown here.
00:05:56;25		The interesting...so it showed that muscle was a sort of semi-crystalline structure at this scale.
00:06:06;26		And these reflections didn't change in spacing when the muscle was passively stretched.
00:06:14;06		Now, I made a mistake at that point.
00:06:17;20		I thought that these reflections probably all came from the actin filaments.
00:06:25;10		So I concluded the actin filaments didn't change in length,
00:06:28;22		but I still had this picture that perhaps the myosin filaments somehow shortened down.
00:06:34;25		Anyway, I finished my PhD and went to MIT to Schmitt's lab,
00:06:41;07		which was a new center for doing electron microscope work,
00:06:47;03		because I wanted to make sure that this double array of filaments really existed.
00:06:53;13		So by fixing, embedding and sectioning a muscle,
00:06:58;14		I was able to look at cross sections in the electron microscope,
00:07:03;15		and lo and behold one could see the double array of filaments very, very clearly.
00:07:09;27		This was a rabbit muscle in rigor.
00:07:12;28		This is a myosin filament, and that's another myosin filament.
00:07:17;16		And then the actin filaments are in this little ring of six of them around each myosin center.
00:07:26;00		So that was very encouraging and then shortly after this, early in 1953,
00:07:34;28		Jean Hanson came to the same lab to learn electron microscopy too
00:07:40;10		because she'd been studying isolated myofibrils as seen in the light microscope.
00:07:47;18		She was zoologist by training.
00:07:50;11		And we decided, we were both doing the same thing essentially,
00:07:55;05		we decided to combine our different approaches, and this worked out very successfully.
00:08:01;25		One of the first experiments we did
00:08:05;00		was to look at myofibrils in the light microscope on a slide with a coverslip over them
00:08:12;00		in the phase contrast microscope, and you get very nice images of the A and I bands. I'll show you one in a moment.
00:08:19;15		And when we irrigated these with solutions known to extract myosin from muscle,
00:08:28;00		we found that the dense material in the A band here was rapidly removed,
00:08:38;24		leaving a sort of ghost fibril which consisted of the original Z lines,
00:08:44;26		and a segment of the filament which we concluded were probably actin.
00:08:53;06		And we later found that applying actin extraction procedures
00:08:59;24		would remove this second set of filaments as well.
00:09:05;26		These are just showing the density plots before and after myosin extraction.
00:09:11;21		So, this was a sort of, quite a revolutionary finding for us.
00:09:17;09		And after a few hours we sort of reoriented our way of thinking about things.
00:09:23;21		And we realized that we were looking at two partially interpenetrating but overlapping arrays
00:09:32;00		of myosin filaments and actin filaments.
00:09:35;27		And that the cross-sections I had seen had been through the overlap region here.
00:09:43;23		So this is what I showed before-the overlap region.
00:09:49;23		In the H zone, which is where the actin filaments have stopped, you just see the thick filaments.
00:09:59;07		And in the I bands you can see the cross-section of the thin filaments.
00:10:03;14		So we then had a quite new model of the structure of the muscle.
00:10:10;04		And in between the two sets of filaments were these cross-bridges
00:10:15;11		that I had postulated must be there to enable
00:10:19;10		the two sets of the actin and myosin filaments to interact with each other.
00:10:24;11		So we published a short paper on this in Nature, but at Schmitt's request
00:10:32;04		we omitted any speculation about how this system would actually work.
00:10:37;20		But what we knew was that there were these two axial series of X-ray reflections
00:10:47;08		which didn't change during passive stretch.
00:10:50;23		And so we thought, "Well maybe they didn't change during contraction either."
00:10:55;22		And that would mean that the two sets of filaments would themselves remain constant in length
00:11:02;00		and that contraction would be achieved by
00:11:05;10		the actin filaments sliding further into the array of myosin filaments.
00:11:10;24		And when we...after a lot of hard work we were able to get sufficient images
00:11:17;10		of muscle at different stages of contraction to prove that point.
00:11:23;07		And you can see, this is a slightly stretched myofibril.
00:11:28;14		Then, we give it a little bit of ATP, and it shortens down a little.
00:11:35;12		The I bands, the clearer regions, become shorter, but the A bands stay constant in length.
00:11:41;03		And as it shortens further still, the I bands get even shorter, and ultimately they almost disappear.
00:11:50;11		We can also extract myosin from a stretched fibril,
00:11:58;01		and we can see the large gap between the ends of the actin filaments.
00:12:02;03		But when, on of course a different fibril, which is contracted, we can see
00:12:08;15		this gap between the actin shortening down and then disappearing.
00:12:12;17		So we can measure the lengths of the I filaments and that was constant during contraction.
00:12:18;03		So, this was written up for Nature and published in late May in 1954,
00:12:26;23		along with another paper by Andrew Huxley, who's actually no relation of mine,
00:12:33;20		and Niedergerke who had used a special interference microscope to look at intact fibers.
00:12:41;11		And they'd found the same thing in so far as the A band stayed constant in length,
00:12:46;24		but their optics weren't such as to be able to see to measure the length of the actin filaments.
00:12:56;23		So that we thought was pretty strong evidence for this type of model.
00:13:05;07		But, it still remained quite controversial for a number of years
00:13:12;15		particularly because I think some of the other workers in the field
00:13:18;03		hadn't sort of got used to this combination of X-rays and EM and phase microscopy.
00:13:23;26		But I think a few more people were probably convinced by some electron micrographs
00:13:31;13		I was able to get two or three years later,
00:13:34;12		which showed the longitudinal sections through the myofibril, through the muscle fiber,
00:13:43;11		in which you can see the thick filaments, very clearly I think,
00:13:49;19		stopping at the end of the A band
00:13:52;18		whereas the thin filaments continue on into the I bands out here.
00:13:58;19		And you can also, at higher magnification, you can see these cross-bridges
00:14:03;29		between the actin and myosin filaments
00:14:07;21		that I'd supposed must be able to produce the relative sliding force that produces contraction.
00:14:16;15		If there's any general conclusions about how you go about a problem you can draw from this,
00:14:24;21		well, I'm not sure that there are.
00:14:27;00		I think I was very lucky because I had a clearly defined problem,
00:14:31;01		and I was able to find some new techniques which provided a way into it.
00:14:37;28		So, if you can repeat that process, you might get lucky like I did.
00:14:43;22		Thank you.

This material is based upon work supported by the National Science Foundation and the National Institute of General Medical Sciences under Grant No. 2122350 and 1 R25 GM139147. Any opinion, finding, conclusion, or recommendation expressed in these videos are solely those of the speakers and do not necessarily represent the views of the Science Communication Lab/iBiology, the National Science Foundation, the National Institutes of Health, or other Science Communication Lab funders.

© 2023 - 2006 iBiology · All content under CC BY-NC-ND 3.0 license · Privacy Policy · Terms of Use · Usage Policy
 

Power by iBiology