Introduction

The video, above right, gives yet another overview of skeletal muscle contraction.

In a previous year, a student asked an interesting question.
 

When we went over the organelles of the cell, some time ago, I mentioned the endoplasmic reticulum, because we would see the specialized form of the ER that appears in muscle cells (the sarcoplasmic reticulum).

The student asked, "if the SR is primarily about storing calcium, what happened to the cellular functions that the ER normally has?"

• Two types of ER:
• Rough ER -
• Appears rough because of presence of ribosomes on its surface.
• Primary function is protein synthesis.
• Smooth ER
• No ribosomes.
• Manufactures lipids and has some other maintenance functions.

(more on this in a bit)

Back to the class: Review

• In this series of classes we will be looking at how skeletal muscles work. To understand how muscles work:
• we must look at muscle structure (to see where the "pieces" are) and
• we must examine the processes that lead to contraction (to see how the "pieces" interact).
• The electrical current transported by the neurons (NOT the AP's!) is passed from motor neurons to skeletal muscle cells (called muscle fibers).
• As a result, the skeletal muscle cells contract (shorten their long axes).
• This electrical current does not supply the energy. It merely initiates the chain of events that begins the mechanical contraction of the muscle cell.

Review of Muscle Structure
 

Went through the gross structure last class. Muscles/fascicles/muscle fibers/intracellular components.  Sarcolemma - name for muscle cell membrane (lipid bilayer).

Courtesy University of Minnesota.

Connective tissue:

• Epimysium - Sheath of connective tissue surrounding a muscle.
• Perimysium - sheath of connective tissue that surrounds the fascicles
• Endomysium - thin connective tissue surrounding each muscle cell

The interior of a skeletal muscle fiber is absolutely packed with organelles. Multiple nuclei Many mitochondria Many myofibrils (myo = muscle).

Myofibrils consist of sarcomeres connected end to end.

• Overlapping regions of actin and myosin (the protein filaments that do the actual contracting)
• Connected by z-disks (another protein).
• Myofibrils are surrounded by the sarcoplasmic reticulum
• SR  is, in fact, the device that is activated by the electrical current and turns on the chemical energy driven contraction of in the sarcomeres.

In this transmission electron micrograph you can see just how crowded the muscle cell interior is. The myofibrils take (blue) up much of the available space. The nuclei are pushed out to the cell membrane (cell membrane = sarcolemma). There is also an abundance of mitochondria (yellow) interspersed between myofibrils. Not much else! (smooth ER = SR) So far, I have found little mention of rough ER in skeletal muscle fibers, except one slide that said rough ER is not present (no room). We know how the cell handles synthesis of myosin, actin, and other proteins in myofibrils but so far no info on how it handles synthesis of non-myofibril proteins.

Here we see a mitochondrion (blue) squished in among some myofibrils (green).  Why do we need a lot of these in a muscle cell?

Back to the the sarcoplasmic reticulum.
 

The SR Surrounds myofibrils Stores, releases, and reabsorbs calcium in the cell Is in close proximity to inpouchings of the sarcolemma called t-tubules.

Courtesy Gary Ritchison, Eastern Kentucky University.

In the drawing, the t-tubules look like pipes that are not necessarily part of the cell membrane.
 

In this TEM, we see the outside of the skeletal muscle sarcolemma with the entrances to the t-tubules.  The t-tubules really are just a continuation of the outer cell membrane (lipid lipid bilayer with ion channels and ion pumps interspersed along it) into the interior of the cell.

The sarcomere (basic contractile unit). Sarcomeres lined up end to end make up a myofibril. A sarcomere goes from z-disk to z-disk. Sarcomeres are made of filaments (actin & myosin) and associated proteins.

 

Here you see a TEM of one myofibril, having sarcomeres laid end-to-end along its length.

Here is another easier to interpret TEM showing  very distinct z-lines (z-disks) dark regions where actin and myosin overlap.  bits of the SR.

Several hundred to several thousand myofibrils are arranged in a group to form one muscle fiber.

In skeletal muscles, one nerve ending serves each fiber.

In this class, we will

• introduce more detail about sarcomere structure and
• step through the sequence of events that make up muscle contraction.

It's easiest to think about sarcomeres in terms of the simple 2-dimensional schematic drawings that you can find all around the Internet. Thick myosin filaments tug on thin actin filaments, pulling z-disks together. Reality is neither 2 dimensional nor quite that simple.

Courtesy of Mark Meyer / 3 Dot Studio

It's more realistic to think about sarcomeres as cylinders ...

Courtesy Gary Ritchison, Eastern Kentucky University.

because that is what they are.  Shown at right is a transverse view of a myofibril.  Note the thick myosin and thin actin filaments.  The arrangement resembles  a crystalline lattice. About 6 actin ring each myosin. Actin are shared by adjacent myosin.

Courtesy Dr. Paul Paolini, San Diego State University

If we zoom in on the actin filament (shown at right), you can see that the actin is anchored to the z disk. A closer view of the actin shows the actin chain plus 2 additional proteins: tropomyosin troponin Both play critical roles in the contraction process.

Courtesy Gary Ritchison, Eastern Kentucky University.

If we zoom in on the myosin filament (shown at right), you can see that the myosin is anchored to the M line (a structural protein in the light region between where the actin and myosin overlap). A closer view of the myosin shows that, in addition to being thicker, it appears  "knobby" at the ends.

Courtesy Gary Ritchison, Eastern Kentucky University.

Upon even closer examination, you can see that the myosin filament is actually an aggregate of myosin tails which, together, give myosin its "thickness." The "knobs" are the myosin heads. They are the tugging units that actually cause contraction to occur.

Courtesy Gary Ritchison, Eastern Kentucky University.

The myosin heads (the orange features at right) move back and forth. They apply power to the actin filaments, which cause them to move over the myosin.

The Sequence of Events in Muscle Contraction (our first look at a myosin motor)

There are loads of animations of the sequence of events that occur when myosin/actin interact in muscle contraction.
 

This one is quite complete, but you have to watch it for a while to see the whole cycle.

Courtesy Biomedical Sciences Department, University of Aberdeen.

Here's another pretty good one. But this one moves a little too fast. The "pink" actin filament has attachment sites for the myosin head (yellow). (inaccuracy in this animation: release of Pi comes too early; see below)

Courtesy Gary Ritchison, Eastern Kentucky University.

The animations provide a sense of how the actin/myosin interact and what actually occurs. The sequence of events is interesting enough that it bears closer examination
(the following is from: Alberts, Bray, et al., Essential Cell Biology).
 

1. Let's start with the myosin head attached to an actin attachment site at the end of the power stroke.

2. ATP binds to the myosin head.  This changes the quaternary structure of the protein myosin head. The change in structure makes the attachment site on myosin no longer fit the attachment site on actin.  The myosin head releases the actin. (Dead organisms enter rigor mortis, since new ATP are not produced to sever the attachment).

3. The release of the myosin head allows it to fold around the ATP.  The folded myosin head hydrolyzes the ATP: ATP ---> ADP + P.  The energy from ATP hydrolysis moves the myosin head approximately 5 nm into the "cocked" position (protein structure goes from a lower energy state to a higher energy state using the energy from the ATP).  Moving to a higher energy state could not have happened without the energy provided by ATP hydrolysis.

4. The change in conformation of the myosin head as it moves to the "cocked" position causes the myosin head attachment site to fit onto the actin again.  If an actin attachment site is nearby, the myosin head and actin will attach (i.e., form a cross bridge). 5. Attachment of the myosin head to the actin further alters the head conformation, allowing the hydrolyzed P to escape.

6. Release of the P releases the potential energy stored in the myosin head conformation.  The head reverts to its lower energy conformation.  This is the "power stroke".  The actin moves with the myosin head, causing the sarcomere to shorten. 7. Head conformation change during the power stroke changes the ADP attachment site to an ATP attachment site. The ADP no longer fits and an attachment site for ATP has been created.  This completes the cycle (we're back at the beginning).

Contractions Larger than 5 nm. So far, we have only accounted for about 5 nm (5 millionth of a mm) of muscle contraction.

• Skeletal muscle cells typically contract about 20% of their length.
• Since sarcomeres are longer than 25 nm, multiple tugs must be involved in each contraction. How does this work?

If you watch the University of Aberdeen animated GIF long enough, you will see that, after the first tug, the myosin head doing the pulling hangs on long enough for another myosin head to attach. In this way, consecutive tugs resemble the hand-over-hand motion that we would use to pull in a rope.

Next Question: How does this all get started?

So why don't our muscles contract continuously?

• The process is turned on by the arrival of the electrical current from motor neurons.
• The process is turned off by the cessation of the electrical current.
• Now we will begin to talk about the "electric switch" that activates mechanical contraction.

When the muscle cell is not contracting, tropomyosin covers the attachment sites located on the actin filament. Since the attachment sites are covered, myosin heads will not attach and the above cycle does not function.

When calcium is present, the Ca attaches to an attachment site on troponin. Attachment of the calcium to the troponin causes (big surprise) a conformational change in troponin that causes a conformational change in the tropomyosin molecule that shifts it to a lower energy conformation. The shift to the lower energy conformation uncovers the myosin head attachment sites on the surface of the actin filament. As long as the calcium is present, the attachment sites will be available and contraction will proceed.

When the calcium leaves the troponin attachment site, the troponin reverts to its original structure (which again becomes lower energy) and the tropomyosin re-covers the myosin head attachment sites.

So, what determines when Ca is present? We'll take that up next time.
 

The above process is summarized in this video.  (there is an error in this animation, too; what is it?)