Other Muscle Proteins, Force of Contraction, and Muscle Cell Repair

Introduction to Today's Reading

The title pretty much says it all for this reading. Today, we will look at

• Some of the other structural proteins which allow a muscle cell to have the structure that it does and that also allow it to behave in ways that we expect (i.e., relax at the end of contraction) and become constructed properly when they are created.
• We will look at how we can measure the force generated by a contracting muscle cell (and in the process, see how the Frank-Starling mechanism in the heart works).
• We will look more closely at how muscles are constructed during fetal development and early childhood and, in the process, see how they repair themselves following injury or exercise and become stronger.

• You should be able to put the pieces that we have learned so far together to tell a coherent story.

Structural components: Muscle - Defined by outer covering of connective tissue (epimyceum) Fascicle - Subunit of muscle defined by perimyceum Motor unit - muscle fibers (cells) within a fascicle activated by a single motor neuron. Muscle fiber - muscle cell  surrounded by endomyceum Myofibril - cylindrical structures with the cell defined by consecutive sarcomeres. Transverse tubules - inpouchings of sarcolemma (cell membrane) Sarcoplasmic reticulum - Sacks containing Ca Dihydropyridine receptors activated by intracellular current Ryanodine receptors activated by DHP act as protein channels in SR that open to release Ca to intracellular fluid surrounding myofibrils. Ca pump - returns Ca to SR Sarcomere - occur end to end in myofibrils; basic unit of contraction. Actin - thin filament having attachment sites for myosin heads. Attachment sites are covered by tropomyosin when no contraction going on Troponin - when Ca attachment sites are occupied by Ca, changes conformation, which changes conformation of tropomyosin, which uncovers MH attachment sites on actin, which initiates contraction. Myosin - thin filaments Myosin heads - when MH attachment sites are available, nearby MH's attach and initiate the contractal sequance from Quiz 3.

Looking more closely:
 

The arrival of a current in a motor neuron to the neuromuscular junction causes the sequence of events you have already learned (VG Ca channels, myosin motors to move vesicles, snare proteins to open vesicles and terminal membrane, release of ACh).

Only excitation occurs at a NM junction. ACh binds to LG Na receptors on the muscle motor end plate, creating the charge gradient (also called the generator potential).  On the muscle side of the neuromuscular junction, the generator potential initiates current that is propagated by action potentials. boosted by VG Na and repolarized by VG K channels.  The current moves along the cell membrane (sarcolemma).

As you learned studying about neurons, acetylcholinesterase continuously breaks down the ACh attached to the receptors on the motor end plate. So, the current through the muscle will cease unless the current from the neuron to the muscle cell continues.
 

Current, propagated by AP's, reaches the cell interior via the T-tubules.  Current activates the DiHydroPyradine receptors (voltage sensors). The DHP activates (opens) the Ryodine receptors (Ca channels in the SR). Ca attaches to troponin to shift tropomyosin to initiate contraction.  Then there is the myosin-actin contraction sequence (which you know). As long as current continues, contraction continues. Termination of current reconforms DHP and closes RyR. Ca pump pumps Ca back into SR. Troponin and tropomyosin return to their conformation prior to contraction.  Myosin-Actin return to original position via elastance of muscle cell.

Other Proteins in the Sarcomere

The "cytoskeleton" is the network of filamentous proteins and carbohydrate microtubules that extend through the interior of a cell. Among other things, the cytoskeleton helps to maintain the shape of the cell, holding the pieces together in their proper positions.
 

Actin and myosin are the components of the cytoskeleton that get the most attention in muscles.

Another component of a sarcomere is Titin.

• "Titin is the largest polypeptide yet discovered (~3.5 MDa).
• Megadalton - atomic weight of 1 million
• Single molecules span from the Z- to M-line (yellow in the diagram, below).
• These have two main functions:
• In the thick (myosin) filament part of the myofibril ... the titin molecule regulates exact myosin assembly by acting as a giant template or "protein-ruler...
• The remainder of the titin molecule forms an elastic connection between the end of the thick filaments and the Z-line.
• These connections give muscle cell its passive tension and they also keep thick filaments centered between Z-discs. (Without this there would be force imbalances in the opposite halves of thick filaments during active contraction)." Source

The figure at right demonstrates how titin provides elastic recovery of muscle length when it has been stretched (by an opposing muscle) or when it has contracted.

(Above text and figure courtesy University of Leeds)

Titin is not the only protein that gives the sarcomere its characteristic form.  Nebulin is shown here.  It is the foundation protein for actin and  appears to provide the template for the actin repeating units.

The "z-disk" is a combination of proteins that acts as an anchoring point for the titin and nebulin and mark the ends of the sarcomere.

What you are seeing above is a schematic simplification. The image at right, which shows half of a sarcomere (from z-disk on the left to m-line on the right). Notice that: the z-disk is a combination of a number of different proteins. The z-disk anchors both the myosin and actin filaments on both sides (the z-disk is the place where sarcomeres are joined). The m-line anchors the myosin but does not appear to anchor the actin.

 
 

Desmin is another important protein.  Desmin ties the myofibrils together.  It also anchors the myofibrils to the sarcolemma.  All of these proteins provide structural stability (they keep the components properly positioned relative to each other) and provide a degree of functionality (template for building actin, abilility of long axis to relax into its original length following contraction).

Force of Contraction: Sarcomere Length and Tension Matters

Force of contraction in a muscle cell depends on the relative positions of the myosin and actin filaments.

• To get maximum force when the current arrives, the tension on the cell (on the myofibrils) must fall within optimum limits.
• It's possible to demonstrate that muscles that start to contract when they are stretched too far generate less force of contraction.
• It's possible to demonstrate that the same thig happens when there is not enough initial tension.
• To get optimum contraction, tension must fall within physiological limits that normally occur in organisms. 
  

These experiments described here have been repeated many times.
 

Muscles are attached to a stand so that passive force can be measured. The force is only due to the amount of stretch applied. The force is recorded. Electrical stimulation is applied to make the muscle contract. The amount of additional force is called active force. The passive force is subtracted from the total force to determine the force of contraction.

The only thing that changes in these experiments is the amount of initial stretch when the muscle is electrically made to contract. This is really measuring how the initial actin and myosin filament positions of the sarcomeres affects strength of contraction.

The results of these tests are summarized in the figure below, right.
 

There is an optimum initial degree of stretching (a). This corresponds to the "at rest" length of the sarcomere.  The MH attachment sites on actin are all close to the myosin, so when they become available, the maximum number of MH's will be involved in the contraction.

• If the initial stretch is greater than normal at rest tension, some of the MH's and actin attachment sites are no longer available to each other (b).
• As the number of possible cross bridges decreases, the maximum possible force of active contraction becomes less.
• (when a MH is attached at an actin attachment site, it is called a "cross bridge").
• If the stretch becomes great enough, very few MH's can reach actin attachment sites, and force of active contraction approaches zero (b, far right).
• If the degree of initial stretch drops below typical at rest tension, the number of cross bridges also decreases as "slack" is introduced into their tension.
• The actin and myosin filaments will bend away from each other and some MH's will not reach actin attachment sites (c).
• If the sarcomeres are actually compressed, some of the MH's will attach to the wrong part of the actin and push the actin filament in the wrong direction.
• Notice the actin filament overlap at (d).
• So, the amount of force that a muscle cell can generate depends upon the proximity of MH's to available actin attachment sites and normal "at rest" tension for skeletal muscles is pretty much in the optimal range.

Look at the left side of the curve. Do you recall the Frank-Starling mechanism when we talked about heart muscle cells?  Does this figure give you any clues about how the F-S mechanism works? (it should!).

Skeletal Muscle Cell Development
 

Embryology.  Every mature muscle cell develops from 100 or so myoblasts (pre-muscle cells) that fuse together in the fetus. That's the reason skeletal muscle cells are multinucleated.  Mature muscle cells cannot divide (complicated cells usually cannot undergo mitosis). Muscle growth is a result of cellular enlargement & not cell division.  Muscle cells have many specialized structures.  In general, cells having a high degree of specialization cannot divide and have a difficult time repairing themselves.

Fetal Development of Skeletal Muscle Cells

Skeletal muscle cells are created from the fusion of myoblasts (Greek: myo = muscle and blast = seed).
 

Myoblasts are slightly differentiated stem cells that combine to create a muscle cell. This occurs during the development of the embryo and continues for a period post birth.

Here is a more detailed figure. The process of fusion begins with the development of gap junctions (direct cell to cell connections of the cytosol).  Over time, the myoblasts physically fuse. During this time, thye can depolarize in the presence of acetylcholine (LG Na channels are present) and they begin to concentrate calcium. The process continues with the fusion of approximately 100 myoblasts per muscle cell. Myofibrils, t-tubules, sarcomere etc. development follows completed fusion.

In this video, we can watch the above process in action.

This diagram contains some new terms.    The "plasmalemma" is a synonym for the cell membrane (i.e., sarcolemma).  The basal lamina (and the unlabeled layer above it) are extracellular layers that separate the cell membrane from the connective tissue.  The basal lamina forms as the cell matures. With the formation of the basil lamina, it is possible to see, for the first time, satellite cells. Satellite cells divide slowly during post natal development. Some of the cells fuse with the muscle fiber, others remain separate.

The article at this link describes satellite cell motility. Satellite cells apparently have the ability to migrate about under the basil lamina!

Satellite Cells Repair Damaged Muscle Cells

The significance of satellite cells is that they allow skeletal muscle cells to be improved and repaired.

• Mature muscle cells cannot divide mitotically, they are just too complicated.
• Muscle growth (hypertrophy) is a result of cellular enlargement & not cell division.
• Muscle cells become bigger through the addition of new myofibrils.
• Muscle cells cannot replace themselves through division, the way simpler cells do.
• This is typical of complex cells (neurons are like this too).
• They also cannot repair themselves without help from other cells (also a shared characteristic with neurons).
• In general, cells having a high degree of specialization cannot divide and have a difficult time repairing themselves.

Despite the presence of all that protective connective tissue, the regular contracting and stretching that skeletal muscles undergo frequently damages the muscle cells.  The satellite cells are the mechanism of skeletal muscle repair. The cycle of damage and satellite-cell-mediated repair is an important mechanism of muscle strengthening.

Here is an undamaged skeletal muscle cell.

1) Z disk; 2) length of sarcomere; 3) length of thick filaments; 4) intramuscular triglyceride droplet; 5) M-line (a cytoskeletal structure to hold the thick filaments in place.  The numerous small black dots are glycogen molecule complexes.

Here again is an undamaged skeletal muscle cell (left) and a sample taken from a runner following a marathon

Here are some other views of exercise induced muscle cell damage.
 

In this electron micrograph, two muscle cell have been split in two.

Here we see the process of muscle repair in action.  (A) Following damage, phagocytic cells (macrophage "inflammatory cells") clean up the mess by ingesting cell debris. (B) Activated satellite cells divide.  (C) Daughter cells are myoblasts. (D) The myoblasts "recapitulate" embryological development. They go through the same steps that occurred during fetal development. They form myofribrils, sarcolemma, SR, etc. (E) the repaired cell.

Here we see a photographic sequence of the muscle cell repair process.

• There is some evidence that activated satellite cells from muscle cells that have been destroyed may replace the dead cell.
• I am not sure about this yet - I found conflicting reports.

When the cell membrane is disrupted, satellite cells migrate into the interior of the cell to begin the repair process.  There is some evidence that satellite cells may be activated by exercise without disrupting the sarcolemma.  Daughter cells (myoblasts) apparently fuse with the sarcolemma (much as they fused during fetal development) and enter the cell to make repairs or build new myofibrils.  Hypertrophy is the construction of new myofibrils within a muscle cell. The addition of new myofibrils increases the diameter of the muscle cell and strengthens the muscle.

Strength Training (e.g., weight training): Hypertrophy (the enlargement of the muscle) is the most observable result.

Strength training has both a neural and a mechanical component.

• Neural adaptation is experienced first and includes:
• Faster motor neuron activation.
• Faster recruitment of muscle cells.
• Improved coordination of the muscle cells that have been recruited.