Sonoma State University - Animal Physiology
Department of Biology - Hanes
Application
Discussion
Reflection
Lecture
Structural Basis of Muscle Contraction
A muscle cell is formed from a large number of myoblasts during development. Myoblasts join end-to-end, merge their membranes and become a syncicium that we call the multinucleated muuscle fiber. Each myoblast contributes several sarcomeres to the muscle. This is the functional unit of muscle and consists of an arrangement of protein from Z-line to Z-line.
A skeletal muscle cell is also called a fiber. It forms from the joining of a number of single nucleus myoblasts and is not replaced if killed. The mature cell is very long with many nuclei around the outer edges. A cut end shows bundles of dots, a longitudinal cut shows alternating light and dark staining bands. The dark band is called the A band (for anisotropic). It is composed of myosin molecules joined together into "thick filaments". Heads of the myosin molecules stand out from the filament like nails from a log, these act as cross-bridges from myosin to actin. Myosin heads hold ATP and are an ATPase. They can also attach to actin if there is no ATP present. In fact, ATP is required to separate myosin from actin. The neck of the myosin molecule stands out from the thick filament and the tail makes up part of the thick filament along with the tails of many other myosin molecules. The light bands are I bands (for isotropic). They are composed of long, thin chains of actin. At the center of each I band is a Z-line, which anchors the actin "thin filaments". The two actin chains are twisted into an alpha helix. In the two grooves formed by the twist lies molecules of tropomyosin which span 7 actin molecules. Attached to one end of the tropomyosin is a troponin molecule. The filaments are all bundled into cylinders called myofibrils that are separated from each other by a flattened, fenestrated sarcoplasmic reticulum. T-tubules are finger-like projections of the sarcolemma that extend down into the cell and come into close, extensive contact with the terminal cisternae of the sarcoplasmic reticulum, usually at the Z-lines. These structures are repeated many times along the muscle fiber. The functional unit of the cell is called a sarcomere. It is the smallest unit that can contract alone, under experimental conditions, and extends from Z-line to Z-line. Note the arrangements of all of these structures in Figures 10-1, 10-2, 10-3 and 10-4 pp362-363.
Actin is a globular protein and chains are composed like two strands of beads on a string that twist around each other. In the two grooves that are formed by the helix lies a filamentous protein tropomyosin 7 actins long and to which is attached a globular troponin complex every 7th actin. Myosin is composed of a neck and tail that is composed of twisted chains, a head region that has a site for ATP binding and a site for calcium binding. The calcium site is modified in different types of skeletal muscle and influences speed of contraction.
Mechanics of Muscle Contraction
Note the length-tension curve Fig 10-8 p 367. Understand why the sarcomere length determines the maximum tension that can be produced and the part of the tendon in developing tension.
ACh binds to receptor gates in the subsynaptic membrane allowing Na+ and K+ exchange between the cell and its environment. This causes the cell to partially depolarize which sets off Na+ gates in the surrounding membrane. These, in turn, set off other Na+ gates further away so that an action potential spreads both directions on the cell to its ends. The T-tubules also carry the action potential deep within the cell with their Na+ gates.
The AP affects a membrane protein called a dihydropyridine receptor in the T-tublule membrane. It now changes conformation and it is in direct physical contact with a ryanodine receptor in the sarcoplasmic reticular membrane. The ryanodine receptors conformational change allows Ca++ stored in the sarcoplasmic reticulum to leak into the cytoplasm. Ca++ is released near the contractile protein. Ca++ reacts with troponin, changing its shape and moving attached tropomyosin out of the grooves of the actin chain. This uncovers the active sites of actin.
The myosin head has ADP incorporated within it. It attaches to the actin active site. This junction allows the head to rotate relieving the spring-like tension in the head. ADP leaves the myosin head during rotation. Almost immediately, a new ATP is incorporated into the head. With ATP present the head can separate from the actin. Upon separation, the head is bent to a more obtuse angle with the body of the molecule (the head is cocked) and locked in a new position as ATP -> ADP. The cycle is ready to start over and does until the active sites of actin are covered again or until the cell runs out of ATP..
After the action potential(s) is/are over, Ca++ is pumped back into the sarcoplasmic reticulum by its Ca++ pumps. Ca++ is removed from the troponin (probably replaced by Mg++) and tropomyosin slips back into the grooves of actin covering the active sites. The myosin heads stay cocked and do not attach to actin. The filaments are free to slip back into position if pulled by the opposing muscle. Within the sarcoplasmic reticulum, much of the Ca++ is attached to a molecule called calsequestrin which removes it from free solution making pumping Ca++ against less of a concentration gradient. Ca++ comes off of the calsequestrin as the free Ca++ level within the sarcoplasmic reticulum is reduced by flow into the cytoplasm. See fig 10-24 p 374.
Know the sequence of events as summarized on p 387-389.
The Transient Production of Force
The length-tension curve is used to determine that muscles with longer actin and myosin filaments (long sarcomeres) develop greater tension per unit cross-sectional area because of the greater extent of overlap possible. If you measure the tension produced by a muscle as it contracts beginning at a variety of lengths, you will find that it can produce the most tension at a moderate length that corresponds with the length at which the most myosin heads are in contact with actin. If stretched too much, some heads have no actin around them; and if stretched too little, myosin begins to bump into Z-lines and actin filaments may overlap.
In extracted muscle, force increases with [Ca+] from 10-8 M to 10-6 M as does the ATPase activity of myosin. Calcium is not required at all for muscle contraction if there is no tropomyosin present. Actin and myosin with ADP in the myosin will attach whether tropomyosin is present or not. In other words, ATP is necessary to separate actin & myosin. At death, when ATP is used up rigor mortis sets in or in other words muscles lock. As autolysis takes place, muscles loosen again.
Mg++ is required for myosin ATPase activity and for relaxation of muscle.
A single sarcomere can be contracted by stimulating the sarcolemma near a single T-tubule with a voltage that is too low to set off an AP.
Aequorin, the Ca++ activated fluorescent molecule, in a large barnacle muscle cell proved that Ca++ was released into muscle cell cytoplasm during a contraction.
Mechanical Properties of Muscle
In an isotonic contraction, the muscle + tendon length shortens, as would happen if one lifted a heavy ball. If a limb moves a lot, the muscle fibers can shorten a great deal and become more inefficient.
An isometric contraction is one in which the muscle + tendon length remains the same. In other words, the muscle can only shorten as much as the tendon stretches. This would happen if you were trying to lift a 1000 pound table top.
Since sarcomeres are arranged in series, each adds to the shortening of the whole muscle. Stronger muscles have many cells arranged in parallel like the jaw muscle. Fast, but less powerful muscles, like the triceps, are composed of fewer longer fibers.
A twitch is the response of a muscle to one stimulus (nerve firing). Treppe is the response of a muscle to a frequency of stimulation that results in a partial relaxation between contractions. Tetanus is the response of a muscle that is stimulated so frequently that it has no time to relax between stimulations.
Energy for contraction comes from ATP. There is only enough present in a cell to last a few seconds. In mammals, creatine phosphate is in equilibrium with ATP with the equilibrium concentration far toward ATP or, in other words, when there is a great deal of ATP and very little ADP, some of the phosphate will be transferred to creatine to make creatine phosphate. When there is more ADP present, creatine phosphate will return its phosphate to ADP to recreate ATP. In most invertebrates the phosphagen is arginine phosphate. These give muscle cells the capability of lasting much longer before metabolism must renew the ATP.
The active state of a muscle cell is measured by the rate of heat evolution during a contraction. It is related to the number of active bridges and influences the load carrying ability of the muscle. See curve p 392.
Fiber types in Vertebrate Skeletal Muscle
There are several subtypes of skeletal muscle. The major types follow. Phasic means that the cells contract and relax in a short time. Tonic means that the cell tends to remain contracted for some time from one stimulation.
|
|
Tonic Oxydative |
Slow phasic Oxydative |
Fast phasic glycolytic |
Fast phasic oxidative |
|
Speed |
very slow |
slow |
Fast |
Fast |
|
Action Pot. |
None (graded) |
Yes |
Yes |
Yes |
|
Fatigue |
little |
slowly |
Rapidly |
Moderately |
|
ATPase Act |
slow |
Moderate |
Rapid |
Rapid |
|
Mitochondria. |
Moderate |
Many |
Few |
Many |
|
Myoglobin |
Lots |
Lots |
Little |
Lots |
|
Innervation |
Multisynap |
1-2 synapses |
1 synapse |
1-synapse |
|
Glycogen |
Little |
Little |
Much |
Little |
|
Force/mm3 |
low |
low |
high |
Intermediate |
|
Size diam. |
Small |
Large |
Very large |
Intermediate |
Note the use of slow oxidative and fast glycolytic muscles in fish. Fig.
10-35, 10-36 p 400-401.
Note the different arrangements of innervation for some invertebrate skeletal
muscle. p 412.
Trophic Effects of Nerves on Muscle.
A denervated muscle cell will revert to a very slow type. It will have ACh receptors all over its membrane. Upon reinnervation, the muscle cell will become the type that the nerve cell previously served and ACh receptors will occur only under the synaptic junction at the same density that they were previously all over the cell membrane.
Cardiac Muscle - Differences from Skeletal Muscle
Note the differences of anatomy and characteristics in Table 10-2 p 415.
Cardiac muscle has a single, central nucleus, branched T-tubules, and gap junctions that permit AP's to travel into adjacent cells. Depolarization and refractory periods may last about 200 msec causing the muscle to remain contracted for that long. Much of the Ca++ for contraction comes not only from the sarcoplasmic reticulum, but also from extracellular fluid. There is a Na+ leak current that will bring the cell to threshold automatically - it does not need nerve stimulation to contract. The action potential carried in the T-tubules allows Ca++ in the dihydropyridine receptors on the tubular membrane (Voltage controlled). The Ca++ that enters from the outside opens the Ryanodine receptors and releases more Ca++ from the sarcoplasmic reticulum. The action potential lasts for nearly 1/4 second. The Ca++ channels are closed by a reduction in membrane voltage as Ca++ activated K+ channels are opened bringin ghte membrane voltage closer to the K+ equilibrium potential.
Smooth Muscle - Differences from Skeletal Muscle
Smooth muscle also has a single, central nucleus, and gap junctions, but no
T-tubules. Smooth muscle is called such because it has no striations. Electron
microscopy has not revealed a pattern of filaments in the cell. Nerve stimulation
comes from autonomic nerves that lie alongside muscle cells and secrete into
the extracellular fluid from varicosities in the nerves. The transmitter diffuses
over several cells. The action potentials are produced by Ca++, not Na+. The
Ca++ is stored outside cells and allowed in after stimulation. The Ca++ binds
to calmodulin which then binds to caldesmon removing
it from its normal binding to actin. This allows actin and myosin to interact.
Caldesmon attached to actin prevents contraction. Tension development
is directly related to the AP and [Ca++]. Again, Ca++ opens ca++ controlled
K+ channels to stop the process.
Application
Be able to draw & label the filaments, etc... of a relaxed and
contracted muscle.
List, in order, the sequence of events in a skeletal muscle
contraction.
Differentiate between an isotonic and an isometric contraction.
Define: active state, treppe, tetanus, length-tension curve
Distinguish between the types of skeletal muscle. Give an example of
each. Actually, muscles may contain several types of muscle cells,
but one type may predominate.
Discussion
Compare skeletal, smooth and cardiac muscle.
Reflection
How is energy provided to the contraction process? What does ATP
actually do?
Compare skeletal, smooth and cardiac muscle anatomically and
mechanistically.