6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors Flashcards
(11 cards)
How do skeletal muscles work?
Skeletal muscles act in antagonistic pairs against an incompressible
skeleton.
Gross structure of skeletal muscle
Skeletal muscle tissue is composed long cells called muscle fibres, bundled together so they run parallel to each other. At each end of the muscle a tendon joins the muscle to the a bone. Skeletal muscle tissue contains both of fast muscle fibres and slow muscle fibres.
Microscopic structure of skeletal muscle
Muscle fibres are highly specialised cells. The plasma membrane, known as the sarcolemma, folds inwards and crosses the interior of the cell - these infoldings are called t-tubules, and allow the wave of depolarisation that is triggered at the neuromuscular junction to spread inward though the cell. Muscle fibres contain long bundles of protein called myofibrils, which are composed of repeating units called sarcomeres. Between the myofibrils (and closely associated with the t-tubules) there is a specialised endoplasmic reticulum called the sarcoplasmic reticulum, as well as mitochondria, granules of glycogen, and nuclei. Muscle fibres are cells without the usual definitive boundaries, so they do not present as clearly separate cells, meaning you may see several nuclei together.
The ultrastructure of a myofibril
Myofibrils are composed of the fibrous proteins actin and myosin. Myosin is thicker than actin, so appears darker under the microscope. Added to which, actin and myosin overlap in places, creating even darker bands. The sarcomere is one unit of these repeating band patterns.
Sections
Myofibrils are shaped like columns, so a longitudinal section will give you a rectangular shape, whereas a transverse section (aka a cross section) will give you a circle. You need to be able to interpret both longitudinal and transverse sections of myofibrils.
The sarcomere under the microscope; longitudinal section
The boundary between adjacent sarcomeres is called the Z line. This is a disc to which the actin filaments are anchored. At the midpoint between two Z lines is the M line - the disc to which the myosin is anchored. This disc is so thin it often does not appear in micrographs, and rarely in diagrams. The M line marks the middle of the sarcomere, and the half of the sarcomere on one side will be the mirror image of the half on the other side.
Each sarcomere is made up of bands: a central I band with an A band either side. The A band marks WHEREVER the myosin is, and appears dark. The I band marks where ONLY the actin is, and appears lighter. Of course, actin and myosin overlap, sliding past each other during contraction, so during contraction the I band gets shorter as less of the ‘unoverlapped’ actin is left. The A band appears dark regardless of whether the actin is overlapping the myosin or not, so the A band does not shorten during contraction. However, there is a central region of the A band - the H zone - where there is ONLY myosin. The M line is in the middle of the Z zone, which is in the middle of the A band. The H zone does get shorter during contraction, as more of the myosin slides over the actin.
The sarcomere under the microscope; transverse section
This obviously depends on where the section is. The only sensible place would be the A band - you would see the cross sections of lots of fibrous proteins. If there is only one type, they will all be myosin. If there are two, the smaller diameter proteins will be actin.
Myofibril contraction.
A wave of depolarisation spreads across the sarcolemma and down the t-tubules, opening voltage-gated calcium ion channels on the sarcoplasmic reticulum, which release calcium ions from eth sarcoplasmic reticulum.
The calcium ions bind to tropomyosin, a helical fibrous protein that is wrapped around actin. The binding of the calcium ions causes tropomyosin to move, exposing the many myosin binding sites all the way along the actin. The myosin heads can now spontaneously bond to the actin’s binding sites, forming ‘actinomyosin bridges’ which link the two proteins together. The act of binding release the potential energy stored in the cocked myosin heads, resulting in the powerstroke: the heads move dragging the actin, and so pulling the Z lines close to the M line.
The head will remain bound to actin until it is released by the binding of an ATP molecule. ATP binds to the myosin head, breaking the actinomyosin bridge. It is the BINDING of the ATP, not it’s hydrolysis, that breaks the actinomyosin bridge. One the actin has been released, then ATP is hydrolysed, releasing energy which is used to cock teh myosin head back to teh original position, ready to bind to the next binding site further along the actin.
The roles of ATP in muscle contraction
ATP has TWO roles:
- The active transport of calcium ions into the sarcoplasmic reticulum.
- The cross-bridge cycle:
Cross-Bridge Formation and Power Stroke.
THIS DOES NOT REQUIRE ATP.
As the myosin head binds to actin to form the actinomyosin bridge, the head releases its ADP + Pi, which helps cause the conformational shape change which triggers the powerstroke, causing muscle contraction.
Cross-Bridge Detachment
THIS DOES REQUIRE THE BINDING OF ATP
A new ATP molecule binds to the myosin head. This causes myosin to release actin (breaks the cross-bridge). Without ATP the myosin can’t detach—this is what causes rigor mortis after death.
Re-cocking the Myosin Head
THIS DOES REQUIRE THE HYDROLYSIS OF ATP
The ATP that bound to the myosin head is hydrolysed into ADP + Pi by the enzyme myosin ATPase (aka ATP hydrolase)
This energizes and re-cocks the myosin head into its high-energy position, ready to form a new cross-bridge.
The role of phosphocreatine in muscle contraction.
Phosphocreatine can quickly phosphorylate ADP to make ATP
ADP + CP → ATP + C
allowing for quicker formation of ATP during exercise.
ATP must then be used to reform the stores of phosphocreatine.
ATP + C → ADP + CP
The structure, location and general properties of slow and fast
skeletal muscle fibres.
Both are found in the same muscle tissue, but in differing proportions, depending on use. Different people (and other vertebrates) will have different proportions in the same muscle tissue.
Fast fibres are ‘fast’ because they have a faster cross-bridge cycle), so can contract more rapidly than slow fibres. Their higher rate of contraction comes from having a thicker diameter, higher concentrations of calcium ions in their sarcoplasmic reticulum, and more myosin ATPases. However it also means fast fibres have to rely on anaerobic respiration (anaerobic respiration is quicker than aerobic respiration, and only anaerobic respiration can supply ATP quick enough for their needs). The resulting lactic acid means that fast fibres soon fatigue (the fall in pH temporarily reduces the ability of the muscle proteins to function).
Slow fibres contract more slowly, but this means that aerobic respiration can keep up with their demand for ATP, so they are less prone to fatigue.
As a result fast fibres allow for quick, powerful activities (eg blinking, sprinting, weightlifting) whereas slow fibres allow endurance (eg long distance running, maintaining posture).
Given that fast fibres mostly do anaerobic respiration they have less need for oxygen, so are served by fewer capillaries and have less myoglobin, making them appear paler. They will however have higher concentrations of glycogen (which they hydrolyse to release glucose - glycogenolysis), as they can only generate 2 ATP per glucose. They will also have more phosphocreatine.
Slow fibres will have denser networks of capillaries, more myoglobin, many more mitochondria, less glycogen and less phosphocreatine.
