Muscle Flashcards
Describe the structure and ultrastructure of skeletal muscle.
Skeletal muscle cells are multinucleated striated fibres, which are created in utero by myoblasts. Muscles are made of bundles of these fibres wrapped in connective tissue sheaths. The striations are caused by sarcomeres.
Define isometric and isotonic twitches.
Isotonic – Contraction with shortening length
Isometric – Contraction with constant length
Lengthening - contraction with increased length
Explain the process of excitation-coupling contraction in skeletal muscle contraction.
Once an AP has fired in the muscle, Ca2+ ions are released from the sarcoplasmic reticulum. These ions bind to the troponin on the actin filament and remove the blocking protein, tropomyosin, exposing the actin binding sites. The myosin filaments can then bind to the actin and from cross bridges. This is what causes the “contraction” of the muscle fibres. The calcium ions then return to the sarcoplasmic reticulum through the use of ATP. This leaves the tropomyosin free to return to its original position, and so the contraction ends and cross bridges are broken.
What occurs in the sarcomere during muscle contraction?
As the muscle contracts, various changes occur to the sarcomere, i.e. the A (myosin only) band and the H band reduce, as the filaments slide over each other. More overlap means more tension and vice versa. However, too much overlap, and the filaments will interfere with each other.
Describe the generation of controlled force through the recruitment of motor units.
As the load increases, more motor units are required to compensate. This process is known as recruitment, and allows more muscle fibres to be involved in a movement.
Explain tetanus
Tetanus is a state of extended contraction via the summation of action potentials. Tetanic tension is much greater than twitch tension, as there is no respite in which the calcium can return to the sarcoplasmic reticulum. As such, tropomyosin is always held in the non-blocking state and so the cross bridges cannot be broken.
Explain fatigue
Fatigue is a state that is caused by repeated muscle stimulation. It prevents using up vast quantities of ATP in breaking and reforming cross bridges to sustain contractions. How much stimulation is required to trigger fatigue depends entirely on the individual’s fitness, muscle fibre type and the length of the contraction.
Describe the aerobic properties of muscle
There are a few different types of skeletal muscle fibres, and they are classified according to their speed and whether they are glycolytic or oxidative.
o Oxidative Fibres
These fibres have more mitochondria for oxidative phosphorylation, and are more vascular to allow for the delivery of O2 and nutrients. They also contain myoglobin to help with oxygen delivery. The fibres are red and tend to be of low diameter.
• Slow oxidative fibres –Resist fatigue
• Fast oxidative fibres - Intermediate resistance to fatigue
Describe the anaerobic properties of muscle
There are a few different types of skeletal muscle fibres, and they are classified according to their speed and whether they are glycolytic or oxidative.
o Glycolytic Fibres
These have very few mitochondria, but have a higher concentration of glycolytic enzymes and glycogen. They have a lower blood supply, and are white fibres with larger diameters.
• Fast glycolytic fibres – Fatigue quickly
Describe the structure and function of smooth muscle
Smooth muscle exists in hollow organs, such as the GI Tract, the bladder, airways etc, and is supplied by the autonomic nervous system. The cells are mononucleate non-striated spindles and can divide throughout life. Like skeletal muscle, they have actin and myosin filaments, except they are arranged diagonally across cells and are anchored to membranes and cell structures by dense bodies.
Explain contraction and relaxation in smooth muscle.
Smooth muscles also form cross bridges between myosin and actin filaments, however the mechanism is different.
As calcium is released from the sarcoplasmic reticulum, it binds to calmodulin, which binds to myosin light chain kinase. This phosphorylates the myosin with ATP and forms cross bridges to actin filaments, resulting in contraction and tension.
Relaxation is brought about through the action of myosin light chain phosphatase, which dephosphorylates the cross bridges. Contraction can be held for longer in smooth muscle due to the low rate of consumption of ATP. This is useful in blood vessels, which must stay open for long periods of time.
Explain the sliding-filament theory of muscle contraction and its relationship to the length-tension properties of muscle.
Sliding filament theory - explains the mechanism of muscle contraction based on muscle proteins that slide past each other to generate movement.
According to the sliding filament theory, the actin (thin) filaments of muscle fibers slide past the myosin (thick) filaments during muscle contraction, while the two groups of filaments remain at relatively constant length.
Length tension relationship – there is an optimal length of filament required for optimal tension generation.
Muscle length for greatest isometric tension = optimal length (lo)
What are the main functions of muscle?
- Generate force & movement
* Allow us to express & regulate ourselves
Describe the cross-bridge contraction cycle
- ATP binds and myosin detaches.
- ATP hydrolysis provides energy for the myosin head to rotate and reattach to actin.
- The power stroke. The power stroke (cross-bridge tilting) begins after Ca2+ binds to troponin to uncover the rest of the myosin binding site.
- At the end of the power stroke, myosin releases ADP, allowing the myosin head to be tightly bound to the actin filament in the rigor state. The cycle is ready to begin once more as a new ATP binds to myosin.
Describe briefly a motor unit
Motor neurons + muscle fibres = motor unit
