Phsiology Of Skeletal Muscle Flashcards
Describe with diagrams the cellular and ultrastructural components of a skeletal muscle cell and how they relate to the contractile mechanism
Connective tissue
- The epimysium surrounds the whole muscle and separates the muscle from surrounding tissues.
- The perimysium divides the muscle into fascicles of muscle cells.
- The endomysium surrounds individual muscle cells.
- The three types of connective tissue fuse at the ends of the muscle to form a tendon that inserts into the bone.
- The connective tissue acts as anchoring points for the contractile proteins to pull against.
Muscle fibre
- Contains multiple nuclei.
- Rich in mitochondria which supply the ATP necessary for contraction, and have well developed sarcoplasmic reticulum.
- The cells appear as striated.
Myofibrils
- Contain the contractile proteins of the cell.
- Composed of myofilaments which account for the striated appearance of skeletal muscle cells.
- The sarcomere is the contractile sub-unit of skeletal muscle cells.
Myofilaments
- Formed from the contractile and regulatory protein molecules that control and produce tension and shortening in the muscle.
Thick myofilament
- Composed of a series of myosin molecules arranged in a regular array.
- Each molecule has a double head pointing away from the middle of the myofilament which sticks out into the space between myofilaments.
- The head has binding sites for actin and ATP.
The ATP binding site is enzymatically active as it breaks down ATP to release the chemical energy which is used to do mechanical work during muscle contraction.
Thin myofilament
- Composed of three different proteins.
- Actin is arranged in a double stranded helix.
- In the groove between the two actin strands is a tropomyosin molecule.
- At regular intervals there are also molecules of troponin which are attached to both actin and myosin.
- Troponin has calcium binding sites and is important in the regulation of contraction.
Describe the sliding filament hypothesis of contraction, naming the molecules involved and summarising the evidence for this hypothsis
- The myofilaments do not become shorter during contraction.
- The thin actin filaments slide over the thick myosin filaments and pull the Z-line behind them.
- This causes each sarcomere as a whole to shorten.
Evidence
- The fact that ATP is needed to bind to the myosin head in order for the myosin head to release from the actin is demonstrated by rigor mortis.
- After death there is no respiration so production of ATP in the body tissues stops.
- In skeletal muscle the calcium pumps on the sarcoplasmic reticulum no longer pump intracellular calcium back into the sarcoplasmic reticulum, and calcium leaks into the muscle cells and the concentration slowly increases.
- The calcium binds to the troponin and causes the myosin head to attach to the tropomyosin, producing the power stroke on release of the ADP and inorganic phosphate, causing contractions.
- As there is no further respiration and no further production of ATP, the myosin heads remain attached to the actin and the muscles remain in a state of contraction.
Describe the events of the cross bridge cycle
- The myosin head must have an ATP molecule bound to it to initiate contraction.
- Myosin ATPase hydrolyses the ATP into ADP and inorganic phosphate. The energy released by this activates the myosin head which cocks into an extended high-energy position. The head temporarily keeps the ADP and inorganic phosphate bound to it.
- The cocked myosin head binds to an exposed active site on the actin filament, forming an actomyosin cross-bridge.
- The myosin head releases the ADP and inorganic phosphate and flexes into a bent, low-energy position, pulling the actin filament with it. This is called the power stroke.
- The myosin head remains bound to the actin filament until it binds a new ATP molecule.
- The binding of a new ATP molecule to the myosin head destabilises the myosin-actin bond, breaking the cross-bridge.
- The myosin head hydrolyses the new ATP molecule, recocks, and attaches to a new active site further down the actin filament, ready for another power stroke. This is known as the recovery stroke.
Describe the main steps in excitation contraction coupling in skeletal muscle
Excitation-contraction coupling refers to events that link action potentials on the sarcolemma to activation of the myofilaments, thereby preparing them to contract.
- A wave of action potentials spreads from the motor end plate in all directions.
- When this wave of excitation reaches the T tubules it continues down them into the cell interior.
- The action potentials open voltage-gated ion channels in the T tubules which are linked to calcium channels in the terminal cisterns of the sarcoplasmic reticulum.
- Calcium channels in the sarcoplasmic reticulum open and calcium diffuses out of the sarcoplasmic reticulum and down its concetration gradient into the cytosol.
- The calcium binds to the troponin of the actin filaments.
- The troponin-tropomyosin complex changes shape and exposes the active sites on the actin filaments making them available for binding to myosin heads.
What are the characteristics of fast twitch fibres (fast glycolytic)
- Large fibres
- Extensive sarcoplasmic reticulum
- Large amounts of glycolytic enzyme
- Less extensive blood supply
- Fewer mitochondria
- White colour
What are the characteristic features of slow twitch fibres (slow oxidative)
- Smaller fibres
- Innervated by small nerve fibres
- More extensive blood supply
- More mitochondria
- Large amounts of myoglobin
- Red colour
What are the characteristic features of fast twitch fibres (fast oxidative glycolytic)
- Intermediate type
- Fast contraction and increased force
- Can utilise oxidative processes
- Red colour
What happens during the latent period in the development of tension
- After stimulation (action potential arriving at muscle fibre).
- Action potential moves over sarcolemma.
- Calcium released from the sarcoplasmic reticulum.
What happens during the contraction phase in the development of tension
- Myosin cross-bridges form.
- Tension rises to peak.
What happens during the relaxation phase of tension development
- Calcium levels drop.
- Number of cross-bridges decline.
- Tension falls.
What is the motor unit
It is the alpha motor neurone and all the motor fibres it innervates.
Name and describe the process used to control contraction force
- Recruitment
- To generate smaller forces and fine control of movement, small number of motor units are recruited.
- The motor command coming down the spine from motor areas of the grain will excite the smaller cell bodies of the slow twitch motor units first as they are more excitable and generate smaller forces.
- As more force is required, more excitation travels down the spine to excite more motor units in the motor pool. This begins to include larger alpha motor neurones innervating fast twitch fibres and generating greater amounts of force.
