Skeletal muscles 2: Skeletal muscle contraction Flashcards
What are the structural components of skeletal muscle?
- They are attached directly to bone or indirectly through ligaments, fascia, cartilage, and skin
- Arteries and veins enter and exit the muscles to bring nutrients and remove waste
- Nerves innervate the muscle bed
- muscle fibres comprise 75%-92%
What are the connective tissues of the muscle?
- Epimysium - surrounds the entire muscle
- Perimysium - surrounds muscle bundles or fascicles
- Endomysium - surrounds each muscle fibre or muscle cell
What is the sarcolemma
It is the plasma membrane of a striated muscle fibre (has longitudinal t-tubules)
Explain the transverse tubular (T-tubular) membrane system
Deep invaginations of sarcolemma “into” the myocyte which conduct propagated action potentials lead to contraction
It is located at the junction of overlap between the A and I bands
What is the appearance of skeletal muscle fibers?
It has a striated appearance due to the highly ordered sarcomeric arrangement. Calls are multinucleated with nuclei lying peripherally. Cells contain myofibrils 1-2um in diameter.
What is the function of the Sarcoplasmic reticulum (SR)?
It is an intracellular compartment that acts as a store for Ca2+.
The terminal enlargements of the SR are known as the terminal cisternae. It is associated with 2 terminal cisternae of SR, known as triad.
The SR provides feedback control required to balance intracellular Ca2+ cycling through the concerted action of three major classes of SR calcium-regulatory proteins:
- luminal calcium-binding proteins for calcium storage
- SR calcium release channels for calcium release
- SR Ca2+ ATPase pups for calcium reuptake
What is the organisation of sarcomeres?
see page 103 for diagram
- A-band: contains both thick and thin filaments (longitudinally overlapping)
- I-band: contains thin filaments only
- Z-line (or disc): an electron-dense region in the middle of the I-band where actin filaments of different “polarity” in each half sarcomere attach.
- H-band: region in the centre of the A-band where there are only thick filaments
- M-line: electron-dense region in the centre of the H-band where myosin filaments of different “polarity” attach.
Explain what the thick filaments are
They are comprised mainly of myosin and they form the A bands of each sarcomere.
The composition:
- 100’s of molecules of myosin
- Oriented in opposite directions
- repeating, staggered array of paired heads
- bare zone (no head) in the middle
Titin acts as an adjustable molecular spring during contraction, contributing to passive force and maintaining the structural integrity of the sarcomere.
Explain what the thin filaments are
They are made out of many single G-actin molecules that form F-actin. Two strands of F-actin twisted together into a helix. Rod-shaped tropomyosin (Tm) molecules lie along the F-actin in a groove.
Each Tm molecule is associated with 3 other proteins which comprise the troponin complex of Toponin Tropomyosin (TnT), Troponin Calcium (TnC), Troponin Inhibitor (TnI).
Nebulin helps to align the actin filaments
TnT - Positions the complex on the Tm molecule
TnC - Contains the Ca2+ binding sites
TnI - Binds actin and “inhibits” the myosin head from binding to the actin binding site
Explain the cross-bridge cycle
- Tight binding in the rigour state. The crossbridge is at a 45* angle relative to the filaments. (Myosin head is attached to myosin binding site on the actin filament)
- ATP binds to the nucleotide binding site on myosin. Mysoin then dissociates from actin
- The ATPase activity of myosin hydrolysis the ATP to ADP and inorganic phosphate (Pi). Both products remain bound to the myosin head (becomes cocked)
- The myosin head swings over and binds weekly to a new actin molecule. The crossbridge now at 90* relative to the filaments.
- The release of Pi initiates the power stroke. The myosin head rotates on its hinge, pushing the associated actin filament past it
- At the end of the power stroke, the myosin head releases ADP and resumes the tightly bound rigour state
What is the role of calcium?
- When calcium levels are low, the tropomyosin molecule prevents interaction between actin and myosin
- When cytosolic Ca2+ concentration increases, four Ca2+ ions bind cooperatively to troponin C causing a conformational change in the troponin complex.
- The tropomyosin molecule then shifts with respect to the actin filament, removing the steric interference to crossbridge interaction
- As long as there are high level of Ca2+ and ATP present, crossbridge cycling will always generate force
Name the different types of summation and tetanus
- Single twitches: Muscle relaxes completely between stimuli
- Summation: Stimuli closer together do not allow muscle to relax fully
- Summation leading to infused tetanus: Stimuli are far enough apart to allow the muscle to relax slightly between stimuli (maximum tension a lot higher than normal)
- Summation leading to complete tetanus: Muscles reaches steady tension (fatigue will cause muscle to lose tension despite continuing stimuli)
What are the actions of the sympathetic nervous system on the skeletal muscles?
- They directly affect the muscles
- The peak tension developed in response to a single stimulus is increased in fast fibres, and the time course is decreased
- Under conditions of stress, the performance of fast contracting muscles is improved
- Opposite effects from catecholamines are seen for slow muscles
- This might explain postural weakness in the muscles of the back and leg following severe stress or fright.
