MSK 2: Muscle Flashcards
What are the three types of muscle?
Smooth muscle
Cardiac muscle
Skeletal muscle
Which type of muscle is under voluntary control?
Skeletal muscle
What influences the contraction of cardiac muscle?
Autonomic nervous system and circulating chemicals.
Which division of the nervous system causes skeletal muscle to contract?
Somatic nervous system
What is the function of skeletal muscle?
To bring about movement
What is a fasicle?
A bundle of myofibres
Order from macroscopic to microscopic: Myofibril, myofilament, myofibre.
Myofibre, myofibril, myofilament.
Which covers fasicles, perimysium or endomysium?
Perimysium
What is the name of the muscle cell plasma membrane?
Sarcolemma
What is a T-tubule?
A tunnel into the centre of the centre of the myofibre.
What types of protein is muscle composed of?
Actin and myosin
What gives the muscle a striated appearance?
The light and dark bands, arranged in sarcomeres.
Label (c)
Actin, myosin, H zone, M line, Z disc, A band, M line,
What is the structure of myosin?
A protein with two globular heads and a tail formed from two alpha helices.
What is the structure of actin?
A protein twisted into helix, each molecule with a myosin binding site. Filaments also contain troponin and tropomyosin.
What is the sliding filament theory?
During contraction the I bands become shorter, the A band remains the same length and the H zone narrows or disappears, due to the filaments sliding over one another.
How are muscle contractions initiated?
- AP opens voltage gated calcium channels.
- Calcium ions enter pre-synaptic terminal and triggers exocytosis of vesicles containing acetylcholine.
- ACh binds to receptors and induces an AP in muscle
- Local currents flow from depolarised region to adjacent region, so AP spreads along surface.
- ACh broken down by acetylcholine esterase.
When the AP is on the surface of the muscle, how is a contraction then activated?
- AP propagates along surface membrane and into t tubules.
- Dihydropyridine (DHP) receptor in T-tubule membrane senses increased voltage and changes shape of protein linked to ryanodine receptor, opening to allow calcium out of the sarcoplasmic reticulum into space around filaments.
- Ca2+ binds to troponin
- Tropomyosin moves allowing crossbridges to attach to actin.
- Calcium is actively transported into the sarcoplasmic reticulum continuously while APs continue.
What is excitation contraction coupling?
- In the presence of calcium ions, troponin moves from the tropomyosin chain.
- Movement exposes myosin binding site on surface of actin chain
- ‘Charged’ myosin heads bind to exposed site
- Binding and discharge of ADP causes myosin head to picot (power stroke), pulling actin towards centre of sarcomere.
- ATP binding releases myosin head from chain
- Hydrolysis to ADP recharges myosin head.
Where is voluntary neural control from?
Upper motor neurons in brain and lower motor neurons in brainstem or spinal cord.
What is a motor unit?
A single motor neuron and all the muscle fibres it innervates.
How many muscle fibres does each motor neuron supply on average?
Around 600
What the three types of motor unit?
Slow (type I)
Fast, fatigue-resistant (type IIa)
Fast, fatiguable (type IIb)
What are the differences between slow and fast types of motor units?
Slow type motor units have smaller diameter cell bodies, smaller dendritic trees, thinner axons and higher aerobic capacity but lower anaerobic capacity.
