Muscle Structure And Contraction.

Question 1

What are the three types of muscles?

Answer 1

• Smooth involuntary muscle.
• Cardiac muscle.
• Voluntary striated/ skeletal muscle.

Question 2

Features of smooth involuntary muscle.
-> what is it stimulated by?
-> how does it contract?
-> where is it found.

Answer 2

• Stimulated by neurones of autonomic NS (no conscious control).
• Spindle shaped fibres and uninucleated.
• contracts slowly and fatigues slowly and contract in different directions.
  Found in: intestine walls, blood vessel walls.

Question 3

Features of cardiac muscle

Answer 3

• atrial and ventricular muscle = contract like voluntary muscle, don’t fatigue
• special excitatory + conductive muscle fibres = control rhythmic HR by conducting waves of electrical excitation across the heart (SAN/AVN)
• uninucleated
• “intercalated disc” between cells = junctions that wallow free movement of ions allowing A.P to pass quickly between muscle fibres.
• powerful contraction but no fatigue
• cells contract simultaneously - no conscious control

Question 4

Features of voluntary striated/ skeletal muscle.

Answer 4

• allows for movement of bones, - muscles attached to bones by tendons which are in elastic - muscle contracts = pulls on tendon = pulls on bone.
• striped - quickly contract + powerful - easily fatigued
• conscious control (somatic) - multinucleated
• muscle contracts in 1 direction

Question 5

Features of muscle contraction.

Answer 5

1- each muscle is made up of muscle fibres -> surrounded by connective tissue
2- each fibre is made of myofibrils
3- each myofibril is made of the proteins : actin and myosin = give striped appearance

Question 6

How muscle fibre contracts - “sliding filament theory”

Answer 6

1- A.P arrives at synapse between motor neurone and sarcolemma (memb) around muscle fibre.
2- causing release of ACh NT across synapse where it binds to specific receptors on sarcolemma memb
3- causing Na+ channels to open, depolarising sarcolemma
4- A.P spreads across muscle fibre via T - tubules
5- causing sarcoplasmic reticulum to release Ca2+ ions into sarcoplasm so they can bind to troponin protein on actin.
6- causing troponin to change shape so they move tropomyosin out of way.
7- so myosin needs to attach to actin binding sites forming actin-myosin cross bridges. Myosin heads bend to pull actin over them (power stroke).
8- Myosin head joins to A.P so it can detach from actin.
9- ATP hydrolysed -> ADP + Pi -> providing energy for bent head to return to original position process repeated as long as there is nervous stimulation.
10- muscle fibre gets shorter and shorter as actin is pulled further over head until max. Contraction occurs.