Ch.6 Muscle Physiology

Question 1

muscle type/classification

Answer 1

1. skeletal, smooth, cardiac
2. smooth, striated (other 2)

Question 2

functions of skeletal muscle

Answer 2

1. body movement
2. maintenance of posture
3. respiration (diaphragm and intercostal contractions)

Question 3

functions of smooth muscle

Answer 3

1. constriction of viscera (peristalsis of gastrointestinal tract)
2. constriction of blood vessels

Question 4

functions of cardiac muscle

Answer 4

heart beat

Question 5

organization of skeletal muscle

Answer 5

one myofibril –> one muscle fiber –> fascicle of muscle fibers (100s) –> bundles of muscle fibres

Question 6

investments of skeletal muscle

Answer 6

1. epimysium: dense collagenous connecti ve tissue that surrounds entire muscle
2. perimysium: collagenous connective tissue that surrounds bundles of muscle fibers called fascicles
3. endomysium: fine sheath of connective tissue composed of reticular fibers that surrounds individual muscle fibers
   epimysium (outside), perimysium (middle - around fascicle), endomysium (inner - surrounds fiber which contains lots of myofibrils)

Question 7

myotendinous junction

Answer 7

finger-like extensions of the muscle fibers insert into the connective tissue of the tendon

Question 8

basal lamina

Answer 8

external lamina - contains glycoproteins and collagen

Question 9

binding of basal lamina to the skeletal myofiber

Answer 9

• cross talk between inside and outside of cell
• basal lamina binds the myofiber via the dystroglycan-containing complex

Question 10

organization of a fascicle of myofibers

Answer 10

• long, cylindrical and striated
• multinucleated with nuclei located at the periphery (why? and how do they have so many nuclei?)

Question 11

myofibrils

Answer 11

• aligned in parallel
• separated by mitochondria (important for providing energy to skeletal muscle movement) and the sarcoplasmic reticulum
• composed of filaments called myofilaments

Question 12

sarcoplasmic reticulum (SR)

Answer 12

• a smooth endoplasmic reticulum
• forms an interconnected network of tubules
• runs longitudinally to the myofibril
• surrounds individual myofibrils with its complex tubule network
• forms chambers called terminal cisternae on either side of the T-tubules
• stores Ca+ when muscle is at rest
• releases Ca+ in the sarcoplasm when muscle is stimulated

Question 13

the triad junction, t-tubules, and voltage gated channel

Answer 13

• a single T-tubule and two terminal cisternae
• T-tubules are deep invaginations of the sarcolemma that run perpendicular to the length of the myofiber (think T has perpendicular lines, so adding T to tubule is the one that runs perpendicularly)
• T-tubule has dihydropyridine receptors that fit with Ca2+ release channels on the foot proteins of the cisterna –> when they are bound together in the triad junction they form a voltage-gated Ca2+ channel
• cycle of contraction and relaxation so that Ca2+ is pumped back for another contraction to take place

Question 14

composition of a contractile unit of myofibrils

Answer 14

• the sarcomere is the contractile unit
• check slide (pg.19) for picture
• composed of myosin (thick) filament, actin (thin) filament, and elastic (titin) filaments

Question 15

myosin (thick) filament

Answer 15

• two head/tail molecules wrapped around each other
• each head has one actin binding site and one myosin ATPase site (ATP will bind and be hydrolyzed)

Question 16

actin (thin) myofilament

Answer 16

• two actin strands wrapped around each other
• troponin complexes within strand
• tropomyosin strands (2) also wrapped around with actin filaments

Question 17

binding of myosin head to actin

Answer 17

• at low Ca2+ concentration, the myosin binding sites on actin are masked by the tropomyosin
• at high Ca2+ concentration, the tropomyosin no longer masks the binding sites and myosin can bind actin
  steps:
  1. sarcoplasmic reticulum releases Ca2+
  2. Ca2+ binds on top of troponin causing a change of its conformation
  3. along with this change, tropomyosin shifts position
  4. this shift exposes the binding site on myosin
  5. now actin can bind to myosin

Question 18

skeletal muscle contraction

Answer 18

1. myosin head attaches to the actin myofilament, forming a cross bridge
2. power stroke: myosin head bends backwards and pulls on the actin filament, dragging it towards the centre of the sarcomere
3. new ATP attaches to the myosin head, and the cross bridge detaches
4. ATP hydrolysis into ADP and Pi leads to the cocking of the myosin head, returning to its original position

Question 19

what leads to shortening of sarcomere?

Answer 19

• pulling of the actin filaments from the myosin heads that are bound to actin
• actin and myosin filaments overlap slightly for relaxed muscles, and then actin filaments on opposite sides end up overlapping when muscle is fully contracted
• NOT caused by actual shortening of either actin of myosin strands, just sliding against each other

Question 20

motor unit

Answer 20

a motor neuron and all the muscle fibers in innervates

Question 21

motor endplate

Answer 21

• a specialized domain of the sarcolemma
• highly excitable
• responsible for initiating the action potential that propagates across the myofiber, causing the muscle contraction

Question 22

excitation-contraction coupling

Answer 22

1. action potential generated is propagated along the sarcolemma and down the T tubules
2. action potential triggers Ca2+ release from terminal cisternae of SR
3. calcium ions bind to troponin - exposes actin active site
4. contraction of myosin fiber, and release of ATP
5. Ca2+ is removed by active transport into the SR
6. tropomyosin blockage restored, muscle fibers relax

Question 23

neuromuscular junction (NMJ)

Answer 23

• junction between the axon terminal of a motor neuron and the motor end plate