Skeletal Muscle

Question 1

Define Sarcolemma

Answer 1

“cell membrane”

Question 2

Define myofibrils

Answer 2

cylindrical structure made up of an end-to-end chain of repeating unit (sarcomeres)

Question 3

Define Sarcoplasmic reticulum

Answer 3

equivalent to endoplasmic reticulum

- Stores calcium, releases calcium into muscle cytosol and resequesters the calcium to end a muscle contraction

Question 4

Define T tubules

Answer 4

Part of the T system

• invaginations of the plasma membrane so the lumen contains extracellular fluid
• Transmits the action potential generated near the end plate through-out the muscle cell.

Question 5

Define Muscle fibers

Answer 5

“muscle cell”

• extend the length of the muscle
• individual muscle fibers are multi-nucleated cells made up of myofibrils.

Question 6

define titin

Answer 6

• Acts as a framework to hold myosin and actin filaments in place
• one end attaches to the Z-line and the other end to myosin thick filament.

Question 7

define dystrophin

Answer 7

Forms a rod that connects the thin actin filaments to a transmembrane protein

• adds strength to muscles partially by connecting fibrils to extracellular matrix
• *mutations in dystrophin causes types of muscular dystrophy**

Question 8

define ryanodine receptor

Answer 8

“calcium release channels”

- activated by DHP channels to open and release calcium from the SR into the myofibril.

Question 9

define dihydropyridine (DHP) receptors

Answer 9

“L-type Calcium channels in T tubules”

• act as voltage sensors.
• when DHP receptor channels open they cause the calcium release channels to open and leading to calcium rapidly leaving the SR and spreads into the myofibrils.

Question 10

Define sarcomere

Answer 10

subunit of myofibril

- area between two Z discs

Question 11

Define topomyosin

Answer 11

attached to actin

- believed to cover active site on actin filaments

Question 12

define the 3 troponin complexes

Answer 12

Troponin I = affinity to actin
Troponin T = affinity to tropomyosin
Troponin C = affinity for calcium ions

Question 13

Define the role of Calcium in contraction of muscle

Answer 13

1) calcium binds to troponin C, which causes tropomyosin to move exposing active site on actin where myosin can bind
2) Calcium pumped back into the Sarcoplasmic reticulum and tropomyosin can move back to cover active site on actin
- ATP must be present to detach the myosin heads from the actin active sites.

Question 14

describe the cross bridge cycle in skeletal/cardiac muscle

Answer 14

1) Attached State = Myosin head bound to actin
2) ATP binds to myosin head, causing the dissociation of actin-myosin complex to form the Released State
3) ATP is HYDROLYZED causing myosin heads to return to their resting conformation
4) A cross-bridge forms and the mysoin head binds to a new position on actin
5) Pi is released. myosin heads change conformation, resulting in the “power stroke”. the filaments slide past each other
6) ADP is released and we return to the attached state with myosin bound to actin.

Question 15

Define rigor mortis

Answer 15

After death as ATP in muscle decreases there is NO ATP THE DETACH THE MYOSIN FROM THE ACTIN and the muscle becomes still

Question 16

describe the initiation of muscle contraction

Answer 16

1) AP intiated at NMJ and spreads through the T-tubule system
2) AP in T tubule causes the release of calcium from terminal cisternae and longitudinal tubules of SR via DHP(L-type Ca chan) and ryanodine receptors (Ca-release chan)
3) Excitatory pulse of Ca cause contraction (one muscle cell twitch)

Question 17

How do you pump Ca back into SR?

Answer 17

Calcium is pumped back into SR via Ca-ATPase pump.

• inhibited by high conc of Ca in SR.
• to increase capacity of the SR to calcium the SR contains calcium binding proteins, calsequestrin.

Question 18

Define Motor unit

Answer 18

one motor neuron the muscle fibers it innervates

• muscles that need fine control have very small motor units (eye muscles)
• muscles that do not need fine control have very large motor units

Question 19

Motor neuron pool

Answer 19

all neurons that innervate a muscle

Question 20

How do you produce a graded muscle contraction?

Answer 20

1) increase number of motor units contracting at any one time
OR
2) increase the rate of stimulation of a single motor unit
- mechanical event (muscle twitch) is much longer than electrical event (AP)
- since refractory period of muscle cell is very short, can cause a second muscle AP while Calcium level in myofibril is still increasing from first AP.
- The strength of muscle contraction depends on the rate of stimulation of AP’s in the muscle cell

Question 21

Define Tetany

Answer 21

continuous state of full activation of muscle due to the electrical event being so much faster than the mechanical event.

Question 22

Define Isometric muscle contraction

Answer 22

muscle does NOT shorten during the contraction

- constant length, but increasing tension

Question 23

Define Isotonic muscle contraction

Answer 23

Muscle does SHORTEN during a contraction but the tension on the muscle remains constant.
- constant tension, but changing length