Skeletal Muscle Flashcards
(23 cards)
Define Sarcolemma
“cell membrane”
Define myofibrils
cylindrical structure made up of an end-to-end chain of repeating unit (sarcomeres)
Define Sarcoplasmic reticulum
equivalent to endoplasmic reticulum
- Stores calcium, releases calcium into muscle cytosol and resequesters the calcium to end a muscle contraction
Define T tubules
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.
Define Muscle fibers
“muscle cell”
- extend the length of the muscle
- individual muscle fibers are multi-nucleated cells made up of myofibrils.
define titin
- 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.
define dystrophin
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**
define ryanodine receptor
“calcium release channels”
- activated by DHP channels to open and release calcium from the SR into the myofibril.
define dihydropyridine (DHP) receptors
“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.
Define sarcomere
subunit of myofibril
- area between two Z discs
Define topomyosin
attached to actin
- believed to cover active site on actin filaments
define the 3 troponin complexes
Troponin I = affinity to actin
Troponin T = affinity to tropomyosin
Troponin C = affinity for calcium ions
Define the role of Calcium in contraction of muscle
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.
describe the cross bridge cycle in skeletal/cardiac muscle
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.
Define rigor mortis
After death as ATP in muscle decreases there is NO ATP THE DETACH THE MYOSIN FROM THE ACTIN and the muscle becomes still
describe the initiation of muscle contraction
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)
How do you pump Ca back into SR?
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.
Define Motor unit
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
Motor neuron pool
all neurons that innervate a muscle
How do you produce a graded muscle contraction?
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
Define Tetany
continuous state of full activation of muscle due to the electrical event being so much faster than the mechanical event.
Define Isometric muscle contraction
muscle does NOT shorten during the contraction
- constant length, but increasing tension
Define Isotonic muscle contraction
Muscle does SHORTEN during a contraction but the tension on the muscle remains constant.
- constant tension, but changing length
