Muscular System Flashcards
What happens when an ACh is releases at a NMJ of skeletal muscle (4 steps)
1) Net sodium entry triggers AP, 2) AP in T-tubule alters shape of DHP receptor, 3) DHP opens RyR Ca release channel in SR and Ca enters cytoplasm, 4) Power stroke/contraction-relaxation cycle
What are the steps in the power stroke cycle
1) Rigor state: Myosin bound to actin at 45 degree angle, 2) ATP binds to myosin, causing it to dissociate from actin, 3) Myosin hydrolyzes ATP, leaving ADP bound to it, 4) Myosin head swigs to bind to new actin at 90 degree angle, 5) Resting state: Phosphate released to initiate power stroke, myosin head rotates and pushes actin past, 6) End of power stroke, myosin head releases ADP and resumes rigor state
What does Ca bind to in skeletal muscle
Troponin
What does tropomyosin do in skeletal muscle
Blocks myosin but rotates out of the way to expose the actin binding site
What stops the contraction-relaxation cycle
ACh is broken down in synaptic cleft by acetylcholinesterase and Ca is put back into SR by Ca++ ATPase uniporter
What are type I skeletal muscle fibers (7 characteristics)
Slow-oxidative: smallest, slowest, longest, fatigue resistant, least force, uses O2, dark red
What are type IIa skeletal muscle fibers (6 characteristics)
Fast-twitch oxidative-glycolytic: Intermediate force and size, fast, fatigue resistant, short, glycolytic but can become more oxidative
What are type IIb/x skeletal muscle fibers (7 characteristics)
Fast-twitch glycolytic: Fastest, largest, most force, short, easily fatigued, more anaerobic than IIa (convert pyruvate into lactose), pale
What factors influence muscle force production
Sarcomere and fiber length (~2.1-2.2um), and summation/frequency of stimulation (activation level)
What are single twitches
Muscle relaxes completely between stimuli
What is summation
Stimuli close together that don’t allow muscles to relax fully
What is unfused tetanus
Muscle tension oscillates around peak force
What is fused tetanus
Muscle tension flatlines at peak force based on Ca release rate that saturates the cross bridge binding sites (all occupied)
What is a motor unit
A somatic motor neuron and all the muscle fibers (same type) it innervates
What are the characteristics of motor units
Vary in number of muscle fibers, all fibers are of the same type, force mainly determined by number of fibers
How do muscles vary muscle force
NS uses frequency coding and twitch summation, as well as motor unit recruitment (smaller and slower recruited first to prevent fatigue)
What is hypertrophy
Muscle fibers get bigger because they add more actin and myosin and thereby can produce more crossbridges
What is atrophy
Muscle loss due to decreased use
What is isotonic contraction
Any contraction where the muscle changes length, two kinds (concentric and eccentric)
What is concentric contraction
When muscle force is greater than the load, the muscle shortens
What is eccentric contraction
When muscle force is less than the load, the muscle lengthens (muscle tries to slow lengthening)
What is an isometric contraction
The muscle contracts but doesn’t change length because the force is equal to the load
What are the 3 muscle load-velocity relationships
Increasing the load decreases velocity of shortening, maximal velocity of concentric contraction when load=0, isometric contraction when load=muscle force
What are characteristics of smooth muscle
No sarcomeres or striations, has actin and myosin, can contract on its own, under endocrine and autonomic control
What initiates contraction in smooth muscle
Signal ligands (membrane receptors) or depolarization or stretch (membrane channels)
What happens when Ca from the ECF binds to membrane receptors in smooth muscle
Increases IP3, activates IP3-R on SR to release calcium
What happens in smooth muscle once Ca is released from the SR (4 things)
1) Binds to calmodulin (CaM), 2) activates myosin light chain kinase (MLCK) 3) Phosphorylates light chains in myosin heads and increases myosin ATPase activity, 4) Active myosin cross bridges slide along actin to create muscle tension
How does smooth muscle relax
Myosin light chain phosphatase or Ca removed by ATPase or NA+ antiporters
What are the 6 characteristics of cardiac muscle
Only in the heart, striated, short, branched, and electrically-linked (in intercalated disks), contract spontaneously, influenced by endocrine and autonomic systems, EC coupling like skeletal muscle
What is the the pathway of cardiac EC coupling (7 steps)
1) AP enters from adjacent cell, 2) Voltage-gated Ca channels open and Ca enters, 3) Ca induces Ca release through RyR channels, 4) Local release sparks Ca, 5) Summed Ca sparks create Ca signal, 6) Ca binds to troponin to start contraction, 7) Relaxation when Ca unbinds and is pumped back into SR for storage (also exchanged with Na by NCX antiporter)
How are smooth muscle fibers organized
In oblique bundles
Which muscle fibers are the fastest at contracting
Skeletal, then cardiac, then smooth
