Download Muscle System Review

Muscular System
5 characteristics:
 Responsiveness: ability to respond to chemical signals, stretch and electrical
changes across the plasma membrane
 Conductivity: ability to pass an electric signal across the muscle fiber
 Contractility: the ability to shorten when stimulated
 Extensibility: the ability to be stretched
 Elasticity: returns to its resting length after being stretched
Muscle Characterisitics
 Voluntary, striated (light at dark bands) muscle attached to bone
 Muscle fibers=myofibers
 Epimysium: surrounds the entire muscle; Perimysium: wraps around one fascicle
(group of muscle fibers); Endomysium: surrounds one individual muscle fiber
 Sarcolemma- plasma membrane of sarcomere
 Sarcoplasm- cytoplasm of a muscle cell
 Single muscle fiber has hundreds of myofibrils that look spotty (because made of
myofilaments, which make up sarcomeres)
o A bands: dark (both thin and thick filaments)
o I bands: light
o H zone: thick filaments only
o I band: thin filaments only
o Sarcomere (smallest contractile unit of a muscle fiber, the functional unit) is
from z disc to z disc
o M line in middle just thick filaments
 Thick filaments: myosin, thin filaments: actin
 Tropomyosin: rod shaped protein that spiral around actin and stabilize it, block
myosin binding sites when contraction is not needed
 Troponin
o TnI- inhibitory subunit that binds to actin
o TnT- binds to tropomyosin and helps position it on actin
o TnC- binds calcium ions
 Sacrcolemma has tunnel like infoldings or transverse (T) tubules that carry electric
current to cell interior
 Sarcoplasmic reticulum: network around each myofibril that stores calcium
 Triad- T tubule and 2 terminal cisternae
Sliding of Filaments:
 Thick and thin filaments don’t change length during shortening, the overlap just
changes as the sarcomeres shorten
 A band stays the same (makes sense, both thick and thin filaments)
 Actin filaments slide past myosin
Motor Units:
 One motor neuron innervates many muscle fibers
 Recruitment:
o Always recruit the smaller motor units first (because if you just need a fine
touch/pressure, then that will do it)
o Largest motor unit is the last one recruited (for extreme forces)
o Smallest motor units are type 1/slow for endurance running; dark meat and
lots of fat for long exercise
o Largest are type 2/fast for explosive motion; white turkey because not much
mitochondria because use lactic acid that makes your muscles hurt after
sprinting
o Exercise increases recruitment of fast and large motor units
o Mismatch- using wrong size of motor unit for a certain force needed
o Motor units die with age and you lose intricate control (loss of muscle as get
older)
Neuromuscular Junction:
 Connection between nerve fiber and muscle cell, ACh released from nerve fiber
stimulates the muscle cell
 1. Local depolarization (end plate potential) ACh opens chemically gated ion
channels, causes Na/K and depolarizes
 2. End plate potential spreads to adjacent membrane areas, voltage gated Na
channels open, if threshold is reached, AP is generated
 3. From there, AP is propagated along the sarcomere to T tubules, where voltage
sensitive proteins stimulate Ca2+ release from SR which is needed for contraction
 4. Calcium binds to troponin, which changes shape and moves tropomyosin away
from the binding sites, allowing myosin to bind to actin
 5. When nervous stimulation ends, Ca2+ pumped back into the SR and binds to
calequestrin- which neutralizes the charge so calcium can keep flooding in without
the gradient having any effect on the flow of calcium
Cross Bridge Cycle:
 Continues as long as there is ATP and Calcium present
 1. Cross bridge forms as an energized myosin head binds to actin
 2. Power stroke occurs as ADP and P are released and the myosin head bends,
causing the actin to slide toward the M line
 3. Cross bridge detachment occurs when ATP attaches to myosin
 4. Myosin head returns to high energy state, “cocking” and ATP is hydrolyzed to ADP
and P
 RINSE AND REPEAT
Relaxation of a Muscle:
 Nerve stimulation ceases and acetylchlolinesterase removes ACh from the receptors
 Active transport needed to pump calcium back into SR to bind to cal.
Rigor Mortis: stiffening of the body beginning 3 to 4 hours after death
-Happens because calcium will still be released, so myosin heads will attach to actin, but
with no ATP, they will not detach from it so the muscles will stay contracted
Length-Tension Relationship
 Amount of tension generated depends on length of muscle before it was stimulated
(textbook lifting experiment)
 Overly contracted beforehand- weak contraction results because the thick filaments
are too close to the Z disc and can’t slide
 Too stretched beforehand- no actin/myosin interaction because they are too far
apart to reach each other
Muscle Twitch (motor unit’s response to a single AP of its motor neuron)
 Latent period: first few milliseconds when EC coupling is occurring
 Period of contraction: cross bridges are active, if tension becomes great enough to
overcome the resistance of the load, the muscle shortens
 Period of relaxation: final phase that is initiated by reentry of Ca2+ into the SR,
muscle tension decreases to zero
 Muscle CONTRACTS faster than RELAXES
Other stuff:
 Myostatin: negative regulator of skeletal muscle mass, limits growth, also limits
cardiac growth
 Loss of myostatin (deficiency) causes uncontrolled muscle mass growth
 Good for animals that need a lot of muscles (dogs that race), but more muscle means
less fat which means less flavor for food and such
 Fused tetanus: when all evidence of relaxation disappears and the contractions fuse
into a smooth, sustained contraction plateau; happens infrequently- superhuman
strength to lift a car for ex.
 Wave/temporal summation: two stimuli delivered in succession, second twitch will
be stronger because second contraction occurred before it relaxed
 Incomplete/unfused tetanus: when wave summation becomes greater and greater
and end up with a sustained but quivering contraction
 Isotonic Contraction: Concentric/eccentric- shortening/lengthening
 Isometric contraction: muscle tenses but doesn’t shorten/lengthen