physiology midterm 2 part 1

cell membrane

sarcolemma

muscle cell

muscle fiber

cytoplasm

sarcoplasm

myosin

thick filament

actin

thin filament

what does a molecule of myosin consist of?

• two identical subunits with heads and intertwining tails

• heads form cross bridges between thick and thin filaments

what are the 2 binding sites of myosin?

• actin-binding site

• myosin ATPase site

tropomyosin

• lies alongside groove of actin spiral

• covers actin sites, blocking cross bridge binding

troponin polypeptide units

• TnT: tropomyosin binding unit

• TnI: actin binding unit

• TnC: Ca 2+ binding unit

what happens during the excited state?

• calcium binds to troponin

• tropomyosin moves off binding site allowing myosin to bind

what muscles types is troponin found in

• skeletal muscle

• cardiac muscle

excitation contraction coupling

events linking muscle excitation to muscle contraction

what events lead to muscle contraction?

• depolarization at motor endplate leads to release of stored Ca 2+ in SR into cytosol

• release of Ca2+ leads to interaction with troponin and sarcomere

what is the link between excitation and contraction?

Ca2+

T tubule

• located at each junction of an A band and I band

• enables rapid conveyance of surface electrical activity into middle of cell where tightly packed myofibrils are

what happens when an action potential travels down a T tubule?

triggers release of Ca2+ out of sarcoplasmic reticulum into the cytosol

what happens when there is a change in T tubule potential?

• dihydropyridine (DHP) receptors change shape

• DHP receptors physically link to ryanodine (RyR) receptors on SR opening RyR channels

• opening of channels allows Ca2+ to flood out of Sr into cytoplasm

what does strength of muscle contraction depend on?

• number of motor units recruited

• tension that is developed in each of those contracting fibers

number of muscle fibers per motor unit

• muscles performing precise, delicate movements: few muscle fibers per motor unit

• muscle performing coarse movements: many fibers per motor unit

recruitmen

when more and more units are activated to carry out strong contractions

what factors do tension that is developed in each contracting fiber depend on?

• frequency of stimulation

• length of muscle prior to onset of contraction

• fatigue

• fiber thickness

single twitch

• muscle fiber restimulated after complete relaxation

• second twitch is same magnitude ad first

twitch summation

• muscle fiber restimulated before complete relaxation

• second twitch added to first twitch

muscle tetanus

• muscle is stimulated so rapidly

• no relaxation between stimuli

• maximal sustained contraction occurs

what causes muscle tetanus?

twitch summation

what happens as a result of muscle tetanus?

• high concentration of Ca2+ in cytosol

• all actin binding sites uncovered and available for binding myosin

oxidative muscle fibers

low-intensity contractions for long periods of time

glycolytic fibers

high-intensity contractions for brief amount of time

what factors determine speed of muscle contraction?

• load-velocity relationship

• myosin ATPase activity in contracting fibers

what determines twitch speed for an individual fiber?

high myosin ATPase content

alpha motor neuron

sends output to regular skeletal muscle fiber during stretching

gamma neuron

send signals to intrafusal muscle fibers to contract

smooth muscle cells

• spindle shaped

• arranged into sheets