specialised tissues Flashcards
skeletal muscle: recall the ultrastructure of skeletal muscle, list functional characteristics, explain the process of excitation-contraction coupling and the cellular mechanisms resulting in contraction
function of skeletal muscle
connect bones and allow movement
name both muscles within an antagonistic pair
flexor and extender
define isotonic contraction
muscle length changes, tension remains the same
define isometric contraction
muscle length remains the same, tension changes
whate are myofibres and what do they do
cylindrical, multinucleated bundle of cells packed with myofibrils which extend length
why is skeletal muscle striated
divided into alternating dark and light bands
A-band
dark bands interesected by darker region (H-zone)
I-band
light bands intersected by dark line (Z-line)
Z-line
made up of a-actin and CapZ; defines lateral boundaries of sarcomere
define sarcomere
functional unit of muscle lying between two Z-lines
what does desmin do
links Z-lines of adjacent myofibrils
actin
polymeric thin filament of two twisted a-helices; displays polarity
myosin
thick filaments (motor proteins); contain numerous globular heads that interact with actin
titin
very large spring-like filaments anchoring myosin to Z-line; associated with thick and thin filaments in relaxed muscle
nebulin
large filaments associated with actin
CapZ and tropomodulin
associated with +ve (nearest Z-line) and -ve (furtherst Z-line) ends of actin respectively
sarcomere diagram
diagram from tissues 7
T-tubules
membrane invaginations into EC fluid
sarcoplasmic reticulum
stores of IC Ca2+
skeletal muscle excitation-contraction coupling
action potential propagates along sarcolemma and down T-tubules → depolarisation activates dihyropiridine receptors (DHPR) → conformational change → ryanodine receptors (RyR) on SR open → Ca2+ released
isotonic contraction: tension and force
tension is greater than force exerted by load so Z-lines become closer
isotonic contraction: sliding filament theory
Ca2+ binding to troponin causes movement of troponin from tropomyosin chain → exposes myosin binding sites on actin chain → charged myosin heads bind to exposed site on actin filament → binding and discharge of ATP causes myosin head to pivot (power stroke) → actin filament pulled towards sarcomere centre → ATP binds → releases myosin head from actin chain → ATP hydrolysed → energy to recharge myosin head
isometric contraction: tention and force
tension is equal to force exerted by load so muscle doesn’t shorten
isometric contraction: sliding filament theory
myosin heads attach at same point as actin pulled in and out; ATP still required to charge heads
define motor unit
single motor neurone all ats its muscle fibres
define recruitment
increasing number of “active” motor units
what does increasing recruitment cause
increase in force
what effect does increasing speed of filament sliding have on number of cross-bridges
fewer cross-bridges
