skeletal, smooth and cardiac muscle

Question 1

describe structure and ultrastructure of skeletal muscle

Answer 1

striated 
muscle fibres
multinucleate
form in utero from mononucleate myoblasts
increase fibre size during growth 

bundles of fibres encased in connective tissue sheaths
cells replaced after injury by satellite cells -> differentiate to form new muscle fibres

Question 2

explain process of excitation-contraction coupling

Answer 2

muscle action potential propagated
Ca2+ released from lateral sac
Ca2+ binding to troponin removes blocking action of tropomyosin
cross-bridge moves
Ca2+ removal from troponin restores tropomyosin blocking action

Question 3

explain mechanics of skeletal muscle contraction

Answer 3

contraction with…

constant length = ISOMETRIC
shortening length = ISOTONIC (concentric)
increasing length = LENGTHENING (eccentric)

Question 4

define isometric

Answer 4

contraction with constant length

Question 5

define isotonic

Answer 5

contraction with shortening length

Question 6

explain sliding filament theory and relationship to length-tension properties

Question 7

describe generation of controlled force by recruitment of motor units

Question 8

explain tetanus

Answer 8

AP is 1-2ms long, but twitch may last up to 100ms
may get more APs during contraction
these add up = SUMMATION

tetanic tension > twitch tension since Ca2+ never gets low enough to allow troponin/tropomyosin to re-block myosin binding sites

Question 9

describe aerobic and anaerobic properties of muscle

Question 10

describe structure and function of smooth muscle

Answer 10

no striations
innervated by ANS, not somatic NS
has X-bridge cycle and uses Ca2+
exists in hollow organs (eg GI tract, uterus, airways, ducts)
spindle-shaped
mononucleate and divide through life
thick myosin and thin actin filaments
filaments arranged diagonally across cells and are anchored to membranes and cell structures by dense bodies (like Z-lines)
filaments still slide together to contract cell

Question 11

explain contraction and relaxation in smooth muscle

Answer 11

increased Ca2+ -> Ca2+ binds calmodulin -> Ca2+-Calmodulin binds to myosin light chain kinase -> kinase phosphorylates myosin X-bridges with ATP -> phosphorylated X-bridges bind to actin filaments -> CONTRACTION + TENSION

relaxes via action of myosin light chain phosphatase -> dephosphorylates X-bridges
persistent stimulation and increased Ca2+ in some smooth muscle

Question 12

how do differences in elastic properties of muscles contribute to force production?

Answer 12

by influencing speed of contractile elements , elastic structures have profound effect on muscle force, power and work

Question 13

define actin

Answer 13

protein found in microfilaments - active in muscular contraction, cellular movement and maintenance of cell shape

Question 14

define myosin

Answer 14

fibrous protein, motor protein
converts chemical energy released from ATP into mechanical energy - this mechanical energy used to pull actin filaments along , causing muscle fibres to contract , therefore, generating movement

Question 15

define titin

Answer 15

large protein
contributes to elasticity of striated muscle fibres
stabilise thick filament, prevent overstretching of sarcomere, and to recoil sarcomere after its stretched

Question 16

how are skeletal muscle fibre types characterised?

Answer 16

fibres are fast/slow-shortening
oxidative or glycolytic ATP forming pathways are used

FAST -> myosin has high ATPase activity
SLOW -> has low ATPase activity

Question 17

oxidative fibres

Answer 17

increased mitochondria -> increased oxid. phosphorylation
increased vascularisation to deliver more O2 and nutrients
contain myoglobin -> increased O2 delivery
fibres are red and have low diameters

Question 18

glycolytic fibres

Answer 18

few mitochondria
increased glycolytic enzymes and glycogen
lower blood supply
white fibres with larger diameters

Question 19

3 types of muscle fibres

Answer 19

slow oxidative (I) - resist fatigue
fast oxidative (IIa) - intermediate resistance to fatigue
fast glycolytic (IIb) - fatigue quickly

Question 20

smooth muscle types

Answer 20

single or multiunit

Question 21

single unit smooth muscle

Answer 21

GIT, uterus, small blood vessels
many cells linked by gap junctions
signals travel between cells
contract synchronously
may contain pacemaker cells
stretch evokes contraction

Question 22

multiunit smooth muscle

Answer 22

airways, large arteries, hairs
few or no gap junctions
richly innervated by ANS
don’t respond to stretch

most smooth muscles in organs are a mixture of single and multiunit populations of cells
means an organ can have a mixture of properties in different areas