lecture 33 - skeletal, smooth (and cardiac) muscle 2 - PoNF

describe the ATP generation for muscle contraction

hydrolysis of ATP - starts in cross-bridges

ATP binds to myosin

dissociates bridges bound to actin

new cycle may begin

what does ATP power as well as contraction

RELAXATION

Ca2+ATPase in sarcoplasmic reticulum

Ca2+ pumped back into SR

contraction ends

define fatigue in terms of muscles

repeated muscle stimulation with no relaxation period

what does muscle fatigue depend on? (3)

fibre type

length of contraction

fitness of individual

when can the muscle contract again after fatigue

when it is relaxed and rested

what does fatigue prevent in muscles

rigor

too much ATP used

can’t activate new cross bridge cycle

what factors cause muscle fatigue during high intensity short duration exercise (3)

increased [K+] out of cell - depolarisation - conduction failure

lactic acid - acidifies proteins - stops x bridge

increased ADP delays myosin detachment

what factors cause muscle fatigue during low intensity long duration exercise (3)

decreased muscle glycogen and blood glucose

dehydration

central command fatigue from cerebral cortex

what are the 3 types of muscle fibres (skeletal)

slow oxidative (I) - resist fatigue

fast oxidative (IIa) - intermediate resistance to fatigue

fast glycolytic (IIb) - fatigue quickly

oxidative muscle fibres (4)

increased mitochondria and ox. phosphorylation

increased vascularisation for O2

myoglobin buffer O2 storage

red fibres - low diameters

glycolytic fibres (4)

few mitochondria

increased glycolytic enzymes and glycogen

lower blood supply

white fibres - large diameters

in terms of muscle fibre recruitment - what happens when there is an increased load

increased need to activate motor units

increased number of active motor units → recruitment order

slow oxidative

fast oxidative

fast glycolytic

aerobic exercise → hypertrophy causes

increased mitochondria

increased vascularisation

increased diameter

anaerobic exercise (strength)

increased diameter

increased glycolysis (glycolytic fibres - fast - explosive movements - think gymnastics)

smooth muscle features

ANS

cross bridge and uses Ca2+

different filaments and excitation contraction coupling

hollow organs

mononucleate - divide through life

thick myosin and thin actin filaments

filaments arranged diagonally

smooth muscle cross bridge activation after increase in [Ca2+] (5)

Ca2+ binds to calmodulin

Ca2+-calmodulin binds to myosin light chain kinase

kinase phosphorylates myosin-cross bridges with ATP

phosphorylated cross bridges bind to actin filaments

contraction and tension

how does smooth muscle relax?

action of myosin light chain phosphatase - dephosphorylates cross-bridges

modes of persistent stimulation and increased [Ca2+] in some smooth muscle eg. blood vessel walls (3)

phosphorylated cross-bridges may be dephosphorylated when still bound to actin

decrease rate of ATP splitting and slow x bridge cycles

maintain tension for long time with low ATP consumption

sources of cytosolic Ca2+

SR

extracellular Ca2+ entering through plasma-membrane channels

skeletal vs smooth number of sites activated

skeletal - all

smooth - some - grade contraction depending on number of APs

tone in smooth muscle

basal level of [Ca2+] to cause constant level of tension

increased stretch effect on contraction

contract harder/faster

smooth muscle types

single unit

multi unit

single unit (GIT, uterus, small blood vessels) (3)

linked by gap junctions - signals move between

may contain pacemaker cells

stretch evokes contraction

multiunit (airways, large arteries, hairs)

few/no gap junctions

ANS innervation

don’t respond to stretch