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?

fibre length of contraction, type and fitness of individual

when can the muscle contract again after fatigue

when it is relaxed and rested

what does fatigue prevent in muscles

vast amounts of ATP being used - muscle would not be able to activate new cross-bridge cycles

what factors cause muscle fatigue during high intensity short duration exercise

- conduction failure due to increased [K+] depolarisation

- increased lactic acid which acidifies proteins

- increased [ADP] and [Pi] inhibits cross bridge cycles delaying myosin detachment from actin filaments

what factors cause muscle fatigue during low intensity long duration exercise

- decreased muscle glycogen

- decreased blood glucose

- dehydration

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

increased mitochondria - increased oxidative phosphorylation

increased vascularisation for delivery of O2

myoglobin - O2 delivery

red fibres - low diameters

glycolytic fibres

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 of muscle fibres

- slow oxidative activated 1st

- fast oxidative 2nd

- fas glycolytic last

aerobic exercise

hypertrophy due to increased diameter, increased mitochondria and increased vascularisation

anaerobic exercise (strength)

increased diameter

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

smooth muscle features

lack of striations

innervated by autonomic nervous system

has a cross bridge and uses Ca2+

filaments and excitation contraction coupling are different

found in structures such as arteries

mononucleate - divide through life

thick myosin and thin actin filaments

filaments arranged diagonally

smooth muscle cross bridge activation

- increased [Ca2+]

- 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

persistent stimulation and increased [Ca2+] in some smooth muscle

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

decrease rate of ATP splitting

slows cross-bridge cycles

maintain tension for long time with low ATP consumption

useful for blood vessel walls which have to stay open for long periods

sources of cytosolic Ca2+

1. sarcoplasmic reticulum

2. extracellular Ca2+ entering the cell through plasma-membrane Ca2+ channels

smooth muscle types

single unit

- gap junctions

- signals travel between cells

- may contain pacemaker cells

- stretch evokes contraction

multi unit

- few or no gap junctions

- richly innervated by CNS

- do not respond to stretch