Muscle contraction Flashcards
twitch contraction
the mechanical response of a muscle fibre to a single action potential
- latent phase due to excitation-contraction coupling
- contraction phase when tension develops due to cross-bridge cycling
- relaxation phase due to the time it takes to get all the calcium ions sequestered
tetanic contraction
if freq. of stimulation is high enough, a tetanic contraction occurs where force is constant(ish) for the period of activation (train of action potentials)
- can get incomplete or complete tetanus
- the muscle is stimulated before it completely relaxes, more Ca+, more cross-bridges form –> summation of tension
- typical of muscle force in functional activities
what happens when the threshold of a motor unit is reached
all of the fibres in that motor unit will generate their maximal force, so the amount of force a muscle generates is modulated by the number of motor units that are activated
skeletal muscle fibres are classified by
- their maximal velocities of shortening (slow, fast or very fast)
- the major pathway they use to form ATP
(oxidative or glycolytic)
myosin isoforms
myosin heavy chain I, IIa, IIx
slow——–> fast
differ in the rates at which they use ATP and therefore the rate of actin-myosin cross-bridge cycling - determines the maximal shortening velocity
3 ways ATP can be synthesised
- phosphorylation of ADP by creatine phosphate
- oxidative phosphorylation of ADP in mitochondria
- glycolytic phosphorylation of ADP in the absence of oxygen
differing features of oxidative fibres and glycolytic fibres
fibres that use oxidative phosphorylation of ADP have a large number of mitochondria, whereas fibres that use glycolytic phosphorylation of ADP have few mitochondria but a large number of glycolytic enzymes and a large supply of glycogen
slow- oxidative fibres (type I)
combine low myosin-ATPase activity with high oxidative capacity
slow but sustained
fast-oxidative-glycolytic-fibres (type IIa)
combine high myosin-ATPase activity with high oxidative capacity and an intermediate glycolytic capacity
produces a lot of force quickly, fatigues faster but somewhat endured
fast-glycolytic fibres (type IIx)
combines high myosin-ATPase activity with high glycolytic capacity
produces POWER, but NOT sustained
what are the 3 types of skeletal muscle fibres?
slow-oxidative (type I)
fast-oxidative-glycolytic (type IIa)
fast-glycolytic (type IIx)
in reality fibres sit on a continuum rather than segregated into different types. Type II fibres tend to be bigger
what does the proportion of muscle fibre types depend on?
- function of the muscle
- species
- individual
all the fibres within a given motor unit are…
of the same fibre type
do small muscle fibres have a higher or lower threshold at which an all or nothing response is generated?
lower
which fibres are activated first?
slow fibres –> fast fibres
