Lecture 12 - Skeletal muscle performance and plasticity Flashcards
Muscle fibres stained for ….
Muscle fibers stained for oxidative enzymes (enzymes involved in oxidative metabolism to generate ATP)
Darker stain muscle fiber
Darker = more oxidative enzymes
Lighter stain muscle fibre
Lighter = less oxidative enzymes
Type I fibres are
Slow twitch, oxidative - Type I fibres
Type IIA fibres are
Fast twitch, oxidative-glycolytic. - Type IIA fibres
Type IIB fibres are
Fast twitch, glycolytic - Type IIB
Type 1 fibres summary
Slow twitch, oxidative - Type I fibres
Dark stain therefore high amount of oxidative symes
Called slow twitch or Type I fibres
They do not fatigue very quickly (fatigue resistant), slow smaller twitches but can remain activated for long periods of time because they use oxidative metabolism, quite small which means that they have less myofilaments on the inside which means that they can generate less force
Type IIA fibres summary
Fast twitch, oxidative-glycolytic. - Type IIA fibres
In between muscle fibres that are a bit of a hybrid between a slow twitch and a fast twitch muscle fibres
Have properties of both the other types, they generate force quickly however they contain both oxidative and glycolytic enzymes and so they are a hybrid
Type IIB fibres summary
Fast twitch, glycolytic - Type IIB
Not many oxidative enzymes, very light fibres, instead use the process of anaerobic glycolysis to produce energy and these are called fast twitch type IIB muscle fibres
Very large, produce very fast powerful twitches but they fatigue very quickly
List of the three key differences between fibre types
Type of myosin expressed
Oxidative versus glycolytic energy production
Type of SERCA pump expressed
Difference between fibre types - type of myosin expressed
Fast or slow utilization of ATP (fast or slow myosin ATPase)
Alters speed of cross-bridge cycling->Fast or slow speed of contraction
Fast or slow rate of utilization of substrate for ATP generation -> Fatigue prone or fatigue resistant
Type IIB fast twitch fibres use anaerobic glycolysis which produces ATP very quickly whereas slow twitch use oxidative enzymes which generates ATP quite slowly, the downside of these fast twitch fibres is that they rely on glycogen and glucose stores in the muscle that are easily depleted so whilst they can generate ATP relatively quickly they can only do this for a short period of time until energy stores run out which is why type IIB are prone to fatigue quickly and slow twitch fibres are fatigue resistant
Difference between fibre types - oxidative versus glycolytic energy production
High oxidative activity (mitochondria) can generate ATP continuously using O2 and substrates from blood but only relatively slowly (also high myoglobin)
Therefore type I fibres have a high density of mitochrondria, downside is that this process is slow , this is good if you want sustained muscle contraction for a long period of time, these can keep producing the ATP that is required for cross bridge cycling
Myoglobin is a molecule that acts as an oxygen store within the muscle, can release when oxygen is needed
High glycolytic activity can generate ATP quickly from muscle glycogen but glycogen stores limited
When stores run out, the muscle fatigues and loses the ability to generate force
Difference between fibre types - type of SERCA pump expressed
Faster or slower clearance of Ca2+ from sarcoplasm into sarcoplasmic reticulum -> faster or slower drop in tension
Force can be terminated more quickly or slowly
Type I: slow oxidative fibres
Slow form of myosin ATPase
High levels myoglobin – “reserve” of O2 (myoglobin binds oxygen and is able to release it when it is needed)
Many mitochondria, high levels of oxidative enzymes
Low density of ryanodine receptors - therefore the response to a single skeletal muscle action potential, the rampant of calcium released is going to be lower
Slow SERCA pump
Graph - Slow force generation and therefore slow twitch for the single twitch, when you are repetitively activating this motor neuron can get a force profile that is very fatigue resistance so you can maintain this steady level of force generation for a very long time
Rich blood supply to supply the energy substrates glucose, free fatty acids and oxygen for oxidative metabolism
Type IIB: Glycolytic fibres
Fast form of myosin ATPase
Low levels myoglobin ( do not need oxygen for the anaerobic glycolytic pathway)
Few mitochondria, low levels of oxidative enzymes
High density of ryanodine receptors (can release a lot of calcium following an action potential into the cytosol)
Fast SERCA pump
Graph - Fast twitch, fast on and fast off which is a good thing when you have a single muscle action potential but if the motor neuron is repetitively active and you get repetitive skeletal muscle activity over five minutes for example as shown in the graph on the right, the force that is able to be generated peaks quite quickly but it fatigues pretty quickly as well - due to the fast form of myosin ATPase
These muscles are generally relying on energy stores that are within the muscle such as glucose and glycogen which are broken down very quickly to produce a lot of ATP very quickly, ATP production and breakdown happens very quickly and can get very quick cross bridge cycling but this runs out very quickly since energy stores can be depleted quickly if you have high intensity muscle activation