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
Type IIA: Intermediate fibres
Fast form of myosin ATPase (breaks down ATP quickly)
High oxidative and glycolytic enzymes
Intermediate speed/fatigue - can generated fast twitches but are less prone to fatigue than the type IIB fibres
Intermediate properties
Muscle fibre distribution
Different proportions of fibres rather than just one type of fibre in one muscle
Power producing but susceptible to fatigue
Higher proportion of Type II glycolytic fibres
High amounts of force very quickly
For example the biceps muscles when doing bicep curls
Low power but resistant to fatigue
Higher proportion of Type I oxidative fibres
Postural muscles are active most of the time, does not need to generate a lot of force but needs to be constantly active to maintain muscle tone in leg form example to stop you from tipping over
Muscle fibre distribution - power producing but susceptible to fatigue
Power producing but susceptible to fatigue
Higher proportion of Type II glycolytic fibres
High amounts of force very quickly
For example the biceps muscles when doing bicep curls
Muscle fibre distribution - low power but resistant to fatigue
Low power but resistant to fatigue
Higher proportion of Type I oxidative fibres
Postural muscles are active most of the time, does not need to generate a lot of force but needs to be constantly active to maintain muscle tone in leg form example to stop you from tipping over
Effects of training - strength training
More actin & myosin (in each skeletal muscle fibre) -> increased fibre diameter (hypertrophy)
-> more actin-myosin interactions (i.e. more cross bridges) -> more force
Effects of training - endurance training
Endurance training -> Increased oxidative capacity-> increased ability for sustained activity (allows you to contract muscles for long periods of time without them becoming fatigued)
more mitochondria (more enzymes)
more capillaries, myoglobin (blood supply to muscle is greater and so can deliver the molecules required more efficiently, can also store more oxygen in the skeletal muscle)
Increased muscle stores of lipid (muscle becomes more efficient at storing fats and using fats for energy productions, ATP production from fatty acids is a slow process but it generates a lot of ATP
Increased ability to use lipids directly from blood
Contraction
Contraction = generation of tension (or generation of force in skeletal muscle)
Isotonic contraction
Isotonic contraction: Contraction with force held constant while muscle length changes
Force is constant throughout the contraction
Isometric contraction
Isometric contraction: Contraction with length held constant
Concentric contraction
Concentric = the force exerted is greater than the external object and you get muscle shortening
Eccentric contraction
Eccentric = the external force is greater than the muscle contraction so the muscle lengthens
Length tension relationship
Isolated skeletal muscle that has many different muscle fibres and has tendons attached to it which are attached to two different devices and one is a force transducer and the other is a micrometer (manipulator that can adjust the length of the muscle fibre by moving it up or down, stretch or recoil it). Measure force by tendon attached to the force transducer and stimulate the skeletal muscle to trigger a skeletal muscle action potential which will cause a twitch, then stretch the muscle and do it again etc etc. passive tension - the longer it gets stretched the more force that has to be put on it to stretch it more, as you stretch a muscle you can measure greater and greater passive tension because of this elastic component as it wants to bounce back when it is stretched, amount of force that you can measure that is exerted on this muscle gets greater and greater as you stretch it more as the muscle is acting like a rubber band, not due to cross bridge cycling, this is only due to the elastic components of muscle. Active tension is when we electrically stimulate the muscle and we get a twitch and the size of the twitch changes based on the length of the skeletal muscle and the relationship is shown in the red graph, 1 is the resting length of skeletal muscle, doesn’t really go below this which is why the graph doesn’t go that far back from this as it is hard to get a muscle below its resting length, curve shows that at the normal resting length you can generate the peak amount of force and muscle twitch, if you shorten the muscle fibre the force of the twitch goes down and likewise if you increase the length of the muscle fibre from resting length as you stretch it more and more the twitch also drops, this curve is due to the cross bridge interactions that go on
Red line is same as black line on the right
If no cross bridges are formed then no force is generated
Length tension curve explained
The amount of force a sarcomere can produce a maximal when overlap between thick (myosin) and thin (actin) filaments is optimal. As the sarcomere lengthens, overlap between actin and myosin is reduced, so the number of cross bridges is reduces and forces therefore almost falls to zero when there is no actin-myosin overlap. Force also declines as myofilaments overlap increases because the thin actin filaments overlap in the centre of the sarcomere and interfere with optimal cross bridge formation. This means that each muscle has an optimal length where it will be strongest, and when either longer or shorter than that length, it will be weaker.
Less overlap of actin and myosin = reduced number of cross bridges = reduced force
Too much overlap = no where for the filaments to move = reduced force
Load-velocity relationship
No load - no load there is very little force generated, shorten muscle very quickly with no load hence the graph point is so high
Medium load - with increasing load the speed at which you can contract that muscle decreases so the curve drops down but load increases as you have a heavier weight
Heavy load - point where load is too heavy so the velocity drops to zero
Very heavy load - so heavy, velocity negative because it is too heavy therefore lengthening of muscle which is an eccentric contraction
Above 0 = concentric and below 0= eccentric contraction
When you are contracting your muscle at maximal velocity as fast as you can muscle is shortening very quickly, the cross bridges have very little time to form and some of them don’t event have any time to form, no load one for example the muscle is contracting so quickly that very few cross bridges actually get to form and the ones that do form only form for a very short period of time, we know that the amount of force you generate is directly proportional to the duration and the number of cross bridges that you can form, as you slow down muscle contraction your cross bridges have more time to form so you can have more of them and can form them for a longer period of time and these cross bridges can generate more force and the force gets even higher as the skeletal muscle stops shortening
Active control of muscle force
Nervous system regulates muscle force by controlling activity in ‘motor units’
Motor unit = a motor neuron and all the muscle fibres it innervates
Motor unit properties
All muscle fibres in one unit are same metabolic type (fast or slow ATPase)
Motor unit size varies from small (~6 fibres) to large (>2000 fibres)
All fibres in unit active at once, so maximum force from a unit depends on size
Size of motor neuron cell body depends on number of muscle fibres in motor unit
Motor unit
Motor unit = a motor neuron and all the muscle fibres it innervates
Muscle force regulated by
changing rate of activity in each unit (rate modulation/mechanical summation)
changing number of units active (recruitment)
Mechanisms of force increase (mechanical summation)
Rate modulation of motor units, change the force produced by one motor unit
Initial twitch expends energy stretching muscle; later ones work on “pre-stretched” muscle so contribute more force to tendon
At high rates, SERCA cannot clear Ca2+ between twitches so no chance for any relaxation; max force is transmitted to tendon, continuously
Recruitment of motor units
Size principle of motor unit recruitment:
Recruitment is orderly: smallest to largest
So smaller motor units more tonically active to give fine graded control of small forces
Larger motor units automatically recruited as required force increases
Probably the most important force-regulation mechanism
