lecture twelve anatomy

what are the main objectives of this lecture?

Compare skeletal muscle fibre types; explain how recruitment and summation control muscle force; briefly describe the length-tension relationship.

what has to happen before myosin can bind to actin?

Ca2+ must bind to troponin, which makes tropomyosin move out of the way so the myosin-binding sites on actin are exposed.

what happens when sarcoplasmic Ca2+ is low?

Tropomyosin covers the binding sites on actin, so myosin cannot bind and cross-bridges cannot form.

what happens when sarcoplasmic Ca2+ is high?

Ca2+ binds to troponin, tropomyosin moves off the actin-binding sites, and myosin can bind to actin to form cross-bridges.

what is a cross-bridge?

A cross-bridge is the connection formed when a myosin head attaches to actin, allowing myosin to pull on actin and generate force.

what are the main steps of the cross-bridge cycle?

Ca2+ exposes actin-binding sites; myosin binds actin; ADP and Pi are released; the myosin head flexes and produces force; ATP binds and myosin detaches; ATP is hydrolysed to re-energise the myosin head.

why is ATP needed in the cross-bridge cycle?

ATP is needed for myosin to detach from actin, and when ATP is broken down into ADP + Pi it re-energises the myosin head for another pull.

what happens if there is no ATP after myosin binds to actin?

Myosin stays attached to actin, so the muscle stays stiff because myosin cannot detach.

what are the two broad types of skeletal muscle fibres?

Slow fibres are better for sustained activity, while fast fibres are better for quick and powerful movements.

what is the difference between fast and slow fibres in cross-section?

Fast fibres tend to be larger because they have more actin and myosin, so they can generate more force; slow fibres tend to be smaller and more endurance-focused.

what are the three key differences between muscle fibre types?

The three key differences are the type of myosin expressed, whether ATP production is more oxidative or glycolytic, and the type of SERCA pump expressed.

how does myosin type affect contraction speed?

Fast myosin uses ATP quickly and cycles cross-bridges quickly, giving faster contraction; slow myosin uses ATP more slowly and contracts more slowly.

what does myosin ATPase do?

Myosin ATPase breaks down ATP on the myosin head, giving energy for cross-bridge cycling.

why does fast myosin ATPase make a fibre fatigue faster?

Fast myosin ATPase uses ATP quickly, so the fibre contracts fast but also burns through energy faster and is more fatigue prone.

what is oxidative energy production?

Oxidative energy production uses oxygen, mitochondria, and substrates from the blood to make ATP slowly but sustainably.

what is glycolytic energy production?

Glycolytic energy production makes ATP quickly from muscle glycogen, but glycogen stores are limited so it cannot be sustained for long.

what is the key difference between oxidative and glycolytic fibres?

Oxidative fibres make ATP more slowly but can keep going for longer; glycolytic fibres make ATP quickly but fatigue quickly.

what does SERCA do?

SERCA pumps Ca2+ from the sarcoplasm back into the sarcoplasmic reticulum, which lowers Ca2+ and helps the muscle relax.

why does SERCA type matter?

Fast SERCA clears Ca2+ quickly so the fibre relaxes quickly; slow SERCA clears Ca2+ more slowly so tension drops more slowly and contraction can be sustained longer.

what are Type I fibres?

Type I fibres are slow oxidative fibres that contract slowly, resist fatigue, and are good for endurance or sustained activity.

what are the main features of Type I slow oxidative fibres?

They have slow myosin ATPase, slow contraction velocity, a longer hinge region, many mitochondria, high oxidative enzymes, rich blood supply, slow SERCA, and high fatigue resistance.

why are Type I fibres fatigue resistant?

They have many mitochondria, high oxidative enzyme levels, lots of blood supply, and myoglobin, so they can keep making ATP aerobically for a long time.

why do Type I fibres contract slowly?

They have slow myosin ATPase, so ATP is used more slowly and cross-bridge cycling is slower.

what are Type IIB fibres?

Type IIB fibres are fast glycolytic fibres that contract very quickly and produce high force, but fatigue quickly.

what are the main features of Type IIB fast glycolytic fibres?

They have fast myosin ATPase, fast contraction velocity, a shorter hinge region, few mitochondria, low oxidative enzymes, fewer capillaries, fast SERCA, and low fatigue resistance.

why do Type IIB fibres fatigue quickly?

They rely heavily on glycolysis and muscle glycogen, which produces ATP quickly but runs out quickly.

why can Type IIB fibres produce force quickly?

They have fast myosin ATPase, fast cross-bridge cycling, and fast SERCA, so they can contract and relax quickly.

what are Type IIA fibres?

Type IIA fibres are intermediate fibres that sit between Type I and Type IIB; they are fast oxidative fibres with both oxidative and glycolytic features.

what are the main features of Type IIA fibres?

They have fast myosin ATPase, intermediate contraction velocity, a mix of oxidative and glycolytic enzymes, and intermediate speed and fatigue resistance.

why are Type IIA fibres called intermediate fibres?

They are faster and stronger than Type I, but more fatigue resistant than Type IIB, so they sit in the middle.

compare Type I, Type IIA, and Type IIB by contraction speed.

Type I is slow; Type IIA is fast; Type IIB is fastest.

compare Type I, Type IIA, and Type IIB by contraction force.

Type I has low force; Type IIA has high force; Type IIB has the highest force.

compare Type I, Type IIA, and Type IIB by fatigue resistance.

Type I has the highest fatigue resistance; Type IIA is intermediate; Type IIB has low fatigue resistance.

compare Type I, Type IIA, and Type IIB by fibre diameter.

Type I is small; Type IIA is intermediate; Type IIB is large.

compare Type I, Type IIA, and Type IIB by ATPase activity.

Type I has low ATPase activity; Type IIA has high ATPase activity; Type IIB has the highest ATPase activity.

compare Type I, Type IIA, and Type IIB by glycolytic enzymes.

Type I has low glycolytic enzymes; Type IIA has intermediate glycolytic enzymes; Type IIB has high glycolytic enzymes.

compare Type I, Type IIA, and Type IIB by aerobic enzymes.

Type I has high aerobic enzymes; Type IIA has intermediate aerobic enzymes; Type IIB has low aerobic enzymes.

compare Type I, Type IIA, and Type IIB by vascularisation.

Type I has high capillary density; Type IIA has moderate capillary density; Type IIB has low capillary density.

compare Type I, Type IIA, and Type IIB by mitochondrial content.

Type I has high mitochondrial content; Type IIA has high mitochondrial content; Type IIB has low mitochondrial content.

compare Type I, Type IIA, and Type IIB by myoglobin content.

Type I has high myoglobin; Type IIA has high myoglobin; Type IIB has low myoglobin.

compare Type I, Type IIA, and Type IIB by colour.

Type I is red; Type IIA is red; Type IIB is white or pale.

why are Type I fibres red?

Type I fibres have lots of myoglobin and a rich blood supply, so they are more oxygen-focused and appear red.

why are Type IIB fibres white or pale?

Type IIB fibres have less myoglobin and fewer capillaries, so they are less oxygen-focused and appear paler.

what does muscle fibre distribution mean?

It means the proportion of different fibre types in a muscle, because most muscles contain a mixture rather than only one fibre type.

are muscles usually purely Type I or purely Type II?

No, most muscles are a mixture of fibre types, but they may have a higher proportion of one type depending on their function.

why does the soleus have lots of Type I fibres?

The soleus helps keep you upright for long periods, so it needs fatigue-resistant Type I oxidative fibres.

why do power muscles have more Type II fibres?

Power muscles need quick, strong contractions, so they tend to have more Type II fibres, which generate force faster but fatigue sooner.

which fibre type is best for endurance?

Type I slow oxidative fibres are best for endurance because they are fatigue resistant and can sustain activity.

which fibre types are useful for power and sprint events?

Type IIA and Type IIB fibres are useful for power and sprinting because they contract quickly and produce more force.

what are the three main energy systems during exercise?

The three main energy systems are ATP-PC for immediate energy, glycolysis for short-term energy, and aerobic metabolism for long-term energy.

what is the ATP-PC system?

The ATP-PC system uses phosphocreatine to rapidly regenerate ATP, giving immediate energy, but it runs out quickly.

what happens to energy use as exercise duration increases?

ATP-PC contributes most at the start, glycolysis increases for short-term energy, and aerobic metabolism becomes more important as exercise continues.

do energy systems ever fully switch off?

No, all energy systems are active all the time; the difference is how much ATP each system contributes.

what does strength training do to muscle fibres?

Strength training increases actin and myosin, which increases fibre diameter by hypertrophy and allows more cross-bridges to form, producing more force.

what is hypertrophy?

Hypertrophy means muscle fibres get bigger, usually because they contain more contractile proteins such as actin and myosin.

does strength training add new muscle fibres?

The lecture notes say there is no evidence here that strength training adds more fibres; it mainly makes existing muscle fibres bigger.

why does more actin and myosin increase force?

More actin and myosin means more possible cross-bridges, and more cross-bridges means more force can be produced.

what does endurance training do to muscle fibres?

Endurance training increases oxidative capacity, meaning the muscle becomes better at sustained activity.

what changes happen with endurance training?

Endurance training increases mitochondria, oxidative enzymes, capillaries, myoglobin, lipid stores, and ability to use lipids from the blood.

why does endurance training increase mitochondria?

Mitochondria are needed for aerobic ATP production, so more mitochondria help the muscle sustain activity for longer.

why does endurance training increase capillaries and myoglobin?

More capillaries improve oxygen delivery from blood, and more myoglobin improves oxygen storage inside the muscle.

what factors can affect fibre type distribution?

Motor unit firing rate, disease, genetics, training, space flight, and aging can all affect fibre type distribution.

how can genetics affect fibre type?

Genetics can make someone naturally more predisposed toward certain fibre proportions, such as being better suited to sprinting or endurance.

how can training affect fibre type or function?

Strength training increases force capacity and fibre size, while endurance training increases oxidative capacity and fatigue resistance.

how can disease affect fibre type distribution?

Disease can change muscle function and fibre composition, such as muscular dystrophy causing fibre-type shifts and weakness.

how can space flight affect muscle?

Space flight reduces loading on muscles, so muscles can waste because they are not working against gravity in the normal way.

what is a motor unit?

A motor unit is one motor neuron and all the muscle fibres it innervates.

what happens when a motor neuron fires?

All muscle fibres in that motor unit activate at once; you cannot activate only some fibres in the same motor unit.

what fibre type are fibres within one motor unit?

All fibres in one motor unit are usually the same metabolic type, such as all Type I or all Type II.

how much can motor unit size vary?

A motor unit can be small with around 6 fibres or large with more than 2000 fibres.

why does motor unit size matter?

Small motor units allow fine control, while large motor units activate many fibres and produce more force.

how does the nervous system regulate muscle force?

It changes how many motor units are active through recruitment and how often each unit fires through summation.

what is recruitment?

Recruitment is activating more motor units, which activates more muscle fibres and increases total muscle force.

what is summation?

Summation is increasing the firing rate of a motor unit so twitches add together and force increases.

how do recruitment and summation work together?

As stimulation increases, smaller units fire faster through summation and larger units are added through recruitment.

what is the size principle of motor unit recruitment?

Motor units are recruited in an orderly way from smallest to largest.

why are small motor units recruited first?

Small motor units allow fine control of small forces, and larger units are only added when more force is needed.

why does recruitment increase force?

Recruitment increases the number of active motor units, so more muscle fibres contract at once and total force rises.

what does the recruitment graph show?

It shows force rising when unit 1 is recruited, then rising again when unit 2 is recruited because more fibres are now active.

what happens at low stimulation frequency during summation?

Each twitch mostly returns to baseline before the next stimulus, so force does not build much.

what happens at higher stimulation frequency during summation?

The next action potential arrives before the muscle fully relaxes, so twitches add together and force increases.

why does force increase during summation?

Ca2+ is not fully cleared between twitches, so Ca2+ stays higher and cross-bridge cycling continues.

what is tetanus?

Tetanus is a maximum sustained contraction at high stimulation frequency, where twitches fuse and there is little or no relaxation.

why can SERCA not fully relax the muscle at high stimulation frequency?

At high firing rates, action potentials arrive too quickly for SERCA to clear Ca2+ completely between twitches.

what are the three possible muscle actions during contraction?

The muscle can shorten in concentric contraction, stay the same length in isometric contraction, or lengthen in eccentric contraction.

what is a concentric contraction?

A concentric contraction is when a muscle shortens while producing force, such as the biceps shortening when lifting the forearm.

what is an isometric contraction?

An isometric contraction is when the muscle produces force but its length stays constant, so there is no visible movement.

what is an eccentric contraction?

An eccentric contraction is when the muscle lengthens while producing force, such as lowering a heavy object.

what does isotonic mean?

Isotonic contractions involve muscle or sarcomere shortening, with speed depending on myosin ATPase activity and contraction velocity.

why is there no external work in an isometric contraction?

Work equals force times distance, and in isometric contraction there is force but no movement distance.

why can isometric contraction produce high force?

Many myosin heads can be attached at the same time, so high cross-bridge cycling and constant tension can occur.

why are eccentric contractions more likely to cause injury?

Eccentric contractions produce high force while the muscle is being lengthened, which can mechanically detach myosin heads and damage muscle proteins.

why do eccentric contractions use less ATP?

The external load helps force the muscle to lengthen, so some detachment is mechanical rather than fully ATP-driven.

what happens to cross-bridges during eccentric contraction?

Myosin binds and tries to generate force, but the load lengthens the muscle and can mechanically pull myosin away from actin.

what is the length-tension relationship?

The length-tension relationship describes how much force a muscle can produce at different sarcomere lengths, depending on actin-myosin overlap.

what is active tension?

Active tension is force produced by cross-bridge cycling, so it depends on actin and myosin overlap.

what is passive tension?

Passive tension comes from stretching the elastic parts of the muscle, so it increases when the muscle is stretched.

what is total tension?

Total tension equals active tension plus passive tension.

where is active tension highest?

Active tension is highest at the optimal resting length where actin and myosin overlap is ideal.

why is force low when the sarcomere is too short?

Actin overlaps too much and the filaments are crowded, so myosin cannot pull effectively and active force is reduced.