Module 632 Lecture 7 JCS Muscle types, structure, activation and energy use
MODULE - 632 Lecture 7 • Lecture outcomes: • At the end of this lecture a student will be aware of: • 1)the different types of muscle • 2)that basic function of muscle is to produce force and movement • 3) that a variety of muscles exist where the outputs – force and movement occur to different degrees • 4) that most muscles work by being attached to a skeleton but that, • 5)some work between or within non-skeletal tissues– e.g. heart, vascular • 6)most muscles work in pairs, • 7)that muscles are attached to skeletons through tendons or tendon- like structures • 8)that muscles move skeletons • 9)that pairs of muscles move joints, • 10) the ultrastructure of striated muscle • 11) the energy sources for muscle contraction and • 12) How muscle contraction of is activated
Three main types of vertebrate muscle • Smooth (smooth muscle myosin II – 1 isoform) • Smooth appearance (no cross-striations) • Involuntary • Blood vessels, gut, sphincters • Skeletal (striated muscle myosin II – 8 isoforms, including a cardiac isoform in ‘slow’ muscle) • Striated appearance • Voluntary control • Biceps, triceps, quadriceps etc. • Cardiac (cardiac myosin II – 2 isoforms a & b) • Sarcomeric structure (striated) - not as ordered as skeletal. • Rhythmic contractions • Highly specialised function
Striated skeletal muscle is very diverse: • Within an organism (e.g. human muscles): • Structural architectures (pennate, styloid, long/short sarc.) • Fibre types – many muscles contain a mix of the two Type 1 – slow – postural/slow to fatigue Type II –fast • Myosin isoforms (fast, intermediate and slow ATPase activities)
Muscles usually work in antagonistic pairs Vert. upper limb muscles - human Frog leg muscles
2006 Note: Not same as handout Flexion, extension, adduction, abduction Muscles are often named by the effects of their action and the bones they attach to. In general: Flexors: bends a joint; move limbs away from ‘corpse’ position Extensors: straightens a joint; returns them to corpse position Adductors: ‘add’ the limb towards the rest of the body (pulls the body to wards the midline) Abductors: moves them away from the midline
Muscle has a hierarchical structure: Each muscle is a contractile organ: it contains: muscle fibres blood vessels peripheral ends of nerves/muscle endplates fibrous connective tissue/tendons and is covered with a connective layer. Each muscle fibre is a multinucleated single cell (a syncitium) It contains approx. 1000 myofibrils; specialised contractile organelles, which run the length of the fibre. Each myofibril consists of serial contractile units known as sarcomeres.
Muscle has a hierarchical structure UK ‘fibre’
0.5mm Acto-myosin in muscle : Myosin containing, thick filament 1 mm 0.5 mm Actin containing, thin filaments 0.1 mm Sarcomere acto-myosin “cross-bridges”
Filament sliding causes muscle to shorten Light micrograph myofibril Electron micrograph sarcomere Myosin molecules (purple bars) move over the F-actin (turquoise).This movement is powered by ATP.
Highly specialised striated muscles (1): • Many specialised muscles exist in different animalse.g. • Asynchronous insect flight muscle - drives insect wingbeats at >200Hz (Drosophila 220Hz). • Tympal muscles that allow crickets to sing • Molluscan “Catch” muscle - keeps shell closed for long periods • Molluscan adductor muscle - fast closing of shell for swimming.
Highly specialised striated muscles (2): Asynchronous insect flight muscle - isometric; requires Ca++ +applied strain to activate- contracts in an oscillatory fashion at frequencies >200Hz myosin.
Pecten maximus Catch muscle Adductor muscle Highly specialised striated muscles (3):Molluscan catch + adductor muscles: Catch muscle – keeps shell closed with minumum energy requirement (slow) Adductor muscle – for swimming.
Forces produced by the muscle are easily measured How clams etc. can close their shells – a single muscle working against a stiff elastic hinge Highly specialised striated muscles (4):
Crossbridge Cycle All muscle contraction is powered by the cyclical interactions of myosin and actin – the so-called crossbridge cycle. Myosin is an ATPase. By coupling its ATPase to a conformational change, dependent on binding actin we can get: Mg.ATP + H2O Mg.ADP + Pi + H+ + mechanical work The crossbridge cycle (more in the next lecture) consists of: - a biochemical cycle (changes in nucleotide state - ATP, ADP etc. - and in protein binding actin + myosin) and, - a biomechanicalcycle (conformation of the motor molecule – myosin) that are completely functionally inter-dependent. .
Energy Sources for Muscle Contraction (1) : • For the crossbridge cycle outputs: • Mg.ATP + H2O Mg.ADP + Pi + H+ + mechanical work • the primary energy source is clearly ATP (produced by the mitochondria) • For very short bursts of activity you will use up your ATP pool. The ATP needs to be replaced. • How? • immediate stores of energy – creatine phosphate • new ATP production: from glycolysis (glucose, glycogen) – but product is lactic acid from oxidative phosphorylation (ATP production by the mitochondria)
Energy Sources for Muscle Contraction (2) : The energy sources used are reflected by the physiological properties of skeletal muscles: Vetebrate skeletal muscle contains two major types of fibres that differ in: - speed of contraction – ‘Fast’ (type 2) or ‘slow’ (type 1), and - their major energy supply - their neural activation – ‘twitch’ and ‘phasic’ (see later). Fast fibres (for sprinting) are ‘glycolytic’ Slow fibres (for slower movements, maintained peformance – e.g. over long distance or time outputs and posture etc) are ‘oxidative’ Most skeletal muscles are a combination of ‘fast’ and ‘slow’ fibres. There is also natural variation in the proportion of these fibre types between individuals in particular muscles – sprinters vs marathon runners. Fibre-typing is now a routine part of assessing ‘olympians’ ‘Red’ and ‘white’ meat reflect these differences. Red – mostly oxidative; white is mostly glycolytic.
Energy Sources for Muscle Contraction (3) : (Fast fibres predominantly) For fairly short bursts of activity you will use up an energy reserve (effectively an ATP storage pool) of creatine phosphate, Cr.P + Mg.ADP Mg.ATP + Cr Cr = creatine This reaction is readily reversible; the energy from CrP is released very quickly (a few seconds) allowing sprints. In insect muscles: Arginine.P replaces Creatine.P
Energy Sources for Muscle Contraction (4) : ATP ADP Pi PCr Cr Mg2+ Ca2+ Total (mmol/kg) 5 0.8 3 25 13 10 1 Free (mM) 4 0.02 2 25 13 3 0.0001 Note: More Cr.P than ATP Once Cr.P is used up the high requirement for more ATP is met by glycolysis (end-product lactic acid) So these muscle fibres can work anaerobically for brief periods, but accumulate lactic acid. Typically this metabolism predominates in muscles used for sprints - fast glycolytic fibres predominate. After exercise ceases the ATP and PCr must be regenerated and lactic acid metabolised.
Energy Sources for Muscle Contraction (5) : Type 1 fibres: For long periods of sustained work – type 1/oxidative/ tonic or slow twitch fibres i.e. These muscle fibres require a supply of oxygen to enable oxidative phosphorylation to go on in the mitochondria to produce a continuous supply of ATP. Energy sources here are primarily glucose (glycolysis and the TCA cycle) and over longer periods the glycogen stores. Requires the mitochondria. Mitochondria occupy about 30% of volume of the heart and these muscle fibres. It is the high(er) concentration of cytochromes/myoglobin etc. which give these fibres their red colour
Neuromuscular junction – muscle endplate Transmitter is usually acetylcholine Muscle activation by nerves (1)
Muscle activation by nerves (2) Events Leading to Muscle Contraction e.g. acetylcholine
3-D section of a skeletal muscle cell Muscle activation by nerves (3)
Muscle activation by nerves (4) • Control by nerves – action potentials in motor neurons • Neuromuscular junction • Muscle plasma membrane depolarises • Propagates down ‘T’ tubules into centre of fibre • ‘T’ tubule close to sarcoplasmic reticulum (SR) • Di-hyropyridine receptor ryanodine receptor • Calcium release from SR - induces further calcium release • Ca++ binds to troponin complex – troponin C (part of the sarcomeric thin filament)