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

Striated muscle

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

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)

Striated skeletal muscle is very diverse:

Striated Skeletal Muscle Architecture:

Unipennatepenna - Lat. wing or

featherfeather pen!

fusiformfusus - Lat. spindle bipennate

styloidstulos - Gk. pillar

triangular

Muscles usually work in antagonistic pairs

Frog leg muscles Vert. upper limb muscles - human

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

2006Note: Not same as handout

tibialis cranialisgastrocnemius

muscle

tarsal joint

Muscles work in opposing pairsoften called the flexor and

extensor muscles

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’

Muscle – Fibre – myofibril – myofilaments

Myosin containing, thick filament

Actin containing, thin filaments

acto-myosin “cross-bridges”

Acto-myosin in muscle :

Sarcomere

0.5m

0.5 m

0.1 m

1 m

Filament sliding causes muscle to shorten

myofibril

sarcomere

Light micrograph

Electron micrograph

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 animals e.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.

Highly specialised striated muscles (3):

Molluscan catch + adductor muscles:

Catch muscle Adductor muscle

Pecten maximus

Catch muscle – keeps shell closed with minumum energy requirement (slow)

Adductor muscle – for swimming.

How clams etc. can close their shells – a single muscle working against a stiff elastic hinge

Forces produced by the muscle are easily measured

Highly specialised striated muscles (4):

Crossbridge CycleAll 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 biomechanical cycle (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

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 (4) :

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

Energy Sources for Muscle Contraction (5) :

Neuromuscular junction – muscle endplate

Transmitter is usually acetylcholine

Muscle activation by nerves (1)

Events Leading to Muscle Contraction

e.g. acetylcholine

Muscle activation by nerves (2)

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)