MusclesMuscles

Muscles work by contracting.Muscles pull.A pair is needed to move a bone two ways.An antagonistic pair.

A muscle is made up of bundles of muscle cells.

Muscle cells are made of bundles of myofibrils. These are groups of protein fibres.

Muscle cells also have many mitochondria, transverse tubules and an extended set of sacs called the sarcoplasmic reticulum. What are they likely to be for?

Myofibrils are made of protein fibres. They appear banded under the microscope.

The banding relates to how two protein fibres overlap. They form units called sarcomeres.

The thin fibres are made of actin.

The thick fibres are made of myosin.

Contraction happens when the myosin and the actin interact by forming cross links.

First there has to be an impulse from a motor neurone.

Describe what happens here.

An impulse arrives at the neuro-muscular junction.Acetyl choline is secreted.The muscle membrane depolarises.What then happens to the acetyl choline?

The actin has sites for the myosin to link to it.But, the sites are covered by proteins called troponin and tropomyosin.

The calcium binds to the complex.It moves part of it out of the way.

The myosin can then bind.This triggers it to change its position.It shifts to the side.

The myosin slides over the actin.The muscle gets shorter as the proteins overlap more.

Sarcomeres shorten during contraction.

The calcium is pumped out of the cytoplasm.The protein complex on the actin goes back to its normal position.The myosin cannot bind.The muscle contraction stops, this is relaxation.

Put these in the correct order and then learn the sequence.

DepolarisationSlidingTroponinVesiclesInteractionCalciumImpulseCalcium

ImpulseCalciumVesiclesDepolarisationCalciumTroponinInteractionSliding

The Twitch

Z -lin e Z -lin eH -zo n e I-b an d A -b a n d

R es t

C o n trac ted

T h ic k fi lam en tT h in filam en t

Since I made such a pigs ear of teaching this yesterday, which bands will change in size when the muscle contracts?

Z -lin e Z -lin eH -zo n e I-b an d A -b a n d

R es t

C o n trac ted

T h ic k fi lam en tT h in filam en t

Myosin heads bind to the actin forming cross links.

The heads change position pulling the actin over the myosin. ADP is released.

ATP binds with the myosin head and this releases it from the actin.

The ATP is hydrolysed, returning the head to its original position.

Fatigue

The ATP…Pumps sodium and potassium across membrane after the action potential.Pumps Calcium back into the sarcoplasmic reticulum.Knocks the myosin heads off actin binding sites and primes the head for the next movement.Is used in the manufacture of acetyl choline in the neuromuscular junction.

Calcium allows the myosin heads to attach to actin binding sites.

The myosin heads change position, sliding the actin over the myosin – the power stroke. ADP is released.

ATP attaches to the myosin heads and they detach from the actin binding sites.

The ATP hydrolyses, releasing energy. The Myosin heads’ position reset ready for the next power stroke.