Biology Skeletal muscles

How do Skeletal muscles enable movement ?

exerting a pulling force on incompressible skeleton

What are muscles and bones connected by?

Tendons

what are agnostic pairs?

muscles that pull in opposite directions, when one contracts, the other relaxes

An example of agnostic pair?

Biceps contract when triceps relax (pulling the arm up)

What is the structure of skeletal muscles ?

Sarcolemma (cell surface membrane), sarcoplasm (cytoplasm), sarcoplasmic reticulum (endoplasmic reticulum)

Bundel of muscle fibres (muscle cells)

What does skeletal muscle do ?

moves the skeleton

What are features of muscle fibres?

All muscle fibres are elongated cells that contain many nuclei

Sarcoplasm contains organised arrangement if contractile protines that form myofibrils

Sarcolemma has t tubules (deep fold in cell that carries action potential)

many mitochondria in sarcoplasm generate ATP for muscle contraction

What is a myofibril?

long rod like structure of protein filaments, (the protein filaments are thin actin and thick myosin)

what is a sarcomere?

What happens to it during contraction?

Between two Z lines

section of myofibrils that shorten during muscle contraction as myosin and actin slide past each other

What the z line?

What happens during contraction?

boundary between sarcomeres

Actin filaments attach

Contraction: z lines get closer together

What is the M line?

Central point of each sarcomere

Myosin filaments attach

What is the A band?

Region of sarcomere where the myosin filaments extend

Remains the same size during contraction because myosin filaments don’t change in length

What is the H band?

region of sarcomere containing only myosin filaments

Shrinks during muscle contraction due to overlap between actin and myosin

what is the I band?

Reigion of the sarcomere containing only actin

Shrinks during muscle contraction due to overlap

What strcutures of a muscle can be seen using an electron microscope?

Nuclei and banding

Name all the diffrent bands and there loations.

Why cant you see details of muscle structure using electron microscope?

Resolution is too low

What are visible features of the strucutre of myofibrils that you’d see e.g colours

A band - Dark (overlap of myosin and actin)

I band - light (actin only)

Z line - Dark

When does the sarcomere shorten?

During muscle contraction

What are features of actin filaments?

actin myosin binding sites

Tropomiosine (protine) attached

Where does the globular head of myosine bind?

actin myosine binding site

What does tropomyosin do when attached to actin?

blocks binding site (that myosin head binds to during contracion)

Describe the sliding filament theory of muscle contraction (6 mark)

• action potential arrives at neuromuscular junction

• Calcium ions are released from the sarcoplasmic reticulum

• Ca+ diffuse into sarcoplasm

• Ca+ bind to tropomyosin

• causing tropomyosin to move exposing actin myosin binding sites

• Cross bridges formed as myosin heads bind to actin

• Movement causes myosin head bend releasing ADP and pi

• ATP binds to myosin heads causing them to release from actin

• (incomplete flash card)

what is a neuromuscular junction?

Specialised synapses located at the junctions between neurones and muscles

Presynaptic cell is motor neurone

Postsynaptic is muscle cell

What happens after action potential arrives at the neuromuscular junction?

• Calcium ions released from the sarcoplasmic reticulum and diffuse into sarcomere

• Ca+ bind causing tropomyosin to move exposing actin binding sites

• Myosin heads bind to form cross bridges

• Myosin head bends pulling actin filaments causing power stroke

• ATP binds to myosin heads causing them to release from actin

• ATP hydrolase hydrolyses ATP

• Myosin heads return to original position

What is a power stroke?

When myosin heads bend causing them to release ADP and Pi, pulling the actin towards centre of sarcomere (m line)

What is a cross bridge?

When myosin head binds to actin

What happens when Myosin head bends?

Releases ADP and Pi and pulls actin filaments causing power stroke

What does ATP do in the sliding filament theory ?

Binds to myosin head causing them to release from actin

What does ATP hydrolase enzyme do?

hydrolyses ATP providing energy that allows myosin head to go back to original position

What happens when a nerve impulse is no longer stimulated?

Muscle stops being stimulated:

Calcium ions taken up by sarcoplasmic reticulum

Actin myosin binding sites are blacked , myoxine cannot bind to actin

Filaments can be pulled apart (slide back) to resting

Why is ATP important in muscle contraction?

Resets the position of myosin heads

Return Ca+ to sarcoplasmic reticulum after contraction via active transport

Where does ATP in muscle contraction come from?

Aerobic respiration: due to oxygen supply only sustain low intensity exercise

Anaerobic respiration: small amounts quickly , lactate build up so only short periods

Phosphocreatine: rapid production over short periods

What is phosphocreatine?

stored by muscles , used for rapid regenerate of ATP over short periods

Transfers a pi ion to ADP

ADP + phosphocreatine =

ATP + creatine

What are features of fast twitch muscles?

Contract rapidly

Use anaerobic respiration

Fatigue quickly due to lactate

Few capillaries

Low in myoglobin (due to low oxygen requirements)

What is myoglobin?

Red pigment like haemoglobin, stores oxygen in muscles

Which muscles are better for short bursts of high intensity activities? Why?

Fast twitch

short: anaerobic respiration causes the build up of lactate so fatigue quickly

High intensity: contract rapidly with great force as they generate ATP quickly

What are features of slow twitch fibres?

Contract slower

Aerobic respiration for ATP

Fatigue slowly (less lactate production)

Dense capillary network

High in myoglobin and haemoglobin

Lots of mitochondria

what colours are fast and slow twitch

slow : darker because richer in myoglobin

Fast : lighter - less myoglobin

What are the differences between slow and fast twitch? (5 mark)

Fast vs Slow

Fast have rapid contraction , slow have contract slowly

Fast most ATP from anaerobic , slow most ATP from aerobic

Fast fatigue rapidly (lactate) , slow fatigues slowly

Fast less mitochondria, slow many mitochondria

Fast large glycogen and phosphocreatine stores , slow less

Fast large store of calcium ions in SR, slow small store

Fast fewer capillaries, slow more capillaries

Fast high ATP hydrolase activity , slow lower activity

What happens as a result of repeated contraction of muscles?

Muscle fatigue due to :

Production of lactate during anaerobic respiration

ATP running low

reduction in force over time

Decrease in calcium ion availability

Describe the investigation into rate of muscle fatigue

Apparatus : Hand grip strengthener, stop watch, participant

Method: bring two handles together as many times as possible over 20 seconds

1. Record number of successfully squeezes

2. Rest 10 secs

3. Repeat 3 times

4. Repeat with other people

5. plot graph x - time elapses 20,40 , etc , y - number of successful squeezes

why is there a decrease in calcium ion availability and how does this effect muscle contractions?

Causes muscle fatigue

Less calcium ions released (this is because less are returned to SR)

Less troponin molecules activated

fewer actin myosin cross bridges

weaker contractions

What are limitations of the investigation into muscle fatigue?

Only looks at fatigue in hands:

Other skeletal muscles fatigue at different rates

Intensity may not be high enough to cause fatigue for some individuals

What is troponin?

Protine complex that attaches to actin

Ca+ binds to receptors on it

Causing it to change shape

Causing tropomyosine to move exposing binding sites