Topic 6—B: Nervous coordination-4. Muscle contraction

Question 1

Myosin filaments

Answer 1

• They have globular heads that are hinged so they can move back and forth
• Each myosin head has a binding site for actin and a binding and a binding site for ATP

Question 2

Actin filaments

Answer 2

• They have binding sites for myosin heads, called actin myosin binding sites
• Another protein called tropomysosin is found between actin filaments
• It helps myofilaments slide past eachother

Question 3

What does the myosin head need to do for myosin and actin filaments to slide past eachother?

Answer 3

• the myosin head needs to bind to the actin myosin binding sites on the actin filament

Question 4

When is the actin myosin binding. Site blocked by tropomysoin ?

Answer 4

In a resting (unstimulated muscle)
- This means myofilaments can’t slide past eachother because the myosin heads can’t bind to the actin filaments

Question 5

Process of muscle contraction (stages)

Answer 5

1. Arrival of an action potential- when an action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma.
   - Depolarisation spreads down the t tubules to the sarcoplasmic reticulum
   - This causes the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm
   - This influx of calcium ions into the sarcoplasm triggers muscle contraction
   - calcium ions bind to a protein attached to tropomyosin causes the protein to change shape
   - This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament
   - This exposes the binding site, which allows the myosin head to bind
   - The bond formed when a myosin head binds to an actin filament is called an actin-myosin cross bride
2. Movement of the actin filament -
   Calcium ions also activate the enzyme ATP hydrolase, which hydrolyses (break down) ATP (into ADP + Pi) to provide the energy needed for muscle contraction.
   The energy released from ATP causes the myosin head to bend, which pulls the actin filament along in a kind of rowing action
3. Breaking of the cross bridge
   Another ATP molecule provides the energy to break the actin-myosin cross bridge, so the myosin detaches from the actin filament after it’s moved.
   - The myosin head then returns to its starting position, and reattaches to a different binding site further along the actin filament
   - A new actin-myosin cross bridge is formed and the cycle is repeated (attatch, move, detach, reattach)
   - Many actin-myosin cross bridges form and break very rapidly pulling the actin filament along- which shortens the sarcomere, causing the muscle to contract
4. Return to resting state
   - When the muscle stops being stimulated, calcium ions leave their binding sites and are moved by active transport back into the sarcoplasmic reticulum (this needs ATP too)
   - This causes the tropomyosin molecules to move back, so they block the actin-myosin binding sites again
   - Muscles aren’t contracted because no myosin heads are attached to actin filaments (no actin-myosin cross bridges)
   - The actin filaments slide back to their relaxed position, which lengthens the sarcomere

Question 6

Sliding filament theory (stages)

Answer 6

• Myosin head attaches (forms cross bridges) to the actin at the binding site (actin myosin cross bridge formation)
• Power stroke- Myosin head pivots, pulling the actin filaments, sliding it towards the M line
• Myosin head detaches and moves back into ‘cocked’ position

Question 7

What is tropomyosin?

Answer 7

• It’s a protein that covers the myosin head binding sites on the actin filament
• Calcium ions attatch to tropomyosin which changes its shape so it moves and exposes the myosin head binding sites

Question 8

ATP (stages in muscle contraction)

Answer 8

1. ATP is bound to the myosin head
2. Myosin head hydrolyses ATP into ADP and Pi pulling the myosin head back into a cocked position
3. The myosin head binds to actin, forming a cross bridge/ actin myosin bridge
4. Releasing ADP and Pi causes the power stroke. Actin molecules slide towards M line
5. Binding of a new molecule of ATP releases the myosin head from actin and a new cycle begins

Question 9

What is the role of ATP in muscle contraction?

Answer 9

• Hydrolysis of ATP-> ADP and Pi provides the energy to pull the myosin head to cocked position ready for another cycle
• Cross bridge formation occurs
• ADP and Pi detaching from myosin head- power stroke occurs - This changes tertiary structure, hydrogen bonds, 3D shape so its no longer complementary
• ATP binds to myosin head- causes cross bridge to break, detaching myosin head from actin
• ATP is required to actively transport calcium ions back into the sarcoplasmic reticulum

Question 10

How is ATP continually generated so exercise can continue?

Answer 10

1. Aerobic respiration
   - Most ATP is generated via oxidative phosphorylation in the cells mitochondria
   - Aerobic respiration only works when there’s oxygen so it’s good for long periods of low intensity exercise e.g. a long walk
2. Anaerobic respiration
   - ATP is made rapidly by glycolysis
   - The end product of glycolysis is private which is converted to lactate by lactate fermentation
   - Lactate can quickly build up in muscles and cause muscle fatigue
   - Anaerobic respiration is good for short periods of hard exercise e.g. a 400m sprint
3. ATP-phosphocreatine (PCr) system
   - ATP is made by phosphorylating ADP ( adding a phosphate group taken from PCr
   - PCr is stored inside cells
   - ATP-PCr system generates ATP very quickly
   - PCr runs out after a few seconds
   - It’s used during short bursts of vigorous exercise e.g. tennis serve
   - The ATP-PCr system is anaerobic so it doesn’t need oxygen and it’s alactic (doesn’t form any lactate)

Question 11

Equation for phosphorylation of ADP by PCr

Answer 11

ADP + PCr-> ATP + Cr (creatine)
- Some of the creatine gets broken down into creatinine which is removed from the body via kidneys
- Creatinine levels can be higher in people who exercise regularly and those with a high muscle mass
- High creatinine levels may also indicate kidney damage

Question 12

What are the 2 types of muscle fibres skeletal muscles are made up of?

Answer 12

• Slow twitch
• Fast twitch

Question 13

Slow twitch muscle fibres

Answer 13

• They contract slowly
• They can work for a long time without getting tired
• This makes them good for endurance activities e.g. a long distance run
• High proportions of slow twitch muscle fibres are found in the muscles you use for posture, such as the muscles in the back and the calves
• Energy is released slowly through aerobic respiration in slow twitch muscle fibres
• They have lots of mitochondria and blood vessels to supply the muscles with oxygen
• The mitochondria are mainly found near the edge of the muscle fibres so that there’s a short diffusion pathway for oxygen from the blood vessels to the mitochondria
• They are rich in myoglobin (red coloured protein that stores oxygen)

Question 14

Fast twitch muscle fibres

Answer 14

• They contract very quickly
• They get tired very quickly
• This makes them good for short bursts of speed and power e.g. sprinting
• High proportions of fast twitch muscle fibres and found in muscles you use for fast movement such as legs, arms and eyes
• Energy is released quickly through anaerobic respiration using glycogen in fast twitch muscle fibres
• They also have stores of PCr so that energy can be generated very quickly when needed
• They have few mitochondria/ blood vessels
• They don’t have much myoglobin either so they can’t sore much oxygen
• This gives them more of a whitish colour