15. Nervous coordination and muscles

Question 1

How does the nervous system coordinate in animals?

Answer 1

Uses nerve cells to pass electrical impulses along their length. They stimulate their target cells by secreting chemicals, known as neurotransmitters, directly on to them. This results in rapid communication between specific parts of an organism. The responses produced are often short lived and restricted to a localised region of the body.

Question 2

Give an example of nervous transmission

Answer 2

Reflex action, such as the withdrawal of the hand from an unpleasant stimulus.

Question 3

What is a neurotransmitter?

Answer 3

One of a number of chemicals that are involved in communication between adjacent neurones or nerve cells and muscles.

Question 4

How does the hormonal system coordinate in animals?

Answer 4

Produces chemicals that are transported in the blood plasma to their target cells. The target cells have specific receptors on their cell surface membranes and the change in concentration of hormones stimulates them. This results in slower, less specific form of communication between parts of an organism. The responses are often long lasting and widespread.

Question 5

Give an example of hormonal coordination?

Answer 5

Control of blood glucose concentration, which produces a slower response with a long term and widespread effect.

Question 6

What is a ‘neurone’?

Answer 6

Neurones are specialised cells adapted to rapidly carrying electrochemical changes called nerve impulses from one part of the body to another.

Question 7

Describe a ‘cell body’ of a neurone

Answer 7

Contains all the usual cell organelles, including a nucleus and large amounts of RER, associated with production of proteins and neurotransmitters.

Question 8

Describe the ‘dendrons’ of the neurone

Answer 8

Extensions of the cell body which subdivide into smaller branched fibres, called dendrites, that carry nerve impulses towards the cell body.

Question 9

Describe the ‘axon’ of the neurone?

Answer 9

A single long fibre that carries nerve impulses away from the cell body.

Question 10

Describe the ‘schwann cells’ of a neurone

Answer 10

Surround the axon, protecting it and providing electrical insulation. They also carry out phagocytosis and play a part in nerve regeneration. Schwann cells wrap themselves around the axon many times, so that layers of their membranes build up around it.

Question 11

Describe the differences between the hormonal and nervous system of coordination

Answer 11

Hormonal- communicate by hormones. Nervous- communicate by nerve impulses.
Hormonal- transmit in blood. Nervous- transmit by neurones.
Hormonal- transmission is slow. Nervous- transmission is rapid.
Hormonal- travels to all parts of the body, but only target cell respond. Nervous- travel to specific parts of the body.
Hormonal- response is widespread.
Nervous- Response is localised.

Question 12

Describe the ‘myelin sheath’ of a neurone

Answer 12

Forms a covering to the axon and is made up of the membranes of the Schwann cells. These membranes are rich in a lipid known as myelin.

Question 13

Describe the ‘node of Ranvier’ of neurones

Answer 13

Constructions between adjacent Schwann cells where there is no myelin sheath. The constructions are 2-3 um long.

Question 14

What is a sensory neurone?

Answer 14

Transmits nerve impulses from a receptor to an intermediate or motor neurone. a they have one dendron that is often very long. It carries the impulse towards the cell body and one axon that carries it away from the cell body.

Question 15

What are motor neurones?

Answer 15

Transmits nerve impulses from a relay neurone to an effector, such ad a gland or muscle. Motor neurones have a long axon and many short dendrites.

Question 16

What is a relay neurone?

Answer 16

Transmits impulses between neurones, they have numerous short processes.

Question 17

How is the movement of ions, such as sodium and potassium, across the axon membrane controlled?

Answer 17

• The phospholipid bolster of the axon plasma membrane prevents sodium and potassium ions from passing through it.
• Channel proteins span the phospholipid bilayer. These proteins have ion channels, with ‘gates’ which can be opened or closed so that sodium/potassium ions can move through by facilitated diffusion. Some. channels remain opened all the time.
• Sodium potassium pump: Carrier proteins actively transport potassium ions into and sodium ions out of the axon

Question 18

Define nerve impulse

Answer 18

A self-propagating wave of electrical activity that travels along the axon membrane. It is a temporary reversal of the electrical potential difference across the axon membrane.
The reversal is between 2 states, the resting potential and action potential.

Question 19

What is the resting potential?

Answer 19

The difference in electrical charge maintained across the membrane of the axon of the neurone when not stimulated.
The inside of an axon is negatively charged, relative to the outside, due to membrane controls. The resting potential ranges from -50-90 mV. In this condition the axon is said to be polarised.

Question 20

Describe how the resting potential is established

Answer 20

-The sodium potassium pump actively transports sodium ions out of the axon and potassium ions into the axon.
-3 sodium ions move move out for every 2 potassium ions that move in.
-Despite Na+ & K+ both being positively charged, the outward movement of sodium ions is greater than the inward movement of potassium ions, creating an electrochemical gradient.
-The sodium ions begin to diffuse back naturally into the axon while the potassium ions begin to diffuse back out of the axon.
-However, most of the gates in the channels that allow the potassium ions to move through are open, while most of the gates in the channels that allow the sodium ions to move through are closed.
The axon is polarised. Inside = negatively charged.

Question 21

Describe how the action potential is generated

Answer 21

• Stimulus if a sufficient size is detected by a receptor in the nervous system, causing a temporary reversal of charge either side of the axon membrane.
• The channels in the axon change shape, and open to allow sodium ions to diffuse into the neurone down the sodium ion electrochemical gradient.
• If the potential difference meets the threshold value of -55mV, more sodium ion channels open.
• Depolarisation- The negative charge of -65mV becomes a positive charge of +40mV.
• Repolarisation- At a PD of +40mV, the sodium ion channels close and potassium ion channels open. The membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the cone gratin gradient. This starts to get the membrane back to its resting potential.

Question 22

What is ‘hyperpolarisation’?

Answer 22

Potassium ion channels are too slow to close so there’s a slight ‘overshoot’ where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the testing potential (less than -70mV)

Question 23

Why does the action potential move along the neurone as a wave of depolarisation?

Answer 23

• When an action potential happens, some of the sodium ions diffuse sideways.
• This causes sodium ion channels in the next region of the neurone to change and sodium ions diffuse into that part.
• This causes a wave of depolarisation to travel along the membrane.
• The wave moves away from the parts of the membrane in the refractory period because these parts can’t fire an action potential.

Question 24

How does an action potential pass along a myelinated axon?

Answer 24

The fatty sheath of myelin around the axon acts as an electrical insulator, preventing action potentials. There are breaks in the myelin insulation, called nodes of Ranvier, and action potentials occur at these points. Localised circuses arise between adjacent nodes of Ranvier and action potentials jump from rode to rode in a process known as saltatory conduction.

Question 25

Does action potential travel faster down the axon of a myelinayed neurone or an unmyelinayed neurone?

Answer 25

Myelinated neurone: in an unmyelinayed neurone the events of depolarisation have to take place all the way along an axon and this taxes more time.

Question 26

Give the factors which affect the speed which an action potential travels

Answer 26

• Myelin sheath
• Diameter of the axon
• Temperature

Question 27

How does the myelin sheath affect the speed in which an action potential travels?

Answer 27

The myelin sheath acts as an electrical insulator, preventing the action potential forming in the part of the axon covered in myelin. It does however jump from 1 node of Ranvier to another, saltatory condition. This increases the speed of conductance.

Question 28

How does the axon diameter affect the speed in which an action potential travels?

Answer 28

The greater the diameter of axon, the faster the speed of conductance. This is due to less leakage of ions from a large axon.

Question 29

How does the temperature affect the speed in which an action potential travels?

Answer 29

• The higher the temperature, the faster rate of ion diffusion, the faster the nerve impulse.
• The energy for active transport comes from respiration. Respiration is controlled by enzymes. Enzymes function rapidly at higher temperatures, until denaturation (where nerve impulses fail to conduct).
• Temperature is an important factor in response times in cold blooded animals whose body temperature varies in accordance with the environment.
• Temperature also affects the speed & strength of muscle contractions.

Question 30

What is the threshold value?

Answer 30

A certain level of stimulus which triggers an action potential, and therefore a nerve impulse.

Question 31

How can an organism perceive the size of a stimulus?

Answer 31

• By the number of impulses passing in a given time. The more impulses generated, the larger the stimulus.
• By having different neurones with different threshold values. The brain interprets the number and type of neurones that pass impulses as a result of a given stimulus and thereby determines its size.

Question 32

What is the refractory period?

Answer 32

once an action potential has been generated in any region of the axon, that is a period afterwards when an inward movement of sodium ions is prevented because the sodium voltage gated channels are closed. During this time it is impossible for a further action potential to be generated.

Question 33

What’s the purpose of the refractory period?

Answer 33

• it insures that action potentials are propagated in one direction only: Action potentials can only pass from an active region to a resting region. This is because action potentials cannot be propagated in a region that is refractory, they can only move in a forward direction. This prevents action potentials from spreading out in both directions.
• it produces discrete impulses: due to the refractory period, a new action potential cannot be formed immediately behind the first one.
• It limits the number of action potentials: As action potentials are separated from one another this limits the number of action potentials that can pass along an axon in a given time, and limits the strength of the stimulus that can be detected.

Question 34

Explain how the refractory period insures that the nerve impulses are kept separate from one another.

Answer 34

During the refractory period the sodium voltage gated channels are closed so no sodium ions can move inwards and no action potential is possible.

This means that there must be an interval between the one impulse and next.

Question 35

State the all or nothing principle

Answer 35

There is a particular level of stimulus that triggers an action potential.
At any level above this threshold, a stimulus will trigger an action potential that is the same regardless of the size of the stimulus. Below the threshold, no action potential is triggered.

Question 36

Earthworms have unmyelinated axons and so to increase the speed of conduction of action potentials these are relatively large in diameter.
Suggest two reasons why mammals do not require large diameter axons to achieve rapid transmission of action potentials.

Answer 36

Mammals have myelinated neurons and so have saltatory conduction. Mammals are endothermic and their constant, high body temperature increases the rate of diffusion of ions across the axon membrane and hence the speed of conduction of the action potential.

Question 37

Describe the structure of a synapse

Answer 37

Synapses transmit information, not impulses, from one neuron to another by means of neurotransmitters. Neurons are separated by a small gap called the synaptic cleft. The neuron releases the transmitter is called the presynaptic neuron. The axon of this neuron and is in a swollen potion known as the synaptic knob. This possesses many mitochondria and large amounts of endoplasmic reticulum. These are required in the manufacture of the neurotransmitter which takes place in the axon.The neurotransmitter is stored in synaptic vesicles.Once the neurotransmitter is released from the vesicles, it transports to the postsynaptic neuron, which possesses specific receptor proteins on its membrane to receive it.

Question 38

Give the main features of synapses

Answer 38

• Unidirectionality
• Summation (rapid build up of neurotransmitter in the synapse)
• Inhibition

Question 39

What is spatial summation?

Answer 39

A number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone. Together they trigger a new action potential.

Question 40

What is temporal summation?

Answer 40

A single presynaptic neuron releases neurotransmitter many times over a short period.

If the concentration of the neurotransmitter exceeds the threshold value of the post synaptic neuron then a new action potential is triggered.

Question 41

Describe how inhibitory synapses prevent an action potential

Answer 41

-The presynaptic neurone releases the type of neurotransmitter that binds to chloride ion protein channels in the post synaptic neurone.
-The neurotransmitter causes the chloride ion protein channels to open.
-Chloride ions move into the postsynaptic neuron by facilitated diffusion.
-The binding of the neurotransmitter cause the opening of nearby potassium protein channels.
potassium ions move out of the postsynaptic neuron into the synapse.
-This combined effect of negatively charged chloride ions moving in, and positively charged potassium ions moving out, makes the inside of the postsynaptic membrane more negative and the outside more positive.
-this membrane potential increases to as much as -80mV.
This is called hyperpolarisation and makes it less likely that a new action potential will be created because a larger influx of sodium ions is needed to produce one.

Question 42

State the functions of synapses

Answer 42

Act as junctions allowing…

• A single impulse along one neuron to initiate new impulses in a number of different neurons at a synapse. This allows a single stimulus to create a number of simultaneous responses.
• A number of impulses to be combined at a synapse.This allows nerve impulses from receptors reacting to different stimuli to contribute to a single response.

Question 43

what neurotransmitter is released at cholinergic synapse?

Answer 43

Acetylcholine

Question 44

Describe the mechanism of transmission across a cholinergic synapse

Answer 44

1. The arrival of an action potential at the end of the presynaptic neurone causes calcium ion protein channels to open and Ca2+ enter the synaptic knob by facilitated diffusion
2. This causes synaptic vesicles to fuse with the pre-synaptic membrane, releasing acetylcholine into the synaptic cleft.
3. Acetylcholine diffuses along the narrow synaptic cleft quickly because the diffusion pathway is short. Acetylcholine binds to receptors on sodium ion protein channels in the membrane of the postsynaptic neurone. This causes sodium ion protein channels to open, allowing sodium ions to diffuse in along a concentration gradient.
4. This generates a new action potential in the post synaptic neurone.
5. Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid, which diffuse back across the synaptic cleft into presynaptic neurone. The rapid breakdown of acetylcholine also prevents it from continuously generating a new action potential in the post synaptic neurone, and so leads to discrete transfer of information across synapses.
6. ATP is released by mitochondria is used to re-combine choline and ethanoic acid into acetylcholine. This is stored in synaptic vesicles for future use. Sodium ion protein channels close in the absence of acetylcholine in the receptor sites.

Question 45

Explain how presynaptic neuron is adapted for the manufacture of neurotransmitter

Answer 45

It possesses many mitochondria and large amounts of endoplasmic reticulum

Question 46

Explain how the postsynaptic neuron is adapted to receive the neurotransmitter

Answer 46

It has receptor molecules for the neurotransmitters on its membrane.

Question 47

If a neurone is stimulated in the middle of its axon, an action potential will pass both ways along it to the synapses at the end of the neuron. However the action potential will only pass across the synapse at one end. Explain why

Answer 47

Only one end can produce neurotransmitter and so this end alone can create a new action potential and the neurone on the opposite side of the synapse. At the other end there is no neurotransmitter that can be released to pass along the synapse and so no new action potential can be set up.

Question 48

When walking along the street we barely notice the background noise of traffic. However we often respond to loud noises such as the sound of a horn.From your knowledge of summation explain the difference.

Answer 48

The relatively quiet background noise of traffic produces a low-level frequency of action potentials in the sensory neurons from the ear. The amount of neurotransmitter released into the synapse is insufficient to exceed the threshold in the postsynaptic neurone and so the noise is filtered out. Loud noises create a higher frequency and the amount of neurotransmitter released is sufficient to trigger an action potential in the postsynaptic neuron so there is a response. this is an example of temporal summation. In which a single presynaptic neurone releases neurotransmitter many times over a very short period.

Question 49

Suggest an advantage in responding to high-level stimuli but not to low-level ones

Answer 49

Reacting to low-level stimuli that present little danger can overload the central nervous system and so organisms may fail to respond to more important stimuli.High-level stimuli need a response because they are more likely to represent danger.

Question 50

Give examples of how drugs affect the action of neurotransmitters at synapses

Answer 50

• Some drugs, called agonists, have the same shape as neurotransmitters so they mimic that action at receptors. This means more receptors are activated, for example nicotine mimics acetylcholine and binds to nicotine cholinergic receptors in the brain.
• Some drugs, called antagonists, block receptors so they can’t be activated by neurotransmitters. This means fewer receptors can be activated.
• Some drugs inhibit the enzyme that breaks down neurotransmitters.This means there are more neurotransmitters in the synaptic cleft to bind to receptors and they’re there for longer.
• Some drugs simulate The release of neurotransmitter from the pre-synaptic neuron so more receptors are activated
• Some drugs inhibit the release of neurotransmitters from the presynaptic neuron so fewer receptors are activated, eg alcohol

Question 51

Where is cardiac muscle found?

Answer 51

The heart.

Under unconscious control.

Question 52

Where is smooth-muscle found?

Answer 52

The walls of blood vessels and the gut.

Unconscious control.

Question 53

Where is skeletal muscle found?

Answer 53

Makes up the bulk of body muscle invertebrates. It is attached to bone and acts under voluntary conscious control.

Question 54

How are muscles adapted for contraction?

Answer 54

• Grouped into myofibrils which have strong collective force.
• The separate cells are fused together into muscle fibres.These muscle fibres share nuclei and cytoplasm, called sarcoplasm, which is mostly found around the circumference of the fibre and contains a large concentration of mitochondria and endoplasmic reticulum.

Question 55

What are the two types of protein filament that make up myofibrils?

Answer 55

• Actin: thin and consist of 2 strands twisted around each other.
• Myosin: thick and consists of long rod shaped tails with bulbous heads.

Question 56

When looking at myofibril under an electron microscope why can you observe a pattern of dark and light bands?

Answer 56

• Dark bands contain thick myosin filaments and some overlapping actin filaments: A bands
• Light bands contain thin actin filaments only: I bands.

Question 57

What is the Z line?

Answer 57

The end of each sarcomere

Question 58

What is the M line?

Answer 58

The middle of each sarcomere, and the middle of myosin filaments.

Question 59

What is the H zone?

Answer 59

Around the M line, only contains myosin filaments.

Question 60

What is skeletal muscle used for in the body?

Answer 60

Movement.

Muscles act as effectors and are stimulated to contact by neurones.

Question 61

Why are muscles referred to as ‘antagonistic pairs’?

Answer 61

• The contracting muscle is the agonist

- The relaxing muscle is the antagonist

Question 62

What is tropomyosin?

Answer 62

An important protein found in muscle, form long thin strands that wrap around the actin filament.

Question 63

What are the 2 types of muscle fibre?

Answer 63

• Slow twitch fibres

- Fast twitch firbes

Question 64

Compare slow twitch and fast twitch fibres

Answer 64

Slow- Contract slowly, Fast - Contract quickly.

Slow- Muscles used for posture, Fast- Muscles used for fast movement.

Slow- Endurance activities, Fast- Speed and power activities

Slow- Work for long time without getting tired, Fast- Get tired easily.

Slow- Energy released through aerobic respiration. Lots of mitochondria and blood vessels supply oxygen, Fast- Energy released quickly through anaerobic respiration using glycogen. There are few mitochondria or blood vessels.

Slow- Red colour, rich in myoglobin, storing oxygen. Fast- white, don’t have much myoglobin, can’t store oxygen.

Question 65

What is a neuromuscular junction?

Answer 65

The point where a motor neurone meets skeletal muscle fibre. Many junctions along a muscle, for rapid and coordinated contraction when stimulated by action potentials.

Question 66

What is a motor unit?

Answer 66

All muscle fibres supplied by a single motor neurone act together as a single functional unit. If only a slight force needed only a few units stimulated. If greater force required, larger units stimulated.

Question 67

Explain how the neuromuscular junction receives a nerve impulse.

Answer 67

When a nerve impulse is received at the neuromuscular junction, the synaptic vesicles fuse with the pre-synaptic membrane and release their acetylcholine. The acetylcholine diffuses to the postsynaptic membrane, altering its permeability to sodium ions, Which enter rapidly depolarising the membrane. The acetylcholine is broken down by acetylcholinesterase to ensure the muscle is not over stimulated. The acetyl and ethanoic acid diffuse back into the neurone, where they’re recombined too corn acetylcholine using energy from mitochondria.

Question 68

Compare a neuromuscular junction to a cholinergic synapse

Answer 68

Junction- Only excitatory. Synapse- May be excitatory or inhibitory.

Junction- Only links neurones to muscles. Synapse- Links neurones to neurones or other effector organs.

Junction- Only motor neurones involved. Synapse- motor, sensory and intermediate neurones.

Junction- action potential ends here. Synapse- another action potential may be produced along a post synaptic neurone.

Junction- acetylcholine binds to receptors oh membrane of muscle fibre. Synapse- acetylcholine binds to receptors on membrane of post synaptic neurone.

Question 69

Suggest a reason why there are numerous mitochondria in the sarcoplasm

Answer 69

Muscles require much energy for contraction. Most of this energy is released during the krebs cycle and electron transport chain in respiration. Both these take place in mitochondria.

Question 70

If we cut across a myofibril at certain points, we see only thick myosin filaments. Cut at different points and we see only thin actin filaments. At other other points we see both types of filament. Explain why.

Answer 70

The actin and myosin filaments lie side by side in myofibril and overlap at the edges where they meet. If cut where they do not overlap we see one or other filament only. If cut where they do overlap both filaments can be seen.

Question 71

Explain how slow twitch fibres are adapted to contract slowly.

Answer 71

Slow twitch fibres have myoglobin to store oxygen, much glycogen to provide a source of metabolic energy, a rich supply of blood vessels to deliver glucose and oxygen, and lots of mitochondria to produce ATP

Question 72

How are fast-twitch fibres adapted for quick muscle contraction?

Answer 72

Fast twitch fibres have thicker and more numerous myosin filaments, a high concentration of enzymes involved in anaerobic respiration and a store of phosphocreatine to rapidly generate ATP from ADP and anaerobic conditions.

Question 73

Why do skeletal muscles have to occur and act in antagonistic pairs?

Answer 73

The contraction of skeletal muscle will move a part of the skelton in one direction, but the same muscle can’t move it in the opposite direction. Muscles can’t push, they only pull. To move the limb in the opposite direction requires a second muscle working antagonistically to the first one. In doing so it stretches it’s partner muscle, returning it to its original state ready to contact again. They pull in opposite directions, when one is contracted the other is relaxed.

Question 74

Give evidence for the sliding filament mechanism

Answer 74

When the muscle contracts the following changes occur to a sarcomere:
-I band narrows
-Z lines move close together, sarcomere shortens
-H zone becomes narrower
The A band remains the same width, as the width of this band is determined by length of myosin filaments, it follows that the myosin has not become shorter. This discounts the theory that muscle contraction is due to the filaments themselves shortening.

Question 75

What are the 3 main proteins involved in the sliding filament mechanism?

Answer 75

-Myosin, made up of 2 types of protein:
A fibrous protein tail, arranged into a filament of several hindered molecules.
A globular protein head, forming 2 bulbous structures at one end.

• Actin is a globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical stand.
• Tropomyosin forms long thin threads that are wound around actin filaments.

Question 76

Describe the sliding filament process of muscle contraction, part 1: muscle stimulation

Answer 76

• An action potential reaches many neuromuscular junctions simultaneously, causing calcium ion protein channels to open and calcium ions to diffuse into the synaptic knob.
• The calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft.
• Acetylcholine diffuses across the synaptic cleft and binds with receptors on the muscle cell surface membrane, causing it to depolarise.

Question 77

Describe the sliding filament process of muscle contraction, part 2: muscle contraction

Answer 77

• An action potential travels deep into the river through a system of T tubules that are extensions of the cell membrane and branch throughout the sarcoplasm.
• The tubules are in contact with endoplasmic reticulum of the muscle, which has actively transported calcium ions from the sarcoplasm leading to very low Ca2+ concentration in sarcoplasm (muscle cytoplasm).
• The action potential opens the calcium ion protein channels on the endoplasmic reticulum, and calcium ions diffuse into the sarcoplasm down conc gradient.
• The calcium ions cause the tropomyosin molecules that were blocking the binding site on the actin filament to pull away.
• ADP molecule attached to the myosin heads mean they’re in a state to bind to the actin filament and form a cross bridge.
• Once attached to the actin filament, the myosin heads change their angle, pulling the actin filament along, releasing a molecule of ADP.
• An ATP molecule attaches to each myosin head, causing it to become detached from the actin filament.
• The calcium ions then activate the enzyme ATPase, which hydrolyses ATP to ADP, providing the energy for the myosin head to return to its original position.
• The myosin head, once more with an ADP molecule attached, reattached itself further along the actin filament and the cycle is repeated as long as the concentration of calcium ions in the myofibril remains high.
• As the myosin are joined tail to tail in 2 oppositely facing sets, the movement of one set of myosin heads is in the opposite direction to the other set. This means the actin filaments to which they’re attached also move in opposite directions.
• The movement of actin filaments in opposite directions pulls them towards each other, shortening the distance of the 2 adjacent Z lines. The effect of this process taking place repeatedly and simultaneously throughout a muscle is to shorten it and bring about movement in the body.

Question 78

Describe the sliding filament process of muscle contraction, part 3: muscle relaxation

Answer 78

• When a nervous stimulation eases calcium ions are actively transported back into the endoplasmic reticulum using energy from the hydrolysis of ATP.
• This re absorption of calcium ions allows tropomyosin to block the actin filament again. Myosin heads are now unable to bind to actin filaments, contraction ceases, muscle relaxes.

Question 79

Muscle contraction requires considerable energy, how is this supplied?

Answer 79

From the hydrolysis of ATP to ADP and an inorganic phosphate.

Question 80

What is the energy used for in muscle contraction?

Answer 80

• The movement of the myosin heads

- The reabsorption of calcium ions into the endoplasmic reticulum by active transport

Question 81

Generating ATP aerobically requires oxygen. In a very active muscle the demand for ATP, and therefore oxygen, is greater than the rate that the organism can supply oxygen. How can ATP be generated in this circumstance?

Answer 81

Anaerobic respiration. This is achieved by using a chemical, phosphocreatine and more glycolysis.
Phosphocreatine cannot supply energy directly to the muscle so instead regenerates ATP. Phosphocreatine is stored in muscle and acts as a reserve supply fo phosphate why is available immediately to combine with ADP and reform ATP. The phosphocreatine store is replenished using phosphate from ATP when muscle is relaxed.

Question 82

Explain how the shape of myosin is adapted to muscle contraction

Answer 82

Myosin is made of 2 proteins. The fibrous protein is long and thin in shape, enabling it to combine with others to form a long thick filament along with which the actin filament can move. The globular protein forms 2 bulbous heads at the end of a filament. This shape allows it to exactly fit recesses in the actin molecules to which it can become attached. It’s shape also means it can be moved at an angle. This allows it to change it’s angle when attached to the actin, moving it along, causing contraction of muscle.

Question 83

Trained sprinters have high levels of phosphocreatine in the muscles. Explain the advantage of this.

Answer 83

Phosphocreatine stores the phosphate that is used to generate ATP from ADP in anaerobic conditions. A sprinters muscles often work so strenuously so that oxygen supply cannot meet the demand. The supply of ATP from mitochondrial during aerobic respiration therefore ceases. Sprinters with the most phosphocreatine have an advantage because ATP can be supplied to their muscles for longer and they can perform better.

Question 84

During the contraction of a muscle sarcomere, a single actin silent moves 0.8 um. If hydrolysis of a single ATP provides enough energy to move an actin filament 40 nm, calculate how many ATP molecules are needed to move the actin filament 0.8um.

Answer 84

1 ATP= moves actin 40nm
Total distance be moved by actin = 0.8um= 800nm
Number of ATP required to move 800nm = 800/40 = 20 ATPs

Question 85

Dead cells can no longer produce ATP. Soon after death, muscles contract, making the body stiff- ‘rigor mortis’ From your knowledge of muscle contraction, explain why rigor mortis occurs after death.

Answer 85

One role of ATP in muscle contraction is to attach to the myosin heads, thereby causing them to detach from the actin filament and making the muscle relax. As no ATP is produced after death, there’s none to attach to the myosin which therefore remains attached to the actin, leaving the muscle in a contracted state.