Synapses and Muscle Contraction (Chapter 15)

Question 1

What is a synaptic cleft?

Answer 1

The very small gap between two neurones where they meet (20nm wide)

Question 2

What is a synapse?

Answer 2

The parts of the two neurones near to the synaptic cleft and the synaptic cleft

Question 3

What is synaptic transmission?

Answer 3

When molecules of neurotransmitter (transmitter substance) are released to stimulate the next neurone

Question 4

What can impulses not do?

Answer 4

Jump across synapses

Question 5

Describe the general way in which synaptic transmission occurs

Answer 5

1) an action potential occurs at the CSM of the presynaptic neurone
2) the action potential causes the release of neurotransmitter molecules into the synaptic cleft
3) the molecules of neurotransmitter diffuse across the cleft and bind temporarily to receptors on the postsynaptic neurone
4) the postsynaptic neurone responses to all the impulses arriving at any one time by depolarising - if the overall depolarisation is above its threshold, then it will send impulses

Question 6

What does the cytoplasm of the presynaptic neurone contain?

Answer 6

Vesicles of neurotransmitter e.g. noradrenaline and acetylcholine (ACh)

Question 7

What are cholinergic synapses?

Answer 7

Synapses that use ACh as the neurotransmitter molecule

Question 8

Describe how synaptic transmission occurs at a cholinergic synapse (presynaptic neurone)

Answer 8

1) as an action potential occurs at one place on an axon, local circuits depolarise the next piece of membrane, stimulating the opening of Na+ voltage-gated channels ∴ propagating the action potential
2) in the part of the membrane of the presynaptic neurone that is next to the synaptic cleft, the arrival of the action potential causes Ca2+ voltage-gated channels to open
3) Ca2+ diffuses into the cytoplasm of the presynaptic neurone down a steep electrochemical gradient
4) the influx of Ca2+ stimulates vesicles containing ACh to move to the presynaptic membrane (exocytosis) and fuse with it, emptying their contents into the synaptic cleft
5) the ACh diffuses across the membrane

Question 9

Describe how synaptic transmission occurs at a cholinergic synapse (postsynaptic neurone)

Answer 9

1) molecules of ACh temporarily bind with receptor proteins on the CSM of the postsynaptic neurone - a part of the receptor protein molecule has a complementary shape to part of the ACh molecule
2) this changes the shape of the protein, opening channels through which Na+ can pass
3) Na+ diffuse into the cytoplasm of the postsynaptic neurone and depolarise the membrane - the action potential is transmitted if the threshold potential is reached

Question 10

Why is there a very steep Ca2+ electrochemical gradient in the presynaptic neurone?

Answer 10

Bc there are virtually no Ca2+ in the cytoplasm but many in the tissue fluid surrounding the synapse

Question 11

Why are the receptor proteins on the postsynaptic CSM chemically-gated ion channels?

Answer 11

Bc they are stimulated by chemicals (neurotransmitters) and not by voltage change

Question 12

What would happen if ACh remained bound to the postsynaptic receptors?

Answer 12

The Na+ channels would remain open and the postsynaptic neurone would ∴ be permanently depolarised

Question 13

What happens to prevent permanent depolarisation of the postsynaptic neurone and waste of ACh?

Answer 13

ACh is recycled

1) the synaptic cleft contains acetylcholinesterase, which catalyses the hydrolysis of each ACh into acetate and choline
2) the choline is taken back into the presynaptic neurone where it is combined with acetyl CoA to reform ACh
3) the ACh is then transported into the presynaptic vesicles, ready for the next action potential

Question 14

When and only when does the depolarisation of the postsynaptic neurone lead to the generation of an action potential?

Answer 14

If the p.d. is above the threshold for that neurone - if not,, then there is no action potential

Question 15

How is the chance that an action potential is generated and an impulse is sent in a postsynaptic neurone increased?

Answer 15

If more than one presynaptic neurone releases ACh at the same time or over a short period of time

Question 16

What is a neuromuscular junction and what happens here?

Answer 16

• Where a motor neurone forms a motor end plate with each muscle fibre + the synapse
• Here, an action potential is produced in the muscle fibre, which may cause it to contract

Question 17

What are the 4 roles of synapses?

Answer 17

1) to ensure one-way transmission
2) to allow integration of impulses
3) to allow the interconnection of nerve pathways
4) to be involved in memory and learning

Question 18

How do synapses ensure one-way transmission?

Answer 18

• Impulses can only pass in one direction at synapses because a neurotransmitter is released on one side and its receptors are on the other side
• ∴ no way that a chemical transmission can occur in the opposite direction

Question 19

How do synapses allow integration of impulses?

Answer 19

1) each sensory neurone has many branches at the end of its axon that form synapses with many relay neurones
2) the cell body of each motor neurone is covered with the terminations of many relay neurones
3) motor neurones only transmit impulses if the net effect of the relay neurones is above the threshold at which it initiates action potentials
4) so, if the depolarisation of the postsynaptic membrane does not reach the threshold, no impulse is sent in that neurone
5) ∴ impulses with low frequencies do not travel from sensory neurones to reach the brain ∴ the brain is not overloaded with sensory information

Question 20

What does the interconnection of many nerve pathways mean for the nervous system?

Answer 20

That synapses allow a wider range of behaviour than could be generated in a nervous system in which neurones were directly ‘wired up’ to each other

Question 21

What are the two ways in which synapses allow the interconnection of many nerve pathways?

Answer 21

1) individual sensory and relay neurones have axons that branch to form synapses with many different neurones
- ∴ information from one neurone can spread out throughout the body to reach many relay neurones and many effectors, especially in danger
2) there are many neurones that terminate on each relay and motor neurone as they have many dendrites to give a large surface area for many synapses
- ∴ one neurone can integrate the information coming from many different parts of the body (essential for decision-making in the brain)

Question 22

How are synapses involved in memory and learning?

Answer 22

1) if your brain frequently receives information about two things at the same time (e.g. sound of voice and sight of face), then new synapses form in your brain that link the neurones involved in the passing of information along the particular pathways from your ears and eyes
2) ∴ in the future, when you hear the voice, information flowing from your ears along this pathways automatically flows into the other pathway too, s that your brain pictures the face which goes with the voice

Question 23

What is the disadvantage of synapses?

Answer 23

• Synapses slow down the rate of transmission of a nerve impulse that has to travel along to or more neurones
• Responses would be much quicker if action potentials generated in a receptor travelled along an unbroken neuronal pathway from receptor to effector, rather than having to cross synapses on the way

Question 24

What is striated muscle?

Answer 24

• Type of muscle tissue that makes up the many muscles in the body attached to the skeleton
• Only contracts when stimulated to do so by impulses that arrive via motor neurones ∴ it is neurogenic

Question 25

What is cardiac muscle?

Answer 25

• The muscle in the heart

- It is myogenic ∴ it contracts and relaxes automatically with no need for impulses arriving from neurones

Question 26

What is smooth muscle?

Answer 26

• Muscle found in organs e.g. gas exchange system, alimentary canal, in the walls of arteries, veins and arterioles
• Most smooth muscle is neurogenic, however smooth muscle in arteries also contracts when stretched by pressure of blood

Question 27

What does the nervous system ensure about muscles?

Answer 27

That the behaviour of each muscle is coordinated with all the other muscles so that they can bring about desired movement without causing any damage

Question 28

What is a muscle made up of?

Answer 28

1000s of muscle fibres

Question 29

What is a muscle fibre?

Answer 29

• A very specialised cell with a highly organised arrangement of contractile proteins in the cytoplasm, surrounded by the CSM
• It is a syncytium bc it is multinucleate

Question 30

Describe the organelles of a muscle fibre

Answer 30

• It has more than one nucleus
• Sarcolemma = CSM
• Sarcoplasm = cytoplasm
• Sarcoplasmic reticulum (SR) = endoplasmic reticulum

Question 31

What are transverse system tubules (T-tubules)?

Answer 31

• Deep infoldings in the CSM into the interior of the muscle that run close to SR
• They have a large surface area

Question 32

What do the membranes of the SR have?

Answer 32

Huge numbers of protein pumps that transport Ca2+ into the cisternae of the SR

Question 33

What does the sarcoplasm contain?

Answer 33

Many mitochondria, which carry out aerobic respiration, generating ATP required for muscle contraction, often packed tightly between the myofibril

Question 34

What are the striations on muscle fibre produce by?

Answer 34

• A very regular arrangement of many myofibrils in the sarcoplasm
• Each myofibril is striped in exactly the same way and is lined up precisely against the next one

Question 35

What is each myofibril made up of?

Answer 35

Thick and thin filaments - parallel groups of thick filaments lie between groups of thin filaments

Question 36

What are thick and thin filaments made up of?

Answer 36

Protein - thick mostly made of myosin, thin mostly made of actin

Question 37

What is the A band?

Answer 37

The area of thick filaments (darker) including the overlap (the darkest part) - the length of the thick filament

Question 38

What is the I band?

Answer 38

Only thin filaments (lighter) - distance between adjacent thick filaments

Question 39

What is the H band?

Answer 39

Only thick filaments, not including overlap - distance between thin filaments at rest

Question 40

What does the Z line do?

Answer 40

Provides attachment for actin filaments - it is an anchoring point

Question 41

What does the M line do?

Answer 41

Provides attachment for myosin filaments

Question 42

What is the sarcomere?

Answer 42

• The part of the myofibril between the two Z lines

- The contractile unit of muscles

Question 43

Why is the Z line also called a Z disc?

Answer 43

Bc myofibrils are cylindrical ∴ the Z line is a disc separating one sarcomere from another

Question 44

Describe the structure of the thick filament

Answer 44

• Composed of many molecules of myosin (a fibrous protein with a globular head)
• The fibrous portion helps to anchor the molecule into the thick filament
• Each myosin head is an ATPase
• Within the thick filament, many myosin molecules all lie together in a bundle with their globular heads all pointing away from the M line

Question 45

Describe the structure of the thin filament

Answer 45

• The main component is actin (a globular protein)
• Many actin molecules are linked together to form a chain
• Two of these chains are twisted together to form a thin filament
• A fibrous protein called tropomyosin is also twisted around the actin chains
• A protein called troponin is attached to the actin chain at regular intervals

Question 46

How do muscles cause movement?

Answer 46

By contracting

• The sarcomeres in each myofibril get shorter as the Z discs are pulled closer together (the sliding filament model of muscle contraction)
• The energy for the movement comes from ATP molecules that are attached to the myosin heads

Question 47

Describe how a muscle contracts

Answer 47

1) Ca2+ are released from stores in the SR and bind to troponin molecules, stimulating them to change shape
2) the troponin and tropomyosin proteins move to a different position ∴ exposing parts of the actin molecules which act as binding sites for myosin
3) the myosin heads bind with these sites, forming cross-bridges between the thick and thin filaments
4) the myosin heads tilt puling the actin filaments along towards the centres of the sarcomere (power stroke) - here, ADP and Pi are released
5) the heads hydrolyse ATP molecules, which provide enough energy to force the heads to let go of actin
6) ATP hydrolyses to ADP + Pi, and the heads tip back to their previous positions and bind again to the exposed sites on the actin
7) the thin filaments have moved as a result of the power stroke

Question 48

What happens after the muscle has made its first movement?

Answer 48

1) as the thin filaments have now moved, myosin heads now bind to actin further along the thin filaments closer to the Z disc
2) they tilt again, pulling the actin filaments even further along, then hydrolyse more ATP molecules so that they can let go again
3) this goes on and on, so long as the troponin and tropomyosin are not blocking binding sites and so long as the muscle has a supply of ATP

Question 49

When happens at the synapse of a neuromuscular junction?

Answer 49

1) an impulse moves along the axon of a motor neurone and arrives at the presynaptic membrane
2) the arrival of Ca2+ through voltage-gated channels stimulates the exocytosis of vesicles containing ACh to travel and fuse with the presynaptic membrane
2) ACh diffuses across the neuromuscular junction and binds to receptor proteins on the sarcolemma (postsynaptic membrane)
3) the binding of ACh stimulates the ion channels to open, so that Na+ enter to depolarise the membrane and generate an action potential in the sarcolemma

Question 50

What happens at the muscle fibre before the contraction occurs?

Answer 50

1) impulses pass along the sarcolemma and along the T tubules towards the centre of the muscle fibre (the membranes of the SR are very close to the T tubules)
2) the arrival of the impulses causes Ca2+ channels in the membrane to open
3) Ca2+ diffuse out, down a very steep conc gradient, into the sarcoplasm surrounding the myofibrils
4) Ca2+ binds with the troponin molecules
5) this changes the shape of the troponin molecules, causing the troponin and tropomyosin to move away and expose the binding sites for the myosin heads
6) the myosin heads attach to the binding sites on the thin filaments and form cross-bridges

Question 51

What happens when there is no longer any stimulation from the motor neurone?

Answer 51

1) there are no impulses conducted along the T tubules
2) released from stimulation, the Ca2+ channels in the SR close and the calcium pumps move Ca2+ back into stores in the SR
3) as Ca2+ leaves their binding sites on troponin, tropomyosin moves back to cover the myosin-binding sites

Question 52

What happens when the muscle is in a relaxed state?

Answer 52

1) there are no cross-bridges between thick and thin filaments
2) troponin and tropomyosin are sitting in a position in the actin filament that prevents myosin from binding
3) there is nothing to hold the filaments together ∴ any pulling force applied to the muscle will lengthen the sarcomeres so that they are ready to contract (and shorten) again

Question 53

What does each skeletal muscle in the body have?

Answer 53

An antagonist - a muscle that restores sarcomeres to their original lengths when it contracts

Question 54

How is ATP used in muscle contraction?

Answer 54

• A contracting muscle uses a lot of ATP
• The very small quantity of ATP in the muscle fibres in a resting muscle is used up rapidly once the muscle starts to contract
• More ATP is produced by respiration - both aerobic respiration inside the mitochondria and when that cannot supply ATP fast enough, also by lactic fermentation in the sarcoplasm

Question 55

What is the other source of ATP for muscles?

Answer 55

• ATP produced by creatine phosphate (kept in stores in the sarcoplasm)
• Creatine phosphate is their immediate source of energy once they have used up the small quantity of ATP in the sarcoplasm

Question 56

How is creatine phosphate a source of ATP?

Answer 56

• A phosphate group can quickly and easily be removed from each creatine phosphate molecule and combined with ADP to produce more ATP
• creatine phosphate + ADP = creatine + ATP

Question 57

What happens once the demand for energy in the muscles has slowed down/stopped?

Answer 57

• ATP molecules produced by respiration can be used to ‘recharge’ the creatine
• creatine + ATP = creatine phosphate + ADP

Question 58

What happens if energy is still being demanded by the muscles and there is no ATP spare to regenerate the creatine phosphate?

Answer 58

The creatine is converted to creatinine and excreted in the urine

Question 59

What is the structure of muscles in decreasing size?

Answer 59

Muscle
Muscle fibre
Myofibril
Sarcomere

Question 60

What happens to the length of the A band during contraction?

Answer 60

Nothing

Question 61

What happens to the length of the H band and I band during contraction?

Answer 61

They shorten