Muscle Contraction

Question 1

How does the central nervous system coordinate muscular movement?

Answer 1

-the CNS receives sensory information and decide what kind of response is needed
-if the response needed is movement, the CNS sends signals along neurones to tell skeletal muscles to contract

Question 2

What is the sarcolemma?

Answer 2

The cell membrane of muscle fibres

Question 3

What is the sarcoplasm?

Answer 3

The cytoplasm of a muscle fibre

Question 4

What are transverse (T) tubules?

Answer 4

-inwards folds in the sarcolemma that help to spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre

Question 5

What is the sarcoplasm if reticulum and what is it’s role?

Answer 5

-the endoplasmic reticulum in muscle fibres
-it stores and releases calcium ions that are needed for muscle contraction

Question 6

Explain two other features of muscle fibres

Answer 6

-have lots of mitochondria = provide the ATP that’s needed for muscle contraction
-they are multinucleated

Question 7

What are myofibrils?

Answer 7

-long cylindrical organelles
-they’re made up of protein and specialised for contraction

Question 8

What are thick myofilaments made up from?

Answer 8

The protein myosin

Question 9

What are thin myofilaments made up from?

Answer 9

The protein actin

Question 10

In the structure of a muscle, what is the A band?

Answer 10

-dark bands containing thick myosin, with some overlapping thin actin filaments

Question 11

In the structure of a muscle, what is the I band?

Answer 11

Light bands containing actin only

Question 12

What is the Z line?

Answer 12

Marks the end of each sarcomere

Question 13

What is the M line?

Answer 13

The middle of each sarcomere, the middle of myosin filaments

Question 14

What is the H zone?

Answer 14

Only contains myosin filaments, around the M line

Question 15

Draw the structure of a muscle with the bandings labelled

Question 16

What is the sliding filament theory?

Answer 16

-myosin and actin filaments Slide over one another to make the sarcomeres contract- the myofilaments themselves don’t contract
-the simultaneous contraction of lots of sarcomeres means the myofibrils and muscle fibres contract, then return to their original length as the muscle relaxes

Question 17

Describe the structure and features of cardiac muscle and give an example of where it is found

Answer 17

-cardiac muscle contracts on its own, it’s myogenic, but rate is controlled involuntarily by the automatic nervous system
-found in the walls of your heart
-made of muscle fibres connected by intercalated discs
-muscle fibres are branched to allow nerve impulses to spread quickly
-each muscle fibre has one nucleus
-muscle fibres cylindrical and about 0.1mm long
-see some cross-striations
-fibres contract rhythmically and don’t fatigue

Question 18

Describe the structure and features of skeletal muscle and give an example of where it is found

Answer 18

-skeletal muscle contraction is controlled consciously
-made up of muscle fibres with many nuclei
-muscle fibres many centimetres long
-see regular cross-striations
-contract very quickly, used for speed and strength, but fatigue very quickly
-some muscle fibres contract slowly and fatigue slowly for endurance and posture

Question 19

Describe the structure and features of smooth muscle and give an example of where it is found

Answer 19

-involuntary muscle contraction is controlled unconsciously
-also called smooth muscle because has no striations
-it is found in the walls of your hollow organs e.g the gut, blood vessels
-each muscle fibre has one nucleus
-muscle fibres are spindle shaped with pointed ends, around 0.2mm long
-muscle fibres contract slowly and don’t fatigue

Question 20

Explain the structure of myosin and actin in relation to how they bind to each other

Answer 20

-myosin filaments 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 site for ATP
-actin filaments have binding sites for myosin heads, called actin-myosin binding sites
-2 other proteins called tropomyosin and troponin are found between actin filaments. These proteins are attached to each other and they help myofilaments move past each other

Question 21

Explain the process of how muscle contraction (sliding filament theory) works

Answer 21

1) when an action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma, spreading down the T tubules to the sarcoplasmic reticulum

2) this causes the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm

3) calcium ions binds to troponin, causing it to change shape. This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament

4) this exposes the binding site, which allows the myosin head to bind

5) the bond formed when a myosin head binds to an actin filament is called an actin-myosin cross bridge

6) calcium ions also activate the enzyme ATPase, which breaks down ATP to provide the energy needed for muscle contraction

7) the energy released from ATP moves the myosin head, which pulls the actin filament along in a kind of rowing action

8) ATP also provides the energy to break the actin-myosin cross bridge so the myosin head detaches from the actin filament after it’s moved

9) the myosin head then reattaches to a different binding site further along the actin filament. A new actin-myosin cross bridge is formed and the cycle is repeated

10) many cross bridges form and break very rapidly, pulling the actin filament along, which shortens the sarcomere, causing the muscle to contract

11) the cycle will continue as long as calcium ions are present and bound to troponin

12) when the muscle stops being stimulated, calcium ions leave their binding sites on the troponin molecules and are moved by active transport back into the sarcoplasmic reticulum, using ATP

13) the troponin molecules return to their original shape, pulling the attached tropomyosin molecules with them. This means the tropomyosin molecules block the actin-myosin binding sites again

14) muscles aren’t contracted because no myosin heads are attached to actin filaments

15) the actin filaments slide back to their relaxed position, which lengthens the sarcomere

Question 22

What is a neuromuscular junction?

Answer 22

A synapse between a motor neurone and a muscle cell

Question 23

How to neuromuscular junctions work and which neurotransmitter do they use?

Answer 23

-use the neurotransmitter acetylcholine which binds to receptors called nicotinic cholinergic receptors
-they work the same way in which synapses between neurones do- they release neurotransmitters which triggers depolarisation in the postsynaptic cell
-depolarisation of a muscle cell causes it to contract, and acetylcholinesterase stored in clefts on postsynaptic membrane is released to break down acetylcholine

Question 24

Explain what effect a chemical/drug will have on an organism if it blocks the release of a neurotransmitter in a synapse

Answer 24

-this will prevent an action potential from being passed on to the muscle, so the muscle won’t contract
-this can be fatal if it affects the muscles involved in breathing e.g the diaphragm and intercostal muscles. If they can’t contract, ventilation can’t take place and the organism can’t respire aerobically

Question 25

How is ATP generated for muscle contraction in aerobic respiration?

Answer 25

-most ATP 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 walking or jogging

Question 26

How is ATP generated for muscle contraction through anaerobic respiration?

Answer 26

-ATP is made rapidly by glycolysis
-the end product of glycolysis is Pyruvate, which is converted to lactate
-lactate can quickly build up in the muscles and cause muscle fatigue (where muscles can’t contract as forcefully)
-anaerobic respiration is good for short periods of hard exercise e.g sprint

Question 27

How is ATP generated for muscle contraction through ATP-Creatine Phosphate (CP) system?

Answer 27

-ATP is made by phosphorylating ADP-adding a phosphate group taken from CP
-CP is stored inside cells and the ATP-CP system generates ATP very quickly
-CP runs out after a few seconds so it’s used during short bursts of vigorous exercise e.g a tennis serve, supports a further 2-4 seconds of muscle contraction
-the ATP-CP system is anaerobic (doesn’t need oxygen) and is alactic (doesn’t form any lactate)