1
Q
What is a synaptic cleft?
A
A very small gap between two neurones at a synapse
2
Q
What is a synapse?
A
A point at which two neurones meet but do not touch; the synapse is made up of the end of the presynaptic neurone, the synoptic cleft and the end of the postsynaptic neurone
3
Q
How do impulses travel across a membrane?
A
- Impulses cannot ‘jump’ across a synapse
- Instead molecules of a transmitter substance or neurotransmitter are related to stimulate the next neurone
4
Q
What is a transmitter substance?
A
A chemical that is released from a presynaptic neurone when an action potential arrives, and that then diffuses across the synaptic cleft and may initiate an action potential in the postsynaptic neurone
5
Q
What is a postsynaptic neurone?
A
The neurone on the opposite side of a synapse to the neurone in which the action potentials arrives
6
Q
What is a presynaptic neurone?
A
A neurone ending at a synapse from which neurotransmitter is secreted when an action potentials arrives
7
Q
Describe an impulse travelling across a synapse
A
- An action potential occurs at the cell surface membrane of the first neurone or presynaptic neurone
- The action potential causes the release of molecules of transmitter substance into the cleft
- The molecules of transmitter substance diffuse across the cleft and bind temporarily to receptors on the postsynaptic neurone
- The post synaptic neurone responds to all the impulses arriving at any one time by depolarising; if the overall depolarisation is above its threshold, then it will send impulses
8
Q
What is acetylcholine (ACh)?
A
A transmitter substance found, for example in the presynaptic neurone at neurotransmitter junctions
9
Q
What is noradrenaline?
A
A neurotransmitter substance
10
Q
What are examples of different transmitter substances?
A
-Nonadrenaline and Acetylcholine (ACh) are found through the nervous system
11
Q
What are examples of different transmitter substances?
A
- Nonadrenaline and Acetylcholine (ACh) are found through the nervous system
- Dopamine, glutamic acid and gamma-aminobutyric acid (GAMA) occur only in the brain
12
Q
What are cholingeric synapses?
A
A synapse at which the transmitter substance is acetylcholine
13
Q
Describe action potentials in more detail
A
- As an action potential occurs at one place on an axon, local circuits depolarise the next piece of membrane, stimulating the opening of sodium ion voltage gated channel and so propagating the action potential
- In the part of the membrane of the presynaptic neurone that is next to the synaptic cleft, the arrival of the action potential causes calcium ion voltage gated channels to open
14
Q
What is the significant of the calcium ion voltage gated channels to open in the membrane of the presynaptic neurone?
A
- Therefore the action potentials causes not only sodium ions but also calcium ions to diffuse out of the cytoplasm of the presynaptic neurone
- There are virtually no calcium ions in the cytoplasm, but many in the tissue fluid surrounding the synapse
- This means that there is a very steep electrochemical gradient for calcium ions
15
Q
What happens as a result of the influx of calcium ions?
A
- The influx of calcium ions stimulate vesicles contain ACh to move to the presynaptic membrane and fuse with it, emptying their contents into the synaptic cleft
- Each action potential causes just a few vesicles to do this and each vesicle contains up to 10,000 molecules of ACh
- The ACh diffuses across the synaptic cleft, usually in less than 0.5ms
16
Q
Describe the cell surface membrane of the postsynaptic neurone
A
The cell surface membrane of the postsynaptic neurone contains receptor proteins
17
Q
What is significant about the receptor proteins in the cell surface membrane of the postsynaptic neurone?
A
- Part of the receptor protein molecule has a complementary shape to part of the ACh molecule, so that ACh molecules can temporarily bind with the receptors
- This changes the shapes of the protein, opening channels through which sodium ions can pass
- Sodium ions diffuse into the cytoplasm of the postsynaptic neurone and depolarise the membrane
18
Q
What is different about these receptor proteins?
A
These receptor proteins with their channels are chemically gated ion channels as they are stimulated to open by chemicals (neurotransmitters) and not by a voltage change
19
Q
What would happen if the ACh remained bound to the postsynaptic receptors? How is this avoided?
A
- The sodium channels would remain open, and the postsynaptic neurone would be permanently depolarised
- The ACh is recycled to prevent this from happening and also to avoid wasting it
20
Q
What does the synaptic cleft contain?
A
The synaptic cleft contains an an enzyme, acetylcholinesterase, which catalyses the hydrolysis of each ACh molecule into acetate and choline
21
Q
What is acetylcholinesterase?
A
An enzyme that rapidly breaks down acetylcholine at synapses
22
Q
What is an action potential dependent on?
A
- The depolarisation of the postsynaptic neurone only leads to the generation of an action potential if the potential difference is above the threshold for that neurone
- If not then there is not action potential
- The chance that an action potential is generated and an impulse sent in the postsynaptic neurone is increased if more than one presynaptic neurone relates ACh at the same time or over a short period of time
23
Q
Where is research on synapses?
A
- Much often research on synapses has been done at synapses between motor neurone and a muscle, not those between two neurones
- The motor neurone forms a more end plate with each muscle fibre and the synapse is called a neuromuscular junction
- Such synapses function in the same way as describe above
- An action potential is produced in the muscle fibre which may cause it to contract
24
Q
What is the disadvantage of synapses?
A
- Synapses slow down the rate of transmission of a nerve impulse that has to travel along two or more neurones
- Responses to a stimulus 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 not the way
25
What is a motor end plate?
The ending of an axon of a motor neurone where it forms a synapse with a muscle
26
What is a neuromuscular junction?
A synapse between the axon of a motor neurone and a muscle
27
How do synapses allow integration of impulses?
1. Each sensory neurone has many branches at the end of its axon that form synapses with many relay (intermediate) neurones
2. The cell body of each motor neurone is covered with the termination 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. If the depolarisation of the postsynaptic membrane does not reach the threshold, no impulse is sent in that neurone
5. One advantage of this is that impulses with low frequencies do not travel from sensory neurones to reach the brain
6. This means that the brain is not overloaded with sensory information
28
How do synapses allow the interconnection of nerve pathways?
1. Synapses allow a wider range of behaviour than could be generated in a nervous system in which neurones were directly 'worded up' to each other
2. They do this by allowing the interconnection of many nerve pathways
29
What is one way that synapses allow the interconnection of many nerve pathways?
1. Individual sensory and relay neurone have axons that brach to form synapses with many different neurones
2. This means that information from one neurone can spread out throughout the body to reach many relay neurones and many effectors as happens when we respond to dangerous situations
30
What is another way that synapses allow the interconnection of many nerve pathways?
1. 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
2. This allows one neurone to integrate the information coming from many different parts of the body, something that is essential for decision making in the brain
31
How are synapses involved in memory and learning?
- If your brain frequently receives information about two things at the same time, say the sound of particular voice and the sight of a particular face then it is thought that new synapses form in your brain that link the neurone involved in the passing of information along the particular pathways from your ears and eyes
- In future when you hear the voice, information flowing from your ears along the pathway automatically flows into the other pathway room, so that your brain 'pictures' the face which goes with the voice
32
What is the appearance in the light microscope of striated muscle?
Stripes (striations) at regular intervals
33
What is the appearance in the light microscope of cardiac muscle?
Stripes (striations) at regular intervals
34
What is the appearance in the light microscope of smooth muscle?
No striations
35
What is the cell structure of striated muscle?
Multinucleate (syncytium)
36
What is the cell structure of cardiac muscle?
Uninucleate cells joined by intercalated discs
37
What is the cell structure of smooth muscle?
Uninucleate cells
38
What is the shape of cells in striated muscle?
Long, unbranched cylinder
39
What is the shape of cells in cardiac muscle?
Cells are shorter with branches that connect to adjacent cells
40
What is the shape of cells in smooth muscle?
Long unbranched cells that taper at either end
41
What is the organisation of contractile proteins inside the cell of striated muscle?
Organised into parallel bundles of myofibrils
42
What is the organisation of contractile proteins inside the cell of cardiac muscle?
Organised into parallel bundles of myofibrils
43
What is the organisation of contractile proteins inside the cell of smooth muscle?
Contractile proteins not organised into myofibrils
44
What is the distribution in the body of striated muscle?
Muscles attached to the skeleton
45
What is the distribution in the body of cardiac muscle?
Heart
46
What is the distribution in the body of smooth muscle?
Tubular structures e.g. blood vessels (arteries, arterioles and veins), airways, gut, Fallopian tubes (oviducts), uterus
47
What is the control of striated muscle?
neurogenic
48
What is the control of cardiac muscle?
myogenic
49
What is the control of smooth muscle?
neurogenic
50
Describe striated muscle
- This type of muscle makes up the many muscles in the body that are attached to the skeleton
- Striated muscle only contracts when it is stimulated to do so by impulses that arrive via motor neurones and muscle tissue like this is described as being neurogenic
51
What is myogenic?
The cardiac muscle in the heart is myogenic as it contracts and relaxes automatically with no need for impulses arriving from neurones
52
Where is smooth muscle found?
Throughout the body in areas such as in the gas exchange system alimentary canal and in the walls of arteries, arterioles and veins
53
How does most smooth muscle contract?
- When it receives impulses in motor neurones
- However smooth muscle in arteries also constricts when it is stretched by the pressure of blood surging through them and this happens without any input from the nervous system and this type of muscle is called smooth, because unlike the other two types of muscle tissue it has no striations
- Smooth muscle does NOT form smooth linings of tubular structure such as the trachea and arteries; the lining of these structures is always formed by an epithelium
54
What happens when a muscle contracts?
- It exerts a force on a particular part of the body, such as a bone
- This results in a particular response
- The enormous system ensures that the behaviour of each muscle is coordinated with all the other muscles, so that together they can bring about the desired movement without causing damage to any parts of the skeletal or muscular system
55
What is the sarcolemma?
The cell surface membrane of a muscle fibre
56
What is the sarcoplasm?
The cytoplasm of the muscle fibre
57
What are the transverse system tubules or T-tubules?
(also know as T-system tulle or T-tuble) infolding of the sarcolemma that goes deep into a muscle fibre
58
What are myofibrils?
One of many cylindrical bundles of this and thin filaments inside a muscle fibre
59
What is a muscle made up of?
1. A muscle such as a bicep is made up of thousands of muscle fibres
2. Each muscle fibre is a very specialised 'cell' with a highly organised arrangement of contractile proteins in the cytoplasm, surrounded by a cell surface membrane
3. Some biologists prefer not to call it a cell because it contains many nuclei
4. Instead they prefer the term syncytium to describe the multinucleate muscle fibre
60
What are the different parts of the muscle fibre?
- The cell surface membrane is the sarcolemma
- The cytoplasm is sarcoplasm
- The endoplasm reticulum is the sarcoplasmic reticulum (SR)
- The cell surface membrane has many deep infolding into the interior of the muscle fibre called transverse system tubules and these run close to the sarcoplasmic reticulum
61
What do the membranes of the sarcoplasmic reticulum have?
Huge numbers of protein pumps that transport calcium ions into the cisternae of the SR
62
What does the sarcoplasm have?
- It contains large number of mitochondria, often packed tightly between the myofibrils
- These carry out aerobic respiration, generating the ATP required for muscle contraction
63
How is a muscle organised?
1. An individual muscle is composed of hundreds of muscle fibres
2. Each muscle fibre is composed of many myofibrils
3. A myofibril has a distinctive banding pattern due to microfilmanets
64
What are the different types of muscle?
1. Voluntary muscle aka as striated or skeletal muscle (these muscles are neurogenic and so it only contracts when it is stimulated by impulses
2. Involuntary muscle aka smooth muscle, also neurogenic
3. Cardiac muscle is myogenic and so contracts without needing an impulse from neurones
65
What is voluntary muscle?
1. An individual muscle is made up of hundreds of muscle fibres
2. Each muscle fibre is bounded by an membrane called the sarcolemma
3. The sarcolemma has many deep infolding not the interior of the muscle fibre called transverse system tubules or T tubules
4. The endoplasmic reticulum inside the muscle 'cell' is called sarcoplasmic reticulum SR
5. The cytoplasm is called sarcoplasm
6. The T-tubules run close to the SR
7. The membranes of the SR have high numbers of proteins pumps that transport Ca2+ ions into the cisternae of the SR
66
Why does a muscle fibre have stripes?
- These are produced by a very regular arrangement of many myofibrils in the sarcoplasm
- Each myofibril is striped in the exact same way and is lined up precisely against the next one producing a pattern
67
What is a myofibril composed of using an electron microscope?
- Made up of filaments
- Parallel groups of thick filaments lie between groups of thin ones
- Both thick and thin filaments are made up of protein
68
What are thick filaments made up of?
Myosin
69
What are thin filaments made up of?
Actin
70
What is the A band? What colour is it?
- The whole length of myosin filament with the overlap of actin
- A dark band has both actin and myosin microfilaments
71
What is the H zone?
Consists of myosin only
72
What is the I band?
- Consists of actin only
| - Light band and has only actin microfilaments
73
What is the Z line?
Dark vertical lines through the centre of each actin filament
74
What is the M line?
Provides an attachment for the myosin filament
75
How do you define a sarcomere?
It is from 1 Z line to another (the part of a myofibril between two Z lines)
76
What is the shape of a myofibril?
Myofibrils are cylindrical in shape and so the Z line is in fact a disc seriating one sarcomere from another and is also called the Z disc
77
What happens to the lines and bands during muscle contraction?
1. I band is reduced as actin slides between myosin
2. A band remains the same size: myosin length is unchained
3. H one is reduced as overlap between actin and myosin increases
78
What happens to the lines and bands during muscle contraction?
1. I band is reduced as actin slides between myosin (decreased in length and disappear completely in fully contracted muscle)
2. A band remains the same size: myosin length is unchanged (stay the same length and appear darker due to the overlap)
3. H one is reduced as overlap between actin and myosin increases (decreased in length and disappear completely in fully contracted muscle)
79
Describe thick filaments
1. Thick filaments are composed of many molecules of myosin, which is a fibrous protein with a globular head
2. The fibrous portion helps to anchor the molecule into the thick filament
3. Within the thick filament many myosin molecules all lie together in a bundle with their globular heads all posting away from the M line
- Each myosin molecule, tail and two protruding heads
- Each thick filament, many myosin molecules
80
Describe thin filaments
1. The main component of thin filaments, actin is a globular protein
2. Many actin molecules are linked together to form a chain
3. Two of these chains are twisted together to form a thin filament
4. Also twisted around the actin chains is a fibrous protein called tropomyosin
5. Another protein, troponin is attached to the actin chain at regular intervals
- Two chains of actin filament twisted together to form a chain and this is held together by a protein called tropomyosin
- Another protein troponin is attached to the actin
81
What happens to the sarcomere is muscle contraction?
Decreases in length, the whole muscle shortens and pull on the bone
82
How do muscles cause movement?
- Muscles cause movement by contracting
| - The sarcomeres get shorter as the z discs are pulled closer together
83
Describe the muscle at rest
1. At rest actin myosin binding site is blocked by tropomyosin, held in place by troponin and myosin heads cannot bind to actin filaments
2. When the muscle is relaxed, tropomyosin and troponin are sitting in a position in the actin filament that prevents myosin from binding
84
What happens with the calcium ions?
1. Ca2+ binds to the troponin changing its shape which causes tropomyosin to unblock the binding site for the myosin head
2. Myosin head binds to the actin forming a cross bridge
85
Where does the energy for the the sliding filament model of muscle contraction start?
- The energy for the movement comes from ATP molecules that are attached to the myosin heads
- Each myosin head is an ATPase
86
What happens with the calcium ions?
1. When a muscles contracts Ca2+ ions are released from stores in the SR
2. Ca2+ binds to the troponin changing its shape which causes tropomyosin to unblock the binding site for the myosin head
- The troponin and tropomyosin proteins move to a different position on the thin filaments, so exposing parts of the actin mooches which act as binding sites for myosin
3. Myosin head binds to the actin forming a cross bridge
87
What happens once the cross bridge between the two types of filament is formed?
1. The myosin head bends, pulling the action filament along towards the centre of the sarcomere, overlapping more with the myosin
2. This is the power stroke releasing the ADP and Pi
88
What happens with free ATP?
1. Free ATP then binds to the myosin head and is hydrolysed providing enough energy to force the head to detach from the actin, and thus breaking the cross bridge
2. The head top back to their previous position and binds gains to another exposed site on the actin
- With continued stimulation the cycle is repeated
89
What happens if stimulation ceases?
- When there is no longer any stimulation from the motor neurone, there are no impulses conducted along the T-tubules
- Released from stilton, the calcium ion channels in the SR close and the calcium pumps move calcium ions back into stories in the SR
- As calcium ions leave their bonding sites on troponin, tropomyosin move back to cover the myosin binding sites on the thin filaments
1. If stimulation ceases Ca2+ is pumped back into sarcoplasmic reticulum
2. Troponin and tropomyosin return to original positions
3. Muscle fibre is relaxed
90
How does skeletal muscle contract?
1. Skeletal muscle contracts when it receives an impulse from a neurone
2. An impulse moves along the axon of a motor neurone and arrives at the presynaptic membrane
3. A neurotransmitter, generally acetylcholine, tissues across the neuromuscular junction and binds to receptor proteins on the postsynaptic membranes which is the sarcolemma (the cell surface membrane of the muscle fibre)
91
What does the binding of acetylcholine stimulate?
The ion channels to open, so that sodium ions enter to depolarise the membrane and generate an action potential in the sarcolemma
92
What happens to the impulses?
1. Impulses pass along the sarcolemma and along the T-tubules towards the centre of the muscle fibre
2. The membranes of the sarcoplasmic reticulum are very close to theT-tubules
3. The arrival of the impulses causes calcium ions channels in membrane to open
4. Calcium ions diffuse out down a very steep concentration gradient into the sarcoplasm surrounding the myofibrils
93
What do the calcium ions do?
1. The calcium ions bind with the troponin molecules that are part of the thin molecules, which cause
94
What happens when there are no cross-bridges between thick and thin filaments?
- The muscle is in a relaxed state
- There is nothing to hold the filaments together so nay pulling force applied to the muscle will lengthen the sarcomeres that they are ready to contract (and shorten again)
- Each skeletal muscle in the body has an antagonist (a muscle that restores sarcomeres to their original lengths when it contract e.g. the triceps is the antagonist of the biceps)
95
Summarise the power stroke
1. Myosin head groups attached to surrounding actin filaments forming cross bridge
2. Head group bends; thin filament moves along and overlaps thick filament more; this is the power stroke; ADP and Pi released
3. Cross bridge broke as now ATP attached to myosin head
4. Head group moves backwards as ATP is hydrolysed to ADP and Pi and then repeat 1 and 2
96
Why do muscles need ATP and where does it come from?
- A contracting muscle uses a lot of ATP and 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 reparation, both aerobic respiration inside the mitochondria and when that cannot be supplied fast enough, also by lactic fermentation in the sarcoplasm
97
What other source of ATP do muscles have? Where is it kept?
1. Muscles also have another sources of ATP; produced from as substance called creatine phosphate
2. They keep stores of this substance in their sarcoplasm
98
What is creatine phosphate?
-It is their intermediate source of energy once they have used the small quantity of ATP in the sarcoplasm
-A phosphate group can quckily and easily be removed from each creatine phosphate molecule and combined with ADP to produce more ATP
creatine phosphate + ADP --> creatine + ATP
99
What happens to the creatine later on?
1. Later when the demand for energy has slowed down or stopped, ATP molecules produced by respiration can be used to 'recharge' the creatine
creatine + ATP --> creatine phosphate + ADP
2. In the meantime, however if energy is still being demanded by the muscles and there is no ATP spare to regenerate the creatine phosphate, the creatine is converted to creatinine and excreted in urine
100
Summarise the events at the neuromuscular junction
1. An action potential arrives
2. The action potential causes the diffusion of calcium ions into the neurone
3. The calcium ions cause vesicles containing acetylcholine to fuse with the presynaptic membrane
4. Acetylcholine is released and diffuses across the synaptic cleft
5. Acetylcholine molecules bond with the receptors in the sarcolemma, causing them to open channel proteins for sodium ions
6. Sodium ions diffuse in through the open channels in the sarcolemma. This depolarises the membrane and initiates an action potential which spreads along the membrane
101
Summarise the events in the muscle fibre
7. The depolarisation got the sarcolemma spreads down T-tubules
8. Channel proteins for calcium ions open and calcium ions diffuse out of the sarcoplasmic reticulum,
9. Calcium ions bind to troponin. Tropomyosin moves to expose myosin-binding sites on the actin filaments. Myosin heads form cross-bridges with thin filaments and the sarcomere shortens
102
Why is myosin?
The contractile protein that makes up the thick filaments in striated muscle
103
What is actin?
The protein that makes up the thin filaments in striated muscle
104
What is a sarcomere?
The part of the myofibril between to Z discs
105
What is tropomyosin?
A fibrous protein they is part of the thin filaments in myofibrils in striated muscle
106
What is troponin?
A calcium-binding proton that is part of the thin filaments in myofibrils in striated muscle
Z A Level Biology 2
flashcards
Decks in class (18)
# Cards
-
Biological Molecules15
-
DNA31
-
The Mitotic Cell Cycle39
-
Inherited change 291
-
Enzymes38
-
Meiosis38
-
Inherited Change15
-
Practical Skills30
-
Selection and Evolution84
-
Photosynthesis47
-
Genetic Technology128
-
Coordination64
-
Coordination 2106
-
Coordination 364
-
Practical skills 225
-
Homeostasis94
-
Homeostasis 266
-
AS Experiments19
