1
Q
In animals what are the two types of information transfer that are used to coordinate the body’s activities?
A
- Nerves that transmit information in the form of electrical impulses
- Chemical messenger, hormones that travel in the blood
2
Q
What is the mammalian nervous system made up of?
A
- The brain and the spinal cord, which form the central nervous system (CNS)
- The cranial and spinal nerves, which form the peripheral nervous system (PNS)
3
Q
Describe neurones
A
- Cranial nerves are attached to the brain and spinal nerve to the spinal cord
- Information is transferred in the form of nerve impulses, which travel along neurones at every high speeds
- Neurones carry information directly to they target cells
- Neurones coordinate the activities of sensory receptors such as those in the eye, decision-making centres in the CNS and effectors such as muscles and glands
4
Q
What are the three types of neurone and their function?
A
- Sensory neurones: transmits impulses from receptors to the CNS
- Intermediates neurones (relay or connector): transmits impulses from sensory neurones to motor neurones
- Motor neurones: transmits impulses from the CNS to effectors
5
Q
Describe the structure of a motor neurone
A
- The cell body of a motor neurone lies within the spinal cord or brain
- The nucleus of the neurone is always in its cell body
- Dark specs seems in the cytoplasm are small regions of rough ER that synthesise proteins
- Thin cytoplasmic processes extend form the cell body and some are very short and often have many branches (dendrites)
- The axon is much longer and conducts impulses over long distances
6
Q
Why does a motor neurone have many highly branched dendrites?
A
To give a large surface area for the endings of other neurons
7
Q
What is an axon?
A
A long cytoplasmic process of a neurone
8
Q
What is a dendrite?
A
A short cytoplasmic process of a neurone that receives nerve impulse from other neurones
9
Q
What is in the cytoplasm of an axon (motor)?
A
- Within the cytoplasm of an axon there are some organelles such as mitochondria
- The ends of the branches of the axon have large numbers of mitochondria, together with many vesicles contained chemicals called transmitted substances
- These vesicles are involved in passing impulses to an effector cell such as a muscle cell or a gland
10
Q
Describe the structure of a sensory neurone
A
- Same basic structure as motor neurone
- But has one long axon with with a cell body that may be near the source of stimuli or in a welding of spinal nerve know as a ganglion
11
Q
Describe relay neurones
A
Found entirely within the CNS
12
Q
How are axons protected?
A
- For most of their length, the axons of motor and sensory neurones are protected within nerve
- Some surrounded by thick dark rings which is myelin which are made by specialised cells Schwann cells that surround the axons of some neurones
13
Q
What is myelin?
A
A substance that surrounds many axons, made up of many layers of the cell surface membranes of Schwaan cells
14
Q
What are Schwann cells?
A
A cell which is in close association with a neurone, whose cell surface membrane wounds around and around the axon of the neurone to form a myelin sheath
15
Q
Do all axons have myelin?
A
- Not all axons are patented by myelin, ones which aren’t are unmyleinated neurones
- About two-thirds of our motor and sensory neurones are unmyleinated
16
Q
How is mylein made?
A
- Myelin is made when Schwann cells wrap themselves around the axon all along its length
- The Schwaan cell spirals around, enclosing the axon in many layers of its cell surface membrane
- This enclosing sheath, called the myelin sheath is made largely of lipid together with some proteins
- The sheath affects the speed of conduction of the nerve impulse
17
Q
What are the small uncovered areas of the axon between Schwann cells called?
A
- Nodes of Ranvier
- They occur about every 1-3mm in human neurone and the nodes themselves are very small around 2-3 mum long
18
Q
What are nodes of Ranvier?
A
A short gap in the myelin sheath surrounding an axon
19
Q
What is a reflex arc?
A
The pathway taken by an action potential leading to a reflex action: the action potential is generated in a receptor, passes along a sensory neurones into the brain or spinal cord and then along a motor neurone to an effector
20
Q
What is a relfex action?
A
A fast, automatic response to a stimulus; reflex actions may be innate (inborn) or learned (conditioned)
21
Q
Describe a reflex arc
A
- A reflex arc is the pathway along which impulses are transmitted from a receptor to an effector without involving ‘conscious’ regions of the brain
- Some reflexes may have no relay neurone and the impulses passes directly from the sensory to motor neurone
- There are also reflex arcs in the brain egg, those controlling focusing and how much light energy the eye
22
Q
What happens in a reflex arc?
A
- Within the spinal cord, the impulse will also be passed on to other neurones which take the impulse up the cord to the brain
- This happens at the same time as impulses are travelling along the motor neurone to the effector
- The effector therefore responds to the stimulus before there is any voluntary response to involving the conscious regions of the brain
- This type of reaction to stimulus is called a reflex action - It is a fast automatic response to a stimulus; the response to each specific stimulus is always the same
- Reflex actions are a very useful way of responding to danger signals such as the touch of a very hot object on your skin or the sight of an object flying towards you
23
Q
Describe the transmission of nerve impulses
A
- Neurones transmit electrical impulses
- These impulses travel very rapidly along the cell surface membrane from one end of the cell to the other and are NOT a flow of electron like an electric current
- Rather the signals are very brief changes in the distribution of electrical charge across the cell surface membrane called action potentials caused by the very rapid movement of sodium ions and potassium ions into and out of the axon
24
Q
What is an action potential?
A
- A brief change in the potential difference across cell surface membranes of neurones and muscle cels caused by the inward movement of sodium ions, followed by the outward movement of potassium ions; it rapidly travels along the length of the neurone
- A rapid and fleeting change in potentials difference across the membrane
25
What is the Na/K pump?
1. The inside of a resting axon has a slightly negative electrical potential compared with the outside
2. Active transport creates a concentration gradient across the cell membrane for Na + and K + ions
3. This pump removed 3 Na+ from the cell for every 2 K + ions it brings into the cell
4. K+ diffuses back out again much faster than Na+ diffuses back in
5. Therefore an overall positive change outside the membrane compared with the negative charge inside
26
How is the resting potential maintained?
1. Na+ are pumped out of the axon
2. At the same time, K+ are rough into the cytoplasm
3. Both processes involved moving the ions against their concentration gradients
4. The Na/K pump removes 3 Na+ from the cell for every 2 K+ it brings into the cell
5. Moreover K+ diffuse back out faster than Na+ diffuse back in
6. There is an overall excess of positive ions outside the membrane compared with the inside
27
What is the potential difference usually?
- 60mV and -70mV as the electrical potential of the inside of the axon is between 60 and 70MV lower than the outside
- This difference is the resting potential
28
What is the resting potential?
The difference in electrical potential that is maintained across a neurone when it is not transmitting an action potential; it is normally about -70mV inside and maintained by the sodium potassium pump
29
Describe the sodium potassium pump
1. The resting potential is produced and maintained by the sodium-potassium pumps in the cell surface membrane
2. These constantly move sodium ions, Na+ out of the axon and potassium ions K+ into the axon
3. The sodium potassium pumps are membrane proteins that use energy from the hydrolysis of ATP of move both of these ions against their concentration gradients
4. Three sodium ions are removed from the axon for every two potassium ions brought in
30
What are the protein channels for sodium and potassium like?
1. The membrane has protein channels for potassium and for sodium which are open all the time
2. There are far more of these for potassium than for sodium
3. Therefore, some potassium diffuses back out again much faster than sodium diffuses back in
4. In addition there are many large negatively charged molecules inside the cell that attract the potassium ions reducing the chance that they will diffuse out
- The result of these effect is an overall excess of negative ions inside the membrane compared with outside
- The membrane is relatively impermeable to sodium ions but there are two things that influence the inward movement of sodium ions during an action potential
- There is a steep concentration gradient, and also the inside of the membrane is negatively. charged which attracts positively charged ions
- A 'double' gradient like this is called an electrochemical gradient
31
What is an electrochemical gradient?
A gradient across a cell surface membrane that involves both a difference in concentrations of ions and a potential difference e.g. the entry of sodium ions into neurones
32
What is depolarisation?
The reversal of the resting potential across the cell surface membrane of a neurone or muscle cell, so that the inside becomes positively charged compared with the outside
33
When does depolarisation occur?
- If something happens to reduce this difference in charge across the membrane, then the neurone is said to be depolarised
- Depolarisation can be caused, by a change in temperature, change in light intensity, change of chemicals in the air
34
What are voltage dependent channels?
- These channels have gates that open and close in response to changes in membrane potential
- A voltage gated channel is a channel protein through a cell membrane that opens or closes in response to changes in electrical potential across the membrane
35
Describe depolarisation
1. A stimulus causes the opening the voltage gated channels in the cell surface membrane of the axon which allow sodium ions to pass through
2. As there is a much greater concentration of sodium ions outside the axon that inside, the sodium ions enter through the open channels
3. To begin with only a few channels open and this changed the pd across the membrane which becomes less negative on the inside
4. This depolarisation triggers some more channels to open so that more sodium ions enter
5. There is more depolarisation, and if the pd reaches -50mV then many more channels open and the inside reaches a potential of +30mV compared with the outside
- As Na+ continue to flood in, the inside of the axon continues to build up positive charge until it reaches a potential of +40mV compared to the outside
36
Why is depolarisation an example of positive feedback?
A small depolarisation leads to a greater and greater depolarisation
37
What do action potentials need to be generated?
- Action potentials are only generated if the potential difference reaches a value between -60mV and -50mV and this value is the threshold potential
- If it is less than this, then an action potential does not occur
38
What is repolarisation?
Returning the potential difference across the cell surface membrane of a neurone or muscle cell to normal following the depolarisation of an action potential
39
Describe repolarisation
1. After about 1ms, all the sodium ion voltage gated channels close, so sodium ions stop diffusing into the axon
2. At the same time, the potassium ion channels open and potassium ions therefore diffuse out of the axon, down their concentration gradient
3. The outward movement of potassium ions removes positive charge from inside the axon to the outside, thus retuning the potential difference to normal (-70mV), this is called repolarisation
40
What actually happens in repolarisation?
- The potential difference across the membrane briefly becomes even more negative than the normal resting potential
- The potassium ion channels then close and the sodium ion channels become responsive to depolarisation again
- The sodium potassium pump continues to pump sodium ions out and potassium ions in and this helps to minutia the distribution of sodium ions and potassium ions across the membrane so that many more action potentials can occur
1. So many K+ leave the axon that the potential difference across the membrane briefly becomes even more negative than the normal resting potential (hyperpolarisation)
2. The K+ channels then close and the Na/K pumps begins to act again, restoring the normal distribution of Na+ and K+ across the membrane and therefore restoring the resting potential
41
What is the refractory period?
A period of time during which a neurone is recovering from an action potential, and during which another action potential cannot be generated
42
Describe the transmission of action potentials
1. Once an action potential is generated, it sweeps along the cell membrane
2. When Na+ have rushed in to a depolarised region of the neurone, they are attracted sideways, as the regions on either side of the depolarised area have a more negative charge
3. This depolarises these region and so the action potential travels along the neurone in theory
43
Describe what happens during the action potential
1. An action potential at any point in an axon's cell surface membrane triggers the production of an action potential in the membrane on either side of it
2. The temporary depolarisation of the membrane at the site of the action potential causes a 'local circuit' to be set up between the depolarised region and the resting regions on either side of it
- Na+ flow sideways inside the axon, away from the charged region towards the negatively charged regions on either side
3. This depolarises these adjoining regions and so generates an action potentials on them
44
Why are new action potentials only generated ahead and not behind?
1. In practice a new action potential is only generated ahead of but not behind it
2. This is because the region behind it will still be recovering from the action potential it has just has and its distribution of Na+ and K+ ions is not yet back to normal and the sodium ion voltage gated channels are shut and cannot be stimulated open, however great the stimulus
45
What are the consequences of refractory periods?
- The axon is unresponsive in this period
- Action potentials are discrete events; they do not merge into one another
- There is a minimum time between action potentials occurring at any one place on a neurone
- The length of the refractory period determines the maximum frequency at which impulses are transmitted along neurones
46
What is all or nothing?
- The action potentials remain exactly the same size as it travels along the neurone
- When a resting neurone is depolarised the depolarised is either enough to produce a full sized action potential or not big enough to produce any action potential at all
- You cannot have a big or small action potential
- You either have a standard one or not one at all
- Action potentials all have the same amplitude
47
What is the speed of transmission in an axon?
In any one axon the speed of axon potential transmission is always the same
48
What makes action potentials different?
The difference about the action potentials resulting from a strong and a weak stimulus is their frequency
49
What does a strong stimulus do?
- Produces a rapid succession of action potentials, each one following along the axon just behind its predecessor
- Likely to stimulate more neurones that a weak stimulus
- The sensory neurone is activated and transmits impulses to the CNS
50
What does a weak stimulus do?
- Results in fewer action potentials per second
- It may result in action potentials passing along just one or two neurone, a strong stimulus could produce action potentials in many more
- The cells are not depolarised very much and the sensory neurone is not activated to send impulses
51
How does myelin affect speed of conduction?
- Mylien speeds up the rate at which action potential travel by insulting the axon membrane (0.5 compared to 100ms-1)
- Sodium and potassium ions cannot flow through myelin sheath so action potentials can only occur at the nodes of Ranvier where all the channel proteins and pumps proteins are concentrated
52
What is saltatory conduction?
- Conduction of an action potential along a myelinated axon, in which the action potential jumps from one node of Ranvier to the next
- Along a myelinated axon the local circuits exist from one node to the next so action potentials jump from one node to the end (1-3mm)
- This type of conduction can increase the speed of transmission by up to 50 times that in an unmyleinated axon of the same diameter
53
How does diameter affect the speed of transmission?
1. Thick axons transmit impulses faster than thin ones, as their resistance is much less
54
What is an example of unmyleinated and myleinated axons?
- Earthworms have no myelinated axons, have a small number of very thick unmysleinated ones that run all along their body
- A bird pecking at an earthworm that has just emerged from its burrow acts as a stimulus to set up impulses in these giant axons, which travel very quickly along the length of the body, stimulating muscles to contract
- The rapid response may help the earthworm to escape down into its burrow
55
What is a receptor cell?
- A cell which is sensitive to a change in the environment and that may generate an action potential as a result of a stimulus
- Can be specialised cells e.g. chemoreceptors in the taste buds which detect a specific type of stimulus and influence the electrical activity of a sensory neurone, rods and cones
- Found in sense organs
- Other receptors e.g. touch receptors are found at the ends of sensory neurones
56
What are receptor cells?
Transducers, converting energy in one from e.g. light heat or sound into energy in an electrical impulse in a neurone
57
What is a chemoreceptor?
A receptor cell that responds to chemical stimuli; chemoreceptors are found in the taste buds on the tongue, in the nose and in blood vessels where they detect changes in oxygen and carbon dioxide concentrations
58
What is receptor potential?
A change in the normal resting potential across the membrane of a receptor cell, caused by a stimulus
59
Describe the tongue
- Covered with papilla and with each taste bud between 50 and 200 receptor cells that are sensitive to chemicals in liquids and food that dissolve in saliva
- Each chemoreceptor in the taste buds is covered with receptor proteins that detect these different chemicals
- There are several different receptor proteins each detecting a different type of chemical and giving us a different sensation
60
Describe what happens on the tongue with salt
- Stimulus: sodium chloride in food
1. Entry of sodium ions depolarises the membrane in the sensory zone
2. This stimulates calcium ion channel proteins to open
3. Entry of calcium ions stimulates movement of vesicles and release of neurotransmitter by exocytosis
4. If stimulation is above threshold, impulses travel to the brain along the sensory neurone
61
What also happens with salt on the tongue
- Chemoreceptors in the taste buds that detect salt are directly influenced by sodium ions
1. Sodium ions diffuse through highly selective channel proteins in the cell surface membrane of the microvilli and this leads to depolarisation of the membrane
2. The increase in positive charge inside the cell is the receptor potential
3. If there is sufficient stimulation by sodium ions in the mouth then the receptor potential becomes large enough to stimulate the opening of voltage gated calcium ion channels
4. Calcium ions enter the cytoplasm and lead to exocytosis of vesicles contains neurotransmitters from the basal membrane
5. The neurotransmitter stimulates an action potential in the sensory neurone that transmits impulses to the taste centre of the cerebral cortex of the brain
62
How do other chemoreceptors work in the taste buds?
1. Other chemoreceptors in the taste buds use different methods
2. The cells that are sensitive to sweet have protein receptors that stimulate a G protein, which citrates an enzyme to produce many molecules of cyclic AMP
3. Cyclic AMP acts as a second messenger activating a cascade to amplify the signal leading to the closure of potassium ion channels
- This also depolarises the membrane
63
Describe the structure of a motor neurone
- Nucleus of neurone in cell body
- Dendrites
- One long axon
- Many mitochondria in cell body
- Much RER in cell body
- Synaptic knobs
- Schwaan cells
- Nodes of Ranvier
64
Describe and explain the transmission fo an action potential in a myelinated sheath
1. Na+ channels open
2. Na+ enters the cell
3. Inside pd becomes less negative
4. Na+ channels close
5. K+ channels open
6. K+ moves out of cell
7. Inside pd becomes less negative
8. Local cities
9. Myelin sheath insulate / prevent ion movement
10. Action potential/ depolarisation only at nodes of Ranvier
11. Saltatory conduction/ action potential jumps from node to node
12. One way transmission
13. Refractory period
Z A Level Biology 2
flashcards
Decks in class (18)
# Cards
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Biological Molecules15
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DNA31
-
The Mitotic Cell Cycle39
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Inherited change 291
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Enzymes38
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Meiosis38
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Inherited Change15
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Practical Skills30
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Selection and Evolution84
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Photosynthesis47
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Genetic Technology128
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Coordination64
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Coordination 2106
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Coordination 364
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Practical skills 225
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Homeostasis94
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Homeostasis 266
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AS Experiments19
