6.5 Neurons and synapses Flashcards
1
Q
Diagram of the pain reflex
A
2
Q
Neuron is another name for a ___
A
Nerve cell
3
Q
How many neurons does the human body have?
A
Between 80 billion and 90 billion of them.
4
Q
Neurons transmit signals in the form of ___
A
Electrical impulses
5
Q
Diagram of a neuron
A
6
Q
What is the nervous system used for?
A
Communication throughout the body and communication within the brain that generates higher brain functions.
7
Q
What are the axons of some neurons coated with?
A
A myelin sheath
8
Q
What is a myelin sheath?
A
- An insulating layer that speeds up the transmission of a nerve impulse.
- This fatty layer is composed of compacted layers of the Schwann cell membrane, which is mostly lipid, but also contains several proteins.
- These play important roles in maintaining the structure and compaction of the myelin and adhesion of the sheath to the axon.
9
Q
What is the node of Ranvier?
A
A gap between the adjacent Schwann cells
10
Q
What is the difference between the speed of a nerve signal or action potential propagated along a myelinated axon vs. one that is not myelinated?
A
A nerve signal or action potential propagated along a myelinated axon can move at speeds of up to 120 m/s, whereas in the case of an axon which is not myelinated, the speed can be as slow as 1 m/s.
11
Q
What does it mean when we say that a signal is propagated?
A
It moves down the length of the axon towards the terminals.
12
Q
How does the myelin shealth speed up the rate of impulse transmission?
A
- The myelin sheath forces the nerve signal to jump from one node of Ranvier to the next, which accounts for the faster speed of impulse transmission.
- This is called saltatory conduction of nerve impulses.
13
Q
Diagram showing continuous (left column) versus saltatory (right column) conduction
A
14
Q
Where are the nucleus and most of the organelles in a neuron located?
A
In the cell body
15
Q
Nerve impulses are transmitted at a faster rate along myelinated neurons because ___
A
The impulse has to jump from one node of Ranvier to another due to the presence of the myelin sheath.
16
Q
In which direction are nerve impulses propagated along a neuron?
A
From cell body to axon terminal, where the signal may be sent onwards.
17
Q
What is the name of the structure labeled A in this diagram?
A
Axon
18
Q
What kind of gradient does a neuron have across its membrane?
A
A gradient of sodium (Na + ) and potassium (K + ) ions
19
Q
Describe the sodium and potassium gradient across a neuron’s membrane when it is not transmitting a signal
A
- When a neuron is not transmitting a signal/at rest, the resting potential is negative due to the accumulation of more sodium ions outside the cell than potassium ions inside the cell.
- There are also some proteins with a negative charge located inside the neuron.
20
Q
Diagram showing the resting potential of a neuron
A
21
Q
What is the resting potential?
A
The ion gradient causes an electrical imbalance between the inside and outside of the neuron, known as the resting potential.
22
Q
Is the membrane of an axon more permeable to sodium or potassium ions?
A
The membrane is more permeable to K + (potassium) ions than to Na + (sodium) ions.
23
Q
Explain the factors that contribute to a negative resting potential of −70 mV.
A
- A Na + /K + pump transfers Na + ions out of the cell and pumps K + ions back in.
- For every turn of the Na + /K + pump, three Na + ions are transferred to the outside, but only two K + ions are pumped back into the neuron.
- The combination of all of these factors results in the overall loss of positive ions from the neuron, which in turn contributes to the development of the negative resting membrane potential of −70 mV.
24
Q
Describe the methods of transport across the membrane of a neuron
A
- The sodium (Na + ) and potassium (K + ) ion channels are passive transport channels: the movement of the ions through these channels is driven by a concentration gradient.
- However, the sodium/potassium pump is an active transport protein, requiring energy in the form of ATP to move the ions against their concentration gradient.
25
How does the sending of electrical messages by neurons work?
- Neurons are not always at rest.
- When neurons send electrical messages, it occurs through a particular pattern of changes in the membrane potential.
26
When is a neuron depolarized?
- When it is stimulated.
- For example, when you touch something with your fingers, the dendrites of a neuron in the tips of your fingers will become depolarized.
27
What is depolarization caused by?
- The opening of Na + (sodium) channels, which allows the rapid influx of Na + ions (passive movement of Na + ions into the cell).
- Since there is a concentration gradient across the neuron membrane (there is a higher concentration of Na + ions outside), the change is rapid.
- The membrane potential of −70 mV changes quickly to a positive value of around +30 mV.
28
What happens once an area of the neuron has been fully depolarized?
- The change in potential causes voltage-gated K + channels to open.
- Voltage-gated means that the trigger to open the protein channels is a membrane potential of +30 mV.
- As a result of these channels opening, K + (potassium) ions that are at a higher concentration inside the neuron diffuse out and result in a decrease in membrane potential, a process called repolarisation.
29
Once the K+ ion channels open, how long do they stay open?
- The K+ channels remain open until the membrane potential becomes at least as negative as the resting potential.
- However, in many cases, the membrane potential becomes even more negative than the resting potential for a brief period (approximately 2 ms); this is called hyperpolarisation.
30
What is the reason for hyperpolarization?
Not all K + channels close immediately after the resting potential has been reached.
31
What happens right after hyperpolarization?
That part of the neuron enters a refractory period and cannot be depolarised (to generate an action potential) as its Na + channels are inactivated.
32
Diagram of the stages of the action potential and the order in which voltage-gated Na + and voltage-gated K + ion channels open and close.
Note that when a channel is closed in this figure, this is indicated by a v-shaped arrow, showing that an ion may try to pass through the channel, but is blocked because the channel is closed.
.
33
What is an osciloscope and what can it show?
An oscilloscope trace of a neuron shows a graph of the membrane potential values (in mV) as it would appear on the screen of an oscilloscope.
(An oscilloscope is a device that registers electrical voltages and displays them on a screen as time passes.)
34
What do you need to be able to annotate and analyze about an osciloscope?
- Annotate an oscilloscope trace to show the resting potential, action potential (depolarisation and repolarisation), threshold potential, and refractory period.
- Analyze oscilloscope traces showing the resting potential and action potentials (depolarisation and repolarisation).
35
Diagram of an oscilloscope trace showing resting potential and action potential.
36
What happens once a neuron is depolarised to about –50 mV?
The depolarisation will rapidly rise to +30 mV and the action potential will occur (that is, a nerve impulse is sent).
37
What is the threshold potential?
The –50 mV level.
38
What will any depolarization over the threshold potential cause?
- An action potential.
- This is called the all-or-nothing principle.
39
What happens if a neuron does not reach a threshold potential of around –50 mV?
The neuron will not depolarise enough to send an impulse.
40
What happens if a neuron reaches the threshold potential?
There is a positive feedback effect in the membrane, and nearby Na + channels open to further depolarise the neuron and instigate the action potential.
41
Define the threshold potential
- The minimum level to which a membrane potential must be depolarized to trigger an action potential.
- The threshold potential varies between –55 mV and –40 mV.
42
Where can an all-or-nothing effect be seen?
At synapses
43
Depolarization
Na + channels open, Na + diffuse to the inside of the neuron
44
Repolarization
Na + channels close and K + channels open, allowing K + to diffuse out.
45
All-or-nothing principal
Neurons need a threshold potential (around –50 mV), otherwise, the neuron cannot be depolarised.
46
For a neuron with a resting membrane potential at –70 mV, an increase in the net movement of Na+ into the neuron's cytoplasm would result in ___
The depolarization of the neuron.
47
During an action potential, what accounts for the depolarisation of the neuron?
Opening of voltage-gated Na+ channels.
48
During an action potential, what accounts for the repolarisation of the neuron?
Diffusion of potassium out of the neuron
49
On the oscilloscope trace shown, what is the correct label for the region labeled A?
Refractory period
50
What needs to be done after the depolarization of the axons?
In a neuron with a long axon, the nerve signal needs to be propagated from one end to the other end, where it can transfer information to another neuron via a synapse.
51
Diagram of the propagation of a nerve signal.
It is a section of an axon, where, to the left, it has a resting potential and, further to the right, it has become depolarised.
The signal will propagate to the right along the axon.
52
What happens when Na + from the part of the axon that was depolarised previously diffuses to the right? What then happens when it reaches the threshold potential? (look at diagram)
- When Na + from the part of the axon that was depolarised previously diffuses to the right, it makes the membrane potential in that area less negative.
- When the membrane potential in that area reaches the threshold potential, Na + channels open and in turn allow more Na + to flow into the cytoplasm of the axon and cause depolarization.
53
What effect does the increased sodium ion concentration in the cell have? (propagation of a nerve signal)
- It disturbs the concentration gradient that normally exists and establishes a new concentration gradient.
- On the inside of the cell, Na + concentration is high because it just diffused into the cell, therefore it will diffuse to the right and left inside the axon because that is where the Na + concentration is still low.
- On the left side of the axon, the Na + channels are inactive (refractory period) so no depolarization can occur.
- To the right, the increased sodium depolarises that part of the axon and creates a new action potential.
- This process repeats itself until the end of the axon is reached.
54
What are nerve impulses?
Action potentials propagated along the axons of neurons.
55
What causes the propagation of nerve impulses?
Local currents that cause each successive part of the axon to reach the threshold potential.
56
How is an action potential initiated?
Once depolarisation has started, an action potential will only be initiated if the threshold potential is reached.
57
What causes an action potential to be propagated along a neuron?
Sodium inside the neuron diffuses along the neuron, depolarising the neuron in that area until it reaches threshold potential.
58
What are synapses?
Connections between neurons and other neurons, neurons and glands, neurons and muscles, and neurons and sensory cells (receptors).
59
What does a synapse consist of?
A presynaptic neuron (before the synapse) and a postsynaptic neuron (after the synapse), with a gap of about 20 nm between them.
60
What does a synapse prevent?
The movement of a nerve signal from one neuron to another.
61
Diagram of the propagation of a nerve signal at a synapse
62
When does a nerve signal still need to pass across a synapse?
- For example, a signal from your big toe needs to travel to your brain to inform the brain that you have bumped into a chair.
- That signal must travel from neuron to neuron several times.
- So, how does an electric signal in a presynaptic cell ensure that another nerve signal is propagated in the postsynaptic neuron?
- The key is a chemical called a neurotransmitter that diffuses from the presynaptic neuron to the postsynaptic neuron.
63
Diagram of the 10 steps necessary to propagate a nerve impulse from one neuron to another neuron across a synapse
64
Steps 1-5 of the propagation of a nerve signal at a synapse
1) A neurotransmitter is synthesised and stored in vesicles of the presynaptic neuron.
2) Action potential reaches the presynaptic terminal.
3) Depolarisation of the presynaptic terminal causes the opening of the voltage-gated calcium (Ca 2 + ) channels.
4) Influx of Ca 2 + through channels.
5) Ca 2 + causes vesicles (containing neurotransmitters) to fuse with a presynaptic membrane.
65
Steps 6-10 of the propagation of a nerve signal at a synapse
6) Neurotransmitter is released from presynaptic neuron into the synaptic cleft via exocytosis.
7) Neurotransmitter binds to receptor molecules on the postsynaptic membrane.
8) Opening or closing of postsynaptic ion channels (for example if Na + channels are opened, Na + ions will enter the postsynaptic neuron and depolarise the neuron, possibly initiating an action potential).
9) Membrane potential of postsynaptic neuron changes (for example, depolarised if sodium channels are opened), which affects whether the postsynaptic cell may have an action potential.
10) Retrieval of vesicular membrane from the plasma membrane in the presynaptic neuron.
66
Does the neurotransmitter enter the presynaptic cell?
No, it causes ion channels to open.
67
Explain postsynaptic potentials
- Depending on which channels open, the neurotransmitter may cause the postsynaptic cell to be excited (depolarised, and closer to threshold) or inhibited (hyperpolarised, and farther from threshold potential).
- These changes in postsynaptic membrane potential are called postsynaptic potentials.
68
When many excitatory postsynaptic potentials occur in a neuron at once, ___
They may add up to initiate an action potential, and information has been passed from one neuron to the next.
69
Propagation of action potential across a synapse
The propagation of an action potential across a synapse involves the depolarisation of the presynaptic neuron, which, in turn, triggers a cascade of reactions that lead to the release of a neurotransmitter into the synapse.
70
How long does the propagation of a nerve impulse from one neuron to another take?
About 5-10 ms
71
What is the crucial factor in this transmission from one neuron to the next?
The neurotransmitter
72
How many different neurotransmitters are there found in the human body?
40
73
Give examples of different neurotransmitters often found in synapses connecting nerves and muscle fibres.
Acetylcholine and noradrenaline
74
What does acetylcholine consist of?
A choline and an acetyl group.
75
Explain how acetylcholine works
- In the presynaptic cell, choline and an acetyl group are combined by the enzyme choline acetyl transferase and stored in vesicles.
- When the acetylcholine is released into the synaptic cleft by exocytosis and diffuses across to the postsynaptic cell, it binds for a brief period of time to the receptors on the postsynaptic neuron.
- This causes sodium ion channels to open and the postsynaptic membrane to be depolarised. If threshold is reached, an action potential will occur.
76
What happens once the neurotransmitter acetylcholine is released?
- An enzyme called acetylcholinesterase breaks down the neurotransmitter into choline and acetate.
- The liberated choline is returned to the presynaptic neuron where it is coupled with another acetate to reform acetylcholine
77
Diagram of acetylcholine reabsorption and re-use
78
The secretion of acetylcholine by neurons at synapses by exocytosis ensures that ___
The action potential is propagated to the postsynaptic neuron.
79
The timely reabsorption of acetylcholine ensures that ___
The intensity and duration of the signal being sent is controlled.
80
What is the overall role of neurons and synapses?
Neurons transmit the message and synapses modulate the message.
81
What does modulating the message mean and how can synapses do this?
- This means controlling the intensity and duration of the signal across the synapse.
- This can occur through reabsorption of the neurotransmitter by the presynaptic neuron.
82
Give an example of a type of drug that modulates the message by controlling the intensity and duration of the signal across the synapse through reabsorption of the neurotransmitter by the presynaptic neuron.
Antidepressant drugs called selective serotonin reuptake inhibitors (SSRIs) act in this way.
83
Explain how SSRIs work
Antidepressant drugs called selective serotonin reuptake inhibitors (SSRIs) act in this way: they prevent presynaptic neurons from absorbing the neurotransmitter serotonin from the synapse, allowing it to affect the postsynaptic neuron for a longer period of time.
84
Give an example of how modulation can also occur when enzymes in the synapse break down a neurotransmitter
- Another older type of antidepressant, called monoamine oxidase inhibitors (MAOIs) modulate synapses in this way: monoamine oxidase is an enzyme that breaks down serotonin.
- When it is inhibited by these types of antidepressants the serotonin is able to affect the postsynaptic neuron for a longer period of time.
85
Neurotransmitters are released from axon terminals via ___
Exocytosis
86
Neural transmission across a synaptic cleft is accomplished by ___
Impulses causing the release of a neurotransmitter signal and its diffusion across the gap.
87
What is the function of acetylcholinesterase?
Remove acetylcholine from the synapse
88
What happens when a neurotransmitter binds to a receptor on a postsynaptic neuron?
An ion channel opens, allowing depolarisation or hyperpolarisation of the postsynaptic neuron.
89
What are neonicotinoids?
Derivatives of nicotine used as insecticides.
90
How do neonicotinoids work?
- They can bind to nicotinic acetylcholine receptors in cholinergic synapses (synapses that use acetylcholine as the neurotransmitter).
- Once a neonicotinoid enters the nervous system, it binds irreversibly with the receptors, preventing acetylcholine from binding.
- Additionally, acetylcholinesterase cannot break down these compounds.
91
What is the effect of neonicotinoids?
- The synaptic transmission is permanently blocked or, in other words, the nerve signals cannot be propagated to the postsynaptic nerve.
- When this happens in the brain, the result can be paralysis or even death.
92
Diagram of imidacloprid (an insecticide) binding to postsynaptic receptors.
93
Describe the effect of neonicotinoids on humans and other mammals vs. on insects
- In humans and other mammals, the binding of these compounds to the receptors is not as strong as in insects.
- Thus, humans and other mammals are less affected by these compounds than insects.
94
Why are neonicotinoids useful as insecticides?
Most of the cholinergic receptors in insects are located in the brain.
95
Give an exaple of a neonicotinoid that is used as an insecticide
Imidacloprid, which has an annual turnover of more than 1 billion dollars (US).
96
Describe the effects that neonicotinoids might be having on beehives and wild bee populations
- Worldwide, the loss of beehives and wild bee populations has seen an enormous rise since 2006.
- Since then scientists have been conducting research to find the causal factor of this population decrease.
- Recent research has suggested a potential toxicity of neonicotinoids in honeybees and other beneficial insects, even at low levels of exposure.
97
Why are humans less affected than insects by neonicotinoids?
Binding between neonicotinoids and postsynaptic receptors is not as strong in humans as it is in insects.
98
Name the receptors that neonicotinoid pesticides bind to in insects.
Cholinergic receptors/Acetylcholine receptors/Nicotinic receptors
