Neurons and that Flashcards
1
Q
What are neurons?
A
They are specialised cells that function to transmit electrical impulses within the nervous system
2
Q
How does the nervous system respond and detect stimuli?
A
It converts sensory information into electrical impulses in order to rapidly detect and respond to stimuli
3
Q
What are the three categories of roles for neurons?
A
Sensory, motor and relay
4
Q
What are the three basic components of neurons?
A
Dendrites, axon and soma
5
Q
What are dendrites?
A
Short branched fibres that convert chemical information from other neurons or receptor cells into electrical signals
6
Q
What is an axon?
A
An elongated fibre that transmits electrical signals to terminal regions for communication with other neurons or effectors
7
Q
What is a soma?
A
A cell body containing the nucleus and organelles. Where essential metabolic processes occur to maintain cell survival.
8
Q
What is a myelin sheath?
A
A fatty white substance which acts as an insulating layer surrounding the axon on some neurons
9
Q
What does the myelin sheath do?
A
The myelin sheath improves the conduction speed of electrical impulses along the axon, but requires additional space and energy.
10
Q
Where do electrical impulses travel from and to?
A
Electrical impulses travel from the dendrites to the axon terminal.
11
Q
How do neurons generate and conduct electrical signals?
A
Neurons generate and conduct electrical signals by pumping positively charged ions (Na+ and K+) across their membrane.
12
Q
What is a membrane potential?
A
A membrane potential is when the unequal distribution of ions on different sides of the membrane that creates a charge difference.
13
Q
What is a resting potential?
A
A resting potential is the difference in charge across the membrane when a neuron is NOT firing
14
Q
In a typical resting potential, what is happening on the inside of the neuron?
A
The inside of the neuron is more negative relative to the outside (approximately -70mV)
15
Q
True or false, the maintenance of a resting potential is a passive process?
A
False, it is an active process
16
Q
What does it mean if it is an active process?
A
It is ATP dependent
17
Q
What is the maintenance of a resting potential controlled by
A
Sodium potassium pumps
18
Q
What is a sodium potassium pump?
A
The sodium potassium pump is a transmembrane protein that actively exchanges sodium and potassium ions (antiport)
19
Q
How does the sodium potassium pump work?
A
It expels 3 Na+ ions for every 2 K+ ions admitted. (additionally, some K+ ions will leak back out of the cell
20
Q
What does the sodium potassium pump create?
A
The sodium potassium pump creates an electrochemical gradient whereby the cell interior is relatively negative compared to the extracellular environment.
21
Q
Why is the cell interior negative compared to the extracellular environment during resting potential?
A
There are more positively charged ions outside of the cell and more negatively charged ions inside the cell
22
Q
What is needed for the exchange of sodium and potassium ions and why?
A
The hydrolysis of ATP as it is an energy dependent process
23
Q
What are the ionic gradients for resting potentials? (Na+, K+, Cl-, A-)
A
Inside
- Na+ = 15mM
- K+ = 150mM
- Cl- = 10mM
- A- = 100mM
Outside
- Na+ = 150mM
- K+ = 5mM
- Cl- = 120mM
- A- = 0mM
24
Q
What are action potentials?
A
Action potentials are the rapid changes in charge across the membrane that occur when a neuron IS firing
25
What are the three stages of action potentials?
Depolarization, repolarization and a refractory period.
26
What is depolarisation?
Depolarisation refers to a sudden change in membrane potential from a relatively negative to positive internal charge.
27
How is depolarisation triggered?
It is triggered in response to a signal initiated at a dendrite which results in the sodium channels opening within the membrane of the axon.
28
What happens after the opening of sodium channels in depolarisation and why?
There is a passive influx of sodium when the sodium channels open due to the Na+ ions being more concentrated outside of the neuron.
29
What does the influx of sodium cause?
The influx of sodium causes the membrane to become more positive (depolarisation)
30
What is the mV before and after depolarisation
Before = -70mV
After = +30mV
31
What is repolarisation?
Repolarisation refers to the restoration of a membrane potential after depolarisation (i.e restoring a negative internal charge)
32
What does the influx of sodium result in, in repolarisation?
Following an influx of sodium, potassium channels open within the membrane of the axon.
33
What causes a passive efflux of potassium?
As K+ ions are more concentrated inside the neuron, opening potassium channels causes a passive efflux of potassium
34
What does the efflux of potassium cause?
The efflux of potassium causes the membrane potential to return to a more negative internal differential (repolarisation)
35
What is the mV before and after repolarisation?
Before = +30mV
After = -80mV
36
What is the refractory period?
The refractory period refers to the period of time following a nerve impulse before the neuron is able to fire again?
37
Where are sodium and potassium ions found in a normal resting state?
Sodium ions are predominantly outside of the neuron and the potassium ions are mainly inside. (Resting potential)
38
What happens to the ionic distribution in the refractory period?
The ionic distribution is largely reversed due to the sodium influx in depolarisation and the potassium efflux in repolarisation.
39
What must happen before a neuron can fire again?
Before a neuron can fire again, the resting potential must be restored via the antiport action of the sodium potassium pump.
40
What is the mV before and after the refractory period?
Before = -80mV
After = -70mV
41
What are nerve impulses?
Nerve impulses are action potentials that move along the length of an axon as a wave of depolarisation
42
When talking about nerve impulses, when does depolarisation occur?
Depolarisation occurs when ion channels open and cause a change in membrane potential
43
What is special about the ion channels that occupy the length of the axon?
They are voltage gated
44
How do voltage gated channels open?
They open in response to changes in membrane potential
45
Why in nerve impulses is depolarisation described as a wave?
Depolarisation at one point of the axon triggers the opening of ion channels in the next segment of the axon. This causes depolarisation to spread along the length of the axon.
46
What is the principle that action potentials are generated within the axon?
the all or none principle
47
What is required for an action potential to always occur?
1. The action potential must be of the same magnitude
2. A minimum electrical stimulus is generated (-55mV)
48
What is the minimum stimulus known as?
The threshold potential
49
What happens if the threshold potential is not reached?
If the threshold potential is not reached, an action potential cannot be generated and hence the neuron will not fire.
49
What is the threshold potential?
1. -55mV
2. The level required to open voltage gated ion channels
50
What triggers threshold potentials?
Threshold potentials are triggered when the combined stimulation from the dendrites exceeds a minimum level of depolarisation
51
How is the activation of voltage gated ion channels triggered in the next axon section?
If the overall depolarisation from the dendrites is sufficient to activate voltage gated ion channels in one section of the axon, the resulting displacement of ions should be sufficient to trigger the activation of voltage-gated ion channels in the next axon section.
52
What is an oscilloscope?
Oscilloscopes are scientific instruments that are used to measure the membrane potential across a neuronal membrane
53
How is data from an oscilloscope displayed?
Data is displayed as a graph, with time (in milliseconds) on the X axis and membrane potential (in millivolts) on the Y axis
54
How long will a typical action potential last for?
Roughly 3 - 5 milliseconds
55
What is the oscilloscope reaction to resting potential?
Before the action potential occurs, the neuron should be in a state of rest (approx. -70mV)
56
What is the oscilloscope reaction to depolarisation?
A rising spike corresponds to the depolarisation of the membrane via sodium influx (up to roughly +30 mV)
57
What is the oscilloscope reaction to repolarisation?
A falling spike corresponds to the repolarisation via potassium efflux (undershoots to approx. -80 mV)
58
What is the oscilloscope reaction to the refractory period?
The oscilloscope trace returns to the level of the resting potential (due to the action of the Na+/K+ pump)
59
How does the myelin sheath increase the speed of electrical transmissions?
Via saltatory conduction
60
What is the difference in how action potentials act on unmyelinated neurons and myelinated neurons?
Unmyelinated neurons
- action potentials propagate sequentially along the axon in a continuous wave of depolarisation
Myelinated neurons
- action potentials hop between the nodes of ranvier
61
What is myelin composed of?
Myelin is a mixture of protein and phospholipids that is produced by glial cells
62
What are the depolarized gaps in the myelin sheath called?
Nodes of Ranvier
63
Are myelinated or unmyelinated neurons have a faster speed of electrical conduction and by how much?
myelinated neurons are faster by a factor of up to 100 fold
64
What is an advantage and disadvantage of myelination?
Advantage - It improves the speed of electrical transmission via saltatory conduction
Disadvantage - It takes up significant space within an enclosed environment
65
What is the colours of the nervous system?
Myelinated axon tracts appear as white matter. The other areas appear as grey matter.
66
What does the grey matter of the nervous system consist of?
Grey matter consists of neuronal cell bodies and dendrites, as well as support cells (glial cells) and synapses
67
How do nerves transmit electrical impulses?
Nerves transmit electrical impulses by changing the ionic distribution across the neuronal membrane
68
When can electrical signals not be conducted?
When a semi-permeable membrane is absent
69
What do you call the physical gaps that separate neurons from other cells?
Synapses
70
How do neurons transmit information across synapses?
Neurons transmit information across synapses by converting the electrical signal into a chemical signal.
71
What is the process of synaptic signal transmission?
1. Action potential arrives at axon terminal
2.Voltage gated Ca+ channels open
3. Ca+ enters the presynaptic neuron
4. Ca+ signals to neurotransmitter vesicles
5. Vesicles move to the membrane and dock
6. Neurotransmitters released via exocytosis
7. Neurotransmitters bind to receptors
8. Signal initiated in postsynaptic cell
72
What are neurotransmitters?
Neurotransmitters are chemical messengers released from neurons and function to transmit signals across the synaptic cleft.
73
When are neurotransmitters released?
Neurotransmitters are released in response to the depolarisation of the axon terminal of the presynaptic neuron
74
What can a neurotransmitter do and how?
Neurotransmitters bind to a receptor on post-synaptic cells and can either trigger (excitatory) or prevent (inhibitory) a response
75
What response can a neurotransmitter trigger on a neuron?
Stimulation or inhibition of an electrical signal (nerve impulse)
76
What response can a neurotransmitter trigger on a glandular cell?
Stimulation or inhibition of secretion (exocrine or endocrine)
77
What response can neurotransmitters trigger on muscle fibre?
Stimulation or inhibition of muscular contraction/ relaxation
78
When and where is acetylcholine released?
1. It is released at neuromuscular junctions and binds to receptors on muscle fibres to trigger muscle contraction.
2. It is released within the autonomic nervous system to promote parasympathetic responses.
79
How is acetylcholine created?
Acetylcholine is created in the axon terminal by combining choline with an acetyl group. (this is derived from mitochondrial Acetyl CoA)
80
Where is acetylcholine stored and why?
Acetylcholine is stored in vesicles within the axon terminal until released via exocytosis in response to a nerve impulse
81
How is a post-synaptic cell activated?
Acetylcholine activates a post-synaptic cell by binding to one of two classes of specific receptor
82
Why must acetylcholine be continually removed from the synapse?
Otherwise it causes overstimulation which can lead to fatal convulsions and paralysis
83
What is acetylcholinesterase and what does it do?
Acetylcholinesterase is a synaptic enzyme breaks down acetylcholine into its two component parts
84
Where is acetylcholinesterase released/embedded?
Acetylcholinesterase is released from the presynaptic neuron or is embedded on the membrane of the post-synaptic cell.
85
What happens to the choline liberated from acetylcholinesterase?
The liberated choline is returned to the presynaptic neuron where it is coupled with another acetate to reform acetylcholine.
86
Where do neurotransmitters bind to neuroreceptors?
Neurotransmitters bind to neuroreceptors on the post-synaptic membrane of target cells and open ligand-gated ion channels
87
What are graded potentials?
Graded potentials are when the opening of ligand gated ion channels cause small changes in membrane potentials
88
What does excitatory neurotransmitters cause and how?
Excitatory neurotransmitters cause depolarisation by opening ligand-gated sodium or calcium channels.
89
What does inhibitory neurotransmitters cause and how?
Inhibitory neurotransmitters cause hyperpolarisation by opening ligand-gated potassium or chlorine channels.
90
What determines if a threshold potential is reached?
The combined action of all neurotransmitters acting on a target neuron determines whether a threshold potential is reached
91
When will a neuron fire?
A neuron will fire if overall there is more depolarisation than hyperpolarisation and a threshold potential is reached
92
When will a neuron not fire?
A neuron will not fire if overall there is more hyperpolarisation than depolarisation and a threshold potential is not reached.
93
What is the threshold required to open voltage gated ion channels in a typical neuron?
approximately -55mV
94
What is a receptor?
A receptor is a protein macromolecule which an agonist molecule has to combine to initiate a response
95
How long do ligand gated ion channel receptors take to respond?
milliseconds
96
What is a synaptic transmission?
How neurons communicate via the chemical transfer of an impulse from one neuron to another
97
What type of channel is a GABAa receptor?
The GABAa Receptor is an Anion channel
98
What happens to adrenoceptors?
They are bound and activated by the neurotransmitters/hormones adrenaline and noradrenaline
99
What is the physiological effect of activation of a1?
Vasoconstriction of blood vessels
100
What is the physiological effect of activation of a2?
Presynaptic inhibition of noradrenaline in the CNS relaxation
101
What is the physiological effect of activation of b1?
Increased heart rate and cardiac muscle contraction?
102
What is the physiological effect of activation of b2?
Dilation of the bronchi and increase heart rate and cardiac muscle contraction (lesser extent than b1)
103
What is the physiological effect of activation of b3?
Thermogenesis in skeletal muscle, lipolysis
104
Tell me about adrenaline?
Binds/activates all adrenoceptors causing a full sympathetic physiological response
105
Tell me about isoprenaline
Binds/activates b1 and b2 adrenoceptors causing tachycardia (big side effect) and bronchodilation
106
Tell me about salbutamol
Binds/activates b2 adrenoceptors causing bronchodilation. This is the desired therapeutic effect for asthma.
Salbutamol is indicated to treat the acute symptoms of asthma
107
What are the five stages of GPCRs?
1. A hormone/neurotransmitter outside the cell binds to the GPCR on the cells surface
2. This binding changes the shape of the GPCR
3. The GPCR then activates a G protein inside the cell
4.The activated G protein splits into parts that can move and activate other proteins in the cell
5. These activated proteins trigger various responses inside the cell, like making new molecules or changing cell behaviour
108
What is the name of the cell found on the axon?
Schwann cell
109
When will action potentials stop?
When synapse is reached
