L25 & 26: Neurotransmission Flashcards
1
Q
What are the two key components on neuronal tissue?
A
1) Neurones
2) Glial cells (neuroglia)
2
Q
Which component of neuronal tissue is excitable?
A
Neurones
3
Q
What is an axon? What are its functions?
A
Elongated part of neurone. Can be myelinated or non-myelinated.
Functions:
1) Transmits electrical information to effectors
2) Axon terminals release neurotransmitters
4
Q
What is the function of dendrites?
A
Receive impulses across a synapse or from stimuli.
5
Q
What is the role of glial cells?
A
Support and protection for other cells
6
Q
What are three functional types of neurones?
A
1) Sensory (afferent) neurons
2) Interneurons (association neurons)
3) Motor (efferent) neurons
7
Q
What is the role of afferent neurons?
A
Conduct signals from receptors to the CNS
8
Q
What is the role of efferent neurons?
A
Conduct signals from the CNS to effectors, such as muscles and glands
9
Q
What are the three structural types of neurones?
A
1) Multipolar
2) Bipolar
3) Unipolar
10
Q
What distinguishes the structural types of neurones from one another?
A
The number of processes coming directly off the cell body.
Multipolar: multiple processes coming off cell body. One axon and many dendrites
Bipolar: two processes coming off cell body, single dendrite which branches, and a single axon
Unipolar: just one process coming off cell body – this branches into axon and dendrites
11
Q
Which is the most abundant structural type of neurone?
A
Multipolar. This is a major cell type in the CNS.
12
Q
Which structural type of neurone is only common in dorsal root ganglia of the spinal cord and sensory ganglia of the cranial nerves?
A
Unipolar
13
Q
Which structural type of neurone is rare, found only in special sensory organs (olfactory mucosa, eye)?
A
Bipolar
14
Q
Fill in the gaps:
__1__ cells surround the cell body, __2__ cells surround peripheral axons (forming myelin sheath).
A
1) Satellite
2) Schwann
15
Q
Which neuroglial cells regulate the exchange of materials between the cell body and the interstitium?
A
Satellite cells
16
Q
__1__ cells myelinate peripheral axons, __2__ myelinate CNS axons.
A
1) Schwann
2) oligodendrocytes
17
Q
What is a neurilemma?
A
The outermost nucleated layer of a Schwann cell
18
Q
What is a node of ranvier?
A
A gap in the myelin sheath of a nerve, between two adjacent Schwann cells
19
Q
Define axoplasm
A
Cytoplasm of an axon
20
Q
Define axolemma
A
Plasma membrane of an axon
21
Q
What are the three properties of neuronal cells?
A
1) Excitability
2) Conductivity
3) Secretion
22
Q
What is meant by “neuronal cells are excitable”?
A
They are able to respond to stimuli
23
Q
What is meant by “neuronal cells are conductive”?
A
They are able to generate and propagate electrical signals known as action potentials
24
Q
What is meant by “neuronal cells can secrete”?
A
They are able to release chemical neurotransmitters that are used to bridge gaps at synapses between neurones
25
What is a resting potential?
The electrical difference across the membrane of a neuron
26
How is the resting potential generated?
Transport proteins (sodium-potassium pumps) move Na+ out of the cell and K+ into the cell at a ratio of 3 Na out for ever 2 K in. This means the cell is more positive outside than inside.
27
What is an action potential?
When a stimulus reaches a resting neuron, a generator potential is generated. If this reaches a threshold, the neuron transmits the signal as an impulse called an action potential.
28
When a signal from a stimulus is received, ions cross back and forth across the neuron's membrane, causing electrical changes (generator potential). How does a stimulus provoke this?
The stimulus causes sodium channels (gated ion channels) in the neuron's membrane to open, allowing the Na+ ions that were outside the membrane to rush into the cell. The inside of the cell becomes more positive.
29
What is an action potential threshold?
The generator potential from a stimulus must be strong enough to reach the threshold, resulting in the depolarisation of the cell. Otherwise, the action potential will not be transmitted.
30
What is the difference between generator potential and action potential?
Generator potential is the stationary depolarisation of a receptor that occurs in response to a stimulus. When a generator potential reaches the appropriate threshold, an impulse is fired along the afferent nerve, which is known as the action potential.
31
After being depolarised, how is the receptor repolarised?
Peak voltage of the action potential causes gated sodium channels to close and potassium channels to open. This means the sodium stays in the cell and the potassium rushed out, resulting in a polarisation that is opposite to the initial charge (resting potential) of the cell.
32
Why does the cell become hyperpolarised?
Overshoot: When the potassium gates finally close, there is more K+ outside than there is Na+ inside, meaning the inside charge drops below the resting potential.
33
What happens during the refractory period?
Sodium potassium pumps are activated again, returning the cell to it's resting potential.
34
# Fill in the gap:
The sodium-potassium pump moves sodium to the __?__ of the cell.
outside
35
# Fill in the gap:
The sodium-potassium pump moves potassium to the __?__ of the cell.
inside
36
# Fill in the gap:
During a resting potential, the cells cytoplasm is __?__ relative to the outside of the cell.
negatively charged
37
# Fill in the gap:
At the peak of action potential, the cell's cytoplasm is __?__ relative to the outside of the cell.
positively charged
38
On a graph showing the transmission of a nerve impulse, what is shown by the ascending limb of the action potential peak?
Depolarisation
39
On a graph showing the transmission of a nerve impulse, what is shown by the descending limb of the action potential peak?
Repolarisation
40
On a graph showing the transmission of a nerve impulse, what is shown by the shallow trough following the action potential peak?
Hyperpolarisation
41
Following an action potential, what is the name given to a cell's hyperpolarisation and subsequent return to resting potential, during which time the Na+/K+ pumps are reactivated?
Refractory period
42
How does an action potential propagate?
The local flow of current from AP region depolarises the adjacent membrane, which causes gated Na+ channels to open and triggers a new AP in that membrane. This then depolarises the next region of membrane, and so forth.
43
What ensures unidirectional (anterograde) flow of an action potential?
The refractory period of the preceding membrane means it cannot be depolarised. This prevents retrograde flow (back firing)
44
What factors affect the transmission speed of an impulse?
Fibre diameter
Extent of myelination
45
How does fibre diameter affect the transmission speed of an impulse?
Large fibres transmit impulses much quicker, due to the larger surface area.
46
How does myelination affect the transmission speed of an impulse?
Myelination speeds up transmission. Myelination produces saltatory conduction, which is up to 18 times faster than unmyelinated conduction.
47
Which of the following is likely to be the most efficient transmitter:
A) Small unmyelinated fibre
B) Large unmyelinated fibre
C) Small myelinated fibre
D) Large myelinated fibre
D) Large myelinated fibre
48
In salutatory conduction, where is the action potential generated?
At the nodes of Ranvier
| Fast Na+ diffusion occurs between nodes
49
What is a soma?
Cell body of a neuron
50
What is an axon hillock?
Connects soma (cell body) to axon
51
# Fill in the gap:
__?__ stimuli cause lower action potential firing frequency.
Weak
52
# Fill in the gap
__?__ stimuli cause higher action potential firing frequency.
Strong
53
Where is an impulse integrated?
At the soma (cell body)
54
Where is an impulse converted?
At the hillock
55
Where is an impulse conducted?
By the axon
56
How is an electrical signal propagated between two different neurones?
Presynaptic neurone passes the signal to the postsynaptic neurone via a chemical or electrical synapse.
57
What are the 3 types of synapse?
Axodendritic
Axosomatic
Axoaxonic
58
Which synapse is the quickest at transmitting an impulse?
Electrical synapse
59
How does an electrical synapse transmit the impulse?
The membranes of the pre- and post-synaptic cells are close together, separated by a gap junction. Electrical currents pass between cells directly through ion channels.
60
What type of synapse is found in retina, cardiac muscle intercalated discs, and in some areas of cerebral cortex?
Electrical synapse
61
Most synapses are chemical synapses. In chemical synapses, how is an impulse transmitted?
A chemical neurotransmitter is stored in vesicles in presynaptic neurones.
The neurotransmitter is released in response to an action potential.
The neurotransmitter diffuses across the synapse and can elicit a local potential (synaptic potential) in the postsynaptic neuron.
62
# Fill in the gap:
Synaptic transmission can be excitatory or __?__
Inhibitory
63
What determines whether a synaptic transmission is excitatory or inhibitory?
Type of neurotransmitter released, and the type of neurotransmitter receptor on the postsynaptic membrane
64
What is the functional class of acetylcholine?
Excitatory to vertebrate skeletal muscles; excitatory or inhibitory at other sites
65
What is the functional class of noradrenaline?
Excitatory or inhibitory
66
What is the functional class of dopamine?
Generally excitatory; may be inhibitory at times
67
What is the functional class of serotonin?
Generally inhibitory
68
What is the functional class of GABA (gamma aminobutyric acid)?
Inhibitory
69
What is the functional class of glycine?
Inhibitory
70
What is the functional class of glutamate?
Excitatory
71
What is the functional class of aspartate?
Excitatory
72
What is the functional class of Substance P
Excitatory
73
What is the functional class of Met-encephalin?
Generally inhibitory
74
What are EPSPs?
EPSPs: Excitatory Postsynaptic Potentials
75
What are IPSPs?
IPSPs: Inhibitory Postsynaptic Potentials
76
Give some examples of EPSPs
Acetylcholine
Noradrenaline
Dopamine
Substance P
Glutamate
Aspartate
77
Give some examples of IPSPs
Noradrenaline
Serotonin
GABA
Glycine
Met-enkephalin
78
What would be the neurotransmitter for an excitatory cholinergic synapse?
Acetylcholine (ACh)
79
What is the neurotransmitter used for an inhibitory GABAergenic synapse?
GABA
80
Whatg is the neurotransmitter used for an excitatory adrenergic synapse?
Noradrenaline
81
Explain how an impulse is transmitted across an excitatory cholinergic synapse.
1. Action potential (depolarisation of the membrane) opens calcium channels in presynaptic membrane.
2. Calcium floods into presynaptic cell, triggers release of ACh from vesicles.
3. ACh diffuses through presynaptic membrane into synaptic cleft.
4. ACh interaction in ACh receptors (Na+ channels) on postsynaptic membrane triggers Na+ influx.
5. This produces an EPSP, can become an action potential if threshold is reached (-55mV) in the postsynaptic neurone
82
What is the effect of a calcium channel blocker (e.g. conotoxin)?
Prevents calcium from penetrating presynaptic membrane. This means the ACh is not released and the nerve impulse fails to propagate across the synapse.
83
Explain how an impulse is transmitted across an inhibitory GABAergic synapse.
1. Action potential triggers release of GABA which crosses synapse
2. GABA receptors trigger opening of Cl- channels and Cl- influx which produces an IPSP
3. The postsynaptic neuron is now less likely to reach threshold and less likely to fire an AP
84
Explain how an impulse is transmitted across an excitatory adrenergic synapse.
Neurotransmitter is noradrenaline. Crosses the synapse and binds to NA receptor on the postsynaptic membrane. Receptor releases G protein, which converts ATP to cAMP (second messenger) which binds to ion channels (causing depolarisation), activates cytoplasmic enzymes and transcribes for new enzymes. This system allows amplification of the response (signal transduction cascade)
85
How can the signal stop?
1) Diffusion of neurotransmitter away into ECF
2) Reuptake of neurotransmitter by presynaptic neuron for recycling
3) Degradation of neurotransmitters by enzymes
86
What does acetylcholinesterase do?
Degrades ACh to acetyl and choline.
Choline is recycled via reuptake mechanisms to produce more ACh, by acetylation via Acetyl-CoA.
87
What effect do anticholinesterase inhibitors (acetylcholinesterase inhibitors) have?
prevent the degradation of Ach, thus increasing the level and duration of acetyl choline in the synapse.
88
Alzheimer's disease is caused by a shortage of cholinergic neurones. What might be a useful treatment?
Anticholinesterase inhibitor, as it increases the level and duration of acetylcholine
89
How are acetylcholinesterase inhibitors useful in the treatment of myasthenia gravis?
MG is caused by autoimmune block of ACh receptors, so it is helpful to increase the level and duration of ACh using anticholinesterase inhibitors
90
How does cocaine act on neurons?
It is a dopamine reuptake inhibitor and allows dopamine to accumulate in synaptic cleft in the CNS, producing an addictive CNS stimulant effect
91
How does fluoxetine (prozac) act on neurons?
It is a serotonin reuptake inhibitor potentiating the effect of released serotonin, causing downregulation of postsynaptic serotonin receptors, eventually stabilising mood.
92
What is synaptic delay?
Transmission delay owing to diffusion across the synaptic cleft
93
Explain the reason behind the following statement:
"The greater the number of synapses in a neural pathway, the slower the conduction of impulses to the final destination".
Each synapse causes a short transmission delay, owing to diffusion across the synaptic cleft. Each synapse adds to the cumulative delay.
94
What is synaptic fatigue?
Synapses show fatigue after prolonged repetitive activity. It is presumed that depletion of neurotransmitter stores is the cause.
95
What is neural integration?
The integration of information from synapses. It is based on the sum of the type, magnitude, and frequency of the presynaptic potentials.
96
What is temporal summation?
Intense stimulation by one presynaptic neuron causes firing of the postsynaptic neuron.
97
What is spatial summation?
Simultaneous stimulation by several presynaptic neurons causes firing of the postsynaptic neuron.
98
What is presynaptic inhibition?
Where a presynaptic neuron is suppressed by another presynaptic neuron.
E.g. Neurone 1 releases iGABA, preventing voltage-gated Ca2+ channels from opening in neurone 2. Neuron 2 releases less or no neurotransmitter.
99
How does the nervous system distinguish qualitative information? (How do we know whether the stimulus is a sound, touch, or taste?)
Depends on which neurons fire. The brain "knows" what type of sensory information travels on each fibre.
100
How does the nervous system gather quantitative information? (E.g. how strong a stimulus is)
1) Threshold value - different neurons have different thresholds. Weak stimuli on excite specific neurons.
2) Stronger stimuli cause a more rapid firing rate. CNS obtains stimulus strength from firing rate. Absolute refractory periods vary between neurons.
101
What is a diverging neural circuit?
Where one presynaptic cell synapses on multiple postsynaptic cells. Amplifies the signal, passes signal to multiple CNS regions
102
What is a converging neural circuit?
Multiple presynaptic cells input onto one postsynaptic cell. Allows effective stimulation/inhibition of postsynaptic cell. Allows single motor neurone to receive input from different brain regions.
103
What is a reverberating neural circuit?
Cells stimulated in linear sequence, but one cell restimulates the first cell to continue process. (e.g. breathing and short-term memory)
104
What is a parallel after-discharge neural circuit?
Input cell stimulates several pathways which all stimulate the output cell to fire at a very high frequency (sometimes for a longer duration that the original stimulus). (e.g. precise activities like calculations)
105
What is a motor unit?
A unit that comprises of individual motor neurons, their motor end plates and corresponding muscle fibres.
106
The axons of motor neurons branch into fibres, each ending with a motor end plate near the membranes of muscle fibres.
What neurotransmitter is used at synapses formed by the motor end plate?
Acetylcholine
107
# Fill in the gap:
"Somatic motor neurons are __?__ at their distal end"
branched
108
At neuromuscular junctions, how many ACh molecules bind to each NAChR?
2
109
When 2 ACh molecules to an NAChR, what is the result?
Nonspecific ion channel is opened and sodium ions flood into muscle fibre. Some potassium leaves, but the overall result is a local excitatory potential (known as the end plate potential (EPP).
110
How is the end plate potential propagated?
Via transverse T tubules
111
What does the end plate potential (EPP) cause?
EPP opens voltage-sensitive calcium channels in sarcoplasmic reticulum, releasing stored Ca2+ into sarcoplasm, causing muscle contraction.
112
How is a neuromuscular junction event stopped?
Acetylcholinesterase degrades ACh to stop signal
113
How can drugs affect neuromuscular junctions?
1. Inhibition of vesicular ACh release
2. Nicotinic receptor blockade (non-depolarising)
3. Nicotinic receptor blockade (depolarising)
114
What does botulinum toxin do?
Inhibits vesicular ACh release. It reduces muscle contraction/induces paralysis
115
What does botox do?
Inhibits vesicular ACh release. Used to reduce facial wrinkles. Used clinically to treat dystonia and similar disorders.
(Dystonia is a condition that causes muscle spasms and contractions)
116
What does curare (arrow poison) do?
Blocks nicotinic receptors, prevents ACh from acting. Causes paralysis and respiratory arrest.
117
Why is curare (a deadly paralytic poison) safe to eat? How must it be administered to have an effect?
Safe to eat because it cannot cross mucous membranes. It needs to be administered directly to the blood stream to bypass mucous membranes.
118
What does suxamethonium (sux) do?
Blocks nicotinic receptors, prevents ACh from acting. Used as a muscle relaxant in anaesthesia (intubation). Causes depolarisation, but is AChE-resistant. Ca2+ is initially released as sux depolarises the muscle.
Eventually Ca2+ is taken back up into sarcoplasmic reticulum. Ca2+ cannot be released again because nAChRs are blocked by Sux (ACh cannot bind) causing muscle flaccidity.
