Week 11 Flashcards
1
Q
Neuronal function most directly depends on a changeable permeability to which of the following?
A
Ions
2
Q
Membrane channels which are selective (only allow passage of one type of ion) and have two alternatives, i.e. open and closed, are called ______ _________ channels.
A
Blank 1: gated
Blank 2: ion
3
Q
A potential difference exists across every cell’s plasma membrane. The side of the membrane exposed to the cytoplasm is the _________ charged pole, while the side exposed to the extracellular fluid is the _________ charged pole.
A
Blank 1: negatively or negative
Blank 2: positively or positive
4
Q
The resting membrane potential of a neuron is a _________
number, implying that the inner part of the cell is more negative than the outer part of the cell.
A
Blank 1: negative
5
Q
The imbalance created by the sodium-potassium pump and ion channels for Na+ and K+ are two factors that help in explaining which of the following?
A
The inside of a cell is more negatively charged than the outside.
6
Q
When a neuron is stimulated(n) _________
charge is carried by ions because cells are aqueous solutions.
A
Blank 1: electric
7
Q
The sodium-potassium pump allows ________
K ions to enter into the cell for every _________
Na ions that are taken out of the cell.
A
Blank 1: 2 or two
Blank 2: 3 or three
8
Q
Ion channels in the membrane that are open all the time are called _____ channels.
A
Blank 1: leakage
9
Q
The resting potential of many vertebrate neurons ranges from ________
mV to ______
mV.
A
Blank 1: -40
Blank 2: -90
10
Q
A normal vertebrate neuron’s resting potential is a negative value for which of the following reasons?
A
The inside of the cell is negative with respect to the outside.
11
Q
Why is the inside of a neuron more negatively charged than the outside environment?
A
The sodium-potassium pump establishes and maintains a difference in ion concentrations across the membrane
Certain types of ion channels in the cell membrane are more abundant than others
12
Q
Which of the following illustrates how the sodium-potassium pump maintains an electrochemical gradient?
A
The sodium-potassium pump establishes concentration gradients for Na+ and K+ by transporting Na+ out and K+ into the cell.
13
Q
Which of the following are proteins that enable the diffusion of specific ions via pores that span the membrane?
A
Ion channels
14
Q
Why is the cell membrane more permeable to K+ than to Na+?
A
Because K+ ion channels are more numerous in the cell membrane
15
Q
The _________
membrane potential of a neuron arises due to the action of the sodium-potassium pump and the differential permeability of the membrane to Na+ and K+ due to ion channels.
A
resting
16
Q
Why is the sodium-potassium pump important?
A
Because it contributes to the long-term maintenance of resting potential
Because it establishes a concentration gradient
17
Q
The equilibrium potential of the membrane in a neuron arises from a balance between the ________ force and the ___________ force as they relate to K+ ions.
A
diffusional force and the electrical force
18
Q
_______ _______ are membrane proteins which form pores through the cell membrane and allow for the diffusion of ions across the membrane.
A
Blank 1: Ion
Blank 2: channels
19
Q
In order to determine the equilibrium potential for an ion at any given concentration, a scientist would use the ________
equation.
A
Blank 1: Nernst
20
Q
The neuron membrane is
A
more permeable to K+ because it contains more ion channels for K+.
21
Q
The resting membrane potential of a neuron can change in response to a stimuli resulting in two types of potential. Choose the two types of potentials from the list below.
A
Graded potentials
Action potentials
22
Q
Which of the following are most important to the resting potential of a neuron?
A
The differential permeability of the membrane to Na+ and K+
Actions of the sodium-potassium pump
23
Q
Small changes in the membrane potential that do not spread far from the site of the stimuli are known as which of the following?
A
Graded potentials
24
Q
Which of the following helps to establish ion concentration gradients and maintain resting membrane potential?
A
The sodium-potassium pump
25
The __________
potential for an ion is reached when a balance between electrical and diffusional forces along the cell membrane for the ion exists.
Blank 1: equilibrium
26
The balance between the diffusional force and the electrical force across the plasma membrane produces an _______ _________ , which can be measured by using the Nernst equation.
equilibrium potential
27
Small changes in membrane potential caused by the opening and closing of gated ion channels are called ______
Graded potentials
28
When a neuron's resting membrane potential is changed in response to stimuli, two types of changes can be observed: ________
potentials and ________
potentials.
Graded potentials
Action
29
Graded potentials are small continuous changes in the
membrane potential.
30
Why is the sodium-potassium pump important?
Because it contributes to the long-term maintenance of resting potential
Because it establishes a concentration gradient
31
Choose the two types of gated ion channels.
Voltage gated
Chemically gated
32
The activation of which of the following causes small changes in membrane potential known as graded potentials?
Gated ion channels
33
Permeability changes of the membrane are measurable as ________
or ____________
of the membrane potential.
Blank 1: depolarizations
| Blank 2: hyperpolarization
34
When a neuron is stimulated, gated membrane channels
Open
35
The two types of gated channels in membranes are _______ gated and ________ gated.
Blank 1: chemically or ligand
| Blank 2: voltage
36
A ligand-gated channel is one that is regulated by
the binding of small molecules such as neurotransmitters.
37
Changes in the ____________
| of the membrane are measurable as depolarization and hyperpolarization.
Permeability
38
Graded potentials are small continuous changes in the
membrane potential.
39
________ occurs when the membrane becomes less negatively polarized. ___________
occurs when the membrane becomes more negatively polarized.
Blank 1: Depolarization
| Blank 2: Hyperpolarization
40
Which of the following describes gated-membrane channels when a neuron is stimulated electrically or chemically?
They are open.
41
A dendrite may make connections with several axons. Depolarizations and hyperpolarizations of the dendrite membrane caused by signaling molecules produced by the connecting axons are added together. This phenomenon is called ______.
summation
42
A ______ gated channel is one controlled by the binding of small molecules such as hormones and neurotransmitters.
Blank 1: ligand or chemically
43
A change in the membrane potential from -70 mV to -75 mV is a
Blank 1: hyperpolarization
44
The ability of graded potentials to combine is called
Blank 1: summation
45
The action potential has three phases: a ____ phase, a _____
phase, and an ______ phase.
Blank 1: rising
Blank 2: falling
Blank 3: undershoot
46
A ligand-gated channel is one that is regulated by
the binding of small molecules such as neurotransmitters.
47
When a neuron _________, it gets closer to the threshold potential, whereas __________
moves a neuron farther away from the threshold potential.
Blank 1: depolarizes
| Blank 2: hyperpolarization
48
What prevents the membrane potential from reaching +60mV during the rising phase of an action potential?
The inactivation gate of the Na+ closes.
49
The rising phase, falling phase, and undershoot phase are the three phases of a(n)
action potential
50
Which of the following are part of the falling phase of an action potential?
The membrane becomes repolarized
K+ diffuses out of the cell
The K+ channel opens
51
A ___________ gated channel is one controlled by the binding of small molecules such as hormones and neurotransmitters.
Chemically or ligand
52
During the production of an action potential, the activation of a sodium voltage-gated ion channel results in which of the following?
Influx of Na+ into the cell
53
To return a voltage-gated Na+ channel to the resting state, the membrane must be
depolarized.
54
A(n) _________ potential is a membrane potential that can open voltage-gated Na+ channels causing depolarization of the cell and triggering a(n) _______
potential.
Blank 1: threshold
| Blank 2: action
55
Which of the following are part of the rising phase of an action potential?
The Na+ channel rapidly opens
The membrane potential shifts towards the equilibrium potential for Na+
56
The membrane of a neuron is able to repolarize after voltage-gated K+channels open for which of the following reasons?
The efflux of K+ is able to counteract the effects of the Na+ channel.
The high concentrations of K+ inside the cell causes an efflux.
57
During the falling phase of an action potential, the opening of the K+ channels causes which of the following to occur?
K+ to diffuse out of the cell
58
When the threshold voltage is reached, the _______
| gate of the voltage-gated sodium channels open and Na+ flows into the cell.
Blank 1: activation
59
All of the following are true for a voltage gated Na+ channel returning to its resting state except which one?
Opening of activation gate
60
A membrane potential that is large enough to open voltage-gated Na+ channels and trigger an action potential is called a(n) __________
potential.
threshold
61
Which of the following leads to termination of the rising phase?
The inactivation gate of the Na+ channel closes
62
The major effect that occurs when the activation gate of the K+ channel opens is which of the following?
Repolarization of the membrane
63
The termination of the ______ phase of an action potential is achieved by the closing of the Na+ channel inactivation gate.
rising
64
The opening of the activation gate of the K+ channel causes a ______
of the cell membrane as a result of an _______
of K+ ions.
Blank 1: repolarization or repolarizing
| Blank 2: efflux, exit, or departure
65
A voltage-gated Na+ ion channel in its resting state has its ______
gate open, and the ________
gate closed.
Blank 1: inactivation
| Blank 2: activation
66
When the threshold voltage is reached, the ______
| gate of the voltage-gated sodium channels open and Na+ flows into the cell.
Blank 1: activation
67
Action potentials are separate, all-or-none events that result from which of the following?
The passive diffusion of ions
68
The major effect that occurs when the activation gate of the K+ channel opens is which of the following?
Repolarization of the membrane
69
The inactivation gate is open and the activation gate is closed during the resting state for membrane channels called ______ -gated _________
ion channels
Blank 1: voltage
| Blank 2: sodium, Na+, or Na
70
During the _______ refractory period, stimulation produces action potentials of reduced amplitude. During the _______ refractory period, the membrane cannot be stimulated.
Blank 1: relative
| Blank 2: absolute
71
Which of the following describes the events of action potentials?
They are all or none events.
They are separate events.
72
Which of the following explains why action potentials do not add together?
After firing, Na+ channels are in an inactivated state.
73
The termination of the ________
| phase of an action potential is achieved by the closing of the Na+ channel inactivation gate.
Blank 1: rising
74
The generation of a(n) _______ potential causes changes in the cytoplasm such that there is a little more Na+ and a little less K+ than at rest.
action
75
While active transport is important to maintain the cell membrane ion gradient, only passive transport in the form of ion diffusion is required for _______
potentials.
Blank 1: action
76
The membrane
(can/cannot) be stimulated during the absolute refractory period of an action potential. In contrast, the membrane
(can/cannot) be stimulated during the relative refractory period.
Blank 1: cannot
| Blank 2: can
77
The nature of ______ potentials is such that they do not add together or interfere with one another, as _______ potentials do.
action
| Blank 2: graded
78
The _______ __________ generated by a membrane channel do not back up because after firing, the Na+ channels remain in an inactivated state and are refractory to stimulation.
Blank 1: action
| Blank 2: potentials
79
At the end of an action potential, the cytoplasm contains a little (more/less) K+ and a little
(more/less) Na+ than it did at rest.
Blank 1: less
| Blank 2: more
80
Which of the following is most responsible for action potentials?
The passive diffusion of ions
81
Which of the following best approximates the amplitude of an action potential at the end of an axon?
The same amplitude as the first action potential
82
An action potential is propagated along the axon pathway by which of the following?
Recreating the action potential in adjacent stretches of axon membrane
83
Why does an action potential signal propagate along the length of an axon (without backing up)?
Because the Na+ channels that have just fired are still refractory to stimulation
84
If an axon is increased in diameter or if it is myelinated, then there will be an increase in the ________ of nerve impulses.
velocity
85
While active transport is important to maintain the cell membrane ion gradient, only passive transport in the form of ion diffusion is required for _______
potentials.
action
86
Which of the following describes how an axon's diameter affects the rate of the action potential?
Axons with larger diameters will have faster moving action potentials
87
An _______ ________ that originates at an axon base is recreated along the membrane of an axon pathway until the end of the axon is reached.
Blank 1: action
| Blank 2: potential
88
The amplitude of an action potential is (increasing, decreasing, constant) as it travels down an axon.
Blank 1: constant
89
The _____ _______ generated by a membrane channel do not back up because after firing, the Na+ channels remain in an inactivated state and are refractory to stimulation.
Blank 1: action
| Blank 2: potential
90
Axons whose diameters are ________ have less resistance to current flow because electrical resistance is inversely proportional to cross sectional area.
Blank 1: larger, wider, or bigger
91
Which of the following factors can increase the velocity of a nerve impulse?
Increasing the diameter of an axon
Myelination of the axon
92
Compared to axons with smaller diameters, the speed of action potentials in axons with larger diameters will be
Greater
93
Which of the following would conduct action potentials faster?
Myelinated axons
94
An action potential is propagated along the axon pathway by which of the following?
Recreating the action potential in adjacent stretches of axon membrane
95
Larger diameter axons have less resistance to current flow because electrical resistance is in ______
proportion to cross sectional area.
Blank 1: inverse
96
Action potentials are produced only at the nodes of Ranvier when the axons are which of the following?
Myelinated
97
Unmyelinated axons conduct action potentials
| (faster/slower) than myelinated axons.
Slower
98
_________ conduction occurs in myelinated axons when Na+ move into the cell and the charge moves quickly through the cytosol to the next node, continuing the _______ potential.
Blank 1: Saltatory
| Blank 2: action
99
Action potentials seem to jump from one node of Ranvier to the next due to which of the following?
Depolarization spreads quickly below the insulated myelin.
100
Axons whose diameters are ______ have less resistance to current flow because electrical resistance is inversely proportional to cross sectional area.
larger
101
In myelinated axons, action potentials are produced only at the ______ of ______.
Blank 1: nodes
| Blank 2: Ranvier
102
conduction is a type of conduction in myelinated axons in which impulses appear to jump from one Node of Ranvier to the next.
Saltatory
103
The rapid spreading of the _________
stimulus beneath the myelin insulation creates the impression that action potentials jump from node to node in myelinated axon.
Blank 1: depolarization
104
What is the brain?
• The centre of the nervous system
• Centralised control of the sensory, somatosensory, and autonomic nervous systems
• Acts through control of muscle activity (neural) and release of hormones (chemical)
2% mass and 20% energy.
Brains carry out vitally important functions but a theme I want to develop today is that they are energetically costly.
105
What is the difference between Sensory and somatosensory?
Sensory – specific organ (like taste) vs. somatosensory – all over the body
106
Why does an animal need a brain?
* Compare 3 organisms: 2 unicellular and 1 multicellular.
* Point of comparison is to look at the sorts of challenges each of these organism's face when they are interacting with their environment.
* This is not meant to represent an evolutionary transition from one organism to the next.
107
What is Somatosensory?
denoting a sensation such as pressure, pain or temperature which can occur anywhere in the body.
108
How does Computation without a brain in E. coli?
Start with the smallest: E.coli. Bacteria that lives in the intestines (helps with the production of vitamin K).
Daily task: find food.
Loves glucose but it will consume lactose (if it is present). Each of which requires a different biochemical pathway to breakdown.
Challenge is to only produce the proteins for digestion that are necessary.
E.coli doesn’t need a brain to make this decision and can rely on biochemistry.
I won’t go over this in detail but I am sure that most of you remember that in the presence of glucose the bacteria will only transcribe and translate proteins for the metabolism of glucose; when there is no glucose and lactose present, the bacteria will transcribe and translate proteins for the metabolism of lactose.
It does not need a brain to do this and it uses the very famous lac operon region of its genome – it can compute with biochemistry.
109
Basically, we end up with a situation like this where the lac operon is capable of
coding for the correct production of proteins for the relevant sugar. Again, it does this using biochemistry.
110
What do E.coli rely on?
* E. coli very small and can rely on biochemistry because it is tiny and lives in an equally tiny world.
* Small world that it encounters is likely to be very similar and predictable.
111
What happens when organisms get bigger?
* As organisms get bigger, chemistry starts too not be so effective.
* We can see this in paramecium – still unicellular but 300,000 x bigger and experiences a much bigger world.
112
What does paramecium activate?
Paramecium also active foragers and are looking for bacteria to absorb and digest.
They have an escape mechanism that is important because they live in a bigger area and will likely face more threats vs bacteria.
113
How do paramecium move?
They use cilia to move which are small hair like projections.
paramecium are very fast at moving.
This response is too quick for chemical signaling as we will see.
114
How long does chemical signalling in E.coli take?
o Chemical signaling in E. coli only takes 4ms to cross the whole cell; however, in paramecium it takes 40s which is way to slow for an escape response to happen.
115
What does E.coli use?
• They use electrical signaling. They use mechanoreceptors which detect stretch etc. and they use these to detect other paramecium and potential threats. When these mechanoreceptors are activated, they open and allow the influx of positively charged ions into the cell which causes depolarisation like what we saw with action potentials. This causes the cilia to beat and move the paramecium = escape response
116
Compare chemical vs. electrical signaling
• As organisms get bigger, electrical signaling becomes more and more useful and dominant because it is much quicker. However, if we think about mammal nervous systems, they rely on both chemical and electrical signaling. They typically use chemical signaling, for example hormones, to cause slower changes and fast electrical signaling when the message needs to be delivered quickly – like the reflex responses we saw in the recorded material.
117
Why is electrical signalling so expensive?
Long term maintenance and regulation of ion concentrations on the inside and outside of the cell - Na+/K+/ATPase pumps – 50% in mammalian brain.
118
What is chemical communication?
cheap, simple, slow.
| works for short distances, slow messages.
119
What is electrical communication?
very fast, needs more ‘kit’, energetically expensive. good for fast messages over large distances.
120
What are C.elegans?
Nervous system has 2 important roles:
Sense the environment
Execute decision by controlling other neurons
Critical role of a brain:
To have centralized control over the actions of the body to ensure it is not in danger.
Still very small (compared to us) but way bigger than paramecium.
121
Outline some features of multicellular organisms?
a multicellular organism with a brain of 302 neurons and 959 somatic cells. Lives close to the soil surface and feeds on bacteria in vegetable matter. World = more complex – needs a more complex repertoire of sensory responses = needs to forage effectively.
122
Are C.Elegans multicellular or unicellular?
• C.Elegans is multicellular so unlike our other 2 examples, it has to coordinate its responses across multiple cells.
123
Unlike E. coli and paramecium, C.elegans encounter what?
encounters a world where predators, changes in humidity, very low O2 levels (hypoxia) etc all tell the C.elegans it needs to escape.
124
Who has a brain?
Building blocks of NS are found in some very simple organisms.. E.g. Na+ channels found in Choanoflagellates.
Some sponges have the ability to propagate electrical impulses and a very recent finding, which I have added as a post lecture read found that sponges may have cells that might be the precursors of neurons – this was a novel, and very exciting finding. Then there is jelly fish + box jellies have very simple nerve nets which we looked at in the offline material.
A nerve net is where neurons are present and connected to one another without central coordination (no brain).
Within the group bilateria that brains probably first evolved – bilateria (organism with bilateral symmetry).
All of these invertebrates and vertebrates must share a common ancestor that had the elements of a brain.
125
Why do brains get bigger?
* As bodies get bigger = increase in foraging area (generally speaking) and so does the lifespan of the organism.
* In general, this means that as animals get bigger and covering more distance, they are encountering and dealing with more and more sensory information in their environments.
* This information can come from the external and internal environment – bigger animal means more internal cells that need to be maintained.
126
What does a longer lifespan mean?
• A longer lifespan means more opportunities for forming memories and the need to be more flexible with behavioral strategies.
127
What happens as the inhabited area increases?
As the inhabited area increases the bigger the brain and the lifespan also increases. More important to have memories, otherwise it may but organisms in danger. E.g. areas with low oxygen or predators.
128
What do pretty much all multicellular animals have?
Pretty much all multicellular animals have a nervous system and this helps them to control and coordinate their bodies and interact with their environment.
129
What information do sensory systems gather?
Sensory systems gather info about the surrounding and also the internal environment. Key point here is that they tell the animal about changes in the external and internal environment.
130
What is the role of motor systems?
Motor systems act on muscle to bring responses and movements. They act to initiate changes in behaviour. In the middle there is a central processing unit, and that interprets sensory information and relays that information to the motor system.
131
How are nervous systems organised?
Stimuli--- sensory-- central processing--- motor-- muscles(movement)
132
What is the structure of a basic neuron?
Cell body- the cell's life-support center.
Dendrites- receive messages from other cells.
Axon- passes messages away from the cell body to other neurons,muscles and or glands.
Neural impulse- electrical signal travelling down the axon.
Myelin sheath- covers the axon of some neurons and helps speed neural impulses.
Axon terminals.
133
What is the Ionic basis when measuring membrane potential?
- --Difference in ion concentrations between inside and outside
- --Differences in membrane permeability to ions
--Salts exist as ions in aqueous solution
134
How does potassium move in the neuron?
∴ two forces: concentration pulls K+ out negative charge pulls K+ in
Equilibrium where these are in balance
135
How do ions move across the membrane?
1. Permeability
- > Cell membrane is impermeable to ions
- > Ions travel through protein pores (passive movement)
- > Protein pores determine the permeability of the membrane
- > Leaky K⁺ channels determine the RMP
- > Moment by moment regulation
TAKE HOME
-> K⁺ > Na⁺ leaky channels = resting membrane more permeable to K⁺
136
What is the process of active pumping in ion movement?
- > Long term regulation
- > Sets up the potential difference across the membrane
-> Not what maintains concentrations gradients moment by moment
137
What happens during Hyperpolarization?
- --Less K+ leaves the cell
- --K+ prefers more negative
- --Less + ve will be lost from the cell
- --Return to RMP
138
What happens during depolarisation?
- --Less electrical gradient bringing K+ back into the cell
- --Less + ve coming into the cell
- --Return to RMP
139
What is a passive process?
Hyperolarization and depolarisation
140
What is a neurons mode of communication?
Nerve impulses/action potential/spikes are neurons mode of communication
141
What does a nerve impulse result in?
(= action potential = spike)
142
What happens in the absolute refractory period?
- -Na+ permeability increases (channels open)
| - - Na+ channels close (inactivate)
143
What happens in the relative refractory period?
- --K+ permeability increases (channels open)
| - -K+ channels close
144
What are the key features of an axon?
Self sustaining
Refractory – inactivation of Voltage gated sodium channel and opening of K+ channels
Bi-directional
Leaky – leaky channels are always open
145
What does the axon do in response to a signal?
the soma end of the axon becomes depolarized.
146
Once the soma end of the axon becomes depolarised what happens?
The depolarisation spreads down the axon. Meanwhile, the first part of the membrane repolarises. Because Na+ channels are inactivated and additional K+ channels have opened the membrane cannot depolarize again.
147
During saltatory conduction what does the nodes of ranvier contain?
high density of voltage gated sodium channels.
148
Where do opposite charges exist?
Opposite charges exist either side of membrane to produce resting membrane potential
149
What is action potential?
Action potential is the brief reversal of the charges due to activation of voltage-gated Na+ channels - depolarisation
150
What is repolarisation due to?
Repolarisation due to inactivation of Na+ channels and activation of voltage-gated K+ channels
151
What makes an action potential travel?
Action potential travels due to internal +ve charge triggering Na+ channels ahead of action potential
152
How do the voltage gated Na+ channels function?
At resting potential- closed but capable of opening.
Open activated- from threshold to peak potential (-50 mV to +30mV).
Closed and not capable of opening (inactivated)- from peak to resting potential (+30mV to -70mV).
153
Where is the dendrites found?
axodendritic synapse.
| dendrodendritic
154
Where is the cell body (soma) found?
axosomatic synapse
155
Where is the axon terminal found?
axoaxonic synapse
156
What happens during temporal summation?
- --In temporal summation postsynaptic potentials at the same synapse occur in rapid succession.
- --Can occur with IPSP's as well as with EPSP's.
157
What is spatial summation?
- ---In spatial summation multiple postsynaptic potentials from different synapses occur about the same time and sum.
- --EPSP's and IPSP's will also add up to cancel each other's effects
- --The EPSP's of an individual synapse are often not strong enough to generate an AP
158
Outline the sliding filament theory.
- > Muscle contractions controlled by Ca2+
- > Tropomyosin blocks the cross bridge binding site on actin
- > Ca2+ binds to troponin
- > Releases tropomyosin
- > Exposes actin binding site
- > Myosin can bind to actin binding site – cross bridge formation
159
How are muscle contractions controlled in vertebrates?
Vertebrates
Action potential in motoneuron axon
Neurotransmitter release from nmj (acetylcholine)
[Muscle cells are electrically excitable; have resting potential]
Excitatory junction potential (ejp) in muscle cell
[= end plate potential (epp)]
Action potential in muscle cell
Travels along Transverse tubules (T tubules)
Release of Ca2+ from sarcoplasmic reticulum
Initiates contraction
Ca2+ brings about “excitation-contraction coupling”
= Transduction stage
160
What is the function of the motor neurons in vertebrates?
Many motoneurons for a whole muscle
| Each motor neuron innervates only one muscle fibre.
161
What is recruitment?
Recruitment:
- -Weak contraction – few motoneurons (motor units);
- -Strong contraction – many motoneurons (motor units).
162
What is the size principle?
‘Size principle’: small motoneurons recruited before larger ones
163
How are muscles contraction controlled in invertebrates?
---Action potential in motoneuron axon
---Neurotransmitter release (glutamate)
---Excitatory junction potential in muscle cell
[Often no impulse in muscle cell]
- --Release of Ca2+ from sarcoplasmic reticulum
- --Initiates contraction
164
Outline what happens in the insect flight muscle.
- -Typically, one motor neuron impulse produces one contraction.
- -- Most insects can only contract muscles up to ~100 Hz
- -Wing beat frequencies can be >1000 Hz
- --This is a higher frequency than motor neurons can fire impulses
- --Some invertebrate striated muscle can contract independently, not synchronised to motor neuron impulses.
- --Wing muscles can contract more frequently than motor neurons fire impulses.
165
What are specializations do Cnidarians e.g. Hydra & Aglantha have?
Specializations:
sensory neuron groupings
axon tracts – groups of axons
giant axons – large diameter and fast impulses for escape response
166
outline the properties in the Platyhelminthes e.g. Planaria
-> Brain at the front of the body – formed by collections of neurons
-> Clearly defined nerve cord: collections of axons (tracts) and cell bodies (ganglia)
-> Nerve cord allows the brain to coordinate movement
-> Brain + ventral nerve cord =
Central Nervous System (CNS)
```
CNS = motor neurons and interneurons
PNS = Sensory neurons
```
167
What special properties do arthropods have?
```
Specializations:
specialized sensory organs
body becoming more condensed
regional specialization
local control
giant axons
```
168
PNS components include bundles of axons called _______ and groups of neuron cell bodies outside the CNS called ______
Blank 1: nerves
| Blank 2: ganglia
169
The function of the _______ nervous system is to carry information into and out of the ________ nervous system.
Blank 1: peripheral, PNS, or pns
| Blank 2: central, CNS, or cns
170
Spinal nerves separate into _______ and ______
| components at the spinal cord.
Blank 1: sensory
| Blank 2: motor
171
Sensory neurons enter the ______ surface of the spinal cord while motor neurons leave from the ______ surface of the spinal cord.
Blank 1: dorsal
| Blank 2: ventral
172
Cell bodies of ______
neurons are located outside the spinal cord but cell bodies of _________
neurons are located inside the spinal cord.
Blank 1: sensory
| Blank 2: motor
173
Nerves and ganglia are the 2 components that make up the _______
nervous system.
Blank 1: peripheral
174
Which system routes information to and from the brain and spinal cord?
Peripheral nervous system
175
At the ______ _______, spinal nerves are separated into sensory and motor components.
spinal cord
176
The two neurons involved in the efferent motor pathways of the sympathetic and parasympathetic divisions of the autonomic nervous system are _______
neurons and ________
neurons.
Blank 1: preganglionic
| Blank 2: postganglionic
177
The axons of ______
neurons enter the dorsal surface of the spinal cord, while the axons of ______ neurons exit from the ventral surface of the spinal cord.
sensory
| motor
178
In the PNS, the medulla oblongata, sympathetic division, and parasympathetic division are components of the ______
nervous system.
autonomic
179
Cell bodies of sensory neurons are located ______
the spinal cord, while cell bodies of motor neurons are located ______
the spinal cord.
Blank 1: outside
| Blank 2: inside, in, or within
180
The cell body of which of the following is located in the CNS and its axon synapses with the autonomic ganglion?
Preganglionic neuron
181
The axon of this neuron synapses with a smooth muscle, cardiac muscle, or gland cell. Its cell body is located in the autonomic ganglion.
Postganglionic neuron
182
The lumbar and thoracic regions of the spinal cord are the originating points for the preganglionic neurons of the _______division of the autonomic nervous system.
Blank 1: sympathetic
183
The efferent motor pathway of both divisions of the autonomic nervous system involve two neurons that are called which of the following?
Postganglionic neuron
Preganglionic neuron
184
Postganglionic neurons in the parasympathetic division of autonomic nervous system release ________
at their synapses.
Blank 1: ACh or acetylcholine
185
Components of the autonomic nervous system include which of the following?
Medulla oblongata
Parasympathetic division
Sympathetic division
186
The sympathetic chain consists of two parallel chains of ganglia located on either side of the _______ _______ whose axons innervate visceral organs.
Blank 1: spinal
| Blank 2: cord
187
Characteristics of the preganglionic neuron associated with the autonomic nervous system include which of the following?
- -Cell body is in the CNS
- --Axon synapses with autonomic ganglion
- --ACh released at synapse
188
How does the release of neurotransmitters by the adrenal gland prepare the body for action?
By increasing metabolism
189
Postganglionic neurons have axons that can synapse to which of the following?
with a smooth muscle, cardiac muscle, or gland cell
190
A parasympathetic ganglion chain similar to the sympathetic ganglion chain cannot exist because preganglionic parasympathetic neurons originate at the ________
and the ________ regions of the spinal cord.
brain
| sacral
191
The preganglionic neurons of the sympathetic division begin in the ________
and _______
regions of the spinal cord.
Blank 1: thoracic
| Blank 2: lumbar
192
Postganglionic neurons in the sympathetic division of the autonomic nervous system release ________
at their synapses.
Blank 1: norepinephrine
193
What is the name used to refer to the two parallel chains of ganglia located just outside the spinal cord whose axons innervate different visceral organs?
Sympathetic chain
194
When action potentials stimulate the adrenal gland, the cells of the medulla secrete the hormone
epinephrine (adrenaline)
195
Increased digestive activity and the slowing of the heart are nerve effects caused by the ________
division of the autonomic nervous system.
Blank 1: parasympathetic
196
Preganglionic axons of parasympathetic neurons terminate in ganglia located
within internal organs.
197
The lumbar and thoracic regions of the spinal cord are the originating points for the preganglionic neurons of the ________ division of the autonomic nervous system.
Blank 1: sympathetic
198
In general, the _________
division of the PNS inhibits digestion and relaxes muscles and tubules, while the _______ division stimulates digestion, contracts muscles, and constricts tubules.
Blank 1: sympathetic
| Blank 2: parasympathetic
199
The effects of the parasympathetic nerve includes all of the following except which one?
Increased sweat gland secretion
200
Choose all statements that describe muscarinic receptors in cardiac muscle cells.
- --They are G protein activating receptors.
| - ---They bind ACh.
201
The tenth pair of cranial nerves, known as the _______ nerves, are responsible for control of ______ rate, breathing, digestion and the release of various hormones.
vagus
| heart
202
Sympathetic stimulation causes constriction of blood vessels in which of the following areas?
Viscera
| Skin
203
Increased digestive activity and the slowing of the heart are nerve effects caused by the _______ division of the autonomic nervous system.
parasympathetic
204
In a neuromuscular junction, the receptor for ACh allows for the influx of which of the following ions?
Na+
205
Match cranial nerves (denoted by roman numerals) to their function.
I-
II-
X-
I- responsible for smell
II- responsible for vision
X- control of heart rate and breathing.
206
ACh has an inhibitory effect on the pacemaker cells of the heart because its binding causes an outward diffusion of which of the following ions?
K+
207
Why does pretty much every multicellular animal have a nervous system?
this helps them to control and coordinate their bodies and interact with their environment.
208
How do sensory systems gather info?
gather info about the surrounding and also the internal environment. Key point here is that they tell the animal about changes in the external and internal environment.
209
what do motor systems do?
Motor systems act on muscle to bring responses and movements. They act to initiate changes in behaviour. In the middle there is a central processing unit, and that interprets sensory information and relays that information to the motor system
210
What does things quickly and why?
Nervous systems do things very quickly and most animals can respond to stimuli extremely quickly. One of the reasons the nervous system can work so fast is that it uses electrical signalling. Using chemical signalling as well, in the form of neurotransmitters, but electrical signals are typically the fastest.
211
How long does a typical action potential last?
A typical action potential, the sort of unit of code of the nervous system, typically lasts about a millisecond.
212
What is transduction?
where energy in one form is transferred into energy in another form and in terms of sensory systems, that could be chemical, electromagnetic, mechanical, olfactory, etc. And these are converted into electrical signals that the nervous system can use. Electrical signals in the motor system, coming from axons, need to be converted into mechanical energy. By mechanical energy we mean muscle movement
213
Why is communication between neurons important?
Communication- between sensory and central processing and between central processing and motor systems are vastly important for the nervous system, and largely are formed by something called synapses.
214
What is likely to happen when the nervous system gets excited?
Excitability of the nervous system- the likelihood of parts of the nervous system being active.
215
Outline the nervous system.
the cell type that we are dealing with is obviously the neuron. The neuron use electrical signals to communicate. Have dendrites that receive information from other cells and then this message is passed to the cell body or the soma. The cell body houses the nucleus and much of the metabolic machinery. This info is then transmitted along the axon to the axon terminal. Now axons carry information over distances and contact other neurons or muscles or other effector cells. And some neurons have a myelin sheath surrounding them which helps increase the speed of neural impulses. And some do not have this sheath.
216
Lots of different shapes of neurons but all the different bits of a neuron, the dendrites axon etc are all doing the ______ ______.
same thing
217
What is the membrane potential?
is the electrical potential difference or the voltage between the inside and outside of the cell. And all cells have a membrane potential but it is typically much bigger in neurons than it is in other types of cells.
218
What is the normal resting potential?
-65 to -75 millivolts normal resting potential- depending on the neuron and species.
219
What do neurons act like?
Neurons act like a battery, with a store of electrical energy waiting to be tapped into, and thats the potential difference or the voltage difference, between the inside and the outside. The neuron uses this difference to go about a lot of its different functions. This is important.
220
How do you measure this voltage or potential difference between the inside and the outside of the cell?
use a fine glass microelectrode.
221
What is a fine glass microelectrode?
That is a fine glass pipette which can be inserted into the inside of the cell, or in this case the neuron, we are essentially measuring the voltage difference or the potential difference, between the inside and the outside. So inside we have this fine glass microelectrode and outside we have a reference electrode and it compares the voltage between the two A typical glass microelectrode will have a tip diameter of around 0.1 micrometers which is around 10 thousandths of a millimeter. If the pipette is filled with something conductive such as potassium chloride, you can use it like a wire pushed inside the cell. It makes electrical contact using the tip.
222
What is another way to measure the voltage or potential difference between inside and outside of the cell?
we can measure this is using a patch electrode
223
How do we measure with a patch electrode?
has a wider diameter typically 1 micrometer and again this electrode is connected to some measuring device. The patch electrode is sucked onto the outside of the cell and using various tricks you can punch a hole in the membrane so that the electrode is now in contact with the inside of the cell. So the patch electrode and the glass microelectrode are the two main electrodes that we use to make contact with the inside of the cell.
224
What is the glass microelectrodes typically used for?
Typically the glass microelectrodes are used for measuring the activity of a whole cell or a whole neuron
225
What is the patch clamp typically used for?
patch clamp is used to measure the individual activity of a given receptor on the cell membrane. Can't be used to measure the whole cell activity.
226
What is the Most important cation that accounts towards the resting membrane potentials and why?
is K+. K+ concentration is much higher intracellularly compared to extracellularly, and this specific numbers vary between neurons and different species. These ions move down their concentration gradient, so they want to move out across the membrane.
227
What is the typical concentration inside and outside the neuron of K+ ions?
So we have a typical concentration inside of 150 millimolar, and a typical concentration of K+ of five millimolar outside of the membrane.
228
Where will K+ ions want to move?
So K+ will want to move down its concentration gradient and leave the neuron. Now potassium moves extracellularly, through leaky potassium channels, and these are always open.
229
what is the charge of the inside of the neuron and why?
However, the inside of the neuron is negatively charged compared to the outside, and because K+ is positively charged, the electrical gradient draws K+ ions back into the cell. So as the K+ leaves the neuron, it takes some of the positive charge with it, meaning that the inside becomes more negatively charged, and this attracts the K+ back into the neuron. So we have a concentration or a chemical and an electrical gradient acting on K+.
230
why is an equilibrium achieved?
There is an equilibrium where these two forces are in balance, which we call the Nernst potential,, and this calculates the equilibrium potential of a given ion.
231
What is the Nernst equation?
Simple version of the equation suggests that Ek, which is the equilibrium potential for K+, =58 log concentration outside/ concentration inside. Now the 58 comes from the full Nernst equation, and it is calculated based on a mixture of different chemical and physical measurements. And the number 58 millivolts is sometimes seen as 61. This number can vary depending on the valency of the ion in question.
232
What is the valency of sodium and potassium indicated by the Nernst equation?
So sodium and potassium, having a valency of 1, whereas calcium has a valency of two. So the Nernst potential or the equilibrium potential is the membrane potential at which these two forces, the concentration in the electrical gradient, or collectively we call the electrochemical gradient, are in balance.
233
Why at equilibrium potential is the net movement 0?
At equilibrium potential, the net movement will be 0. The equation tells us that the Nernst/ equilibrium potential for K+ is -90 millivolts, so at -90 the net movement of K+ is 0. So the movement of potassium ions is the key thing that regulates the resting membrane potential, moment by moment. The reason that the resting membrane potential is not -90 millivolts is because theres is this small involvement of other ions, in particular Na+ ions
234
In contrast to K+, where is Na+ concentration higher?
Now in contrast to K+, Na+ is higher extracellularly than it is intracellularly. We have typical concentrations of 150 millimolar outside of the cell and then 15 millimolar inside of the cell, and again this varies depending on the neurons and varies depending on the species.
235
What does higher Na+ concentration extracellularly mean?
What this does however, emphasises is that we evolved from marine life where Na+ concentration is higher. Key here is that Na+ is higher outside than it is inside. This concentration gradient means that Na+ ions are desperate to move into the cell, and also the electrical gradient, because the inside of the cell is more negatively charged, will mean that Na+ wants to move inside the cell. So the electrochemical gradient is strongly favouring the movement of sodium intracellularly. This will keep going until the inside is more positive than the outside.
236
What is the equilibrium potential for sodium?
The equilibrium potential for sodium, where everything is in balance, and there is no net movement of sodium, is +60 millivolts. That means when the inside of the neuron is at +60 millivolts, there will be no net movement of Na+ ions .
237
So how does Na+ and K+ ions move to reach their respective Nernst potentials?
So K+ ions are trying to get the membrane potential down to -90 millivolts and Na+ ions are trying to get their membrane potential to +60 millivolts, to reach their respective Nernst potentials
238
What is the resting potential closer to and why?
Resting potential is closer to the equilibrium potential for K+ because the resting membrane is more permeable to K+ ions than it is to Na+ ions. When we look at the number of leaky channels for K+ compared to Na+, there are many more for K+ so the permeability to K+ is higher in the resting membrane. So it is highly permeable to K+, but only slightly permeable to Na+ at rest. This is what shapes the resting membrane potential.
239
How do ions move across the membrane?
The cell membrane is impermeable to ions and in order for ions to gain access to the cell, they have to be transported either passively, so not requiring ATP, or actively using ATP, using proteins on the cell membrane surface. So leaky K+ channels are passive, so they dont require ATP, and this allows K+ to diffuse out of the cell, down its concentration gradient, but also back into the cell, along its electrical gradient. There are also some Na+ leaky channels but nowhere near as many when compared to the leaky K+ channels. Key point here is that the moment to moment regulation of the resting membrane potential is mediated by these leaky K+ channels.
240
What does the passive movement of ions across the cell membrane help with?
The passive movement of ions across the cell membrane- the long term regulation of the difference in ion concentration between the inside and the outside of the neuron is regulated by active pumping of ions, so requiring ATP
241
What does the ATPase active pump pump in and out and why?
pumps three Na+ ions out of the cell and 2 K+ ions into the cell. Shows that more positive current is leaving the cell than positive current coming back into the cell. Helps to set up the potential difference across the membrane leaving the inside of the cell more negatively charged when compared to the outside. So it sets that potential difference, or voltage difference, between the inside and the outside of the membrane.
242
What maintains the concentration gradient?
This is not what maintains the concentration gradient moment by moment, that is the leaky channels.
243
What has some recent evidence suggested about the brain?
However , some relatively recent evidence has shown that around 50% of the brain's energy expenditure and typically around 30% of overall energy expenditure for a species, goes towards maintaining Na+/K+ ATPase pumps. So they are important in long term regulation of concentration gradients. When ions move across the membrane, it really is only tiny amounts of ions that are moving. It has a big functional effect on the potential difference between the inside and the outside, but has very little effect on the actual number of ons. So typically there isn't actually a change of concentration of ions when they move intra to extracellularly, but this very small amount of ions does have a big functional effect in setting up the membrane potential.
244
What would happen if you were to poison or pharmacologically antagonise, block the sodium/potassium ATPase pump?
the resting potential doesn’t just go away, it will eventually but only once the concentrations have been diminished and that takes a long time. The passive movement across the ion channels determines their resing membrane potential moment-by moment and can maintain it for quite a long time. In fact some evidence has shown that if you poison the sodium/potassium ATPase pump of a giant squid axon, that axon can still generate around 100,000 action potentials or impulses before the internal sodium concentration increases by 10%.
245
What is the resting potential?
where the neuron is at rest and has that negative membrane potential of around -70 millivolts.
246
How do neurons communicate with other neurons or effector organs?
To communicate with other neurons or effector organs, for example, neurons use receptor potentials, and action potentials. To do this and for neurons to communicate the permeability of the neuronal membrane needs to change. And the membrane potential can become depolarised, so it becomes more positive, so if the membrane was to move from -20 millivolts to +50, that’ll be depolarisation, but also neurons can hyperpolarise, and that means become more negative. So if the membrane potential was to move from -70, the typical resting membrane potential, down to -90 millivolts, we would say the cell has been hyperpolarised.
247
What will happen after a brief depolarization?
there will be a sharp return to the resting membrane potential.we call this type of response a receptor potential.
248
The membrane is
self-adjusting
249
How are the changes in the membrane mediated?
The changes in potential in the membrane are mediated via different classes of protein channels in the membrane. So these channels effectively act like a door, when they are open this allows those ions to flow in and cause a change in voltage, and we call this an increase in permeability to a given ion
250
In neurons, how many gated channels are there?
2
251
What are these gated channels?
One is a chemically gated channel, also known as a ligand gated channel. Another one is called a voltage gated channel. VGC opening and close depending on the voltage of the membrane. Now for these receptor potentials these are mediated by chemically gated or ligand gated ion channels. So with receptor potentials, the amount of ligand binding to the receptor, causing an opening of the receptor will mediate an increased size of the receptor potential. Whereas action potentials, are mediated mostly by voltage gated ion channels.
252
What will happen if the depolarisation is a little bit bigger and it crosses a certain threshold?
you get an action potential, or an impulse or a spike and these are often used interchangeably. These action potentials typically last one to three milliseconds, but they can be longer depending on the cell type.
253
So you get a very fast change in the membrane potential, what level is the threshold when this happens?
typically the threshold is around 10 millivolts above the resting level
254
How much does the action potential typically overshoot by?
The action potential typically overshoots 0 millivolts and becomes positive, so the inside of the cell becomes more positive compared to the outside.
255
What is the 'all or nothing' law?
because they are the same amplitude and duration each time. These action potentials are used by neurons as their unit of information. If the stimulus strength is increased, the amplitude of the action potential does not increase. So how does the nervous system code the intensity of a given stimulus- the trick that the nervous system uses is that the strength of the stimulus is coded into the frequency of the action potentials that are generated. Therefore, the stronger the stimulus, the higher the frequency of action potentials. We therefore say that the nervous system is frequency modulated and not amplitude modulated.
256
What happens when small current pulses or stimuli don't reach the threshold?
However, small current pulses or stimuli that don’t reach the threshold evoke purely electrotonic or graded passive potentials, and these are sometimes also called receptor potentials. In this case, the size of the change of the potential is proportional to the size of the stimulus. So these are amplitude modulated. A small graded potential results from local changes in ionic conductance and when it spreads along a stretch of membrane, it becomes exponentially smaller.
257
What do all action potentials start with?
So all action potentials begin with these receptor potentials depolarising the membrane above a certain threshold potential which converts that receptor potential into an action potential. So the receptor potential basically has to reach a certain threshold in order to trigger an action potential.
258
What happens to neurons which are smaller in relation to their length?
So neurons which are smaller in relation to their length, such as neurons in the brain, have only receptor potentials and longer neurons utilise receptor potentials to trigger the action potential.
259
Over short distances what happens to receptor potentials?
So over short distances, receptor potentials are great but over longer distances action potentials are needed.
260
How can we generate an all or nothing response?
So we do an experiment where we are injecting some current intracellularly that does not elicit enough of a change in the membrane potential to elicit a full-blown action potential, we get these graded responses. If enough of the current is injected to take the membrane potential above the threshold, an action potential will occur. These action potentials are all the same amplitude and therefore are coded in terms of their frequency and not their amplitude- all or nothing response.
261
Why are impulses used?
Impulses are used to carry impulses along axons.
262
Where are axon potentials found?
found only in axons
263
Where is a an impulse usually first generated?
Usually an impulse is first generated in the region that we call the axon hillock. Dendrites will receive information and this will trigger receptor potentials which will reach the axon hillock and if they are large enough they will trigger an action potential to move along the axon. usually , that is because there is a relatively high density of the voltage gated sodium channels in that region, so that it allows a large influx of sodium.
264
What is a property of unmyelinated axons?
Unmyelinated axons are leaky, where current can flow across the membrane, so current is lost.
265
What is meant by current in axons?
By current here is where the ions are crossing the membrane, so the ions being charged carry this current. That means the subsequent point at which an impulse needs to be regenerated needs to be close to where it was last regenerated. So teh impulses have to be generated close together along the axon.
266
What is wrapped around the axon?
Around this leakiness is to wrap the axon in an insulator, known as myelin.
267
What do myelination and saltatory conduction increase?
increase conduction velocity by about 10 times.
268
What is another way that you can increase the speed of conduction?
The other way that you can increase the speed of conduction is by increasing the diameter of the axon. This is what invertebrates have to do to increase conduction velocity as they do not have myelin.
269
Only vertebrates have
myelinated axons.
270
What does a wider diameter of the axon mean?
A wider diameter of the axon means less resistance in the axon, and therefore it is easier for the current to flow.
271
What does two neurons in close contact allow?
his is a typical situation that would allow one neuron to communicate with another.
272
Where is the site of transmission between two neurons?
the synapse
273
What will the presynaptic terminal of an axon of one neuron contact?
will contact another dendrite of another neuron in the most basic situation. Now axons can contact other somas, so the cell body, or axons, but mostly contact dendrites.
274
What are some more rarer more complex connections?
In some instances, which are slightly more complex and not as common, you also get axon cell body, and dendrite to dendrite and axon to axon connections.
275
What do we have at the synapse?
Looking at the synapse, we have the presynaptic terminal of one neuron then at the bottom we have a postsynaptic dendrite of another neuron.
276
But this resting membrane potential does ____ between different _____, _____ and other ____.
Blank 1- vary
Blank 2-neurons,
Blank 3- species
Blank 4-cells
277
What does the resting potential depend on?
Now this resting membrane potential depends on two things; t depends on the concentration of ions between the inside and the outside of the membrane, salts liek sodium chloride become separate ion is the solution, and these ions then have a charge associated with them, and then the second thing is the difference in membrane permeability to those particular ions, as that will change how easily these ions can cross the membrane.
278
Why are impulses needed?
Impulses are used to carry impulses along axons. Action potentials are only found in axons. Usually an impulse is first generated in the region that we call the axon hillock
279
How is a response triggered?
Dendrites will receive information and this will trigger receptor potentials which will reach the axon hillock and if they are large enough they will trigger an action potential to move along the axon. usually , that is because there is a relatively high density of the voltage gated sodium channels in that region, so that it allows a large influx of sodium. Unmyelinated axons are leaky, where current can flow across the membrane, so current is lost. By current here is where the ions are crossing the membrane, so the ions being charged carry this current. That means the subsequent point at which an impulse needs to be regenerated needs to be close to where it was last regenerated. So teh impulses have to be generated close together along the axon. Around this leakiness is to wrap the axon in an insulator, known as myelin.
280
What are the different types of receptors?
There are different types of receptors on the postsynaptic membrane, and the simplest of these are ligand gated ion channels. For the most part at rest with no ligand bound, these ligand gated ion channels are closed and dont let any ions through. Their open or closed state depends on whether a certain neurotransmitter is bound to them. They are specific to certain neurotransmitters
281
What happens when a neurotransmitter binds?
When a neurotransmitter binds, they open and allow the movement of ions into the dendrite, or in some cases out of the dendrite and this type of channel is referred to as a ligand gated ion channel. There are also G protein coupled receptors on the postsynaptic membrane. The neurotransmitter acetylcholine being released by the presynaptic terminal.
282
What will happen when the gated ion channels in the postsynaptic membrane bind acetylcholine?
sodium will move into the cell. This produces a very brief depolarisation in the postsynaptic cell. Sodium is a positively charged ion or a cation. That response only lasts a few milliseconds.
283
Why do the neurotransmitter have relatively poor affinity?
The neurotransmitter has relatively poor affinity and doesnt bind for a very long, so the ion channel closes quickly and the resting membrane potential is restored. We call this response a postsynaptic potential. In this case, it is an excitatory signal and this is excitatory because the neurotransmitter has caused depolarisation, and sometimes this is shortened to EPSP. another thing to note is that these ligand gated ion channels are also different to the voltage gated channels. The voltage gated channels have voltage sensor on and they open or close depending on the voltage of the membrane, whereas ligand gated ion channels need to have a ligand bound to open or close. It is important to note that not all responses are excitatory.
284
In contrast what does inhibitory synaptic transmission cause?
but causes an opposite response as shown by the negative postsynaptic potential. So the neurotransmitter is causing hyperpolarisation. The neurotransmitter here is the AA glycine. This is just an example of an inhibitory neurotransmitter.
285
What do the presynaptic vesicles release and what else happens?
So the presynaptic vesicles has released glycine and bound post-synaptically to its receptor. In this case the ion channels are specific to chloride ions which have a negative charge, therefore making the intracellular compartment of the postsynaptic neuron negatively charged. They move intracellularly and cause an inhibitory response postsynaptically. This is called hyperpolarisation, where it becomes more negative. So this takes the membrane potential down and means that the neuron will be less likely to fire. Sometimes this is shortened to IPSP. there are other examples of neurotransmitters that lead to hyperpolarisation such as GABA, inhibitory neurotransmitter in the central nervous system. GABA works slightly different and the channels are specific to potassium ions. As potassium is in higher concentration inside the cell, it diffuses out down its concentration gradient when the GABA channel opens. There are lots of different excitatory and inhibitory neurotransmitters. Each can have excitatory or inhibitory effects depending on the ion concentrations, intra-versus extracellularly. It depends on the receptor that it is binding to. There are some examples of neurotransmitters that have various receptors that can cause opposite responses.
286
What do the axons connect?
Axon connects a dendrite which is the case most of the time, but we can also get axosomatic synapses where the axon will contact the cell body of another neuron. Then we also have axoaxonic synapses where an axon will contact an axon. Dendrodendritic connections- where a dendrite will contact a dendrite and Dendrosomatic connections- where dendrites will contact the soma or the cell body. Any combination you can really think of is actually possible. So a postsynaptic cell isn't just going to receive information from one synapse, it will receive information from many synapses sometimes thousands or even hundreds of thousands of synapses. With this many dendrites and therefore this many possible connections, a lot of processing will need to take place.
287
What do neurons receive signals from?
Neurons receive signals from three different other neurons, A and B being excitatory and C being inhibitory. We are measuring the output from this neuron at the axon hillock, which remember is where the action potential are generated because of their high density of voltage gated sodium channels. On the X-axis we have time in milliseconds and membrane potential in millivolts on the Y-axis. We can see that A when it fires once is an example of an excitatory postsynaptic potential, caused by an excitaory presynaptic neuron. Blue dotted line is the threshold in the postsynaptic neuron. So the depolarisation elicited by A is going to need to reach that threshold to elicit an action potential. And sometimes the excitatory postsynaptic potential is not big enough to cross this threshold. If you only get one excitatory signal from one of the connections, and that is because there is some random noise in the nervous system, and this acts to protect the postsynaptic neuron from that noise. If you get two excitatory signals from the same neuron, so A fires quickly in succession, it can add to the top of the previous and so you get a little bit more depolarisation. If you get a succession of exitatory signals, they can build up and take the membrane potential above threshold, so you get an action potential. We call this temporal summation, where signals are received from the same neuron in succession. This can occur with inhibitory signals as well as excitatory signals, so it can cause more hyperpolarisation.
288
What is spatial summation?
is where multiple postsynaptic potentials from different synapses occur within a given time window and sum together. So with our last example of temporal summation, that was where the same neuron fires two times in a row, or maybe more, and this example is where different neurons are firing and those and the effects of all of those different neurons are summed together. So again we have a postsynaptic neuron with three connections, A,B and C, with A and B being excitatory and C being inhibitory. We are again measuring the membrane potential at the axon hillock. So first there is an excitatory signal from A which doesnt reach threshold. And neither does an excitatory signal from B. C causes hyperpolarisation. So that lowering of the membrane potential and making it more negative. SS- when two excitatory signals from different synaptic connections occur, for example with A and B, which are both excitatory signals, you can see that spatial summation leads to an action potential beig generated. When A and B fire close together, then depolarizations are summed and this can lead to an action potential. A and C- when A which is an excitatory signal and C an inhibitory signal fire they cancel each other out and this is also an example of spatial summation. Both of these types of interactions are called summation because the individual effects are summed. In a normal central nervous system it is normally a combination of several hundred or several thousand presynpatic neurons that will have the most effect. Often that is taught in quite a simple case where one neuron interacts with another neuron. In the majority of cases the excitatory postsynaptic potentials of an individual synapse are often not string enough to reach threshold and therefore generate an action potential.
289
What is another type of neural integration?
presynaptic facilitation
290
What is presynaptic facilitation?
this is where you have axoaxonic synapses where axons are influencing the presynaptic neuron. In this example, we have an excitatory presynaptic neuron, C, ith an excitatory axon synapsing onto it named E. we also have another excitatory neuron with no axoaxonic connection called D. we are again measuring the membrane potential in millivolts at the axon hillock, and you can see the resulting trace at bottom. So we have an axoaxonic connection between neuron E and C. 1st e.g. where only C is firing it may not be enough to reach the membrane potential threshold, but if E also fires, it can lead to an increase in the membrane potential to reach threshold. So with axoaxonic synapses, the neurotransmitter from the presynaptic neuron modulates the axon terminal by affecting the amount of calcium that enters in response to electric signals. In this e.g. where we have presynaptic facilitation with a neurotransmitter released from E, is causing more calcium to enter neuron C. and this leads to more vesicles releasing their respective neurotransmitter into the synaptic cleft. This is just another kind of interaction that is fundamental to how synapses communicate and interact. Final e.g. of neural integration is presynaptic inhibition. So presynaptic inhibition is the opposite of what we just saw and involves axoaxonic connections again where one neuron is excitatory and the other is inhibitory. F is an excitatory synapse and H is inhibitory. If F and H fire at the same time H will inhibit F. it dies this by reducing the amount of calcium that can enter F, and then therefore that will mean that less vehicles will release the neurotransmitter into the synaptic cleft, and therefore less neurotransmitter will bind to postsynapticly and graph shows that when F and H fire at the same time causes no change in membrane potential.
291
What is muscle movement mediated by?
Muscle movement is mediated by neuron to muscle connection. Muscles work by shortening and working against some kind of skeleton.
292
Outline how muscles are connected.
We start with the muscle which is usually connected to the skeleton via tendons. If we look inside a muscle there are bundles of muscle fibres, each one being a single cell that has a nucleus. These muscle fibres have a striped appearance. If we look inside one of those muscle cells, they have even smaller bundles of myofibrils. Zoom in on one of those myofibrils, and we get myofilaments. Those myofilaments behaviour is what produces muscle contraction- myosin and actin. This is divided into sarcomeres which are around 1-1.5 micrometers long. These sarcomeres are bound by what we call Z lines. These striations come about because of the way actin and myosin filaments are arranged. The other bands that we can see in muscle are H, I and A bands. During contraction the I and H band shorten and get narrower. This was some of the first evidence to show how the mechanism of muscle contraction works.
293
How is actin linked?
Actin is linked across on the Z line, those are interweaved with thicker orange myosin filaments. The stripes come about because of the different densities of these structures. Actin on its own looks lighter under the scanning electron microscope. The myosin and the actin together look darker. The shortening of the muscle involves a sliding of the actin filaments against the myosin filaments. Each fibre gets shorter but not fatter.
294
How is calcium controlled?
calcium ions are stored in the sarcoplasmic reticulum and are released by signals from motor neurons and this connection between the motor neuron and a muscle is called the neuromuscular junction. The cell bodies of motor neurons in most vertebrates and invertebrates lie in the central nervous system. They have axons that run to the muscle. So all this initial work on synapses, was done on the neuromuscular junction because they are bigger and easier to study. So in this electron microscopy image, we see this in finer detail. At the top we have the axon terminal and here you can see a lot of these synaptic vesicles that contain the neurotransmitter. These foldings in the membrane of the muscle cell, at the bottom, helps with increasing surface area. This increases the receptor density on the postsynaptic membrane, or in this case the muscle cell. At the bottom you can see those contractile proteins. At the neuromuscular junction an action potential will arrive at the presynaptic terminal, causing voltage gated calcium channels to open, leading to an influx of calcium into the presynaptic terminal. This will lead to the vesicles releasing neurotransmitter into the synaptic cleft, in this case acetylcholine. Acetylcholine will diffuse across the synaptic cleft and bind to receptors on the postsynaptic muscle fibre. This will open the ligand gated ion channels and allow the influx of sodium. This will depolarise the muscle cell. This is what we call an excitatory junction potential.
295
What happens when this action potential propagated along the sarcolemma of the skeletal muscle?
And it will enter the T tubules and depolarise them. Which causes voltage gated calcium channels on the sarcoplasmic reticulum to open and release calcium. The release of calcium from the sarcoplasmic reticulum allows the initiation of contraction. This calcium brings about excitation-contraction coupling, and we call this the transduction stage.
296
What is the transduction stage in muscle contraction?
This transduction stage is simply where the signal or the action potential is transduced into an effect. In terms of skeletal muscle, transduction is electrical to chemical to electrical to mechanical. So electrical being the action potential arriving at the presynaptic terminal, chemical being the neurotransmitter being released, then that obviously opens ligand gated ion channels which causes an action potential in the muscle cell, being the electrical component, and then the mechanical being contraction at the end.
297
How are the muscle cells arranged in most vertebrate skeletal muscle?
In most vertebrate skeletal muscle, each muscle fibre is innervated by only one motor neuron. however , there are hundreds of motor neurons that may innervate the whole muscle, but only one innervates a given fibre. And collectively we cell a motor neuron, and the muscle fibres it innervates the motor unit.
298
How is contraction strength controlled?
So contraction strength is controlled by recruitment, and control of contraction strength follows the size principle and this means that smaller muscle groups with lower numbers of motor neurons will be activated first before larger muscle groups with more motor units. This means that as the force required for movement increases, so does the size of the motor units recruited.
299
How does muscle contraction start in invertebrates?
In invertebrates it starts off with the same- the action potential arriving at the end of a motor neuron’s axon. Again this cell body lies in the central nervous system and when the action potential arrives at the neuromuscular junction, this time it releases a different excitatory neurotransmitter called glutamate. And that will produce junction potential in the muscle cell, which is a graded response and not an all or nothing response like we saw with action potentials. The difference in invertebrates is that they don't often actually use impulses here like what we saw in vertebrates. So it is the excitatory junction potential that initiates the release of calcium from the sarcoplasmic reticulum, which will then initiate contraction. So not too different from what we see in invertebrates.
300
What is a nervous system?
is essentially an assembly of neurons and interconnections, and there are various forms of nervous systems in different animal groups. There are some common principles that we can identify. Nervous systems are very rarely preserved in fossil records. Theories of nervous system evolution are based on interpretation of the anatomy and molecular genetics of living groups, which is a risky proposition because all groups are like today are highly evolved and none can be taken as representing a primitive condition. Comparative studies of living animals show that the neurons of nervous systems of all animals, although diverse in form, are quite similar in their functional properties. For example, the neurons of all phyla have common molecular bases for their excitability and intracellular communication, with homologous voltage gated channels and synaptic mechanisms. The genetic controls of nervous system development show striking homologies in a wide range of phyls. The major changes in the evolutionary history of nervous systems, appear to have involved changes in the complexity of organisations of neurons into systems, rather than changes in the neurons themselves.
301
Outline the properties of Cnidarians.
includes Hydra and jellyfish. simple nervous system in which all neurons are similar and linked to one another in a web or something we call a nerve net. Are sessile so they dotn move much and typically have a radial symmetry with no obvious front or back. They also tend to have relatively few neurons, and those neurons are really quite simple connections. One of the simplest connections would be a direct connection between the cell in the body wall attached directly to the muscle. Slightly more complex, we have a body wall and a muscle cell, so being the contractile cell. And hydra have sensory cells that contact interneurons, which link sensory to motor neurons, and that feeds onto the muscle cell. And this gives some more flexibility and some sophistication. We call these kind of connections a nerve net which is a loose array of nerve connections over the body surface. Or if you like nerve nets are where the neurons are present and connected to one another without a central coordination.
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What are some specialisations within Cnidarians nervous systems?
So even this very simple nervous system has some specialisations, including sensory neuron groups, such as around the mouth of the bell. Some of the axons form what we call tracts, into groups of axons, not just nets. Another thing that we see is some specialised giant axons which have a very large diameter and conduct impulses very quickly. If your axons are myelineated, you can increase conduction velocity, which increases information transfer speed, by increasing the diameter of the axon. Because that reduces resistance inside the axon. Now some of these giant axons are used in escape swimming in jellyfish. And these specialiations allow coordinated movements, for movements such as feeding.
303
Outline Bilateria systems.
Bilateria are organisms with bilateral symmetry, and include most of the organisms you’d think of as an animal. All of these invertebrates, fish, amphibians and mammals all share a common ancestor that had the rudiments of a centralised nervous system. So two major trends characterised the evolution of nervous systems in the bilaterally symmetrical phyla of animals, centralisation and cephalisation, the centralisation of nervous systems refers to a structural organisation, in which integrating neurons are collected into central integrating areas rather than being just randomly dispersed like what we saw in the Cnidaria, with the nerve sets, and cephalisation is the concentration of nervous system structures and functions at one end of the body and mostly in the head.
304
Outline Platyhelminthes systems
some of the most simple bilateral animals, which again basically means that they have a very obvious left and right side and a distinctive head, and the nervous system of these animals with bilateral symmetry have more obvious structures.
Flatworm called the Planaria- they live in bodies of fresh water in this country. And there is now something that we can actually call a brain. These are formed from collections of neurons either side of the head forming this brain structure. The front of the animal is going to be the bit that comes into contact with new environments before the rest of the body. That is where sensory structures and sensory systems evolved and are going to be more present. Such as eyespots which allow the flatworm to detect light and respond by moving away from this light stimulus, something we call negative phototaxis. So this information will go from the eyespot to the brain, and then initiate movement away from the light. We have this central coordinator- so the sensory information passes to the brain, which is the central coordinator, which then initiates a behaviour. In this case it activates the motor system to cause a movement away from the light. From the brain we can see the appearance of a number of nerve cords which run the length of the animal, and then we also have a transverse nerve cord going across. These nerve cords are made up of the axons of neurons. There are also cell bodies of neurons scattered along the length. These nerve cords and neurons allow the brain to coordinate movement in the rest of the body. Result of this setup with a brain at the head, and distinctive nerve cords is that we see the appearance of something that we can actually call the central nervous system. Generally speaking the central nervous system tends to contain the motor neurons and the interneurons, whereas the sensory neurons tend to be located outside of the central nervous system in what we call the peripheral nervous system. Flatworms of the phylum Platyhelminthes, have both a central nervous system, made up of a small brain and two nerve cords and peripheral nervous system containing a system of nerves that extend throughout the body.
305
All of the subsequent evolutionary changes in nervous systems, can be viewed as a
series of elaborations on the characteristics already present in flatworms.
306
Outline annelid systems.
they have a clear front and back but now they are also segmented. Animals with relatively complex central nervous systems exhibit two major forms of central nervous system organisation: ganglionic nervous systems, which are characteristic of protostomes and columnar nervous systems which are characteristic of vertebrates and other deuterostomes. Leech- we have a brain at the front which is more sophisticated in the leech than it is in the flattworms, and we see the appearance of more sophisticated sensory structures. So there is an increase in the quantity and range of sensory information available to the animal and then obviously that sensory information will again be processed by the brain. From the brain we have nerve cords running down the body but this time there are two nerve cords. Each of those nerve cords is solid and structure and they run along the ventral surface of the body. So there are two of these nerve cords and they are solid and ventral. Those nerve cords are made up of the axons of neurons, for example neurons coming from the brain and another feature of these nerve cords id that they pass around the oesophagus, which is a very persistent feature in nervous systems of this kind. The nerve cords are made up of the axons of neurons, but not all of the neuron cell bodies are in the brain, some of them are condensed into these segmental structures, called the segmental ganglia. So along the nerve cord we have these segmentally arranged gamglia, which are groupings of neurons. These segmented ganglia are arranged in a way that the cell bodies of the neurons lie in the ganglia, and the axons and the dendrites are out in the periphery. So the peripheral nerves go out to the rest of the body, so for example to innervate muscles. These peripheral nerves contain both sensory and motor axons, so they contain information bringing back in to the ventral nerve cord with sensory systems, but also information going out to the periphery via motor axons. Leading from each of the ganglia are peripheral nerves, which are nerves leading out to the rest of the body, for example to the muscles. And these peripheral nerves are made up of the axons of motor neurons controlling muscles and the axons of sensory neurons bringing sensory information back into the central nervous system. So the structure of what we see here isnt too dissimilar to what we saw in the Platyhelminthes, just more of it and more of it is organised into this distinct structure.
307
Outline the nervous system of arthropods.
Arthropods which include myriapods, crustaceans, insects and arachnids. The nervous system that you see in these groups are similar to what we saw in the annelid groups with more complex sensory and motor structures, which is largely because they have much higher motor demands. For example the control of jointed limbs and more complicated sensory systems because they encounter more information. Just as a reminder, this is the next example of this is our ganglionic nervous systems. What we also see is a lot of autonomy in different regions of the nervous systems, so parts of the nervous systems have a major role to play in controlling certain structures, like the legs for example. So we have specialised sensory organs, regional specialisation, local control, and we will also see that there are giant, wide diameter axons in these nervous systems. In some cases you can see that the body has become more condensed.
308
What is the nervous system of a locus?
The nervous system of the locust- and hopefully you notice straight away that the structure is relatively similar to what we saw in the annelids, so we have a brain at the front and then we have a double solid ventral nerve cord running down the body, and thats again similar to what we saw in the annelids.
309
What does the central nervous system of arthopods consist of?
In arthropods the central nervous system consists of a chain of segmental ganglia. Ganglia are swellings containing discrete aggregations of nerve cell bodies and various processes. The chained ganglia are linked by paired bundles of axons called connectives, the central nervous system of an arthropod, such as the locust or a cockroach, consists of an anterior brain and a ventral nerve cord that is linked to the brain by connectives encircling the oesophagus. Again, similar to what we saw in the annelids. The ventral nerve cord is a chain of ganglia linked by connectives, one ganglion for each thoracic and abdominal body segment. So each ganglion of the central nervous system of an arthropod consists of an outer rind and an inner core. So the rind consists mostly of cell bodies of neurons and is largely devoid of axons and synapses. Indeed, nearly all neuronal cell bodies of arthropods are confined to the rinds of the central nervous system ganglia. However, the major exceptions here being cell bodies of the sensory neurons, and many of them are located out in the peripheral nervous system. The inner core of each ganglion contains two regions, a region of synaptic contacts between axons and dendrites , and this is called the neuropil, and a region of tracts of axonal processes within the ganglion. So we have two regions, the neuropil and region of tracts of axonal processes within the ganglion and you can see that again in this cross section. So in arthropod or in other ganglion nervous systems, such as we saw in the annelids, there are four terms for a bundle of nerve axons, depending on where the bundle is located. In the peripheral nervous system, a bundle of axons is a nerve. Between the ganglia, the central nervous system, it is a connective. Within a ganglion it is a tract and between right and left sides of a bilaterally symmetrical ganglion, it is a commisure. The terms nerve, tract and commissure have the same meanings for vertebrate nervous systems, but vertebrate central nervous systems do not have connectives. Here in the arthropods there is much more specialisation in the neurons that make up these segmental ganglia we saw in the annelids, with more regional specialisation. And there are three of these thoracic ganglia and in an insect, the neurons in these ganglia control the three pairs of legs and the wings. At the back we have the abdominal ganglia, and these are specialised in controlling processes in the abdomen, including the muscles. A similar nervous system to the annelids but with far more regionalised specialisations.
310
What is a chordates system?
Chordates- so vertebrate central nervous systems in contrast to the arthropods, are classified as columnar because they consist of a continuous column of neural tissue with cell bodies in synaptic areas intermingled. Vertebrate nervous systems are usually more complicated than invertebrate nervous systems, and superficially they can seem quite different, but there are some common features. Now vertebrates are obviously bilaterally symmetrical, and again we have a lot of the sensory structures with the head, similar to what we’ve seen before.
311
So in the vertebrates we can distinguish a central nervous system
consisting of a brain and spinal cord and a peripheral nervous system that includes axons of motor neurons and the sensory neurons.
312
The spinal cord, unlike the nerve cords of invertebrates, is
dorsal rather than ventral.
313
Outline some aspects of the spinal cord.
It is also single rather than double or even multiple.
314
What does the central nervous system start with?
The central nervous system starts as a hollow tube of cells during development. There is a hole up through the middle that is hollow and that is called the spinal canal. It can be divided into two regions: the inner grey matter and the outer white matter. So things are the opposite way round here when compared to invertebrate ganglia, because the grey matter contains the cell bodies of the neurons, which are on the inside, and the white matter is the axons which are on the outside. White matter is labelled as white matter because of the presence of myelin, so myelin gives its white appearance. So the axons of the motor neurons leave through the ventral root and go to the muscles. The sensory axons come in through the dorsal roots at the top of the image. So information comes in from the dorsal root and goes through the ventral root. If we look inside the spinal cord, we can see interneurons, which are entirely inside the spinal cord. And then motor neurons which head out of the spinal cord through the ventral roots. Together the interneurons and the start of the motor neurons make the central nervous system. Now the sensory neurons have their cell bodies outside of the central nervous system, much like we saw in the invertebrates. 2these cell bodies of the sensory neurons, are in sensory ganglia called the dorsal root ganglion. And their axons then run into the central nervous system. Motor neuron axons and sensory axons make up the peripheral nervous system, so the start of the motor neurons are in the central nervous system, and then the latter part of the motor neurons are in the peripheral nervous system. Vertebrate nervous systems are just organised a little bit differently. We have a simple reflex arrangement here, with a sensory interneuron and a motor neuron going back out, and that is just the same as what we saw in the Cnidaria.
315
What are aspect of the fish brain?
analysed fossils from fish that swam 500 million years ago have revealed a lot about the early evolutionary stages of the vertebrate brain. They are small brains, but they contain the main three divisions that characterise the brains of all contemporary vertebrates. So the back, or the hindbrain, which was and is still the major component of these fish brains, contains the cerebellum, the pons and the medulla oblongata. The hindbrain links to the spinal cord and plays a big role in motor movement. It acts as an important relay between the rest of the brain and the spinal cord, particularly for things like locomotor movements. The cerebellum grows out of hindbrain, small in fish, but it gets a lot bigger in mammals. And the cerebellum is a very sophisticated processing region. It receives info from inside and outside of the animal and uses that information to stabilise movement. The cerebellum is very important in skilled motor tasks. Ones that require learning, recent evidence has started to highlight other roles for the cerebellum and not just coordinating movement. So the pons largely plays a role in descending motor control. The medulla oblongata plays a specific role in the control of autonomic and respiratory systems. mid brain and forebrain are largely devoted to reception and processing of sensory information. The midbrain is composed of the optic tectum, which receives and processes visual information. Within the mid brain the superior colliculus is largely responsible for visual integration, whereas the inferior colliculus is largely responsible for auditory integration. Whereas the forebrain contains the olfactory lobe, which is devoted to the processing of olfactory information. The thalamus, which is also in the forebrain, is a sensory relay are for visual, auditory, somatosensory and gestatory systems. Then the forebrain also contains the hypothalamus and the pituitary, which plays an important role in maintaining bodily homeostasis, so keeping things stable within the organism.
316
Outline aspects of the mammals brain
we have the exactly the same regions. We have a hindbrain with the cerebellum on top, the midbrain and the forebrain, but the cerebrum is massively expanded to form te cerebral hemispheres. If we pop our fishbrain underneath and colour-code, we see that we have the exact same regions.
317
Put the steps of an action potential in order.
1-stimulus moves voltage above threshold.
2-voltage-gated sodium channels open
3-sodium enters the cell
4-cell depolarizes
5-voltage-gated sodium channels close and voltage-gated potassium channels open.
6-potassium leaves the cell
7-cell hyperpolarizes
8-voltage-gated potassium channels close
9-sodium-potassium pump restores resting potential
