11. Neurons, Synapses And Signaling Mechanisms Flashcards
1
Q
What are ion channels?
A
Specialised proteins in a cell membrane that regulate the passage of ions down a concentration gradient.
2
Q
What type of transport are ion channels?
A
Passive transport.
3
Q
What are gated ion channels?
A
A type of ion channels that controls the flow of ions across a cell membranes in response to specific stimuli.
4
Q
What are 3 types of stimulus that control gated ion channels?
A
- Changes in voltage (neurons).
- Ligand binding.
- Mechanical forces.
5
Q
What is a ligand?
A
Any molecule or atom that binds reversibly to a protein.
6
Q
The opening and closing of gated ion channels alters…?
A
- The permeability of the membrane to particular ions.
- Subsequently the altering of the membrane potential.
7
Q
When is resting potential achieved?
A
When the cell membrane reaches the electrochemical equilibrium potential.
8
Q
What are voltage-gated ion channels?
A
Ion channels that open and close based on stimuli associated with voltage changes across the plasma membrane.
9
Q
What 2 types of cells are associated with voltage gated ion channels?
A
- Neurons.
- Muscle cells.
10
Q
After activation, voltage gated ion channels have an…?
A
Inactivation period that prevents the channel from responding to repeated stimulus.
11
Q
Why is there an inactivation period in voltage gated ion channels?
A
To prevent triggering of the channel from repeated stimuli.
A self-regulatory process.
12
Q
What is depolarisation?
A
A change in cell membrane potential such that the inside of the membrane is made less negative to the outside.
There is a decrease in polarity difference
13
Q
What is hyperpolarisation?
A
A change in cell membrane potential such that the inside of the membrane becomes more negative relative to the outside.
There is an increase in polarity difference
14
Q
What is a graded potential?
A
A shift in cell membrane potential in response to hyperpolarisation or depolarisation.
The larger the stimulus the greater the graded potential
15
Q
What is induced by a graded potential?
A
A small electrical current that dissipates as it flows along the membrane.
Graded potentials decay with time and distance
16
Q
When are action potentials generated?
A
When depolarisation reaches a specific threshold, shifting the membrane potential sufficiently.
17
Q
What type of graded potential decreases the chance of an action potential?
A
A hyperpolarisation graded potential.
18
Q
What occurs to sodium channels to reach the action potential threshold?
A
- Voltage gated sodium channels open, enabling the flow of Na+ into the cell.
- The influx of Na+ causes further depolarisation.
- A positive feedback loop occurs as this depolarisation causes even more sodium channels to open.
19
Q
What is the approximate threshold value for action potentials in the neurons of many mammals?
A
About -55mV (milliVolts)
20
Q
How does stimulus strength affect an action potential?
A
An action potential is all or nothing - it does not have different levels of strength according to the stimulus.
Rather, the existence of an action potential is based entirely on the threshold value.
21
Q
Action potentials spread along…?
A
Axons.
22
Q
What are the 6 phases of triggering an action potential?
A
- Resting state. Gated Na+ and K+ channels are closed.
- Depolarisation opens some Na+ channels.
- Rising phase of action potential. Depolarisation causes more Na+ gated channels to open, starting a positive feedback loop as the threshold is reached.
- Falling phase of action potential. Peak action potential causes a signal to be transmitted. Na+ channel close while K+ channels open.
- Undershoot. Hyperpolarisation makes the cell more negative.
- Cell returns to its resting potential.
23
Q
What is the refractory period in neuron gated ion channels?
A
The downtime between one action potential and another, where a hyperpolarised membrane will not respond to any more stimulus.
24
Q
What is the neuron refractory period linked with?
A
The temporary blockage/inactivation of sodium channels.
25
What is the function of an action potential?
An action potential acts as a nerve signal, being conducted along an axon and transmitted to the target tissue.
26
What is the process of action potentials conveying information between and along excitable cells?
Conduction.
27
Upon stimulation, action potentials will be...?
Stimulated, inhibited, or modulated in some way.
28
The magnitude and duration of an action potential are...?
The same at each position along the axon.
29
What quality of an action potential is directly proportional to the strength of its stimulus?
The frequency.
*E.g. a loud noise (strong stimulus) causes more frequent action potentials in auditory neurons*
30
What is the main purpose of the refractory period preventing continued stimulus activation?
To prevent action potential reversal, allowing signals to propagate in only one direction along the axon.
31
Where are voltage gated sodium channels found on the axon?
In the nodes of Ranvier, where they have direct access to the extracellular fluid.
32
Where are action potentials generated due to the location of sodium ion channels?
Only at the nodes of Ranvier.
Each current generated then travels within the insulated axon to the next node.
33
What are electrical synapses?
Gap junctions that allow electrical currents to flow directly from one neuron to another.
34
What is the more common type of synapse?
The chemical synapse.
35
How do chemical synapses function?
They release chemical neurotransmitters from the presynaptic neuron into the synapse to reach the postsynaptic neuron.
36
How are neurotransmitters packaged?
They are packaged into synaptic vesicles at the axon terminals *(+ dendrites)*.
37
What are the 4 steps of an action potential initiated chemical neurotransmission across the synapse?
1. Action potential depolarises the plasma membrane at the synaptic terminal, opening voltage gates that allow Ca2+ to diffuse in.
2. Increased Calcium ions cause the synaptic vesicles to fuse with the terminal membrane and release the neurotransmitters through exocytosis.
3. Neurotransmitters cross the synapse before binding to and activating ligan-gated ion channels *(receptors)* in the postsynaptic membrane of the receiving neuron.
4. This results in a graded potential of the postsynaptic neuron *(localised depolarisation or hyperpolarisation)*.
38
How would removal of calcium affect neuron communication?
1. The creation and transmission of the action potential would be unaffected.
2. The transmission of the action potential *between neurons* would be negatively affected.
39
What is an excitatory postsynaptic potential (EPSP)?
A postsynapse potential that brings the neuron's membrane potential closer to its firing threshold.
40
What is inhibitor postsynaptic potential?
A postsynapse potential that brings the neuron's membrane potential **further** from its firing threshold.
41
What is summation *(neurons)*?
The result of incoming EPSP and IPSP.
42
What is the Axon hillock?
A specialised part of the cell body of a neuron that connects to the axon.
43
Where does the summation, determining threshold breach in a neuron, occur?
In the axon hillock.
44
When is the threshold of a neuron breached during summation?
When the EPSPs are strong enough to overcome the IPSPs.
45
Can a single neuron receive multiple signal inputs?
Yes.
Both excitatory and inhibitory inputs from multiple neurons can be received.
46
How are neurotransmitters disposed of after completing their signalling?
In some combination of the following:
1. Diffuse away from the synapse.
2. Direct degradation by enzymatic hydrolysis at the synapse.
3. Reuptake at the presynaptic neuron - repackaged and reused.
47
What is the complexity of the nervous system of a cnidarian *(e.g. hydra)*?
Nerve cells form a decentralised nerve net.
48
What is the complexity of nervous system of an echinoderm *(e.g. starfish)*?
Nerve cells are bundled into fibres called nerves.
49
What is the complexity of the nervous system of bilateral animals like the planarians *(flatworms)*?
Neurons cluster into an anterior brain that processes information.
50
What is the complexity of the nervous system of arthropods?
In addition to a brain, they have peripheral ganglia located along the ventral nerve cord.
51
What is the complexity of the nervous system of molluscs?
Highly developed with complex brains containing millions of neurons.
52
What is the complexity of the nervous system of vertebrates?
The brain and spinal cord comprise the CNS, with neurons in the rest of the body forming the PNS.
53
What is the nervous system complexity of porifera?
Sponges have no nervous system and no nerves.
54
What 2 types of structures respond to a stimulus sent from the CNS?
1. Glands.
2. Muscles.
55
What is vasopressin?
A hormone of the posterior pituitary.
Also known as antidiuretic hormone (ADH).
56
What stimulates the release of vasopressin (ADH)? *3 steps*
1. Chemoreceptors detect fluctuations in CO2 and O2, as well flucations in blood pH.
2. Chemoreceptors transmit information as action potential to the CNS.
3. CNS sends action potential to the hypothalamus to stimulate release of vasopressin.
57
Sensory information travels to the CNS as...?
Motor commands travel from the CNS as...?
Both as action potentials!
58
What stimuli causes mechanoreceptors to trigger an action potential?
1. Sound.
2. Touch.
3. Motion.
59
What stimuli causes chemoreceptors to trigger an action potential?
1. Solutes.
2. Tastes.
3. Smells.
60
What stimuli causes thermoreceptors to trigger an action potential?
1. Heat.
2. Cold.
61
What stimuli causes electromagnetic receptors to trigger an action potential?
1. Light.
2. Electricity.
62
What stimuli causes pain receptors to trigger an action potential?
1. Noxious chemicals.
2. Extreme temperatures.
63
All sensory process begin with...?
Stimuli *(which represent forms of energy)*.
64
What does a non-neural sensory receptor do?
Convert stimulus energy into a change in membrane potential, regulating the output of action potentials to the CNS.
It does not generate an action potential itself, but instead converys information to an *(afferent)* sensory neuron.
65
What does a neural sensory receptor do?
*A neural sensory receptor is a sensory receptor that is also a neuron.*
It generates action potentials that travel along an axon to the CNS.
66
What are afferent neurons?
Sensory neurons that carry nerve impulses from sensory stimuli towards the CNS.
67
What are efferent neurons?
Neurons that carry signals from the brain to the PNS in order to initiate an action.
68
What is the effect of all sensory receptors?
To open or close ion channels.
69
What is a receptor potential?
A change in the membrane potential of a sensory receptor.
70
What increases the size of the receptor potential?
The intensity of the stimulus.
71
What do larger receptor potentials cause in sensory neurons?
More frequent action potentials.
72
What do larger receptor potentials cause in sensory non-neurons?
*Usually* the release of more neurotransmitters.
73
What are skeletal muscles?
Bundles of fibres that function using the voluntary system to enable the movement of vertebrates.
74
A muscle fibre is composed of many...?
Fibrils, packaged into orderly units.
75
What is the sarcolemma?
The plasma membrane of a muscle cell.
76
How many cells make up a muscle fibre?
Each muscle fibre is a single cell.
77
What is the site of action potential conduction, which triggers muscle contraction?
The sarcolemma.
78
Many skeletal cells are...?
Multinucleated.
79
How large are multinucleated skeletal cells?
1. Diameters up to 100μm.
2. Lengths up to 30cm.
80
What is the sarcoplasm?
The cytoplasm of a muscle cell.
81
How much of total human body mass is comprised of skeletal muscle?
30 to 40%
82
What is a myofibril?
A long cylindrical structure *(fibril)* that lies parallel to the muscle fibre - located inside the muscle fibre.
83
What is the approximated diameter of a myofibril?
1.2μm
84
How many myofibrils can be found in one muscle fibre?
Hundreds to thousands.
*Because of their small diameter*
85
How long are myofibrils?
The entire length of the muscle fibre, running from end to end.
86
What are sarcomeres *in muscle fibres*?
The thick and thin filament repeating sections of myofibrils.
87
How is a sarcomere region determined?
Sarcomeres are delineated by Z lines: a sarcomere is the region from one Z line to the next Z line.
88
What causes the striated pattern of skeletal *and cardiac* muscle?
Sarcomeres.
89
What are the different fibre types present in sarcomeres?
1. Thick filaments.
2. Thin filaments.
90
What makes up thin filament?
Two strands of polymerised actin strands coiled around one another.
*Contains the regulatory proteins troponin and tropomyosin*
91
What makes up thick filament?
A staggered array of myosin molecules.
92
What is polymerisation?
The process of small molecules *(monomers)* chemically combining to produce large network/chain molecules called polymers.
93
What is the purpose of the Z line *in the sarcomere*?
To define the boundary of each sarcomere.
94
What is the purpose of the M line *in the sarcomere*?
To mark the middle of the sarcomere.
*Contains a protein called myomesin*
95
What is the H zone *in the sarcomere*?
An area between the M line and Z line that contains only thick filaments *(myosin)*.
96
What is the I band *in the sarcomere*?
A light coloured region containing only thin *(actin)* filaments.
97
What is the A band *in the sarcomere*?
A darker coloured region containing both thin and thick filaments.
98
During muscle contaction, the myofilaments do not...?
Change length.
99
How do myofilaments contract?
They slide across each other, shortening the distance between Z Lines and, therefore, shortening the length of the sarcomere.
This results in contraction of the muscle fibre.
100
What happens to the A band and I band during muscle contraction?
The A band remains the same width while the I band shortens because the actin filaments run along each other *(as the Z lines move closer together)*.
101
The myofibrils are attached to...?
The sarcolemma at their ends.
This means that when the filaments slide across each other, the entire muscle contracts.
102
How many sarcomeres contract across a myofibril?
All of them.
The filaments all slide in unison.
103
At full contraction, what occurs to the myofilaments?
The thin filaments overlap, and the muscle essentially 'shrinks'.
104
Do sarcomeres and myofibrils line up with other sarcomeres and myofibres across a muscle fibre?
Yes.
105
Describe the amount of sarcomeres per muscle fibre?
1. There are multiple sarcomeres per myofibril repeating the entire length.
2. There are multiple myofibrils per muscle fibre, with each sarcomere lined up across the width of the muscle cell.
106
What 2 proteins regulate binding sites on actin filaments?
1. Tropomyosin.
2. Troponin complex.
107
How do the 2 proteins regulate muscle contraction?
They block or allow actin and myosin interactions.
108
What is the function of tropomyosin?
It covers the myosin binding sites on actin filaments, preventing myosin interaction.
109
What is the function of the troponin complex?
It binds with Ca2+, removing tropomyosin and exposing the myosin binding sites.
110
What is the function of actin in muscle contraction?
It forms thin filaments that can slide.
111
What is the function of myosin in muscle contraction?
It uses energy to pull on actin filaments.
112
What does ATP enable in muscle contraction?
The myosin head of the thick filament to 'climb' the thin actin filament.
113
What are the 5 steps of cross-bridge muscle contraction?
1. The actin filament binding site is exposed to calcium, forming a troponin and calcium complex. This complex removes the tropomyosin and exposes the active site.
2. The myosin head 'sits up' and forms a cross-bridge with the actin filament binding site.
3. *During the power stroke* the ADP and phosphate are released, producing energy and enabling the myosin head to pull the actin filament towards the centre of the sarcomere.
4. A new ATP molecule attaches to the myosin head, causing it to detach from the active site of the actin filament.
5. The ATP hydrolyses to ADP and phosphate, and the myosin returns to its resting position.
114
What is the myosin head?
The 'motor unit' of the thick filament that binds to the actin filament.
115
When is the myosin head in a low energy state/resting state?
When bound to the energy molecule ADP and phosphate.
116
Each end of a thick filament contains how many heads of myosin?
Approximately 300.
117
How fast can a myosin head form and reform a cross bridge?
At least 5 times per second.
118
Do myosin heads coordinate?
No.
They all fire off their ATP independly and create cross bridges at slightly different times.
119
Why aren't myosin heads in sync?
To prevent filaments from sliding back into their original position.
120
What stimulates muscle contraction?
Action potentials from motor neurons.
121
What is the sarcoplasmic reticulum (SR)?
- The equivalent of the endoplasmic reticulum.
- Located in muscle cells.
- Full of calcium.
122
What are t tubules?
Invaginations of the plasma membrane *(of skeletal or cardiac cells)* that extend down into the cell.
123
Skeletal t tubules put the sarcolemma in close contact with...?
The sarcoplasmic reticulum (SR).
124
What common organelle is in abundance in muscle cells?
Mitochondria.
To produce the large amount of ATP required for muscular contraction.
125
What are the 7 steps of contraction of a muscle fibre as initiated by action potential?
1. The neurotransmitter released by the motor neuron binds to receptors on the muscle fibre's sarcolemma, triggering an action potential.
2. The action potential spreads down muscle t tubules, coming in close contact with the sarcoplasmic reticulum (SR), where it triggers the SR to open calcium ion channels.
3. Ca2+ flows from the SR into the cytosol of the muscle fibre.
4. Calcium binds with the toponin complex and triggers contraction.
5. When motor neurons' input ceases, receptors in the synaptic cleft remove the neurotransmitter from the synapse.
6. SR proteins pump calcium out of the cytoplasm back to the SR, restoring resting potential.
7. Muscle returns to rest.
126
What is the neurotransmitter that triggers muscle contraction?
Acetylcholine.
127
How is acetylcholine removed from the synapse?
Some combination of the following:
1. Degraded by acetylcholinesterase.
2. Dissipates in the synaptic cleft.
3. Reuptake may occur.
128
What is one function of acetylcholine?
Trigger muscle contraction.
129
What is slow-twitch muscle?
Muscle that contracts slowly but can continue contracting for a long time.
*Good for endurance activities like long distance running or cycling*
130
What is fast-twitch muscle?
Muscle that contracts quickly but consumes a lot of energy and tires quickly.
*Good for rapid movements like jumping to catch a ball or sprinting*
131
What are the different types of muscle?
1. Slow twitch muscle.
2. Fast twitch muscle.
132
How much faster contracting are fast-twitch muscles than slow-twitch muscles?
2 to 3 times faster.
133
What are oxidative fibres?
Muscle fibres that rely on aerobic respiration to produce ATP.
They produce low power contractions over long periods and are slow to fatigue.
134
What are glycolysis fibres?
Muscle fibres that rely on glycolysis to produce ATP.
They generate fast contractions and have a fast rate of fatigue.
135
What helps transform muscle contraction into movement?
A skeletal system.
136
What are antagonistic pairs?
Muscle pairs where one muscle contracts while the other relaxes.
*E.g. biceps and triceps or extensor and flexors*
137
What is the point of antagonistic pairs?
To prevent muscles from working against each other.
138
Animal muscles are...?
Anchored to a skeleton.
139
What are the different types of skeletons?
1. Endoskeletons.
2. Exoskeletons.
3. Hydrostatic skeletons.
140
What are endoskeletons?
Internal structural frameworks made of bones or cartilage that provide support and protection for an animal's body.
141
What are exoskeletons?
Rigid external coverings made of chitin or other materials that support and protect an animal's internal organs.
142
What are hydrostatic skeletons?
A system of fluid-filled compartments held under pressure within an organism's body that provide structural support and allow for movement.
143
What type of skeleton do vertebrates have?
An endoskeleton.
144
Are all endoskeletons made of bone?
No.
145
What type of skeleton do bivalves have?
Exoskeletons *(made of calcium carbonate)* that are enlarged by adding rings to the outer edge.
146
What type of skeleton do insects and arthropods have?
Exoskeletons that must be moulted and regrown as the animal grows bigger.
147
What type of animals have hydrostatic skeletons?
Soft-bodied invertebrates such as worms and jellyfish.
148
What do annelids *(such as earthworms)* use to move?
Muscles control their form and movement through changing the shape of their hydrostatic skeleton.
These muscles *(longitudinal and circular)* act together in peristalsis to enable movement.
149
An additional function of chitin exoskeletons is...?
Acting as a water-impermeable barrier, protecting the animal from desiccation.
