Chapter 3- Neurophysiology Flashcards
(84 cards)
1
Q
Neurophysiology
A
Study of chemical and electrical signals in neurons
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Q
Which region of the neuron receives chemical information?
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The dendrites
3
Q
Which part of the neuron integrates/processes information?
A
Integrated and processed in the cell body/axon hillock. The axon hillock can decide to send an impulse/action potential or decide not to.
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Q
Which part of the neuron transmits/conducts information?
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The axon
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Q
Action potential
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Action potentials are brief (transient) but large changes in the membrane potential, where the inside of the cell becomes positively charged, and a rapid electrical signal travels along the axon.
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Q
Intracellular communication
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Signals travel within cells/neurons
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Q
Intercellular communication
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Signals travel between cells/neurons
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Q
Neurotransmitter
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A chemical messenger between neurons- released at synapse
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Q
Phospholipid bilayer
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Cell membranes are phospholipid bilayers. A layer of fat on the inside of the membrane separates intracellular from extracellular and prevents some things from entering/leaving. The layer of fat is hydrophobic. There is a phosphate layer on the outside of the membrane that is hydrophilic.
10
Q
What is a neuron’s membrane surrounded with?
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Surrounded by fluid (mainly water) on both sides, intracellular fluid/cytosol and extracellular fluid (outside the neuron). Ions are dissolved in this fluid.
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Q
Membrane proteins
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Membrane proteins are embedded in the bilayer- these proteins transport things across the membrane. Like the membrane, proteins have hydrophobic and hydrophilic regions
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Q
Cation
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Ion with positive charge
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Q
Anion
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Ion with a negative charge
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Q
Electricity
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The movement of ions. Ions can generate an electrical signal when they move across a membrane.
15
Q
What type of molecule are ion channels and pumps?
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Proteins
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Q
Differences between ion channels and pumps (2)
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- Ion channels open to let an ion flow through in either direction. Ion pumps only transport in one direction (only go in or only out)
- Ion pumps require ATP, ion channels don’t
17
Q
Ways for ions to move across the membrane (2)
A
- Electrostatic pressure
2. Diffusion
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Q
Diffusion
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Ions move from regions of high concentration to low concentration, down the concentration gradient. It occurs until you have an equal concentration on both sides of the membrane- with a membrane, ions will redistribute over time. Diffusion is more relevant for ion channels, since they don’t use ATP. Diffusion is the chemical driving force of ions moving across the membrane.
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Q
When a neuron is at rest, which ions have a high concentration on the outside of the membrane?
A
Cations- Na+, Ca++
Anions- Cl-
20
Q
When a neuron is at rest, which ions have a higher concentration on the inside?
A
Cations- K+
There are also negatively charged proteins
21
Q
Electrostatic pressure
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Ions move across an electric field because they are charged- opposite charges attract, like charges repel. This is the electrical driving force of neurons moving across the membrane.
22
Q
Membrane voltage differential
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Inside of the cell is more negatively charged than the space immediately outside the cell
23
Q
How does electrostatic pressure affect selective ion channels?
A
Cations move into the cell
Anions move out of the cell
24
Q
Resting membrane potential
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The charge of a neuron in the absence of any other external input. It ranges from -60- -70 mV
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Sodium potassium pump
Uses energy to move 3 Na+ ions out and 2 K+ ions in for every energy molecule (against their gradient). This is necessary because nothing can happen in the cell when concentrations are equal
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How is the resting membrane potential generated/maintained?
K+ channels. Allow positively charged K+ ions to leave cell down concentration gradient, creates a negative charge inside the cell. When they're open, K can flow in either direction.
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Why would ion channels open/close? (3)
1. A ligand/chemical can bind a receptor
2. Temperature sensitive- might open if something is very hot or cold- the channels can change shape in response to temperature
3. Voltage sensitive- some channels open when they become less negative
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When anions flow into the cell, how does membrane potential change?
hyperpolarization (cell becomes more negative)
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When cations flow out of the cell, how does membrane potential change?
Hyperpolarization
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When anions flow into the cell, how does membrane potential change?
hyperpolarization (cell becomes more negative)
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When cations flow into the cell, how does membrane potential change?
Depolarization
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Parts of an action potential (5)
1. Stimulus causes a small depolarization of the neuron to the threshold voltage (-40 to -55 mV)- action potential triggered once it gets to the threshold
2. Depolarization- cell becomes more positive
3. Repolarization- when the membrane potential becomes negative
4. Hyperpolarization- when the membrane potential gets so negative that it undershoots the RMP
5. Resting state, the membrane returns to RMP
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All or none property
Information is encoded by number of action potentials, not size. Action potentials either happen or they don’t happen- no such thing as a big or small action potential. A very active neuron will generate a lot of action potentials very quickly, transferring information by the number of action potentials
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What occurs in resting membrane potential before an action potential is generated?
K+ channels are open, Na+ channels are closed. A small depolarizing stimulus causes membrane potential becomes less negative and approach threshold potential (-40 mV)
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What happens to voltage gated channels at threshold?
Voltage gated Na+ channels open/activate (because of change in membrane voltage). Na+ ions rush into the cell, and membrane potential becomes positive
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Absolute refractory period
After threshold, Na+ channels will inactive after about 1 millisecond. During inactivation, the channel is closed and is temporarily unable to open again. This means that no sodium can enter the cell and no action potential can be generated under any circumstances. Occurs during repolarization
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Relative refractory period
During hyperpolarization- not impossible for an action potential to be fired, but difficult since K+ channels are open. The neuron can generate another AP once it gets to RMP.
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What happens to potassium channels during depolarization?
As membrane depolarizes, voltage gated K+ channels slowly open/activate. K+ flows out of cell, and the membrane hyperpolarizes. Voltage gated K+ channels close but other K+ channels stay open. Na+ reset occurs.
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Active propagation
This occurs in an unmyelinated axon. The action potential is slow (10 m/s) but does not weaken. Sodium ions enter locally and depolarize the neuron, which then depolarizes the adjacent region to open more voltage gated sodium channels to regenerate the action potential down the axon. Takes many small steps down the axon- some chance of failure at every step, which isn't good because we want the action potential to keep flowing.
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Passive propagation
Occurs in a myelinated axon. Action potential triggered at axon hillock, moves passively through the myelinated segment- quick (150 m/s), but signal weakens as it travels. After sodium channels open, myelin channels the depolarization down the axon interior. Depolarization spreads within the axon very rapidly. Then, depolarized Na+ channels open to recreate the action potential at the new node. At the Node of Ranvier, it regains full charge through active means (Na/K channels).
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What type of axons tend to be myelinated?
Really long axons tend to be myelinated. Jumping to gaps makes the signal travel much faster
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Tetrodotoxin (puffer fish) and saxitoxin (algae) effect
Block voltage gated sodium channels- no action potential generated.
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Batrachotoxin (frog) effect
forces sodium channels to stay open
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Agitoxin (scorpion) and beta-bungarotoxin (snake) effect
Block voltage gated potassium channels
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Parts of the synapse (3)
The synapse consists of the presynaptic axon terminal, synaptic cleft where chemical signal is released, and the postsynaptic dendritic spine
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Presynaptic side of the axon terminal
The axon terminal is the output zone. Axon terminals from one neuron release chemical signals onto many other neurons (the dendritic spines)
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What happens when an action potential reaches the presynaptic axon terminal?
Voltage gated calcium channels open, and calcium enters the terminal. Calcium causes synaptic vesicles to fuse with the presynaptic membrane, which releases neurotransmitter into the synaptic cleft through exocytosis
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What causes calcium to enter the cell when calcium gated channels open?
There is more calcium outside the cell than inside, so it enters due to the electrochemical gradient.
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Vesicle
packets of neurotransmitters
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What proteins are located at the axon terminal (3)
1. v-snares, attached to the vesicle
2. t-snares, attached to the presynaptic membrane
3. Synaptotagmin, attached to the vesicle
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Role of snare proteins
These proteins serve as tethers. When the v-SNAREs attach to the t-SNAREs, the vesicle is considered docked and ready to be released. Synaptotagmin wraps around the snare complex and pulls vesicles toward the membrane to trigger the final fusion of the vesicle with the membrane.
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Role of synaptotagmin
Synaptotagmin acts as a calcium sensor. When calcium ions enter the axon terminal, they activate synaptotagmin, which will wrap around the snare complex and pulls vesicles toward the membrane. This will trigger the fusion of vesicles and presynaptic membranes so the neurotransmitter can enter the synaptic cleft.
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What is the general function of neurotransmitters?
Neurotransmitters are the chemicals that are the basis for communication between neurons.
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Glutamate
Excitatory neurotransmitter
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GABA
Inhibitory neurotransmitter
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Dopamine
Neurotransmitter involved in motor control and reward
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Norepinephrine
Neurotransmitter involved with stress
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Acetylcholine
Neurotransmitter involved with muscle contraction
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Serotonin
Neurotransmitter involved with stress and many other functions
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How many neurotransmitters does a neuron use?
Neurons only release a selection of neurotransmitters- one, two, or sometimes 3 different types
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How are neurotransmitters produced?
A specific enzyme (protein) is involved in producing each neurotransmitter. Often, that enzyme converts an amino acid we derive from our diet into a neurotransmitter. Neurons must synthesize their neurotransmitter and move it into vesicles.
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How do neurons cross the synaptic cleft?
They diffuse across the fluid in the synapse to the postsynaptic membrane. They will float around until they get to a binding site, where they fit like a lock into a key- they don't "know" which receptor they're going to.
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What happens to extra neurotransmitter? (3)
1. Degraded by enzymes
2. Taken up by presynaptic terminals or astrocytes (tripartite synapse). The presynaptic neuron can participate in reuptake, where the presynaptic neuron removes neurotransmitter from the synaptic cleft and recycles it.
3. Neurotransmitter can also diffuse away
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What composes the postsynaptic side of the synapse?
The dendrites (branches) and spines act as the input zone where information is received. A neuron has many dendrites/spines. Each neurotransmitter binds with its own receptor on the postsynaptic side (GABA binds with a GABA receptor, etc.).
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Dendritic spines
Mushroom shaped protrusions from dendrites. Each spine represents a synapse where information is received
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Functional categories of postsynaptic receptors (2)
1. Ionotropic receptors
| 2. Metabotropic receptors
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Ionotropic receptors
These types of receptors are ligand gated ion channels. They are chemically activated by neurotransmitter binding to open Cation (Na+, K+, Ca +2), or anion (Cl-) channels. This type of receptor has a fast response.
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Metabotropic receptors
A type of G-protein coupled receptor- after a neurotransmitter binds, the receptor activates a G-protein, which is coupled to enzymes/ion channels. The G-protein can then open an ion channel or activate enzymes to trigger other biochemical reactions within the cell. The action of these receptors is slower, but has more widespread effects in the cell.
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How do G-proteins work?
G proteins can open ion channels themselves, but they can also activate other internal chemicals that will open ion channels. The neurotransmitter is the first (external) messenger that arrives at the postsynaptic cell, and the next chemical signal activated by the G-protein is considered the second messenger. Second messenger systems can amplify and prolong the synaptic signals that a neuron receives.
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Ligand
Any substance that can bind to a target protein. Can include neurotransmitters and drugs.
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Agonist
An agonist can act like a neurotransmitter and bind to a receptor to open it (has the same effect as the neurotransmitter). Could be used to treat someone with a neurotransmitter deficiency.
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Inverse agonist
Binds to a neurotransmitter receptor, but has the exact opposite effect.
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Antagonist
These molecules bind to receptors but don’t activate them. They just block agonists/neurotransmitters from binding to the receptors. Can be used if the neuron makes too much neurotransmitter. Epilepsy is usually caused by too much neurotransmitter activity, patients can be prescribed a glutamate antagonist.
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Postsynaptic potentials
Postsynaptic potentials are brief changes in the membrane potential of the postsynaptic cell after a neurotransmitter is released into the synapse. Neurons receive hundreds of these, so they can generate an action potential when integrated. However, postsynaptic potentials can be excitatory or inhibitory.
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Excitatory postsynaptic potential
When an excitatory presynaptic neuron fires, it releases a neurotransmitter that causes a small local depolarization in the postsynaptic neuron as cations enter the cell. This has an excitatory effect on the postsynaptic neuron because it moves closer to threshold. Usually, many EPSPs are required to trigger an action potential- how tall it is depends on how many ion channels are opened
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Inhibitory postsynaptic potential
With an inhibitory postsynaptic potential, the postsynaptic neuron experiences an increase of the resting membrane potential (hyperpolarization). This decreases the probability of the neuron firing an action potential. This results from chloride anions entering the cell (or from cations leaving).
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Electrochemical transmission definition
The chemical aspect refers to the presynaptic cell releasing neurotransmitter onto the postsynaptic cell. The electrical aspect refers to the excitation or inhibition of the postsynaptic cell, and the possibility of altering the biochemical processes within the postsynaptic cell via G-proteins
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What is the path of EPSPs and IPSPs within the cell?
Excitatory or inhibitory postsynaptic potentials spread over dendrites (input zone) and cell body to the axon hillock (where the action potential is actually generated, if it occurs).
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Summation
combining signals received from other neurons
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Integration
Translating signals into a decision on whether to send an output to the next neurons
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Spatial summation
The summation of potentials from different locations on the cell body arriving at the same time. Two EPSPs occurring at the same time at synapses close to the axon hillock will produce a larger sum than those occurring from further away. All potentials are summed together, and an action potential is only generated if the sum is enough to depolarize the cell to threshold.
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Temporal summation
Postsynaptic potentials last a few milliseconds before fading away, so they overlap more and have a bigger summation if they occur closer together. This is called temporal summation.
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How does integration of synaptic inputs occur?
Many excitatory and inhibitory inputs are summed. If the sum reaches threshold potential, an action potential is triggered. Action potentials begin at the axon hillock (initial segment of the axon). The size of the summation is influenced by when the input arrives and what part of the cell the input arrives at (the basis of spatial and temporal summation).
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Whether a PSP is excitatory or inhibitory depends on
The type of neurotransmitter released and the type of voltage gated channel that opens.
