Nerve/Synapse Flashcards
What are the three chracteristic structures of all neurons?
- Cell Body(soma)
- Dendrites
- Axon
What is the Soma/Cell body?
-This is where the nucleus, mitochondria and ribosomes are
-Metabolism occurs here
-Keeps the neuron alive (you can chop of the dendrites and axons but if you chop the soma the neuron will die)
What dendrites?
-Recieve inputs coming from other neurons
-Bigger branching dendrites means that the neuron can recieve a huge number of synapses and information from other neurons
What is an axon?
-Enables the neuron to send messages to other neurons
How many xons does each neuron have?
Only one
How long are axons?
Can be very long going from the tow to the brainstem or they can be really short
How does information flow through neurons?
Information flows from dendrites into the cell body and then down the axon
What are synapses?
Places where other neurons can input information to another neuron
Where are synapses found?
On the dendrites of neurons
What is the resting membrane potential of a typical neuron?
-70mV compared to the outside of the neuron
Why is the resting membrane potential of a neuron -70mV?
The inside of the neuron contains a small excess of negatively charged ions
Are there more Na+ ions inside the neuron or outside the neuron?
Outside, sodium want to go into the neuron
Are there more K+ ions inside or outside the neuron?
Inside, the potassium ions tend to want to flow out of the neuron
Is there more Cl- inside or outside of the neuron?
Outside, Cl- tends to want to flow into the neuron
What is the membrane of the neuron permeable to at rest ?
The neuron membrane at rest is permeable to potassium ions and impermeable to sodium ions, chlorine ions and other physiological ions
Explain why the inside of the neuron is slightly negative?
At rest, the membrane of the neuron is permeable to K+ ions. Since the concentration of K+ ions is higher inside the neuron, the K+ ions tend to flow out of the neuron. This leaves behind impermeable negatively charged ions in the neuron
What is an electrical gradient?
Determines the direction ions will flow based on charges
Ex. More positive charge inside a cell = more negative ions will want to flow in the cell
How does the electrical gradient react when K+ ions flow out of the neuron?
The electrical gradient tends to pull K+ ions back into the cell
When does the potassium stop leaking out of the cell?
When the electrical gradient and concentration gradient are equal. This results in the membrane potential of -70mV
-At this point potassium ions continue to flow out of the neuron but they also continue to flow into the neuron (the rate at which they flow in/out are equal to eachother ) this maintains the -70mV potential
How do we determine when the potassium has reached an equilibrium?
Nernst Equation
What would happein if you add potassium to the outside of the cell, until the concentration of potassium outside and inside the cell are equivalent?
-The membrane potential would be 0mV
-There would be no concentration gradient for potassiums to flow out of the neuron
What would happen if we decreased the difference between the concentrations of potassium outside/inside the cell?
- The membrane potential equilbrium would be closer to zero
-There would be less of a concentration gradient and thus less K+ ions would flow out of the neuron
What makes the neurons membrane impermeable to ions at rest?
The membrane is made out of phospholipids which make it impermeable to ions
What makes the neuron membrane permeable to potassium ions at rest?
The membrane is covered in ion channels that allow potassium ions to pass through the membrane at rest
What are the potassium ion channels of the neuron membrane known as?
“leak channels”
Since they give the cell its leaky property and they are not gated and always remain open
Does the membrane ion channels for other types of ions?
Yes, but these channels are gated and remain closed when the membrane is at rest
Why is the resting membrane potential more positive than the equilibrium of the potassium ions?
The neuron is not perfectly impermeable to all ions so it lets a few Na+ ions in. These Na+ ions push the membrane to be slightly more positive
The resting membrane potential is between…
The equilibrium potential of potassium and the equilibrium potential of sodium but it is much closer to that of potassium since they are permeable
If we had a cell that perfectly impermeable to all ions except K+ what would the resting membrane potential be?
-90mV
At rest is the membrane at an equilibrium potential?
No, but it is between the Na+ and K+ equilbrium potentials (closer to K+) (dominant permeability wins)
What is the point of the sodium potassium pump?
Since sodium is constantly leaking into the neuron and potassium is constantly leaking out of the neuron their concentration gradients are gradually decreasing and eventually there will be no more concentration gradients or membrane potential
-The sodium-potassium pump helps maintain the concentration gradients
What determines the membrane potential of the neuron?
-Concentration gradients
-permeability of ions
What permeabiliy makes the greatest contribution to the membrane potential?
-The dominant permeability
-Ex. At rest, the dominant permeability is K+. When active dominant permeability changes to other ions
Functions of the sodium-potassium pump?
- Maintains/Creates concentration gradient of sodium and potassium
-Maintains resting membrane potential
Where does the energy that drives the nervous sytem come from?
The concentration gradient of created by the sodium potassium pump is a source of potential energy used by neurons to send electrical signals through the nervous system
-The energy comes from ATP driving the sodium potassium pump
How does the sodium-potassium pump work?
- Hydrolyzes ATP into ADP
- Uses the energy of the hydrolysis to pump 3 Na+ ions from inside the cell out and 2 K+ ions from outside the cell back in
What type of transport is the sodium-potassium pump?
Active transport(requires energy) because it is pumping against the concentration gradient
How do axons propagate information from one neuron to another?
Through action potentials
What is used to send signals to other neurons?
In neurons the resting membrane potential is used to send signals
What is an action potential?
Electrical signal that neurons send
Where does the action potential start?
At the initial segment (point where the axon attaches to the cell body)
Where does the action potential end in terms of the axon ?
The presynatpic terminal (end of the axon)
What is an action potential?
Brief change in the membrane potential
What happens to the membrane potential during an action potential?
Membrane goes from its resting potential of -70mV up to a more positive potential. Then is comes back down to -70mV
Memebrane is depolarizing
When the membrane goes from its resting potential -70mV up to +30mV
Polarized Membrane
When the membrane is at -70mV
Depolarized membrane
When the membrane is at its positive level
Repolarizing membrane
When the membrane goes from +30mV back down to -70mV
Hyperpolarizing Membrane
When the membrane potential goes below -70mV
What is the threshold potential?
The minimum amount that the membrane of the initial segment must be depolarized to in order for an action potential to occur
What happens if the membrane is depolarized to the threshold potential ?
An action potential will be propagated down the axon
What happens if the membrane is depolarized below the threshold potential ?
No action potential will be propagated
Does how high above the threshold potential the depolarization is affect how large the action potential is?
No, as long as the depolarization is above the threshold potential you will get a FULL action potential
What causes the depolarization of the initial segment and production of an action potential?
Sodium ions flowing into the cell through voltage-gated sodium channels
What are the three properties of sodium channels?
- Closed at the resting membrane potential, but open when the membrane depolarizes
- Selective for Na+
- The open channel rapidly inactivates, stopping the inflow of Na+ ions
Where are leak channels located on the neuron?
Leak channels are found everywhere on the neuron
but they are less numerous than Na+ voltage-gated channels on the axon
Where are voltage-gated sodium channels located on the neuron?
They are found on the axon and initial segment
What is the absolute refractory period and what does it determine?
-The time it takes a voltage-gated sodium channel to go from its inactivated state to its closed state
-Determines how fast neurons can fire action potentials
How does depolarization of the membrane lead to more sodium channels opening(positive feedback)?
- If we depolarize the initial segment to the threshold potential, a small fraction of sodium channels will open
- This further depolarizes the membrane resulting in the activation of even more sodium channels
What is the threshold potential in terms of sodium channels?
It is the point at which you have enough sodium channels open to trigger even more sodium channels to open
What happens when the sodium channels inactivate?
The membrane drops back to its normal resting potential(-70) (repolarizes)
-Potassium will leak out of the initial segment until the membrane reaches -70mV
What is the dominant permeability during action potentials?
The sodium ions
-This means that the membrane potential will be closer to that of the equilibrium potential of Na+ during actioon potentials
What is the dominant permeability ion during repolarization?
K+
T/F: At the peak of the action potential the membrane potential is positive?
True, there are more positive ions in the initial segment
what are voltage-gated potassium channels and when are they open/closed?
-Channels permeable to potassium
-Closed at the resting membrane potential
-Open when the membrane is depolarized (more positive then -70mV)
Sodium vs Potassium voltage-gated channels
Sodium:
- As soon as the membrane potential reaches the threshold the voltage-gated sodium channels open really fast
-At the peak of the action potential the sodium channels inactivate
Potassium:
- Open more slowly once the membrane potential reaches the threshold
-The potassium channels are maximally open during the repolarizing phase of the membrane(when the sodium channels are inactive)
What is the purpose of voltage-gated potassium channels?
-Allows potassium to flow out of the intial segment faster and bring the membrane potential back down to resting
-Ensures the action potential is rapid
Why do action potentials need to be rapid?
To transmits as much information as possible
Why does the membrane hyperpolarize after an action potential?
Because both voltage-gated potassium channels and leakage channels are letting potassium flow out of the neuron
How does the sodium potassium pump affect the action potentials sent by neurons?
When the neuron fires an action potential sodium enters the neuron and potassium leaves eventually this would lead to no concentration gradients. The Sodium-potassium pump helps maintain the concentration gradients not only for the resting membrane potential but also for action potentials
What would happen if the sodium potassium pump stopped working?
- No concentration gradient
-No action potentials could be fired
How is the action potential propogated down the axon?
- Depolarization of the initial segment
- Positive charge of the initial segment attracts the negative resting membrane potential further down the axon
- The positive charge will move toward the negative part of the axon and depolarize that segment
- This will then continue down the entire axon
The action potential attracts both sides of the segment?
The positive charge that flows in will be attracted to both the left and right side of the segment, because the left side of the segment that was the previous action potential will be back to -70 mV
Why doesn’t the action potential start going back the way it came?
Because of the sodium channel inactivation
-The region may get depolarized but it won’t generate an action potential because the sodium channels are inactivated
Why do sodium channels need to line the entire axon?
If they did not the action potential would not spread the entire way down the axon
What is the relative refractory period?
Period where the axon may fire another action potential but it is unlikely
When is the relative refractory period?
After the action potential when the membrane potential is slightly more negative due to the two kinds of potassium channels being open
How do neurons convey information through action potentials if they are all the same?
Neurons convey information to other neurons by varying how fast they fire the action potentials(frequency)
Can an action potential be large or small?
No, every single action potential fired is identical they just vary by frequency
How does Puffer Fish Toxin (TTX) affect neurons?
TTX plugs up voltage-gated sodium channels which prevents neurons from firing action potentials and results in death
How does Batrachotoxin from phylobates frogs affect neurons?
Batrachotoxin makes the sodium channels irreversibly open, this causes the neurons to fire action potentials constantly and you will die from a seizure
How do local anesthetics such lidocaine used by dentists affect neurons?
Lidocaine is injected into nerves of the mouth and blocks sodium-gated ion channel. Sensory neurons in your tooth will no longer fire when the dentist starts drilling it (no pain)
How do antiepileptic drugs affect sodium voltage gated ion channels?
These drugs block voltage-gated sodium channels
What is a seizure?
When the neurons in your brain start depolarizing too much and generating too many action potentials
When are fast action potentials useful?
If a projectile is being thrown at us
Thin axons vs Fat axons
Thin axons:
- Propagate action potentials very slowly
Fat Axons:
-Propagate action potentials more rapidly
What is the squid axon?
A FAT axon that runs down the middle of the squid and allows squids to shoot itself away rapidly when it sees a predator
How do neurons in our brain that are small have high conduction velocity?
They are wrapped in cells called myelin
What is myelin?
Cells that wrap neurons and allow axons to propagate more rapidly
-Allows us to make compact axons that can transmit signals rapidly
Myelin in the PNS vs the CNS
PNS:
-Myelin is made up of Schwann cells
CNS:
-Myelin is made up of oligodendrocytes
Is there myelin at the initial segment ?
No
What are the Nodes of Ranvier?
Gaps in the myelin where the axon is exposed
What axons tend to be myelinated?
Long axons as they are sending signals over a long distance
What is found at the nodes of ranvier?
A high density of voltage gated sodium channels
Ar there voltage-gated sodium channels where the myelin wraps around the axon?
No
How are action potentials fired in myelinated axons?
- Initial segment goes from -70mV to +30mV
- Myelin acts as a n insulator and allows the charge produced by the initial segment to spread out further/faster to the next node of ranvier
- As the charge spreads out over the myelinated portion of the axon, the membrane potential gets closer to the resting potential and then before it reaches the resting potential there is a node that brings the potential back up to +30mV
Myelinated axon vs Unmyelinated axon
Myelinated:
- Regeneration of the signal only happens periodically at the nodes of Ranvier
Unmyelinated:
- Must continuosly regenerate the action potential
What is multiple sclerosis?
An autoimmune disease in which the immune system attacks the myelin in the CNS made up of oligodendrocytes
What happens when a myelinated axon is demylinated?
The axon will propagate action potentials abnormally or will fail to propagate action potentials
What are the symptoms of MS?
Movement abnormalities, Cognitive Deformities
What happens in MS patients after the axon is demyelinated?
Initially after an episode of demyelination there is a recovery period where myelin will grow back and the axon recovers but after time as the axons are continuously stripped of myelin the axons become damaged and the patient will no longer recover
What is white matter of the brain?
Region of the brain where myelinated axons are bundled together
What is gray matter of the brain?
Region of the brain where the cell bodies, dendrites and synapses are
Where do most synapses occur?
On dendrites of neurons
What are axodendritic synapses and what are the two types?
Synapses that are found between dendrites and axons
1. Spine synapse
2. Shaft synapse
What are spine synapses?
-Excitatory synapses
-Vast majority of synapses in neurons
-Bind to the spine of the dendrite
What is a shaft synapse?
-Mainly inhibitory synapses
-Bind directly onto the shafts of the dendrite
What are axosomatic synapses?
Synapses tat are on the cell body
-Usually inhibitory
What are Axoaxonic synapses?
The pre synaptic terminal of one neuron is making a synapse with the presynaptic terminal with the axon of another neuron
Pre-synaptic vs Post-synaptic?
Pre-synaptic: everything that is upstream of the synapse
Post-synaptic: everything downstream of the synapse
What axons are typically responsible for the flow of information in neurons?
Axodendritic and Axosomatic
What is divergence?
The idea that neurons only have one axon but that axon can be sending signals all across the nervous system by branching
-Each neurons synapses with multiple other neurons
What happens when an action potential reaches a branch point of an axon?
The full blown action potential will go down both branches with the same frequency
-The neuron will only send one signal by each branch will receive the same signal and it will go to different parts of the body
Is the presynaptic terminal attached to the postsynaptic spine?
No
What is the synaptic cleft?
The space btween the presynaptic terminal and the postsynaptic spine
Why is the synaptic cleft so small?
Because neurotransmitter diffuses across this space and it the space is too large it would take to long for the neurotransmitter to reach the postsynaptic spine
What are the presynaptic vesicles and where are they located?
Vescicles made up of plasma membrane that contain neurotransmitter
-Located in the presynaptic vesicles
What are the two types of presynaptic vesicles?
- Reserve pool of vesicles (randomly distributed throughout the presynaptic terminal)
- Vesicles lined up along the presynaptic terminal facing the cleft
What reserve pool vesicles ?
-not directly involved in synaptic transmission (backup supply)
-If the synapse is really active and we need more vesicles these vesicles can be called into play
What are “docked” vesicles ?
The vesicles lined up along the presynaptic terminal and facing the cleft, they are tethered to the membrane adjacent to the synaptic cleft
Which type of vesicles are directly involved in synaptic transmission?
“Docked” vessicles
What are “active zones” ?
Regions where the “docked” vessicles are lined up along the presynaptic terminal
What are postsynaptic densities?
Regions in the postsynaptic membrane that are right under the active zone. These regions are darker because they are packed full of proteins that are important for synaptic transmission
What two ion channels are found at the synapse?
- Voltage-gated calcium channels
- Ligand-gated ion channels
Where are voltage-gated calcium channels found?
The presynaptic terminal, right next to the docked vesicles in the active zone (calcium channels and the vesicles are part of a protein complex)
When are voltage-gated calcium channels opened/closed?
Closed: At the resting membrane potential
Open: when the membrane of the presynaptic terminal is depolarized
How do ions flow when the calcium voltage-gated channel is opened?
The inside of cells has a lower concentration of calcium ions than the extracellular solution so Ca2+ flow into the presynaptic terminal when the channel is open
Why do cells keep the intracellular calcium concentration low?
Because calcium is a signal to activate intracellular processes, the smallest change in calcium will trigger biochemical events in that initiate transmission in the presynaptic terminal
How are ligand-gated ion channels activated?
By the binding of small molecules to the binding sites on their extracellular surfaces
Once the ligand binds this causes the ion channel to open up and allow ions to flow through it
What happens when a synapse is active?
- Action potential propagates down the axon, then depolarizes the membrane of the presynaptic terminal
- When the terminal depolarizes that activates the voltage-gated calcium channels and calcium starts flowing into the neuron
- The calcium triggers a series of biochemical events that lead to the fusion of a few of the docked vesicles with the presynaptic membrane
- Fused vesicles then open and dump their neurotransmitters into the synaptic cleft
- Neurotransmitters diffuse across the cleft and bind to binding site of the ligand-binding channels in the postsynaptic membrane
- When ligand-binding channels open ions can flow in/out of the postsynaptic membrane
Where are ligand-gated ion channels found?
The postsynaptic spine
What happens after calcium enters the presynaptic terminal ?
Calcium binds to a protein of the protein complex this results in a conformation change of the protein which then activates a sequence of events that causes the vesicle to fues with the membrane and dump its contents into the synaptic cleft
What is the vesicle recycling process?
Vesicle fuses to the membrane releases its contents then will get pinched back up again and filled with neurotransmitters again
How does Botox act on the vesicle fusion process?
Botox is taken up by presynaptic terminals and it will chew up the proteins that are part of the calcium-vesicle complex. This prevents the vesicle fusion process from occuring because the protein can no longer undergo a conformational change. Now the synapse is no longer functional.
How does Tetanus toxin affect the vesicle fusion process?
Tetanus toxin is taken up by the presynaptic terminals of the neurons. Tetanus then degrades the proteins of the vesicle-calcium channel complex preventing vesicle fusion.
How does black widown spider venom act on the vesicle fusion process?
Causes the vesicles to start fusing with the plasma
-Opposite affect of Botox and Tetanus toxin
What are the two possible postsynaptic responses to neurotransmitter?
- Excitatory Postsynaptic potential(EPSP): depolarizes the postsynaptic membrane
- Inhibitory postsynaptic potential(IPSP): hyperpolarizs the postsynaptic membrane
Where are excitatory synapses found?
-Mainly on spines of dendrites
-Most synapses in the brain
What happens when an excitatory synapse is active?
Makes the postsynaptic cell more depolarized, as a result this pushes the postsynaptic cell closer to the action potential and makes it more likely to fire an action potential
Where are inhibitory synapses found?
Mainly on the shafts of the dendrite
What happens when an inhibitory synapse is active?
The postsynaptic cells hyperpolarizes and moves further from the action potential threshold and it becomes less likely to fire an action potential
What is the main excitatory neurotransmitter in the brain?
Glutamate
What is glutamate?
An amino acid and neurotransmitter found in virtually all of the excitatory synapses of the brain
What are two types of ligand-gated ion channels in the postsynaptic spine?
-NMDA receptors
-AMPA receptors
Also called neurotransmitter receptors because they have binding receptors for neurotransmitters and are activated by neurotransmitters
What are ionotropic receptors?
Ion channels that open in response to binding of small molecules(neurotransmitters) to receptor sites on their external surfaces.
Ex. AMPA and NMDA
What axons are typically excitatory?
Long axons that carry info from one part of the nervous system to the other or from one body part to another
What are AMPA receptors responsible for?
Excitatory depolarization (EPSP)
T/F: AMPA receptors typically have several binding sites for glutamate to help open the ion channel?
True
How do excitatory synapses work?
- Action potential in the presynaptic neuron gets to the presynaptic terminal via the axon
- Calcium channels open and calcium flows into the terminal and the vesicles fuse and release glutamate
- Glutamate diffuses across the cleft anf binds to the AMPA receptors
- AMAP channel activates and is permeable to sodium ions
- Sodium ions then flow intot the postsynaptic spine and depolarize it (EPSP)
What receptors are activated by glutamate?
AMAP receptors and NMDA receptors
What receptors are responsible for EPSP?
AMPA receptors
How much does a typical EPSP depolarize the membrane by?
1-2mV
Can a single EPSP trigger an action potential?
No, because in order to trigger an action potential the membrane must be depolarized to -50mV
If EPSPs are too small to trigger an action potential, how are action potentials triggered?
A bunch of EPSPs occur simultaneously which depolarizes the initial segment enough and leads to an action potential (a typically neuron needs about 50-100 EPSPs at once)
When is the NMDA receptor closed/open?
Closed: At resting potential (-70mV). A magnesium ion goes into the pore and blocks ions from entering the pore
Opened: When glutamate binds to the NMDA receptor the channel opens. Magnesium will only come out of the pore if the postsynaptic membrane is already depolarized
What does the NMDA receptor allow to flow thorugh its channel?
When open the NMDA receptor allows calcium ions to diffuse into the postsynaptic terminal
Why does the magnesium ion only leave the pore once the membrane of the postsynaptic terminal has been depolarized?
Because a depolarized membrane is more positive and thus repels the magnesium ion which is why it leaves
What does the calcium do once it enters the postsynaptic terminal?
Triggers a series of biochemical events that increase the strength of the synapse
When are only AMPA receptors active and how does this affect the strength of the synapse?
At membrane potential (-70mV) only AMPA receptors allow the flow of ions. Although glutamate binds to both NMDA and AMPA receptos there will be no current through the NMDA receptor due to magnesium blocking the pore
When are both AMPA and NMDA receptors activated, how does this affect the strength of the synapse?
Both AMPA and NMDA receptors allow the flow of ions when the postsynaptic terminal is depolarized.
-Glutamate binds to both NMDA and AMPA receptors and the depolarized membrane forces the Mg out of the NMDA pore.
-This results in a current flowing through both of the receptors which leads to a strengthened synapse
What is synaptic plasticity?
-The idea that synapses can change and can get stronger
What is changing the strength of the synapses?
The basis of learning and memory
T/F: When you learn new things you synapses become stronger?
True
What is long-term potentiation?
What occurs when synapses in the brain change in strength
How does long-term potentiation work?
Synapses that are really active get stronger and are able to be activated by less action potentials due to bigger EPSPs
Synapses that are not really used are weaker and require more action potentials to be activated
How does the synapse get stronger?
The influx of calcium by NMDA receptors causes more AMPA receptors to get inserted into the membrane
-This means that the same amount of glutamate will make an even bigger EPSP because there are more AMPA receptors (more flow of ions)
How long do changes in synapses last?
Can last for days/hours because you can learn things that you can remember for a long time
What is exocitotoxicity?
When the concentration of glutamate in the brain is high and the glutamate is toxic to neurons
T/F: Glutamate levels are tightly regulated in the brain and glutamate is cleared out of the synaptic cleft very quickly?
True
How can a stroke cause exocitotoxicity?
When someone has a stroke certain regions of the brain get no blood/oxygen the neurons in that region die. When neurons die they depolarize and release glutamate. The glutatmate diffuses to surrounding areas of the brain and activate AMPA and NMDA receptors. This causes a lot of calcium to flow into neurons, the increase in calcium inside the neurons causes neurons to die in other regions of the brain
Where do inhibitory synapses come from?
Come from neurons within the same region
What are the two functions of inhibitory synapses?
- Act as a break in excitation
- Can shape and sculpt firing pattern of excitatroy neurons
Why do inhibitory synapses create breaks?
Neurons make synapses with several other neurons and those neurons also make synapses with hundreds of other neurons, this creates a positive feedback loop where if there is no break it can get out of control and cause a seizure
What is the main inhibitory neurotransmitter in the brain?
GABA
Where are vesicles filled with GABA found?
Vesicles with GABA are found in the presynaptic terminals of inhibitory synapses
What are the receptors for inhibitory synapses neurotransmitter in the postsynaptic membrane called?
GABAa receptors
How do GABAa receptors work?
- GABA is released by the presynaptic terminal of the inhibitory synapse
- GABA binds to the GABAa receptor and activates the channel to open
- The GABAa channel allows Cl- ions to flow into the postsynaptic membrane
- When Cl- ions flow into the membrane the neuron is hyperpolarized and pushed further from the action potential threshold
How do Barbiturates and Benzodiazepines and Ethanol affect GABA receptors?
Both of these compounds bind to the GABAa receptor and make the receptor more responsive to GABA. This results in more IPSPs and makes you sleepy(increasing inhibition)
What happens when there is a bunch of activity in excitatory synapses but no activity in inhibitory synapses?
The neuron will fire an action potential
What happens when there is no activity in the excitatory synapses and lots of activity in the inhibitory synapses?
The neuron will not fire an action potential
What happens when both excitatory and inhibitory synapses are active?
An action potential will be fired if the excitatory synapses have enough excitation to overcome the inhibitory synapses
How do neurons communicate with each other?
Through action potentials
If a neuron is excitatory what will it release at its presynaptic terminal ?
Glutamate
If a neuron is inhibitory what will it release at it presynaptic terminal ?
GABA
Can neurons be both excitatory and inhibitory?
No, only one or the other
How are inhibitory synapses activated?
The dendrites of inhibitory neurons are covered with excitatory synapses that activate it to fire action potentials
What are Metabotropic Receptors?
Receptors that are found at the synapses but that are not ion channels.
How are metabotropic receptors activated?
By neurotransmitters but they do not form ion channels
What type of receptors are Metabotropic?
G-protein, Coupled receptors, GPCRs
What are local synapses ?
When a neuron has a synapse that feedsback to the orignal synapse that is activating the neuron
How do inihibitory neurons attached to the original neuron work?
- Orignal neuron will fire 10 action potentials, these action potentials will excite all of the neurons attached to the axon including the inhibitory neuron
- Now that the inhibtory neuron is activated. It is bound to the dendrites of the first neuron and will put a limit on how many action potentials the neuron can fire
How are metabotropic receptors activated?
Glutamate binds to the binding site of metabotropic glutamate receptors
What happens when glutamate binds to the metabotropic receptors?
The binding of glutamate induces a conformational change in the receptor which activates it
What happens when the metabotropic receptor is activated?
It transmits a message to the inside of the cell that activates a sequnce of proteins inside of the cell that results in the synthesis of second messenger molecules
What are second messenger molecules and give examples?
-Small molecules that can diffuse within the cell
-They are a chemical signal spreading out on the inside of the cell
-Ex. Cyclic Am-P
What do second messengers do?
- Bind to ion channels and cause them to open
- Can activate proteins(ex. protein kinases)
- Can bind to proteins that then migrate into the nucleus of the cell and regulate the transcription of genes
Describe the second messenger mediated response?
- Metabotropic receptor is activated by glutamate
- Receptor then activates proteins
- The protein synthesize second messenger molecules
- Second messenger molecules build up inside the cell
- Second messenger molecules diffuse throughout the cell
- Second messenger molecules activate other proteins and ion channels
- Now that protein has to do its thing
Why are metabotropic receptors considered slow synaptic responses?
Longer process compared to AMPA and NMDA receptors
-Can take several days to turn off
What receptors are found at excitatory synapses?
-AMPA receptors
-NMDA receptors
-Metabotropic glutamate receptors
What happens when an excitatory synapse is activated?
- Causes fast EPSPS
- Causes slow changes mediated by metabotropic receptors
What happens when an inhibitory synapse is active?
- Causes fast IPSPs
- Causes slower changes mediated by metabotropic GABA receptors (GABAb)
What receptors are found at inhibitory synapses?
-GABAa (fast)
-GABAb(slow)
T/F: glutamate and GABA activate both ionotropic and metabotropic receptors?
True
Glutamate activates: AMPA, NMDA and metabotropic glutamate receptors (all excitatory)
GABA activates: GABAa and GABAb (all inhibitory)
T/F: Majority of neurons in the brain release GABA or Glutamate neurotransmitters?
True
What substances other than GABA and glutamate that neurotransmitters in the brain release?
What are these substances called?
-Dopamine
-Serotonin
-Norepinephrine
-Neuropeptides
-Endorphins
These substances are known as neuromodulators
Describe neurons that release neuromodulators?
Neurons that release neuromodulators often originate in the small brainstem or midbrain and have axons that project diffusely throughout the brain
What does norepinephrine do ?
When you sleep norepinephrine neurons do not fire but when you are awake the neurons fire at a steady pace.
-If you have to suddenly pay attention to something the norepinephrine neurons start firing faster
-Norepinephrine regulates sleep/wake cycles and attention
How does norepinephrine get all over the brain if it only comes from a tiny cluster of neurons?
The norepinephrine neurons have axons that extend throughout the brain and when they release norepinephrine they spritz it everywhere
How does Ritalin influence norepinephrine neurons?
Given to people with ADD ritalin influences the firing of norepinephrine neurons
What does serotinin have to do with?
Mood
What do neurons that release dopamine involve?
Reward and learning
How do drugs impact dopamine?
All drugs and substances of abuse cause an increase of dopamine in the brain
Describe neuromodulators?
Neuromodulators are a small number of neurons but their effects are global and they modulate overall brain state. They affect how the entire brain is working (ex. How you feel, whether you are feeling motivated, tired)
