W1: What is Cognitive Neuroscience: Information Processing in the Brain Flashcards
Brain is made up of
neurons
What are neurons composed of? (3)
- Cell body
- Axon
- Dendrites
Diagram of a neuron
What are nerve cells or neurons specialised to do?
are specialized to generate and propagate electrical signals
What does a neuron’s cell body contain? (2)
- contains nucleus
- contains organelles (e.g., endoplasmic reticulum, ribosomes, Golgi apparatus, mitochondria)
Neurons long extension from the cell body is called a
axon
Neurons’s short branches extending from the cell body is called
dendrites
What is the purpose of a neuron’s axon?
carries nerve impulses (sends info) away from the cell body
What is the purpose of dendrites?
Dendrites receive information from
the axonal endings arising from other nerve cells.
There are proteins in the neuronal membrane known as
ion pumps & ion channels
Diagram of ion channels and pumps:
Ion channel and ion pumps support
action potential propgation down the axon of the neuron
The ion channel and ion pump control
the movement of ions (charged atoms) from the inside of the neuron (intracellular) to the outside of the neuron (extracelluar).
Cell membrane is a lipid bilayer that surrouds the neuron and keeps stuff
separate from outside to the inside
Action potentials (those electrical impulses that send signals around your body) are nothing more than a temporary shift (from negative to positive) in the
neuron’s membrane potential caused by ions suddenly flowing in and out of the neuron.
Diffusion
Ion channels allow
specific ions to move through the neuronal membrane
There are 2 forces that determine the movement of ions inside and out of the cell (2)
- concentration/diffusion (high to low concen grad)
- electrical (negative <–> positive; pos goes to neg, neg goes to pos)
The fluid inside and outside of the neuron contains different types of ions such as (4)
- Sodium (Na+)
- Potassium (K+)
- Chloride (Cl-)
- Large negative ions (A-)
Diffusion
Whats happening in A? (10)
- In A we put equal amount of K+ (orange) and Cl- (Green) in a solution which becomes KCL
- We put equal amount of KCL in chamber 1 and in chamber 2
- Purple membrane which allows movement of K+ allows to move through
- K+ ions moves from chamber 1 to chamber 2
- It makes chamber 2 more positive as K+ carries its charge
- K+ then drawn to chamber 1 since chamber 2 more positive than chamber 1
- There is a diffusion gradient pushing back to chamber 1
- There is electrical force pushing back to chamber 1 as chamber 2 is positive and chamber 2 is negative so things are positive attract to things are negative
- It re-establishes the equilibrium
- Some K+ ions move to chamber 1 and some move to chamber 2 based on electrical forces and diffusion until molecules are evenly spaced out
What is diffusion?
the random movement of particles towards a state of equilibrium
When does a diffusion concentration gradient occur? (2)
A concentration gradient occurs when the concentration of particles is higher in one area than another.
In passive transport, particles will diffuse down a concentration gradient, from areas of higher concentration to areas of lower concentration, until they are evenly spaced.
Neurons, like all living cells, are surrounded by a plasma membrane that is
imperable to ions.
Neuron’s imperable membranr property allows to
maintain different concentration of ions between the inside and outside of the cell by preventing the passive diffusion of ions from regions of high to low concentration:
In a typical neuron at rest, there is a large difference in the concentration of ions such as
sodium and potassium between the intracellular and extracellular environments
The plasma membrane is composed of a
lipid bilayer
The hydrophobic interior of the lipid bilayer of the cell membrane prevents
the movement of ions across the membrane:
The only way ions can diffuse across plasma membrane is by passing through
specialized channels that permit the movement of ions while excluding others (ion channels)
The ion channels can be in a ___ or ___ state
These channels can be in an open or closed state
When the neuron is at rest, most ion channels are closed. However, a few potassium
channels are open, and potassium ions are therefore free to diffuse out of the cell into the extracellular environment
Note: channels can be highly selective; the potassium channels do not permit the passage of
sodium ions.
Since only a few sodium channels are open at rest, very few sodium ions flow into
the interior of the neuron.
How can Electrical Potential Across Membranes be Measured? (2)
The electrical potential across the membrane can be measured using a pair of electrodes
An oscilloscope is used to continuously display the potential difference between the reference and recording electrode:
If we consider only potassium ions, which are relatively free to cross the membrane, we notice the intracellular concentration remains higher than the extracellular concentration at rest
This is because K+ ions pulled by 2 different and opposing forces (2)
- First, a diffusion force acts to pull potassium ions down their concentration gradient toward the exterior of the cell. The movement of potassium ions out of the cell results in a depletion of the internal positive charges causing the potential to become increasingly negative.
- An electrical force, produced by K+ ions toward the negative cell interior generates an electrical gradient that brings potassium back to the interior of the cell.
The diffusion and electrical forces come to balance where
electrical gradient is balanced by diffusion gradient -this is electrochemical equilibrium
For K+ at electrochemical equilibrium there is no
net movement of potassium ions
W
When both electrodes are in the bath outside of the cell,
no difference in potential in potential is measured between them and the display registers 0 mV.
However, when the recording electrode enters the cell, the display records a membrane potential of about
60mV –> this value represents the resting membrane potential of the neuron
Resting MP is the difference in
in electrical potential acrss the membrane during an inactive period.
When the concentration of K+ ions in KCl solution is same in chamber 1 and chamber 2 then
no electrical potential will be meausred across the membrane (V = 0)
If the concentration of K+ ions is not the same on chamber 1 and chamber 2
then electrical potential will be generated (-58 mV) as K+ flow down concentration gradient (diff) to cham 1 to cham 2 and electrical force moving K+ to cham 2 and 1
What is the purpose of the sodium-potassium pump?
Helps to maintain a higher concentration of Na+ ions outside the cell and higher concentration of K+ ions inside the cell (at resting)
Proposed mechanism of Na+ and K+ (6)
- The cycle begins with the pump open to the inside of the cell – in this forum the pump has a relatively high affinity for sodium ions which bind to the pump:
- When bound by sodium, the pump is a substrate for phosphorylation (addition of phosphoryl group to a mole) by the high-energy molecule, ATP. The energy requirement in this step is estimated to account for 20-40 percent of the brain’s energy consumption:
- The phosphorylation triggers a conformational change in the pump. In the new conformation, the pump has a low affinity for sodium which it releases outside of the cell.
- The pump now has a relatively high affinity for potassium which binds to the pump:
- Potassium binding triggers the dephosphorylation of the pump. Without the phosphate group, the pump returns to its original conformation:
- Back in its original conformation, the pump has a relatively low affinity for potassium, which it releases into the cell:
In sodium-potassium pump, For every three sodium ions that are taken out of the cell, the pump transports
2 potassium ions into the cell.
Why is sodium-potassium pump said to be electrogenic? (2)
Fore every 3 Na+ out of cell, pump takes 2 K+ into cell
net loss of one positively charged ion for each round of pumping results in a very small electrical current.
At rest a neuron is
negatively charged (~-65mV)
What is a recording electrode?
one to record membrane voltage
What is stimulating electrode?
it inject current that can be used to push the membrane voltage toward more positive (depolarizing) or more negative (hyperpolarizing) voltages.
If the stimulus is sufficient to push the membrane potential past the firing threshold for neurons, then
action potential is generated.
Action potentials are a all-or-none event meaning:
amplitude of an action potential is independent of the EPSP that is used to generate it.
APs are all or nothing event
However,
if the amplitude or duration of the EPSP is increased the number of action potentials propagated per second increase (known as spiking rate)
Each AP lasts for
1 millisecond
Any suprathreshold stimuli will produce
produce an action potential of similar amplitude and duration:
If negative current is injected in neuron then
an inhibitory post-synaptic potential (IPSP) occurs because the neuron becomes less likely to fire an action potential (hyperpolarized)
If a positive current is injected in a neuron then
an excitatory post-synaptic potential (EPSP) occurs because the neuron becomes more likely to fire an action potential (depolarized)
If the EPSP exceeds threshold (~-50mV) an
action potential occurs
Voltage-gated or voltage-sensitive ion channels play an important role in
generating action potentials.
There are 2 types of voltage-gated channel (2)
Na+ and K+
When EPSP’s amplitude or duration increased then you don’t get bigger AP but
more APs
Voltage-gated channels open when
membrane becomes depolarised above threshold due to EPSPs
Na+ channel open and close faster than
the slower K+ channels
Action potential generation (8)
- Resting potential: When a neuron is at rest, only the so-called “leak” potassium channels are open, establishing the resting potential
- Rising phase: During this phase, the increasingly positive shift in MP is driven by the opening of progressively more and more voltage-gated Na+ channels and the entry of Na+ ions into the neuron. This inward Na+ current depolarizes the membrane voltage
- Overshoot phase: The membrane potential is now at its most positive state, it has overshot 0mV. At this positive potential, two processes are occurring simultaneously. a. First, the voltage-gated Na+ channels that initially activated during the rising phase begin to close. As a result, Na+ conductance starts to decline.
b. Potassium channels begin to open driving the MP back toward the equilibrium potential for potassium. These voltage gated potassium channels differ from leak potassium channels in that they are normally closed at resting potential but open in response to depolarization (thus the term voltage-gated)
i. Now the AP is in its repolarizing phase which means the MP is rapidly returning to the resting potential.
- Overshoot phase: The membrane potential is now at its most positive state, it has overshot 0mV. At this positive potential, two processes are occurring simultaneously. a. First, the voltage-gated Na+ channels that initially activated during the rising phase begin to close. As a result, Na+ conductance starts to decline.
- Falling phase: During this phase, activation of voltage-gated K+ channels is at max and the number of open Na+ channels is dramatically reduced. The AP repolarizes beyond the MP.
- Undershoot phase: The undershoot occurs because most voltage-gated K+ channels are still open such that total K+ conductance of the neuron is greater than when the membrane is at its steady state
- Recovery: During this phase, the MP returns to the original steady-state MP. This occurs as the delayed K+ channels that were opened during AP now close. The MP is now determined by the other channels normally open at resting MP.
Voltage-gated channels are present at the
axon hillock.
When an action potential is generated it moves down
the axon.
Voltage-gated channels are present at the Nodes of Ranvier where the
action potential is regenerated.
The axons of many neurons is surrounded by a myelin sheath. This helps the
conduction of the action potential
Multiple sclerosis is a
disease of myelin
Communication between neurons occur at specialized junctions called
synapses
The most common type of synapse is the
chemical synapse
Puffer fish contains tetrodoxin that blocks
voltage-gated Na+ channels and will die
The arrival of an action potential at a presynaptic axon terminal sets off a
cascade that results in the release of neurotransmitter molecules into the synapse
Synaptic vesicles are either binded to the or… (2)
cytoskeleton in reserve pool or free in the cytoplasm.
Steps of synaptic transmission
- When the axon terminal of is depolarized, voltage-gated calcium channels open and Ca+ ions rush into axon terminal.
- Some Ca+ ions bind to a protein on synaptic vesicle membrane, called synaptotagmin, nearest to the active zone which makes synaptic vesicles draw even closer to the presynaptic membrane
- The vesicles fuse with the axon terminal membrane and release their neurotransmitter which diffuses across the synaptic cleft
- Some of the NT molecules bind to the special receptor molecules in the postsynaptic membrane
- The response of the post-synaptic cell (either excitation or inhibition) depends on specific
NT and receptor combination
Example of that the response of the post-synaptic cell (either excitation or inhibition) depends on specific NT and receptor combination
For NT acetlycholine that binds to specific receptor it generates an excitatory post-synaptic response (EPSP)
- Eventually, NT are inactivated or removed rapidly from the synaptic cleft so that transmission is
brief and accurately follows pre-synaptic signal
How is acetlycholine removed from synaptic cleft?
an enzyme in syanptic cleft called acetlycholinesterase breaks down acetylcholine into choline and acetate.
Not all NT are broken down by enzymes in synaptic cleft but many NTs are (3)
taken up into presynaptic terminal by special proteins called transporters (on pre-synaptic terminal
This process is known as reuptake.
Reuptake not only cuts off synaptic activity prompty, but also allows the pre-synaptic terminal to recycle NT molecules.
Neurotransmitter receptors are membrane proteins in postsynaptic cell that have a region for
binding neurotransmitter and an ion channel
What happens to the ions when an appropriate NT binds to this neurotransmitter receptor - (2)
causes the opening of the ion channel component
Ions flow through the ion channel changing the electrical potential of the post-synaptic cell
Glutamate in synapse is terminated as it is taken up by
glial cells, where it is recycled and used again.
If NT does not get taken away or recycled (reuptake) than there is a more than normal
effect of NT
Agonist vs antagonists drugs - (2)
An agonist is a drug that binds to the receptor, producing a similar response to the intended chemical and receptor (mimics actions of NT)
Whereas an antagonist is a drug that binds to the receptor either on the primary site, or on another site, which all together stops the receptor from producing a response (stops NT from binding to NT receptor)
Prozac prevents reuptake of
serotonin making effect of serotonin stronger
The correct balance of excitatory and inhibitory signals is necessary for the
proper working of the nervous system.
Excitatory and inhibitory postsynaptic potentials (EPSP & IPSP) are integrated by neurons. If the neuron is sufficiently excited, an action potential is ……The action potential is then….. - (2)
generated at the axon hillock.
propagated down the axon.
The post-synaptic potentials that are caused by NT chemicals can be either depolarizing, often (but not always) resulting in an - (2)
excitatory post-synaptic potential (EPSP)
or hyperpolarizing resulting in an inhbitatory post-synaptic potential (IPSP).
2 EPSPs give rise to +
2 EPSPs + 1 IPSPs stops - (2)
AP
stops AP from happening (incorrect balance of EPSPs and IPSPs for AP to occur)
Epilsey is disturbance between
excitatory and inhbitatory inputs in brain - EPSP and IPSP
Post-synaptic potentials generally move passively along the dendritic membrane gradually becoming
smaller as they spread.
Since post-synaptic potentials become smaller as they spread
post-synaptic potentials from distant synapses will decay more than
PSPs from synapses closer to the integration zone at the axon hillock
What is the axon hillock?
inputs from other neighboring neurons combined at soma in axon hillock which controls the firing of an AP down the axon
The postsynaptic APs produced at most synapses are usually well below the
the threshold for generating postsynaptic APs
