Neural communication Flashcards
A healthy neuron has a resting membrane potential (or voltage) of
between -60 and -80 mV
Neuronal communication is chemical
sodium Na+ and potassium K+ move in and out of the membrane
Neuronal communication is electrical
Ions are charged positively Na+ K+
as they move in and out of the cell, they change the potential of the membrane. It is relative!
Electrical and chemical gradients
charge/ions wants to flow from high concentrations to low
Equilibrium between electrical and chemical gradients
When they are at odds, one against each other. = 0mV
Membrane potential
Electrical gradient pushing one way and chemical pushing other
The cell membrane
Phospholipid bilayer
Hydrophilic heads and hydrophobic tails
Lipid bilayer is tightly packed
Passive transport through bilayer
Very very small molecules can pass the layer without channels or pumps
Channels
Facilitates passive diffusion (along chemical gradient) for bigger molecules
Pumps
Active transport.
They push ions against their chemical gradient: requires ATP.
Slower than channels
Consumes 2/3rds of all neuronal energy
The sodium potassium pump
The sodium potassium pump
Pushes 3 Na+ out and 2 K+ in
(more Na+ out of cell than K+ into)
Inside gets negative
Conformation of protein
When a protein changes shape so that only one ion can bind
Potassium leak channel
Constantly open, K+ can move freely via this channel
K+ moves more to outside
Builds up potential - makes cell more negative
After the cell pumps K+ to the inside
K+….
Gets out of the cell, according to the chemical gradient, through leak
cell gets more and more negative
Electrical force starts trying to push K+ back into the cell through channel BUT…
electrical force (pushing K+ in) is as high as chemical gradient (pushing K+ out) = equilibrium
When electrical gradient (pushing k into cell) equals force of chemical gradient (pushing k out of cell)…
resting membrane potential of -70mV
Postsynaptic Potentials
When a neurotransmitter molecule binds to a postsynaptic receptor. It can have two localized effects: Depolarize or hyperpolarize the membrane
Excitatory postsynaptic potential (EPSP)
Depolarizes membrane!
The membrane potential goes from -70 to -67 mV.
Increases the likelihood that the postsynaptic neuron will fire an action potential (AP).
Inhibitory postsynaptic potential (IPSP)
Hyperpolarizes membrane!
The membrane potential goes from -70 to -72mV.
Decreases the likelihood that the postsynaptic neuron will fire an action potential (AP)
The transmission of postsynaptic potentials (PSPs) is
Graded means…
The more NTs are binding to more receptors the stronger the voltage change will be
The transmission of postsynaptic potentials (PSPs) is
Rapid means…
PSPs travel like an electrical signal along an uninsulated wire (dendrite)
The transmission of postsynaptic potentials (PSPs) is
Decremental means…
It decays along the dendrite
Temporal summation of postsynaptic potentials
EPSPs and IPSPs voltages sum over time until threshold of excitation is reached (-55)
Spatial summation
If 1 EPSP and 1 IPSP are released into one axon, the voltages are neutralized. They are summed.
Threshold of excitation (-55) is reached when…
the sum of PSP’s reaches the axon initial segment and is sufficient to depolarize the membrane above -55
What is an action potential?
Massive momentary reversal of the membrane potential - from -70 to +55 mV (reversal of the polarity at the membrane from negative to positive)
Action potential steps
Resting potential -> depolarization -> repolarization -> hyperpolarization phase
Voltage gated (or activated) ion (mostly sodium) channels are responsible for..
Action Potential generation and conduction
Na+ channels open and Na+ goes into cell when…
Potential reaches -55 mV -> threshold of excitation
Na+ goes into the cell and it becomes positive.
After this, the activation gate…
Innactivated the channel. Automatic built in inactivation - ball and chain that blocks gate without closing
What is the absolute refractory period?
it is when Na+ channels stay inactivated (after action potential) until membrane goes back to resting potential
After threshold of excitation, sodium channels open, and after that…
Potassium channels open and finish opening at the peak of action potential. K+ flows to outside.
At the peak of AP K+ channels finish openning and…
Sodium channels innactivate (ball and chain)
At the end of AP curve, at the start of hyperpolarization…
K+ channels start closing
The slow closing of voltage-gated K+ channels at the end of AP leads to
hyperpolarization phase and the relative refractory period
Relative refractory period
Happens during hyperpolarization. The cell can fire but requires more EPSPs
After AP Na+/ K+…..
Na+/ K+ pumps restores ion balance over time (slow) back to -70
Potassium pumps don’t play a role in the firing of AP because…
They are too slow
What is hyperpolarization (after AP)?
After AP, when K+ (potassium) channels start to close. cell is between -70 and -75
What makes the AP an all of nothing event?
The voltage gated ion channels, that only open in a certain voltage
Depolarization
When Na+ channels open and action potential happens. curve going up
Repolarization
after K+ channels finish opening and sodium channels close. Curve going down.
cascading signal of the axon, AP doesn’t spread in all directions
Channels are opened by the previous ones
Voltage spreads in all directions but the action potential goes straight due to the channels behind it being inactive (ball and chain blocks NA channels)
myelinated axon - nodes of Ranvier
Na+ gated channels ONLY at the nodes of ranvier in a myelinated axon
These nodes are between the myelin sheaths
myelin sheaths
located between nodes of ranvier
Why do myelinated axons conduct action potentials faster?
Because of Saltatory conduction and because less Na+ gated channels need to open for the conduction to happen.
Saltatory conduction
Channel in the node of ranvier is activated and the electricity generated is insulated inside the myelinated sheath, where the it spreads passively, and at almost the speed of light
terminal buttons (boutons) have…
vesicles filled with neurotransmitters
What does the action potential do to terminal boutons?
Action potential depolarizes bouton and causes NT to release
How does an action potential depolarize a bouton?
AP Causes voltage-gated Ca++ channels to open
- Ca++ causes vesicles to fuse with membrane
- When the vesicle fuses -> Neurotransmitters released into synapse
Receptors are often just closed channels
ligand-gated ion channels - that open when they bind with NTs
When an NT binds to a receptor it can…
causes the flow of Na+ inside (depolarization = EPSP) or K+ outside (hyperpolarization = IPSP)
More chloride (Cl-) outside than inside of cell IPSP causes Cl- ...
to flow into the cell
Are action potentials graded?
No, APs always look the same
PSPs (Postsynaptic potentials) strength is amplitude modulated, which means…
a stronger signal has stronger potential
Action potentials strengths are frequency modulated, which means..
The stronger the signal, the more frequent APs fire
Rapid: PSPs?
APs?
PSPs - yes
APs - less so
Are APs decremental?
No
