(4) neural communication I Flashcards
What does it mean to say that neuronal communication is an electrochemical process?
Chemical:
- primarily result of 2 ions: sodium (Na+) & potassium (K+)
- ions move into/out of cell, but not freely
Electrical:
- ions positively & negatively charged (Na+ & K+ positive)
- as they move into/out of cell, change potential at membrane
How are the chemical and electrical gradients for a neuron like at rest?
Chemical gradients:
- ions want to flow from high to low concentration
Electrical gradients:
- charge/potentials want to flow from high to low concentration
What two proteins are responsible for the resting membrane potential? How so?
- Sodium-Potassium Pump
- embedded in cell membrane
- pumps 3 Na+ out & 2 K+ in
- creates 2 chemical gradients:
- push Na+ from outside to inside
- push K+ from inside to outside
- electrical gradient: inside negative charge of -1 & negative in respect to outside
- Potassium “leak” channels
- K+ moves freely via open K+ “leak” channels
- Na+ channels closed: can’t move freely across membranes
How is the resting potential established?
- certain molecules & ions permitted via channels & pumps
- Na+/K+ pump push more Na+ out than K+ into cell -> inside more negative than outside
- but K+ can move freely thru leak channels -> K+ wants to move w/ chemical gradient out of cell
- but moving K+ makes cell more negative
- result: flow of K+ stops when force of electrical gradient = chemical gradient
- chemical force push K+ out = electrical force push K+ in
- end result: resting membrane potential of -70mV
What are channels?
protein holes allowing passive diffusion (along chemical gradient)
What are pumps?
actively push ions against their chemical gradient
- requires energy
What two proteins are responsible for the action potential? How so?
- Voltage-activated sodium ion channels (Nav)
- normally closed
- channels open when reach threshold
- K+ channels
What is an action potential (AP)?
rapid, brief reversal of polarity at membrane, from negative to positive (-70 to -55mV)
- main method of brain communication - all-or-none
How does AP occur?
when sum of EPSPs & IPSPs that reaches axon initial segment sufficient to depolarise membrane above threshold of excitation
- threshold of excitation = -55mV
What is stage 1 of AP?
Depolarisation (rising phase):
- Na+ channels open → Na+ move in
- chemical gradient push in: Na+ higher concen. inside
- electrical gradient push in: neg. charged inside
- cell membrane flips from neg. to pos.
- but Na+ channels have built-in inactivation, shuts off automatically, after ~1ms
- Na+ channels stay inactivated until membrane goes back to resting potential, leads to absolute refractory period
What is the absolute refractory period?
no more action potentials until reset
What is stage 2 of AP?
Repolarisation:
- K+ channels always open, even more open during AP
- membrane now pos., so K+ ions leave cell membrane to return it to neg. resting membrane potential
- chemical gradient push out: K+ higher concen. inside
- electrical gradient push out: pos. charge inside - slow closing of voltage-gated K+ channels lead to hyperpolarisation & relative refractory period
- Na+/K+ pump restores ion balance over time (slow)
What is stage 3 of AP?
Hyperpolarisation:
- relative refractory period
- membrane potential below resting membrane potential
What is the relative refractory period?
cell needs more EPSPs to fire AP
Why does conduction only happen in one direction along the axon?
- due to Na+ channels
- one direction: channels closed and ready to be opened
- other direction: channels in absolute refractory period, inactivated state
- thus AP can only spread in 1 direction
What is effect of subthreshold stimulation of axon?
excitatory potential produced, not sufficient to elicit AP
What is effect of suprathreshold stimulation of axon?
excitatory potential produced that exceeds threshold of excitation & produces AP that continues undiminished down axon
What are the differences between conduction in unmyelinated vs. myelinated axons?
Unmyelinated axon:
- Na+ channels everywhere
- Na+ channels open causing charge to travel across axon & adjacent channels to open
- many channels need to be open, AP deteriorates
Myelinated axon:
- similar principles to unmyelinated axons
- Na+ channels present only at Nodes of Ranvier
- regenration of AP happens
- fewer channels to be opened
What are the four key differences between postsynaptic potentials (PSP) and action potentials (AP)?
Graded
PSPs - yes: stronger signal cause bigger EPSPs
APs - no: always same shape
Strength
PSPs - amplitude modulated (AM): larger PSP, stronger signal
APs - frequency modulated (FM): faster frequency, stronger signal
Rapid
PSPs - yes: instantenous
APs - less so: regeneration slows it down
Decremental
PSPs - yes: decays as it spread out
APs - no: constantly regenerated, do not decay
What is the phopholipid bilayer?
- cell membrane guardian
- tightly packed
- hydrophobic & hydrophilic
- keeps out dangerous entities
What is hydrophilic?
interacts w/ polar molecules (those similar to water)
What is hydrophobic?
interacts w/ non-polar molecules (those like oil)
What is a postsynaptic potential (PSP)?
when NT binds to postsynaptic receptor, 1 of 2 localised effects:
- excitatory postsynaptic potential (EPSP)
- inhibitory postsynaptic potential (IPSP)
What is EPSP & IPSP summation?
EPSPs & IPSPs sum spatially & temporally
- many EPSPs & IPSPs needed to sum up to reach AP
- spatially: 2 simultaneous sum to produce greater one
- temporally: 2 in rapid succession summate to produce larger one
What is an excitatory postsynaptic potential (EPSP)?
- depolarise membrane: decrease potential (closer to 0)
- increase likelihood that neuron will fire action potential (AP)
What is an inhibitory postsynaptic potential (IPSP)?
- hyperpolarise membrane: increase potential (further from 0)
- decrease likelihood that neuron will fire AP
