(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
