exam 1 review slides (stuff I didn't already do)

presynaptic neuron sends…

an electrical signal from axon hillock to axon terminal

at synapse…

electrical signal gets converted into chemical signal

post synaptic neuron converts…

chemical signal into electrical signal

leak ion channels

always open

set resting membrane potential

gated ion channels

require a trigger to open/close

• voltage & ligands

neurons are…

polarized

• electrical/membrane potential is abt -70mV

membrane potential

separation of charges across membrane

current

movement of ions across membrane through ion channels

• causes membrane potential to become + or - (depending on ion movement)

hyperpolarization

neural potential is (or is becoming) more negative than resting membrane potential

depolarization

neural potential is (or is becoming) more positive than resting membrane potential

when neuron is at rest…

there is no net flux/ movement of ions even though ions are still moving

• they just enter and leave neuron at same rate

• achieved by diffusive force & electrical force

why are neurons polarized

differential distribution of ions across membrane

4 ions contribute to resting membrane potential

Na+, K+, Cl-, A- (proteins)

concentration of Na+ & Cl- is…

greater outside the cell

concentration of K+ & protein (A-) is….

greater inside the cell

2 sets of forces work in…

opposition to contribute to membrane potential

1. homogenizing forces

2. opposing forces

homogenizing forces

forces promoting equal distribution of ions across membrane

• concentration gradients

• electrostatic pressure

opposing forces

differential permeability & Na+/K+ pump

electrostatic pressure

force exerted by attraction of oppositely charged ions or by repulsion of similarly charged ions

• promotes even distribution of ions

accumulation of charge is dispersed by

• repulsion of like charges

• attraction of opposite charges

differential permeability

K+ & Cl- pass readily through resting membrane through leak channels

• membrane is only slightly permeable to Na+ bc there are very few channels open for Na+

charged proteins _____ cross membrane

cannot

sodium-potassium ATPase pump

• maintains Na+ & K+ concentration gradients

• uses ATP to move Na+ & K+ against concentration gradient

• aka Na-K ATPase

in Na-K ATPase it transfers…

3 Na+ out of neuron for every 2 K+ moved into neuron

Na-K ATPase pump is…

electrogenic- causes net transfer of 1 positive ion

Na-K ATPase pump affects…

resting potential

resting membrane potential

• abt -70mV

• relatively large K+ leak conductance

• very low sodium conductance

• high chloride concentration outside cell

• negatively charged proteins inside cell

resting membrane potential is determined by

• concentration gradient

• electrical gradient

• relative permeability

• sodium potassium pump

concentration gradient & electrical gradient…

don’t always go in same direction

concentration gradient’s diffusive force…

pushing potassium OUT

electrical gradient has…

electrical force pushing potassium IN

if diffusive force is greater…

there will be net outward potassium movement

if electrical force is greater….

there will be net inward potassium movement

no net flux of K+ when…

2 forces are equal & opposite

equilibrium potential

Vm that provides a force that is equal & opposite to diffusive force

• membrane potential where there is no net flux

• each ion has own equilibrium potential that depends on concentration gradient

• ions move in direction that brings membrane closer to eq potential

when neuron is at resting potential…

no ion is at equilibrium

all permeable ions will contribute…

to setting the resting membrane potential

the more permeable the ion…

the more influence it will have on resting membrane potential

resting potential is between…

the equilibrium potentials for K+ & Na+

at rest potassium is moving

out as the positive driving force

at rest sodium is moving

in as the negative driving force

Na+ is driven in by

both electrostatic forces & its concentration gradient

K+ is driven in by

electrostatic forces & out by its concentration gradient

Cl- is close to

equilibrium

sodium-potassium pump

active forces that exchanges 3 Na+ inside for 2 K+ outside

generation of resting potential

requires interaction of 4 mechanisms

• selective permeability to K+

• diffusion along concentration gradient

• electrostatic forces

• sodium potassium pump

selective permeability to K+ (resting potential)

allows K+ to move out & keeps other ions from moving in

diffusion along concentration gradient (resting potential)

acts to move K+ out

electrostatic forces (resting potential)

opposes diffusion gradient: brings system to equilibrium at voltage close to K+ equilibrium potential

radioactive labeling

• label Na+ & K+

• measures degree of permeability (chxns in radioactivity inside cell over time for each ion)

• showed Na+ permeability is only abt 5% that of K+

changing ion concentrations in extracellular fluid

• Increase Na+: no effect on resting potential

• increase K+: more positive inside → depolarizes membrane

voltage gated ion channels

• opened/closed by chxns in membrane voltage

3 classes of voltage gated ion channels

Na+ & K+ : generate action potentials

Ca+: synaptic transmission

membrane potential

separation of charges across membrane

current

movement of ions across membrane through ion channels

• causes membrane potential to become + or -

at resting membrane potential…

membrane is much more permeable to K+ than to Na+ bc cell has many more channels open for passive K+ traffic than for passive Na+ traffic

neuron membrane is relatively impermeable to…

Na+ at rest & those that do leak are actively pumped back in

membrane potential of neuron at rest is relatively constant despite…

great pressure for Na+ to flow into cell

action potential

electrical signal used for neuronal communication

• active response of neuron to depolarizing input

• all-or-none

• responsible for long-range transmission of info within nervous system

• transient chxn in membrane voltage from negatove resting potential to positive voltages

every component of action potential is due to…

functioning of voltage-gated ion channels

• ion currents flowing through voltage gated channels

measured action potential currents

have measured ionic current in response to a large depolarization step that would normally cause an Action potential

depolarization

• early, inward current

• delayed, outward current

tetrodotoxin (TTX)

blocks voltage gated Na+ channels by blocking early current

early current is due to

Na+ influx

tetraethylammonim (TEA)

blocks voltage gated K+ channels by blocking late current

late current is due to

potassium efflux

threshold

at this point action potential becomes all or none

below threshold…

Na+ influx due to applied depolarization is balanced by K+ efflux which leads to small fluctuations in membrane potential

self regenerative

if a few extra Na+ ions enter, the added Na+ channels & depolarization become self regenerative

upstroke

strong Na+ influx (open Na+ channels= strong driving force)

weak K+ efflux (K+ channels are not open yet= weak driving force)

net sodium entry in upstroke causes

depolarization- inside of cell becomes more positive

downstroke

strong K+ efflux (K+ channels open= strong driving force)

weak Na+ influx (Na+ channels have inactivated= weak driving force)

• at high levels of membrane depolarization

net potassium efflux causes

repolarization - inside cell becoming less positive/more negative

what currents underlie after hyperpolarization

K+ channels are slow to open & close, therefore K+ current outlasts action potential & hyperpolarizes membrane below resting membrane potential

potassium efflux results in

afterhyperpolarization

afterhyperpolarization

part of action potential when membrane potential is “more negative” than at rest

• continued potassium efflux (until K+ channels have time to close) pushes membrane towards Ek

threshold current

Na+ influx

upstroke current

Na+ influx

downstroke current

Na+ off & K+ on

afterhyperpolarization current

K+ efflux

K+ current outlasts…

action potential & hyperpolarizes membrane below resting membrane potential

potassium channels have___ gate

1; open when depolarized & stay open until neuron gets back to resting potential

sodium channels have ___gates

2: activation & inactivation

• inactivation gate results in sodium channels closing even though neuron is still depolarized

sodium channel inactivation causes

absolute refractory period

absolute refractory period

occurs when voltage-gated sodium channels are inactivated (from peak until cell reaches resting potential)

• it is impossible to generate another action potential, no matter how much stimulation is applied

relative refractory period

time after an action potential when enough Na channels have recovered from inactivation to trigger an action potential

K efflux is still active & cell is hyperpolarized (stimulus is needed to reach threshold)

during relative refractory period…

a very strong depolarization can evoke an action potential

2 important characteristics of refractory

Absolute RP & relative RP

absolute RP

Ensures 1-way propagation of action potential

• action potential is initiated at axon hillock & travels in 1 direction towards terminal (doesn’t reverse back towards soma)

relative RP

rate of firing (frequency of action potentials) is related to intensity of stimulation

• if stimulation is intense enough, neuron can fire during relative refractory period

• if not, neuron will only fire when both absolute & relative refractory periods have run their course

duration of stimulus is coded by

number of APs (action potentials)

number of ions that flow during action potential is…

extremely small in relation to total number of ions inside & around neuron

a single action potential has little effect on…

relative concentrations of various ions

resting ion concentrations are rapidly re-established

by movement of ions

list of properties of action potentials

initiated at axon hillok

threshold potential

all or none

non-decremental (staying at the same strength or size without decreasing over time or distance)

refractory periods

very rapid

non-decremental as it relates to action potentials

stay at the same strength or size without decreasing over time or distance

myelinated nerves

large nerve fibers are wrapped in myelin that is formed by Schwann cells (PNS) or olgiodendrocytes (CNS)

• acts to insulate & increase effective membrane resistance (less ions leak out of axon between nodes)→ increase velocity (speed)

greater # of myelin wrappings =

greater resistance to current flow

Nodes of Ranvier

voltage gated channels are restricted to these breaks in myelin

Sodium channel are concentrated at

nodes of ranvier