WEEK 2 READINGS Electrical signals in neurons · Depolarization: resting membrane potential of -70 becomes more positive o Either positive ions enter the cell or negative ions leave the cell o Occurs when Na+ channels open o Na+ equilibrium potential = +60mV · Hyperpolarization: membrane potential become more negative o Either positive charge leaves the cell, or negative charges enter · Occurs when K+ channels open o K+ equilibrium potential = - 90 mV . Membranes change their potential by selectively changing the permeability of different ions (goldman equation) . When only one gated ion channel opens, it makes the membrane much more permeable to that ion reducing goldmans down to the Nernst equation · Changes in membrane potential occur after only a small number of ions flow; concentrations are not greatly altered o > membrane permeability; not ion concentration; causes a membrane to deviate from resting membrane potential · Graded potentials: depolarizing or hyperpolarizing signals that travel within a neuron from a gated ion channel o Decrease in strength over distance o Cannot travel over large distances · Action potential o Generated at the axon hillock if combination of excitatory (depolariz- ing) and inhibitory (hyperpolarizing) signals is greater than the thresh- old potential (usually -55 mV)
o Summation at the axon hillock can be spatial summation or temporal summation Signals in the axon . Action potential depend on the action of voltage gated ion channels · Excitatory graded potentials reaching the action potential depolarize the membrane there and increase the likelihood of voltage gated Na channels to open Depolarization phase . If the signal is greater than the threshold potential, enough sodium channels open to depolarize the membrane and trigger an action potential · Activation gate opens o Na+ channels have a positive feedback loop > as some open, the membrane becomes more depolarized increasing the chance of Na+ channels opening > further depolarizing · Na+ channels are only in high concentration in the axon, so no action-poten- tials are triggered in the cell body or dendrites Repolarization phase · Na+ channels close at +30 mV before Na+ reaches its equilibrium potential · Inactivation gates close · Depolarization also causes K+ channels to open . They open more slowly, just before Na+ channels close o Flow of K+ out of the cell causes repolarization Na+ channels return to original state (inactivation get open, ac- tivation gate closed hyperpolarization since they are slow to close or remain open after resting potential is reached after-hyperpolarization phase
· potential slowly returns to resting potential as K+ channels close · Na+/K+ ATPase restores resting membrane potential by transferring K and Na back where they came from Action potential conduct across long distances · Action potentials are all or nothing · Travel unidirectionally down he axon - triggering another action potential at a node of Ranvier o Nodes contain high density of voltage gated channels o In the internodal sections, current travels elctrotonically which is faster, and with less degradation o Signal travels faster