4- Action Potentials + Conductance

2 types of channels found in a neuron

1. “leak” channels: always open for K and Na in the resting cell, responsible for the resting potential

2. gated channels: can exist as open or closed

2 types of gated channels

1. voltage-gated: open or close when the membrane charge (voltage) changes

2. ligand-gated: open when a ligand (chemical substance) binds to the channel

what happens when a resting neuron is stimulated

generation of action potential:

creates a disruption in the membrane → opening of voltage-gated channels for Na⁺ → membrane potential to move towards E_Na

any change that moves the membrane potential closer to 0 is called

depolarization

describe what’s happening at 3

incoming Na⁺ establishes positive charge on interior of membrane called the overshoot, which peaks at between +30 and +50 mV

describe what’s happening at the repolarization

Na⁺ voltage-gated channels close and K⁺ voltage-gated channels open → outward flux of K⁺ → membrane repolarized to original resting potential

describe what’s happening at hyperpolarization (4)

b/c not all of the K⁺ voltage gated channels close in time, the potential becomes more negative than the normal resting potential (hyperpolarized)

graph of action potential shape is determined by what

different voltage-dependent and time-dependent gating properties of Na⁺ and K⁺ channels

T/F: Na⁺ and K⁺ channels do not open at the same time

true, Na⁺ opens first

in an action potential, Na⁺ and K⁺ channels flow in or out

• Na⁺ channels: in

• K⁺ channels: out

what’s the all or none principle

once threshold is reached, an action potential reaches full amplitude

what’s the absolute refractory period

the time during which it is impossible to generate a second action potential

why does the absolute refractory period exist

due to sodium channel inactivation:

• sodium channels close to an inactivated state that takes 2-3 msec to relax to a true closed state

• the closed state can be reopened by a stimulus but the inactivated state cannot

how does local anesthetic use sodium channel inactivation

local anesthetics are hydrophobic amines (ex: lidocaine) that act at a defined site on Na⁺ channels and lock the channel in its inactive conformation

what’s the relative refractory period

the time during which a second action potential can be generated but would require a stronger stimulus to reach threshold

describe the mechanism of relative refractory period

• the larger stimulus is necessary to open enough Na+ channels to override the efflux of K+ whose permeability is still above the resting state

• however, action potential will have a reduced amplitude b/c the influx of Na⁺ is partially counteracted by the K⁺ efflux

• the amplitude will return to normal when voltage gated K⁺ channels have completely closed

how does an action potential move along an axon

via conduction

describe how describe conduction occurs in this axon pictured

influx of Na⁺ → Na⁺ will diffuse laterally along the inside of the membrane to the right → neutralizes the inside negative charge in adjacent axon segment and raises potential to threshold → initiating an action potential in this segment, while outward flux of K⁺ is repolarizing previous segment

the spread of an action potential as it moves down the axon in segments is called

regenerative current, aka a new action potential generated in each axon segment

T/F: conduction is unidirectional

true, demonstrating the refractory period

where can the Na⁺ and K⁺ current can only flow through the membrane in myelinated neurons

at the nodes of Ranvier, b/c the myelin sheaths act as insulators

since Na⁺ and K⁺ current can only flow at nodes of Ranvier, the action potential has to

“jump” from node to node, aka the saltatory conduction → thus conduction is faster within myelinated neurons

describe the conduction in a leaky membrane

• a leaky (to ions) membrane loses current as the potential spreads passively down the fiber

• the leakier the cell, the spread of the potential becomes limited

what is the length constant γ

quantifies the distance that a graded electrical potential can travel passively down an axon before it decays to ~ 37% of its original amplitude

what does r_m and r_i mean

• r_m = resistance to ion current through the membrane

• r_i = resistance to ion flow in the axoplasm; viz axial resistance

what does high length constant (γ) mean

low r_i and/or high r_m, the axon will depolarize to threshold further down from where the injection occurs → increase in conduction velocity

what increases length constant (γ)

1. myelination → increases r_m

2. larger axon diameter → decreases r_i

describe mechanism of demyelinating diseases (ex: multiple sclerosis)

• removal of myelin sheath exposes the underlying membrane which lacks vg-Na⁺ channels, but has many vg-K⁺ channels

• inward Na⁺ current at the nodes is dissipated in the demyelinated region by the outward K+ current, preventing the depolarization from reaching the Na⁺ channels in next node → conduction failure