Neurons

first basic principle of electricity

opposite charges are attracted to each other

second basic principle of electricity

energy is required to keep opposite charges separated across a membrane

third basic principle of electricity

energy is liberated when the charges move toward one another

fourth basic principle of electricity

when opposite charges are separated, the system has potential energy

voltage

a measure of potential energy generated by separated charge

how is voltage measured

between two points in volts or millivolts

what is another name for voltage

potential difference

how does charge difference affect voltage across a plasma membrane

creates a potential, a greater difference means higher voltage

current

flow of electrical charge (ions) between two points

what can a current be used for

to do work

currents flow is dependent on

voltage and resistance

resistance

hindrance to charge flow

insulator

substance with high electrical resistance

conductor

substance with low electrical resistance

Ohm’s law

gives relationship of voltage, current, resistance (current = voltage/resistance)

current is ____ to voltage

directly proportional

greater the voltage

greater the current

current is ____ to resistance

inversely proportional

the greater the resistance

the smaller the current

3 potentials

membrane potential, action potential, graded potential

membrane potential =

voltage difference across membrane

chemically gated (ligand-gated) channels

open only with binding of a specific chemical (ex. neurotransmitters)

voltage-gated channels

open and close in response to changes in membrane potential

mechanically gated channels

open and close in response to physical deformation of receptors, as in sensory receptors

can membrane potential change?

can change relative to resting membrane potential

depolarization

inside of membranes becomes less negative than resting membrane potential (ex. Na+ enters cell)

hyperpolarization

inside of membrane becomes more negative than resting membrane potential (ex. Cl- enters cell or K+ leaves cell)

two types of signals

graded potentials and action potentials

4 phases of an action potential

resting state, depolarization phase, depolarization phase, hyperpolarization

generating a resting membrane potential depends on

differences in K+ and Na+ concentrations inside and outside cells and differences in permeability of the plasma membrane to these ions

what triggers an action potential to fire

threshold: membrane depolarizes by 15 to 20mV, neuron becomes excited, triggers voltage gated channels to open

what does “action potentials are all or none” mean

an action potential either happens completely or doesn’t happen at all

what is the principle way neurons send signals

action potentials

what is an action potential

a brief reversal of membrane potential with voltage change of about 100 mV

what is an action potential called in neurons

a nerve impulse

do action potentials decay over distance

no, unlike graded potentials

where do action potentials occur

in muscle cells and the axons of neurons

what causes an action potential to occur

the opening of specific voltage-gated channels

how do action potentials allow long-distance communication

they carry neural signals over long distances without decaying

if action potentials are all or none, how can the brain distinguish weak and strong stimuli

stronger stimuli cause more frequent action potentials, weaker stimuli causes fewer

resting state of action potential

all gated Na+ and K+ channels are closed

which channels are open at resting state

only Na+ and K+ leakage channels

what is the resting state of gated Na+ and K+ channels

all voltage-gated channels are closed

what maintains the resting membrane potential

leakage channels and ion distribution

what are the two gates on a Na+ channel

activation gate and inactivation gate

what does the Na+ activation gate do

opens with depolarization, allowing Na+ into the cell

what does the Na+ inactivation gate do

blocks the channel after it opens, stopping further Na+ entry

what is the Na+ inactivation gate’s state at rest

open

How many voltage-sensitive gates does a K+ channel have

one

when does the K+ channel gate open

slowly in response to depolarization

what happens during depolarization

voltage-gated Na+ channels open, and Na+ rushes into the cell

what happens to the Na+ gates during depolarization

activation and inactivation gates are open initially

how does Na+ influx cause more depolarization

Na+ entering makes the inside of the cell less negative

what is the threshold for an action potential

approximately -55 to -50 mV

what happens when threshold is reached

positive feedback opens many voltage-gated Na+ channels

what causes the large action potential spike

rapid Na+ influx through voltage-gated Na+ channels

when is the membrane potential at the action potential peak

approximately +30 mV

what happens to Na+ channels during repolarization

inactivation gates close, stopping Na+ influx

what happens to Na+ permeability

it decreases toward the resting level

what stops the action potential spike from rising

Na+ channels become inactivated

what happens to voltage-gated K+ channels during repolarization

they open

which ion leaves the cell during repolarization

K+ exits down its electrochemical gradient

what is repolarization

the membrane returns toward its resting potential

why does hyperpolarization occur

some K+ channels remain open, allowing excess K+ to leave the cell

what happens to the inside of the membrane

it becomes more negative than at the resting state

what is hyper polarization

a slight dip below the resting membrane voltage

what happens to Na+ channels during hyperpolarization

they begin to rest

why does the membrane become more negative during hyperpolarization

excessive K+ efflux occurs while some K+ channels remain open

repolarization resets…

electrical conditions, not ionic conditions

after repolarization…

Na+/K+ pumps (thousands of them in an axon) restore ionic conditions

how is the strength of a stimulus encoded

all action potential are alike and are independent of stimulus intensity

how frequently can action potentials fire

stimulus strength and refractory periods influence frequency

two types of refractory periods

absolute refractory and relative refractory

absolute refractory period

time from the opening of the Na+ channels until the resetting of Na+ channels

what does the absolute refractory period do

prevents the neuron from generating an action potential, ensures that each action potential is separate, enforces one-way transmission of nerve impulses

relative refractory period

the threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events

the interval following the absolute refractory period when

sodium channels have returned to resting state, some potassium gates are open, repolarization and hyperpolarization is occurring

conduction velocity =

speed of axon potential traveling down axon

conduction velocity is influenced by

axon diameter and presence of a myelin sheath