Lecture 3: Equilibrium, Resting Membrane, and Action Potentials

resting membrane potential (RMP)

potential difference across the membrane when the cell is not generating an action potential

• results primarily from diffusion potentials

RMP is generated by…

• K+ diffusion through leak channels (K+ leaks out, leaving a negative charge behind)

• selective membrane permeability

• Na+/K+ ATPase maintains ion concentration gradients that allow diffusion potential to develop

largest contributor to RMP

K+

minor contributor to RMP

Cl-

Membrane permeability is very low at rest, so ___ and ___ contribute very little to RMP during rest.

Na+ and Ca2+

Typical value of membrane potentials

-70 to -90 mV

The RMP os always close to the __________ equilibrium potential.

K+

Why is the RMP negative?

the membrane is more permeable to K+ than Na+, which allows K+ to diffuse out of the cell and leave behind impermeant intracellular anions

K+ equilibrium potential and typical value

the point at which movement of K+ into the cell because of negative electrical potential is balanced by diffusion of K+ out of the cell due to the concentration gradient

-85 to -90 mV (that much lower than the extracellular environment)

K+ equilibrium potential is the point at which movement of K+ _____ the cell because of negative electrical potential is balanced by diffusion of K+ ____ of the cell due to concentration gradient.

into

out

At K+ equilibrium diffusion force is ________ electrical force.

equal to

What term describes the difference between the measured membrane potential and the ion’s calculated equilibrium potential?

driving force

When the driving force is negative, ion will [enter/leave] the cell if it is a cation, and [enter/leave] the cell if it is an anion. (and why)

enter

leave

the membrane potential is too negative, so this will try to bring it towards equilibrium

typical equilibrium potentials (Na+, Ca2+, K+)

ENa+ = +65 mV

ECa2+ = +120 mV

EK+ = -85 mV

describe depolarization in terms of membrane potential

membrane potential is less negative

describe hyperpolarization in terms of membrane potential

membrane potential is more negative

action potentials

transmit information in nervous system and all muscle

occurs in excitable cells

• rapid depolarization followed by repolarization

During an action potential, any mention of an inward or outward current regards the movement of a [positive/negative] charge.

positive

threshold potential

less negative than RMP

membrane potential at which voltage-gated Na+ channels open rapidly, initiating an action potential

overshoot

portion of an AP where the membrane potential is positive

undershoot

portion of AP where the membrane potential is more negative than RMP

refractory period

period during which another AP can’t be generated

AP steps

1. RMP is -70 mV; K+ conductance (eflux) is high (K+ channels almost fully open); Na+ conductance is low

2. rapid depolarization (upstroke); membrane is depolarized to threshold; voltage-gated Na+ channels are rapidly opened, Na+ influx

3. repolarization; inactivation on Na+ channels close (ends upstroke) and K+ channels open (allows repolarization)

   1. undershoot (hyperpolarization); K+ conductance is higher than at rest

absolute refractory period (ARP)

• overlaps with most of AP

• No secon AP can occur because Na+ channels are inactivated

  • once inactivation closes the gates, they can’t open again until repolarization

relative refractory period (RRP)

• from the end of ARP until through most of hyperpolarization

• a stronger-than-normal stimulus is required because the membrane is repolarized

  • stronger-than-normal stimulus needed because the membrane is more negative during hyperpolarization, and threshold must be reached

voltage gated Na+ channels

• resting state: RMP, activation gate is closed

• activated state: activated by a signal, activation gate opens

• inactivated state: inactivation gate closes

  • return to RMP and time required to reset

  • refractory to subsequent stimulus

True or False: “The amplitude and shape remain constant for a given cell type” is a characteristic of an AP.

True

characteristics of AP

• amplitude and shape remain constant for a given cell type

• propagation

  • depolarization occurs at adjacent areas on the membrane

• all-or-none response

  • stimulus either reaches threshold and generates a full AP or it does not generate an AP at all

Propogation of APs

• APs start close to the cell body of a neuron, spread down the axon via local currents

• 1. initial area of axon depolarized to threshold, AP fires, cell interior positive

• 2. positive charges inside cell flow towards negative charges in adjacent areas of cell- those areas the deolarized to threshold

  • 3. at this point, the initial areas has repolarized

As local current spreads, some [positive/negative] current is lost across the membrane. This current leakage shows that the membrane isn’t a perfect…

positive

insulator

conduction velocity

how fast can the membrane respond, and how far can current spread?

Conduction velocity depends on two properties. What are they?

time constant and length constant

What constant determines how quickly a membrane depolarizes?

time constant

A [smaller/larger] time constant allows the membrane to reach threshold faster and results in faster action potential conduction.

smaller

What constant is influenced by membrane resistance and membrane capacitance?

time constant

high membrane resistance → [more/less] current leaks out, but membrane voltage changes slowly → [smaller/larger] time constant

less

larger

high membrane capacitance → [more/less] charge is required before voltage changes → [larger/smaller] time constant

more

larger

How does time constant eddect conduction velocity?

• large time constant → membrane changes slowly → conduction velocity decreases

• small time constant → membrane changes quickly → conduction velocity increases

Which constant determines how far local current spreads before it dies out?

length constant

larger length constant → adjacent membrane reaches threshold more [easily/difficultly] → conduction velocity [increases/decreases]

easily

increases

Which constant is influenced by membrane resistance and internal resistance?

length constant

high membrane resistance → current [stays inside/leaves] the axon → current spreads [further/shorter] → length constant [increases/decreases]

stays inside

further

increases

low internal resistance → current flows [difficultly/easily] inside the axon → current spreads farther → length constant [increases/decreases]

easily

increases

How does length constant effect conduction velocity?

• large length constant → conduction velocity increases

• small length constant → conduction velocity decreases

Increasing axon diameter is a strategy used by the body to do what? How does it work, and what are its cons/limits?

increase conduction velocity

• decreases internal resistance

• increases length constant

• current travels farther

cons

• requires very large nerves

• limited by anatomy

Myelination is a strategy used by the body to do what? What all does it do?

increase conduction velocity

• increases membrane resistance

• decreases membrane capacitance

• current spreads farther

• membrane depolarizes faster

• saltatory conduction

AKA much faster transmission

In order for myelination to [increases/decreases] conduction velocity, the current would need to travel [farther/shorter] and the membranes will need to depolarize [slower/faster].

increase

farther

faster

How does myelination increase conduction velocity?

by making the nerves larger

• larger nerves have lower internal resistance, so the current spreads farther

What is evolutions solution for larger nerves not being very practical to increase conduction velocity?

wrap the nerve in myelin, which acts as insulation

• myelin prevents current from leaking across the membrane, which increases membrane resistance, and the current is forced to travel inside the axon

  • myelin decreases membrane capacitance → membrane reaches threshold quickly at each node (membrane depolarizes at each node) → AP appears to jump from node to node (saltatory conduction)

Increasing axon diameter increases conduction velocity because…

it decreases internal resistance, which allows the current to travel farther

Increasing myelination increases conduction velocity by…

• increasing membrane resistance, which allows less current to leak out

• decreases membrane capacitance, which allows the membrane to depolarize faster