A&P Test 3 : Excitable Cell Physiology

The human body is electrically

• neutral

Voltage

the measure of potential energy generated by separated electrical charges

Potential Difference

• Voltage is always measured between two points

Current

• The flow of electrical charge from one point to another

Resistance

• the hindrance to charge flow provided by substances through

  which the current must pass

Ohm’s Law

• Current (I) = voltage (V) / resistance (R)

Ion Channels

• these channels are selective as to the type of ion (or ions) it allows to pass

Chemically Gated Channels

• open when the appropriate chemical (in this case a neurotransmitter) binds

• also known as ligand-gated channels

Voltage-gated Channels

• open and close in response to changes in the membrane potential

Mechanically Gated Channels

• open in response to physical deformation of the receptor

Resting Membrane Potential

• potential difference in a resting neuron

What does the Sodium-potassium Pump do?

• stabilizes the resting membrane potential by maintaining the concentration gradients for sodium and potassium

What can change membrane potential?

• anything that alters ion concentrations on the two sides of the membrane

• anything that changes membrane permeability to any io

Changes in membrane potential can produce?

• Graded Potentials

• Action Potentials

Graded Potentials

• usually incoming signals operating over short distances

Action Potentials

• long-distance signals of axons

Depolarization

• a decrease in membrane potential

• The inside of the membrane becomes less negative (moves closer to zero) than the resting potential

• includes events in which the membrane potential reverses and moves above zero to become positive

Hyper polarization

• an increase in membrane potential

• The inside of the membrane becomes more negative (moves further from zero) than the resting potential

• reduces the probability of producing nerve impulses

What is the voltage of the resting membrane potential of a stereotyped neuron?

• -70mV

Where do graded potentials occur?

• dendrites

Where does the action potential occur?

• axon hillock or axon initial segment

Nernst Equation

• calculates the electrical potential that is in equilibrium with the concentration gradient of an ion

• Ex : E_ion = RT / zF ln ([out]/[in]) = 61/z log([out]/[in])

• R = universal gas constant

• T = absolute temp (k)

• F = faraday constant

• z =charge

What does the Nernst equation calculate?

• calculates the electrical potential that is in equilibrium with the concentration gradient of an ion

For a given ion, what two characteristics determine its equilibrium potential?

• The concentration gradient

• The valence of the ion

The membrane is?

• Selectively Permeable

• @ rest

  • lots of K= is leaking out thru channels

  • only a little bit of Na+ is leaking in

  • moderate amount of Cl- is leaking in thru leak channels

If an ion has a high concentration gradient, but the permeability of the membrane to that ion is zero, how much of the ion is crossing the membrane?

• None of the ion is crossing the membrane

If an ion has a high concentration gradient, but the permeability of the membrane to that ion is high, how much of the ion is crossing the membrane?

• A large amount of the ion is crossing the membrane

At rest, what is the membranes permeability to K+?

• high

At rest, what is the membranes permeability to Na+

• low

At rest, what is the membrana’s permeability to Cl-

• moderate

How does the GHK equation differ from the Nernst Equation?

• GHK - calculates actual resting potential of cell by taking into account multiple ions and their relative permeabilities

• Nernst Equation - calculates the theoretical equilibrium potential for a single ion across a cell membrane

How does permeability affect diffusion of an ion?

• directly controls the rate and ease with which an ion diffuses across a membrane

When an excitable membrane is at rest, how does the membrane’s permeability to K+ compare to Na+ and Cl-?

• K+ is significantly higher

The -70mV resting membrane potential is due to high permeability to ____ and low permeability to ___

• potassium (K+) and sodium (Na+)

What physiological assumption do we make regarding the diffusion that contributes to the membrane potential?

• constant-field assumption

What mechanism allows us to make this assumption

• active maintenance of gradients by Na+/K+ ATPase

What directions do Na+ and K+ diffused across the cell membrane?

• Sodium ions (Na+) diffuse into the cell, while potassium ions (K+) diffuse out of the cell

How many Na+ and K+are pumped per ATP hydrolyzed by the Nat/K+ATPase?

• What direction are Na+ and K+ pumped across the membrane?

• Is this along or against the direction of the concentratic gradients?

• 3 Na+ ions pumped out against its concentration gradient

• 2 K+ ions pumped in against its concentration gradient

• 1 ATP hydrolyzed

If the Nat/K+ATPase were to quit working, what would happen to the Na+ and the K+ gradients?

• What would happen to the diffusion of Nat and Kt?

• What would happen to the membrane potential?

• he chemical gradients for both ions will dissipate

• the net diffusion of both ions will eventually stop

• the membrane potential will depolarize toward zero

Do positive ions entering the cell (e.g. Na+) cause a positive or negative electrical potential?

• postive shift

Do positive ions leaving the cell (e.g. K+) cause a positive or negative electrical potential?

• negative electrical potential

Do negative ions entering the cell (e.g. CI) cause a positive or negative electrical

• negative shift

Is the sum of the equilibrium potentials resulting from the Na+, K+, and Cl concentration gradients equal to the resting membrane potential?

• Do the concentration gradients alone explain the voltage of the resting membrane potential?

• No, the resting membrane potential is not a simple arithmetic sum or average of the individual equilibrium potentials

• No, concentration gradients alone do not explain the resting membrane potential; they must be paired with selective membrane permeability and active transport

What is the resting membrane potential due to?

• unequal distribution of ions across the cell membrane

Which two variables of diffusion are necessary for this current?

• Which variable do we assume does not change (under normal physiological conditions)?

  • Why?

• Which variable is changing?

  • Why?

• Concentration Gradient and Electrical Gradient

• We assume the concentration gradient does not change

• The permeability of the membrane is changing

Do action potential degrade with distance?

• no

An AP in a neuron is called?

• a nerve impulse an generated only in axons

Steps for generating at action potential?

• 1. Resting State : All gated Na+ and K+ channels are closed

• 2. Depolarization : Na+ channels open

• 3. Repolarization : NAa+ channels are inactivating, and K+ channels open

• 4. Hyperpolarization : Some K+ channels remain open, and Na+ channels reset

Absolute Refractory Period

• This period, from the opening of the Na+ channels until the Na+ channels begin to reset to their original resting state

Relative Refractory Period

• follows the absolute refractory period. During the relative refractory period, most Na+ channels have returned to their resting state, some K+ channels are still open, and repolarization is occurring.

Saltatory Conduction

• APs are triggered only at the gaps

Group A fibers

• are mostly somatic sensory and motor fibers serving the skin, skeletal muscles, and joints

• largest diameter, thick myelin sheaths, and fast conduct impulses

Group B fibers

• are lightly myelinated fibers of intermediate diameter.

• average rate of transmitting impulses

Group C fibers

• have the smallest diameter. They are nonmyelinated, so they are incapable of saltatory conduction and conduct impulses at a leisurely pace

Where does the action potential occur in a neuron?

• originates at the axon hillock (the "trigger zone") and then propagates down the axon toward the axon terminals

• Draw and label an action potential.

What state is the neuron in during the resting membrane potential?

• polarized

Depolarization

• a cellular process where the electrical charge inside a cell becomes less negative (or more positive) relative to the outside, reducing the voltage difference across the plasma membrane

Repolarization

• the biological phase where a cell's membrane potential returns to a negative resting state after a depolarization event

Hyperpolarization

• a change in a cell's membrane potential that makes the inside of the cell more negative relative to the outside

What is the permeability of K+ relative to Na+?

the plasma membrane of a cell (such as a neuron) is 50 to 100 times more permeable to potassiumKthan to sodium 𝑵𝒂+

K+ leak channel (open or closed)

• What is the relative permeability and what direction is K+ flowing through this channel?

• K+ leak channels are always open

• flowing out of cell

Voltage-gated Na+ channel (open or closed)

• Activation gate (open or closed)

• Inactivation gate (open or closed)

• What is the relative permeability and what direction is Na+ flowing through this channel?

• Voltage-gated Na+ channel : Open (conducting), high membrane permeability

• Voltage-gated Na+ channel : Closed (non-conducting), very membrane permeability

Voltage-gated K+ channel (open or closed)

• What is the relative permeability and what direction is K+ flowing through this channel?

• Channel is open : high permeability, flow out of cell

• Channel is closed : low permeability, no flow

What is the permeability of K+ relative to Na+?

• 50 to 100 times more

Depolarization : K+ leak channel?

• open

• high permeability

• flowing out of cell

Depolarization : Voltage-gated Na+ channel?

• open (activation and inactivation gate)

• high permeability

• flowing into cell

Depolarization : Voltage-gated K+ channel?

• low permeability

• leaves the cell

• opens very slowly

What is the permeability of Na+ relative to K+?

• 10 to 20 times

End of Depolarization : K+ Leak Channel

• high permeability

• flowing out

End of Depolarization : Voltage-gated Na+ channel?

• Activation Gate : open

• Deactivation Gate : closed

• no permeability and no flowing direction

End of Depolarization : Voltage gated K+ channel?

• increasing permeability

• flowing out

Resting Potential : What is the permeability of Na+ relative to K+?

• K+ has much more permeability than Na+, as the Na+ channels are closed.

Repolarization : K+ leak channel

• highly permeable

• flowing out

Repolarization : Voltage-gated Na+ Channel

• Activation Gate : Open

• Deactivation Gate : closed

• non permeable

• not flowing in or out

Repolarization : Voltage-gated K+ channel

• highly permeable

• flowing out

Repoarization : What is the permeablility of K+ relative to Na+?

• K+ highly permeable

• Na+ nonpermeable

Hyperpolarization : K+ leak channel

• highly permeable

• flowing out

Hyperpolarization : Voltage-gated Na+ channel

• Activation gate : closed since below voltage threshold

• Deactivation gate : open

• non permeable, but if a big enough stimulus comes around it will open

Hyperpolarization : Voltage-gated K+ channel

• open, but closing slowly

• moderate permeability

• leaving cell

What is the permeability of K+ relative to Na+?

• high K+ permeability relative to Na+, but transitions to moderate K+ permeability higher compared to Na+

How does the permeability of K+ during hyperpolarization compare to the permeability of K+ at rest?

• Permeability is higher during hyperpolarization than at rest

How far does the change in voltage due to an action potential spread across the membrane?

• What variable for diffusion limits the spread of this voltage?

• A short distance along the membrane, but due to multiple voltage-gated Na+ channels its distance is increased

  • distance is the limiting variable

How far does the change in voltage need to travel?

• far enough to depolarize the adjacent section of the axon membrane to threshold

What happens when the voltage reaches the next voltage-gated Na+ channel?

• the Na+ channel opens if the membrane reaches threshold and allows for depolarization

How does the action potential conduct a signal along the axon of the neuron?

• What analogy was used to describe this process

• chain reaction of depolarization described as falling dominos

Graded potentials

• short-lived, localized changes in membrane potential

• can be either depolarizations or hyperpolarizations

Where on the neuron do graded potentials occur?

• dendrites and cell body (soma)

What is a chemically-gated (ligand-gated) channel?

• What kind of stimulus causes this channel to open?

• A chemically-gated channel is an ion channel in the cell membrane that opens or closes when a specific chemical messenger (ligand) binds to it.

• chemical stimulus

What is a mechanically-gated channel?

• What kind of stimulus causes this channel to open?

• A mechanically-gated channel is an ion channel in the cell membrane that opens or closes in response to physical deformation of the membrane.

• mechanical (physical) stimulus

Excitatory G.P.

• influx of Na+ ions entering the cell

• depolarization

• increases probability that A.P. is triggered

Inhibitory G.P.s

• efflux of K+ ions leaving cell

• influx of Cl- ions entering the cell

• hyperpolarization

• decreases prob. that A.P. is triggered

An excitatory G.P. causes what kind of change in voltage?

• Describe the flow of ions that causes an excitatory GP?

• How does this affect the prob. of triggering an A.P.

• An excitatory graded potential causes depolarization, making the membrane potential less negative (more positive).

• Na⁺ channels open, allowing Na⁺ to flow into the neuron

• increases the probability of triggering an action potential

An inhibitory G.P. causes what kind of change in voltage?

• Describe two ways the flow of ions can cause an inhibitory G.P.?

• How does this affect the prob. of triggering an A.P.?

• causes hyperpolarization

• K⁺ exits the neuron

• Cl⁻ enters the neuron

• decreases the probability of triggering an action potential

What is the relationship between the magnitude of the stimulus and the number of chemically-gated (or mechanically-gated) channels that are open?

• As stimulus strength increases, more ion channels open, resulting in a greater change in membrane voltage (a larger graded potential).

What is the relationship between teh number of chemically-gated (or mechanically-gated) channels that are opened and the amplitude of the graded potential?

• Applying the principles learned from GHK-equation, what is the relationship between membrane permeability and the amplitude of the graded potential?

• More channels open → greater membrane permeability → more ion movement → larger graded potential (up to a limit determined by the ion's equilibrium potential).

What is the relationship between the magnitude of the stimulus and amplitude of the graded potential?

• As stimulus strength increases, the amplitude of the graded potential increases

What is summation?

Describe how temporal summation occurs?

• How does the frequency of repeated stimuli affect the summation of G.P. amplitude?

• Higher stimulus frequency → greater temporal summation → larger combined graded potential → increased likelihood of triggering an action potential (if the potentials are excitatory)