VF: Term 1 Cell Excitability

What are the two types of passive transport?

Simple and Facilitated

What are the characteristics of passive transport?

Transport is only driven by the concentration/electrical gradient.

No energy required

Simple diffusion is NOT saturable.

Facilitated diffusion is saturable

What are the two types of Active transport?

Primary and secondary

What are the characteristics of Active transport?

-Transport is conducted against a concentration/electrical gradient

- Energy required.

-Transport is saturable

What are the characteristics of Primary Active transport?

• Uses ATP directly as an energy source for transport

• Mechanism: The substrate(s) binds to a carrier protein, which passes it/them through the membrane and releases it/them at the opposite side

What is the name of the leak ion channels?

Non-gated ion channels

What is an example of a primary active transport system?

The Na+/K+ ATPase pump.

What is the resting membrane potential (RMP) of most neurons?

Around -70 mV.

What is the principle of electroneutrality?

The total number of positive and negative charges is equal in both intracellular and extracellular fluids.

Which ion primarily determines the resting membrane potential?

Potassium (K+).

Why does K+ have a larger impact on the resting membrane potential than Na+?

K+ leak channels allow more K+ efflux compared to Na+ influx.

What is the role of the Na+/K+ ATPase pump?

It moves 3 Na+ out and 2 K+ in, maintaining the resting membrane potential.

What does the Nernst equation calculate?

The equilibrium potential for a specific ion.

What is the equilibrium potential for K+ in most cells?

Around -90 mV.

What is the equilibrium potential for Na+?

Around +61 mV.

How does an increase in extracellular K+ affect the membrane potential?

It depolarizes the cell.

How does a decrease in extracellular K+ affect the membrane potential?

It hyperpolarizes the cell.

What initiates an action potential?

A depolarizing stimulus that reaches the threshold.

Which ion channels open first during an action potential?

Voltage-gated Na+ channels.

What happens during depolarization?

Na+ rushes into the cell, making the membrane potential more positive.

What happens during repolarization?

K+ exits the cell, restoring a negative membrane potential.

What is the purpose of hyperpolarization?

It ensures that the neuron does not immediately fire another action potential.

What type of ion channel is always open?

Leak (non-gated) ion channels.

What type of ion channel opens in response to voltage changes?

Voltage-gated ion channels.

What ion movement causes the repolarization phase of the action potential?

Efflux of K+.

What prevents Na+ channels from reopening immediately after an action potential?

The inactivation gate.

What is the absolute refractory period?

A period during which no new action potential can be initiated.

What is the relative refractory period?

A period during which a stronger-than-usual stimulus can initiate another action potential.

How does hyperkalemia affect cell excitability?

It depolarizes the cell, increasing excitability.

How does hypokalemia affect cell excitability?

It hyperpolarizes the cell, decreasing excitability.

What condition is characterized by episodes of muscle weakness due to K+ disturbances?

Hyperkalemic periodic paralysis (HYPP).

What is the consequence of hypocalcemia on sodium channels?

It increases sodium conductance, leading to hyperexcitability.

Why does hypercalcemia reduce cell excitability?

It stabilizes Na+ channels, making it harder for depolarization to occur.

What clinical sign is associated with severe hypokalemia in cats?

Ventroflexion of the neck.

What is the consequence of a channelopathy that prevents Na+ channel inactivation?

Prolonged depolarization.

What is the difference between graded and action potentials?

Graded potentials are variable in strength and decay over distance, while action potentials are all-or-none and travel without losing strength.

Where do graded potentials typically occur?

In the dendrites and soma of neurons.

Where do action potentials typically occur?

In the axon.

What determines whether a graded potential triggers an action potential?

Whether it reaches the threshold at the axon hillock.

What is the all-or-none principle?

An action potential either happens completely or not at all.

What is overshoot in an action potential?

When the membrane potential becomes more positive than 0 mV.

What happens when an action potential reaches the axon terminal?

It triggers the release of neurotransmitters.

How does myelination affect action potential conduction?

It speeds up conduction by allowing saltatory conduction.

What is saltatory conduction?

The jumping of action potentials between nodes of Ranvier.

Which cells produce myelin in the central nervous system?

Oligodendrocytes

Which cells produce myelin in the peripheral nervous system?

Schwann cells.

What effect does demyelination have on action potentials?

It slows down or stops conduction.

What disease is associated with demyelination in the central nervous system?

Multiple sclerosis.

What disease is associated with demyelination in the peripheral nervous system?

Guillain-Barré syndrome.

What happens if extracellular Na+ concentration increases significantly?

It may enhance depolarization, making the cell more excitable.

What happens if extracellular Cl- concentration increases?

It can cause hyperpolarization, reducing excitability.

How does extracellular Ca2+ influence the threshold potential?

High Ca2+ raises the threshold, making it harder to trigger an action potential.

What is negative membrane potential?

Extracellular fluid is considered the reference in relation to intracellular cellular fluid. The intracellular potential is negative in comparison to the o mV of the extracellular.

What is the generation of the resting membrane potential?

ion concentration gradient causes the efflux of K (out of the cell) and the influx of Na (inside the cell)

K ions diffuse through leak channels more easily than Na ions.

What is the different between Cations and Anions

Cations are positive

Anions are Negative

Why the exchange of electrical charges between the extracellular and intracellular compartments by the Na/K pump is considered?

Exchanges more positive charges out of the cell

What is the purpose of the Nernst Equation?

Diffusion of ions based on chemical gradient will continue until the counteracting electrical gradient equal the chemical gradient

The electrical potential of this balanced state for one univalent ions.

How does the membrane potential change during depolarization?

The membrane potential difference deceases or the membrane potential is less negative or more positive.

How does the membrane potential change during hyperpolarization?

The membrane potential difference increases, or the membrane potential is more negative or less positive.

What is action potential?

Initiated when the cell membrane is depolarized to the threshold.

What happens if the depolarization of the cell membrane does not reach the threshold?

A local potential or graded potential

How does the depolarized membrane reverse its polarity?

At its peak due to the inactivation of Na channels and the early repolarization due to efflux of K.

What ion transport causes an axon’s repolarization phse of the action potential?

Efflux of K

What are the two basic electrical signal types?

Graded and action potentials

What is a graded potential?

A variable strength signal that travels over short distances and loses strength as they travel through the cell

Moves to the integrating region within a neuron (trigger zone or axon hillock) then changes to action

What is action potential?

A very brief, large depolarization that travels long distances through the neuron without losing strength.

Rapid signaling over long distances (toe to brain)

Describe Depolarization during of the action potential.

Influx of Na depolarizes the cell membrane

The electrical gradient promotes the influx of Na but at the end overshoots

What stops the depolarization and initiates repolarization?

The inactivation gate responds to depolarization, this gate closes and the potential is reversed for a short time.

Repolarization: is the efflux of K repolarizes the cell membrane

What is the driving force during repolarization?

A positive membrane potential, the concentration and electrical gradients for K. The electrical gradient opposes the diffusion at the membrane potential become negative

The K channel gat closes too late which causes repolarization to go past the resting potential causing hyperpolarization

What is the consequence of the channelopathy that interferes with the closure of the inactivation gate in Na voltage gate channels of a stimulated excitable cell?

The depolarization phase is prolonged

What is the Absolute refractory period?

A period of an ongoing action potential during which no stimulus can initiate another action potential

What is the Relative refractory peroid?

follows the absolute refractory peroid during which a strong stimulus can initiate another action potential

How are action potential important for electrical events of the cell membrane?

During the relative refractory period, stronger stimuli are required to trigger an AP. Important for the heart.

Explain the “ALL” in the All or Non Principle.

Both Na and K channels are activated and deactivated by certain membrane potential levels this sequence alwasy generates a full action potential

Explain the “NON” in the All or Non principle.

Stimuli, not reaching the threshold potential has no effect

What does HYPP stand for in horses?

Hyperkalemic peroidic paralysis

What is hyperkalemia?

When there is to much K in the blood

What is hypercalcemia?

To much calcium in the blood

What causes HYPP in horses?

A genetic mutation that doesnt allow Na channel to close and the cells become overstimulated which results in a increase of K in the blood.

What are the three ways to identify the K chemical gradient?

Using Numbers

Using dots

Using the blue traingle

How do you observe the change of the K concentration gradient strength under Higher K concentration?

Higher K concentration is the ECF: the strength decreases

How do you observe the change of the K concentration gradient strength under Lower K concentration?

The strength increases