Neuroscience: Excitability and Ion Gradients

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Questions and Answers

What does the Nernst equation primarily calculate?

The overall membrane potential of a cell

The permeability of different ions across the membrane

The equilibrium potential for a specific ion (correct)

The concentration gradient of ions

What is the equilibrium potential (Eion) of sodium ion given its concentrations inside (15mM) and outside (150mM) the cell?

+61.5mV (correct)

-15mV

+15mV

-61.5mV

Which statement accurately describes the relationship between equilibrium potential and concentration gradient?

A higher concentration gradient leads to a higher equilibrium potential regardless of ion charge.

If the concentration gradient is high, the equilibrium potential is low.

Equilibrium potential is independent of the concentration gradient for all ions.

If the concentration gradient is zero, the equilibrium potential is also zero. (correct)

What is a key factor that differentiates equilibrium potential from membrane potential?

Membrane potential is influenced by ion permeability across the membrane. Signup and view all the answers

Which constant value is used in the Nernst equation and applies specifically at 37°C?

61.5 mV Signup and view all the answers

What is the primary condition for maintaining membrane potential during homeostasis?

Constant relative concentration of ions Signup and view all the answers

How does the movement of ions affect the electrochemical gradient?

It establishes a difference in charge across the membrane. Signup and view all the answers

What happens when Na+ ions accumulate inside the cell?

It repulses further movement of Na+ inward due to charge imbalance. Signup and view all the answers

What defines the concept of membrane potential?

The potential energy created by a charge difference between two environments. Signup and view all the answers

What is the significance of electrostatic forces relating to ion distribution across the membrane?

It contributes to the overall charge distribution across the membrane. Signup and view all the answers

What is the effect of axon diameter on action potential propagation?

Wider axons lead to faster action potential propagation. Signup and view all the answers

How does myelination affect action potential propagation?

Myelination acts as an insulator, speeding up propagation. Signup and view all the answers

What characterizes electrical synapses compared to chemical synapses?

Electrical synapses avoid delays in transmission. Signup and view all the answers

What are fast post-synaptic responses characterized by?

Immediate opening of ligand-gated ion channels. Signup and view all the answers

What happens during slow post-synaptic responses?

A signaling cascade is initiated by G-protein coupled receptors. Signup and view all the answers

What occurs when the permeability of an ion increases at resting membrane potential?

The resting membrane potential moves towards the ion's equilibrium potential. Signup and view all the answers

What does the Goldman-Hodgkin-Katz equation primarily calculate?

Resting membrane potential. Signup and view all the answers

Under resting conditions, what is the net movement of ions across the membrane?

NET FLUX = 0. Signup and view all the answers

How does the uneven distribution of ions contribute to membrane potential?

It creates a concentration gradient for ions. Signup and view all the answers

Which ion has the highest permeability based on the provided data?

Potassium (K+) Signup and view all the answers

What effect does a change in ion concentration gradients have on membrane potential?

It can change the flux of ions across the membrane. Signup and view all the answers

Which of the following statements accurately reflects the role of neurons?

Neurons detect stimuli and convert them into electrical signals. Signup and view all the answers

Which statement regarding the relationship between ion permeability and resting membrane potential is incorrect?

Increased permeability causes ions to move against their concentration gradient. Signup and view all the answers

What is the definition of excitability in a cell?

The ability of a cell to send and receive electrical signals. Signup and view all the answers

If the concentration of sodium inside a neuron is increased from 15mM to 50mM, how does this affect the sodium concentration gradient?

The sodium concentration gradient decreases. Signup and view all the answers

What occurs when ions move down a concentration gradient?

Ions move from high concentration to low concentration. Signup and view all the answers

Which of the following statements about ion channels is TRUE?

Ion channels only allow passive transport. Signup and view all the answers

Which of the following ions has the highest concentration outside a typical neuron?

Sodium Signup and view all the answers

What is the primary mechanism of transport through ion channels?

Passive transport only by facilitated diffusion. Signup and view all the answers

What is the primary difference between voltage-gated and ligand-gated ion channels?

They open in response to different stimuli. Signup and view all the answers

Which of the following ions is found in the lowest concentration inside a neuron?

Calcium Signup and view all the answers

What is the primary function of dendrites in a neuron?

Input signals to the neuron Signup and view all the answers

Which area of the neuron is responsible for generating action potentials?

Axon hillock Signup and view all the answers

What primarily occurs at the axon terminal?

Signal output Signup and view all the answers

What are graded potentials characterized by?

Decreasing intensity over time and distance Signup and view all the answers

How does the membrane potential change during depolarization?

It becomes more positive Signup and view all the answers

Which phase is characterized by potassium efflux?

Repolarization phase Signup and view all the answers

What is the term for the gaps between two neurons filled with interstitial fluid?

Synaptic cleft Signup and view all the answers

What term describes the change in membrane potential that is restricted to the local area where ions are moving?

Graded potential Signup and view all the answers

Which of the following describes the refractory period where a greater stimulus is required to reach the threshold?

Relative refractory period Signup and view all the answers

What occurs at the presynaptic cell?

Sends signals to the next neuron Signup and view all the answers

Study Notes

Excitability and Concentration Gradients

Excitability is a cell's ability to send and receive electrical signals across the plasma membrane.
A concentration gradient is a difference in the concentration of a substance between two compartments.

Ion Concentrations

Typical ion concentrations inside and outside a neuron are:
- Potassium: 140mM inside, 5mM outside
- Sodium: 15mM inside, 150mM outside
- Chloride: 10mM inside, 120mM outside
- Calcium: 0.008mM inside, 5mM outside

Calculating Concentration Gradients

Gradient = [Ion]_outside - [Ion]_inside
Moving down a concentration gradient means moving from high to low concentration
Moving up a concentration gradient means moving from low to high concentration

Ion Channels

Ion channels transport ions across the membrane.
Ion channels are specific, only allowing particular ions to pass through.
Example: Na⁺ channels only allow Na⁺ ions to pass.
Ion channels are passive (ions follow the concentration gradient).
Ion channels can be gated (open/close) by changes in voltage or chemical messengers.
- Voltage-gated - open and close in response to changes in voltage across the membrane
- Ligand-gated - open and close in response to a specific molecule (ligand) binding to the channel.

Homeostasis

In a healthy person, the relative concentration of any one ion is constant.
The movement of molecules across concentration gradients is a tiny fraction of the total molecules present in the cell and interstitial fluid.
The concentration of sodium (Na⁺) is much higher outside the cell than inside the cell (approximately 10 times greater).

Ions and Charge

Ions are charged particles.
If ions are distributed unevenly, then there is an uneven distribution of charge across the membrane.
If ions are permeable & being transported across the membrane, the charge distribution across the membrane also changes.

Membrane Potential

Membrane potential is a form of potential energy caused by a difference in charge between two environments.
When ions move across the membrane, they change the electrochemical gradient.
Cells use this stored energy (membrane potential) for various cellular processes.

Measuring Membrane Potential

Membrane potential can be measured experimentally using a voltmeter with electrodes inside and outside the cell.

Electrochemical Equilibrium

Electrochemical equilibrium is a balance of forces that determine whether or not ions flow across a membrane.
Electrostatic repulsion (same charges repel) and concentration gradients (difference in concentration) determine the net flux.

The Nernst Equation

The Nernst equation calculates the equilibrium potential for an ion.
- E_ion = 61.5 mV / z • log₁₀ [ion]_out / [ion]_in where:
  - E_ion = equilibrium potential for the ion
  - z= charge of the ion
  - log₁₀[ion]_out / [ion]_in = ratio of ion concentration outside to inside
  - 61.5mV = constant assuming cell is at 37°C

Common Ion Equilibrium Potentials

Some common ion equilibrium potentials are:
- Potassium (K⁺): -89.0mV
- Sodium (Na⁺): +61.5mV
- Chloride (Cl^-): -66.0mV

Equilibrium Potential vs. Membrane Potential

Equilibrium potential is not the same as membrane potential.
Membrane potential also depends on ion permeability across the membrane.
- Equations such as the Goldman-Hodgkin-Katz equation and Parallel conductance equation consider ion permeability to calculate membrane potential.

Movement of Ions at Resting Membrane Potential

At resting membrane potential, the net flux of ions is zero.

Relationship between Resting Membrane Potential and Equilibrium Potentials

Resting membrane potential is closer to the equilibrium potential for an ion if the membrane is more permeable to that ion.

Graded Potentials

Graded potentials occur at the dendrite resulting from transient changes in the membrane potential.
They decrease in intensity over time and distance.
Dendrites mainly contains ligand-gated channels.

Types of Graded Potentials/ Synaptic Potentials

EPSPs (excitatory postsynaptic potentials): Depolarizing
IPSPs (inhibitory postsynaptic potentials): Repolarizing

Graded potentials are decremental

Changes in membrane potential are restricted to the local area where ions are moving.
The further away from the point of origin, the smaller the change in membrane potential.

Summation of Graded Potentials

No summation, temporal summation, spatial summation, and EPSP-IPSP cancellation are different ways graded potentials can be summed.

Phases of an Action Potential

Action potential is triggered when the membrane potential reaches a threshold.
Action potentials are rapid, transient changes from resting membrane potential.
Action potentials are triggered at the axon hillock
Phases include: resting membrane potential, depolarization, repolarization, after-hyperpolarization phase

Depolarization Phase

The depolarization phase is characterized by rapid sodium entry.

Repolarization Phase

The repolarization phase is accelerated by potassium efflux, as voltage-gated potassium channels open.

Action Potential Propagation

Action potentials are recharged at each segment of the axon by the influx of Na+.

Factors Affecting Action Potential Propagation

Wider axons lead to faster action potential propagation
Myelination increases the speed of action potential propagation - acts as electrical insulation.

Chemical and Electrical Synapses

Chemical synapses use neurotransmitters to transmit signals between neurons.
Electrical synapses transmit signals directly through gap junctions.

Postsynaptic responses at chemical synapses

Fast responses involve ligand-gated ion channels that open immediately after neurotransmitter binding.
Slow responses involve G-protein coupled receptors that trigger a downstream signaling cascade, eventually opening ion channels.

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Description

Explore the concepts of excitability and concentration gradients in neurons. This quiz covers typical ion concentrations and how to calculate concentration gradients. Test your understanding of ion channels and their role in neural signaling.