Excitable Cells and Membrane Potentials

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Questions and Answers

What are excitable cells primarily capable of generating?

Membrane potentials

Graded potentials

Electrochemical gradients

Action potentials (correct)

Which mechanism is responsible for restoring the resting membrane potential after an action potential?

Voltage-gated ion channels

Passive diffusion of ions

Na+-K+ ATPase pump (correct)

Graded potential integration

What primarily influences the membrane potential in excitable cells?

Only cation channels

Equal distribution of ions

Active transport of all substances

Unequal distributions of ions (correct)

What is primarily required for the generation of an action potential?

Voltage-gated ion channels Signup and view all the answers

Which statement about the action potential is true?

It can travel along the axons of neurons. Signup and view all the answers

What is the primary function of the Sodium-Potassium ATPase?

To maintain concentration gradients of Na+ and K+ Signup and view all the answers

How does the permeability of a cell to K+ compare to its permeability to Na+ under normal conditions?

Cells are more permeable to K+ than Na+ by a factor of 40 Signup and view all the answers

What effect does an increase in Na+ permeability have on the membrane potential?

The equilibrium potential will move towards Na+ levels Signup and view all the answers

What is the typical membrane potential of a resting cell?

-70 mV Signup and view all the answers

Which ion primarily sets the membrane potential in a resting cell?

K+ Signup and view all the answers

What is the main assumption when using the Nernst equation to calculate equilibrium potential?

The conductance is infinite for each single ion. Signup and view all the answers

Which equation is used to calculate the equilibrium potential in a more realistic scenario involving multiple ions?

Goldman-Hodgkin-Katz Equation Signup and view all the answers

What role does the Na/K pump play in neuronal activity?

Prevents dissipation of sodium and potassium gradients. Signup and view all the answers

During intense neuronal activity, what effect does the influx of sodium ions have?

It triggers an increased activity of the Na/K pump. Signup and view all the answers

What is the typical resting membrane potential difference for a neuron?

–90 mV Signup and view all the answers

How do cells manage permeability to different ions over time?

Cells change their permeability based on ion concentration. Signup and view all the answers

What is the primary consequence of an electrogenic Na/K pump in neurons?

It aids in the establishment of resting membrane potential. Signup and view all the answers

Which of the following ions is primarily involved in generating action potentials?

Sodium ions (Na+) Signup and view all the answers

What is the typical resting potential of a neuron?

-70 millivolts Signup and view all the answers

Which ion is primarily pumped out of the neuron by the sodium-potassium pump?

Sodium Signup and view all the answers

What condition is met when a neuron is at resting potential?

There is a negative charge inside the neuron Signup and view all the answers

What maintains the electrical gradient across the neuron's membrane?

Sodium-potassium pump activity Signup and view all the answers

What occurs to potassium ions during resting potential?

They slowly leak out of the cell. Signup and view all the answers

Which combination of ions is involved in maintaining the resting membrane potential?

Sodium and potassium Signup and view all the answers

In the equilibrium state of a neuron, what occurs?

Ions constantly move with no net change in charge. Signup and view all the answers

What is the role of the electrical gradient for sodium ions?

It pulls sodium ions into the cell. Signup and view all the answers

How does the permeability of the membrane influence resting potential?

Higher permeability allows more sodium to enter. Signup and view all the answers

What is the significance of an electrochemical gradient in a neuron?

It drives the action potential generation. Signup and view all the answers

What primarily determines the amplitude of graded potentials?

The strength of the triggering event Signup and view all the answers

Where is the action potential (AP) primarily triggered in a neuron?

At the axon hillock Signup and view all the answers

What mechanism occurs when two graded potentials from one neuron happen closely together in time?

Temporal summation Signup and view all the answers

What type of summation involves the simultaneous arrival of graded potentials from multiple presynaptic neurons?

Spatial summation Signup and view all the answers

What occurs during the generation of an action potential at the axon hillock?

Opening of voltage-gated Na+ channels Signup and view all the answers

What is a characteristic of action potentials compared to graded potentials?

They obey the all-or-none law Signup and view all the answers

How does optogenetics manipulate neuronal activity?

By expressing light-activated ion channels in neurons Signup and view all the answers

What is the role of refractory periods in action potentials?

To prevent the backward propagation of action potentials Signup and view all the answers

What is the role of the nodes of Ranvier in an axon?

They are the sites where action potential is regenerated. Signup and view all the answers

What is saltatory conduction?

The jumping of action potentials from node to node in myelinated axons. Signup and view all the answers

What characterizes local neurons?

They produce graded potentials instead of action potentials. Signup and view all the answers

What happens to the myelin sheath in multiple sclerosis?

It is completely destroyed. Signup and view all the answers

What is the primary function of graded potentials?

To generate action potentials when the threshold is reached. Signup and view all the answers

What is true about the myelin sheath?

It insulates axons and aids in rapid impulse conduction. Signup and view all the answers

How do graded potentials behave in response to stimulation?

They vary in magnitude according to the strength of the stimulus. Signup and view all the answers

What is the common myth about brain use?

Only 10 percent of neurons are active at any moment. Signup and view all the answers

Study Notes

Excitable Cells

Excitable cells change their membrane potential explosively and reversibly, generating action potentials.
The cell membrane transports substances and has a resting membrane potential, which stores energy.
Ion channels are present in the membrane.
Electrochemical gradients occur across the membrane.
The Na⁺-K⁺ ATPase pump is important for this process.
Action potentials involve voltage-activated ion channels.
Graded potentials integrate neuronal inputs.

Membrane Potential

The membrane potential is the difference in electrical charge across the membrane.
Body fluids are electrically neutral, containing both anions and cations.
Ions (Na⁺, Ca²⁺, K⁺, Cl⁻, protein anions, phosphate anions) are not evenly distributed between intracellular and extracellular fluids.
The resting membrane potential is approximately -70 mV.

Resting Membrane Potential

The distribution of major ions determines the resting membrane potential (Em).
Typical ion concentrations are: Sodium (Na⁺) 145mM extracellular, 14 intracellular; Potassium (K⁺) 4mM extracellular, 150 intracellular; Chlorine (Cl⁻) 120mM extracellular, 5 intracellular; Calcium (Ca²⁺) 2mM extracellular, 10⁻⁴ intracellular.
Intracellular Ca²+ content typically is 2mM, but most of it is bound within organelles.

Methods for Recording Neuron Activity

Intracellular microelectrodes and reference microelectrodes are used to measure neuron activity.
Electrical signals, recorded with the aid of an amplifier, appear on a computer monitor.

Membrane Permeability

The cell membrane has a property called permeability, which influences substance transfer across it.
Factors involved in this permeability include molecular size, lipid solubility, and membrane thickness.
Lipid solubility and molecular size affect membrane permeability.

Transport Proteins

Transport proteins move ions and large molecules across the plasma membrane.
Channel proteins create water-filled pores.
Carrier proteins never form an open channel between membrane sides.
Channel proteins can be Gated (open/closed), Open (always open), or pores.
Carriers can be Uniport (transport only one substrate); Symport (moves two or more substrates in the same direction), Antiport (moves substrates in opposite directions).

Gated Membrane Channels

Channels are protein complexes determining the membrane permeability based on voltage difference.
The permeability of sodium and potassium channels is voltage-dependent.
Examples include voltage-gated channels, ligand-gated channels, and gap junctional channels.

Transport Proteins: Passive and Active Transport

Transport proteins facilitate passive and active transmembrane transport.
Passive transport occurs through diffusion.
Active transport uses energy (ATP) to move substances against concentration gradients.
Transport proteins help the movement of substances through the membrane.

Pore Channels

Pore channels are selectively permeable to specific ions.
The selective permeability of ion channels is affected by ion size, charge, and channel structure.
Ions (K+, Na+) flow through these ion channels influenced by concentration differences.

Membrane Potential (Difference)

ICF (intracellular fluid) is in equilibrium with ECF (extracellular fluid).
However, cells are not in electrical equilibrium across the membrane.
The uneven distribution generates an electrical potential across the membrane or membrane potential.
The membrane is an insulator.

Impermeable Membrane

A membrane is impermeable to ions if the cell membrane does not allow the selective transfer of ions across it.
Both cell and solution remain electrically neutral.

Selective Permeable Membrane

K⁺ leak channels allow K⁺ leakage out of the cell down the concentration gradient.
An electrical gradient is created to oppose K⁺ leak, maintaining the resting potential.
The electrical gradient opposes the concentration gradient and maintains the resting membrane potential (equilibrium potential for K⁺).

Nernst Equation

The Nernst equation calculates the equilibrium potential for an ion.
Em values for ions are calculated using this equation, applying the ion concentrations inside and outside the cell.
This calculates equilibrium potential for an ion.

Equilibrium Potential for Different Ions

Equilibrium potential values for essential ions, Na+, K+, Cl-, Ca²⁺, are provided.
The extracellular and intracellular ion concentrations are relevant for calculating these values.

Resting Membrane Potential Difference

The resting membrane potential difference is the membrane potential of a neuron at rest.
The resting membrane potential is determined by the equilibrium potentials for different ions and the membrane's permeability to these ions.
The action potential occurs when the membrane potential depolarizes above a threshold.

Na⁺/K⁺ Pump

The Na⁺/K⁺ pump maintains ion gradients.
It is electrogenic because it moves more positive charge out of the cell than in, contributing to the resting membrane potential.
During periods of intense activity, it increases to cope with the influxes of Sodium.

Electrical and Chemical Driving Forces

K+ sets the membrane potential.
Specific ion concentration values are given; Na+ (145 mM), K+ (160mM), Ca²⁺ (2mM), and Cl− (3mM)
The electrochemical gradients play a role in determining the membrane potential.

The Membrane Potential Depends Mostly on K⁺

In normal conditions, most cells are highly permeable to potassium (K⁺).
The equilibrium potential for K⁺ is close to the resting membrane potential.
A small leak of Na+ inside the cell causes a slightly more depolarized state.

Ion Pumps Maintain Ion Gradients

To maintain these gradients, processes like the Na⁺/K⁺ ATPase pump are needed.
It pumps out more positive charge than it pumps in, contributing to the resting membrane potential.

The Sodium-Potassium ATPase is an Electrogenic Pump

The sodium-potassium pump is an electrogenic pump that moves more positive charge out of the cell.
It helps maintain the resting membrane potential.

Sodium and Potassium Gradients for a Resting Membrane

Ion concentration gradients and membrane permeability play a role in resting membrane potential.
The sodium-potassium pump actively maintains these gradients.

Changes in Resting Membrane Potential

Depolarization: membrane potential becomes less negative.
Repolarization: return to resting membrane potential.
Hyperpolarization: membrane potential becomes more negative.
These changes follow a stimulus and are specific for different cells.

The Action Potential

The action potential is a rapid change in membrane potential.
It is characterized by depolarization and repolarization phases.
It follows the all-or-none law, meaning the amplitude and velocity are independent of stimulus strength.
The action potential's precise form and timing varies between different cell types.

The Nerve Impulse

Electrical messages are transmitted down the neuron axon.
Action potentials are regenerated along the axon, preventing weakening of the signal.
Nerve impulse speed depends on the type of axon.

The Nerve Impulse (Continued)

The brain's ability to interpret nerve impulse timing varies.
The speed and control of nerve impulses are critical for information transfer.

Refractory Periods

Neurons have periods when they cannot or have difficulty producing another action potential.
The absolute refractory period: time in which another action potential cannot be generated.
The relative refractory period: time the neuron may be able to generate another action potential but takes a higher than usual stimulus.

Propagation of an Action Potential

Action potentials propagate in one direction, preventing backward travel.
The refractory period restricts backward action potential traveling along the axon.

Action Potential Propagation in Myelinated Axons

Myelinated axons use saltatory conduction.
Action potentials "jump" between nodes of Ranvier to quickly propagate and save energy.

Local Neurons

Small neurons communicate locally via graded potentials.
Local neurons don't generate action potentials, instead using graded potentials to transmit signals between cells.
They summate to reach action potential threshold.

Graded Potentials

Graded potentials vary in strength and size.
They are local changes in voltage.
They are crucial because they initiate action potentials.

Temporal Summation

Temporal summation happens when graded potentials from the same pre-synaptic neuron occur in close succession- resulting in a cumulative impact on the post-synaptic neuron.
The graded potentials summate when they occur closely in time.

Spatial Summation

Spatial summation happens when graded potentials from several pre-synaptic neurons arrive at the trigger zone at nearly the same time.
The graded potentials arrive at the trigger zone, combining to influence the post-synaptic neuron.

Summary

Summarizing, the membrane potential is influenced by equilibrium of ions and membrane permeability, assisted by electrogenic ion pumps.
The action potential is initiated in the axon hillock/initial segment and propagates to other areas, obeying the all-or-none law.

Optogenetics

Optically controlled proteins in neurons alter the organism's actions by reacting to light.
Light-sensitive proteins change neural activity.
Optogenetic techniques open new avenues for studying and manipulating brain function.

Summary-Additional

The resting state of a neuron is maintained by electrochemical gradients and the Na⁺/K⁺ pump.
Action potentials have key phases: depolarization, repolarization, and the refractory period, which are impacted by the sodium and potassium channels.
There are different types of channels involved and they are voltage activated.
Electrical gradients and concentration gradients are factors influencing ionic movements.
Myelin and nodes of Ranvier regulate action potential propagation speed.

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Description

This quiz explores the complex processes of excitable cells, focusing on action potentials and membrane potentials. Learn about ion channels, gradients, and the significance of the Na⁺-K⁺ ATPase pump in maintaining the resting membrane potential. Test your knowledge on how these concepts integrate into neuronal signaling.