Physiology of Excitable Cells

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Questions and Answers

What characterizes excitable cells?

They generate a constant ion flow across their membranes.

They change their membrane potential in a reversible manner. (correct)

They lack voltage-gated ion channels.

They have a fixed membrane potential.

Which component is primarily responsible for generating an action potential?

Sodium-Potassium ATPase pump.

Resting Membrane Potential.

Electrochemical gradients.

Voltage-gated ion channels. (correct)

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

To create an equal distribution of ions across the membrane.

To initiate action potentials.

To maintain the resting membrane potential. (correct)

To generate graded potentials.

What does membrane potential primarily depend on?

Electrochemical gradients of ions. Signup and view all the answers

Which of the following best describes graded potentials?

They result from the integration of neuronal inputs. Signup and view all the answers

What is the primary ion that determines the resting membrane potential in cells?

K+ Signup and view all the answers

What effect does increased permeability to Na+ have on the equilibrium potential?

Moves the equilibrium potential away from K+ Signup and view all the answers

What is the equilibrium potential of K+ typically in a cell?

-90mV Signup and view all the answers

Which statement about the Sodium-Potassium ATPase is true?

It is an electrogenic pump Signup and view all the answers

What typically happens to the membrane potential if K+ permeability decreases?

The membrane potential becomes less negative Signup and view all the answers

Where does the action potential begin in a motor neuron?

At the axon hillock Signup and view all the answers

Which statement accurately describes the all-or-none law?

Action potentials are equal in intensity within a given neuron. Signup and view all the answers

What contributes to the back-propagation of action potentials?

Synaptic plasticity involvement Signup and view all the answers

What is the primary role of the initial segment of the axon?

Generation of action potentials Signup and view all the answers

During which period can a neuron partially respond to stimuli with a lowered amplitude action potential?

Relative refractory period Signup and view all the answers

What primarily determines the characteristics of action potentials in different neurons?

Types of protein channels present Signup and view all the answers

In terms of action potential propagation, what is a direct consequence of the absolute refractory period?

Action potentials cannot be generated. Signup and view all the answers

What defines the location where sensory neurons trigger action potentials?

Receptor and axon junction Signup and view all the answers

What is the function of the nodes of Ranvier in myelinated axons?

To regenerate the action potential Signup and view all the answers

What does saltatory conduction primarily enhance in nerve signaling?

The speed of impulse transmission Signup and view all the answers

How do local neurons differ from larger neurons?

They do not produce action potentials and have shorter axons Signup and view all the answers

What is the cause of multiple sclerosis according to the information provided?

Destruction of the myelin sheath Signup and view all the answers

What type of potentials do local neurons produce when stimulated?

Graded potentials Signup and view all the answers

What role do graded potentials play in the generation of an action potential?

They provide the necessary threshold stimuli Signup and view all the answers

What kind of information do local neurons exchange?

Information only with close neighbors Signup and view all the answers

What is the primary composition of the myelin sheath?

Fats and proteins Signup and view all the answers

What factors determine the amplitude of graded potentials?

The strength of the triggering event and the distance from their origin Signup and view all the answers

Where is the trigger zone for action potentials located?

In the axon hillock or initial segment Signup and view all the answers

What defines the all-or-none law of action potentials?

Action potentials are initiated only if the threshold is reached Signup and view all the answers

Which process involves multiple graded potentials occurring simultaneously from different presynaptic neurons?

Spatial summation Signup and view all the answers

What is the result of activating channelrhodopsin-2 (ChR2) with blue light?

It allows the passage of Na+ and K+ ions, generating depolarizing stimuli Signup and view all the answers

Which period prevents the generation of another action potential immediately after one has occurred?

Absolute refractory period Signup and view all the answers

Which ions are primarily involved in the generation of action potentials?

Na+ and K+ Signup and view all the answers

What do graded potentials represent in neuronal function?

Inputs that are integrated to potentially trigger an action potential Signup and view all the answers

What characterizes the resting potential of a neuron?

The resting potential is approximately -70 millivolts. Signup and view all the answers

Which ions are primarily involved in maintaining the resting membrane potential of a neuron?

Sodium and potassium Signup and view all the answers

What role does the sodium-potassium pump play in neurons?

It helps maintain the electrical gradient by transporting ions against their concentration gradient. Signup and view all the answers

During the resting state of a neuron, what is true about the potassium channels?

Potassium channels are partially closed, allowing some leakage. Signup and view all the answers

How does the electrical gradient affect sodium and potassium ions in a neuron?

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

What does the resting membrane potential depend on?

The selective permeability of the membrane to different ions. Signup and view all the answers

What happens to the net electric charge across the membrane at equilibrium?

There is a constant flow of ions with no net change in electric charge. Signup and view all the answers

What is the primary function of ion channels in the resting membrane potential?

To selectively control the passage of specific ions across the membrane. Signup and view all the answers

Why is the inside of a neuron negatively charged during resting potential?

Because potassium ions leak out, carrying a positive charge with them. Signup and view all the answers

What results from the balance of the electrical and concentration gradients in neurons?

The maintenance of an electrochemical gradient. Signup and view all the answers

Study Notes

Biological Psychology 1: Excitable Cells

Excitable cells change their membrane potential explosively and reversibly, generating action potentials.
Basic concepts: the cell membrane and transport of substances through it (Protein Carriers and Active Transport).
Resting membrane potential: an energy store.
Ion channels are involved in electrochemical gradients across the membrane.
The Na+-K+ ATPase Pump is crucial.
Action Potentials: involve voltage-activated ion channels.
Graded Potentials: the integration of neuronal inputs.
Methods for Recording Neuron Activity: use intracellular and reference microelectrodes connected to an amplifier and a computer.

Membrane Potential

The membrane potential depends on unequal ion distributions across the membrane.
Body's electrical neutrality is maintained by both anions (-) and cations (+).
Key ions include Na+, Ca++, K+, Cl-, Protein anions-, and Phosphate anions.
Ions are not evenly distributed between ECF and ICF (extracellular and intracellular fluids).
Membrane potential diagram shows voltmeter measuring electrical difference (mV) across the cell membrane between the inside and outside.

Resting Membrane Potential

Em = -70 mV (millivolts)
Ions are not evenly distributed: concentration and electrical gradients influence the potential.
Intracellular ion concentrations (mM): Na+ (14), K+(150), Cl-(5), Ca++(10^-4).
Extracellular ion concentrations (mM): Na+(145), K+(4), Cl-(120), Ca++(2).
Ion ratios: Sodium (10:1), Potassium (1:30), Chloride (24:1), Calcium (2x10^4:1).
Intracellular Ca++ content is largely bound or sequestered in organelles (Mitochondria and sarcoplasmic reticulum).

The Membrane of a Neuron

The neuron membrane is primarily composed of phospholipid molecules, forming a bilayer.
Protein molecules are also embedded in the membrane.

Cell Membrane Permeability

Membrane permeability is dependent on lipid solubility, molecular size, membrane surface area, and membrane thickness. Concentration gradients exist across the membrane.

Transport Proteins

Transport proteins move ions and large/non-lipophilic molecules across the plasma membrane.
Channel Proteins create water-filled pores for specific ions to pass through.
Carrier Proteins never form an open channel; can be classified into uniport, symports, and antiports.

Gated Membrane Channels

Voltage-gated channels: open or close in response to changes in the membrane potential.
Ligand-gated channels: open or close in response to a specific molecule (ligand).
Gap junctional channels: allow for direct communication between adjacent cells.

Transport Proteins and Membrane Transport

Transport proteins mediate passive and active transport across the cell membrane.
Passive Transport: simple diffusion, facilitated diffusion.
Active Transport: requires energy (ATP) to move substances against their gradient.

Pore Channels

Pore channels are selectively permeable to different ions.

Membrane Potential (Difference)

The unequal distribution of ions creates an electrical potential difference between the inside and outside of the cell.
The membrane acts as an insulator.

Impermeable Membrane

An artificial cell with an impermeable membrane demonstrates the concept of electrical and concentration gradients.

Selective Permeable Membrane

An artificial cell with a selective permeable membrane demonstrates how electrical gradients can stop ions from leaving.

Nernst Equation

The equation calculates the equilibrium potential (Eion) for an ion based on its concentration gradient.
- 61 / Z * log ([ion]out/[ion]in) where:
  - Z is the charge of the ion.
  - Ion values are in concentration outside and inside the cell
  - Eion is in mV

Equilibrium Potential of different Ions

Membrane potentials depend on relative permeabilities to specific ions.

Nernst Equations

The Nernst equation calculates Equilibrium potentials (Eion) for different ions based on their concentration gradients.

Resting Membrane Potential Difference

The electrical gradient and concentration gradient work together to maintain the resting membrane potential.

Forces Acting on Sodium and Potassium Ions

The membrane selectively allows permeability of sodium, potassium, calcium, and chloride through channels.
Sodium channels are closed at rest while potassium channels are partially closed and allows flow of potassium.

Ion Channels

Sodium-Potassium Pumps are a protein complex that constantly pumps 3 sodium ions out and draws 2 potassium ions in to maintain electrical gradients.

Electrical and Concentration Gradients

The electrical gradient and concentration gradient work together to pull Na+ into the cell and K+ into the cell.

Resting Membrane Potential (RMP) Summary

An electrical membrane potential exists, negative inside and positive outside.
The RMP results from an equilibrium between electrical and chemical gradients (electrochemical gradient).
The membrane's permeability to ions determines the RMP.
The Sodium/Potassium Pump maintains the RMP over time.

The Action Potential

Action potentials are rapid changes (depolarization and repolarization phases)in membrane potential.
The action potential is generated through a rapid depolarization and repolarization process.
Stimuli will increase permeability.

The Nerve Impulse

The electrical message travels down the axon, regenerating at points along the axon to minimize weakening of the signal.
Nerve impulses range in speed from <1 m/second to 100 m/second.

The Nerve Impulse: Summary

The brain doesn't need precise timing for touch messages but does need precise timing for messages in vision and movement.
Nerve impulse properties are well-suited for information transfer in the nervous system.

Action Potentials: Summary

Stimulus will increase depolarization of membrane, eventually producing a massive depolarization above threshold, which brings on the action potential.
Action potentials have an All-or-None law that applies to their amplitude and velocity; this is dependent on intensity of stimulus.

AP Threshold

Rapid depolarization triggers action potential.
Threshold varies by neuron, but is consistent for a given neuron.

Resting and Action Potentials

The graph shows the phases of the action potential from rest through depolarization to afterhyperpolarization.
Shows phases of open and closed ion channels.

Action Potential Phases

The graph shows the different phases of action potentials including the threshold, depolarization, repolarization, and after-hyperpolarization phases.

Voltage-Activated Channels

Permeability depends upon voltage differences across the membrane.
When sodium channels open, positively charged sodium ions rush in producing nerve impulses.

The Movement of Sodium and Potassium Ions During an Action Potential

The graph shows the movement of ions (Na+ and K+) during neuronal action potential, highlighting the phases of depolarization, repolarization, and after-hyperpolarisation.

Refractory Periods

After an action potential, a neuron enters a period where it cannot or needs a stronger stimulus to create a new action potential. —Absolute : The first part when the membrane cannot produce an action potential. —Relative: The second part where a stronger-than-usual stimulus is needed to produce an action potential

The Movement of Sodium and Potassium: Summary

After an action potential, sodium channels close rapidly, and potassium channels open, returning the neuron to its resting state.
Potassium ion flow from the concentration gradient takes with it positive charge.
The sodium-potassium pump restores the original distribution of ions.

Restoring the Sodium-Potassium Pump

Re-establishing the sodium-potassium gradient in the axon takes time.
An unusual number of action potentials may cause sodium buildup, which is toxic.

Blocking Sodium Channels

Local anesthetic drugs block sodium channels, preventing action potentials, and nerve impulses.
Tetrodotoxin from pufferfish blocks sodium channels, leading to respiratory failure and muscle paralysis.

The AP originates at and propagates from the Axon Hillock/Initial Segment

The axon hillock and initial segment possess a high density of voltage-gated ion channels, acting as the trigger zone for action potentials.

The All-or-None Law

Action potential amplitude and velocity are independent of stimulus intensity; they have consistent intensity and speed for a given neuron.
Characteristics of action potentials vary between neurons due to differences in protein channels.

Propagation of Action Potential:

Action potentials are initiated at the axon hillock or trigger zone of the neuron.
Propagation is the transmission of the action potential down the axon.
Sensory neurons have trigger zones near receptors; motor and interneurons initiate at axon hillocks.

Refractory Period

Action potentials cannot summate; the refractory period prevents backward conduction.

AP Propagation

Action Potential transmission can be likened to falling dominoes, each domino (axon section) in a different position (phase).
Simultaneous recordings show that each segment is experiencing a different phase of the action potential.

Local Neurons

They have short axons and don't typically produce action potentials.
When stimulated, they create graded potentials that vary in magnitude and don't follow all-or-none law.
They depolarize or hyperpolarize in proportion to the stimulus, often for integration with other neurons.

Graded Potentials

Graded potentials can be depolarizations or hyperpolarizations.
Amplitude and distance are proportional to the triggering event.
Graded potentials are the synaptic signaling for other neurons.
Summation occurs when multiple graded potentials from a single or multiple neurons cause change in membrane potential from one or same location to increase signal strength.

Optogenetics

Using transgenic technique, introduce light-sensitive proteins to specific neurons.
Light stimulation activates these channels, enabling scientists to trigger specific behaviours.

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Description

This quiz explores the fundamental concepts of excitable cells, including action potentials, membrane potentials, and the role of various ion channels and pumps. Test your understanding of how these cells function and the mechanisms behind their electrical activity.