Ion Channels in Vertebrates

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Questions and Answers

What is the primary characteristic of the charge distribution across the cell membrane?

Equal distribution of positive and negative ions on both sides.

Significant negative charge buildup outside the cell.

Small buildup of negative ions inside and positive ions outside. (correct)

Heavy positive charge buildup inside the cell.

What did Hodgkin and Huxley's experiment utilize to measure the voltage across the plasma membrane?

Recording microelectrode and voltmeter (correct)

A laser system

Electrophysiology via light sensors

A temperature probe

Which ion is primarily found in higher concentrations in the extracellular fluid (ECF)?

Ca2+

Mg2+

K+

Na+ (correct)

What does the potential energy across the membrane depend on?

The difference in charge across the membrane (A) Signup and view all the answers

What is the role of voltage-gated Na+ channels in vertebrates?

To initiate action potentials by allowing Na+ influx (B) Signup and view all the answers

Which of the following is a characteristic of voltage-gated Ca2+ channels?

They open in response to membrane depolarization. (B) Signup and view all the answers

What is a notable feature of the cytosol's composition in relation to ion distribution?

It is rich in negatively charged proteins. (A) Signup and view all the answers

Which of these ions is NOT primarily found in the extracellular fluid (ECF)?

K+ (A) Signup and view all the answers

What is the primary cation found in the cytosol?

K+ (C) Signup and view all the answers

Which type of ion channels randomly open and close?

Leak channels (D) Signup and view all the answers

How do Na+/K+ ATPases maintain the resting potential of a cell?

They pump out Na+ and bring in K+. (B) Signup and view all the answers

What is the main function of voltage-gated sodium channels?

To propagate action potentials (A) Signup and view all the answers

What characterizes the inactivation gate of sodium channels?

It blocks sodium movement when closed. (D) Signup and view all the answers

What role do calcium channels play in physiological processes?

They initiate synaptic transmission. (D) Signup and view all the answers

What ensures K+ ions mostly leak out of the cell?

Number of K+ leakage channels (A) Signup and view all the answers

What is the structure of voltage-gated sodium channels composed of?

Three subunits and 24 transmembrane segments (D) Signup and view all the answers

Which component is primarily responsible for the function of the pore in voltage-gated sodium channels?

α1 subunit (A) Signup and view all the answers

What ensures that most anions cannot leave the cell?

They are attached to non-diffusible molecules. (D) Signup and view all the answers

What initiates the opening of ligand-gated ion channels?

Binding of a chemical stimulus (C) Signup and view all the answers

In the context of ion channels, what does the term 'mechanically gated' refer to?

Channels that open in response to mechanical stimuli (A) Signup and view all the answers

How many Na+ ions are expelled for every K+ ion imported by the Na+/K+ ATPase?

3 Na+ per K+ (A) Signup and view all the answers

What is the main challenge for Ca2+ channels in selecting Ca2+ in the presence of Na+?

Ca2+ must compete against a higher prevalence of Na+. (B) Signup and view all the answers

Study Notes

Ion Channels

Ion channels are proteins in cell membranes that allow passage of ions.
They are crucial for nerve impulses and other cellular functions.
Various types exist, including leak channels, ligand-gated, mechanically gated, and voltage-gated channels.

Aims and Objectives

The aim is to define types of ion channels in vertebrates.
To comprehend the molecular structure and function of vertebrate voltage-gated Na+ and Ca2+ channels.
To define key Ca2+ channel types and their locations.

Why Do Ion Channels Exist?

A buildup of negative ions inside the cell membrane and an equal positive buildup outside creates a resting membrane potential.
This potential difference (measured in mV) is crucial for cell function, serving as potential energy.
The difference in charge across the membrane directly relates to the potential energy level.

Evidence for Membrane Potential

Hodgkin and Huxley (1952) used microelectrodes to measure this resting membrane potential.
A recording microelectrode is inserted inside the cell, a reference electrode is placed outside in the extracellular fluid, and connected to a voltmeter for accurate readings of the voltage difference across the membrane.

Unequal Ion Distribution

The extracellular fluid (ECF) is rich in sodium (Na+) and chloride (Cl−) ions.
The cytosol is rich in potassium (K+) and negatively charged molecules (phosphate and proteins) that cannot readily leave the cell.
K+ leakage channels enable more K+ ions to diffuse out of the cell compared to Na+ ions entering.
Na+/K+ ATPase pumps maintain this unequal distribution by actively pumping Na+ out of the cell and K+ into the cell.

Types of Ion Channels

Leak channels: Randomly open and close; more numerous in neurons.
Ligand-gated channels: Open when a signaling molecule (ligand) binds. Found on dendrites and cell bodies.
Mechanically gated channels: Open in response to physical stimuli (e.g., touch, vibration). Found in sensory receptors.
Voltage-gated channels: Open in response to changes in membrane potential. Crucial for propagating nerve impulses.

Neuronal Ion Channels

The plasma membrane of neurons contains various types of ion channels (chemically gated, voltage gated Na+, and voltage gated Ca2+).
These ion channels are strategically located along the neuron to facilitate signaling.

Voltage-gated Sodium Channels

They are crucial for nerve impulse generation.
Composed of three subunits, with one α subunit forming the pore.
The α subunit also has activation and inactivation gates.
The β subunit has a smaller size (~30 kDa).

Sodium Channel Domains and Functions

The channel has 24 transmembrane segments organised into 4 domains (S1-S6).
The domains facilitate voltage sensing and form the pore.
A 2000- amino acid protein chain forms the channel.

Molecular Mechanism of Fast Inactivation

Inactivation gates close the channel after opening rapidly.
Amino acid residues in the inactivation gate form a latch that keeps the channel closed.

Calcium Channels

The α1 subunit forms the pore; α2, β, γ, and δ subunits are auxiliary subunits.
These subunits are not directly involved in voltage-gating or Ca2+ conduction but regulate the channels.
Various types of Ca2+ channels exist (L, N, P, Q, R, T).

Ca2+ Signals Initiate Physiological Processes

Ca2+ channel activation by electrical signals initiates cellular events, including synaptic transmission and muscular contraction.
Ca2+ channels regulate numerous cellular processes.

Structure of Calcium Channels

Calcium channels consist of a large protein with four repeated domains.
Each domain contains six transmembrane segments and intracellular loops—crucial for signaling and protein binding.

Example - Calmodulin

Calmodulin is a protein that binds to Ca2+ and regulates various cellular processes.
Calmodulin binds Ca2+ in response to increased levels, which initiates changes in cellular activity.

Types of Calcium Channels

L-type: Long lasting, slow inactivation, high activation threshold. In cardiac muscle.
T-type: Transient, fast inactivation, low activation threshold. In SA node, endocrine cells, etc.

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Description

This quiz covers the types, structures, and functions of ion channels in vertebrates. It delves into voltage-gated channels and the importance of membrane potential for cell function. Test your understanding of the physiological roles these proteins play in cellular communication.