BS2015 Block 2 lecture 2

Assessment Overview

  • The exam in January will cover Blocks 2, 4, and 5.

  • Block 1 has been assessed via an essay.

  • Block 3 will be assessed by work session.

  • All content within Blocks 2, 4, and 5 may appear in the end-of-module assessment.

  • Date for the exam will be included in the timetable once finalized by the exam timetable team.

Exam Format

  • The exam will be conducted on campus.

  • No note sheets or group sheets provided in this year's exam; students are expected to learn material.

Importance of Understanding Material

  • Students are encouraged to ask questions if they do not understand the material discussed in lectures.

  • Preparation for work sessions next week to ready students for the exam.

Capacitance in Neuronal Signals

  • Definition of Capacitance:

    • A property of electrical circuits that allows storage of electrical charge.

    • Example: Two metal plates with an insulator in between constitute a capacitor.

    • In neurons, capacitance relates to how much charge must be added to change membrane potential.

  • Role of Capacitance in Neurons:

    • Neurons behave as electrical circuits where the plasma membrane has capacitance and resistance.

    • Capacitance helps slow changes in membrane potential allowing for proper integration of signals.

  • Composition of a Capacitor:

    • Two conductors (intracellular and extracellular solutions) are separated by an insulator (plasma membrane).

    • Conductors allow ion movement which contributes to neuronal signaling.

RC Circuit Dynamics

  • Resistance and Conductance:

    • An ion channel acts as a resistor in biological membranes, affecting ion movement.

    • Open ion channels result in less resistance, facilitating ion flow.

  • Current Injection Analysis:

    • When current is injected into a cell, it leads to a change in potential.

    • Initially, current charges the capacitor before traversing the membrane.

  • Ohm's Law:

    • Voltage ($V$) = Current ($I$) x Resistance ($R$).

  • Current Flow Dynamics:

    • The behavior of electric current changes the membrane potential and is pivotal in understanding neuronal signaling.

Capacitance Effects on Voltage Change

  • When a current is injected, the capacitor must first charge, leading to a delay before a change in voltage is observed.

  • Once charged, current can flow through the ion channels, altering voltage readings.

  • An important aspect is that the time taken to change voltage (time constant, $ au$) is influenced by the capacitance of the cell.

Influencing Factors on Capacitance

  • Factors Determining Capacitance:

    • Surface Area: Increased diameter allows for more charge storage, increasing capacitance.

    • Insulator Thickness: Thicker membranes reduce the ability to store charge, thereby decreasing capacitance.

  • Capacitors in membranes are paramount for fast response to stimuli and action potential generation.

Types of Neuronal Signaling

  • Passive Conduction:

    • Involves changes in membrane potential due to passive current flow without action potentials or reaching thresholds.

    • Understanding passive conduction is essential for analyzing neural activity.

  • Importance of Understanding Neural Activity:

    • Understanding how neural signals operate is crucial for diagnosing and treating neurological disorders.

    • Knowledge of neuron signaling contributes to advancements in brain-computer interfaces and prosthetic developments.

  • Disruptions can lead to conditions like multiple sclerosis and motor neuron diseases, hindering proper bodily control.

Time Constant ($ au$)

  • Definition:

    • The measure of how quickly a membrane potential changes in response to a stimulus.

  • Determining Factors:

    • Influenced by membrane resistance and capacitance (Equation: $ au = Rm imes Cm$).

  • Effect on Neuron Signals:

    • Shorter time constants allow for quick signal transmission, whereas longer time constants can enable temporal summation for action potential generation.

Length Constant ($

ho$)

  • Definition:

    • Represents the distance over which the electrical signal decreases to 37% of its original value.

  • Calculating Length Constant:

  • Can be determined from the exponential decay of voltage change.

  • Length constant ($
    ho$) and its implications on signal propagation are crucial for understanding neuronal function.

Summary of Key Points

  • The interplay between capacitance, resistance, time constants, and length constants play vital roles in neuronal signaling.

  • Active participation in practical work sessions will enhance comprehension of these principles.

  • Continuous engagement with foundational concepts is necessary for success in upcoming assessments and overall understanding of neuroscience principles.