Neural Signaling

Neural Signalling DP IB Biology: HL Study Notes

Contents

  • Neurones: Function & Structure

  • Nerve Impulses

  • Nerve Impulses: Skills

  • Synapses

  • Action Potentials

  • Interpreting Oscilloscope Traces: Skills

  • Nerve Conduction Velocity

  • Synaptic Transmission

  • Neurones in the Brain

Neurones: Function & Structure

The Nervous System

  • Structure of the Human Nervous System:

    • Central Nervous System (CNS): Comprises the brain and spinal cord.

    • Peripheral Nervous System (PNS): Includes all nerves in the body.

  • Functions:

    • Enable responses to surroundings and body coordination/regulation.

    • Information transmission via electrical impulses through neurones.

  • Nerve Definition: A bundle of neurones that extends from the CNS to sense organs and every part of the body.

  • CNS as Central Coordinator: Receives and sends impulses to and from various body parts.

Neuronal Structure

  • Key Features of Neurones:

    • Long, main fiber known as an axon.

    • Axon is often insulated by Schwann cells which form the myelin sheath preventing impulse loss.

    • Cell Body: Contains the nucleus and other cellular structures.

    • Dendrites: Extensions from cell bodies that facilitate connections with other neurones, forming communication networks.

Types of Neurones

  • Three Main Types:

    1. Sensory Neurones: Carry impulses from receptors to the CNS.

    2. Relay (Intermediate) Neurones: Located entirely within the CNS and connect sensory and motor neurones.

    3. Motor Neurones: Transfer impulses from the CNS to effectors (muscles or glands).

  • Structural Differences:

    • Motor Neurones:

    • Large cell body, located in the spinal cord or brain.

    • Contains a nucleus in the cell body.

    • Highly-branched dendrites for greater surface area.

    • Relay Neurones:

    • Short, highly branched axons/dendrites.

    • Sensory Neurones:

    • Cell body branches in the middle.

    • Single long dendron to cell body and a long axon away from cell body.

Nerve Impulses

Generating the Resting Potential

  • Impulse Definition: A momentary reversal of electrical potential difference across the neurone membrane, not a constant electrical current.

  • Resting Potential:

    • Exists at approximately -70mV, indicating that the inside of the axon is more negatively charged compared to the outside.

    • Maintained by:

    1. Active Transport: Sodium ions (Na⁺) are moved out and potassium ions (K⁺) are moved into the axon via sodium-potassium pumps (3 Na⁺ out for every 2 K⁺ in).

    2. Diffusion Rates: Greater diffusion of K⁺ out than Na⁺ in due to permeability differences.

  • Membrane Potential: Voltage across a membrane that causes resting potential; negative due to negatively charged proteins.

Resting Membrane Potential Maintenance

  • Ion Channels:

    • Sodium and potassium channels facilitate diffusion across the membrane.

    • Neurone membrane less permeable to Na⁺.

  • Consequence:

    • Higher concentration of positive ions outside the neurone, contributing to negative resting potential.

Initiating Nerve Impulses

  • Polarisation: Resting neurone membrane that is negatively charged.

  • Depolarisation: Required for action potential generation which transforms resting potential to about +40mV.

  • Action Potential Characteristics: A change in ion movement across the axon membrane leading to the electrical signal of a neurone when stimulated.

Skills on Nerve Impulses

Speed and Transmission

  • Features Affecting Speed:

    • Myelination: Myelinated neurones conduct impulses faster due to the insulating effect of the myelin sheath, enabling saltatory conduction.

    • Axon Diameter: Wider axons facilitate quicker conduction due to decreased resistance.

  • Example: Squid axons (1mm wide) vs human axons (4-100µm wide) demonstrate significant speed variation, emphasizing myelination benefits.

The Role of Myelination in Impulse Speed

  • Myelin Structure: Formed by Schwann cells wrapping around the axon, providing electrical insulation and high resistance.

  • Nodes of Ranvier: Uninsulated sections of axon allowing saltatory conduction whereby impulses jump between nodes for faster transmission.

Statistical Analysis of Nerve Transmission

  • Correlation vs Causation: Understand the distinction between correlation (relationship) and causation (influence).

  • Data Collection: Analyze relationships using correlation coefficients and graphs, assessing variables such as axon diameter and myelination on speed.

Interpreting Results

  • Statistical Methodology:

    • Pearson's correlation coefficient (r) is used to quantify the strength of relationships.

    • Coefficient of determination (R²) is calculated from r² to identify predictive strength of variables.

Synapses

Synapse Structure

  • Defined as the junction where two neurones meet, separated by the synaptic cleft.

  • Function: Propagate electrical impulses through chemical messengers (neurotransmitters).

Synaptic Transmission Process

  • Triggering Mechanism: Arrival of an impulse causes depolarisation at the presynaptic membrane leading to an influx of calcium ions via calcium channels.

  • Neurotransmitter Release: Vesicles fuse with presynaptic membrane, releasing neurotransmitters like acetylcholine (ACh) into the synaptic cleft.

  • Binding Process: Neurotransmitters diffuse across the cleft, binding to postsynaptic receptors, opening sodium channels, and generating potential action in the postsynaptic neurone.

  • Enzymatic Breakdown: ACh is broken down by acetylcholinesterase to prevent unchecked stimulation.

Neurotransmitter Functions and Effects

  • Unidirectionality of Synapses: Impulses flow in one direction only due to neurotransmitter action, preventing retrograde impulse travel.

  • Examples of Neurotransmitters: Over 40 known, including dopamine, noradrenaline, and ACh—essential in establishing postsynaptic potentials.

Inhibitory Postsynaptic Potentials

  • Neurotransmitters can inhibit action potential generation (hyperpolarisation), preventing impulse propagation.

  • Inhibition Mechanism: Potassium channels opening leads to potassium ions diffusing out of the cell, making it negatively charged, preventing threshold level achievement.

Summation of Neurotransmitter Effects

  • Temporal Summation: Multiple impulses in quick succession enhance NT effects to potentially reach action potential.

  • Spatial Summation: Multiple simultaneous impulses from different synaptic knobs contribute post-synaptic potentials sufficient to trigger an action potential.

Action Potentials

Depolarisation and Repolarisation

  • Mechanism: Upon stimulation, sodium channels open, sodium influx leads to depolarisation, creating an action potential.

  • Propagation Process: Action potentials trigger adjacent sections of the axon through the same ion channel opening mechanism.

  • Repolarisation: After action potential peak, potassium channels open, re-establishing resting membrane potential.

  • Refractory Period: Time during which neuron can't be re-stimulated to prevent backtracking of impulses.

Action Potential Graphing

  • Represents depolarisation (upward spike) and repolarisation phases, highlighted by the threshold potential around -50mV for action potential generation.

Interpreting Oscilloscope Traces: Skills

Membrane Potentials of Neurones

  • Measurement: Achieved via electrodes across the neurone membrane; visualized on oscilloscope graphs.

  • Graphs Representations: Resting potentials appear as horizontal lines (-70mV) while spikes correspond to action potentials.

Nerve Conduction Velocity

Saltatory Conduction Mechanism

  • Myelin sheaths increase conduction speed; sodium/potassium ion diffusion is inhibited, allowing action potentials to jump between nodes of Ranvier in saltatory conduction, increasing impulse speed.

Factors Influencing Neuronal Signal Transmission

  • Exogenous chemicals can disrupt normal synaptic transmission; examples include neonicotinoids (block ACh receptors) and cocaine (block neurotransmitter reuptake).

Inhibitory Synapses and Their Role

  • Inhibitory synapses provide balance to neuronal circuits, ensuring only pertinent impulses are transmitted and reducing random activity.

Neurones in the Brain

Perception of Pain

  • Pain receptors (nociceptors) are activated by stimuli like high temperatures, acids, or chemicals (capsaicin).

  • Resulting signals travel via sensory neurones to CNS for processing and protective responses.

Interaction of Neurones in the Brain

  • Cerebrum: Largest part controlling conscious activities and complex behaviors.

  • Emergent Properties: Result from complex neuronal interactions, influencing consciousness and cognitive functions.