PSYCH201: Tue 11/03 W3;07

  • Overview of Topics

    • Introduction to synaptic plasticity will occur tomorrow.
    • Today's focus is on neuronal communication from Module 1.2 and 1.3 of CALAD.
    • Chapter 2 deals with synapses, which will be discussed later.

  • Cell Membrane Fundamentals

    • Every cell has a cell membrane, which acts as a barrier to prevent leakage of cytoplasmic substances and restricts entry of external substances.
    • Maintains the intracellular environment while allowing essential nutrients, like glucose, to pass through via specific mechanisms.
    • Diffusion: Substances soluble in fat (e.g., cortisol) can freely pass through the membrane.
    • Active Transport & Channels: For water-soluble substances (e.g., salts, sugars), specialized pumps or channels are required.

  • Resting Membrane Potential

    • All cells, including neurons, have a difference in electrical charge across their membrane, termed the resting membrane potential.
    • Inside the cell is negatively charged relative to the outside, primarily due to the presence of negatively charged proteins and nucleic acids, leading to a typical resting potential of approximately −70-70 mV.
    • The differences in charge result from the selective permeability of the membrane.

  • Neuronal Communication

    • Neurons can transmit signals quickly over long distances, illustrated by the example of a motor neuron reaching its terminal in the toe from the body.
    • Neurons operate via electrical impulses generated by the rapid flow of ions across membranes, resembling ripples in water when activated.

  • Activation and Depolarization

    • Depolarization occurs when the inside of the neuron becomes less negative (e.g., reaching −60-60 mV) due to sodium influx.
    • Sodium channels allow sodium ions to flow into the neuron, driven by concentration and electrical gradients, causing the inside to become more positive (action potential firing).
    • Hyperpolarization refers to the potential becoming more negative, which can also occur under stimulation.

  • Ion Channels

    • Specific ion channels are present in neurons, allowing selective flow of ions like sodium (Na+) and potassium (K+).
    • Sodium channels open at certain voltages, allowing for rapid internalization of sodium ions, contributing to action potential generation.

  • Action Potentials

    • The action potential is an all-or-nothing response; it either occurs fully or not at all.
    • The sequence of events begins when sufficient stimulation reaches the threshold, causing mass sodium influx and subsequent potassium efflux to restore balance post-firing.
    • Sodium-Potassium Pump restores resting conditions by actively transporting sodium out and potassium back in using energy.

  • Propagation of Action Potentials

    • Domino Effect analogy describes how action potentials propagate along the axon, as each sodium channel opened triggers the next along the membrane.
    • This leads to a rapid and forthcoming signal generation moving down the axon.

  • Myelination

    • Myelinated neurons have an insulating layer (myelin sheath), allowing action potentials to jump from one node (Node of Ranvier) to another, significantly speeding up conduction (up to 100 m/s).
    • This jumping mechanism is termed saltatory conduction.

  • Refractory Period

    • The refractory period is crucial to prevent continuous firing of action potentials; it consists of the absolute refractory period and the relative refractory period which makes the neuron temporarily unresponsive after firing.

  • Clinical Connection

    • Discussed connections to local anesthetics like lidocaine, which block sodium channels, preventing action potentials from firing and thus pain perception during dental procedures.
    • The implications of blocking sodium channels systemically can lead to severe dangers including fatal consequences.

  • Conclusion

    • Understanding neuronal communication forms the basis for further studies on neurotransmission and synaptic plasticity.
    • Future discussions will expand on synaptic mechanisms that follow action potential generation.