Neurons, Synapses, and Signaling Overview

Historical Perspective on Animal Electricity

  • Luigi Galvani (~1750):

    • Conducted experiments on dissected frog muscle, proposing theory of animal electricity from observing nerve activity.
    • Suggested that animal tissue generates electricity.

  • Alessandro Volta:

    • Demonstrated that electrical current can be produced by contact between different metals, challenging Galvani's theory.

The Galvani-Volta Controversy

  • 1794 Experiment:

    • Contact between nerve sections and muscle causes contraction, supporting Galvani’s view.

  • 1797 Experiment:

    • Contact between sciatic nerves contracts both legs, showcasing interaction.

  • Carlo Matteucci:

    • In 1830s supported Galvani’s view with experimental evidence using a galvanometer.

Membrane Potentials and Action Potentials

Membrane Potential
  • Definition: The voltage across the plasma membrane of a cell (difference in electrical charge).
  • Resting potential:
    • Potential when neuron is not sending signals, typically around -70 mV.
  • Action potential:
    • Brief signal transmitted along neurons used for information transfer.
Maintaining Resting Potential
  • At equilibrium, both electrical and chemical forces across the membrane equilibrate, known as equilibrium potential.
    • K+ Ion Channels: Majority open at resting state.
    • Concentration Gradient: Difference in ion concentration maintained by Na+/K+ ATPase pump (3 Na+ out, 2 K+ in).
  • Example:
    • For K+, concentration is typically 120 mM inside and 4.5 mM outside, leading to EK of -88 mV.

Ion Movement and Membrane Changes

  • Driving Force:
    Analysis of how membrane potential (Vm) relates to equilibrium potential (EK) shows whether ion movement will depolarize (less negative) or hyperpolarize (more negative) the neuron.
Types of Changes in Membrane Potential
  • Depolarization:
    • Increase in membrane potential (less negative) caused by Na+ influx.
  • Hyperpolarization:
    • Decrease in membrane potential (more negative) caused by K+ efflux or Cl- influx.

Action Potentials (AP)

  1. Generation of AP:
    • Occurs when a depolarization surpasses a threshold, leading to activation of voltage-gated Na+ channels.
  2. Phases of AP:
    • Rising Phase: Rapid depolarization due to Na+ influx.
    • Falling Phase: K+ efflux leads to repolarization.
    • Undershoot: Membrane potential briefly becomes more negative than resting potential (hyperpolarization).
Propagation of Action Potentials
  • Unidirectional Conduction:
    • APs travel in one direction due to inactivation of Na+ channels behind the depolarization zone.
  • Myelination:
    • Myelin sheaths around axons increase signal transmission speed through saltatory conduction at nodes of Ranvier.

Neuronal Communication

Overview
  • Chemical Synapse:
    • Involves neurotransmitter release, receptor binding, and generation of postsynaptic potentials (PSPs).
  • Types of PSPs:
    • EPSP (Excitatory): Depolarizes membrane towards threshold.
    • IPSP (Inhibitory): Hyperpolarizes membrane away from threshold.
    • Summation: Takes place via temporal (rapid succession) and spatial (simultaneous different synapses) summation of EPSPs and IPSPs.
Neurotransmitter Mechanisms
  • Release:
    • Action potential opens Ca2+ channels, triggering neurotransmitter release.
  • Clearance:
    • Neurotransmitters are cleared by diffusion, reuptake, or enzymatic breakdown.

Conclusion

  • Understanding the balance of excitatory and inhibitory signals is critical for neural circuits, affecting everything from reflexes to complex behaviors.
  • Neuronal structure and function provide foundational insights into how information is processed in the brain.

Key Makeovers in Neuroscientific Understanding

  • Santiago Ramón y Cajal's neuron doctrine established that individual neurons are the fundamental units of brain functionality, with their morphology pivotal in defining their roles.
  • Historical context of discoveries from Galvani to Cajal illustrates the evolution of our understanding of neurobiology.