#15 - Neurons Synapse

Historical Perspective of Animal Electricity

  • Luigi Galvani (~1750): Proposed the theory of animal electricity after observing frog muscle contractions induced by electricity

  • Alessandro Volta: Challenged Galvani, suggesting electrical current arises from contact between different metals

  • Experiments showed a frog leg could be stimulated by electric shock, leading to debates over the nature of animal electricity

    • Galvani's experiments showed that nerves stimulated muscle contractions directly, while Volta argued it was due to electrical contact with metals

Neuron Basics

  • Cell Membrane Potential: Every cell has a voltage across its plasma membrane called membrane potential, essential for neuron communication

    • Resting Potential: The membrane potential of a neuron at rest, typically between -80 mV to -65 mV

    • Action Potential: An all-or-none signal that transmits information along neurons over long distances

Equilibrium Potentials

  • Nernst Equation: Used to calculate the equilibrium potential for individual ions, e.g., K+

    • The formula: Ex (mV) = (RT/zF) ln [x] outside / [x] inside

    • Example for K+:

    • EK = 61.5 log (4.5/120) = -88 mV

  • At equilibrium: electrical and chemical forces balance

Action Potentials

  • Mechanism:

    • Begins with depolarization: Membrane potential rises due to Na+ ion influx through voltage-gated channels.

    • Threshold: A critical level of depolarization that must be reached to generate an action potential.

    • Following depolarization, repolarization occurs when K+ ions exit the cell.

    • Refractory Periods: Ensure unidirectional propagation of the action potential, preventing backward movement.

Neuronal Communication

  • Neurotransmitters: Chemicals like acetylcholine released at chemical synapses to transmit signals between neurons.

    • Types of Postsynaptic Potentials: Excitatory (EPSPs) that depolarize the membrane and inhibitory (IPSPs) that hyperpolarize it.

  • Graded vs Action Potentials: Graded potentials are variable and localized changes in membrane potential, while action potentials are uniform and propagate along axons.

Structural Components of Neurons

  • Dendrites: Receive signals and funnel them to the soma (cell body).

  • Axon: Transmits action potentials; begins at the axon hillock, where action potentials are generated.

Special Features of Neurons

  • Myelin Sheath: Insulates axons, increasing the speed of action potentials through saltatory conduction at nodes of Ranvier.

  • Node of Ranvier: Gaps in the myelin sheath critical for the rapid transmission of electrical impulses.

Summation of Postsynaptic Potentials

  • Temporal Summation: Rapid successive firing of EPSPs can trigger an action potential.

  • Spatial Summation: EPSPs from different sources can combine to reach threshold potential.

  • The balance of excitatory and inhibitory inputs determines whether an overall action potential occurs.

Key Figures in Neuroscience Research

  • Camillo Golgi & Santiago Ramón y Cajal: Pioneered the neuron doctrine, identifying neurons as the functional units of the nervous system, utilizing Golgi's staining technique to visualize neuron structure.

Neurotransmitter Mechanisms

  • Vagusstoff: The initial discovery of acetylcholine by Otto Loewi during experiments with heart contractions, leading to understanding synaptic transmission.

  • Chemical vs Electrical Synapses: Different mechanisms through which neurons communicate, emphasizing the role of neurotransmitters in chemical synapses.

Conclusion

Understanding neurons and electrical signaling is foundational to understanding complex behaviors and physiological responses in animals, bridging the gap between historical insights and contemporary neuroscience.