Neurophysiology Notes

Neuroscience: Neurophysiology Notes

Overview of Neurophysiology

  • Neurophysiology involves the generation, transmission, and integration of neural signals.
  • The nervous system operates as a bio-electrical communication system.

Neuronal Structure and Function

  • Components of Neurons:
    • Cell Body: Contains the nucleus and cellular machinery.
    • Dendrites: Input zone where neurons collect and integrate information.
    • Axon: Conducting zone where information is transmitted over long distances.
    • Myelin Sheath: Insulates the axon for faster signal propagation.
    • Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid conduction.
    • Axon Hillock: Integration zone where the decision to produce a neural signal is made.
    • Synapse: Output zone where neurotransmitters are released to communicate with other neurons.

Membrane Potential

  • Definition: All cells maintain an electrical potential difference (membrane potential) across their cell membranes due to the uneven distribution of ions.
  • Key Factors Affecting Membrane Potential:
    • The presence and concentration of ions (Na+, K+, Cl-, Ca2+) both inside and outside the cell.
    • The permeability of the cell membrane to different ions.
    • The state of ion channels (open, closed, or inactivated).

Ion Transport Mechanisms

  • Membrane Transport Molecules:
    • Channels: Allow specific ions to pass.
    • Gated Channels: Open or close in response to stimuli, allowing ions to flow based on concentration and electrical gradients.
    • Pumps: Actively move ions against their concentration gradients, such as the Na+/K+ pump.

Resting Membrane Potential

  • Ion Distribution:
    • More K+ ions inside (~140mM) versus outside (~5mM)
    • More Na+ ions outside (~145mM) versus inside (~12mM)
  • Mechanisms Maintaining Resting Potential:
    • The Na+/K+ pump keeps higher concentrations of K+ inside and Na+ outside.
    • At resting, the membrane is more permeable to K+ than Na+.

Action Potential (AP)

  • Definition: A rapid reversal of the membrane potential that occurs when a neuron transmits a signal.
  • Phases of AP:
    1. Threshold: Membrane potential reaches about -50mV; slow depolarization triggers voltage-gated Na+ channels to open.
    2. Rising Phase: Rapid influx of Na+ leads to depolarization—membrane potential climbs to around +40 mV.
    3. Falling Phase: Na+ channels inactivate; K+ channels open, K+ exits, causing repolarization.
    4. Undershoot Phase: Membrane potential briefly becomes more negative than resting potential due to continued K+ efflux before returning to -70mV.
  • Propagation of Action Potential:
    • Local change in membrane potential causes neighboring sections to depolarize, leading to AP movement along the axon (recording wave of eletricity).
    • Saltatory Conduction: In myelinated axons, AP jumps between nodes of Ranvier, greatly increasing conduction speed (30 - 150 m/s).

Synaptic Transmission

  • Process of Synaptic Transmission:
    1. AP triggers depolarization at the presynaptic terminal, leading to Ca2+ influx.
    2. Ca2+ causes fusion of neurotransmitter vesicles with the membrane, releasing neurotransmitters into the synaptic cleft.
    3. Binding to postsynaptic receptors opens ion channels, creating excitatory (EPSP) or inhibitory (IPSP) postsynaptic potentials.

Types of Receptors

  1. Ionotropic Receptors:

    • Fast response; ligand-gated ion channels
    • Directly allows ions to flow across the membrane, leading to synaptic transmission.

  2. Metabotropic Receptors:

    • Slow response; G-protein coupled receptors
    • Activate intracellular signaling, leading to longer-term changes in cell activity.

Summary of Learning Outcomes

  • Explain how resting membrane potential arises.
  • Describe the phases and mechanisms of action potential generation.
  • Explain AP propagation and the significance of saltatory conduction.
  • Discuss vesicle release mechanisms and the critical role of Ca2+.
  • Distinguish excitatory and inhibitory postsynaptic potentials and their influence on action potential generation.
  • Distinguish between ionotropic and metabotropic receptors, including their functions and speed of action.