Notes on Neuron Physiology: Part 1

Study Notes on Neuron Physiology: Part 1

Overview of Peripheral Nerves

  • General Structure:
    • Peripheral nerves consist of:
    • Nerve: The whole structure.
    • Fascicles: Bundles of nerve fibers.
    • Nerve Fibers or Neurons: Which can be myelinated or unmyelinated.
    • Important to note the presence of blood vessels within peripheral nerves as they are crucial for nerve function and health.

Structure of a Neuron

  • Neuron Components:
    • Cell Body (Soma):
    • Contains organelles necessary for protein synthesis (e.g., endoplasmic reticulum, Golgi complex).
    • Dendrites:
    • Antenna-like projections.
    • Function: Receive input from multiple neurons, suggesting a complex network of synapses (not just one-to-one connection).
    • Contain numerous receptors (mainly ligand-gated ion channels) that react to neurotransmitters.

Neurotransmitter Interaction

  • Role of Neurotransmitters:
    • Release of neurotransmitters will lead to:
    • Excitatory Response: Increases likelihood of firing an action potential.
    • Inhibitory Response: Decreases likelihood of firing an action potential.

Axon Structure and Function

  • Axon:
    • Distinguished intracellular region known as the
    • Axon Hillock (Spike Initiation Zone):
      • High concentration of voltage-gated sodium channels crucial for initiating an action potential.

Cytoskeletal Proteins in Neurons

  • Types of Cytoskeletal Proteins:
    • Neurofilaments: Types of intermediate filaments, providing structure and mechanical stability to axons.
    • The expression of neurofilament genes correlates with axonal diameter.
    • Thicker axons result in faster conduction velocity.
    • Microtubules: Formed from tubulin, essential for axonal transport of proteins and organelles.

Implications in Neurodegenerative Diseases

  • Abnormal neurofilament assembly:
    • Diseases such as Amyotrophic Lateral Sclerosis (ALS) are linked to neurodegenerative conditions associated with impaired assembly or function of neurofilaments.
  • Axonal Transport and Motor Proteins:
    • Axonal transport can affect intracellular transport processes.
    • Kinesin: A motor protein that moves materials from the cell body toward the axon terminal (anterograde transport).
    • Operates in a manner similar to trains on tracks, moving vesicles along microtubules.
    • MAP1C (Dyenin family): The motor protein responsible for retrograde transport (transport from axon terminal back to cell body).

Diseases Associated with Axonal Transport

  • Charcot-Marie-Tooth Disease:
    • A peripheral neuropathy caused by mutations in kinesin, affecting axonal transport and causing functional impairments.
  • Alzheimer's Disease:
    • Proposed connection to transport disorders. Poor neuronal trafficking potentially results in inability to transport necessary materials, affecting cellular health.

The Impact of Axonal Injury

  • Upon injury to an axon, the following occurs:
    1. Synaptic Terminal Degeneration: Initial response to injury when the synapse fails.
    2. Wallerian Degeneration:
    • Followed by degeneration of the axon and loss of the myelin sheath.
    • Microglia (CNS) and Macrophages (PNS) then clear debris.
    1. Chromatolysis: Degeneration of the endoplasmic reticulum in the neuron’s cell body in response to injury.
    2. Transneuronal Degeneration:
    • Retrograde Transneuronal Degeneration: Neurons upstream from the injured neuron also degenerate.
    • Anterograde Transneuronal Degeneration: Neurons downstream undergo degeneration as a consequence.

Conclusion

  • Understanding the structure and function of neurons is crucial to comprehend how injuries and diseases affect the nervous system.
  • The complexity of neuronal connections and the role of both structural components and transport mechanisms highlight the intricate nature of peripheral nervous system physiology.