Notes on Synapses and Glial Cells

Synapses

  • Definition: Synapse is the junction where one neuron communicates with another neuron or with an effector cell.
  • Types:
    • Chemical Synapses: Most common, involving neurotransmitter release and receptor binding.
    • Electrical Synapses: Direct electrical signal transmission through gap junctions.
Chemical Synapses
  • Structure:
    • Presynaptic terminal releases neurotransmitters from synaptic vesicles.
    • Synaptic cleft: A small gap between the presynaptic and postsynaptic neurons.
    • Postsynaptic terminal has receptors that bind neurotransmitters.
  • Process:
    • Action potentials reach the presynaptic terminal, causing Ca2+ influx.
    • Neurotransmitters are released into the synaptic cleft, diffuse across, and bind to postsynaptic receptors.
    • This binding initiates postsynaptic potentials (graded potentials) and causes synaptic delay.
Electrical Synapses
  • Structure:
    • Neurons are connected by gap junctions.
  • Features:
    • Allow rapid signaling with no synaptic delay.
    • Generally facilitate synchronous activity in groups of neurons.

Glial Cells

  • General Characteristics:
    • Non-excitable support cells found in CNS and PNS.
    • Comprises about half the volume of the nervous system.
    • Capable of mitosis, protect and nourish neurons.
Types of Glial Cells in the CNS
  1. Astrocytes:
    • Star-shaped, most abundant.
    • Help form the blood-brain barrier, regulate tissue fluid, and provide structural support.
  2. Oligodendrocytes:
    • Form myelin sheaths around multiple axons in CNS.
    • Allow faster action potentials.
  3. Microglia:
    • Immune cells that engulf debris and infectious agents.
  4. Ependymal Cells:
    • Line brain and spinal cord cavities, produce cerebrospinal fluid (CSF).
Types of Glial Cells in the PNS
  1. Satellite Cells:
    • Surround neuron cell bodies in ganglia, electrically insulate, and regulate nutrient exchange.
  2. Neurolemmocytes (Schwann Cells):
    • Envelop PNS axons with myelin, allowing faster action potential propagation.

Myelination

  • Definition: The process where axons are wrapped with myelin made from glial cells, enhancing electrical insulation.
  • Key Features:
    • Myelin appears glossy-white due to high lipid content.
    • In PNS: Neurolemmocytes wrap axons individually; in CNS: Oligodendrocytes can myelinate many axons.
  • Functions:
    • Increases conduction velocity of action potentials along axons.
    • Myelination gaps (Nodes of Ranvier) facilitate saltatory conduction.

Axon Regeneration

  • PNS Regeneration:

    • Possible if the neuron cell body and enough neurilemma remain intact.
    • Successful regeneration is more likely with less damage and shorter distance to innervate.

  • Steps:

    1. Trauma severs axon.
    2. Distal portion degenerates (Wallerian degeneration).
    3. Regeneration tube formed by neurilemma and endoneurium.
    4. Axon regenerates along the tube.

  • CNS Regeneration:

    • Very limited due to growth-inhibiting molecules from oligodendrocytes, crowding of axons, and scarring.

Clinical Considerations

  • CNS Tumors: Often originate from glial cells or supporting tissues, with the potential for being benign or malignant.
  • Myelin-Related Disorders:
    • Multiple Sclerosis: Autoimmune demyelination in CNS, leading to inflammation and loss of function.
    • Guillain-Barré Syndrome: Peripheral nerve myelin loss, causing muscle weakness, often reversible.

Review Questions

  1. What is a synapse and its types?
  2. How does the structure of chemical and electrical synapses differ?
  3. What are the main types of glial cells and their functions in the CNS and PNS?
  4. Describe the process of myelination in the nervous system.
  5. What factors affect axon regeneration in the PNS and CNS?