Brain/beh Chap2 Part A

Protective Glial Cells

Astrocytes

  • Astrocytes are a prominent type of glial cell in the brain.

    • Often mistakenly referred to interchangeably with all glial cells.

    • Numerous types of glial cells exist; astrocytes constitute one subtype.

  • Name Origin

    • "Astrocyte" derives from the word "star" due to their star-like shape.

    • Structural features include a centralized soma (cell body) and multiple processes extending outward.

  • Staining Markers

    • Astrocytes can be stained for observation in research studies.

    • An example of a staining marker is spleal fibrino acetic protein (not essential to memorize).

  • Subtypes of Astrocytes

    • There are various subtypes of astrocytes, each functional depending on their location and condition in the brain.

    • They perform diverse roles in various brain environments.

  • Functions of Astrocytes

    • Structural Support: They help maintain the physical structure of the brain by keeping neurons in their proper locations.

    • Radial glial cells assist in neural migration during development and eventually transform into astrocytes.

    • Chemical Environment Regulation: Responsible for regulating extracellular fluid concentration to maintain an optimal environment for neurons.

    • Clear excess molecules to regulate chemical balance.

    • Metabolic Support: Astrocytes provide energy (glucose/lactate) to neurons, crucial for nervous system function.

    • Maintenance of Blood-Brain Barrier (BBB): Play a role in preserving the integrity of the BBB, essential for brain protection.

    • Role in Learning and Memory: Astrocytes support synaptic connections, thus impacting cognition and higher-order brain functions.

  • Interaction with Neurons

    • Astrocytes assist in the chemical signaling process but do not initiate the formation of synapses.

    • They maintain synaptic connections through their end feet, surrounding synapses and aiding in their function.

    • Unique structural characteristics: Unlike neurons, astrocytes do not have a unidirectional flow of information.

    • Astrocytes can adapt functions of their processes based on environmental needs.

Metabolic Processes

  • Neurons are high-energy consumers, utilizing significant portions (40-50%) of body's energy, primarily glucose.

  • Astrocytes facilitate energy supply by:

    • Transporting glucose from blood, processing it, and converting it into lactate (usable by neurons).

    • Storing glycogen for emergency energy use, which can be converted to lactate if glucose levels drop.

  • Use of lactate by Neurons

    • Neurons metabolize lactate to produce ATP, the energy currency within cells.

    • Astroglycogenic support can provide a temporary reserve of energy during short-term crises like cardiac arrest.

Myelinating Glia

  • Oligodendrocytes and Schwann Cells: Two types of glial cells involved in the formation of myelin.

    • Myelin: An insulating fatty substance that wraps around axons.

  • Myelination Differences

    • Oligodendrocytes:

    • Found in the central nervous system (CNS); can myelinate multiple axons or multiple segments of a single axon.

    • Their extensions wrap around parts of axons, creating myelin sheaths.

    • Play a role in supporting neuronal health and communication.

    • Schwann Cells:

    • Located in the peripheral nervous system (PNS), can myelinate only one segment of a single axon.

  • Nodes of Ranvier

    • Gaps between myelinated sections on axons, where action potentials can be regenerated.

Function of Myelination

  • Myelination increases conduction velocity of nerve impulses.

  • Saltatory Conduction: Ions generate electrical signals predominantly at nodes of Ranvier, leading to faster signal transmission by "jumping" between nodes.

  • Myelin also releases trophic signals to support axon maintenance and health.

Microglial Cells

  • Microglia serve as immune cells within the brain and spinal cord.

    • Originate from progenitor cells in the periphery, migrating to the brain during development.

  • Primary Functions

    • Immune Surveillance: Detects infections or debris in the brain.

    • Activation Phase: Swell and change processes upon detecting problems, releasing signals to recruit other glial cells (e.g., astrocytes) for support.

    • Conduct phagocytosis to remove cellular debris, dead neurons, and pathogens.

  • Role in Recovery and Inflammation

    • Facilitate recovery after injury by coordinating other cells involved in healing.

    • Involved in inflammatory responses but can lead to chronic inflammation if overactivated, contributing to neurodegenerative diseases.

Blood-Brain Barrier (BBB)

  • The BBB maintains the distinct chemical environment necessary for neuronal function by controlling entry into the brain.

    • Discovered by Paul Ehrlich, and further studies clarified its nature.

  • Structure of BBB

    • Composed of endothelial cells around blood vessels with tight junctions preventing large or harmful molecules from entering the CNS.

  • BBB Permeability

    • Semipermeable: Small molecules and lipid-soluble substances can cross (e.g., glucose, certain hormones), while large or hydrophilic molecules cannot directly enter.

    • Transporter proteins may facilitate specific substance transport across the BBB.

  • Circumventricular Organs (CVOs)

    • Areas in the brain with a weaker BBB that allow for monitoring of bloodstream conditions (e.g., hormone levels) while still protecting neuronal environments.

  • Importance of Astrocytes in BBB

    • Astrocytes help maintain the integrity of BBB by regulating tight junctions between endothelial cells.