CNS Histology

These flashcards cover key vocabulary and concepts from the lecture on CNS histology, focusing on the anatomy and functions of nervous tissue.

Neurons

Cells that transmit electrochemical signals in the nervous system.

Neuroglia

Support cells in the nervous system that insulate, nourish, support, and protect neurons.

Blood-brain barrier

A protective barrier that prevents certain substances from entering the brain from the bloodstream.

Astrocytes

Star-shaped glial cells that regulate interstitial fluid composition and support blood-brain barrier integrity.

Oligodendrocytes

Glial cells that form myelin sheaths around axons in the central nervous system.

Microglia

The smallest and rarest type of glial cells, acting as the primary immune defense in the central nervous system.

Ependymal Cells

Cubic epithelial cells lining the ventricles of the brain, involved in production and secretion of cerebrospinal fluid.

Myelin

A fatty substance that insulates nerve fibers and enhances the speed of nerve impulse transmission.

Purkinje Cells

Large neurons found in the cerebellum, important for motor coordination.

Neurite

A generic term for the projections of a neuron, including dendrites and axons.

Neuronal Response to Injury

The process involving neuronal and glial cells in the repair and regeneration following nerve injury.

Detailed Anatomy of a Neuron

1. Soma (Cell Body): Contains the nucleus and organelles (e.g., Nissl bodies/RER).

2. Dendrites: Branched processes that receive inputs via synapses.

3. Axon: Long process conducting impulses away from the soma.

   • Axon Hillock: The trigger zone for action potentials.

   • Nodes of Ranvier: Gaps in myelin where Na^{+} channels are concentrated.

   • Synaptic Terminals: Release neurotransmitters into the synaptic cleft.

Classification of Neuroglia by System

1. Central Nervous System (CNS):

   • Astrocytes

   • Oligodendrocytes

   • Microglia

   • Ependymal Cells

2. Peripheral Nervous System (PNS):

   • Schwann Cells: Form myelin in the PNS (1:1 ratio with axon segments).

   • Satellite Cells: Surround and support cell bodies in ganglia.

Functional Roles of Astrocytes

• Blood-Brain Barrier (BBB): Create "end-feet" that wrap around capillaries.
• Homeostasis: Regulate K^{+} concentrations (spatial buffering) and pH.
• Metabolism: Provide glucose/lactate to neurons.
• Repair: Form glial scars (astrogliosis) after injury.
• Neurotransmission: Take up and recycle glutamate.

Detailed Layers of the Cerebellar Cortex

1. Molecular Layer: The outermost layer containing dendrites of Purkinje cells and axons of granule cells (parallel fibers), plus Stellate and Basket cells.
2. Purkinje Cell Layer: The middle layer consisting of a single row of large Purkinje cell bodies.
3. Granular Layer: The innermost, densest layer containing billions of Granule cells and Golgi cells.

Purkinje Cells: Morphology and Function

• Structure: Possess massive, fan-like dendritic trees that extend into the molecular layer.
• Output: The only output from the cerebellar cortex; they send inhibitory (GABAergic) projections to the deep cerebellar nuclei.
• Function: Critical for refining motor movement and coordination.

Microglia: Origins and Immune States

• Origin: Unlike other glia, they derive from mesodermal yolk sac progenitors.
• States:
  • Ramified (Resting): Surveying the environment with long processes.
  • Amoeboid (Activated): Retract processes to become phagocytic, releasing cytokines and clearing debris.

Ependymal Cells and the Choroid Plexus

• Location: Line the ventricles and the central canal of the spinal cord.
• Structure: Often possess cilia to circulate cerebrospinal fluid (CSF) and microvilli for absorption.
• Choroid Plexus: Specialized ependymal cells and tufts of capillaries that actively secrete CSF.

Myelination: CNS vs. PNS

• CNS (Oligodendrocytes): One oligodendrocyte can myelinate up to 50 different axon segments across multiple neurons.
• PNS (Schwann Cells): One Schwann cell myelinates only a single segment of one axon.
• Function: Increases conduction velocity via Saltatory Conduction.

Structure of the Blood-Brain Barrier (BBB)

1. Endothelial Cells: Connected by tight junctions (claudins/occludins) to prevent paracellular leakage.
2. Basement Membrane: A continuous extracellular matrix layer.
3. Pericytes: Cells embedded in the basement membrane that regulate capillary blood flow.
4. Astrocyte End-feet: Provide biochemical signals to maintain tight junction integrity.

Neurite Types and Polarized Transport

• Axons vs. Dendrites: Axons are specialized for output (high microtubule stability), while dendrites are specialized for input.
• Anterograde Transport: Movement from soma to terminal via Kinesin.
• Retrograde Transport: Movement from terminal to soma via Dynein (used for recycling and signaling).

Neuronal Response to Axonal Injury

1. Wallerian Degeneration: The distal segment of the axon breaks down.
2. Chromatolysis: The cell body swells, and Nissl bodies disperse as protein synthesis shifts toward repair.
3. Regeneration (PNS): Schwann cells form Bands of Büngner to guide the regrowing sprout.
4. Regeneration (CNS): Limited by inhibitory factors from oligodendrocytes (Nogo) and physical glial scars.

Types of Synapses

1. Chemical Synapse: Utilizes neurotransmitters; involves a synaptic delay but allows for signal modulation.
2. Electrical Synapse (Gap Junction): Composed of Connexons; allows direct ionic flow and near-instantaneous bidirectional communication.