Neurohistology

Early Foundations of Neurohistology

  • Theodor Schwann (1810 – 1882):

    • Schwann was a German scientist who collaborated with Matthias Schleiden.
    • Cell Theory (1839): He proposed that all living things are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells.
    • Application to Neurology: Schwann stated, ‐All animal tissue is made up of cells.‐ This led to the conclusion that the brain is also comprised of cells.
    • Myelinating Cells: He discovered the cells responsible for myelination in the Peripheral Nervous System (PNS), now known as Schwann cells.

  • Franz Nissl (1860 – 1919):

    • Nissl was a German psychiatrist and neuropathologist.
    • Nissl Stain: He discovered a cationic dye capable of staining the nucleus and clumps of material within the cytoplasm called Nissl Bodies.
    • Scientific Utility: This staining technique allowed researchers to distinguish neurons from glial cells and facilitated the study of the cytoarchitecture of specific brain regions.

  • Camillo Golgi (1843 – 1926):

    • Golgi was an Italian pathologist.
    • Silver Nitrate Stain (1873): He discovered a stain that coated the entire neuron, enabling researchers to see the cell body (soma) as well as all neurites (dendrites and axons).

  • Santiago Ram%n y Cajal (1852 – 1934):

    • A Spanish neuroscientist and histologist.
    • He utilized Golgi's silver nitrate stain, but diluted it further to study the intricate circuitry of the brain.

The Neuron Doctrine and Historical Debates

  • The Golgi vs. Cajal Debate:

    • Continuous Theory (Golgi): Golgi concluded that neurites were fused together like a network of vessels.
    • Discontinuous Theory (Cajal): Cajal concluded that nerves communicate by contact rather than continuity.
    • Nobel Prize: Both scientists received the Nobel Prize in Physiology in 19061906.

  • The Neuron Doctrine:

    • Proposed by Heinrich Wilhelm Gottfried von Waldeyer-Hartz in 18911891.
    • Core Theses:
      • The nervous system consists of individual cells called neurons, which possess their own membranes and act as fundamental signaling units.
      • The connections established between these neurons are highly specific.

Cellular Anatomy of the Neuron

  • Primary Components:

    • Cell Body / Soma / Perikaryon: The central metabolic hub of the neuron.
    • Axon: A process that carries action potentials away from the cell body.
    • Dendrites: Processes that carry action potentials into the cell body.

  • The Neurosoma (Cell Body):

    • Cell Membrane: The plasma membrane contains integral proteins, peripheral proteins, filaments of the cytoskeleton, carbohydrate chains (forming glycoproteins), cholesterol, and phosphoglyceride molecules.
    • Nucleus: Contains DNA, the nucleolus, nuclear satellites, and the accessory body of Cajal.
    • Chromatin State: Neurons exhibit euchromatin (active DNA) and heterochromatin. Neurons are noted to be approximately 10×10\times more active than other cells.
    • Barr Bodies: Sex chromatin found in the nucleus of certain cells.
    • Organelles:
      • Nissl Bodies: Also known as chromatophilic substance; consists of Rough Endoplasmic Reticulum (RER), Ribosomes, and Polysomes.
      • Mitochondria: Provide cellular energy.
      • Lysosomes: Involved in waste processing.
      • Golgi Complex: Involved in protein packaging (not present in dendrites).
      • Smooth Endoplasmic Reticulum (SER): Involved in lipid synthesis.

Specialized Axonal and Dendritic Structures

  • Axon Features:

    • Axon Hillock: The cone-shaped region where the axon joins the cell body; notably lacks Nissl bodies.
    • Initial Segment (IS): The region immediately following the hillock. It features a dense electron-dense undercoating (UU) observed at high magnification (70,000×70,000\times).
    • Trigger Zone: Area responsible for generating action potentials.
    • Telodendria: The terminal branches of the axon (terminal arborization).
    • Terminal Boutons: Also called synaptic knobs or axon terminals; these represent the site of communication with other cells.
    • Terminal Environment: Lacks microtubules but contains synaptic vesicles filled with neurotransmitters. The inside surface has a dense protein coating and numerous mitochondria for energy. It contains Ca2+Ca^{2+} channels.

  • Dendrite Features:

    • Neurons typically have 77 to 88 primary dendrites.
    • Primary dendrites branch between 44 and 66 times.
    • Spines: Small protrusions added to increase the surface area for synaptic contact.
    • Organelles: Dendrites contain similar organelles to the soma, but specifically lack the Golgi complex.

Classification of Neurons

  • Classification Criteria:

    • Number of neurites (processes).
    • Dendrite branching pattern.
    • Connections (sensory, motor, or interneuron).
    • Axon length.
    • Neurotransmitter type.

  • Classification by Neurites:

    • Unipolar: One process.
    • Bipolar: Two processes (one axon, one dendrite).
    • Pseudounipolar: Features a single process that splits into two (common in sensory receptors).
    • Multipolar: Many dendrites and one axon (most common).

  • Classification by Dendrite Pattern (Examples):

    • Rat neocortex Martinotti cell, Bipolar cell, and Pyramidal cell.
    • Mouse neocortex Pyramidal cell, Hippocampus Schaffer collateral-associated neuron, and Cerebellum Golgi cell.
    • Cat brainstem vertical cell.
    • Mouse retina ganglion cell and Cerebellum Purkinje cell.

  • Other Classifications:

    • Connections:
      • Primary sensory neurons.
      • Interneurons.
      • Motor neurons.
    • Axon Length:
      • Golgi Type I: Long axons that form tracts; referred to as principal cells.
      • Golgi Type II: Short axons that form local circuits; referred to as interneurons.

Neurotransmission and Synapses

  • Terminology:

    • The Synapse: A specialized structure where two neurons come close enough to pass chemical signals. Described in 18971897 by Sir Charles Scott Sherrington.

  • Neurotransmitter Types:

    • Excitatory (Open Na+Na^{+} channels):
      • Cholinergic (Acetylcholine).
      • Glutamate.
    • Inhibitory (Open Cl−Cl^{-} channels):
      • Gamma-aminobutyric acid (GABA).
      • Glycine.

  • Synapse Types by Location:

    1. Axoaxonic: Axon to axon.
    2. Axodendritic: Axon to dendrite (the most common type, aimed at firing the neuron).
    3. Axosomatic: Axon to cell body.
    4. Dendrodendritic: Dendrite to dendrite (often used to modify signaling).
    5. Motor End Plate: Neuromuscular junction where a motor neuron stimulates a skeletal muscle fiber.

  • Classification by Morphology (Gray's Types):

    • Gray Type I Synapse: Features round synaptic vesicles and an asymmetrical membrane (the postsynaptic membrane is thicker). These are typically excitatory.
    • Gray Type II Synapse: Features ovoid synaptic vesicles and symmetrical membranes. These are typically inhibitory.

Glial Cells

  • General Characteristics:

    • Glial cells provide physical and metabolic support and outnumber neurons (90%90\% of cells in the brain).

  • Types of Glial Cells:

    • Astrocytes: The most abundant glia.
      • Fibrous Astrocytes: Found in white matter; have thin, fewer processes; involved in metabolic transfer.
      • Protoplasmic Astrocytes: Found in gray matter; have thicker processes; provide metabolic intermediates.
      • Functions: Scaffolding, storage of glycogen, takes up extra K+K^{+}, removes neurotransmitters, forms the Blood-Brain Barrier, and provides insulation.
    • Oligodendrocytes:
      • Form myelin in the Central Nervous System (CNS).
      • One cell can myelinate several neurons (approx. 4040 to 5050 neurons) and up to 6060 internodal segments.
      • May remove K+K^{+} from the environment.
    • Ependymal Cells:
      • Choroid Epithelial Cells: Cover the choroid plexus and form Cerebrospinal Fluid (CSF). Feature microvilli and basal invaginations.
      • Ependymocytes: Line the ventricles; possess cilia/microvilli to move and absorb CSF.
      • Tanycytes: Found in the 3rd ventricle; possess long basal processes; transport chemicals to the hypophyseal portal system.
    • Microglial Cells:
      • Formed from monocytes.
      • Act as the macrophages (scavengers) of the CNS.

Axonal Transport and Myelination

  • Degeneration and Transport History:

    • Wallerian Degeneration: Described by Augustus Volney Waller (18162˘01318701816 \u2013 1870); axons cannot be sustained when separated from the cell body.
    • Axonal Transport: Proposed by Paul Alfred Weiss (18982˘01319891898 \u2013 1989).

  • Transport Mechanisms:

    • Anterograde (Orthograde): Transport from cell body to terminal.
      • Slow: 1–10 mm/day1 – 10\,mm/day (used for repair).
      • Fast: 100–400 mm/day100 – 400\,mm/day (transports membrane-bound vesicles).
      • Motor Protein: Kinesin.
    • Retrograde: Transport from terminal to cell body.
      • Speed: 50–200 mm/day50 – 200\,mm/day.
      • Purpose: Destruction of old organelles.
      • Motor Protein: Dynein.

  • Myelin Composition:

    • Composed of 70%70\% lipid (phospholipids and cholesterol) and 30%30\% protein.
    • Contains Myelin Associated Glycoprotein (MAG), which acts like glue.
    • Cells: Schwann cells (PNS) and Oligodendrocytes (CNS).
    • Neurolemma: The outer part of the Schwann cell area.

  • Myelin Formation and Structures:

    • Jelly Roll Theory: Proposed by Richard Bunge (19681968) to describe how myelin wraps around axons.
    • Nodes of Ranvier: Gaps in the myelin sheath that facilitate Saltatory Conduction (fast signaling).
    • Conduction Speed Factors: Myelinated axons are faster than unmyelinated. Large diameter axons are faster than small diameter axons.
    • Schmidt-Lanterman Clefts: Small amounts of cytoplasm trapped between myelin membranes due to incomplete fusion.

  • Myelinization Sequence:

    • Begins at the 4th4^{th} month of gestation and continues into adulthood.
    • Order: Sensory paths first, then motor; projection neurons before association neurons (interneurons); central telencephalon before poles; occipital pole before temporal or frontal poles.

Clinical and Peripheral Structures

  • Multiple Sclerosis (MS):

    • A chronic autoimmune disease primarily affecting the CNS.
    • The immune system attacks myelin, causing demyelination, inflammation, and resulting in multiple scars (lesions or plaques).
    • Typically affects individuals aged 20–4020 – 40.

  • Ganglia:

    • ANS Ganglion: Contains multipolar neurons with eccentric (off-center) nuclei and few satellite cells. Includes Small Intensely Fluorescent (SIF) cells.
    • Craniospinal Ganglion (e.g., Dorsal Root Ganglion/DRG): Contains pseudounipolar neurons (except for CN VIII, which is bipolar). Features a centrally placed ‐owl-eyed‐ nucleus and many satellite cells (which are flattened Schwann cells).

  • Nerve Coverings:

    • Endoneurium: Delicate connective tissue (CT) around individual axons.
    • Perineurium: CT that groups axons into fascicles; acts as a diffusion barrier.
    • Epineurium: CT that surrounds the entire nerve. May contain adipose tissue to act as a shock diffuser and dissipate stress.

  • PNS Regeneration:

    • Following injury, distal nerve fiber degeneration occurs.
    • Schwann cells proliferate and axons grow to re-establish connections with muscle fibers.