Chapter 2 Objectives - Exam 1

1. Basic Units of the Nervous System

  • Neurons (8.6×10108.6 \times 10^{10} in the human brain): Primary signaling cells that receive, process, and transmit electrical and chemical signals.

  • Glial Cells (1:11:1 ratio with neurons): Support cells that provide structural, metabolic, insulating, and immune functions.

2. Key Staining Techniques & Discoveries

  • Nissl Stain (Franz Nissl): Stains cell bodies and rough ER (Nissl bodies). Differentiates neurons from glia and highlights brain cytoarchitecture.

  • Golgi Stain (Camillo Golgi): Stains full individual neurons (1%−5%1\%-5\% of total) using silver chromate, revealing soma, axons, and dendrites.

  • Neuron Doctrine (Santiago Ramón y Cajal): States that neurons are discrete, individual units that communicate by contact across gaps, disproving the continuous network theory (Reticular Theory).

3. Neuronal Soma & Organelles

  • Neuronal Membrane: Phospholipid bilayer (5 nm5\,nm thick) with embedded channels and receptors regulating ion movement.

  • Soma (Cell Body) (10−50 μm10-50\,\mu m): Contains cytosol (K+K^+ rich) and key organelles:

    • Nucleus: Holds DNA; site of mRNA transcription.

    • Rough ER & Free Ribosomes: Sites of protein translation.

    • Smooth ER & Golgi Apparatus: Lipid synthesis, intracellular Ca2+Ca^{2+} buffering, protein folding, sorting, and packaging.

    • Mitochondria: Perform cellular respiration to produce energy (ATPATP).

4. Cytoskeleton

  • Microtubules (20 nm20\,nm): Made of tubulin; serve as tracks for fast axoplasmic transport.

  • Neurofilaments (10 nm10\,nm): Intermediate filaments providing structural strength.

  • Microfilaments (5 nm5\,nm): Made of actin; located near the membrane for dynamic structural plasticity.

5. Axons, Synapses, & Transmission

  • Axon Structure:

    • Axon Hillock: Spike-initiation zone where action potentials originate.

    • Axon Proper & Collaterals: Main shaft and branches transmitting signals to target cells.

    • Axon Terminal: Presynaptic region containing neurotransmitter-filled vesicles and mitochondria (lacks microtubules).

  • Synaptic Transmission Steps:

    1. Action potential reaches axon terminal.

    2. Depolarization opens voltage-gated Ca2+Ca^{2+} channels, causing Ca2+Ca^{2+} influx.

    3. Ca2+Ca^{2+} triggers vesicle exocytosis, releasing neurotransmitters into the synaptic cleft (20−50 nm20-50\,nm gap).

    4. Neurotransmitters bind to postsynaptic receptors, inducing a response.

  • Dendrites: Input antennae covered in dendritic spines that isolate chemical signaling events.

6. Neuron Classification

  • By Neurites: Unipolar (1 process), Bipolar (2 processes), Multipolar (many dendrites, 1 axon).

  • By Shape: Stellate (star-shaped) vs. Pyramidal (pyramid-shaped); spiny vs. aspinous.

  • By Function: Primary sensory neurons, motor neurons, or interneurons.

  • By Axon Length: Golgi Type I (long projection neurons) vs. Golgi Type II (short local circuit neurons).

  • By Neurotransmitter: Cholinergic, Glutamatergic, GABAergic, Dopaminergic, etc.

7. Glia & Non-Neuronal Cells

  • Astrocytes: Most common glia; clear neurotransmitters, buffer extracellular K+K^+, and contribute to the blood-brain barrier.

  • Oligodendrocytes (CNS) & Schwann Cells (PNS): Form myelin sheaths around axons; gaps called Nodes of Ranvier enable fast saltatory conduction.

  • Microglia: Resident immune phagocytes that clear debris and pathogens.

  • Ependymal Cells: Line brain ventricles and central canal; produce and circulate cerebrospinal fluid (CSF).

  • Vascular & Meningeal Cells: Control cerebral blood flow and form protective outer membranes (dura, arachnoid, pia mater).