Nervous Tissue and CNS Structure Notes (ACBS 400A/500A)

CNS and PNS: Basic Organization

  • CNS (Central Nervous System)
    • Consists of: Cerebrum, Cerebellum, Brainstem, Spinal cord
    • Function: central processing unit of the nervous system
  • PNS (Peripheral Nervous System)
    • Consists of: Nerves, Ganglia, Sensory receptors
    • Function: conveys sensory signals about external and internal environment to CNS; conveys motor signals from CNS to peripheral effectors

Autonomic Nervous System (ANS)

  • Not under voluntary control
    • Regulated by the hypothalamus
  • Components include many specialized neural elements in visceral organs, smooth muscle, and glands

Neurons: Structure and Key Features

  • Neuronal cell body (soma/perikaryon)
    • Contains typical organelles; abundant rough endoplasmic reticulum and polysomes (Nissl substance)
  • Axon
    • Each neuron has a single axon
    • Originates from the axon hillock (trigger zone)
    • Axon hillock → initial segment (just distal to hillock)
    • Axon branches at a distance from the cell body; form synapses with other neurons, muscle cells, or glands
  • Dendrites
    • Primary receptive sites for impulses from other neurons
    • Number varies depending on neuron type

Action Potentials Through Neurons

  • Action potentials generated at the axon hillock
  • Propagation speed along the axon: v ∈ [0.5, 120] m/sv \,\in\, [0.5,\,120] \,\mathrm{m/s}
  • Myelination and conduction
    • Large axons are myelinated in CNS and PNS
    • Smaller axons are not myelinated
    • Myelinated fibers conduct impulses faster; longer internodes yield faster conduction velocities
  • Axon branching
    • If linked to more than one recipient, axon branches form synapses with all recipient neurons
  • Neurons and division
    • Neurons do not divide once maturity is reached
  • Neural tissue color
    • Grey matter: high population of neuron cell bodies
    • White matter: predominantly myelinated axons

Classification of Neurons

  • Unipolar (primary)
    • Single stem process bifurcates into peripheral and central processes
    • Innervates peripheral tissues; carries somatic and visceral sensory information to CNS
  • Bipolar
    • Found in retina, spiral ganglion of the cochlea, vestibular ganglion, olfactory epithelium
    • Peripheral processes: sensory receptors; central processes: conduction to CNS
    • Olfactory exception: terminal branch forms dendritic bulb; cilia act as receptors detecting airborne chemicals
  • Multipolar
    • Most prevalent
    • Multiple processes; typically 1 axon and many dendrites; process length varies by function

Neuroglia: Schwann Cells (PNS) and Oligodendrocytes (CNS)

  • Schwann Cells (PNS)
    • Small and outnumber neurons; nuclei clearly visible on histology due to small size
    • Capable of division
    • Form myelin sheath concentrically around axons (internodes)
    • Node of Ranvier: gap between internodes
    • For smaller-diameter axons, may be non-myelinating but still associate with axons
  • Oligodendrocytes (CNS)
    • Small glial cells present in both white and gray matter
    • Have multiple processes that extend to adjacent axons to form myelin sheaths
    • Myelinate most axons with diameter > 1 μm1\ \mu\mathrm{m} (large) to speed conduction

Nerve Fibers and Conduction Velocity

  • A nerve fiber = axon + myelin sheath
  • Classification by diameter, conduction speed, and function
    • Aalpha: largest; fastest conduction; innervates skeletal muscle; heavily myelinated
    • Abeta, Agamma, Adelta: progressively smaller and less myelinated
    • B fibers: smaller than A; myelinated
    • C fibers: unmyelinated; slow conduction
  • Sensory fibers: numbered (I, II, etc.); Ia is the largest; IV is nonmyelinated and smallest

Neuroglia: Microglia

  • Make up about 10–20% of all glia
  • Macrophages of the CNS; first line of defense
  • Upon activation: proliferate and become phagocytic
  • Roles: clear debris from injury site; phagocytosis; nitric oxide production

Neuroglia: Astrocytes

  • Star-shaped cells with many long processes; nuclei appear pale and oval
  • ~50% of glial population in CNS
  • Functions
    • Structural and metabolic support for neurons
    • Release neurotrophic factors (nerve growth factors)
    • Extracellular adhesion molecules aid axon/dendrite elongation
    • Processes cover much of neurons, synapses, internodal areas, and capillaries
  • Perivascular endfeet
    • Cover capillaries; important for glucose transport, regulation of extracellular environment, glutamate metabolism, and maintenance of the blood-brain barrier (BBB)
  • Homeostasis and protection
    • Maintain optimal extracellular environment; exchange ions via ionic channels
    • Take up extracellular K+ during neuronal excitation to prevent excessive buildup
    • Help prevent buildup of neurotoxic substances
    • Participate in repair after injury; cell swelling indicates hypoxia, trauma, or hypoglycemia

Neuroglia: Astrocytes and the Blood-Brain Barrier

  • Perivascular endfeet and the BBB
    • Astrocyte endfeet contribute to the BBB maintenance and regulation
  • Ion and neurotransmitter homeostasis
    • K+ buffering during neural activity; glutamate clearance to prevent excitotoxicity

Neuroglia: Ependymal Cells

  • Cover ventricles and central canal of the CNS; choroid plexus
  • Forms selective barrier between nervous tissue and CSF
  • Junctional complexes allow modulation of CSF secretion/absorption
  • CSF production sites
    • Choroid plexus
    • Ependymal lining of ventricles and central canal
    • Pia mater outer glial limiting membrane that covers the CNS surface

Cerebrospinal Fluid (CSF)

  • Movement and location
    • CSF leaves the ventricular system via the lateral aperture of the 4th ventricle into the subarachnoid space
    • It can enter the central canal of the caudal medulla oblongata and spinal cord
    • In the subarachnoid space, CSF drains into the dorsal sagittal sinus
  • Functions
    • Protects brain and spinal cord from trauma (meningeal protection)
    • Buoyancy: reduces the effective weight of the brain
  • Choroid epithelium and barrier role
    • Gatekeeper of the choroid plexus that forms the blood-CSF barrier
    • Capillaries act as gatekeepers as part of the BBB
  • CSF composition
    • Approximately 99%99\% water
    • Produced via ion gradients: OH- combines with CO2 to form bicarbonate
    • Reaction: OH−+CO<em>2→HCO</em>3−\mathrm{OH^-} + \mathrm{CO<em>2} \rightarrow \mathrm{HCO</em>3^-}
    • Basal surface: H+ exchange for extracellular Na+; Na+ pumped into ventricles across the apical surface
    • Charge balance achieved by Cl- and bicarbonate movement

Blood-CSF Barrier

  • Location: Choroid plexus in lateral, 3rd, and 4th ventricles
  • Structure
    • Pia mater invaginates and is covered by choroid epithelial cells facing the ventricle; microvilli present
    • Vascular component follows choroid plexus and shows fenestrations; choroid plexus epithelium sealed by tight junctions
  • Function
    • Regulates movement of substances between blood and CSF
    • Requires carrier proteins to transport essential molecules

Blood–Brain Barrier (BBB)

  • Distribution
    • Present throughout nearly all CNS tissue
    • Exceptions: choroid plexus, hypophysis (pituitary), median eminence, pineal gland, area postrema
  • Capillary structure
    • Fenestrations in some exception regions allow larger molecules to pass
    • Continuous tight junctions between endothelial cells; absence of fenestrations in most CNS capillaries
    • Very few pinocytotic vesicles; transcellular transport is the main route for many solutes
  • Maintenance and transport
    • Astrocyte perivascular end feet help maintain BBB integrity
    • Solute transport across BBB is generally transcellular
    • Water-soluble molecules require transporters/carriers (facilitated diffusion)
    • Large neutral amino acids require energy for transport; Na+/K+-ATPase pumps provide energy for transport processes
  • Ionic selectivity and transport dynamics
    • Anions are electrostatically attracted to water’s H+ side; cations attracted to water’s O2 side
    • Lipid bilayer barrier favors selective permeability; ions crossed via active/facilitated mechanisms as needed

Practical and Conceptual Takeaways

  • BBB restricts entry of many substances; pharmacological delivery to CNS often requires transporter-mediated routes or alternative strategies
  • Astrocytes and perivascular end feet are crucial for BBB integrity and ion homeostasis
  • Myelination increases conduction velocity; larger diameter and thicker myelin generally enhance speed
  • Neurons are largely non-dividing after maturity; glial support and neurotrophic factors are critical for maintenance and repair
  • CSF dynamics provide protection, nutrition, and waste clearance for CNS tissue

Review and Study Prompts

  • Compare and contrast CNS vs PNS organization and components
  • List major glial cell types and summarize their CNS vs PNS roles
  • Describe how CSF is produced, circulated, and reabsorbed; identify key barriers (blood-CSF vs BBB)
  • Explain how conduction velocity is determined by fiber diameter and myelination
  • Identify and explain exceptions to the BBB and their physiological rationale

Key Formulas and Quantitative Details

  • Axon conduction speed range: v ∈ [0.5, 120] m/sv \,\in\, [0.5,\,120] \,\mathrm{m/s}
  • Axon diameter threshold for myelination relevance: d \;>\, 1\ \mu\mathrm{m} (myelination common for larger axons)
  • CSF water content: CSF≈99% H2O\text{CSF} \approx 99\% \,\mathrm{H_2O}
  • Carbonic chemistry in CSF production: OH−+CO<em>2→HCO</em>3−\mathrm{OH^-} + \mathrm{CO<em>2} \rightarrow \mathrm{HCO</em>3^-}
  • BBB transport considerations: transporters for water-soluble molecules; Na+/K+-ATPase for energy-dependent transport; transcellular routes dominate for many solutes

Final Thoughts

  • This material integrates anatomy (CNS/PNS organization), cellular biology (neuron and glial cell types), electrophysiology (action potentials and conduction), and neurophysiology (CSF dynamics and barriers) to explain how the CNS is protected while remaining responsive and metabolically supported.