Microscopic Structure of the CNS Ayman Abu-Tabanja, DDS, PhD.

Cells of the CNS Two major cell types Nerve cells (Neurons) 2. Neuroglial cells (Glia)

Neuron Doctrine (19th Century) Neuron is a functional & structural unit The indiviuality of neurons NS is Not a syncytium

The Neuron

Neuronal Types Based on neurites number Neurons are classified into:

Golgi Classification Golgi type I: large neurons within brain region 2. Golgi type II: small local circuit neurons

Neuron cytology Nucleus Central Vesicular Prominent nucleolus

Neuron cytology Cytoplasm Cell body rich in Mitochondria, Golgi complex, RER Neurites rich in filamentous organells microfilaments : contractile actin neurofilaments : structural component microtubules : provide a track for transport of proteins

Neurites Dendrites: Tapers from cell body Dendritic spines Axon: Uniform diameter Missing in some interneurons Collateral branches Synaptic terminals

Myelin Interrupted sheath of myelin covers of most of the axon Laid down by……….. increases the velocity of conduction in nerve fibers

Neuronal Synapses

Neuronal Synapses Gray’s type I: excitatory Asymmetrical Gray’s type II: Inhibitory Symmetrical

Neuronal Synapses: chemical

Neuronal Synapses: electrical Gap junctions Faster conduction than chemical synapse Low resistance pathway Not polarized

Neuroglia Astrocytes Oligodendrocytes Microglia Ependyma

Astrocytes Perivascular feet Internal Glial limiting membrane External Glial limiting membrane GFAP Functions: Supporting framework Electrical insulators Supports blood brain barrier Buffers glutamate and K+ Phagocytic activity

Oligodendrocytes Small cell body Conspicuous cytoplasm Long thin processes rich in microtubules Two types: Interfascicular Perineural

Microglia Smallest glial cells Elongated nuclei Several short branched processes Evenly distributed in gray & white matter Immune effector cells

Ependyma single layer Simple cuboidal to columnar epithelium Lines the ventricular system Posses microvilli and cilia

Ependyma 1. Ependomocytes majority ciliated no tight junctions Tanycytes Present in the Floor of 3rd ventricle Send chemical signal to blood capillaries of hypothalamus

Ependyma 3. Choroidal epithelial cells - Cover choroid plexus - Ciliated - Metabolically active - Tight junctions - Forms Blood-CSF barrier

Blood brain barrier

Circumventricular organs: - Median eminence - Pineal body - Neurohypophesis - Area postrema - Subfornical organ - organum vasculosum of lamina terminalis Allows sampling of systemic blood for regulation of fluid balance and cardiovascular homeostasis