2 cell types:

1. Neuron — functional cell; cell body (soma) + dendrites (input) + axon (output). Has Nissl bodies (RER). Long processes covered with myelin (= speed insulation).
2. Neuroglia (Glia) — supporting cells, ~10× more than neurons.
• CNS: Astrocytes (BBB, support), Oligodendrocytes (myelin), Microglia (immune), Ependymal (CSF lining).
• PNS: Schwann cells (myelin), Satellite cells (around ganglion neurons).

Synapse = junction where neurons communicate (chemical or electrical).

Histology hint: CNS = 'grey matter' (cell bodies) + 'white matter' (myelinated axons). PNS = nerves (axon bundles) + ganglia (cell body clusters).

NERVOUS TISSUE — NEURONS & GLIA

Nervous tissue is highly specialised for rapid signal transmission. It has two main cell populations: neurons (functional) and glia (support). It has VERY little extracellular matrix.

I. NEURONS:
• Cell body (Soma / Perikaryon): contains nucleus + Nissl bodies (clusters of rough ER + free ribosomes) + Golgi + neurofibrils (intermediate filaments).
• Dendrites: Multiple, short, branched. Receive signals; covered with dendritic spines.
• Axon: Single long process. Begins at axon hillock (no Nissl). Ends at terminal boutons (synapses).
• Myelin sheath: Insulating wrapping by oligodendrocytes (CNS) or Schwann cells (PNS). Gaps = Nodes of Ranvier → saltatory conduction.

NEURON CLASSIFICATION:
• By processes: Multipolar (most), Bipolar (retina, olfactory), Unipolar/Pseudounipolar (DRG sensory).
• By function: Sensory (afferent), Motor (efferent), Interneuron.

II. GLIAL CELLS (CNS):
• Astrocytes: Star-shaped; end-feet support the blood-brain barrier, whose seal is formed by capillary endothelial tight junctions; structural support; recycle neurotransmitters; respond to injury (gliosis).
• Oligodendrocytes: Make myelin; one cell can wrap many internodes on multiple axons.
• Microglia: Resident immune cells; phagocytose debris, prune synapses.
• Ependymal cells: Cuboidal/columnar with cilia; line ventricles + central canal; with capillaries form choroid plexus → produce CSF.

III. GLIAL CELLS (PNS):
• Schwann cells: Make myelin (1 cell = 1 internode of 1 axon). Also engulf unmyelinated axons.
• Satellite cells: Surround neuron cell bodies in ganglia; supporting + nutritive role.

IV. ORGANIZATION OF NERVOUS TISSUE:
• CNS:
– Grey Matter = neuron cell bodies + dendrites + glia + capillaries. Cortex of cerebrum/cerebellum, central H of spinal cord, deep nuclei.
– White Matter = myelinated axons (the white colour is the myelin lipid).
• PNS:
– Nerves = bundles of axons (fascicles) wrapped by 3 CT layers:
• Endoneurium (around each axon).
• Perineurium (around fascicles).
• Epineurium (around whole nerve).
– Ganglia = clusters of neuron cell bodies outside CNS:
• Sensory ganglia — dorsal root ganglia + cranial nerve ganglia. Round nuclei + clear cytoplasm; satellite cells around.
• Autonomic ganglia — sympathetic chain + parasympathetic ganglia. More irregular cells.

V. SYNAPSE:
• Junction where signal passes from neuron to next cell (neuron, muscle fibre, gland cell).
• Chemical synapse (most): pre-synaptic terminal contains synaptic vesicles with neurotransmitter; AP → Ca²⁺ influx → vesicle fusion → NT released into synaptic cleft → binds receptors on post-synaptic membrane.
• Electrical synapse: gap junctions → direct ion flow; very fast (cardiac, smooth muscle, some CNS).

HISTOLOGICAL STAINS:
• H&E: Routine; nuclei + Nissl bodies basophilic.
• Cresyl violet (Nissl stain): Specific for Nissl bodies.
• Silver impregnation: Black neuron processes.
• Luxol Fast Blue: Myelin.
• GFAP (immunostain): Astrocytes.