Neurons and Glia

What two major cell types make up the human brain?

Neurons — specialised for electrical/chemical signalling.

Glial cells — support, regulate and protect neurons; include astrocytes, microglia, oligodendrocytes/Schwann cells, and ependymal cells.

What were the competing historical theories explaining how neurons are connected?

Reticular theory (Golgi): neurons fuse into a continuous network.

Neuron doctrine (Cajal): neurons are individual cells that communicate via contact, not continuity.

The electron microscope eventually proved Cajal right by showing synaptic gaps.

Electron microscopy gave nanometre-scale resolution → could directly visualise synapses and membranes.

What is the “Brainbow” technique and what does it allow?

Combines fluorescence microscopy with genetic manipulation (e.g., Cre-Lox) to make individual neurons express different coloured fluorescent proteins.

Produces multicolour labelling → visualises neuron morphology, connectivity, and glial interactions in exceptional detail.

Brainbow uses stochastic recombination so each cell expresses unique fluorophore combinations.

Why do we study microanatomical features of the brain?

To understand connectivity, synaptic organisation, disease pathology (e.g., synapse loss, demyelination), and cell-type specific functions.

These microscopic features drive global brain function and dysfunction.

What are the three main structural components of a prototypical neuron?

1. Cell body (soma)

2. Dendrites — receive inputs

3. Axon — conducts action potentials

What is the primary function of neurons?

To process and transmit information using electrical impulses (action potentials) and chemical signalling (neurotransmission).

Neurons exhibit polarity—information flows dendrite → soma → axon.

What organelles are found in the neuronal soma?

Nucleus

Rough ER (Nissl substance)

Smooth ER

Golgi apparatus

Mitochondria

Neurons are extremely energy-demanding → dense mitochondrial content.

What three structures make up the neuronal cytoskeleton?

Microtubules — transport cargo (kinesin/dynein), maintain axon structure

Neurofilaments — provide tensile strength

Microfilaments (actin) — support growth cones, synaptic plasticity

Microtubules = dynamic; neurofilaments = stable, abundant in large axons.

What are the key features of axons?

Specialised for impulse conduction.

Possess axon hillock (AP initiation area rich in voltage-gated Na⁺ channels).

Often myelinated for rapid conduction.

Axons lack RER → proteins must be transported from soma.

What do dendrites do?

Receive synaptic inputs via post-synaptic receptors.

Form a dendritic tree—critical for integrating thousands of synaptic signals.

Dendritic spines change in learning & memory.

What is neurotransmission?

The process by which neurons communicate using chemical messengers released at synapses.

Requires:

Presynaptic neurotransmitter synthesis & storage

Vesicle fusion

Receptor activation postsynaptically

How can neurons be classified by structure?

Unipolar

Bipolar

Multipolar

Based on dendrite/axon arrangement and morphology.

Structural class often aligns with function—e.g., bipolar neurons in retina.)

How can neurons be classified by gene expression?

Based on genes coding for neurotransmitter synthesis enzymes, vesicle transporters, ion channels, etc.

What are the major neurotransmitter phenotypes?

GABA (inhibitory)

Glutamate (excitatory)

Dopamine

Serotonin

Noradrenaline

Acetylcholine

Determined by enzyme expression—e.g., GAD for GABA, ChAT for ACh.)

What are glial cells?

Supportive non-neuronal cells essential for maintaining homeostasis, structural integrity, immune defence, and myelination.

Four major classes: astrocytes, microglia, oligodendrocytes/Schwann cells, ependymal cells.

What are the main roles of astrocytes?

Most numerous glial cells.

Regulate extracellular ion balance.

Enclose synapses.

Remove neurotransmitters from synaptic clefts.

They form part of the blood-brain barrier and supply metabolic fuel to neurons via lactate shuttling.)

What do microglia do?

CNS immune cells; act as phagocytes.

Remove dead neurons/glia.

Remodel synaptic connections.

Guide neuronal migration.

They originate from yolk sac macrophages, not neural ectoderm.

What are the functions of ependymal cells?

Line ventricles of brain and spinal cord.

Form barrier between tissue & CSF.

Regulate CSF osmolality.

Contribute to CSF flow via cilia.

Modified ependymal cells in choroid plexus produce CSF.

What do oligodendrocytes and Schwann cells do?

Produce myelin, which insulates axons and increases conduction speed.

Oligodendrocytes: CNS; myelinate multiple axons.

Schwann cells: PNS; myelinate one axon segment each.

Schwann cells also support regeneration; oligodendrocytes do not → CNS regeneration is limited.)

Compare oligodendrocytes and Schwann cells.

Location: Oligodendrocytes = CNS; Schwann = PNS.

Myelination pattern: Oligodendrocytes myelinate multiple axons; Schwann cells myelinate one segment.

Regeneration: Schwann cells promote axon regrowth via growth-permissive environment; oligodendrocytes inhibit regeneration.

Origin: Both from neural tube derivatives except microglia.

Match glial cells to their functions

Astrocyte → Regulate extracellular environment

Microglia → Phagocytosis of debris

Oligodendrocyte → Myelination in the CNS