W8 - Neurons and Glia Flashcards
What is the reticular theory vs the neuron doctrine?
The human brain is comprised of both neurons and glial cells - these are broad categories, comprising numerous cell types with different structures and functions.
Reticular theory:
Golgi proposed that neurons were fused together to form a continuous reticulum (or network)
Neuron doctrine:
Cajal proposed that neurons were not continuous but communicated by neuron-neuron contact
What is the histological staining of neural tissue?
FIXATION
Brain tissue is fixed for preservation and
subsequently embedded (e.g. paraffin,
frozen)
SECTIONING
A microtome can cut slices from a block of
embedded brain tissue accurately and continuously.
What is the difference between Nissl stain and the Golgi stain?
NISSL STAIN
The Nissl stain – comprised of a basic dye (e.g.
cresyl violet) – stains the nuclei and Nissl
bodies of neurons. These contain rough endoplasmic reticulum. It helps differentiate between neurones and glial cells.
GOLGI STAIN
The Golgi stain – comprised of a silver
chromate solution – stains neurons and their
projections. These are shown in much greater detail.
What difference does the light and electron microscopy make?
Neuron doctrine was supported by growing body of scientific evidence – definitive evidence came with the resolving power of
the electron microscope…
Light:
* Standard light microscopy has a limit of resolution of 0.1 uM
* Space between neurons approximately 0.02 uM (or 20 nM)
Electron:
* Electron microscopy has a limit of resolution of 0.1 nM
* Our insights into the fine structure of neurons have come from electron microscopy
Neurons are not continuous but are discrete
individual units
What are brainbow mice?
Fluorescence microscopy and genetic manipulation techniques (e.g. Cre-Lox) allow us to see brain regions and individual neurons/glial cells in breath-taking detail
These mice are engineered with three different fluorescence proteins. One colour of the DNA is expressed from each copy of DNA construct.
Because brainbow mice express only 3 different proteins, it would involve the collection of 3 separate scans at 3 different wavelengths.
What is a neuron composed of?
Neurons are the information processing cells within the nervous system, highly specialised for the conduction and transmission of electrical and chemical signals.
I. CELL BODY (SOMA) - home to the neurone’s organelles
II. AXON - conducts nerve impulses/action potentials
III. DENDRITES - receives synaptic input from other neurones.
What is the cell body (soma) organelle?
The cell body (or soma) of a neuron contains the same organelles found in all human cells including:
Nucleus - contains DNA, site of gene transcription
Rough endoplasmic reticulum (RER) - ribosomes are attached to the outer surface. The RER is a major site of protein synthesis in neurones.
Smooth endoplasmic reticulum (SER) - heterogenous and performs different functions in different locations. Eg. Some SER is continuous with the RER
Golgi apparatus - site of post-transcriptional modification.
Mitochondrion - site of cellular respiration and production of ATP.
What is the neuronal cytoskeleton?
The cytoskeleton is the internal ‘scaffolding’ that gives a neuron its characteristic shape. It
is comprised of microtubules, microfilaments and neurofilaments.
MICROTUBULES
A polymer of the protein tubulin – located in axons and dendrites and important in
axoplasmic transport
MICROFILAMENTS
A polymer of the protein actin – found throughout the neuron but particularly
abundant in axons and dendrites
NEUROFILAMENTS
A type of intermediate filament – particularly abundant in axons and important in regulating axonal shape
Promising biomarker for neurodegenerative disorders (e.g. Alzheimer’s)
What are axons?
Axons are highly specialised neuronal projections that conduct nerve impulses (or action potentials) within the nervous system –
comprised of various regions:
Axon composition:
Axon hillock – tapers away from the soma to
form the initial segment of the axon
Axon ‘proper’ – axon can branch to form axon
collaterals (and recurrent collaterals)
Axon terminal – site at which the axon comes
into contact with other neurons at a synapse
Glial cells are able to myelinate axons:
Myelin is a membranous sheath that wraps around and insulates axons
Gaps in myelin sheath are Nodes of Ranvier – highly enriched in voltage-gated Na+ ion channels
What are dentrites?
- Dendrites are highly specialised neuronal projections that receive synaptic inputs from other neurons
- Dendrites of a single neuron are collectively termed a ‘dendritic tree’
- Dendrites of some neurons are covered with specialised structures termed ‘dendritic spines’ – small sacs of membrane that protrude from
the dendrites of some cells to receive synaptic input - Dendritic spine structure is sensitive to type and amount of synaptic activity
A number of conditions have been associated with abnormal dendritic spine number (e.g. Alzheimer’s disease, schizophrenia).
What is neurotransmission?
Neurotransmission is the fundamental process that drives information transfer between
neurons and their targets.
Classification of neurones impacts it’s structure and function.
What role do the number of projections and dendrites play in the classification of neurons?
Neurons can be classified based on neuronal structure and gene expression:
NUMBER OF PROJECTIONS
Neurons can be classified by the total number of
projections (or neurites):
- Single projection = unipolar
- 2 neurites = bipolar
- Multiple neurites = multipolar
DENDRITES
Neurons can be classified by their dendritic trees and dendritic spines:
Star shaped stellate cells and pyramid shaped pyramidal cells resides in the cerebral cortex.
Those with dendritic spines are termed spiny and those without are termed spinous.
What role do connections and axon lengths play in neuron classification?
Neurons can be classified based on neuronal structure and gene expression:
CONNECTIONS
Neurons can be classified by their connections – sensory, motor and interneurons:
- interneurones are the most abundent - neurones that synapse with other neurones in the brain.
AXON LENGTH
Neurons can be classified by axon length – Golgi type I and Golgi type II:
Golgi type I = neurones that extend from one end of the brain to the other. (Pyramidal cells)
Golgi type II = Axons do not extend beyond the vicinity of the cell body are termed Golgi Type II neurones. (Stellate cells)
What is gene expression?
Neurons can be classified based on neuronal structure and gene expression:
GENE EXPRESSION
Neurons can also be classified by the neurotransmitter that they use – these differences arise due to the differential expression of proteins involved in neurotransmitter synthesis, storage and release.
GABA = GABAnergic neurones
Glutamate - Glutamatergic neurones
Acetylcholine (ACh) - Cholinergic neurones
Noradrenaline
Glutamate
Dopamine
What are Glial cells?
Glial cells are the “support cells” within the nervous system and can be classified into four
categories based on structure and function.
ASTROCYTES
MICROGLIA
EPENDYMALCELLS
OLIGODENDROCYTES/SCHWANN CELLS
