Neurons and Glia

Question 1

How does one prepare neural tissue for histological staining?

Answer 1

Fixation- brain tissue is fixed with something like paraformaldehyde for preservation and subsequently embedded (e.g. paraffin, frozen)
Sectioning- once you have your fixed brain you can section it using a microtome which can cut slices from a block of embedded tissue

Question 2

What are the Nissl and Golgi stains?

Answer 2

Nissl stain- comprised of a basic dye (e.g. cresyl violet)- stains the nuclei and Nissl bodies of neurons
Golgi stain- comprised of a silver chromate solution- stains neurons and their projections

Question 3

What are neurons and what does it comprise of?

Answer 3

Neurons are the information processing cells within the nervous system, highly specialised for the conduction and transmission of electrical and chemical signals
At its simplest, a prototypical neuron comprises:
- Cell body (soma)- home to the neurons organelles
- Axon- conducts nerve impulses
- Dendrites- specialised to receive synaptic inputs from other neurons

Question 4

What are the cell body (soma) organelles?

Answer 4

The cells body or soma of a neuron contains the same organelles found in all human cells including:

- Nucleus
- Rough endoplasmic reticulum
- Smooth endoplasmic reticulum
- Golgi apparatus
- Mitochondrion

Question 5

What is the neuronal cytoskeleton and what 3 different components is it comprised of?

Answer 5

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, run longitudinally 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

Question 6

What are axons and what are they comprised of?

Answer 6

Axons are highly specialised neuronal projections that conduct nerve impulses (or action potentials) within the nervous system- comprised of various regions
Axons are comprised of:
- 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
Some 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

Question 7

What are dendrites?

Answer 7

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)

Question 8

What is neurotransmission and how does it occur?

Answer 8

Neurotransmission is the fundamental process that drives information transfer between neurons and their targets
This occurs at the synapse, between the axon terminal of a presynaptic terminal and dendrites of a post-synaptic neuron
The gaps between the myelinated axon, termed Nodes of Ranvier, are rich in voltage-gated Na+ channels allowing them to propagate an action potential
The axon terminal releases neurotransmitters through the process of exocytosis
Dendrites contain post-synaptic receptors

Question 9

How can neurons be classified according to the number of neurites?

Answer 9

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):
A neuron with one neurite is termed unipolar
A neuron with two is termed bipolar
A neuron with multiple neurites are multipolar (most neurons in brain are multipolar)

Question 10

How can neurons be classified according to dendritic spines?

Answer 10

Dendrites- neurons can be classified by their dendritic trees and their dendritic spines:
Start shaped stellate cells and pyramid shaped pyramidal cells reside in the cerebral cortex
Those with dendritic spines are termed spiny and those without are aspinous

Question 11

How can neurons be classified according to their connections?

Answer 11

Connections- neurons can be classified by their connections- sensory, motor and interneurons (which synapse with other neurons within the brain and spinal cord):
Sensory neuron
Interneuron
Motor neuron

Question 12

How can neurons be classified according to axon length?

Answer 12

Axon length- neurons can be classified by axon length- Golgi type I and Golgi type II
Neurons with axons that extend from one part of the brain to another are of the Golgi type I category e.g. pyramidal cells
Neurons with axons that do not extend beyond the vicinity of the cell body are Golgi type II e.g. stellate cells

Question 13

What are glial cells?

Answer 13

Glial cells are the 'support cells' within the nervous system and can be classified into four categories based on structure and function
Astrocytes
Microglia
Ependymal cells
Oligodendrocytes/schwann cells

Question 14

What are astrocytes and their function?

Answer 14

Astrocytes are star-shaped glial cells that function to regulate- in a number of ways- the extracellular environment of the brain
Astrocytes are the most numerous type of glial cell within the human brain
Astrocytes regulate the extracellular environment in the brain by, for example, enclosing synaptic junctions and actively removing neurotransmitters from the synaptic cleft which might interfere with normal neuronal function

Question 15

What are microglia and their function?

Answer 15

Microglia are a type of glial cell that function as phagocytes within the nervous system to remove neuronal and glial debris
Microglia- which account for approximately 5-15% of total CNS cell number depending on anatomical region- are broadly distributed in the brain and spinal cord
They can change from an ameboid shape to a ramified shape
Microglia have been shown to function in:
- Phagocytosis of neuronal and glial debris (e.g. sites of injury)
- Synaptic connection remodelling
- Directing neuronal migration during brain development

Question 16

What are ependymal cells and their function?

Answer 16

Ependymal cells are a type of glial cell that provide the lining of the ventricular system of both the brain and spinal cord
Ependymal cells line the ventricular system and act as a physical barrier separating brain tissue from cerebrospinal fluid (CSF)
Ependymal cells have been shown to function in:
- Osmotic regulation of cerebrospinal fluid via the uptake of ions and water molecules
- Flow of cerebrospinal fluid from the lateral ventricle to the third and fourth ventricles
- Directing cell migration during brain development
Accordingly, deficits in ependymal cell function have been linked with the severe neurological condition hydrocephalus

Question 17

What are Oligodendrocytes and Schwann cell and how do they differ?

Answer 17

Oligodendrocytes and Schwann cells are glial cells that function to provide myelin- a membranous sheath around axons- to neurons in the nervous system
Oligodendrocytes and Schwann cells differ in their location and in some characteristics
- Oligodendrocytes are situated in the central nervous system (CNS)
- Schwann cells are situated in the peripheral nervous system (PNS
One oligodendrocyte contributes myelin to several axons, whilst Schwann cells myelinate only a single axon