NEURO: Neurons And Glia

Question 1

Describe the neuron doctrine theory.

Answer 1

The human brain is comprised of both neurones and glial cells. Neurons are not continuous but are discrete individual units.

Reticular theory: Neurones were fused together to form a continuous reticulum or network.

Neuron doctrine: Neurons were not continuous but communicated by neuron-neuron contact.

Growing scientific evidence supported the Neuron doctrine – definitive evidence came with the development of the resolution power of the electron microscope (light microscopes was unable).

Light microscope:

• Standard light microscopy has a limit of resolution of 0.1 𝝻M
• Space between neurons approximately 0.02 𝝻M (or 20 nM)

Electron microscope:

• has a limit of resolution of 0.1 nM
• insights into the fine structure of neurons have come from electron microscopy.

Question 2

How can neural tissue be examined?

Answer 2

Neural tissue can be examined by fixation and sectioning it.

FIXATION (e.g. by paraformaldehyde) is essential to maintain tissue morphology. Without it, the brain is a similar softness to raw chicken (if we put a bit of pressure on it, it deforms, so we can’t get clean slices).

Brain tissue is fixed and subsequently embedded (e.g. paraffin, frozen). Embedding is important in preserving tissue morphology - providing support for sectioning.

SECTIONING:

• a microtome can cut slices from a block of embedded brain tissue.
• you embed the brain in wax in a particular orientation (coronal/horizontal/sagittal), then you mount it in the microtome and slice it
• a cryostat is specialised microtome which sections frozen tissue which are micrometres in thickness.

Question 3

How can we visualise neural tissue?

Answer 3

By staining. 2 important staining methods include the Nissl stain and the Golgi stain.

The Nissl stain is comprised of a basic dye (e.g. cresyl violet). It stains the nuclei and Nissl bodies of neurones (these comprise RER of neurons). This is useful as it
can differentiate between neurons and glial cells. It labels RNA.

The Golgi stain is comprised of a silver chromate solution and stains neurons with their projections (neurons can be seen in greater detail than Nissl).

Question 4

What advancements have allowed us to see brain regions and individual neurons/glial cells in greater detail?

Answer 4

Advancement in:

• Fluorescent microscopy
• Genetic Manipulation techniques (e.g. Cre-Lox)

These have allowed us to see brain regions and individual neurons/glial cells in greater detail.

Question 5

What is a neurone and what is it’s structure in its simplest form?

Answer 5

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 form, a prototypical neuron comprises:

• Cell body (soma) - home to the neurones’ organelles.
• Axon - highly specialised neuronal projections that conducts nerve impulses/action potentials.
• Dendrites - highly specialised neuronal projections that receive synaptic inputs from other neurons.

Collectively, the axon and dendrites are termed the neurites.

Question 6

Describe the cell body (soma).

Answer 6

The cell body (or soma) of a neuron contains the same organelles found in other cell including:

• Nucleus: spherical in shape and enclosed within nuclear envelope. It contains chromosomes (contain DNA and genes) and is the site of gene transcription leading to the synthesis of proteins which bestow on neurons their unique characteristics.
• Rough Endoplasmic Reticulum: membrane bound organelle which have ribosomes attached to their outer surface. RER is a major site of protein synthesis in neurons.
• Smooth endoplasmic reticulum: Doesn’t have ribosomes. heterogenous (performs different function in different locations). Some are continuous with RER and connect as a site in which proteins are carefully folded giving their 3D structure.
• Golgi Apparatus: stack of membrance enclosed discs, acting as a site of post-translational modification. Chemical modification and subsequent sorting of proteins that are destined for different parts of the neurons such as the axon or dendrites.
• Mitochondria: site of cellular respiration and site of ATP production - fueling most of the chemical reactions of the neurons.

The cytosol is K+ rich.

Soma means bod in Greek. The soma is approximately 20nm in diameter.

Question 7

Describe the neuronal cytoskeleton.

Answer 7

The cytoskeleton is the internal ‘scaffolding’ that gives a neuron its characteristic shape, it is comprised of microtubules, microfilaments and neurofilaments.

• Microtubules (20nm in diameter): are a polymer of the protein tubulin - located in axons and dendrites (running longitudinally) and important in axoplasmic transport.
• Microfilaments (5nm in diameter): a polymer of the protein actin - found throughout the neuron but particularly abundant in axons and dendrites.
• Neurofilaments (10nm in diameter): - a type of intermediate filament - particularly abundant in axons and important in regulating axonal shape.
• Promising biomarker for neurodegenerative disorders (e.g. Alzhemier’s)

Question 8

Describe the axons.

Answer 8

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 where an axon collateral returns to the same cell).
• Axon terminal: site at which the axon comes into contact with other neurons at a synapse. The cytoplasm differs from the axon proper, such as microtubules found in the axon proper do not extend into the axon terminal. It contain synaptic vesicles for neuron to neuron communication across a synapse. The axon terminal is particularly rich in proteins and mitochondria indicating a greater energy demand at the axon terminal.

Glial cells are able to myelinate axons:

• Myelin is a membranous sheath that wraps around and insulates axons to speed up nerve impulses.
• Gaps in myelin sheath are Nodes of Ranvier – highly enriched in voltage-gated Na+ ion channels, allowing the nodes of Ranvier to propagate an action potential.

The initial segment of our axon from the soma is the axon hillock. It is where the EPSP and IPSP are summed, and an action potential is fired/inhibited.

Some axons are 1 mm in length, while others can get up to 1 metre in length.

Question 9

Describe the dendrites.

Answer 9

• Dendrites are highly specialised neuronal projections that receive synaptic inputs from other neurons.
• Dendrites of a single neuron are collectively termed a ‘dendritic tree’. With each branch from the tree termed a ‘dendritic branch’.
• 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 10

What is neurotransmission?

Answer 10

Neurotransmission is the fundamental process that drives information transfer between neurons and their targets. This occurs at a synapse between neurons (axon terminal of pre-synaptic neuron and dendrites of post-synaptic neuron).

Question 11

How can neurones be classified?

Answer 11

Neurones can be classified based on their neuronal structure (number of projections, dendrites, connections, axon length) and via gene expression.

Question 12

Describe how neurones are classified by their structure.

Answer 12

Neuronal structure:
- Number of projections: can be classified by the total number of projections (of neurites) - unipolar, bipolar, multipolar.

• Dendritic trees and dendritic spines (pyramidal cells are always spiny and stellate cells can be spiny or aspinous).
• Connections - can be classified by their connections - sensory, motor, interneurons.
• Axon length - neurones can be classified by axon length - golgi type 1 (pyramidal) and golgi type 2 (stellate). Golgi type 1 neurones extend from one part of the brain to another whereas golgi type 2 neurones do not extend beyond the vicinity of the cell body.

Question 13

Describe how neurones are classified by their gene expression.

Answer 13

Neurones can also be classified by the neurotransmitter that they use - these difference can arise due to the differential expression of proteins involved in neurotransmitter synthesis, storage and release.

They can neurotransmitters such as:

• Acetylcholine (ACh)
• GABA
• Glutamate
• Dopamine
• Serotonin
• Noradrenaline

Question 14

List the kind of membrane proteins that are found on the different membrane.

(SS flashcards)

Answer 14

There are many types of membrane proteins, such as:

• ligand-gated ion channel proteins
• G-protein coupled receptors
• voltage-gated ion channel

Ligand-gated ion channels and G-protein coupled receptors are found mainly on the dendritic membrane.
Voltage-gated ion channels are mainly found on the axonal membrane.

Question 15

What are glial cells?

Answer 15

Glial cells are the ‘support cells’ within the nervous system and can be classified into 4 categories based on their structure and function.

Glial cells (a.k.a glia/neuroglia) are non-neuronal cells in the CNS and PNS that do not produce electrical impulses. They maintain homeostasis, form myelin, and provide support and protection for neurons.

These are:

• Astrocytes
• Microglia
• Ependymal cells
• Oligodendrocytes/Schwann cells

Question 16

What are astrocytes?

Answer 16

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 (chemical content) extracellular environment in the brain by, for example, enclosing synaptic junctions (therefore can restrict this extracellular space and diffusion of neurotransmitters that have been released) and actively removing neurotransmitters from the synaptic cleft (which may otherwise interfere with normal neuronal function).

Question 17

What are Microglias?

Answer 17

Microglia are a type of glial cell that function as phagocytes within the nervous system to remove neuronal and glial cell 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

Upon brain injury, microglia activation occurs - they contract their processes and transition from a ramified shape (maintain immunological stable environment) to an ameboid shape (free movement - scavenger role in phagocytosis). They then proliferate and migrate to the site of injury.

Microglia have been shown to function in:

• Phagocytosis of neuronal and glial debris (e.g. sites of injury)
• Synaptic connection remodelling (microglia scavenging and removing unnecessary synapses)
• Directing neuronal migration during brain development.

Question 18

What are Ependymal cells?

Answer 18

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) filling the ventricles.

Ependymal cells have been shown to function in:

• Osmotic regulation of cerebrospinal fluid (via uptake of ions and water molecules)
• The beating of ependymal cells facilitates the bulk flow of cerebrospinal fluid (from the lateral ventricles to the third and fourth ventricles in the ventricular system before being absorbed into the subarachnoid space)
• Directing cell migration during brain development.

Accordingly, deficits in ependymal cell function have been linked with the severe neurological condition hydrocephalus (excessive accumulation of CSF in the ventricular system).

Question 19

What are Oligodendrocytes and Schwann Cells?

Answer 19

Oligodendrocytes and Schwann cells are glial cells that function to provide myelin – a membranous sheath around axons – to neurons in the nervous system. Myelin functions to speed up the propagation of action potentials down an axon.

Oligodendrocytes and Schwann cells differ in their location and in some other characteristics:

• Oligodendrocytes (pictured) 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.

Question 20

Glossary.

Answer 20

Anterograde transport – A cellular process responsible for the movement of substance from the cell body to the distal parts of a cell
Aspinous neuron – A neuron lacking dendritic spines
Astrocyte – A star-shaped glial cell of the central nervous system
ATP – Adenosine triphosphate, a high-energy molecular found in every cell
Axon – The long thread-like part of a neuron along which impulses are conducted from the cell body to other cells
Axon collateral – Branches off a neuron’s main axon
Axon hillock – The site in the soma where membrane potentials propagated from synaptic inputs are summated before being transmitted to the axon
Axon terminal (bouton) – The enlarged club-shaped endings by which axons make synaptic connections with other neurons or effector cells
Axoplasmic transport – A cellular process responsible for the movement of substances from the soma down the axon
Bipolar neuron – A neuron with two neurites extending from the soma
Cell body – The nucleus-containing part of a cell
Cytoplasm – The material or protoplasm within a living cell, excluding the nucleus
Cytoskeleton – A microscopic network of protein filaments and tubules in the cytoplasm of many living cells, giving them shape and coherence
Cytosol – The aqueous component of the cytoplasm of a cell, within which various organelles and particles are suspended
Dendrite – A short branched extension of a neuron, along which impulses received from other cells at synapses are transmitted to the cell body
Dendritic tree – The structure formed by multiple dendrites from the same neuron
Dendritic spine – A small membranous protrusion from a neuron’s dendrite that typically receives input from a single axon at the synapse, supported by an actin cytoskeleton
DNA – Deoxyribonucleic acid, a self-replicating material which is present in nearly all living organisms as the main constituent of chromosomes. It is the carrier of genetic information.
Endoplasmic reticulum – A network of membranous tubules within the cytoplasm of a eukaryotic cell, continuous with the nuclear membrane. It usually has ribosomes attached and is involved in protein and lipid synthesis.
istology – The study of the microscopic structure of tissues.
Gene – A unit of heredity; a distinct sequence of nucleotides forming part of a chromosome, the order of which determines the order of monomers in a polypeptide or nucleic acid molecule which a cell may synthesise
Gene expression – The processes of DNA transcription and translation
Genetic engineering – The deliberate modification of the characteristics of an organism by manipulating its genetic material.
Genome – The complete set of genes present in a cell or organism
Glial cell – Supporting cells of the nervous system. Three types (astrocytes, oligodendrocytes and Schwann cells) are created from the neuroepithelial cells of the neural tube, and are hence siblings of nerve cells.
Golgi apparatus – A complex of vesicles and folded membranes within the cytoplasm of most eukaryotic cells, involved in secretion and intracellular transport
Golgi stain – A silver staining technique that is used to visualise nervous tissue under light microscopy.
Green fluorescent protein – A protein that glows green under fluorescent light.
Interneuron – A neuron which transmits impulses between other neurons.
Microglia – Immune cells that migrated in to the central nervous system during early development, that function as macrophages.
Microfilament – A small rod-like structure present in numbers in the cytoplasm of many eukaryotic cells.
Microtubule – A microscopic tubular structure present in numbers in the cytoplasm of many eukaryotic cells, sometimes aggregating to form more complex structures.
Mitochondrion – An organelle in which the biochemical process of respiration and energy production occur.
Multipolar neuron – A neuron with multiple neurites extending from the soma
Myelin – A mxture of proteins and phospholipids forming an insulating sheath around neurons.
Neurite – Neuronal processes extending from the soma, namely axons and dendrites
Neurofilament – Intermediate filaments found in the cytoplasm of neurons.
Neurone – A specialised cell that transmits nerve impulses.
Neuronal membrane – The cell membrane of the axon, soma and dendrites of a neuron.
Neurotransmitter – A chemical substance which is released at the end of a nerve fibre by the arrival of an action potential, by diffusing across the synapse or junction, effects the transfer of the impulse to another neuron, muscle fibre, or other structure.
Nissl stain – A method for staining RNA to visualise neurons
Node of Ranvier – A gap in the myelin sheath of a neuron
Nucleus – A dense organelle present in most eukaryotic cells, containing the genetic material.
Oligodendrocyte – A glial cell concerned with the production of myelin in the central nervous system.
Organelle – A specialised structure within a living cell
Perikaryon – The cell body of a neuron, containing the nucleus. Also known as the soma.
Primary sensory neuron – Neurons with neurites in the sensory surfaces of the body.
Protein – Nitrogenous organic compounds which have large molecules composed of one or more long chains of amino acids.
Protein synthesis – The process by which amino acids are linearly arranged into proteins, termed translation.
Receptor – A protein in a cell membrane which responds specifically to a particular neurotransmitter, hormone, antigen, or other substance.
Retrograde transport – A cellular process responsible for the movement of substance from the distal parts of a cell to the cell body
Ribosome – A particle consisting of RNA and associated proteins; they bind mRNA and transfer RNA to synthesise polypeptides and proteins.
RNA – ribonucleic acid, its principal role is to act as a messenger carrying instructions from DNA for controlling the synthesis of proteins.
Schwann cell (neurolemmocyte) - A glial cell concerned with the production of myelin in the peripheral nervous system.
Soma – The cell body of a neuron, containing the nucleus. Also known as the perikaryon.
Spiny neuron – A neuron with dendritic spines
Synapse - A junction between two neurons, consisting of a minute gap across which impulses pass by diffusion of a neurotransmitter.
Synaptic cleft – See ‘synapse’.
Synaptic transmission – The process by which one neuron communicates with another via release and detection of a neurotransmitter.
Synaptic vesicle – A small secretory vesicle that contains neurotransmitter, found inside an axon near the presynaptic membrane, and releases its contents into the synaptic cleft after fusing with the membrane.
Terminal bouton – The specialised presynaptic terminal at the end of an axon.
Transgenic mice – A genetically modified mouse that has had its genome altered through the use of genetic engineering techniques.
Unipolar neuron – A neuron with a single neurite extending from the soma