Composition of the Nervous System Flashcards
Karen Cullen + Richard Ward
How many neurons does the human brain have?
• Made of 100 billion neurons
What is the human brain made of?
- Made of lipids, proteins, white matter (which makes up most of the brain) and grey matter
- Oligodendrocyte membranes make up most of the lipids in the brain
- Most of the brain’s volume is not neurons- the brain is mostly glial cells
How much oxygen does the brain use?
• Uses 20% of body’s oxygen, which is mostly used by grey matter
How much blood supply does the brain use at rest?
• Uses 15-25% of total blood supply at rest
What is the nervous system?
• Nervous system- An organised association of neurons and supporting cells (glia) with its own blood supply (endothelial cells)
What are neurons?
• Neurons- Excitable cell responsible for transfer of information of information via electrical (ionic movement) and chemical communication unidirectionally via specialised junctions (synapses) to other excitable cells
What are endothelial cells?
o Line blood vessels and form the blood brain barrier
Describe how an action is made by the nervous system based on sensory input
o Sensory input (Peripheral nervous system) Integration (Central nervous system) Motor output (peripheral nervous system
Sensory system (afferent-towards)
• Unidirectional signal of neurons from the periphery towards the central nervous system
o The trigger of the signal may be from outside the body or internal which is transduced by a special sense organ/structure and transmitted for interpretation/integration
Motor system (efferent-away)
• Direction of signal is toward the periphery, away from the nervous system and consists of voluntary and involuntary signals
What is the direction of a signal in a neuron?
• Direction of signal passage is constant and unidirectional and goes from the dendrite to the cell body to the axon
o However, many sensory systems don’t have dendrites on their neurons, but a receptive end which is where the signal passage starts
• Neuron signalling-
o Dendrites collect electrical signals
o Soma contains nucleus and organelles that keeps the cells alive
o Axon passes the signal onto dendrites of another neuron or to an effector such as a muscle cell
What is a neuron made of (its microanatomy)?
-Cell body/soma/perikaryon
-Neuronal cytoskeleton
-Dendrites
-Axon hillock
-Axon
-Terminal Bouton
-Synapse
-Neuronal
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Describe what is in the soma of a neuron and the function of these organelles
Contains cytosol- salty, potassium rich solution inside the cell
Contains a 5-10 um nucleus, containing DNA, which is surrounded by the nuclear envelope
Contains a nucleolus, an organelle actively engaged in ribosome synthesis, rich in RNA
Abundant free ribosomes and prominent rough endoplasmic reticulum
A prominent Golgi apparatus for processing and packaging proteins into vesicles that are destined for delivery to different parts of the neuron
Abundant mitochondria which is site of oxidative metabolism
How big is a neuron’s soma?
Typical neuron has 15-20um diameter soma but can be as large as 50um
Describe Nissi bodies in neurons (why they are important, how they can be used for identification, their size)
• The rough endoplasmic reticulum is called Nissi bodies
o Most abundant organelle in the neuronal soma which can be visibly seen in a cell stain and is made up of membrane stacks dotted with ribosomes (which are rich in RNA)
Can tell difference between neuron cell bodies and other cells as nissi bodies are visible in neuron cell bodies
o About 25 nm diameter
o Sites of protein synthesis
o Neurons have massive protein production and very high metabolic activity
o Makes proteins in neurons such as neurotransmitter enzymes, channels, pumps, cytoskeleton…
o Neurotransmitter enzyme, channel and pump synthesis takes up the most energy in the body
Describe ribosomes in neurons (location and function)
o Make proteins
o Are attached to the rough ER or free in the cell
If the protein is destined to reside within the cytosol of the neuron, what ribosome is it made with?
o If the protein is destined to reside within the cytosol of the neuron, then it is made by free ribosomes
If the protein is destined to be inserted into the membrane of the cell or an organelle, what ribosome is it made with?
o If the protein is destined to be inserted into the membrane of the cell or an organelle, it is synthesised on the rough ER
Describe the function of the neuronal cytoskeleton
Provides structural support
Supports movement of proteins and organelles along axons
Contractile properties such as in growth cone extension and dendritic spine formation
What 3 protein types is the neuronal cytoskeleton made of, and what are each of their sizes, location and composition?
Made of 3 protein types
• Neurofilament
o Intermediate filament class
o Measures 10nm
• Microtubules
o Run longitudinally down neurites (axons and dendrites)
o Composed of protein tubulin
o Measures 20nm in diameter
• Microfilaments
o Measure only 5nm in diameter
o Numerous in axons and dendrites
o Made of protein actin
Describe what dendrites are and how long they are
o Dendrites-a lot of shorter fibres extending from the cell body
Rarely longer than 2mm
Have polyribosomes
Dendrites of some neurons are covered with dendritic spines that receive some types of synaptic input
Dendrites are covered with synapses
What is a dendritic branch?
A single dendrite is called a dendritic branch
What is a dendritic tree?
Dendritic tree- the dendrites of a single neuron
What is an axon hillock?
junction between soma and axon where the axon potential fires or does not fire
What is the role and composition of an axon hillock?
Plays essential role in integration and transmission of signals
High density of sodium channels- decision spot on whether axon fires or not
• Region of summation of excitatory and inhibitory neural input: the action potential is usually generated at the axon hillock
Has transmembrane protein barriers made of actin filaments that block free diffusion of proteins from soma to axon and segregates key components such as channels and pumps
What is an axon and what are its dimensions?
Axon length can be mms to meters long
Cytoplasmic extension of the neuron
Very thin (about 1-25 um in diameter in most humans)
May be very long (can be more than 1 meter in humans)
What is the role of an axon?
Carries information from the soma to another neuron or effector cell
Can axons make their own proteins?
Has no ribosomes, so proteins of the axon must be synthesised in the soma and then shipped down the axon
What is a terminal bouton?
The site where the axon comes into contact with the neurons and passes information on to them
Contains synaptic vesicles
Has numerous mitochondria but no microtubules
What is a synapse and what are its components?
Specialised junctions between neurons where information passes from one neuron to another through chemical messages such as neurotransmitters or electrical impulses (communicating by gap junction)
Made of presynaptic neuron, Postsynaptic neuron, Synaptic cleft and Terminal bouton
Where can synapses be found?
Synapses are most often between
• Axon terminal boutons and dendrites: axodendritic
• Axon terminals and perikaryal (neuronal cell bodies)
• Axons: axo-axonic (such as in presynaptic inhibition)
What is the composition of the neuronal membrane?
Composition- phospholipid bilayer and special embedded proteins include ion channels and carrier proteins. Is about 5nm thick.
• Phospholipid bilayer: hydrophilic phosphate heads in extracellular part and hydrophobic lipid tail embedded in the membrane
What is the function of the neuronal membrane?
Function- preserves the highly-controlled internal environment necessary for an action potential and for transmission of the signal from the presynaptic to the postsynaptic neuron or muscle cell
Describe the relative concentrations of key ions across the neuronal membrane
o Potassium- 1 outside:20 inside Higher concentration on inside of cell o Sodium- 10 outside:1 inside Higher concentration outside of cell o Calcium- 10000 outside:1 inside Higher concentration outside of cell o Chlorine- 11.5 outside:1 inside Higher concentration outside of cell
How does the neuronal membrane support ion segregation?
- Passive phospholipid membrane properties
- Passive (don’t need ATP to operate) ion channels
- Active (need ATP to operate) carrier proteins (pumps)
How do passive phospholipid membrane properties support the neuronal membrane’s ion segregation?
(includes what can and can’t diffuse through the lipid bilayer, why they can/can’t and what are the consequences of this)
o Diffusion of ions through the membrane is restricted as some ions don’t move through the lipid bilayer as they are not soluble in the lipid bilayer
Lipid soluble items that freely diffuse through the cell membrane include oxygen, nitrogen, carbon dioxide and alcohols
Water diffuses through the lipid bilayer as it is small and has high kinetic energy (even if it is not lipid soluble)
Ions do not diffuse through the membrane for 2 reasons
• Ions become hydrated in the watery environment and become too large
• Polarities of the phosphate head repel the ion charge
The fact that ions cannot diffuse through the neuronal membrane means that once ions are in or out, they cannot naturally diffuse through and go in or out on their own, but need a channel or pump to transfer
Are most ion channels ion specific?
Yes
What does movement of any ion through its channel depend on?
o Movement of any ion through its channel depends on the concentration gradient and the difference in electrical potential across the membrane
What two types of channels are there?
Gated
Non-gated
What are non-gated ion channels?
o Non-gated ion channels- always open and the ions can always go through
What are gated ion channels?
o Gated ion channels-open or closed depending on stimulus
What are 3 types of gated ion channels and what do they respond to?
Voltage-gated ion channels are open or closed depending on the charge
Ligand-gated ion channels are open or closed depending on the neurotransmitter
Transduction gated are open or closed depending on mechanical forces
Describe when the voltage-gated sodium channel is open or closed and how quickly they do so
o Closed at resting potential (-65mV) o Begins opening between -65mV and -40mV o Sodium flows into the cell o Fully opens at -15mV o Closes at +35mV o Open with little delay o Stay open for about 1 msec then close o Cannot be opened again by depolarization until the membrane potential returns to a negative value near threshold
What determines sensitivity of depolarisation of neurons?
o Density of voltage-gated sodium channels determines sensitivity to depolarisation
Where is there the highest density of sodium voltage-gated channels on a neuron?
o Highest density at axon hillock: allows generation of action potential down the axon
Describe when the voltage-gated potassium channel is open or closed
o Closed at resting potential (-65mV) o Begins to open at +30 to 35mV o Potassium flows out of cell o Closes at resting potential (-65mV) o Do not open immediately: takes about 1msec for them to open
What is active transport?
o Active transport- pumps transport ions across the membrane against the gradient from low to high concentration
What maintains neuronal resting potential and how?
o Pumps maintain the neuronal resting potential (-65mV)
Sodium-Potassium exchanges internal sodium for external potassium
Calcium pumps- actively transports calcium ions out of the cytosol across the cell membrane
o Pumps re-establish the potential after firing
What is the resting potential of a neuron?
-65mV
Describe how information is encoded in neurons
• Information is encoded in the frequency of action potentials of individual neurons as well as the distribution and number of neurons firing action potentials in a give nerve
What is essential for neuronal transmission and what does it result in?
- Ion segregation is essential for neuronal transmission
* Ion segregation results in a charge potential across the neuronal membrane
What is a charge potential?
o Charge potential: difference in concentration of negative and positive ions
When a neuron is polarised, is the inside of the cell more positive than the outside or vice-versa?
• When a neuron is polarised (at rest), the inside of the cell is more negative than the outside
What is an action potential?
• Action potential- when the membrane becomes more positively charged relative to the outside. Lasts about 2 milliseconds
• The action potential is unidirectional and of fixed size and fixed duration
o Sodium channels close behind action potential
o Potassium channels open behind action potential
o Trailing end of axon where refractory period occurs immediately (gets a while to take back to resting potential to where cell can be excited again)
What are the stages of an action potential? Describe each of them
- Resting potential
The voltage across the membrane is about -65 mV at resting potential - Gets triggered
Near the terminal bouton, there are voltage-gated calcium channels
When an action potential fires and gets down to terminal, action potential opens calcium channels which will call the synaptic vesicles to dock and release their neurotransmitters
Neurotransmitter binds to ligand-binding neurotransmitter receptor which opens up sodium channels which starts making the cell depolarised
When the threshold is surpassed (-55mV), an action potential occurs
• An action potential is ONLY caused when the depolarization of the membrane is beyond the threshold
• Threshold-the membrane potential at which enough voltage-gated sodium channels open so that the relative ionic permeability of the membrane favors sodium over potassium. - Rising phase
Sodium channels open and inside of cell becomes more positive as sodium enters in the cell - Overshoot
Voltage is about +35 mV- the part where the inside of the neuron is positively charged with respect to the outside - Falling phase
Potassium channels open and inside of cell becomes less positive, returns to negative
Sodium channels close - Undershoot
Rapid depolarisation causes the inside of the membrane to be more negative than the resting potential - Absolute refractory period
Sodium channels inactive when the membrane becomes strongly depolarised. They cannot be activated again, and another action potential cannot be generated, until the membrane potential becomes sufficiently negative enough to deinactivate the channels (usually 1msec) - Relative refractory period
The membrane potential stays hyperpolarised until the voltage-gated potassium channels close. Therefore, more depolarizing current is required to bring the membrane potential to threshold - Restoration of the resting potential
Describe how unidirectional signal is supported by the microanatomy of the neuron
o Unidirectional signal is supported by the microanatomy of the neuron
Different regions of the neuronal cell have different densities of voltage and ligand gated channels
• High density of ligand-gated ion channels at the dendrites
• High density of voltage-gated ion channels at the axon hillock and along the axon
How many glial cells are there in relation to neurons, and as the brain evolves
Brain mostly made up of glial cells- there are 10x as many glial cells as neurons (approximately)
• Glial to neuron ratio increases as brain evolves
How many synapses might each astrocyte enclose?
• Each astrocyte may enclose more than 2 million synapses
How many axons do oligodendrocytes wrap?
• Oligodendrocytes wrap billions of axons
What are the 2 major glial cell categories?
- Macroglia (astrocytes and oligodendrocytes)
- Microglia
What are two types of macroglia and where do they reside?
-Astrocytes
-Oligodendrocytes
o Central Nervous Systems
What cell type do astrocytes and oligodendrocytes come from?
Come from neural stem cells (like neurons)
How are astrocytes produced?
o Neural stem cell-> Glial Progenitor -> Astrocyte precursor-> Astrocyte OR Neural Stem Cell -> Glial Progenitor -> O-2A cells-> Astrocytes
What are two different types of astrocytes?
o Fibrillary astrocytes in white matter-contact nodes of Ranvier
o Protoplasmic astrocytes in grey matter- closely associated with synapses
How are astrocytes visualised
• Visualisation
o Labelled with immunohistochemistry using antibodies against glial fibrillary acidic protein (GFAP)
What is GFAP?
GFAP is an intermediate filament protein
GFAP is a component of the cytoskeleton
The cytoskeleton confers strength and shape to cells
GFAP is specific to astrocytes
Describe how much space glial cells take up in the brain and why
o Glial cells have a large volume and fill up space in the brain to maintain fluid balance and structural integrity- there is very little space in the brain
What happened to the astrocyte: neuron ratio as the brain evolved?
o As evolution occurred, the increase in number and complexity of astrocytes in more evolved brains is much greater than the increase in neuronal number
What are the functions of astrocytes?
o Provide structural support for the central nervous system
o Invest endothelium and induce blood brain barrier tight junctions
o Provide support for synapse formation and maintenance
o Guide neuronal process growth and regulate neurogenesis
o Maintain biochemical balance around neurons
o Forms scar tissue
o Supports neuroinflammatory system
o Form gap junctions with neighbouring astrocytes and communicate with them
How do astrocytes provide structural support for the central nervous system
Astrocytic processes are joined by cell-cell contacts: confers structural strength
Astrocytes stabilise neuronal configurations (wrap terminals)
Astrocytic processes interweave between neuronal processes
Form glia limitans: dense, fibrous GFAP network around brain that gives it structural integrity
• Astrocytic processes invest outer surfaces of CNS and envelop blood vessels, forming the glia limitans
How do astrocytes invest endothelium and induce blood brain barrier tight junctions?
Astrocytic endfeet wrap capillary: are on more than 90% of endothelial cell surface and induce endothelial cells to form the blood brain barrier
The blood brain barrier is tight junctions between endothelial cells
In the absence of astrocytes, endothelial cells do not form tight junctions
What is a neurovascular unit?
Neurovascular unit-contains endothelial cells, neurons and glial cells
How do astrocytes provide support for synapse formation and maintenance?
Astrocytes isolate neurons and ensure that receptive neuronal surfaces are protected from non-specific influences
Astrocytes support synaptic microanatomy and envelope neuronal terminals
• Astrocyte processes are especially dense in areas of intense synaptic activity
Synaptic remodelling
How do astrocytes remodel synapses?
• Removes degenerating synapses
• Astrocytes can swell or relax depending on water concentration in region due to their aquaporin channels which let water in the cell
o Astrocytic swelling can change synaptic distance, hence changing neuronal firing efficiency
The smaller the synaptic cleft, the more efficient the synapse
How do astrocytes guide neuronal process growth and blood vessel growth and regulate neurogenesis
Astrocytes promote angiogenesis, the growth of new blood vessels
Synapse number and efficacy are significantly compromised in a glia-less environment
Radial astroglial processes provide tracks along which neurons migrate during development
• Provide a temporary scaffold for the migration of newborn cortical and cerebellar neurons
o Long radial fibers extend from the ventricular to the pial surface and serve as guidance cables
Major sources of extracellular matrix proteins, adhesion molecules and neurotrophic factors
Describe how astrocytes maintain biochemical balance around neurons
Glutamate-glutamine cycle
Redistribute potassium during neural transmission
• Has a Sodium-Potassium-ATPase pump (which gets ATP from glucose lactate conversion) which brings potassium in the astrocyte and drives sodium out the astrocyte, helping keep resting potential and keeping ionic balance in order
Remove glutamate and GABA at the synapse
Synthesise glutamate and GABA precursors
Detoxify ammonia
Provide energy substrates (lactate) to neurons
• Astrocytes source of energy that neurons use
• Astrocytes take glucose from the capillary and convert it into lactate, which is given to the neuron as an energy source
Maintains brain/water homeostasis
Envelop synaptic junctions in the brain and restrict the spread of neurotransmitter molecules that have been released
Tripartite synapse model- glia actively communicates with pre and post-synaptic neurons
• Neurotransmitters released by presynaptic neurons do not only signal to post-synaptic neurons and terminal astrocytes, but can also trigger massive calcium waves through astrocytic networks, which could transmit synaptic signals over extensive cortical areas
Describe the astrocyte glutamate-glutamine cycle
Glutamate-glutamine cycle
• Astrocytes remove toxic glutamate from the synaptic cleft and combines it with ammonia
o Glutamate is an excitatory neurotransmitter, and excess glutamate can be neurotoxic
• Through glutamine synthetase in the astrocyte, it is transformed into glutamine, which is not toxic to the neuron, and provides glutamine to the neuron
o Glutamine synthetase requires ATP, which is given by converting glucose into lactate
• In the neuron, glutamine is transformed into glutamate through glutaminase
How do astrocytes form scar tissue?
o Forms scar tissue
Astrocytic scars made of GFAP form around where neurons are injured or dying
• Cells fill with GFAP and extracellular matrix proteins and makes scar stronger and walls off injured area
o GFAP stays within the cell, which becomes enlarged and more rigid
Astrocytic scars can interfere with re-growth of neuronal processes in CNS (partly accounts for poor CNS regeneration)
How do astrocytes form gap junctions with neighbouring astrocytes and communicate with them
Astrocytes propagate calcium waves through gap junctions, which can propagate through entire groups of astrocytes
Glia located at synapses respond to neuronal activity with an increase in calcium
Propagation timescale is slow- takes seconds to propagate
• Several potential functions in vivo
o May prime activity over large distances
o Spread glial activation (such as in inflammation)
o Regulate blood flow in local areas
Extracellular ATP released from stimulated astrocytes activate purinergic receptors on neighbouring astrocytes, which can propagate calcium signalling over significant distances
How are oligodendrocytes produced
o Neural stem cells Glial progenitors O-2A cells Oligodendrocytes
What is the function of oligodendrocytes
o Myelinate axons of the central nervous system- within the brain and spinal cord
o Myelination of axons speeds conduction of impulse
How do oligodendrocytes myelinate axons
Oligodendrocyte process spirals around axon
• Process (myelin sheath) doesn’t lose contact with its oligodendrocyte- if the oligodendrocyte dies the myelin unravels
Cytoplasm is extruded until opposite membranes meet, forming a multi-layered lipoprotein coat
What is myelin?
Myelin-component of oligodendrocyte plasma membrane
Describe the anatomy of the myelin sheath
• Myelin sheath forms internodes, while gaps between these internodes are called node of Ranvier
What are characteristics of fastest condicting axons
Faster conducting axons have thickest myelin sheaths and longest internodes
How does myelination of axons speed up conduction of impulse
Myelin increases resistance across the axonal membrane
Myelin reduces leakage of ions (only a few sodium and potassium ions flow through internodes- most of it happens through nodes of ranvier)
Faster conduction as fewer ions move and less energy is required to segregate ions
• Due to this, unmyelinated axons (such as pain axons) are slower
Sodium channels are concentrated at the node due to myelin sheaths, which makes action potentials faster
What is saltatory conduction?
Saltatory conduction- the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials
What do schwann cells do?
Schwann cells
• Myelinate axons of the peripheral nervous system (motor and sensory axons)
Found surrounding peripheral nerve cell processes where they form the axonal myelin sheath, which improves the conduction velocity of impulses
Have unmyelinated fibres embedded in them
What is the origin of microglia?
o Comes from bone marrow
Bone marrow hematopoietic stem cell Myeloid precursor Monocytes Microglial cell
How are microglia detected?
o Detected by making them eat fluorescent balls and through immunohistochemistry for HLADr
What is the function of microglia?
Main immune defence- patrol brain for damage and disease
Elicit inflammatory responses and phagocytose cellular debris in response to injury
Enter and leave the CNS, widely distributed. Function immunological, phagocytotic
Describe changes in microglia morphology depending on different activation states
- Resting- microglia are ramified (most of the time)
a. Move around and patrol - Activated microglia- filled with lysosomes and make more branches: become ameboid (retract extant branches) and motile
a. Activated by injury, invasion, immune functions or damage to the brain - Become phagocytic and eat damaged cells or invaders-become locomotory
What is the density of microglia
Around 10% of brain cells are microglia
Describe the entrance of microglia to the brain and how was it proven
There is age-dependent entry to the brain (mostly enter before birth (during the second trimester), but a few microglia enter after birth)
• Evidence comes from animal studies and inter-gender human bone marrow transplants (some females have male microglia due to bone marrow transplant from males)
• Microglia invade the brain at birth from the bone marrow
The blood brain barrier prevents monocytes from entering the brain in large numbers in adults
What 3 nervous system types if the nervous system composed of, and what overarching structures are found in each of these systems?
• Central nervous system o Brain Brainstem (medulla, pons, midbrain) Cerebellum, diencephalon, cerebral hemispheres o Spinal cord o Associated structures Ventricles, choroid plexus Blood vessels Meninges Neurocranium, vertebral column • Peripheral nervous system o Cranial o Spinal cord o Somatic motor and sensory o Visceral motor and sensory o Special sensory • Enteric nervous system o Found inside the gut
Why is classifying neurons important?
• Study of external morphological structural characteristics of an individual neuron is valuable at least in two ways
o Provides a picture of the pattern and extent of the nerve cell processes and their immediate relationship with other nerve cells, cytoarchitecture or microcircuitry
o Helps characterise or identify particular cell groups so that correlations can be made between morphological characteristics and other properties
What is cytoarchitecture?
o Cytoarchitecture- the arrangement of cells in a tissue
Describe a timeline of the history of nerve classification including person responsible, year and contribution.
- Jansen (1595): Development of single compound microscope
- Willis (1664): Described macroscopic grey matter and white matter of the brain
- Malphigi (1666): Earliest attempt to visualise the structure of nerve tissue using light microscope
- Leeuwenhoek (1670): Development of simple single lens x200-500 magnification: one of the 1st people to view nerve fibres
- Hooke (Late 1600s): improved the compound microscope
-Many scientists (1820s):
Sufficient resolution on compound microscope to clearly study nerve cell parts and sufficient staining procedures were developed
Indigo, carmine stain developed
- Valentin (1836): Demonstrated cellular nature of neurons
- Purkinje (1837): Demonstrated cellular nature of neurons
Dieters (1865): Observed that the cellular and fibrous nature of nervous tissue were linked
Golgi (1873): Golgi stain developed and demonstrated the diversity of neuronal cell structure and how they were organised with regard to one another
Zeiss, Abbe (late 1800s): Compound microscope corrected for spherical and chromatic aberration
Weigert(1884): Weigert stain
Cajal (1890s): Demonstrate the diversity of neuronal cell structure and how they were organised with regard to one another
Nissl (1894): Nissl stain
Cajal (1903): Reuced silver stain
Do neurons have extensive membrane processes (axons and dendrites)?
Yes
Do neurons form a contiguous network (synapses)?
Yes
Do neurons conduct/direct charge (with their membrane and ion channels)?
Yes
What are distinguishing features of neurons?
Large euchromatic nucleus, usually central Prominent nucleolis Abundant Nissl bodies Filaments and tubules Membrane processes Synapses • Unique to neurons Neurons have many general characteristics similar to other cells, but their ability to form contiguous networks and to direct charge is practically unique
What percentage of the cells in the nervous system are neurons?
Less than 10%
What is a list of supporting cells found in the nervous system?
- PNS Schwann cells
- Satellite cells
- Perineural cells
- CNS astroglia
- Oligodendroglia
- Microglia
What was Jansen’s contribution in nerve classification and when was it?
1595 Development of single compound microscope
What was Willis’s contribution in nerve classification and when was it?
1664 Described macroscopic grey matter and white matter of the brain
What was Malphigi’s contribution in nerve classification and when was it?
1666 Earliest attempt to visualise the structure of nerve tissue using the light microscope
What was Leeuwenhoek’s contribution in nerve classification and when was it?
1670 Development of simple single lens x200-500 magnification
-One of the 1sst people to view nerve fibres
What was Hooke’s contribution in nerve classification and when was it?
Late 1600s Improved the compound microscope
What was Valentine’s contribution in nerve classification and when was it?
1836 Demonstrate cellular nature of neurons
When were nerve cells really able to be studied and why?
1820s Sufficient resolution on compound microscope to clearly study nerve cell parts and sufficient staining procedures were developed
Indigo, carmine stain developed
What was Purkinje’s contribution in nerve classification and when was it?
1837 Demonstrate cellular nature of neurons
What was Dieters’s contribution in nerve classification and when was it?
1865 Observed that the cellular and fibrous nature of nervous tissue were linked
What was Golgi’s contribution in nerve classification and when was it?
1873 Golgi stain developed and demonstrated the diversity of neuronal cell structure and how they were organized with regard to one another
What was Zeiss and Abbe’s contribution in nerve classification and when was it?
Late 1800s Compound microscope corrected for spherical and chromatic aberration
What was Weigert’s contribution in nerve classification and when was it?
1884 Weigert stain
What was Cajal’s contribution in nerve classification and when was it?
1890s Demonstrate the diversity of neuronal cell structure and how they were organised with regard to one another
1903 Reuced silver stain
What was Nissl’s contribution in nerve classification and when was it?
1894 Nissl stain
What are satellite cells
Surround nerve cell bodies located in the ganglia of the peripheral nervous system and have functions similar to astroglia
What are perineural cells
Found in peripheral nerves where they form the perineurium.
Perineurium defines the fascicular substructure of peripheral nerves
What is the CNS ependyma?
Line the ventricles and along with the pia and capillaries form the choroid plexus, which secretes cerebrospinal fluid
What is the meninges?
Membranes which surround the brain and protect it. Composed of varying proportions of flattened epithelial like cells and connective tissue
What is the resolution of a light microscope?
Micrometer
What is the resolution of an electron microscope?
Nanometer
What is necessary for a brightfield light microscope to work?
o In order for brightfield light microscope to work properly there needs to be sufficient contrast, resolution and magnification
Do stains show everything?
• Stains are selective about what they show
o Stains vary in how much internal and external structure they show, how well they show cell processes and whether they stain all or only some of the nerve cells
What is necessary to prepare tissues for a stain?
o In preparing tissues for a stain, a series of steps are required: Tissue selection Fixation in buffered formaldehyde Embedding Sectioning on slides Staining Mounting Viewing
Describe the Haematoxylin and Eosin stain including:
- Description of the different components
- Description of each component’s function and advantages
- Limitations
Haematoxylin Composition: -Flavinoid molecule Specific Function and advantages -Stains nuclei a dark blue-black colour -Can estimate size, numbers, density and distribution of neurons
Eosin:
Composition
-Anionic dye
Specific Function and advantages
-Provides a pink background contrast for cytoplasm and extracellular structures
-Can show some contrast in cell membrane and indirectly show some nerve cell processes
Common advantages:
- Glial and other cells are stained
- Easy to use
- Stains most neurons
- Sometimes proximal staining of processes and tracts
Common limitations-
-No processes or tracts
What are two examples of Nissl stains?
- Toluidine blue
- Cresyl violet
Describe the composition of Toluidine blue
Cationic thiazine dye used on 5um or very thin sections
Describe the composition of Cresyl Violet
Cationic oxazine dye used on 5um or very thin sections
Describe how nissl stains work
Nissl stains are basophilic: they preferentially bind to acidic moieties, such as DNA and RNA, hence why Nissl strains strongly the nucleus (DNA and RNA) and rough endoplasmic reticulum (RNA)
What are the advantages/function and limitations of nissl stains?
Advantages+ function
- Stains the nucleus, nucleolus and ribosomes (including the endoplasmic reticulum)
- Stain shows grey matter and its organisation such as layers, shows cell numbers, density and body size, as well as the beginnings of larger processes
- Relatively simple stain and reliable
- Stains most neurons
Limitations-
- Shows few processes or tracts
- Glial cells may be stained to some extent, such as their nuclei (small dots), also capillary endothelial cell nuclei (elongated structures) and red blood cells: however, glial cell bodies cannot be viewed as they have less abundant rER
What are reduced silver stains made of, their advantages/function and their limitations?
Composition:
-Silver salts followed by a reducing agent on thick sections
Advantages/function-
- Stains all or most neurons
- Cell bodies and neurites are clearly shown hence both grey and white matter are shown
- Stains nucleus membrane
- Stains processes including axons and tracts, their origins and destinations
- Stains glia
Limitations-
- Moderate difficulty of use
- Dense background
What are golgi silver stains made of, their advantages/function and their limitations?
Composition:
-Using silver or mercury salts in blocks of tissue
Advantages/functions
- Stains only a small number of neurons, in particular their dendritic trees
- Gives a clear picture of the cell body shape but not the intracellular structures
- Shows synaptic spines
- Shows glial cells
- Shows full morphology of neurons, including dendritic arbor and axon
Limitations-
- Preparation, cell selection, not all axons stained
- Can be difficult and takes time to use
- Only stains a small number of neurons in the tissue
What are specialised glial detection methods?
-Phosphotungstic stain
-Phosphomolybdic stain
-Heavy metal stains
=Immunohistochemistry stains
-Fluorescence
-Myelin stains are used to view oligodendrocytes and reveal the underlying neuronal axons/tracts wrapped by oligodendrocytes
What are 2 types of myelin stains?
- Luxol fast blue
- Weigert method
What is the composition of luxol fast blue/
Anionic pthalocyanine dye
What is the composition of the Weigert method?
Series of steps involving chromium and copper salts, haematoxylin and a differentiator
Describe the advantages/function of myelin stains and their limitation
Advantages/function
- Useful for indicating fibre tracts and white matter but not cell bodies
- In serial sections large bundles of fibres can be traced but not individual fibres/small groups
- Detects the myelin-rich membrane of oligodendrocytes in the CNS and Schwann cells in the peripheral nerves
- We can infer the location of axons and axonal tracts with a myelin stain
Limitations-
- Moderate difficulty of use
- No neuron cell bodies shown (unless counterstained)
- Axons are not directly labelled- only the location of myelination around axons can be seen
What is a marchi stain?
Describe its class, composition, advangages+functions, and limitations
Degeneration stain-
Composition:
-Osmium in association with another oxidising agent
Advantages and function-
Helps trace the extent of pathways and the localisation of particular pathways within larger regions of white matter
-Visualises degenerating tracts
Limitations:
- Moderate level of usage difficulty
- No neuron cell body unless counterstained
How does preparation for viewing by transmission electron occur?
-Perfused with fixative, fixed in small blocks, sectioned thinly with glass or diamond knife, stained with heavy metals and viewed indirectly in the electron microscope
What are advantages/functions and limitations of transmission electron microscope?
Advantages+functions-
- Demonstrates membranes and intracellular structures
- Shows all neurons
Limitations-
- Not great at tracing extent of any particular cell process and reconstructing 3D aspects of the cell is time consuming
- Need to be specialised to use
What are two different types of uptake markers and what are they?
Retrograde markers- taken up in the axons and transported to the cell body
Anterograde- taken up by the cell body and transported to the axonal synapse region
What are advantages and limitations of uptake markers?
Advantages+functions
- Possible to show where some cells in a region project to and what cells project to that region
- Shows several neurons, including their cell bodies and fibres
Limitations
-Difficult to use
How does microinjection work?
Marker substances can be injected into individual neurons through micropipettes
Marker travels throughout the cytoplasm and processes
-Individual cells can be filled with dyes to examine the cell’s full morphology
What are advantages and limitations of microinjection?
Advantages-
- Can reveal appearance of the cell and its processes
- Correlation can be made between cell shape, location and behaviour
- Usually shows one neuron’s cell body and processes
Limitations-
- Difficult to use
- Can only show a single cell
What is immonstaining and histochemistry and how does it work?
Proteins and molecules in neurons and glial cells can be visualised by reacting them with antibodies, or in chemical reactions to attach other molecules to them which allow them to be seen under the microscope
- Uses antibodies to bind specific antigens in tissue
- Antibodies bound to target antigens are visualised in tissue using a chromogen or a fluorochrome
- Key markers in the CNS for identifying cell types and structures include cytoskeletal proteins, neurotransmitter synthesis enzymes, cell organelles and many others
What are the advantages of immunohistochemistry/immunochemistry and its limitations
Advantages+function
- Can examine correlations between populations if several antibodies and reactions are used
- Can show many neurons
- Only shows particular components of cell body and processes
- Can be used to distinguish neurons from glia, distinguish between different glial cell types and distinguish neuronal categories as antibodies are specific to particular antigens
Limitations-
Difficult to use
What are different methods used for viewing neurons and studying them?
- Haemotoxylin and Eosin stain
- Nissl stains
- Reduced silver stains
- Golgi silver stains
- Myelin stains
- Degeneration stains
- Transmission electron microscope
- Uptake markers
- Microinjection
- Immunostaining and histochemistry
How are neurons drawn schematically?
• Schematic neurons
o Neurons can be represented in a schematic way by a circle (cell body and dendritic tree), a line of varying length (axon) with a small V at the end (the synapse)
What are different ways neurons are classified?
• Ways in which neuronal cells are classified, described or characterised
o Morphologically
Based on their external appearance and/or their location within the nervous system
o Functionally
o Biochemically
o Genetically
o Pharmachologically
What is the checklist for morphological neuron classification?
o Name o Cell body size and shape o Nucleus o Cytoplasm o Processes/Membrane o General location of the overall cell o Particular location of the cell body o Special characteristics o Function o Matter of interest o Connections (if possible)
How do you morphologically classify neurons based on cell body size and shape
Small body- 5 to 10um across • E.g cerebral cortical pyramidal cells and purkinje cells(20um) Medium size body- 10-30 um across • Somatic motor neurons (40um) Large body- 30-100 um across • Somatic sensory neurons (60um)
How do you morphologically classify neurons based on nucleus
Size
Location
Chromaticity
Nucleolus
How do you morphologically classify neurons based on cytoplasm
Features if present may depend on stain used
How do you morphologically classify neurons based on processes/membranes
Polarity
• Number of cell processes (dendrite and axon)
o One-unipolar
E.g. somatic sensory neurons
o 2-bipolar
Some cells in the retina, olfactory cells
o More than 2- multipolar
Interneurons, principal neurons, lower motor neurons
Axon length
• Short axons- Golgi type 2/interneurons
o 90% are this type
o Interneurons are when the axon mostly stays within the grey matter
• Long axons- Golgi type 1/principal projection
o Bridges to grey matter areas: does not stay within one grey matter area
Axon destination
Dendritic tree
• Shape, extent, branching pattern, location, size
• Classification is often unique to a particular par tof the brain
• Spiny dendrites- dendrites that have spines on the receptive side of the synapse
• Aspinous dendrites- dendrites that do not have spikes
How do you morphologically classify neurons based on general location of the overall cell?
Whether the neuron is peripheral (sensory neuron, motor neuron), central (intermediary/integrative neuron) or enteric
How do you morphologically classify neurons based on particular location of cell body?
Particular location of the neuron within the central nervous system grey matter, or within a peripheral or enteric ganglion
How do you morphologically classify neurons based on connections?
Primary sensory neurons- cells with connections to sensory surfaces of the body
Motor neurons- have axons that form synapses with the muscles and command movements
Interneurons- only form connections with other neurons
What are the 2 parts of the CNS?
o External elevations, depression and attachments
o Internal grey and white matter
Is grey matter layered?
• Grey matter is composed of several layers
Where is grey matter found?
o Lamina (e.g. cerebral cortex, cerebellar cortex)-neurons may be arranged in layers
o Nuclei (e.g. amygdala, mammillary nuclei)-clusters of similar types of neurons or a collection of a few neuronal cell types
o Irregular collections (spinal cord grey matter)
o Ganglia
o Columns
Describe the composition of the cerebral neocortex
• Neurons form 6 layers (lamina) in the neocortex parallel to the surface of the brain functionally arranged in columns (100-200 um wide)- the laminae are separated by a relatively neuron-free border (although glia and axons are still present)
o Layers I,II,III and IV mainly receive afferent fibres from other areas of cortex and brainstem
o Layers V and VI mainly provide efferent fibres that pass to the white matter
• The layers vary as you travel across surface of the membrane- % of particular sorts of cells changes
o Differences in areas of cell number and distribution
Width of the cerebral cortex is 3-5mm
Describe layer I of the cerebral neocortex
Layer I- no neurons, lots of myelinated axon, looks yellow (fatty) and has a high density of astrocytes: in a nissi stain, only the nuclei of glia and endothelial cells can be seen
Describe layers II and III of the cerebral neocortex
Layers II and III- can’t be separated easily- medium pyramidal (triangular) neurons
Describe layer IV of the cerebral neocortex
Layer IV looks paler- major input area from the thalamus(lots of axons) especially in sensory areas-generally has smaller neurons
Describe layer V of the cerebral neocortex
Layer V has big dark pyramidal neurons (output neurons) especially in motor cortex (called Betz cells)
Describe layer VI of the cerebral neocortex
Layer VI-last one before subcortical white matter-made of medium sized neurons
Describe the composition of the cerebellum cortex
• 3 layers of grey matter
o Granular
o Purkinje
o Molecules
What is proportion and organisation of grey matter correlated with?
Function
What does the grey matter include?
• Grey matter is defined by the presence of nerve cell bodies
o Also includes presence of:
Dendrites of projection neurons and small interneurons
• Most dendritic trees will be limited to grey matter
Axons of interneurons
Proximal axons of projection neurons
Distal axons of projection neurons from elsewhere
Projection neuron axons in transit/passing through
Synapses
Protoplasmic astrocytes
Capillaries
Neuronal cell bodies
• Interneurons (microcircuits)
• Projection neurons (macrocircuits)
o Also has a good blood supply
What is grey matter the site of?
Information processing
What is microcircuitry?
o Microcircuitry- organisation of neuronal processes and their connections within the grey matter
What is macrocircuitry?
o Macrocircuitry- connections between several groups of grey matter
What is the checklist for the description of grey matter
o Name o Size o Shape o Location o Relationships o Subregions o Substructure o Particular cell components o Microcircuitry o Input o Output o Functional correlations
What is the colour of grey matter when it’s stained?
• When stained, is pale (unless counterstained)
Where is white matter found?
White matter is given a wide range of names depending on location • Locations can include: o Tract o Peduncle o Fasciculus o Lemniscus o Commissure o Funiculis o Capsule o Central o Medullary
What is white matter characterised by?
• Characterised by: o Axons of projection neurons (myelinated or unmyelinated) o Oligodendrocytes o Other supporting cells o Some capillaries
White matter is the site of what?
• Site of information transfer
What is a checklist used to describe white matter?
o Name o Size o Shape o Extent o Location o Regions traversed o Substructure/components o Beginning o Cells of origin o End o Course/relationships o Correlation with function
What is the colour of white matter when it is stained?
• When stained, is usually dark and often black
Where are cell bodies of neurons in the PNS located?
• Cell bodies of neurons in the peripheral nervous system are located in ganglia or specialised tissue
Describe the somatic sensory neuron in:
- Polarity
- General location
- Axon length
- Size/shape of cell body
- Particular location of cell body
- Picture (on phone, timestamp 20/08/2019 at 11:31
Polarity: Unipolar General location: Peripheral sensory Axon length:Long Golgi 1 Size/shape of cell body:Large globular Particular location: Dorsal root ganglion
Describe the lower motor neuron in:
- Polarity
- General location
- Axon length
- Size/shape of cell body
- Particular location of cell body
- Picture (on phone, timestamp 20/08/2019 at 11:31
Polarity: Multipolar General location: Peripheral motor Axon length: Long golgi 1 Size/shape of cell body: large fusiform Particular location: Ventral horn and spinal cord
Describe the cerebral cortex pyramidal cell in:
- Polarity
- General location
- Axon length
- Size/shape of cell body
- Particular location of cell body
- Picture (on phone, timestamp 20/08/2019 at 11:31
Polarity: Multipolar
General location: Central
Axon length: Long golgi 1
Size/shape of cell body: Medium pyramidal
Particular location: Internal pyramidal layer cerebral cortex
Describe the purkinje cell in:
- Polarity
- General location
- Axon length
- Size/shape of cell body
- Particular location of cell body
- Picture (on phone, timestamp 20/08/2019 at 11:31
Polarity: Multipolar General location: Central Axon length: Long golgi 1 Size/shape of cell body: Medium glogular Particular location: Purkinje layer cerebellar cortex
Describe the cerebellar granule cell in:
- Polarity
- General location
- Axon length
- Size/shape of cell body
- Particular location of cell body
- Picture (on phone, timestamp 20/08/2019 at 11:31
Polarity: Multipolar General location: Central Axon length: Short interneuron golgi 2 Size/shape of cell body:Small Particular location: Granule layer cerebellar cortex
Describe the olfactory neuron in:
- Polarity
- General location
- Axon length
- Size/shape of cell body
- Particular location of cell body
- Picture (on phone, timestamp 20/08/2019 at 11:31
Polarity: Bipolar General location: Peripheral sensory Axon length:Longish Size/shape of cell body:Small Particular location: Olfactory mucosa
Describe the composition of the spinal cord and draw it
Picture on phone- timestamp 20/08/2019 at 11:38
• Grey matter of spinal cord is divided into o back (dorsal horn) at the top for somatosensory input o front (ventral horn) at bottom for motor output-lower motor neurons are found in the ventral horn
What is the cytoarchitecture of basal nucleus?
• Basal nucleus- cluster of neurons
What is the cytoarchitecture of the lamina and its differences throughout?
o The size and density of neurons varies between layers
o Thickness of the layers varies
o Neuronal morphology differs between layers
o Layers of neurons may be separated by relatively neuron-free layers. These layers are typically rich in glial cells and myelinated neuronal processes
o Connectivity: input and output to each layer differs
What is the distance of the synaptic cleft?
10-40nm
What is the diameter of the spinal cord?
1.5-2.5mm
