Cells of the nervous system

L2 and 3

What are the 2 main ways of classifying neurons?

by structure // by gene expression

What’s the point of classifying neurons?

85 billion neurons in brain so we can’t study all of them - we need broad types to know how they’ll work electrically, chemically, in different regions of the brain etc

What are ways of classifying a neurone by structure?

• Number of neurites

• Dendritic geometry (expansive? one plane?)

• Connections - where do they project? (efferent? afferent?)

• Axon length (local? long projection through body?)

What are ways of classifying a neurone by gene expression?

• underlies structural differences

• defines NT expression (excitatory? inhibitory?)

How does the number of neurites affect neuron classification? What are the 3 types?

• unipolar - one neurite

• bipolar - two neurites

• multipolar - lots of neurites

what is an example of a pseudounipolar neuron? What type of information does it carry?

dorsal root ganglion

sensory information through the afferent neuron

Which 2 structures make up an axon?

peripheral process and a central process

Considering a unipolar neuron has a single neurite ie a small area for receiving synaptic input, is it highly or not at all specialised in function? How does this affect the quality of the relay of information ?

highly specialised function

make it very reliable

What differentiates a pseudounipolar neuron from a unipolar one?

single process that branches into two functional parts vs a single, undivided process

What makes up a ganglion?

group of neuronal cell bodies in the periphery = bulge

How does the structure of a pseudounipolar neuron inform on the transmission speed of its message?

single axon + dendrite = no divergence/integration of sensory information means very quick relay of info bc no delay

Where are bipolar neurones often found?

the retina

What is the structure of a bipolar neurone?

dendrites on both edges of the neurone/either side of the cell body

Considering a bipolar neuron has two neurites ie a small area for receiving synaptic input, is it highly or not at all specialised in function? How does this affect the quality of the relay of information ?

highly specialised function

make it very reliable

How many contacts does a multipolar neurone receive?

about 150 000 contacts

What type of neurite structure is found in most neurones in the brain?

multipolar

What is the size of the area for receiving synaptic inputs in multipolar neurones? How does this affect the quality of relay of info?

large

high levels of convergence/integration

What type of neuron is a Purkinje cell?

multipolar neurone

What types of neurons are stellate and pyramidal neurons?

multipolar neurons

How does the shape of a pyramidal neuron inform on its function?

distinct dendritic domains have different properties and receive input from distinct sources - two dendrites go off with several small branches off each

Where do sensory neurons project?

dorsal nerves, horns, ganglions

Do all sensory neurons go through the spinal cord?

no, eg ear sensory nerves go straight to brain, wouldn’t make sense to go down to spinal cord

Where do motor neurons project?

ventral nerves, horn, ganglion

Where do interneurons project?

within CNS

What are 2 types of interneurons?

• relay/projection neurons

• local interneurons

What are the functions of the two distinct types of interneurons?

• projection: connect brain regions

• local: short axons that process info in local circuits

How far can projection/relay neurons extend?

stay in CNS but can go from brain, motor cortex, spinal cord so from very top to very bottom of NS

What’s an example of where a local interneuron can extend?

within motor cortex eg

What is the largest class of neurons?

interneurons

Is there space between neurons? What fills it?

no space, glial cells

What’s the space between glia and neurons?

EC space of about 20 nm

Can glial cells proliferate throughout life? (regenerate in adults)

YES

What are 2 ways of classifying glial cells?

• depending on which part of the nervous system the cells are in

• depending on their roles

What are 3 types of glial cells within the CNS and their functions?

• homeostasis: astrocytes

• myelinating cells: oligodendrocytes

• phagocytic: microglia

Which glial cells are found within the enteric nervous system? What’s their function?

enteric glia - homeostasis

What are 3 types of glial cells within the PNS and their functions?

• homeostasis: satellite cells

• myelinating cells: Schwann cells

• phagocytic cells: Schwann cells and macrophages

What’s the role of homeostatic glial cells?

provide nutrients, ions etc to maintain neurons

What are Schwann cells similar to in terms of function?

dust bin! hoover up debris in their phagocytic function

What’s the function of immune cells of the nervous system?

phagocytosis

What do microglia do?

damaged neuron/cell releasing stuff into local area, destroyed to avoid spread

What are some functions of astrocytes?

• cell glue

• controls env. around neurons

• regulate potassium levels

• fuel suppliers through glycogen stores

• part of the tripartite synapse

• buffer EC K+

• form part of blood brain barrier

• couple neuronal activity to blood supply

What’s a unique marker for astrocytes? Which technique is this?

Glial fibrillary acidic protein (GFAP)

immunohistochemistry

What is the equivalent of a neurite (in neurons) in astrocytes? Do these typically overlap with those of other cells?

a process

no

How do astrocytes act as fuel suppliers?

provide lactate as alternative fuel to glucose

Which part of an astrocyte takes up glucose?

endfeet

What are two routes of ATP supply by astrocytes ? Why are 2 needed?

1 - glucose → ATP when lower metabolic activity

2 - glucose → Glycogen → Lactate → ATP when higher metabolic activity so higher energy needs

Can astrocytes detect neuron activity?

yes, when high activity provide lactate as alternate fuel source to glucose

What would an endfoot in astrocytes be comparable to in neurons ?

presynaptic terminals

What makes up a tripartite synapse?

pre-synaptic neuron, post-synaptic neuron and an astrocyte

What is an astrocyte’s role in a tripartite synapse? How?

terminates NT activity making it an efficient transmission of signals

recycling NT to presynaptic terminals

What is the function of microglia?

surveillance and phagocytosis ie protection of neurons + harmful role in neurodegenerative diseases

What is the primary role of oligodendrocytes? How does this compare to Schwann cells?

form myelin sheaths of CNS axons

form 15-30 processes from cell to myelin sheath // Schwann that only myelinate one cell through one process (in PNS)

What is the primary function of Schwann cells? How does this compare to oligodendrocytes?

form myelin sheaths of PNS axon

form 1 myelin segment to myelinate a single axon

What are the 2 steps of myelin sheath formation?

• oligo cytoplasm wraps around the axon

• cytoplasm squeezed out of layers

What does compaction describe in the process of forming a myelin sheath?

cytoplasm squeezed out of layers

Do oligodendrocytes and Schwann cells (glial cells) detach from the myelin sheath once its complete?

no, stay in contact to stay nourished

Comparison/metaphor for myelin sheath?

jam sponge cake - myelin wrapped around like the sponge then jam squeezed out

What’s the function of myelin? Through which structures?

insulating and enabling saltatory conduction

nodes of ranvier

How many of each type of cell are there in the nervous system ? (neurons, glial cells etc)

around 85 billion

What are neurons?

excitable cells that conduct impulses

What is the function of neurons?

integrating and relaying information within a neural circuit

What is the purpose of Nissl staining?

• distinguish between neurons and glia

• visualise nucleolus of all cells

• visualisation of variation in size, density and distribution of neurons

What does nissl staining dye bind to?

negatively charged RNA

What are Nissl bodies, present in neurons?

organelles in neurons that synthesize proteins and are vital for neuronal function

Synonyms of cell body?

soma and perikaryon

What are organelles for protein synthesis and processing in the soma? What is another essential organelle within the soma?

ribosomes, RER, Golgi apparatus

mitochondria

What does Santiago Ramon y Cajal stain show? Which staining technique is is based on?

detailed structure of individual neurons, including their cell bodies, dendrites, and axons, allowing scientists to visualise synapses

golgi staining

What does a golgi stain show?

neurons, binds to all parts of the neuron

How did golgi and cajal disagree on neuron communication? Who was right?

golgi thought no space, cajal thought small space called synpase

cajal was right

What are 2 types of neurites?

dendrites and axons

What are 3 parts making up the cytoskeleton?

• microtubules

• microfilaments

• neurofilaments

What makes up a microtubule? How are they organised in neurites? Their diameter?

Hollow tube composed of polymers of tubulin

longitudinally down neurites

20 nm

What are 4 parts of the axon? (from soma to synapse)

• axon hillock

• axon initial segment

• axon collaterals

• axon terminal / terminal bouton

At which point of the axon are there the most ion channels?

just after hillock

Where is there more protein making ability in a neuron? How is this reflected by axon structure?

more in cell body than in axon

no RER and fewer free ribosomes

What can the length of an axon be?

from <1 mm to >1 m in length

What can the diameter of an axon be?

1 um - 25 um

What could immunohistochemistry be used for? How does it work?

visualising neurons

primary and secondary antibodies used to bind to proteins of interest that are specific to neurons

What is MAP2 used for? How is it specific to this function?

visualising cell morphology through immunohistochemistry - binds to microtubule associated proteins

What does increased collateral numbers off of somas lead to?

high levels of divergence

Are axons myelinated?

can be

Do microtubules enter the presynaptic axon terminal?

no

What are specialised structures of presynaptic terminal cytoplasm?

• no microtubules

• synaptic vesicles

• specialised proteins (on and in)

• mitochondria

What does synapse mean?

to fasten together

Why are there so many mitochondria in presynaptic terminals?

lots of energy is needed for transport of vesicles/proteins

What are 2 types of presynaptic terminals?

• terminal arbour - ends in fine branches

• boutons en passant - stops off mid axon and links to another neuron

How large is a synapse?

20 um

What is it called when axons branch off? Can these branches also split off?

collateral

yes = even more relationships with neurons

How was slow axoplasmic transport discovered?

by tying a knot in the axon, bulge appeared after a while where stuff could no longer move

How was fast axoplasmic transport discovered?

radioactive amino acids

What are 2 types of axoplasmic transport?

slow and fast

Considering the absence of protein making machinery in the presynaptic terminal, how are proteins found there?

transported from soma through axoplasmic transport

How fast are the two types of axoplasmic transport?

1 mm/day vs 1000 mm/day

What allows axoplasmic protein transport?

vesicles moving through microtubules thanks to kinesin/dynein and ATP

Which protein is involved in anterograde axoplasmic transport? What is the anterograde direction?

kinesin (cute protein video - pulls the vesicle)

soma to presynaptic terminal (left to right)

Which protein is involved in retrograde axoplasmic transport? What is the retrograde direction?

dynein

presynaptic terminal to soma (right to left)

What is HRP? What does it allow to visualise?

Horse radish peroxidase, dye carried by kinesin

anterograde/retrograde movements

What do dendritic branches coming together form?

dendritic trees/arbors

What do dendritic branches allow in terms of number of synapses? What term describes the ability to form several synapses?

1000s of synapses = convergence

In which part of the brain can we find purkinje cells for example?

cerebellum