Cells of the nervous system Flashcards
Describe the dimensions of the brain
100,000,000,000 cells
Packed into 1.5 litres
750g-1.5kg (don’t know the significance of this yet)
Describe neurons
-basic structural and functional unit of the nervous system -information processing unit. -responsible for the generation and conduction of electrical signals -communicate with one another via chemicals released at the synapse. -supported by neuroglia, comprising several different cell types.
Highly organised metabolically active cell involved in cell signalling.
Excitable cells.
What is the ratio of neurons to neuroglia
neuroglia outnumber the neurones by 9:1.
Describe neuronal structure
• Cellular structure of all neurons is similar. • Diversity is achieved by differences in the number and shape of their processes.
What does the diversity of neuronal structure depend on
The location and the function of the neuron
What are the most highly organised, metabolically active secretory cells in the body
Neurons
What is a consequence of the diversity of the neurons
It is this diversity in size, location, function and metabolic activity that makes them more or less vulnerable to degeneration in CNS conditions such as Alzheimers, Parkinsons, MS and ALS. The number of neurons is important too.
Describe the characteristics of the cell body of neurons
-large nucleus -prominent nucleolus -abundant rough ER -well developed Golgi -abundant mitochondria -highly organised cytoskeleton -HIGHLY ORGANISED METABOLICALLY ACTIVE CELL
Numerous lysosomes.
Why do the cell bodies of neurons have well developed golgi
(due to need of transport of proteins over long distances). An extensive Golgi apparatus is in keeping with high amounts of protein trafficking via the secretory pathway.
Why do cell bodies need lots of RER
To make a lot of neurotransmitters.
Which structure of the neuron has a lot of diversity
Dendrites
What is the function of dendrites
To collect information from surrounding excitable cells (neurons) and also sensory receptors and to conduct the signal to the cell body of the neuron.
What does the tree like structure of the dendrites reflect
Where the input is coming from
Where are the pyramidal neurons found
In the neocortex of the cerebral cortex- main output neurons of the nervous system.
Some in the motor cortex send axons down to spinal motor neurones and hence some have long axons.
Describe the dendrites
- major area of reception of incoming information - spread from cell body and branch frequently - greatly increase the surface area of the neuron - often covered in protrusions called spines - dendritic spines receive the majority of synapses - large pyramidal neurons may have as many as 30,000/40,000 spines
What is the consequence of the dendrites increasing the surface area of the neuron
It allows the neuron to receive multiple inputs from different places
What is the principal role of dendrites
To increase the potential for synapse formation
Describe the dendritic spines
Thin dendrites have protrusions called dendritic spines that receive the majority of synapses.
What does the number of dendrites reflect
Accordingly, the number of dendrites reflects the way information is processed in that pathway. A cell with many inputs may condense information from several pathways, whereas a cell with few inputs may be part of a highly conserved parallel pathway.
Describe the primary, secondary and tertiary dendrites
Primary dendrites- thicker, leave the 3 vertices of the pyramidal cell
Divide to form secondary dendrites which divide to form tertiary dendrites.
What is a feature of spines
Many synapses onto them- receives a lot of inputs
Lots of mitochondria.
Describe the plasticity of the spines
One of the most plastic elements of the nervous system
Can be withdrawn, destroyed or produced
e.g alcohol, schizophrenia
What type of neurons are Purkinje neurons
Inhibitory
What type of structure are Purkinje neurons
2D
What are the Purkinje neurons of the cerebellum responsible for
Fine motor movements
Learning and memory
Describe the characteristics of the Purkinje cells
-dendrites have an enormous number of spines (> 80,000/ cell) -human cerebellum has approx15 mill Purkinje cells.
Make up a ‘forest’
How many axons are there per cell
1
Describe axon branching
o One axon per cell but can branch extensively into prominences known as ‘Axon Collaterals’- to connect the different cells.
Close to the target, the axon branches ‘terminal branches’ (terminal arbor).
Branches extensively near the target cell.
Describe the characteristics of axons
-conduct impulses away from cell body -emerge at the axon hillock -usually only one per cell -may branch after leaving cell body and at target -prominent microtubules and neurofilaments
Describe the axon hillock
Where axons emerge from
Appears as a swelling at the cell body
Where the action potential is generated.
Why do axons contain abundant intermediate filaments and microtubules
To maintain tensile strength, to compensate for small diameter to maintain structure.
Microtubules to transport vesicles, mitochondria from cell body to synapse (which can be a long distance, up to a metre!).
What do neurofilaments determine
Neurofilaments determine axon calibre and microtubules are very abundant in neurones.
o Axons range in length from micro m to metres.
Where is the axonal membrane of a myelinated fibre exposed
At the node of ranvier
Describe the purpose of the axolemma and axoplasm
The ion channels within the axolemma enable the axon to conduct action potentials.
The axoplasm contains microtubules, mitochondria and neurofilaments. These organelles play a key role in maintaining the ionic gradients necessary for action potential production and also enable the transport and recycling of proteins away from, and to a lesser extent, towards the cell body.
Why is it important of cable properties of the axon
Need to maintain the same speed of conduction all the way along the axon- why the diameter of the axon remains unchanged.
If not- the axon would be unable to transmit fast trains of impulses.
Describe how the axons are organised into domains
Node – Sodium channels found here. ▪ Paranode – Ends of myelin are bound to the axon to form tight junctions. ▪ Juxta-Paranode – Area adjacent to Paranode where potassium and calcium channels are found.
The fact that the K+ channels are in a different area to the Na+ channels is essential for action potential conduction.
Describe axon terminals
-axons often branch extensively close to target (terminal arbor) -form synaptic terminals with target
Describe the two different types of axon terminals
Two forms of axon terminals: ▪ Boutons – A standard synapse- swelling of the axon
▪ Varicosities – Axons may synapse with many smooth muscles cells so you have multiple swellings containing NT called varicosities. Also contact Purkinje fibres.
What is responsible for the synaptic density (black smudge on EM)
Increased number of proteins (ion pumps, ion channels, post-synaptic receptors, machinery to secrete neurotransmitters, neurotransmitters) contributes to a large electron density.
Describe how synaptic vesicles are packaged and bind to plasma membrane
-synaptic vesicles, packaged in the Golgi and shipped by fast anterograde transport - specialised mechanisms for association of synaptic vesicles with the plasma membrane- docking.
Describe the importance of abundant mitochondria in the synapse
- abundant mitochondria - ~ 45% of total energy consumption is required for ion pumping and synaptic transmission - sensitivity to O2 deprivation
Describe synaptic organisation of the post-synaptic neurone
-neurons receive multiple synaptic input -neurons use a diversity of chemical transmitters, excitatory and inhibitory -competing inputs are integrated in the postsynaptic neuron (neuronal integration).
A single DIGITAL output is produced.
Analogue in- digital out
Describe the different types of axon
Axo-dendritic: Axon synapses on dendrite (often excitatory)
Axo-somatic: Axon synapses on cell body (often inhibitory)
Axo-axonic: Axon synapses on axon (often modulatory) changes the action potential going down that axon.
What constitutes the vast majority of the cerebral cortex
Synaptic processes
Describe the neuronal cytoskeleton
- in the human adult axons range in length from micrometers to up to a meter -highly organised cytoskeleton is required (microfilaments, intermediate filaments, microtubules) -neurofilaments play a critical role in determining axon caliber -microtubules are very abundant in the nervous system
CELLS OF THE NERVOUS SYSTEM
Describe the structure of the neurofilaments
Tight bundles of fibres, linked by cross-bridges.
Describe anterograde transport
- Anterograde Transport – Transport of materials needed for neurotransmission AWAY from soma:
a. Fast Anterograde – Synaptic vesicles, transmitters, mitochondria. i. 400mm/day. ii. Uses microtubular network and requires oxidative metabolism. iii. Uses specific molecular motors.
b. Slow Anterograde – Bulk cytoplasmic flow of soluble constituents
Describe retrograde transport
Retrograde Transport – Transport of materials TOWARDS the soma (from EC space). a. Fast Retrograde – Return of organelles. i. Transport of substances from EC space. ii. Trophic growth factors, neurotropic viruses. iii. Uses different molecular motors.
Describe the difference between retrograde and anterograde vesicles
-retrograde moving organelles are morpholgically and biochemically distinct from anterograde vesicles
What causes the formation of retraction bulbs (axon swellings) seen in axonal damage such as in multiple sclerosis.
A restriction on the axon causing swellings to occur due to accumulation of vesicles of NT at the restricted point.
Describe pseudounipolar neurons
o Have two fused processes that are axonal in structure. o Tend to be sensory neurones. o The signal received passes directly to the axon terminal without going through the soma.
For example dorsal root ganglion neurons which give rise to the sensation of pain.
Describe bipolar neurons
o Soma is in the same line of action of the axon
One axon, one dendrite
Found in the retina
Also found in the white mater- to maintain cerebrovascular tone.
What are the two types of multipolar neurones
Golgi type 1 and Golgi type 2
Describe Golgi type 1 multipolar neurones
Projection neurones
Influence cells located in a different part of the nervous system and so have long axons
The long axons often project small collateral branches that help to spread information further in the CNS
These are distinct from projection neurons which have connections outside the nervous system- such as afferent sensory neurons from skin receptors and efferent motor axons to muscles or glands
Highly branched dendritic trees.
Most vulnerable to degeneration from environmental or genetic stimuli- most damaged neurons in Alzheimer’s - pyramidal cells
Most metabolically active neurones.
Degeneration of Purkinje cells- ataxia
Degeneration of anterior horn cells- motor neurone
List some examples of Golgi type 1 multipolar neurons
▪ Pyramidal cells – Cerebral cortex – all of cortical output mediated through these. ▪ Purkinje cells – Cerebellum. ▪ Anterior horn cells – Spinal cord.
Describe the Golgi type 2 multipolar neurons
Local interneurons
Have shorter axons that do not leave their cell group
Provide more opportunities for cells within a group or circuit to communicate with each other
Their axons give off many collateral branches
This increases the ability of cells in a circuit to process information
Often modulatory
-highly branched dendritic trees -short axons -axons terminate quite close to cell body of origin -stellate cells of the cerebral cortex and cerebellum
Describe sensory neurons
Example - Dorsal Root Ganglia. ▪ Commonly pseudo-unipolar with one major process that divides into two branches; one running to the CNS, the other to a sensory receptor.
Describe motor neurons
Example – Spinal Motor Neurones: ▪ Conduct impulses from the CNS to effectors (muscles and glands). ▪ Generally multi-polar with large somas.
Describe interneurons
Example – Neurones inside the CNS: ▪ Neurones whose soma and processes remain inside the CNS. ▪ Can be large multipolar neurones or small bi-polar local circuit neurones. ▪ Responsible for the modification, co-ordination, integration, facilitation and inhibition that must occur between sensory input and motor output.
- responsible for modification, coordination, integration, facilitation and inhibition of sensory input.
What can disturbances in function of neuroglia give rise to
Neurodegenerative diseases- neurons can’t function without neuroglia.
Describe the neuroglia
- support cells of the nervous system - astroglia, oligodendroglia, microglia, immature progenitors, ependymal cells, Schwann cells, satellite glia - many and varied functions - essential for the correct functioning of neurons
Describe the astroglia
-multi-processed starlike shape -most numerous cell type -numerous intermediate filament bundles in cytoplasm of fibrous astroglia(GFAP) -gap junctions suggest astroglia-astroglia signalling
List the functions of the astroglia
- Scaffold for neuronal migration and axon growth during development. 2. Formation of blood-brain barrier. 3. Transport of substances from blood to neurons (important for nutrition of the brain).. 4. Segregation of neuronal processes (synapses). 5. Removal of neurotransmitters. 6. Synthesis of neurotrophic factors. 7. Neuronal-glial and glial neuronal signalling 8. Potassium ion buffering 9. Glial scar formation
What is the glia limitans
Barrier between the brain and CSF- it is a barrier consisting of layer upon layer of the foot processes of the astrocytes.
Why is it important that the astrocytes separate the synapses
To prevent the cross-firing of synapses.
If axons of different cells came into contact with one another- their signals would suffer from interference.
How can astrocytes play a role in scar formation
They can act as phagocytes
Describe the importance of the astrocytes in storing and releasing neurotransmitter
Prevents the neurotransmitters from constantly activating post-synaptic neurons and also playing a potentially adjunctive role in chemical neurotransmission.
What is the role of the astrocytes in times of high metabolic demand
To break down the glycogen that they store into glucose.
What is the difference between the end feet of astrocytes on neurons and on blood vessels
End feet on blood vessels; transport (water, K+)
End-feet on neurons; removal of neurotransmitter
Describe astrocyte interaction with blood vessels
- ordered arrangement of astrocytes with minimal overlap - each cell forms a specific territory that interfaces with microvasculature - might include thousands of synapses.
Each can contact a number of different blood vessels, and a number of different neurons, and so can supply a lot of different neurons with glucose.
Describe how different astrocytes communicate with each other
- individual astrocytes occupy their own specific domain.
- communication between cells occurs via GAP junctions (connexin43)- small pore between the cells
- function as a syncytium that allows spreading of reaction and signalling- such as Ca2+ ions
- myriad of fine processes sample the microenvironment and interact with other cell types- ‘calming’ mechanism to control the levels of ions in the brain.
What are the oligodendrocytes
The myelin forming cells of the CNS
Describe the structure of the oligodendrocytes
o TWO main types; Interfascicular and Perineuronal. o Small and spherical nuclei. o Few thin processes. o Prominent Golgi and ER (to make myelin) and no intermediate filaments. o Highly metabolically active.
When is most of the myelin in the CNS made
Between birth and 2 years (with small production until early 20s) must be maintained throughout life. Oligodendrocytes are constantly degrading proteins and making new ones to maintain the myelin/
What is a consequence of lack of oxygen and nutrients for the foetus
Makes less myelin, shows up on the MRI as hypomyelination. Reason why undernutrition can give rise to physical problems of the brain.
Describe the functions of the oligodendrocytes
-production and maintenance of the myelin sheath -each cell produces multiple sheaths (1-40)
Describe myelin
-a lipid rich insulating membrane -up to 50 lamellae (layers) -dark and light bands seen at EM level
Explain how the myelin remains in contact with the oligodendrocytes
Via foot processes (why it is such a metabolically active process to maintain myelin)
Describe the consequences of a loss of myelin
-loss of oligodendroglia and myelin has disastrous consequences -myelin disease states -Multiple Sclerosis (MS) -Adrenoleucodystrophy
Describe adrenoleukodystrophy
Congential
Deficiency of enzyme which makes proteins in myelin
Eventually not compatible with life- can survive for 15 years- end in vegetative state.
What separates microglia from macroglia in terms of their development.
Not derived from centres in the developing brain.
They are derived from monocytes in the reticuloendothelial system in the bone marrow and yolk sac.
Describe the structure of the microglia
o Derived during early development from blood monocytes that invade the brain. o Dense lysosomes, lipid droplets and residual bodies.
What are the microglia
Immune cells of the CNS
List the functions of the microglia
-derived from bone marrow during early development. -resident macrophage population of the CNS -involved in immune surveillance -present antigens to invading immune cells -first cells to react to infection or damage -role in tissue modelling -synaptic stripping
Describe the role of the microglia in plasticity
They remove synapses when they are not required.
How does the microglia remove debris
By turning into phagocytes
Describe the plasticity of the processes of the microglia
Move around all the time, highly plastic, Constanlty survey the brain for invading organisms, or changes in ion concentrations- to maintain exact homeostatic state. Becomes activated when change detected and we see this as an increase in the number of cell bodies, and the shortening of processes.
Each microglia has its own domain.
What do we see in MS
Microglia surrounding the cell bodies of neurons, don’t know whether this is protecting or damaging the neurones, probably protecting.
Describe the ependymal cells
Epithelial cells that line the ventricles of the brain and central canal of the cord.
▪ Structure: o Apical microvilli and cilia. o Prominent gap junctions between other ependymal cells. o NO tight junctions.
Describe the key difference between the PNS and CNS
PNS is phylogenetically more primitive than the CNS. Schwann cells play the role of astrocytes and microglia.
Describe schwann cells
-myelin producing cells of the PNS -each Schwann cell produces only one myelin sheath -surround unmyelinated axons -promote axon regeneratio
Why is it important that we don’t have schwann cells in the CNS
If each axon had a cell body by it, you would increase the size of the brain and spinal cord massively.
