Neurons and glia cells-Lecture 1 Flashcards
Ratio between neurons and glia cells in the human nervous system
1:1
What are neurons (nerve cells)?
Cells specialized for rapid communication and control of the body
What’s it called the network were neurons are connected
Nervous system
How is the NS divided and what is it made of?
-Central nervous syst (CNS): brain and spinal cord
-Peripheral nervous system (PNS): neurons and neuronal connections outside the CNS
What is/are the function of the NS?
-Receive information about the external world (vision, taste, odour and touch)
-Register the internal state of the body (body Tº, blood CO2 level, nutritional status)
-Integrate and process sensory information (ie to analyse and put the info into a meaningful context)
-Control voluntary and involuntary bodily functions
How does the NS communicate with cells or organs?
-Synaptic signalling (form a synapse) → extension from a neuron (axon) to the target cell.
-Neuroendocrine signalling (release hormones into blood stream)
Parts of a neuron.(morphology)
-Dendrites
-Soma
-Axon
-Axon terminal
Define dendrite
Extension from the soma. They receive connections/info from other neurons via synapses
Define soma
Cell body of a neuron, contaning the nucleus and most of the ribosomes. Integration of input.
Define axon
Projection of a nerve cell (neuron) that transduces the information towards the target cell(s) through action potentials according to the summarized input from the soma.
Axon terminals
Formation at the tip of the axon where NT are released into synapses (or blood) to transmit info to the target cell.
Do mature adult neurons undergo cell division?
No, they are post-mitotic
Which cell has de biggest dendritic tree?
Purkinje cell from cerebellum
How many axons do Purkinje cells have?
1
Define membrane potential
Membrane which experiences a different distribution of charges on oposite sides of the membrane, resulting in one being more positive and the other more negative compared to each other.
Why is membrane potential particularly important for neurons and muscle cells than the rest?
Bc in these cases it can mediate fast signals/communication
Are membrane potential and action potential the same thing? Define both terms.
No. An action potential (AP) is a fast/transiet change of membrane potential, which is propagated along the axon and result in release NT at the axon terminal. The membrane potential is the different distribution of charges on opposite sides of the membrane.
What are the usual values/value of a resting membrane potential?
Around -60mV to -70mV
In a depolarization the membrane potential becomes more negative
False, it becomes more +
In hyperpolarization the membrane potential becomes more negative
True
If the membrane potential of a neuron is not stimulated by any NT it’s called the resting membrane potential
T
An axonal action potential involves the propagation of a fast and transient change of the axonal membrane potential from the axonal terminal towards the cell body
F
If the membrane potential becomes more positive compared to the resting membrane potential the neuron is said to be hyperdepolarized
F
Properties of dendrites
-They may be more than 2 mm long
-They can branch
-They increase the neuronal surface area.
Function of dendrites
To form receptive areas on which synapses can be formed.
All neurons form dendrites
F. Exceptions include many sensory neurons, such as dorsal root ganglia cells (DRG).
What does the soma contain?
-Nucleus with the DNA
-Endoplasmatic reticulum (ER): (calcium homesotasis; ribosomes-protein synthesis)
Can synapses occur on the cell soma?
Yes
Is the axon hillock area important for the integration of all received inputs on dendrites and the soma?
Yes
How much can the size of the soma vary?
10x (from 5-100um)
Most synapses on the soma are inhibitory.
T (will be discussed later on)
How much does the summation of inputs need to sum to have an AP?
It neads to reach above -50mV at the axon hilock. It’s a yes or no response.
A neuron can onlu get one input, either excitatory or inhibitory.
F. There can be thousands of synapses on the dendrites and cell body of one neuron.
Only one axon can protrude from the soma.
T. However, it may in turn divide/branch to form axon collaterals
The axon starts at the axon hillock.
T
The length of an axon can exceed a meter
T
What is the task of the axon?
To transduce electrical impulses (AP) towards the target cell
The diameter of the axon will determine how fast the AP can go.
T. Axons of larger diameters transduce signals faster than axon of smaller diameters.
Neurons that have large somas typically have larger/longer axons and high activity.
T
All neurons have classical axons
F. Ex. rods and cones in the retina or some thalamic interneurons
The depolarization/repolarization is controlled by voltage gated sodium and potassium ion channels
T
Most axons are unmyelinated
F
What is myelin? What does it do?
Several layers of lipid membranes. They speed up the transduction of the axon potential and decrease the energy cost of transduction. Myelinated axons can be up to 10x faster than unmyelinated.
Myelin is produced on the neuron
F. It’s produced by the Schwann cells in the PNS or oligodendrocytes in the CNS
The nodes of Ranvier are not on the neuron.
F
The preterminal axon is myelinated.
F. The final segment of the axon (preterminal) is unmyelinated
How are the end of the axon terminals where synapses are formed called?
Boutons
Some myelinated neurons can have varicosities or “en passant” boutons
F. Unmyelinated neurons!
What is paracrine signalling?
It’s a non-synaptic form of signalling where NT are diffused and can affect receptors on multiple surrounding neurons. “En passant” boutons are capable of forming synapses or diffussing NT.
If an axon has “en passant” boutons it won’t have a terminal bouton.
F. They can have both
Define synapse
Point in which a nervous impulse passes from one neuron to another
What’s another name for electrical synapses?
Gap junctions
How are the gap junction channels called?
Connexons. They are built up by connexin proteins.
Electrical synapses
The membranes from 2 neurons are very close and form a direct connection between the cytoplasm of neighbouring cells via gap junction channels (connexons, a type of ion channel built up by connexin proteins)
They are considered to function as an electrical unit since changes in one membrane potential will affect the other through charge flow (=ion flow).
-Rare in neurons but important in muscles for AP.
Chemical synapse
Withe arrival of an AP a pre-synaptic neuron will release a NT into the synaptic cleft through terminal or “en passant” boutons to a post-synaptic neuron. This can cause changes in the post-synaptic cell, for ex. in the membrane potential or regulation of intracellular proteins.
Once the NT has interacted with the receptors it must be cleared from the synaptic cleft to allow another round of synaptic transmission.
Synaptic cleft
Extracellular space between pre- and post-synaptic membranes of very small volume allowing for a small nº of molecules give rise to a relatively high (uM) concentration and facilitate NT diffusion within the cleft which is very rapid (microseconds)
How do NT get cleared from the synaptic cleft?
-Taken up by nearby cells (e.g back into axon terminal or into glia cells (e.g. astrocytes))
-Degraded by enzyme
-Passibely diffusing away from the synapse
What are NT?
Chemical messenger molecule that transmits an impulse from a neuron across the synapse to a target cell
How can you identify a NT?
There are 4 classical criteria:
-It must be synthesized and stored in the pre-synaptic neuron.
-Released by the pre-synaptic neuron upon stimulation.
-Same response must be obtained when the chemical is experimentally placed on the post-synaptic target.
-A mechanism must exist for removing the chemical from its site of action once is its work is finished.
If a chemical from a synapse is released into the blood stream is called a NT
F. Is a hormone, e.g. oxytocin
How can NT be classified?
-Structure
-Effect on post-synaptic cell
What type of NT exist according to their structure?
-Small-molecule transmitters (produced by enzymes):
aa (Glu, GABA, Glycine), quaternary amines (Ach) and monoamines (Da, 5HT, NA)
-Large-molecule transmitter: neuropeptides (opioid peptides (enkephalin), peptide hormones (substance P, vasopressin, oxytocin)
What type of NT exist according to their effect?
-Excitatory transmitters which produce depolarization of the target cell: Glutamate, Substance P
-Inhibitory transmitters, hyperpolarization: GABA, Glycine
-Both (depending on receptor activated): Ach, DA (usually excitatory), 5HT ( usually inh.), NA, endorphin (usually inh.)
Which NT is/are the most importants in CNS?
GABA (hyperpol)
Glutamate (depol)
*Ach, monoamines and neuropeptides play a more modulatory role
Which NT is/are the most importants in PNS?
Ach and NA
Where are NT stored? How are they usually classified?
Synaptiv vesicles in the bouton.
-Small synaptic vesicles: for small molecule transmitters, ex. Glu, GABA, ACh. Released from small clrear synaptic vesicles.
-Large dense core vesicles: for large-molecule transmitters like neuropeptides
How are the vesicles released?
AP opens voltage-gated Ca2+ channels→fusion of the synaptic vesicle with the pre-synaptic plasma membrane . (Lect 9)
Will the extrasynaptic C of NT be lower or higher via diffusion in “en passant boutons”?
Much lower. (non-synaptic, paracrine signalling)
C : synaptic vesicle(100mM)> synaptic cleft (1mM)>extrasynaptic space (0.01-3uM). However we can act in many different neurons unlike synaptic gap.
An excitatory neurotransmitter (NT) will result in a more positive
membrane potential of the post-synaptic neuron
T
Glutamate is the main inhibitory neurotransmitter in the central nervous system
F
Where do synapses form?
Neurons, a gland or muscle cells.
On neurons: axodendritic, axosomatic or axoaxonic (usually axon hillock or presynaptic terminal)
Where do most excitatory synapses happen?
Spines (dendritic outgrowth)
Only excitatory synapses can happen on spines
No. But they’re usually the ones.
How can we distinguish an excitatory synapse
Spine + PSD (post-synaptic density, a network of proteins)
Cytoskeleton description
3 dimensional scaffold of filaments: microtubules, actin microfilaments and neurofilaments. Responsible for morphology changes.
Microtubules
Protein polymers formed by monomers of tubulin. They can elongate (+end) or shorten (-end). Important for morphology and transport (by motor proteins fuelled by ATP).
*Retograde : towards soma
Anterograde: towards axon terminal
Actin microfilaments (F-actin)
Made by polymerization of monomeric globular-actin (G-actin) proteins.
Function: maintain and stabilize plasma membr., morphology by building structures and anchoring membr. proteins for synaptic communication.
-More Dynamic than microtubules → polymerization/depol. to cell shape changes (V or shape)
Ex. actin maintains spine shape and scaffold for the PSD.
Projection neurons are usually inhibitory
F (most use gluatamte)
Interneurons are usually inhibitory
T. GABA or glycine as NT
Pyramidal cells
-In cerebral cortex
-Projection neuron: to brain or spinal cord
-Glutamatergic
Purkinje cell
-Cerebellum
-Johann Evangelist Purkinje
-Large neuron,elaborate dendritic tree
-Gabaergic
Dorsal root ganglia
-Spinal cord
-Sensory neuron (register info and pass to CNS)
-Pseudounipolar morpho.(no dendrites, cell body in dorsal root ganglion outside spinal cord
-Glutamatergic
Glial cells
-CNS and PNS
-Support health, function and communication of neurons
-In CNS: ependymal cells, olgiodendrocytes, astrocytes and microglia
-In PNS: satellite cells, schawn cells
Which are the myelinating cells?
Glia cells:
-Schwann cells in PNS
-Olgiodendrocytes in the CNS
Astrocytes
-NT re-uptake (regulates extracellular environment around neurons, clearing)
-Metabolic support to neurons
-Help maintain BBB
What is the paper of astrocytes in the BBB?
-Help maintain BBB by promoting stabilization of tight junctions formed by endothelial cells → which Restrict passage of molec./pathogens from the blood into the nervous tissue
Microglia cells
-Immune cells in the CNS, can mediate inflammatory processes
-Phagocytic function
Ependymal cells
-Production of CFS in the choroid plexus of the ventricles of the brain
CSF (cerebrospinal fluid)
-Fills ventricles and central canal in the spinal cord
-Function: trauma protection, delivery of nutrients, waste removal
*PNS lacks this protection.
Astrocytes play an important role in clearing some neurotransmitters from the synaptic cleft and thus allowing continued synaptic signaling
T
