Building Nervous System Flashcards
what are the two parts of the nervous system?
CNS and PNS
how many ways can the PNS be spit up? and what are they?
3
how they connect to CNS? cranial or spinal nerves
direction of propagation? afferent or efferent
motor neurone’s target effector? somatic or autonomic (sympathetic or parasympathetic)
why are studies of lower species e.g. rates still useful in the understanding of human brains?
this is because all vertebrates share the same basic architecture of the nervous system. there are just systematic changes in brain structures.
describe from fish to humans, the different change that happen at each stage
fish - some tubes to carry nerve from distal parts of the body to a rostral point
reptilian brain - nerves sorted into specialised modules e.g. light sensitivity = vision, chemrsensitivity = smell etc. bulge on rostral area of spinal cord, connected to the cerebellum
mammalian brain - hypothalamus allowing reaction to more stimuli. thalamus developed allowing hearing, smell and vision to work together. amygdala and hippocampus allow for crude memory and limbic system generates emotion but they aren’t able to be experienced s it is unconscious
human brain - developed 1.5 million years ago. exposure enlargement of specific areas associated with thinking, planning, organisation and communicating. a larger cortex push cerebellum to current position. skull bones used outwards forming high flat forehead and domed head
what are the planes of sections of a human brain?
sagittal
coronal
horizontal
how many lobes of the brain are there and what is the main attribute of each one?
4 frontal ; personality parietal ; abstract mathematical functions occipital ; vision temple ; hippocampus
what is the function of white and grey matter?
grey matter - which is grey due to its lack of myelin, have cell bodies which are the ‘computer zone’
white matter - a connection between cell bodies. all various computing units can talk to each other, share information, divide work up and checks that conclusions made are reasonable. sends information to specialised computing areas
what are gyri and sulci of the brain?
gyri - ridges
sulci - creases
what is the cerebellum involved in and what could result from deficiencies in it?
cerebellum is involved in: balance,movement, reading and writing
dyslexia can be due to problems with the cerebellum
describe one specialised area of the brain
Brodmann's area - it is a systematic map of the brain based on cell types. for example with speed, there are: Broca's area (44) - specialised motor area involved in the mechanical production of speech. controls the lips and the tongue Weirnickes area (22) - involved in grammatical rules for language
what are ‘hard-wired’ areas of the brain and give examples of these
hard wired areas are areas where given appropriate conditions, the cells should develop into their genetically preordained form and function.
the motor cortex is for movement and the visual cortex is for vision
why has evolution made it essential that humans have specific attributes?
vision
ability to move precisely
faculty of speech
these are all so that humans have an evolutionary advantage over rivals so that these attributes and our genes can be selected for and the species live longer
what is the explanation for attributes that have no obvious evolutionary advantages? and give an example of one
playmusic could be an evolutionary advantage as the ability to play music and the intelligence and sensitivity required to play an instrument may be attractive to the opposite sex which means that those genes armoire likely to be passed on
how do musicians differ from the rest of the population ?
there is a hook that is in the motor cortex which is an area that controls the hand. in the dominant hand, from a frontal view, this is the shape of an inverted omega in 90% of the population and the shape of a lowercase omega in 10% of the population which is in concert pianists, suzuki players etc
what is the reason for an increased size of the hook in musicians?
this is due to the neurones sprouting new connections because they must allow for the signals to get to the correct muscles at the correct time in the correct order.
define plasticity
the ability of neurones to create new connections and also destroy old connections in response to new physical demands
what makes the brain such a powerful processor?
each neurone can make 1000-10000synapes
the human brain has 10^11 neurones
thus, 10^14 connections are in the human brain
describe the overall evolution of the brain
in higher mammalian brains, the development of increasingly complex senses triggered the development of a thin layer of cells on the brain’s surface called the cortex. from which, consciousness emerges. the cortex allows for many neural connections with only a small increase in size
what are the three main features of simple cell organisms? give an example of it in action
receptiveness, responsiveness and spontaneity
eg euglena : spontaneous swimming activity, responds to light and the photons transducer by pigment are localised to the eyespot
what comes with multicellularity?
specialisation and the appearance of a nervous system
give an example of a multicellular organism and describe it
sponges
water flows in through the body wall
water flows out of them through the osculum (large hole at the top)
the flow is controlled by the beating of flagellated cells
and is regulated by the myocytes which are specialised cells tat respond to stretch - can do this becuase of their contractility
describe primordial nervous systems
there is an appearance of neurones and the first one is probably a sensorimotor cell. they span from the exterior to the effector cells
what is a hydra?
it is a multicellular organism which feed themselves using tentacles which are also used to locomote through the environment by tumbling
there is a derivation of different types of neurones from ectoderm for example motor neurones and interneurones
motor neurone - receive input from true sensory neurones and output - to effector cells and other motor neruones
interneurones lie between sensory and motor neurones and excitation or inhibition allows for more complex responses
what are the two types of worms?
segmented and non segmented
what do flatworms clustering of neurones show?
- GBCFC
- ganglionisation
- cephalisation
- bilateral symmetry
- fasiculation
- commissures
give an example of a segmented worm
annelids - fusion of longitudinal nerve cords
how many neurones anglia do c.elegans have?
302 neurones, 56 glia
what kind of nerve cords do c.elegans have?
dorsal, ventral and lateral nerve cords
where do most neurones derive from?
from AB cells that have a shared lineage with the hypodermis (skin)
describe the formation of a neurone in insects
the neurogenic region is next to the ectoderm (future skin)
there is then migration of the neurogenic region as gastrulation proceeds
individual neuroblasts delaminate and migrate inward then coalesce.
how are glia and neurones formed in drosophila ?
neuroblasts form ganglion mother cells
ganglion mother cells form neurones and glia
in drosophila, what does delamination form ?
neuroblasts
where is the nervous system on vertebrates?
it is dorsal,
what is the hypothalamus also known as? and what is it close to?
the autonomous integrating centre
it is close to the pharynx (the mouth)
what is the difference between insects and amphibians in the way that the neurogenic region develops?
the neural cells do not laminate
the neural cells stay as one layer called the neuroepithelium which can often be called the neural plate
where does the nervous system of all vertebrates arise from?
the neural plate
how do cells become neural precursors differ in insects and vertebrates?
in insects, single cells become neural precursors
in vertebrates, whole groups of cells/sheets of cells become neural precursors
where do neural precursors first form in worms, insects and vertebrates?
at the surface. they develop next to the skin on one side and next to the mesoderm on the other side
define cell differentiation
the process by which cells become different from each other and acquire specialised properties.
what is cell differentiation governed by?
changes in the gene expression, which dictate the repertoire of protein synthesised.
what can gene expression of a cell be governed by?
intrinsic (transcription factors) and extrinsic (morphogens) factors
how does the neurogenic region arise?
through inhibition of BMP signalling
describe the process of BMP inhibition and explain what the homologue for it is in insects
BMP signalling is inhibited by an antagonist - chords
BMP and chordin are proteins that have been high conserved through evolution
in insects, the BMP homologue is app and the chords homologue is short-gastrulation (sog)
what does the dpp/sog network do ?
it broadly dictates dorsal and ventral sides of the body in insects/worms. where dpp is inhibited by sog, the neurogenic region arises.
at what point does the neural plate form in insects?
the neural plate broadly forms where dpp inhibits sog
at what point does the neural plate form in vertebrates?
the neural plate broadly forms where chordin inhibits BMP (same as in insects but reverse)
what is the molecular pathway for the development of a neural plate cell (ectoderm)?
the ectoderm can form either an epidermal cell or a neuronal cell
define neurulation
when the neural plate rolls up to form the neural tube, so the vertebrate development progresses
what is SMAD 1?
part of a secondary signalling cascade
how does the neural plate cell become epidermalised?
BMP must bind to SMAD to phosphorylate it. this would then activate a pathway enabling the cell to become an epidermal keratinocyte
what happens to the fate of a neural plate cell in the absence of BMP?
BMP can’t activate SMAD1 so SMAD 1 can’t enter the nucleus. this means that a different set of transcription factors are unregulated which starts their transcription. this can cause the differentiation of a cell to a neurone.
what are the epidermalising transcription factors and what condition is needed for this to happen?
MSX1, GATA and Vent with BMP present cause an epidermis to develop
what are the neuralising transcription factors and what condition is needed for this?
XLpok2, SoxD with the absence of BMP
how has the understanding about neuralation formed?
though studies on xenopus. following fertilisation, the egg undergoes cleavages and gives rise to 3 hollow balls of cells. 3 germ layers will form called the ectoderm, mesoderm and endoderm.
how do we know that the cells in the germ layers are different form each other ?
analysation of the of the transcriptome and proteome in each part, can find 3 big categories of genes then find their proteins and do immunohistochemistry on them to see that the cells are transcribing different proteins.
what is produced when transcription factors e.g. Gse are in the ‘organiser’ ?
in the mesoderm layer, there are specialised mesodermal cells that are induced called the organiser. it expresses special transcription factors and secreted ligands including chordin
under the influence of transcription factors such as Gse, the organiser expresses unique secreted products that are all antagonists of the BMP signal
what do cells in the organiser turn into?
anterior ectoderm, prechordal mesoderm and anterior neural plate. 1 and 2 become axial mesoderm.
define neural induction
the process through which the neural plate forms when BMP antagonists prevent the BMP ligand from stimulating its receptor
what is noticed as the neural plate forms the neural tube?
development of the axial mesoderm
cells in the organiser differentiate into anterior endoderm adaxial mesoderm. these involute, intercalate and undergo convergent extension
after it is induced, dorsal mesoderm involutes and undergoes convergent extension. as it does so, it self differentiates into notochord. convergent extension alters the shape of the developing embryo
as these processes occur, the neural plate grows and elongates making the A-P axis and rolls into the neural tube
what is noticeable about developing nervous systems?
i developing nervous systems, the anterior end/prechordal mesoderm are in close contact with the pharynx and the ventral forebrain
describe the experimental proof for neural induction
in the 1920s, the organiser graft experiment was done by Spemann and Mangold. they grafted the organiser from a donor to a host newt and found that a twinned embryo developed with a complete secondary neural axis.
the secondary neural tube was host derived i.e. showing that the neural tissue is induced from the ectoderm in response to signals from the organiser tissue.
the exile mesoderm and anterior endoderm were donor derived showing that they had self-differentiated from the organiser.
is there any support foremother studies for spemann’s organiser?
Henson’s node grafted from quail embryo to chick host and also found that a new axis was induceed in the host
where does the ability to be neural inducing or the mesodermal dorsalisation come from?
from the ability to synthesise and secrete various BMP antagonists e.g. chordin, noggin, follistatin etc.
how were BMP antagonists in the organiser discovered?
by BMP antagonists having all of it’s mRNA extracted from organiser cells, reverse transcribing them to cDNA then testing each to look for a gene/protein that would mimic the organiser’s ability to induce a secondary neural plate
what experiments can be done to prove a molecule is a neural inducer?
-molecule must be expressed the organiser
-molecule must be secreted and act on adjacent cells
-over-expression of the molecule at an ectopic site should lead to induction of the secondary axis
-inhibition of activity should prevent axis formation
these types of experiment show that BMP signalling is NECESSARY for neural induction
what are the 2 types of experiments that show that BMP antagonists give neural tissue ?
- express BMP antagonist ectopically
2. dominant negative experiments with BMP receptors
what are the 2 types of experiments that show that BMP antagonists give neural tissue ?
- express BMP antagonist ectopically
2. dominant negative experiments with BMP receptors
where are the majority of the neurones in the brain?
in the cerebellum and the cortex
what are the steps in embryogenesis?
CNS PNS Midbrain Hindbrain Spinal cord
where does all of the nervous site arise from?
the neural plate
at what point in development do the main parts of the nervous system become established?
easy in embryogenesis (3-5 weeks in humans)
how do we know that all the main different parts of the nervous system begin to be established very early in embryogenesis?
as the organiser/node begins to differentiate into the axial mesoderm, it involutes and undergoes convergent extension and extends under the newly induced neural plate.
if development is stopped at this point, and look with molecular markers, can see that the neural plate is expressing markers that are aper confined to the forebrain (early neural plate=anterior in character)
this is because BMP antagonists induce neural tissue that has anterior identity. BMP antagonists are made in the early node and in the prechordal mesoderm have different characteristics, providing signals that promote proliferation and growth of neural cells and their ‘posteriorisation’
the posterior nervous system then develops as the node regresses, posteriorly. axial mesoderm laid down in its wave. it induces proliferation and growth of the neural plate/neural tube.
this idea is known as the activation transformation model
describe the main concept of the activation transformation model
-neural inducing molecule induce anterior tissue
these molecules initially come from the early node/organiser cells but are maintained in the prechordal tissue
-other signals from later node/organiser promote growth and posteriorisation
ANTERIOR AND POSTERIOR SIGNLAS ACT ANTAGONISTICALLY
what is the model basis for the activation transformation model?
- BMP inhibitors from the organiser tissue block the formation of epidermis, ventrolateral mesoderm and ventrolateral endoderm
- Wnt inhibitors in the anterior of the organiser allow the induction of head structures
- a gradient of caudal factors specify the regional expression of Hox genes
where are BMP antagonists and Wnt antagonists maintained?
BMP antagonists and Wnt antagonists are maintained anteriorly
what are the caudal factors, where are they expressed and what do they do?
FGF, Wnts and RA are expressed posteriorly and promote growth and posteriorisation
how is a core regional pattern in development established?
by placing 2 antagonistic molecules at each end of a forming structure.
how is the nervous system along the AP axis?
it is segmented along the AP axis
what are the two main models of transforming gradients?
the Lewis Wolpert ‘positional information’ / french flag model
the Alan Turing ‘reaction-diffusion’ model
in drosophila, what is segmental identity controlled by?
homeotic (hox) genes - they provide positional information for the drosophila
give 5 facts about the homeotic genes
they are transcription factors
they have a shared homeodomain box with DNA - so is a useful marker
they specify AP segment identity in dropphila
they have been evolutionary conserved
many hox genes develop through adulthood
how can you experimentally find the function of hox genes?
must KO many hox genes to find the function - because especially in humans and mice, there are many of the same gene
what was found when the KO of hox a1 and hox b1 was done?
that they are required to specify rhombomeres 4 (partially) and 5 and the nerves that are normally made from this region
what forms at the edge of the neural plate and ectoderm? what is it for?
a specialised border of cells forms at the edge of the neural plate and the ectoderm. it is crucial for neural crest formation and roof plate formation and dorsal neural tube patterning/differentiation
what are the different steps for the neural crest formation?
there are 5 steps
- an early border is established at the interface of the induced neural plate and surface ectoderm. this begins to express specific transcription factors (msx1) thought to be induced at ‘intermediate’ levels of BMP signalling
- other signals (wnts, FGFs) act together with msx1 to turn on other transcription factors (Pax 3, Zic 1, Pax 7). the combo of these tfs characterise the neural pate border cell
- Wnt signals act together with NpB tfs to up regulate further tfs (c-myc, ld, snail etc) that characters neural crest cels. many of these tfs, we know to give stem-like behaviours - proliferation and multi potency
- in response to c-myc and sox-9, genes that control proliferation and mulitpotency and survival are transcriptionally activated
- at the same time, neural crest cell tfs upregulate a further set of genes that promote epithelial-mesenchymal cell transit (EMT). neural crest cells delaminate from the border region and begin to migrate.
what are neural crest cells also known as?
the 4th germ layer
what do neural crest cells give rise to?
a number of different types of cells of the body including scwann and neurological cells and also the sympathetic and parasympathetic system in the PNS
what are the different cell types that the neural crest cells will develop dependent on?
the position of origin of neural crest cells
timing of generation of neural crest cells (determined by Hox genes)
migratory pathway and the signals they encounter en-route or at the target
what are the three main pathways of neural crest migration in a chick embryo
superficial pathway - (beneath ectoderm) form pigment cells on the skin
intermediate pathway - (via somites) form sensory glia
medial pathway - form sympathetic ganglia and cells of adrenal medulla
do all neural plate border cells for neural crest cells?
no, a few are retained at the border and form roof plate ells. these are important in the in a step if neuralation and dorsal neural the patterning
what happens to the floor and roof plate when a neural tube the is developing into a spinal cord?
a floor plate of non-neuronal cells develops along the ventral midline ad a roof plate f no neuronal cell along the dorsal line
where neuronal cells develop nearest to the floor plate?
neuronal cells whose phenotype is not known
where do future motor neurones commissars neurones differentiate ?
immediately above the floor plate, in the dorsal region, near the roof plate
what do roof plate cells up regulate?
BMPs and Wnts that diffuse into the dorsal neural tube. they induce the expression of a set of tfs (pax 6,7,2 and Lim1) that cause neural tube progenitors to acquire ‘dorsal identities’
what was thought about BMPs that come from the roof plate?
that they were morphogens that induce different types of dorsal cells.
what is actually true about BMPs and Wnts?
recently has been found that they express many different BMPs, each of which induces particular dorsal cell types.
they induce different sets of progenitor cells that will ultimately differentiate to distinct neuronal subsets in the dorsal spinal cord
what is happening to the axial mesoderm at the same time that the neural plate/early neural tube is forming?
the axial mesoderm forms also and comes to lie just below the ventral midline of the neural tube
how do neurones develop around the midline?
neurones develop around the midline, throughout the entire torso-ventral axis (D-V)
where are floor plate cells and the notochord located and what do they secrete?
they are located at the ventral midline and secrete morphogens
what is the test of if notochord and the floor plate cells diffuse
test by grafting a donor notochord or floor plate to an ectopic position is a host embryo. ask if ectopic floor plate / and ectopic neurones are induced. ectopic floor plate and ventral neurones such as motor errors, induced after a graft of the notochord/floor plate
what is the secreted factor made by the notochord and floor plate?
hedgehog which was the name given as the fruit lies look curled up and spiky.
what is the hedgehog gene?
a factor that has been conserved in evolution, that has a homologue in drosophila.
in the 1980s, what did researchers realise about the conservation of hedgehog?
that hedgehog and shh were conserved between invertebrates and vertebrates
where is Shh mRNA expressed?
in the notochord and then the floor plate
is Shh protein or mRNA detected more widely?
Shh protein. this is because it appears in a gradient as it diffuses through the ventral neural tube from ventral to dorsal
what was the 1st evidence that Shh has a diffusion gradient and can therefore be detected more easily then Shh mRNA?
when an antibody was made against the Shh protein, it was shown to diffuse away from the notochord and floor plate, forming a concentration gradient the ventral part of the neural tube. Shh comes from the notochord and the floor plate and Shh signalling induces expression of transcription factors in progenitors cells. these transcription factors confer ‘ventral’ neural tube identities, so the progenitors will ultimately give rise to cells that differentiate into ventral neurons.
how can an ectopic floor plate and ventral neurones be induced?
An ectopic floor plate and ventral neurons are induced after implantation of a shh soaked bead. This is because the bead soaked in purified Shh protein can mimic the effect of notochord/floor plate
the transcription factor ‘code’ dictates later differentiation
as cells (nerves) differentiate, they move laterally
what is the name of the transcription factor that dictates later differentiation?
code
what direction do cells move as they differentiate?
they move laterally
what is the action of Shh at an early stage ?
it acts to confer a DV pattern of transcription factors on progenitor cells
what is the job of tfs upstream?
the transcription factors are the upstream ‘master’ regulators of particular neuronal fate/identity
what patterns the DV axis?
OPPOSING gradients of BMPs and Shh PATTERN THE DV AXIS
in what conditions do cells change there fate?
in accordance with their position in the Cartesian grid. this means that different types of neurones differentiate at the same D-V position along different parts of the AP axis
give 4 examples of what Shh induces in different parts of the body
-forebrain, it induces hypothalamic neurones
-midbrain, it induces dopaminergic neurones
in the hindbrain, it induces serotonergic neurones
-spinal cord and everywhere else, it induces motor neurones
what does the neural tube initially look like and what changes during dark neural tube development?
initially, the neural tube is a single layered neuroepithelium. during early stages neural tube development, some cells continue to ‘span’ the width of the neural tube. their nuclei migrate back and forth at different stages the mitotic cycle.
what are cell that derive from the stem-like neuroepithelium called?
radial glia. they are thought to be the later neural stem cells
where does the division of radial glia occur? and what is the product of this?
in the lumen. it can divide to form 2 radial glial daughters or a radial glia and a proliferating daughter progenitor - depending on the plane of division.
where do the proliferating daughter progenitors and differentiated cells move into after division?
proliferating progenitors move to the adjacent ,mantle zone (largely picked through various tfs e.g. Nkxx, Pax etc)
differentiated cells move away into the marginal zone
what is the difference the amount of neurones that can be produced in proneural vs neurogenic mutants?
mutants lacking proneual activity cannot form neurones but neurogenic mutants form too many neuroblasts
what is the role of notch and what occurs in its mutants?
it controls the number of cells that from neurones. as it is required to prevent a cell from becoming a neuron. in notch mutants, too many neurones are made.
what happens to a cell suppressing notch?
it will become a neuron
how does notch work?
locally as it is a juxtacrine signal. it woks on adjacent cells
what happens in a fly where there are cutters of cells that have activated pro neural genes
these cells are now competent to become neurones - although only a few actually do
what does notch regulate?
lateral inhibition
what does notch regulation include?
involves the transmission of an inhibitory signal between a pair/cluster of cells to prevent cells that receive the signal from adopting a particular cell fate
what is notch and deltas link?
notch is the receptor for the delta signal
describe the steps of lateral inhibition
initially, both cells are equally capable of making and receiving the inhibitory signal.
subsequently, a change or bias is introduced - perhaps randomly- so that 1 cell begins to make more inhibitory signal
consequently, the second cell receives more inhibitory signal and becomes inhibited.
what is needed to stabilise the change of lateral inhibition?
the inhibited cell must be prevented from continuing to send the inhibitory signal
what is the importance of balance in notch signalling?
in a pro neural region, notch signalling is balances. a slight imbalance develops and it is then quickly amplified, leading to the development of a neuronal precursor.
what is special about the progenitors that don’t differentiate?
they have a certain shape
they exist as radial glia
they provide a pool of undifferentiated cells that are used to build up the nervous system over time in embryogenesis
what does the daughter cell of radial glial cells do when it is migrating away?
they use the scaffold provided by its sister to migrate away from the ventricular zone
what is the arrangement of the motor system?
its arranged hierarchically
what are the different layers of the motor system hierarchy?
- highest level :primary motor cortex
- middle level: brainstem
- lowest level: spinal cord
what is the projection of the primary motor cortex? what does it regulate?
projects directly into the spinal cord via the corticospinal tract
regulates the motor tracts that originate in the brainstem
what is the pathway of the brainstem and what does it control?
lateral descending system controls distal limbs
what does the spinal cord modulate and why can it do this?
modulates reflexes automatisms such as walking as well as many other actions. can do this because it contains neuronal circuits.
what neurones have the simplest reflex?
the monosynaptic sensory neurone and motor neurones
what is the majority of reflexes in the body ?
polysynaptic with interneurones
what is the pathway of the brainstem and spinal cord? what do they control?
brainstem = lateral pathway that controls the hands and arms
spinal cord = direct pathway to the muscles and reflexes
what happens if you artificially stimulate the spinal cord,brain stem and the spinal cord?
you would only get twitches [from the speech motor areas (e.g. brooks area) and simple vowel sounds]
what is the motor acton of the basal ganglia and the cerebellum?
they receive information from many different areas if the cortex and project to the motor cortex via the thalamus
they are aware of the situation a person is and they monitor commands going down to the muscles to make sure that they are appropriate
where does the basal ganglia feedback to?
the basal ganglia only feeds back to the motor cortex - subcortical loop
where does the cerebellum feedback to ?
the cerebellum mainly feeds back to the motor cortex but can send its signals down to the brainstem, spinal cord and onto the muscles - direct output onto the spinal cord and is an internal subcortical loop
what happens if you stimulate different areas of the frontal lobes?
it produces movements on the opposite sides of the body
what was done in the mid 20th century to identify motor effects in the frontal lobes?
electrical stimulation was done to identify specific motor effects pf discrete sites in the frontal lobes in different species- especially in humans and primates
(wilder penfreid 1891-1976)
what are association areas in the brain for?
they are to support the primary areas of the brain
what was found about brodmanns area in terms about stimulation eliciting movement?
brodmann’s area 4 was found to be an area in which the lowest intestate stimulation elicited movement - in the primary motor cortex
where is the brodmann’s area located ?
in the primary motor cortex which is located just before the central sulcus (fissure of Rolando)
what is the name of the motor cortex first neurones?
the upper motor neurones (UMN) which carry the motor commands own through the brain, brainstem and the spinal cord
where does the spinal cord originate from?
not the motor cortex as it is from old areas
where is the output of the UMN?
output to lower MN via interneurones
what is the role of UMN?
planning, initiating and directing movements
where do other UMN originate ?
other UMN originate in phylogenetically ancient motor centres of the brainstem - red and vestibular nuclei, superior colliculus reticular formation - regulate
how many UMN pathways are there are what are they?
what is common about them?
two pathways: direct and indirect
direct motor pathway - input to lower MNs from axons extending directly from cerebral cortex
indirect motor pathway - input to LMN from motor centres in brainstem
they both connect to the LMN
why do the UMN pathways both connect to the LMN?
because the LMN have a final common pathway
what does the basal ganglia do? and what does it connect to?
they provide input to the UMN and connects with the motor cortex
what is the role of the basal ganglia?
- helps initiate and terminate movements
- suppresses unwanted movements
- establishes normal level of toe in muscles
what is a disease in which the basal ganglia’s inhibited?
Giles de la Tourettes syndrome - they are unable to terminate movements that the rest of us can
what is the primary role of the cerebellum?
to step in and correct movement if it is not intended.
controls the activity of the UMN
how does the cerebellum connect to the cortex?
connects to the cortex via the thalamus and the brainstem
how does the cerebellum correct movement that is not intended?
it monitors movement for differences in intended and actual movements and if there are any discrepancies, it sends an error signal and tries to reduce the discrepancy
where is the cerebellum located?
at the base of the brain
what is the appearance of the cerebellum?
tight, intricate folds
what 3 things did the ‘Basic Circuitry’ 1967 describe?
the structure of each of the cell types in the cerebellum
their synapse connections
their electrophysiology
what is the name of the 4 parts of the cerebellum?
mossy fibre input
granule cells
purkinje cells
climbing fibres
what is the mossy fibre input part of and where is it from?
part of the WM underlying the cortex
from many different regions of the brain and spinal cord
what do mossy fibres synapse onto?
granule cells in the bottom (granular) layer
what is the proportion of mossy fibre input to granule cells?
granule cells outnumber mossy fibres 50:1
where do granule cells send their axons to?
to the top (molecular) layer where they split into two to produce parallel fibres
what do the parallel fibres of mossy fibres form?
synapses with purkinje cells
what is the name of the largest cel in the cerebellar cortex?
purkinje fibres
where are the cell bodies of purkinje cells?
in the middle (purkinje) layer
what is the purkinje cells main role?
to be the sole output cells of the cerebellar cortex
how many parallel fibre synapses do purkinje cells receive?
around 150,000
where do climbing fibres send their inputs?
a single climbing fibre input goes onto each purkinje cell
what are climbing fibres?
they are the axons of cellist the interior olive, at the base of the brainstem
what do the climbing fibres wrap around? and how many synapses does it form?
the purkinje cell dendritic tree
forms around 1000 synapses - all with the same input signal
what is a simple spike in terms of PC firing ?
it is when the PC fires spontaneously of usually about 5o spike/second
what can improve the firing rate of the PC cells?
parallel-fibre input can increase this rate to >200 spikes/sec
what is a complex spike?
very unusual shape
produced by climbing fibre input
why are complex spikes considered reliable?
because whenever climbing fibres fire, PC also fires
is there an increased or decreased frequency of firing in terms of complex spikes compared to simple spikes?
there is a decreased frequency of firing compared with simple spikes, so has a little effect on output
how does mossy fibre input contrast with climbing fibre input?
they are all wrapped around PC dendrites
acts as one enormous synapse
low frequency of firing
what do unusual circuits do for clinical and experimental observations?
for clinical and experimental observations of cerebellar dagga, can see that it does not cause paralysis but makes movements inaccurate, slow and un coordinated
-similar to the effects of alcohol
what is important to note about the cerebellums influence on moving the body?
cerebellar damage does not lead to paralysis
what is the importance of the fact that cerebellar damage does not lead to paralysis?
this means that other parts of the brain issue movements commands and may carry them out inaccurately
what s the role both the cerebellum?
to ensure movements are carried out properly
what was Brindley’s suggestion?
that the purpose of the cerebellum is to learn motor skills so that when they have been learned, a simple incomplete message can be sent from the cerebrum and will suffice to provoke the execution of the movement
what are the two models that have been made to explain the cerebellums role?
the Marr-Albus model
what does the Murr-Albus model suggest?
that to learn to make accurate movements, you must have information about what you did wrong an error signal
what component of the cerebellum conveys an error signal?
the climbing fibres
what do complex fibres change?
synaptic weight
what do complex fibres affect on the PC dendritic tree?
all the parallel-fibre synapses
what was the rule suggested to explain how complex fibres affect synaptic weight ?
synaptic weight s changed according to the correlation between the parallel fibres signal and the error signal conveyed by the climbing fibres
what is the suggested learning rule?
the de-correlation learning tule states that:
if there is a positive correlation between the parallel fibres signal and the error signal, the synaptic weight would be low
if there is a negative correlation between the parallel fibres signal and the error signal, the synaptic weight would be high
when does ‘learning’ stop?
when there is no longer a correlation between any parallel-fibre signal and the climbing fibre signal
where is there support for the de-correlation learning rule? explain it
in long term depression as a paired stimulation of parallel fibres with stimulation of climbing fibres, synapses with parallel fibres and PC become depressed.
this is consistent with the error signal idea which was that signals are positively correlated with climbing-fibre signals
what is the de-correlation learning rule a version of?
the mean square learning rule and thus is computationally powerful
what is the mean squared learning rule used mostly in?
widely used in signal processing( e.g. biomedical engineering, seismology etc)
what does the mean square learning rule support?
the cerebellum implements supervised learning
what kind of tests have been used to examine the proposed role of the cerebellum in motor learning?
simple motor learning tasks
what is the name of the reflex that is sed to stabilise eye movements?
the vestibule-ocular reflex (VOR)
describe the VOR
when the head rotates, the eyes move in the opposite direction. this keeps the visual world stable and can be mimicked by looking at a target and moving your head
why is VOR so important?
there is a problem when the eye moves relative to the world, the image moves across the retina which blurs vision. this means that visual information is irretrievably lost. this could be bad as you may not be able to see your predator/prey
when do we use VOR?
when we walk
our heads move and down
VOR counter-rotates eyes to keep visual image from moving
what are the learned changes in VOR reflex in monkeys?
eyes initially move too far but after some experience and gain of the VOR, it rests and the eyes move an appropriate distance in relation to head movements – compensating for the altered size of the visual image
what part of the cerebellar cortex is involved in VOR adaptation?
the flocculus
inactivation of this region prevents VOR adaptation
what things is the cerebellum thought to be involved in?
active sensing, sensory prediction and emotional&cognitive processing
what is the basic microcircuitry of the cortex like?
fairly uniform her the entire cerebellum but not identical. this suggests the there is a multiplicity of functions must arise from external connectivity cerebellar zones
how is the external wiring of the cerebellum connected to project to a unique set of neural targets?
PCs in a given parasagittal strip of cortex receive climbing fibre input from a unique region of the inferior olive
PCs in a given parasagittal strip of cortex project to a unique region of the deep cerebellar nuclei
this in turn project to a unique set of neural targets
what can be used to predict the sensory effects of the brain?
the de-correlation learning rule
why is it not possible to tickle yourself?
because the attenuating signal to tickle comes form the cerebellum and so the cerebellum is what is used to predict the sensory effects of tickling movements - then you would know about it therefore not feel it
how to predict the effects of head movements?
cerebellar tuut neurones have been found that respond to passive head movements but don’t respond to very similar active head movements. this can also be used
how can being able to predict own head movements be an advantage ?
be able to distinguish your head movements from movement thats coming from elsewhere
ability to develop a self identity
what is the most basic circuit in the cerebellar cortex?
input=mossy fibres. these synapse onto granule cells, whose axons synapse onto PC. PC provide the output of cerebellar cortex. PCs also receive climbing fibre inputs, which behave in a very unusual way
what does the basic circuitry of the cerebellum do?
climbing fibres copy error signals about movement inaccuracy. this adjusts the weight of the synapses between parallel fibres and PCs, thus altering PC output in a way that makes movements more accurate.
evidence from the adaptation of the VOR consistent with the theory
what are the external connections of cerebellar cortex?
each small region of cerebellar cortex has unique inputs and outputs. the cerebellum can influence a very wide range of neural structures, from spinal cord to many areas of cerebral cortex
how can the external connections of cerebellar cortex be used for sensory and cognitive functions recently proposed for the cerebellum?
the learning rule proposed for ensuring accurate movements can also be applied to other sensory and cognitive tasks, including the ability to predict the sensory effects of ones own movements
define proprioception
the ability to sense stimuli arising within the body regarding position, motion and equilibrium. even with a blind fold, he/she knows through proprioception if an arm is above the head/hanging by the side of the body
what does muscle control require?
excitation of a muscle by alpha motor neurones but also continued feedback of information from each muscle to the nervous system at each instant.
what information is sent to the brain when muscle control is requires?
muscle length which comes from the spindle
muscle tension and the rate of change of muscle tension development
where is information sent from motor neurones going to?
to spinal cord cerebellum and cortex
what creates action potentials in the axon?
Group 1 alpha terminal endings are anulospiral and wrapped the equator of both bag and nuclear chain intrafusal muscle fibres.
Pulling apart of these coils that initiates action potentials in the axon
what information does group 1a sensory endings relay?
Group 1 alpha sensory endings relay information on the dynamic phase of the muscle stretch
describe group 2 endings and what information do the sensory endings relay?
Group 2 ending have a ‘flower spray’ terminal ending embedded in the equatorial regions of the bag fibres – but have anulospiral endings largely confined to nuclear chain fibres
Group 2 sensory ending relay information on the static phase of the muscle stretch ie its final stretch
at what stage during the stretch is the only time that endings respond? when are they free?
Endings only respond during dynamic phase of stretch
Only free after final stretch is reach
what useful information does the CNS receive by viewing group 1 and 2 muscles together?
By viewing them together, CNS receives useful information on muscle activity:
Group 1: rapid burst of AP when muscle is stetching. Not much at static stretch
Group 2: steady state fire rapid Aps. Not much when dynamic stretch is happening
what is the problem with muscle spindles if there is no stretch?
they will be useless as Muscle spindles are only useful when they are under tension
If there is no stretch- no info will go to the brain
How can the system be ore fine-tuned/sensitive when a skilled movement is about to be performed? And less sensitive when not required?
If the itrafusal fibres are exposed to MAOs, (5HT), there will be an increased firing of APs in the gamma motor neurones. This means that the brain is more respondent to that one
If the brain is exposed to NA, there will be less APs fired by the gamma motorneurones thereforethe brain isn’t sent much info on these
what happens to intrafusal fibres if they are stimulated with 5HT?
By slightly stimulating gamma motorneurones with 5HT: intrafusal fibres can be made slightly stiffer and therefore transmit stretches with greater fidelity
what happens to intrafusal fibres if you inhibit gamma motoneurons with NS?
they will be slightly floppier and therefore elastic materials don’t transmit and stretches well, because they stretch
Axons in the spinal cord spray muscle spindle intrafusal fibres on structures that we don’t need. Less muscle firing is taken to the brain as we don’t need this information.
How can we witness muscle spindles working?
By looking at reflexes
describe the tendon jerk reflex
5
This is a unique reflex because it’s the only monosynaptic reflex
Muscle is stretched by a hammer blow in its tendon
Primary sensory ending (group 1a) are activated and send Aps along the limb to the spinal cord
It has a single synapse with homonymous muscles alpha-motorneurones and produces action potentials in it
The muscle contracts in opposition to the stretch and limb jerks as a consequence
describe the tendon reflex and jendrasski’s manoeuvre
4
the ‘spill over ecitation’ also effects the gamma motoneurones pool which make the intrafusal fibres ‘stiffer’ and make them more sensitive
- -jendrassik’s manoeuvre causes excitation in upper segments of the spinal cord
- -excitation spills over to the rest of the spinal cord
- -alpha-motorneurones that supply (L1-5) thus brought slightly toward the threshold and so more excitable.
describe the tonic vibration reflex
7
• group 2 afferents fire at abut 50hz when arm is straight
• group 2 afferents fire at about 20hz when fully flexed
• vibrator applied to tendon ‘drives’ the group 2 afferents in muscle
so they discharge at 100hz ie the vibrator frequency
- sensory and motor cortex says arm is bent by X amount for brain to register (measured in frequency)
- with vibrator, brain registers the vibration so it thinks that the arm is very stretched and so works to try and bend it
- reflex works better in females than in males
describe the stretch reflex
8
- most studied spinal reflex-contraction that opposes the lengthening of a muscle
- Sherrington (‘father of neuroscience’) showed that the reflex required sensory input from the muscle and a return path to the muscle
- Used a new and novel model – the decerebrate cat – surgical transection at the level of the midbrain, between superior and inferior colluculi
- Spindle 1a makes excitatory connections on homonymous muscle (muscle and spindle is in 1) and the synergistic muscles (those that ‘help’ the main muscle contraction)
- Ia’s also act through inhibitory interneurons that innervate antagonistic muscles
- When the muscle is stretched, the Ia firing rate increases
- This causes contraction of the homonymous muscle and synergists and relaxation of antagonistic muscles
- The reflex therefore tends to counteract the stretch enhancing the springiness of the muscle
describe the somatosensory system- where info is processed
what is the central pathway known as?
• Some of proprioceptive information is processed at the level of the spinal cord
• Higher level processing occurs in somatosensory cortex (as is touch info)
Central pathway is known as DORSAL COLUMN – MEDIAL LEMNISCAL
what are the three synaptic relays that are between the periphery and cortex that are in main pathways?
- Central processes of dorsal root ganglia cells synapse on neurones in gracile and cuneate nuclei in lower medulla
- Axons from these nuclei ascend in medial lemniscus and synapse on neurones in ventral posterior lateral nucleus of the thalamus
- Neurones of lateral muscles send axons to primary somatosensory cortex
where are the primary and secondary somatosensory cortices?
are in the anterior parietal lobe and the posterior parietal cortex. S2 is deep within lateral sulcus.
what does lesioning S1 produce?
Lesioning S1 produces proprioceptive deficits (and ability to discriminate size, texture and shape of objects)
where is S1 and what areas does it correspond to?
S1 = in the post central gyrus – corresponding to the Brodmann’s area 1,2 and 3 – and contains 4 subfields area 1,2,3a and 3b
why is body representation distorted in the primary somatosensory cortex (S1)?
Body representation in the primary somatosensory cortex (s1) is distorted (homunculus) in which some body parts are over represented (hand, face) whereas other body parts are underrepresented (trunk, leg). This is due to discriminative properties that are more prominent in some territories (hand, face) then others
how many neurones does the brain have and how many connections can the brain make?
brain has 10^11 neurones and van all make >10^3 connections so there are potentially 10^14 connections in the brain
what are the two theories of specific neuronal activity in the adult organism? who was right?
Weiss (1928) - Resonance theory
Sperry (1939) - Chemoaffinity theory
what was Weiis’s theory of resonance ?
stochastic (random) and diffuse neuronal outgrowth occurs to all targets followed by elimination of non-functional connections
what was Sperry’s hypothesis?
that it was directed and specific outgrown that occurs through axons following individual identification tags carried by cells and fibres of the embryo
what is the tectum known as in a mammal?
Super colliculus
describe Sperry’s experiment
cut optic nerve and removal retina. allows just basal axons to grow back. importantly, the regrowing ‘red’ axons grew through yet ignored the territory normally innervated by the blue axon. this shows that Sperry was right as the axons grew back direct into their right place. however this was done during regeneration
what would be expected if the other theory of axon guidance was right ? what is actually observed?
would expect to see random patterns of axons in the embryo but instead, see that patterns of axon growth in the embryo are highly organised, reproducible and stereotyped
what are guidance cues?
environmental factors that axons can use to find their correct target
what is the anatomy of a growth cone?
the growing tip of the axon cajal proposed to be able to sense cues from within the environment
in what organism were easy experiments to identify location of guidance cues done? and why?
on insects
- relatively simple nervous system
- embryo are easy to observe and manipulate
- in the larger insects (e.g. grasshoppers), individual cells may be ablated using lasers
what was seen from a detailed analysis of a grasshopper?
identification of almost every neurone in the embryonic nerve cord allowing a map of axon projections to be made
what does the reproducibility of axon behaviour show about growth cones?
that growth cones were responding to cues or labels in the environment.
in what situations do pathways in axon guidance seem to change?
when specific axons are encountered
describe what was fund about G axons and A axons during ablation of P axons
G axon stalls in absence of P axon. not due to lack of axons on which to extend as A axons was still present
not due to reduction in numbers of axons as this was not affected when A axons were analysed.
this shows that it just be because the G axon growth cone is looking for specific cells on the OP axon - this follows the labelled pathway hypothesis
what is the labelled pathway hypothesis?
anxious can selectively fasciculate with other axons
axon surfaces carry labels/cues
different axon growth cones express different sets of receptors for such cues
early axons from an axon scaffold on which later axons can extend
establishes axon surfaces a one potential source of guidance cues
axon scaffold are important in vertebrates aswell
give an example of an experiment where axon scaffolds were important in vertebrates
Ghosh and Shatz (1993): development in subplot neurones
subplot neurones project from the core to the thalamus riot to innervation boy LGN neurones
if you ablate the part f the subplot early on, before axons extend, then LGN innervations fails in th ablated region
what kind of paths do pioneers follow?
eg in grasshopper embryo limb
stereotyped paths and growth cones appear to react at specific points in the pathway e.g. in the grasshopper embryo limb, the pioneer Ti1 growth cones make a specific turn at the limbboundar and then again as it approaches a specific cell, CX1.
ablation of CX1 causes the Ti1 growth cone to sell at the other side of the limb boundary
implies that there must be a molecular difference in the environment
what can CX1 be referred to as?
a ‘stepping-stone’ or a ‘guidepost’ cell
what dos patterning information from the early embryo predict?
give three examples in the forebrain, spinal cord and hind brain
that axon tracts will from
forebrain ; axons follow boundaries of domains of patterning gene expression
spinal cord = axons are attached to and follow boundaries in the floor plate
hind brain ; axons follow boundaries of rhombomeres
how many different ways can guidance cues act?
give 2 examples
contact attraction - cell ablations lead to growth cone stalls as if an attractive force has been lost
contact repellant - avoid the limb boundary as if it were inhibitory
chemoattractant
chemorepulsion
what is the anatomy of a growth cone?
3 domains:
Central : microtubules
Traditional : microtubules come into f actin zones
Peripheral : f actin
what are the different kinds of f actin that makes up lamella and filopodia? are they stuck down?
in lamella, the actin bundles are cross linked into a net
in filopodia, the actin bundles are polarised to form larger bundles
neither are stuck down - highly motile
what is the movement in the central area of a growth cone compared to other domains like?
very random in the centre
very organised in the peripheral as there is a continuos flow of f actin from the outside to inside which are recycled which is called the f actin treadmill
what is the f actin treadmill?
a continual flow of f actin from the outside to inside which are recycled
what happens to tubulin in resting growth cones? does this change in any coiditon?
it is dragged sporadically into the filopodia. this happens more dramatically in the presence of an attractive cue
what happens when growth cones come into contact with attractive cues (bead)?
f actin slows and f actin accumulates
this accumulation drags microtubules into the back of the filopodia
what would happen to the growth cone if the attractive cue bead was immobile?
the growth cone would reorganise its microtubules completely establishing a new growth direction - the lamella and filopodia reorganise to seek out a new attractive cue
can growth cones only be attracted?
no they can also be repelled. this was discovered when mixtures of neurones in culture were found to fasciculate only with their one kind. however, not all due to attractive forces. active repulsion as well as adhesion. the connection is maintained due to signals going onto growth cones and telling it not to join
what does dropping a soluble repellent factor into growth cones show? compare this to if there was no reagent
soluble repellent factors dropped into growth cones show that there is less F actin present
no reagent = normal growth
Expel repellent factor = collapse of the growth cone
what are semaphorins? what is their action?
a family of inhibitory guidance cues
secreted/soluble semaphoring can cause growth cones to turn
collapsing effect is primarily on F-actin
how were semaphorins first identified?
biochemical purification of the facts from the retina is responsible for causing the collapse of sensory axons led to the identification of a family of inhibitory molecules now called semaphoring
what is the relationship between the strength of adhesion and amount of axon growth? give examples
there is no relationship
eg
adhesion ; laminin
what kind of substrates do growth cones need?
ones that are permissive for growth, not just have the ability to attach
give an example of a permissive substrate for growth cones and describe is action
laminin, a growth promoting ECM protein which is localised in the optic nerve
laminin does not dictate the direction of axon growth, just gives ‘permission’ for it to grow there
what evidence is there to suggest that laminin is just a permissive factor and not an instructive one?
- blockade of receptors for laminin slows down the growth of retinal axons, but doesn’t change their direction
- gradients of laminin in vitro don’t direct axon growth - in fact it was found that laminin is permissive for growth only when at a specific concentration range. therefore laminin is permissive but not instructive
what are permissive and non-permissive substrates also known as?
contact attractants and contact repellants (respectively)
what factors can channel axon growth? give an example of it at work
non-permissive factors where axons miss limb-body boundary and so grow abnormally.
in mice lacking sema3a, see axons straying into wong territories
what is axon growth a balance between?
permissive and non-permissive factors
what is an ephrin?
a non-permissive factor that is used in the early patterning and to guide axons
what is the name the ephrin and what do they cause? receptor?
Ephs. they cause repulsion between cells which early on, help to compartmentalise the embryo into discrete domains, later they are also used to keep axons out of specific areas in ways similar to semaphorins
also used in a special way to create topographic maps
they also have a remarkable reciprocal pattern of expression in the mammalian embryo
give 3 facts about growth cones
need permissive substrate to grow
can be kept out of regions of the embryos by non-permissive factors
can be channelled by combinations of permissive and non permissive factors
what are commissural axons repelled by?
BMPs which are made y the roof plate
what is the role of BMPs in axon guidance?
they take part in determining which types of neurones are specified in the spinal cord. at a later stage, they playa role in pattering agon pathways
what does purified BMP7 do to commissural growth cones?
they cause commissural collapse and cells expressing it mimic the repulsion of the roof plate. thus, molecules that were used early on to pattern where specific neurones were born but also used later to guide axons - this is also true for Shh
what is netrin and what is it made by?
a secreted protein, similar to laminin which can associate with the ECM. cells expressing the netrin gene can turn commissural axons. made by the floor plate
what is expressed in the midline f the vertebrate nervous system - led to by biochemical purification
cloning of the gene encoding floor plate chemoattractant protein
do all axons need entrain to reach the floor plate?
no, some axons can reach the floor plate without netrin
what does cyclopamine block and what does Smo require?
cyclopamine blocks SHH signalling and Smo is required for SHH signalling. this shows that there are two different pathways leading to the turning of commissures
how are gradients of morphogens re-used to shape axon paths?
early patterning information is used to guide pioneer axons.
chemoattractant and chemorepulsive molecules work together to guide commissural axons to their initial target, the floor plate
what do SHHH and BMPs specify and co-operate with?
they specify neural fate and co-operate with entrain to guide commissural axons
what do SHH and BMPs specify and co-operate with?
they specify neural fate and co-operate with entrain to guide commissural axons
what is a short-long range guidance cue
sema1 acts as a short range cue and blocking of it’s function (with antibodies) leads to axons straying into the wrong areas.
blocking sema2 function also disrupts Ti1 guidance in a way that the gradient of secreted sema2 directs the Ti1 growth cone towards the body
thus the combo of 4 guidance axons at different stages in their pathways are used in axon guidance
what is growth cone navigation broken into stages separated by?
intermediate targets and choice points
what are guidance cues used in combinations useful for?
guidane cues used in combinations would generate man more than the theoretical 20000 possible cues
what could be the problem if the sensitivity of growth cones to a specific cue change as intermediate targets are encountered?
if the sensitivity of a growth cone to a specific cue can change as intermediate targets are encountered, a guidance cue may be responded to in one part of a pathway but ignored in the next
is there any evidence that growth cones are reprogrammed?
axons reprogram when intermediate targets rare encountered. after the transit of the midline,commissural axons lose responsiveness to netrins
axons exposed to ectopic floor plate only after crossing the midline is no longer respondent
this shows that the sensitivity of axons to floor plate only after crossing the midline is crossed
explains, in the hindbrain, commissural axons are able t continue past the floor plate without turning but in the spinal cord, commissural axons turn after crossing the floor plate
what are fluorescent dyes useful for tracing?
lineage tracing, membrane tracking and axon tracing
at what point do commissural axons become sensitive to repellants?
after crossing the floor plate
i.e. after crossing the midline, commissural axons become sensitive to something inhibitory in the floor plae
why don’t the commissural axons go straight n after crossing the hindbrain?
the inhibitory molecules in the floor plate are semaphoric and proteins called slits
as well as being expressed in the floor plate, they’re also expressed in the ventral spinal cord - creating a pathway through which commissural axons grow
thus initially these axons must be sensitive to netrin, but not sensitive to the inhibitors. once across the FP, the sensitivities must switch
why don’t the commissural axons go straight on after crossing the hindbrain?
the inhibitory molecules in the floor plate are semaphoric and proteins called slits
as well as being expressed in the floor plate, they’re also expressed in the ventral spinal cord - creating a pathway through which commissural axons grow
thus initially these axons must be sensitive to netrin, but not sensitive to the inhibitors. once across the FP, the sensitivities must switch
how do growth cone sensitivities get reprogrammed?
the roundabout gene encodes receptor for the inhibitory protein SLIT. high levels of the Robo gene are in axons that do not cross the midline. Comm is at low levels but high levels after crossing. over expression of Robo can lead to the appearance of no comm and so axons only grow straight. over expression of Comm looks like there is no Robo and so axons move back and ooh across the midline as the SLIT protein isn’t being detected by Robo and so this is not stopped. Comm is in control of Robo as Robo acts if Comm is not working
what does the roundabout gene encode for?
when do robo and commissural axons express the most?
encode a receptor for the inhibitory protein SLIT. Robo protein is expressed high levels on axons that do not cross the midline
commissures axons initially express low levels but high levels after they cross.
give an example of a diffusible attractant and a cell surface repellant
diffusible attractant is netrin
cell surface repellant is SLIT
what us the role of Comm in terms of Robo
it is a trafficking molecule - stops Robo from reaching the cell surface meaning that growth cones can’t be influenced by the inhibitory signal from SLIT
what is the appearance of a cell in which Comm is overexpressed/ forced on the neruones
it will resemble a Robo mutant as the axons will be going across the midline, back and forth
what is the vertebrate homolog for Robo and Comm
Robots vertebrate homolog is Robo1but its expressed both before and after crossing the midline
Comm doesn’t have a homolog vertebrate but there is Rig1 which is expressed before the axon crosses the midline and can block then expression of Robo1 signalling until after the midline has been crossed
what happens if there is a loss of Rig1?
there is no commissural axons that reach the midline
how can an axon scaffold be put onto an axon?
by homophobic binding done by cell adhesion molecules (CAMs) fr example Fas2 in insects
what would happen if there is an attempt to adhere a molecule to one that doesn’t usuallyy adhere to others?
aggregation
what could a fas2 mutant or over expression of fas 2 cause?
fas 2 mutant would mean no fas2 and so the axons would be defaciculated
over expression of fas 2 would lead to novel fasciculation
what are CAMs regulated by?
other proteins e.g. BEATS which interfere with CAM-mediated adhesion
what is getting off a scaffold on axons controlled by?
Fas2 adhesion
over expression of fas2 here would mean that the axons would miss their targets ad fail to fasciculate
also BEATS would interfere
what are the two types of target selection for axons and scaffolds?
discrete targets e.g. 'cellular' in DNS
topographic maps ('multicellular')describe the process of target selection by discrete targets using ablation, netrins and fas3
in grasshopper and drosophila, ablating specific target muscle, motor axons don’t leave the true at appropriate times. this suggests that axons are looking for specific labels on their targets.
the use of netrins does the exact same thing as the axons wonder off and don’t go the right way to their targets
using ectopic netrins causes the axons to innervate wrong muscles
fas 3 over expression causes axons to innervate new targets instead
this shows that multiple cues are required to be combined in order to an address level
state Sperry’s 2 theories for the target selection using topographic maps.
speedy and two theories,
1. each axon has a label and there is a complimentary label on each target
2. there is a co-ordinate system which is encoded by gradients of signalling molecules which stamp the latitude and longitude of the cells target. this is then read by complimentary gradients that are expressed in the retinal ganglion cell.
theory 2 was proven to be right
which of sperrys theories provided evidence for the use of topographic maps in target selection?
the stripe assay showed hat cells from a posterior tectum make a non-permissive factor that repels temporal retinal cells
this appears to be due to temporal axons avoiding a repellant factor in the posterior stripes because:
1. the activity was abolished using heat treatments of posterior but not anterior membranes
2. posterior membranes cause temporal growth cones to collapse in vitro
the inhibitory fact tuned out to be 2 ephrin’s that were expressed highly in the posterior to much lower in the anterior
Eph receptors for ephrins A2 nd A5 expressed in the retina high in temporal and low in nasal. this was a counter gradient
mice KO of A2 ad A5cause temporal neurones to project axons into the posterior tectum therefore topographic maps are disordered.
this shows that non-permissive repellant factors are used instructive to direct growth cones to specific places
which of sperrys theories provided evidence for the use of topographic maps in target selection?
the stripe assay showed hat cells from a posterior tectum make a non-permissive factor that repels temporal retinal cells
this appears to be due to temporal axons avoiding a repellant factor in the posterior stripes because:
1. the activity was abolished using heat treatments of posterior but not anterior membranes
2. posterior membranes cause temporal growth cones to collapse in vitro
the inhibitory fact tuned out to be 2 ephrin’s that were expressed highly in the posterior to much lower in the anterior
Eph receptors for ephrins A2 and A5 expressed in the retina high in temporal and low in nasal. this was a counter gradient
mice KO of A2 ad A5cause temporal neurones to project axons into the posterior tectum therefore topographic maps are disordered.
this shows that non-permissive repellant factors are used instructive to direct growth cones to specific places
what are neurotrophins?
a family of growth factors in the nervous system
describe the discovery of the nerve growth factors (NGF)
it has been discovered by by Bueker (1948)
it is a fast growing muscle like cell that might secrete survival factors.
implanted sarcomas
what was NGF discovered as? - the structure and where?
7s-NGF: a2by2 from submaxillary gland
BNGF = dimer is the active component
how can you test if NGF is working?
in a campenot,if NGF is in the chamber, the cells are ok, if not, they die
how can you test if NGF is important for outgrowth/survival?
- take away NGF after middle section is grown, the process dies out therefore important in survival
- grow only on outside, outgrowth occurs – important in outgrowth
does the NGF have tropic or trophic factors?
both tropic (growth direction) and trophic (survival) functions
which high affinity receptor does NGF bind to?
TrkA = high affinity receptor p75-NTR = low affinity receptor
what do TrkA an p75-NTR promote?
TrkA promotes growth, differentiation and movement
p75 promotes cell death/survival
how long after the initial NGF was found, was another GF found? why is this?
20 years. this is because levels of NTs in the nervous system is lows hard to study biochemically
who discovered the first NTs after 20years and what were they?
Yves Alain Barde & George Yancopoulus found:
BDNF (high homology wit NGF)
NT3&NT4/5
what broadly determines whether or not a cell dies? (receptor)
p75
give examples of neurones that exhibit different NT dependency
- -placodal sensory ganglia eg nodose prefer BDNF/NT3
- -crest-derived DRGs have subpopulations that respond to NGF BDNF or NT3
- -sympathetics respond to NGF or NT3 but not BDNF
- -RUFFINI AFFERENTS, BDNF
- -MERKEL NGF, P75
what is evidence for the statement that dependency changes over time?
newly born neurones may have no dependency
NT3 can support many neurone types early in development
arrival at target coincides with new expression of NT by targeted
trig mental neurones: need BDNF and NT3 early
then NGF
then NGF or MSP
what are other survival factors for animals that don’t have NTs?
Glia-derived neurotrophic factors eg GDNF supports midbrain dopaminergic neurones
cytokines eg ciliary neurotrophic factors CNTP
hepatocyte GF
macrophage - stimulating protein MSP
what are target derived factors critical in the formation of?
monosynaptic stretch reflex between motor neurones (MNs) and proprioceptive Ia sensory neurones (PNs)
what do MNs innervating the triceps and pecs develop compared to MNs innervating the CM/LD?
and whites this controlled by?
MNs innervating the triceps and pectoral muscles develop monosynaptic connections directly with PNs whereas MNs innervating the cutaneous maximus (CM) or latissimus doors (LD) receive polysynaptic input from interneurones.
controlled by GDNF secreted by CM and LD which turns on the tf Pea3 in MNs. Pea3 KO results in MNs innervating CM/LD that have the dendritic morphology of Tri and pec innervating MNs and aberrant proprioceptive connection —- i.e. turns them into having ‘monosynaptic connections’
what does circuit completion rely on?
target feedback. this determines the final afters of both dendritic and axonal connections
what does NT3 induce the expression of?
the tf Er81 by Ia proprioceptors.
what does KO of er81 lead to?
KO of Er81 leads to failure of the Ia central projection to reach the ventral horn and form monosynaptic connections with the appropriate MNs.
what are some non-target derived effects?
many neurones themselves synthesis NTs lack of synaptic input can cause loss of target neurones -eg somatosensory barrels anterograde transport of NTs -eg WT3 in retinal ganglion cells paracrine effects -ealry DRG in single cell cultures release NT3 -promotes differentiation
what are the determinants of survival of a neurone?
- primary determinant is whether or not there is co-ordinated electrical activity pre and post synaptically
- altering activity leads to changes in neurone survival
what happens if there is a block to the post neuromuscular junction?
there will be no competition between the synapses so fewer losers i.e. more MNs survive
what converts electrical activity into survival?
the more active a synapse, the more neurotrophic it takes up by membrane recycling
how does membrane recycling link to target tissue mass?
the greater the target mass, the more NT available
where does cell death occur?
everywhere
the production of viable neurones and the degeneration of them is seen at all levels of the NS
what is the name of a programmed cell death?
apoptosis
what does death during synaptogenesis reflect?
reflects competition, make more; kill more
what is neurone survival correlated with?
neurone survival is correlated with the amount of target tissue
how does cell death occur?
most cells die through apoptosis. this is programmed cll death that is an active process.
what is apoptosis inhibited by?
actin myosin D (transcription)
cycloheximide (protein synthesis)
what cleans up the mess made once apoptosis is complete?
macrophages plus neutritic disintegration and removal
what do caspases cleave proteins involved in?
inhibiting apoptosis
DNA repair
cell cycle
nuclear structure
describe the intrinsic pathway of apoptosis
DNA damage and p53, injured mitochondrion, Cytochrome C release and binding of Apaf1, aggregation of Apaf1 and binding of procaspase 9 then activation of procaspase 9 leading to a caspase cascade.
describe the intrinsic pathway of apoptosis
killer lymphocyte, aggregation and cleavage of procaspase and molecules activated caspase 9 leading to a caspase cascade
what genes are requires in C elegans for apoptosis? what is the anti-apoptotic gene?
what is it’s mammalian homologue?
- in C. elegans, ced-3 and ced-4 required
- cells must express both ced-3 and ced-4 to die
- anti-apoptotic genes eg ced-9 (bcl-2)
- mammalian homologues = caspases
what promotes neurone survival?
targets secrete a factor requires for pre-synaptic neurone survival
what happens if there is a withdrawal of NGF from sympathetic ganglion neurones?
SGN have p75NTR, TrkA and low TrKC
Low NGF + BDNF (p75NTR, TrkB) = apoptosis
–blocking with anti-p75NTR an rescue the kid NGF - induced apoptosis
–ie BDNF activating p75NTR is necessary and sufficient for apoptosis in SGNs.
what evidence is there to support the idea that p75NTR is necessary and sufficient for apoptosis?
p75-NTR KO mice have a higher neurone number (21000) compared to an adult wt (17000)
what do neurones need for apoptosis?
the presence of the correct NT and the absence of the wrong one
what activates p75NTR and what does this lead to?
proNGF strongly activates p75NTR - this can lead to apoptosis
so the levels of enzymes to cleave PRO NGF = important too
growth cones can release cleavage enzymes
describe the synthesis and sorting of brain-derived neurotrophic factors in a typical neuron
proBDNF synthesised
if it can bind carboxypeptideE the there is regulated secretion before intracellular cleavage. if it can’t bind carboxypeptideE, leads to consecutive secretion before intracellular cleavage. this leads to release in dendrites/axons (if not cleaved, released as protein)
how does the likelihood of a neurone surviving during development change?
the programmed cell death of neurones during development = activity dependent
neurones that are stimulated by excitatory NTs survive
no stimulation - neurones may die
what does activation of glutamate receptors induce?
local activation of glutamate receptors induce the production of neurotrophic factors (BDNF & NGF) in the post synaptic neurone
where are neurotropic factors released and what do they activate?
neurotrophic factor - released locally at the active synapses, activates receptors in the presynaptic neurone
what is the determinant of survival of MNs?
initially, MNs innervate a single muscle fibre
normally, this is then reduced by competition to a single MN innervating a single fibre (MN fires at the same time fibre contracts)
blocking activity leads to reduction in synapse loss
–more polyneuronally innervated muscle fibres
what converts electrical activity into survival?
if there are 2 axons innervating the same muscle fibre which produces BDNF, the winner out of the 2 would be the one that has local stimulation to convert proBDNF into BDNF. this ensures survival. the loser would be if no proBDNF=converted. this triggers its retraction and if it has no other NMJ’s, then apoptosis would be triggered. this proves that apoptosis during development = activity dependent. neurones that are stimulated survive
why does where receptors are matter?
hippocampus NMDA receptors synaptic and non-synaptic STAU evokes apoptosis reduced by blocking GABA (bicucculine) synaptic receptors = anti-apoptotic extrasynptic receptors= proapoptotic
why does where receptors are matter?
hippocampus NMDA receptors synaptic and non-synaptic STAU evokes apoptosis reduced by blocking GABA (bicucculine) synaptic receptors = anti-apoptotic extrasynaptic receptors= proapoptotic
why do axonal growth cones follow attractive and repulsive cues?
follow cue to target tissue
what does the growth cone do when it contacts the post synaptic membrane?
it stops growing, differentiation in pre-synaptic terminal
at the same time - target cell specialises to create post-synaptic site
what is needed in order to form functional synapses?
the correct receptors being expressed
synapses a the correct location
correct part of the membrane to different into the synapse
receptors that match the target
the correct number of synapses made -this can vary from 1-10,000 per neurone - i.e. synapses need to be made and stabilised
outline synaptogenesis of NMJ
on approach, motor axon differentiates into motor nerve terminal of contact point
schwann cells gap the junction
muscle forms the post synaptic apparatus
what are the morphological changes in synaptogenesis?
small vesicles at the presynaptic membrane
narrow cleft between pre and post synaptic membranes
post synaptic membrane appears thickened (PSD). cf newborn
what are changes when a growth cone turns into pre synapse?
- filopodia retraction, tight junctions formation
- membrane + Ex cell glycoproteins added
- pre-synaptic vesicles, dense ECM, PSD, receptors accumulate in the cleft
when does synaptogenesis occur?
when axons reach their target which is very variable
how does the process of synaptogenesis differ in vitro and in vivo?
synapse density increases for 1 month at post natal dio
cf mouse olfactory bulb increase is at post natal week 1
in vitro, synapses can form on contact
in vivo, there is an extended period
what dictates synapse sites?
approaching growth cones ‘talks’ to targets
site availability may be restricted - astrocytes may cover cell body- dendrites free (olive)
post synaptic cells may have pre-prepared sites - e.g. cadherin/other adhesion molecules
where do AChRs cluster and what does this involve?
initially, ACh receptors are present at moderate levels - myotube surface
innervation ; AChRs cluster in post synaptic membrane
this clustering involves both redistribution of AChR proteins and localised synaptic synthesis ofAChRs
whats one principle feature of synaptogenesis?
aggregation of neurotransmitter receptors
AChRs cluster in developing myptubes
glycine, GABA and glutamate receptors also cluster
what evokes clustering?
denervated and destroyed muscle
\NMJs from where ‘synaptic’ basal lamina persists
a proteoglycan was identified and called agrin
what is AGRIN?
it is a proteoglycan that is responsible for triggering clustering
describe agrin
agrin is purified from T.California clusters AChRs
has multiple binding sites for the ECM/adhesion
made by motor neurones and muscle
describe the mechanism of Agrin action
Again bind to muscle specific kinase (Musk)
this activates Raspyn
Musk KO mice are agrin insensitive
describe the maturation of post synaptic apparatus
shape= junction changes from simple oval plaque to a pretzel likes of branches
topography = junctional membrane changes from a flat sheet to invaginated surface with gutters and folds
ECM: changes to basal lamina over the AchR rich membrane + cytoskeleton apparatus that are under it
Channel function = AchR subunit composition
Molecular partitioning = ion and ligand-gated channels segregate into discrete alternating domains
describe what happens when the presynapse differentiates?
growth cones express Frizzled, FGFR2, Neuexin
eg granule cells express ; Wnt7a, FGF23, Neuroligin
contact ie sigalling recruits vesicles,synaptic proteins to from pre-synaptic site
how many inputs do mature muscle fibres receive?
initially muscle fibres receive multiple inputs
in mature muscle, one input per muscle fibre
every muscle fibre is innervated
nonrandom : competition
how doe the number of climbing fibres per purkinje cell change during development?
during development - 4 CF’s per cell
mature cerebellum -1 CF per purkinje cell
what is synaptic refinement? and connection focusing?
focusing
CF re-organisation
initial contacts on soma
mature contacts on dendrites
b) immature inputs are not segregated
how is the choice of transmitter chosen during synaptic refinement?
choice of transmitter may depend on environment
e. g. parasympathetic/sympathetic neurones (MP)
- -most SYMP are ADR+
- -most PARA are ACH+
transplantation
- -PARA -> SYMP = ADR+
- -SYMP –>PARA =ACH+
how do excitatory synapses in the CNS and NMJ build synapses?
using activity dependent and independent mechanisms to build a synapse
acquisition of presynaptic release machinery + post synaptic NMDA receptors
but if no additional input, they remain silent because they lack sufficient AMPA receptors
activity recruits AMPA receptors the post synaptic domain to activate synaptic activity
what are the silent synapses in developing and mature systems like?
intact, non functional synapses
seen in NMJs
activation like LTP in the mature hippocampus?
what are the two types of memory?
declarative and non-declarative
declarative - memory about certain facts/objects/events
non-declarative - skills that are learnt throughout our lifetime
what are the two types of NTs used in memory?
excitatory - glutamate: depolarisation
inhibitory - GABA - hyperpolarisation
what are voltage gated calcium channels regulated by?
the position of calcium channels and where it is closer or further from the synaptic vesicle release
closer = more sensitive therefore less depolarisation would be required for it to cause a release
what are the proteins that are important for synaptic vesicle release?
synaptobrevin
SNAP25
Syntaxin
when they interact with each other, the synaptic cosine is driven very close to the membrane
therefore modifying these could regulate synaptic vesicle release
what is synaptogamin and what does it do?
it is a calcium sensor. without this, woudnt have calcium channels
there is a high number of these, all with different calcium sensitivities
regulates the amount of calcium released
what are the 3 different kinds of vesicles in and around the active zone?
- readily releasable pool
- very close to the active zone
- released as soon as Ca enters - proximal pool
- function is to restore the active zone (before readily releasable) - reserve/resting pool
- function is to restore the proximal pool
what is the speed of release in the readily releasable and proximal pool like?
The speed of release in the readily releasable pool is very fast
the speed of release in the proximal pool to becoming the readily releasable pool is very slow
what is the problem with there being overstimulation of synaptic vesicle in memory?
overstimulation of the synapse would have little/no readily releasable vesicles - this is because the synapse has changed its responsiveness to the stimulus
very important form of plasticity which is involved in different forms of depressions/habituation
what are the two types of neurotransmitter receptors?
metabotropic - coupled with a G protein
ionotropc - fused with ion channel - this responds to glutamate
what are the 3 different kinds glutamate receptors?
NMDA receptor - more selective for calcium, opens ad there is an influx of calcium. contains a magnesium bound so mis depolarise memo to remove and open
AMPA receptor - Na/K selectivity - binds glutamate, opens, Na influx - depolarisation
mGlut receptor - Gq pathway (cAMP) –> PLC ([Ca]increases)
what are advantages to studying the memory capabilities of invertebrates?
bigger neurones size - easy to patch (clamp)
circuit complexity - simpler (smaller) number of neurones
less temperature dependence - can work in many temperatures
mapping (developmental, genetics, stereotypy)
what are the simple types of memory?
habituation - decrease of responsivity of sensory system when there is a lot of stimulation - usually when its not harmful/useful
sensitisation - NS starts to respond to the same stimulus more than once
give 4 examples of habituation in humans
habituation of eye blink reflex
of repetitive non-harmful stimulus presentation (e.g. living on a noisy road)
of visual attention
of emotional response
what is the model organic for studying memory in simple organisms?
aplysia - they are happy being cool
what are the two important organs in apylasia?
siphon - important for location
Gill - important for breathing
describe the experiment of testing habituation in apylasia
touch the siphon 10-15 times for 10-60 second intervals
this reduces its reflex which occurs at the abdominal ganglion synapse
where can habituation happen?
- synapse between sensory terminal and sensory neurone = desensitised
- synapse between L7 and muscle = desensitised
- synapse between sensory neurones and L7 = desensitised
what does habituation result from?
reduced synaptic strength
what is an experiment that shows the sensitisation in apylasia?
there is an increased gill withdrawal reflex evoked by tail pinch/shock
involves pre-synaptic input from sensory neurones - serotonin and Protein action
what is the mechanism of sensitisation - what model could be used?
the simple bear model
pka phosphorylates and inactivates K+ channels
longer depolarisation means more vesicular release
how does the apylasia learning relate to associative learning
pavlovian conditioning
weak siphon touch (s) paired with strong shock (US) = increased CS
timing is critical
describe the mechanism of paired stimuli effects in lesnring through simple Bear’s model
calcium influx (CS) synergises 5HT effect (US)
when you stimulate sensory neurones with a conditioned stimulus, you have depolarisation which leads to an increase in calcium in the cytoplasm
when you stimulate L29, there is serotonin release
when you pair these 2 stimuli, you get an additional activation of adenylate cyclase (more cAMP molecules)
what are the mechanisms of sensitisation and learning?
multiple intracellular signalling pathways
varying activation thresholds
pre and post synaptic
long-term involves the nucleus (gene expression)
who conducted lymnae study in 2006 showing feeding behaviour?
Kemenes
what is non-synaptic plasticity?
experience-dependent changes to synaptic efficacy not based directly on synaptic changes
describe the single trial associative learning experimental model
there is an attractive feeding source present (US - sucrose) and a neutral stimulus (CS; amyl acetate). with no learning, it doesn’t cause feeding behaviour
after pairing them, one trial later, the snail shows feeding behaviour to amyl acetate
what is the cell type that permits feeding behaviour?
serotonergic cerebral giant cells (CGCs). they contact many cell types and permit feeding but are not involved in feeding behaviour. they are a gatekeeper
they stimulate the muscle to contract during feeding, they spike continuous. paired neurones of CGC = function is to permit feeding behaviour. if it fires, feeding behaviour happens. if it does not fire, it doesn’t happen
what was found 24 hours after the single trials of lymnae?
there was a large depolarisation in neurones 24 hours later and the resting membrane potential increased by about 10mV but this was the only change observed. this shows that after training, CGC neurones depolarise more
what is the problem with the lymnae study in that depolarisation only happened 24 hours later?
in the first 24 hours, there was no change in depolarisation in GCC neurones yet they had learned the behaviour. this could mean that the depolarisation was random and is not actually needed for learning. suggests that CGC neurones are involved in long term memory - not short term.
how can you show the the depolarisation of the membrane is both necessary and sufficient?
- hyperpolarise neurones in traded animals
2. depolarise neurones in untrained animals
what does amyl acetate show in a trained tissue prep?
amyl acetate triggers fictive ‘imaginary’ feeding behaviour
in trained tissue prep:
produces bursts from feeding network behaviour
‘untrained’ amy acetate doesn’t show anything
what does single spikes and depolarisation trigger in CGC neurones? what are the main conclusion that can be drawn about the CGCs role in memory?
single spikes in depolarised CGCs trigger a response
depolarisation increases calcium presynaptically
amy acetate triggers increased activity in feeding behaviour pathways when CGC is depolarised
the main conclusion is that long term memory is partially encoded for at a site distant from to the neurone producing a behaviour
who invented the hebbian synapse?
Donald Hebb
what disease is the hippocampus susceptible in getting early?
Alzheimers
how many different neurones does the Entorhinal cortex contain?
3 different neurones
-dentate gyrus
-CA3 neurones - main output of the hippocampus
-CA1 neurones
CA3 neurones also have 2nd axon that projects to area that has CA1 neurones. synapse between CA3 and CA1 neurones 0 highly studied
describe the LTP phenomenon
usually CA3 to CA1 synapses
high frequency stimulus produces along lasting potential
shows input specificity
shows co-operativity (co-incidence), which does not need HFS
two pathways converging in the same target can both be strengthened if they fire together
one pathway may be quite weak
associative
how does LTP occur?
LTP could occur by pre-synaptic changes
evidence suggests often a post-synaptic event
most indicates a critical role for Ca2+
what causes co-operativity in LTP?
co-operativity may be a result of NMDA properties
NMDA receptors have a voltage dependent magnesium block
so, NMDA receptors need to be indirectly pre-activates by a separate depolarising input
if an essential aspect of LTP occurs army, does it require proteins?
some essential aspects of LTP occur early and do nt require protein synthesis
some are late and do
describe early phase LTP
NMDR-mediated calcium increases and activates calmodulin Kinase 2
PSD protein (2-5%)
multiple catalytic subunits
autophosphorylation and constant activation triggered by calcium n LTP
phosphorylation enhances AMPA currents
KO/ inhibition studies are conflicting
what is AMPAfication ?
the delivery of ready made AMPA receptors to the synapse
describe late phase LTP
long term storage needs protein synthesis
take effect about 1 hour after initiation
cAMP signalling may be critical
is LTP memory?
there are different types of synapses and LTP
mossy fibres - CA3 LTP is nonNMDA and mainly presynaptic
high transmitter release calcium activated/cAMP mediated
perforant path - dentate gyrus does not use camk11
LTP varies in non-hippocampal areas e.g. cortex
what are the different types of memory within LTP?
the hippocampus is mainly new and declarative in function
episodic, semantic, spatial recognition
what is the relationship between LTP and memory?
inhibiting LTP inhibits some memory formation
APS and Morris water maze - time critical effect
mutations of camk11, NMDARs, cAMP pathways all affect aspects of learning
drugs to enhance memory also enhance LTP e.g. nootropics
however AMPAKines in alzheimers - poor success
is LTP necessary and sufficient for memory formation ?
it is only necessary for some memory formation but its not sufficient
how long is the LTD phenomenon? an where is it?
it is an actively evoked., long lasting reduction in synaptic efficacy
examples in the cerebellum where there is low EPSP in purkinje fibres and in the hippocampus where i occurs in the same synapse that LTP occurs in
what are the two types of LTD and how do they differ?R
depotentiation - removal of previous potentiation - i.e. wipes out effect of LTP if LTD is triggered
LTD de novo - had no previous potentiation and so has no synapses with them
what does it mean for LTP to be hebbian ?
hebbian means that its homosynaptic like LTP
non-hebbian means that’s heterosynaptic and so doesnt require pre-synaptic activity.
what are the general mechanisms for LTD induction?
often requires NMDA receptors, but not always (e.g. in a case in the cerebellum)
often evoked by low frequency stimulation but not always
often requires calcium influx and activation of serine/threonine phosphatase (unlike LTP; protein kinases)
often involves glutamate, but also diffuse transmitters e.g. 5HT and endocannabinoids, like anandamide, released post-synaptically to inhibit pre-synaptic transmitter release
what is the input and the output of the cerebellums LTD circuitry?
input – +ve mossy and climbing fibres
output – negative purkinje fibres
what is the route of transport for mossy fibres and climbing fibres in LT?
mossy fibre - granule cells (parallel fibres) - purkinje cells
climbing fibre –purkinje cells
what happens in LTD when climbing fibres have been activated?
theres an increased depolarisation of the purkinje cells
describe purkinje synapses from LTD and PF
a paired pf and cf input to a single purkinje cell evokes LTD
it is int specific
the Albus Mar model suggests that cf input indicates a motor error and so weakens the pf;p synapse
what does LTD in the cerebellum help us to do?
it helps us to learn motor skills
what are the cerebellar LTD mechanisms?
doesn’t involve NMDA receptors
metabotropic Glu-R, AMPA-R and voltage gated calcium channels
needs co-incident activation of both intracellular signalling pathways
PKC phosphorylates AMPA Glu-R subunit (on sights different to in LTP)
reduces currents by endocytosis
AMPA-R internalisation triggered by phosphorylation
endocytosis inhibitors preventLTD therefore prevents memory formation
what is Bienenstock, Cooper, Munro (BCM) theory about LTD?
the synapses are carve when the rest of the cell doesn’t become weakened
where does LTD occur? what is its function?
at the CA3 - CA1 synapse
with low frequency stimulation (1-5Hz)
LTD can reverse a previous LTP depotentiation
it is calcium dependent but so is LTP
what does the likelihood of inducing LTP or LTD i the hippocampus depend on?
the amount of NMDA receptors. if there is a lot, will be LTP receptors
if there is little-no NMDA receptors, LTD is induced
because LTP requires NMDA receptors but LTD doesn’t
what is the phosphatase/kinase balance in the hippocampus like for LTD?
small increase in calcium from NMDA-R triggers more phosphatase action and reduces AMPA-R efficacy
large increases activate more protein kinases, which increase AMPA-R efficacy
and/or other proteins
does LTP+LTD =memory?
no. close but not quite
describe an experiment which shows that LTP and LTD are not the only aspects contributing to memory formation
marmosets (monkeys) were in 3 groups nd put into 2 environments. 2 groups were in an enriched environment (interesting) and 1 group was in a boring environment. looked at the shapes of all different neurones in the post-synaptic goudons.
monkeys in interesting environment had a higher number of dendritic spines and there was a change in the synapses, the umber of the, and position etc. this shows therefore that memory is not just about LTP/LTD
what function is the hippocampus involved in? how do we know this?
involved in new memory formation
damage causes anterograde (inability to form new memories), not retrograde (inability to recall precious memories) amnesia
its also important in spatial memory
how do associated or poorly associated sets of inputs affect synapses?
Associated sets of inputs strengthen synapses but poorly associated sets weaken synapses, including those that may have been strengthened before
what was shown in free-moving was about affects to the LTP/LTD?
Novel environments and novel objects / arrangements influence LTP/LTD respectively
what do stressful stimulus promote? LTD or LTP
LTD
where is long term memory ended? and whites the main job of the hippocampus?
Long-term memory may be encoded outside the hippocampus – its main job is to make new associations and pass them on for storage if appropriate.
what controls the excitability neurones?
excitability is funsatmental to their functions
vast majority of neurones communicate chemically
who first described the chemical communication of neurones?
Sherrington (1906)
what do we need to transmit and control signals in neurones?
transmitters receptors ECM other neurones Glial cells need sodium channels to ope and get an AP
how do sodium channels open?
more positive membrane potential. need a negative resting membrane potential (-70mV)
you need to establish the resting membrane potential
what do we need t establish the resting membrane potential?
pumps/transporters - regulate ionic gradients
channels - (Na2+,Ca2+)
K+ - terminate AP-return cell to resting membrane potential
how can you generate complex repsonses in membranes?
you can generate complex responses by changing what channels, transporters are expressed
how is excitability developed (during embryogenesis)?
resting potential falls during development
new channels,new pumps
glia (astrocytes) increase, concentration of potassium outside decreases from 35-3mM
action potentials appear early - e.g. xenopus from cosureof the neural tube
what are the changes the happen during development?
resting membrane potential becomes MORE NEGATIVE
input resistant FALLS
membrane time constant (MTC) falls
MTC depends on resistance and capacitance
why does MTC change during development?
neurones get baggers can store more charge e.g. ferret -FGN
what are the 2 principle routes for action potential development?
- long calcium dependent then short, sodium dependent
- ->tetrodotoxin-resistant
- ->eg mouse dorsal route ganglion, RohonBeard cells - short sodium dependent
- ->eg mouse spinal cord
what are Rohan-beard cells and what is important for them
mechanosensory cells of lower vertebrate dorsal spinal cord
appearance if delayed rectifier K+ channel important
what is the difference between a rectifier and a delayed rectifier?
a rectifier is a channel that only allows current through one way
a delayed rectifier is a channel that opens some time after its voltage threshold has been reached
outward delayed K+ rectifiers also the change out of neuroses after an AP
appearance of such channels shortens the AP
describe calcium waves
Ca2+ waves and spikes
waves influence process growth and differentiation
e.g. mouse spinal cord neuron expression of enkephalin
e.g. myocyte expression of AChR
e.g. xenopus expression of GABA
what do adult GABA receptors do?
adult GABAa receptors are inhibitory:
GABAa inotropic
GABAb metabotropic
what is the action of GABAa in development ?
in development, GABAa depolarise
immature neurones have high chlorine concentration inside
chlorine efflux causes sodium and calcium influx via VACs
increased expression of K+/Cl- co-ransporter KCC2 (out)
little decreased expression NKCC1 (in)
switches effect to hyper polarising
what is the GABA receptor development itself?
receptor subunits change hippocampus GABAa(a1 green, a2 red)
why is regeneration important?
there is a large number of traumatic injuries in the peripheral nerve (300,000 per annum in Europe only)
spinal cord injury
stroke - due to lack of blood supply
what is the regeneration capacity lower vertebratees like? give an example
lower vertebrates have a high regeneration capacity
eg newt regenerates whole limb if it has been damaged - the bone, muscle skin and nerve!
what is the xenopus tail regeneration triggered by?
BMPs. heavily dependent on signalling molecules BMP and Notch
describe the idea that there is a window of refractory activity
tadpole, cut tail at 42 hours, will regenerate,at 48 hours, will regenerate nut at 47 hours, will not!
can newts regenerate without nerves?
newts can have regeneration without nerves and form non-neurogenic limb production
what is regeneration in higher animals like?
regenerating the PNS
myelinating axons covered in schwann cells
covered in connective tissue - endoneurium
grouped into fascicles
covered by connective issue - perineurium etc
what are the 3 nerve injury classifications?
neuropraxia - preserve nerve conduction distal to injury
axonotmesis - preservation of nerve itself
neuretmesis- whole nerve = severe
what happens to the nerves when it is a very severe injury?
more severe=more nerves would be affected - lose all connections and complete separation of whole nerve
what is the process of regeneration of peripheral nerves?
cell death is more likely closer to soma
-proximal axon and soma- reorganisation and re-expression of immature features, e.g.
tubulins
– distal axon: Wallerian degeneration. axons close to cell soma are okay but al the distal parts are quickly damaged
important to get rid of dying axon to start making a new one
what are the effects of nerve injury/cell death on denervated muscles?
muscle atrophy
AChR reversal to embryonic
MusK increase
extrema input can help prevent atrophy- over time, decreases in calibre of muscle fibres if not, system tried to cause sprouting
what does regeneration involve?
mitosis of schwann cells (supply GFs)
formations of bands of Bungner (row of schwann cells to guide axons
re-growth along dividing schwann cells
sprouting - fibre of adjacent motor neurone may try to innervate it to preventive loss of function
describe what happens when there is a spinal cord injury
Sprouting followed by failed regeneration, and degeneration
Cysts and glial scars form
Recovery of connections difficult
why is recovery of spinal cord injuries so bad?
due to inhibitory myelin
there is an inhibitory area in the spinal cord that doesn’t exist in the PNS
neurones try to avoid oligodendrocytes
CNS neurones avoid oligodendrocytes in vitro
removing myelin/oligos improves regeneration
auto immunisation to myelin proteins improved regeneration
why is recovery of spinal cord injuries so bad?
due to inhibitory myelin
there is an inhibitory area in the spinal cord that doesn’t exist in the PNS
neurones try to avoid oligodendrocytes
CNS neurones avoid oligodendrocytes in vitro
removing myelin/oligos improves regeneration
auto immunisation to myelin proteins improved regeneration
myelin proteins eg nogo-a inhibits axon growth
what is the myelin protein that inhibits axon growth?
Nogo
what are the different kinds of Nogo in its family?
Nogo-a 200kd in olivos and developing neuorones
Nogo b 55kd,in many cells
Nogo c 25kd, muscle
which nogo do fish and salamanders lack and what doe this do for them?
fish ad salamanders lack nogo a
KO or antinogo decreases the inhibition (partially)
what evidence is there to support/oppose the idea that nogo inhibits regeneration?
supporting = putting CNS axons into the PNS environment leads to regeneration. shows that the CNS environment (which has NOGO in it) is the problem
opposing = there is no correlation between nogo receptor and regeneration capacity
transplanted hippocamal neurones grow axons into myelin
much myelin is removed b macrophages after damage
what is wallerian regeneration and what is the difference between it in the PNS and the CNS?
wallerian degeneration is the removal of debris
it is quicker in the PNS then in the CNS
this could be due to myelin
are astrocytes responsible for poor regeneration?
could be because they create a tissue which is inhibitory and fibres and it hard to overcome
it is found in all parts of the CNS
they proliferate at lesions;: glial scars, jumbled astrocytes, attachments to astrocytes impairs region
secrete inhibitory condroitin
sulphate proteoglycans (SPGs)
what is spinal cord bridges used for?
for the repair
biological (sural) and artificial bridges may be used
filled with growth factors, ECM, synthetic matrix etc
what is spinal cord bridges used for? what are they filled with?
for the repair
biological (sural) and artificial bridges may be used
filled with growth factors, ECM, synthetic matrix etc
give 4 methods clinical ways to repair the body cells/neurones after injury and what are there success likelihoods
- transplant foetal cells
a. Parkinsons - may have undesired side-effects. easy trials with conflicting resul
b. Huntingtons - little integration/improvement of graft survival - transplant h-ESC-derived progenitors
- great potential, prep for clinical rials - transplant umbilical cells
- Transplant autologous NS stem/other cells
give 4 methods clinical ways to repair the body cells/neurones after injury and what are there success likelihoods
- transplant foetal cells
a. Parkinsons - may have undesired side-effects. easy trials with conflicting resul
b. Huntingtons - little integration/improvement of graft survival - transplant h-ESC-derived progenitors
- great potential, prep for clinical rials - transplant umbilical cells
- Transplant autologous NS stem/other cells - stem cells in the adult brain
how can you prove that there are new cells being made in the brain using timidine?
inject timeline labelled with its isotope into animal and can see where this is being made – found in brain, new cell proliferation so new DNA was made therefore new ells are made in the brain
what animal shows that new neurones are made in the brain? and in which areas is that possible?
male songbird from SVZ-HVC show that new neurones were produced as an old animal learned to sing
2 areas in the brain that have it:
-area of forebrain SVZ (olfactory epithelium) [less established in humans]
-area of hippocampus (dentate gyrus)
what do rodents have that humans don’t in terms of regenerative capacity?
rodents have more of the ability to regenerate. the olfactory ensheathing cells have the ability to encourage regeneration and is fairly accessible to be studied.
what are the 3 types of stimuli that the visual system is tuned to recognise?
food
predator
mate
what is the morphology of the sensory neurone defined by?
its function
what are the three ways in which the visual system processes level organisation?
positive feedforward - use excitatory neurotransmission
negative feedback - from 2nd layer to 1st layer
negative feedforward
what can the optic nerve be referred to as? and how does it respond to light?
the information bottleneck
by changing spike rate (increasing or decreasing it)
what are the 2 parts of the visual field and what are their roles?
the right and left visual field. the right hemifield activates the left brain and vice versa.
in the right visual field, it activates the left part of the retina which both project onto the left part of the brain. information here goes to the lateral geniculate nucleus and the superior colliculus
what is the main function of the retina?
image acquisition
what is the role of the superior colliculus and the lateral geniculate nucleus?
superior colliculus - responsible for focusing the eye
lateral geniculate nucleus - main info, sends info and projects int to the primary visual cortex where information processing occurs. (processes the visual information)
what are the 2 main pathways in the cortex?
ventral ‘what’ object feature stream - inferior temporal - responsible for processing object identity. starts in the V1 cortex along to the inferior temporal cortex
dorsal ‘where’ spatial location stream -posterior parietal - responsible for space related problems
whites the function of the pupil, lens and fovea?
the pupil regulates the amount of light that falls on the retina
lens focuses image on fovea
fovea is the part of the retina with the highest visual acuity. the rest of the retina has a smaller acuity and contains primarily rods but in the flea, the rods are around the fovea rather than in it so they have a much larger sensitivity to light
where are photoreceptors located in the eye and why?
they are on the opposite side of the eye so when light comes in to an active photoreceptor,it must first travel through the entire tissue
light doesn’t go through the entire retina, goes through specific cells that are transparent- others are white
what cells does light go through in the retina?
muller cells which is a type of glial cell
what is the basic layout of the retina?
consists of 3 neurones and 2 layers of synapses.
1st layer is a layer of photoreceptors with 4 types of cells (rods and 3 types of cones - activated by different colours)
2nd layer is a layer of bipolar cells (inner nuclear layer)
3rd layers a ganglion cell layer - these collect information and send it to the optic nerve
what are the two types of synapses in the visual system?
inner and outer plexiform layers
contain synapses of cells that belong to each of the three layers of the retina
what are the two interneurones within the 2 layers of the synapses in the retina?
their function is essential and they shape the response of photoreceptors, bipolar cells and ganglion cells
inhibitory - mostly glycenergic/gabaergic
what do horizontal and amacrine cells send feedback/feedforward to?
amacrine cells - send feedback to bipolar cells ; inhibit them send feedforward to ganglion cells DONT CONNECT TO PHOTORECEPTORS horizontal cells - send feedback to photoreceptors send feedforward to bipolar cells DO NOT CONNECT TO GANGLION CELLS
when are rods and cones most active?
rods - active at dim light
cones - active at bright light
photoreceptors respond to flashes of light by hyper polarisation (decrease in membrane potential)
what do photoreceptors and bipolar cells have in common?
they both don’t spike
whites a ribbon synapse and what receptor do they relate to?
ribbon synapses are what is able to keep vesicle release permanent in photoreceptors. these constantly release glutamate in the dark
do photoreceptors have all or nothing action potentials or graded potentials?
they had graded potentials and do not spike
what do photoreceptors release?
glutamate. when the light goes up, the release rate goes down and vice versa
where do bipolar horizontal, amacrine and ganglion cells receive input from?
bipolar and horizontal cells receive input from photoreceptors in the OPL
ganglion and amacrine cells receive input from bipolar cells the IPL
which cells hyperpolarise and which ones depolarise in response to light?
the majority of bipolar cells are OFF cells. hen light goes on, these cells hyper polarise because photoreceptors release less glutamate therefore there is less activation of bipolar cells
some of the bipolar cells are ON cels. when light goes on, these cells depolarise
what is the sign inverting synapse in ON bipolar cells? explain the action of metabotropic receptors
instead of expressing ionotrpic glutamate receptors, instead express metabotropic glutamate receptors. these are not coupled to channels directly. instead - they are coupled to a G protein
these cells have a channel that is activated by cGMP. when the metabotropic receptor is activated, the amount of cGMP decreases in the cytoplasm of dendrites which leads to the closure of the channel so the channel hyper polarises.
so when the photoreceptors are active, the amount of glutamate decreases and the metabotropic glutamate receptors are less active which leads to depolarisation
what receptors do OFF and ON cells express?
OFF cells express glutamate receptors (EXCITATORY)
ON cells express metabotropic glutamate receptors (INHIBITORY)
define the receptive field?
an area in the retina (or space) which when illuminated activates a visual neurone
what is the centre surround organisation of the RF? give an example
illumination of the centre and the surround of the retinal feels, leads to responses in the opposite polarities
eg in an OFF cell, if you shine light in the middle of the cell, the cell depolarises, if light is shined on the outside, the cells are hyperpolarised
what kind of cells have centre surround organisation in the receptive field?
bipolar and ganglion cells
activation with spot and annulus leads to responses in different polarities.
where does the central surround organisation of the RF of bipolar cells come from?
from inhibitory feedback from the horizontal cells
where do bipolar cells receive their input and output?
inout directly from photoreceptors located in the centre of the RF and indirectly via horizontal cells
what are ganglion cells designed to respond to in terms of their centre-surround organisation?
illumination differences that occur with the receptive field is what ganglion cells are designed to respond to. not illumination of the whole receptor field
why do we need colour vision?
helps in object recognition. for insects that need to find flowers to survive, wouldn’t be able to do so without colour vision
what is used to activate different bipolar cells?
there are three types of cones with
different spectral sensitivities
These types activate different
populations of bipolar cells
what is the distribution in the retina like?
dorsal (upper) part of retina = green cones
ventral (lower) part of retina = blue cones
wave of light comes in then to opposite sides - green wave at the bottom and blue wave on top
what are the layers of the lateral geniculate nucleus and describe them
6 layers monocular input layers alternate input from each eye P and M ganglion cells remain segregated organised retinotopically
what is the role of the thalamus station?
thalamus ‘relay’ station = somehow processes info from eye to brain
what are the layers of the LGN?
C, IP, IP, C, IP, C
(1,4,6) receive input from the contralateral eye
(2,3,5) receive input from the ipsilateral eye
what do the M and P cells from the layers 1-6 in the LGN do?
1 & 2 = M cells - position
3-6 = P cells - processing information about object identity
all sent to different layers of the visual cortex
how many connections do the ganglion cells make with the LGN projection neurones?
1:1 connections
are there any similarities or differences between the receptive fields of the LGN neurones and that of the retinal ganglion cells?
SAME - they are the same shape and size
DIFFRENT - 60% of synaptic input from cortex ‘back propagation’
local interneurones in LGN
describe the process of object recognition
a few neurones in the higher cortical areas fire
stimulation of the same neurones cause perception of the same object
what is object recognition in orientation invariance?
can recognise objects that are in orientations that we have not previously seen the in before
what is scale invariance in object recognition?
the fact that the brain can recognise objects independently other size
what is the hierarchical model of object recognition?
- Categorisation (e.g. a human, an animal, a vehicle etc.)
2.View-invariant object detection (particular person, a car)
3.Detection of objects from one point of view (e.g. a person from a front)
4.Detection of object parts (e.g. face)
5.Detection of combination of edges and contours
6.Detection of edges
there is an increase in stimulus complexity from 6-1
what are the two key features of the cortical structures?
layering - send their projections to different parts of the brain
columns - (ocular dominance, orientation and direction) and blobs
which inputs in the lateral geniculate nucleus do P and M cells send projections to?
P send projections mainly to 4CB and 4A
M send projections mainly to 4Ca
what do resident cells do?
receive inputs from some layers and project the to different parts of the cortex
what is the purpose often different layers in the LGN?
they help to process information as they each come from different ares
what is the columnar organisation of the cortex like?
ocular dominance
orientation (direction) column
Blobs (colour)
how can you test ocular dominance columns in the visual cortex?
inject radioactive proline in one eye
or
inject radioactive glucose in the cortex and stimulate one eye with light
each column receives input from either the ipsilateral or contralateral eye - helps with 3D vision
what are the 3 types of cells in the V1 cortex?
simple
complex
hypercomplex
what do blobs respond to?
respond to different colour dyes - have neurones that are specifically trained to do this
why re receptive fields of LGN neurones aligned?
to build an elongated simple cells receptive field
have the receptive fields that are downstream of V1 been studied?
they increase in complexity and so are difficult to stay. theres 2 approaches: guessing or using a computational model to predict their response to many images
what is the Jennifer Aniston neurone?
it is a specific neurone that fires in response to seeing a familiar face. this examplee is that people who have seen her before would have a neurone that fired to her, her profile and even someone who is known to be associated with her. formerly known as the grandmother cell
what is the problem with theoretical models that were designed to study hierarchical models?
they are still poor in scale - and orientation invariance
do not take into account feedback from higher cortical areas (‘top down regulation’)
need experimental validation which is difficult, because one need to record pre and postsynaptic responses
what are the main methods of studying vision?
psychophysical methods and illusions
lesions ad other ways to silence neurones or parts of the brain
anatomical studies and morphology (to study connections between neurones
FMRI, electrophysiological recordings and imaging
modelling and theoretical stimulations
what are the model organisms used to study vision?
humans (+) can give verbal feedback (-) not easy to express due to long generation time cant put electrons on them - ethical considerations primates lower mammals (cats, rodents) lower vertebrates (zebrafish) invertebrates (drosophila)
where in the brain do lesions, brain imaging techniques and multi electrode recordings happen?
lesions - take certain parts of the brain - can see if vision is impaired
whole brain imaging techniques - see which part of the brain is activated for a particular task
multi electrode recordings in different brain areas
give an example of the effects of a lesion in a patient
patient following a stroke, had difficulty perceiving certain forms of motion, object recognition and colour perception were in tact
what is fMRI used for? in moving and stationary dots
FMRI is a good way to study brain areas involved in visual processing as it can detect which parts of the brain take in more oxygen and glucose. there are 2 stimuli (moving and stationary). fMRI response is higher for moving dots then it is for stationary dots which shows that the MT+ cortex is involved in seeing moving stimuli
who was the first to describe the morphology of ganglion cells?
Canal 1892
what kind of cell is used to describe the morphology of neurones in the brain?
fluorescent cells. they can drive the expression of fluorescence to label each thing to visualise structural differences and can dissect its circuitry
explain the process of patch clamping
use electrode with holes in it
goes close to the cell but doesn’t penetrate it
creates a negative pressure so forms a tight connection between the membrane and the patch clamp (by creating suction)
higher resistance leads to a higher quality recording
can either record from just one cell or entire area
(+) in control of wha is happening inside the cell e.g. can add a backer and choose what it effects or even use a fluorescent dye that labels morphology of a specific dye so you can gather information of the structure / morphology of a wide range of cells
what is the role of GCaMP3?
reports changes in neuronal calcium concentration and therefore, neuronal activity
fused 2 proteins (M13 and CaM, which interact in the presence of calcium
the entire molecule is stabilised and in the correct conformation due to calcium concentration increasing as the molecule is more active
expressing this in individual neurones can lead to you seeing when neurones are active
what are the benefits and disadvantages of using GCaMP3?
recording the activity of many neurones
can record simultaneously the input and output of a particular neurone (circuit reconstruction)
can easily target a particular neuronal type
(-) image is taken every 4 seconds at a high frequency which takes too long
very slow imaging
faster with electrophysiology
what is the depolarisation by light called and explain it?
channelrhodopsin
a light activation channel (blue light 480nm). when its activated, the channel opens, increase in sodium going in, leads to a depolarisation in the neurone and so causes a spike.
what is the hyper polarisation by light called and explain it?
halorhodopsin (yellow light [570nm]) there is a chloride influx. when the channel opens, due to light shining on it - the chloride influx leads to hyperpolarisation
describe one disease of the eye and the cause and treatments
retinitis pigments (most abundant cause of blindness)
no efficient treatments (no prevention)
the cause is the death of a retina (degeneration), no more photoreceptors leads to the rest of the retina degenerating
treatment = artificial retina - retina (mostly for retinas pigments) and the visual cortex (when the optic nerve didn’t develop or is destroyed)
why is it important to activate the artificial retina as early as possible in the visual pathway?
retina performs complex computations
different ganglions
how can you activate an artificial retina?
by electric stimulation of the channelrhodopsin and halorhodopsin (activation by light)
describe the process of delivering channelrhodopsin/halorhodopsin to bipolar cells to see if it restores vision
there is a layer of ganglion cells that are only accessible part to put the electrodes on and so have access to photoreceptors
halorhodopsin expressed in retinitis pigments retina using yellow light, could activate it, these parts expressed glutamate
response of ganglion cells in the retina with degenerated photoreceptors - key properties of ganglion cells are the same – start spiking when they see yellow light due t receiving input so cells decrease fitting rate when there is no light (OFF cells)
therefore the key difference of having ON/OFF cells in ganglion cells install present. centre surround organisation of RGCs in intact:
increased size of light spot (covering entire field), stops responding
direction selectivity is intact. (direction= spike, opp=no spike). using this method, can restore the function/vision.
i clinical trials ATM -
