Neurons and Synapses PDF

Neurons and synapses geschreven door StephSilentium www.stuvia.com Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, he...

Neurons and synapses geschreven door StephSilentium www.stuvia.com Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Neurons and synapses Content Action potentials.................................................................................................................................3 Types of neurons................................................................................................................................6 Classification of inhibitory neurons.................................................................................................7 l. Morphology classification.........................................................................................................7 II. Electrophysiology classification:..............................................................................................8 III. Molecular classification..........................................................................................................9 Four examples of inhibitory neurons:..............................................................................................9 Chemical synapse part I.................................................................................................................... 10 Structure of the synapse............................................................................................................... 10 Two types of synapses.................................................................................................................. 11 Dendritic spines............................................................................................................................ 12 Axonal Boutons............................................................................................................................. 12 Neurotransmitters........................................................................................................................ 13 Neuromuscular junction............................................................................................................ 13 Central nervous system............................................................................................................. 13 Autonomic nervous system....................................................................................................... 14 Receptors...................................................................................................................................... 14 Neurotransmitter receptors...................................................................................................... 14 Subtypes of neurotransmitter receptors.................................................................................... 15 Neuromodulators and metabotropic receptors......................................................................... 15 Neurotransmitter analogues..................................................................................................... 16 Glutamate receptors................................................................................................................. 16 GABA receptors......................................................................................................................... 17 Chemical synapse part ll.................................................................................................................... 17 EPP: Neuromuscular junction........................................................................................................ 18 EPSP.............................................................................................................................................. 19 IPSP.............................................................................................................................................. 19 Electrical synapse.............................................................................................................................. 20 Parkinson’s Disease........................................................................................................................... 21 Synaptic plasticity part l.................................................................................................................... 24 Long term potentiation (LTP)........................................................................................................ 25 Long-term depression (LTD).......................................................................................................... 26 Spike-timing dependent plasticity (STDP)...................................................................................... 27 Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen LTP in inhibitory synapse............................................................................................................... 27 Homeostatic plasticity................................................................................................................... 27 Morphological changes by plasticity.............................................................................................. 28 Synaptic plasticity part ll................................................................................................................... 29 Short term plasticity......................................................................................................................... 30 Neural Circuit & Signal integration.................................................................................................... 31 Summations.................................................................................................................................. 31 Layer-specific connections in neocortex........................................................................................ 33 Molecular dynamics at the synapse.................................................................................................. 34 Delivery of proteins to synapses.................................................................................................... 35 Sushi belt model........................................................................................................................ 36 Surface diffusion for transmembrane proteins.......................................................................... 36 Local mRNA translation/protein synthesis................................................................................. 38 Neural recording methods................................................................................................................ 38 Pros and cons of recording techniques.......................................................................................... 39 Network properties of neuronal tissue.............................................................................................. 41 Memory in dissociated cortical networks...................................................................................... 43 Extra notes after re-watching all the lectures:................................................................................... 44 Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Action potentials After touching the receptor of, for example, a finger, the membrane potential gets depolarized. So the membrane potential gets less negative. This depolarization is transient (=only lasts for a short period of time) and the membrane potential will go back to its resting state. From the resting membrane potential, if the stimulus is high enough and goes up (above the threshold), action potential takes place. This is depolarization. If the membrane potential gets more negative, hyperpolarization takes place. Action potentials are: Short (about 1 ms) All-or-none Standard shape and amplitude Frequency encodes strength To understand the action potential, you first have to understand the membrane potential in rest. The changes in membrane potential is to achieve equilibrium. Membrane potential is influenced by two underlying mechanisms: Chemical gradient: difference in intra and extra-cellular ion concentrations. Electrical gradient If the K+-channels are open, K+ moves towards the negative side until equilibrium is reached. ~-58 for K+(potassium) and ~58 for Na+ outside the cell. Only a small amount of ions have to pass through the channels for equilibrium so the concentration in- and outside the membrane stays more or less the same. Nernst-equation is used to calculate the rest potential and is influenced by with absolute temperature (T), ion charge (z) the gas- (R) and Faraday (F) constants as well as the equilibrium potentials of the ions in the solution. During action potential more Na+ channels open up, allowing Na+ ions to enter the cell and depolarizes the cell. After depolarization, resting potential settles at a new level where the influx of Na+ is balanced by the efflux of K +. In a neuron several ions play a role , as well as the relative permeability (P) of the membrane (as determined by how open the ion-channels are). This is calculated by the Goldman-equation. In rest the permeability for K+ is higher (negative potential). During the action potential the membrane will be more permeable for Na+ (positive potential). The Na-K+ pump (and other pumps and transporters) regulate the Chemical concentration gradients, but are not directly involved in the action potential generation. It is the conductance of the ion channels that change. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen The opening and closing can be regulated by different types of stimuli: Ligand gating, Protein phosphorylation, Voltage gating, Stretch or pressure. During action potential it is the voltage dependent gating that changes. See figure below: Sequential opening of voltage-gated Na+ and K+ channels generating the action potential. Sodium (Na+) channels: Permissive and non-permissive states. (activation gate opens fast at depolarization, inactivation gate slowly closes at depolarization) An ion channel is open when all gates are in the permissive state. If any of the gates is non-permissive, ions cannot flow. 1. Resting potential: sodium and potassium channels (relatively) closed. 2. Sodium channels open. 3. Depolarization: Activation gates open (positive feedback). 4. Repolarization: Inactivation gates close, Na+ and K+ channels open. 5. Refractory period: Na+ channels closed, K+ still open. (small undershoot). Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen The refractory period is associated with the recovery of the sodium channels from inactivation. To distinguish whether the spikes you measure are coming from only one neuron, you can look at whether there is an refractory period. (If it’s present it indicates it is only from one neuron) Hodgkin-Huxley (HH) Model Description in differential equations of macroscopic currents in the squid giant axon in terms of Na+ and K+ conductance. The electrical circuit: The change in membrane potential along a neuron process during electrotonic conduction decreases with distance. Electrotonic conduction contributes to the direction of propagation of the action potential. Action potentials in myelinated nerves are regenerated at the nodes of Ranvier. Different firing patterns are related to different neuronal functions: 1. Regular spiking (non-fast spiking cells) 2. Fast spiking 3. Intrinsically bursting 4. Chattering Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen In electrophysiology you have 3 ways of recording a single neuron: -Extracellular recording. (only action potential) -Intracellular recording. (also intracellular membrane potential) -Patch clamp recording. Just touch the membrane not inside. (ion channel function or inject current or look at Calcium activity by fluorescence) Which of the three techniques do we use quiz: 1. How does the activity (or inactivity) of one neuron affects the activity of another neuron? Intracellular recording, because you get information about the intracellular membrane potential. (because the change in membrane potential can happen, without action potential and with extracellular recording you only measure action potential) 2. How does an ion channel’s open and closed times depend on the membrane potential? Patch clamp. You can look at opening and closing of ion channels. It is thus more precise than intracellular recording. For extracellular recording there are different methods: Tetrode, Multielectrode array, Neuropixel probe, Mesh electrodes. Types of neurons There are many different cell types and they are connected in a cell-type specific way. Questions you may as while classifying neurons: What cell types exist? And where? Which cell types are interconnected? Where are the synapses of the connections? What are the functions of these connections? Excitatory neurons secrete neurotransmitters that cause membrane depolarization in the postsynaptic neuron. Inhibitory neurons secrete neurotransmitters that cause membrane hyperpolarization in the postsynaptic neurons. So they inhibit the neuron. Both excitatory neurons and inhibitory neurons can contain inhibitory GABAergic receptors. The naming is because of what effect they cause on the postsynaptic neuron. If an inhibitory neuron contains inhibitory GABAergic receptors disinhibition happens, which is the release of an inhibitory constraint that effectively results in an increased activity in the target neurons (for example, principal or projection neurons). Better said, disinhibition refers to the selective and transient reduction of synaptic inhibition of a projection neuron due to suppression of interneuron firing by another population of interneurons. Neocortex – Excitatory neurons It is the outermost part of the cerebral cortex and it has six layers. In layer 1 does not contain excitatory neurons. (inhibitory neurons do exist in layer 1) -Pyramidal cells ( layer 2 t/m layer 6): Apical dendrite, pyramidal shaped soma, multi-polar, regular- spiking. -Spiny stellate cells ( layer 4): No apical dendrite, Multi-polar. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Hippocampal formation Pyramidal neurons are densely packed in one layer and pyramidal cells in the dorsal part are upside down. (pyramidal soma shape is upside down; apical dendrites go down) Inhibitory neurons mostly project locally. Hence, they are often called “interneurons”. Pyramidal cells are sometimes called “principal cells”. Some pyramidal cells send long distance axons to make inter- areal connections. Classification of inhibitory neurons There are three main approaches: I. morphology (shapes, location, synaptic projections) II. electrophysiological properties (firing patterns and ion channels III. molecular properties (gene expression, proteins, peptides) l. Morphology classification -Shapes of cell body and dendritic patterns. In the figure below there are different shapes and patterns of soma and dendrites. So how do these look like in actual imaging? See figure below. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Some classifications of inhibitory neurons, however, are based on their axonal projections and not the dendrites. The axon terminal is important in cell to cell communication through the neurotransmitters it releases into the synaptic cleft. The neurotransmitters that exit the neuron relay signals to the next target cell. The axon initial segment. The axon initial segment (AIS) is a specialized structure in neurons that resides in between axonal and somato-dendritic domains. The localization of the AIS in neurons is ideal for its two major functions: it serves as the site of action potential firing and helps to maintain neuron polarity. II. Electrophysiology classification: Responses to both negative and positive current injections are analysed. Firing patterns Regular spiking: even with a large current injection, the regular spiking will not show high frequency spiking. Fast spiking: Capable of showing high frequency spiking. (>several hundred spikes/sec) and do no show adaptation Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen III. Molecular classification There are diverse patterns of molecular properties (gene expression, proteins, peptides). Particular types of morphology may carry particular types sets of molecules. These markers of cell types are called biochemical markers. Important ones are calcium binding proteins: Parvalbumin (PV) and neuropeptides: Somatostatin (SOM) and Vasoactive intestinal peptide (VIP). Four examples of inhibitory neurons: 1. Basket cell: -Axonal projection to soma and peri somatic regions -Parvalbumin (PV) positive -Fast spiking firing pattern 2. axo-axonic cell” (chandelier cell): -Axonal projection to initial segment of axons -Parvalbumin (PV) positive -Fast spiking firing pattern 3. distal dendrite targeting cells: -Axonal projection to distal dendrite -Somatostatin (SOM) positive -Regular spiking pattern Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen 4. Interneuron projection cell: -Axonal projection to inhibitory neurons -Vasoactive intestinal peptide (VIP) positive -Regular spiking firing pattern. Chemical synapse part I Golgi originally developed the “black reaction” technique for the nervous system. He considered that the nerve cells act together as a continuous web of tissue. neuron doctrine: Individual neurons are the elementary building blocks and signalling elements of the nervous system. Structure of the synapse The synapse is the structure where neurons communicate each other. It is a contact point between neurons, or between neurons and muscle fibres. The chemical synapse means: 1. Specific molecules are released at a synaptic terminal. 2. The molecules bind to receptors. 3. A channel opens in response (directly or indirectly) to the binding of the molecules. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Excitatory neurons release neurotransmitters that excite postsynaptic neurons. Inhibitory neurons release neurotransmitters that inhibit postsynaptic neurons. This means that excitatory neurons do not inhibit target neurons, and inhibitory neurons do not excite target neurons. When a neuron targets multiple postsynaptic neurons, it releases the same neurotransmitters at all of its synapses. Exceptions: But neurons can have multiple neurotransmitters and there are examples of coexistence of excitatory and inhibitory neurotransmitters. Two types of synapses Type 1 asymmetric synapse has a thick postsynaptic density. It is excitatory mostly and typically contact spines and less contact shafts and/or cell bodies. Type 2 asymmetric synapse is usually inhibitory. It contacts cell bodies and dendritic shafts. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Dendritic spines Spine= A dendritic site that mostly receives excitatory synaptic inputs and are mostly found on dendrites of excitatory neurons. -presynaptic cells that connect axon terminals to spines = excitatory -postsynaptic cells that have spines = excitatory Inhibitory neurons often do not have spines and Inhibitory synapses are often formed directly on dendritic shafts, somas (=cell body), or axons. Axonal Boutons Axons typically project over long distances from their sites of origin to communicate and synapse with their targets. Swellings termed axonal varicosities / boutons are typically the sites where synapses occur. Boutons form as terminal bulbs at the end of an axon so terminaux, and/or along the length of individual axons as boutons en passant. terminaux:  a bulb-like structure  axon branch ends there en passant:  a bead-like structure  axon makes synapses there while it passes by the dendrite Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Often found in nature is de tripartite synapse formation where the postsynaptic and presynaptic cell as well as an astrocyte bind together. Astrocyte is a kind of glia cell. Astrocytes contribute to the metabolism of neurotransmitter, removal of extra synaptic neurotransmitters, recycling of molecules, and regulation of the ionic compositions in the synaptic cleft. When a single cell makes synapses on a target cell, it often makes multiple contacts. The contact numbers are cell-type specific. Most have a contact number of 5 whereas basket cells have 10 to 12 contact numbers. Also the contact number means number of boutons, not number of active zones. One bouton can have multiple active zones. These active zones are identified in electron microscopic images. Neurotransmitters Different neurotransmitters are used in different parts of nervous system. Neuromuscular junction Acetylcholine (ACh) is the neurotransmitter that is found at the neuromuscular junction. The neuron is excitatory and is used for muscle contraction. Synaptic vesicles containing Ach migrate in the presynaptic membrane towards the active zone of the neuromuscular junction. At the postsynaptic membrane which is the muscle, there are ACh receptor channels. See figure below. Central nervous system Neurotransmitters found in the central nervous system are Glutamate (Glu) which is excitatory, Gamma-Aminobutyric acid (GABA) which is inhibitory and Glycine (Gly) which is inhibitory. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Autonomic nervous system The main neurotransmitters in the autonomic nervous system are norepinephrine in the sympathetic system and acetylcholine (ACh) in the parasympathetic system. Their effects can be excitatory or inhibitory. Receptors Neurotransmitter receptors neurotransmitter receptors:  They are membrane-spanning proteins.  The region exposed to the external environment of the cell recognizes and binds the transmitter from the presynaptic cell.  The receptors typically influence the opening or closing of ion channels. Ionotropic receptor: Opens ion channels in response to the binding of a neurotransmitter, also called a ligand gated channel. It has a direct effect on receptor channels. Ligand gated channel is one type of channel that opens and closes the gate in response to specific molecules. metabotropic receptor: indirectly modifies ion channels in response to the binding of a neurotransmitter. Binding of the transmitter causes a second-messenger cascade to start and this in turn opens/closes the channel. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Subtypes of neurotransmitter receptors glutamate receptors (GluR)  AMPAR (sensitive to AMPA: α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)  NMDAR (sensitive to NMDA: N-methyl-d-aspartate)  Ca++ permeable AMPAR  mGluR (metabotropic glutamate receptor) GABA receptors (GABAR)  GABAAR  GABABR acetylcholine receptors (AChR)  nAChR at N-M junctions (sensitive to nicotine)  mAChR at CNS (sensitive to muscarine) Table of different receptors Neuromodulators and metabotropic receptors Main neuromodulators in the brain are serotonin (5-hydroxytryptamine (5-HT)), dopamine, acetylcholine (ACh), glutamate and GABA can also be neuromodulators when they bind to mGluR and GABABR, resepectively. Dopamine is linked to reward seeking behaviour and degeneration of dopaminergic neurons is linked to Parkinson’s disease. Serotonin is linked to stabiliztion of moods and feeling of well-being. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Neuromodulators: bind to receptors to alter: the excitability of neurons, the likelihood of transmitter release, the functional state of receptors on postsynaptic neurons. Receptors that neuromodulators bind to are mostly metabotropic receptors. Neuromodulators activate second messenger pathways. Two major families of metabotropic receptors: -G protein-coupled receptor: “trimeric guanine nucleotide-binding” protein. when activated, it produces second messengers. mAchR, GABABR, mGluR, serotonergic R, dopaminergic R belong to this family -Receptor tyrosine kinase: when activated, it triggers a cascade of phosphorylation reactions. Second messenger: Intracellular signalling molecules released by the cell in response to exposure to extracellular signalling molecules (first messengers). Neurotransmitters act as first messengers in neuromodulation. (in this case, the neurotransmitters are called neuromodulators) In some cases, Ca2+ ions from the intracellular store are mobilized and act as second messengers. Neurotransmitter analogues Agonist: A molecule that binds to neurotransmitter receptors and activates them. It mimics the effect of neurotransmitters. Antagonist: A molecule that binds to neurotransmitter receptors and blocks synaptic transmission. Many poisons and medicines are neurotransmitter analogues (found in nature or synthesized). Curare (D-tubocurarine): nAchR antagonist -from the leaves of Strychnos toxifera -used by South American indigenous people on arrowheads to paralyze animals to hunt 𝛼-bungarotoxin: nAchR antagonist -from the venom of a snake Bungarus atropine: antagonist of mAChR -used as a medicine -it dilates pupils Ketamine: NMDAR antagonist -used as an anesthesia benzodiazepine: GABAAR agonist -treatment for anxiety, insomnia, seizures Glutamate receptors Glutamate receptors are the most common excitatory receptors found in the brain. Examples are AMPAR (permeable to Na+ and K+), NMDAR (permeable to Na+, K+, and Ca++) and Ca++ permeable AMPAR. AMPAR Presynaptic voltage-gated Ca channels are activated upon arrival of action potentials to the terminal. Exocytosis of vesicles containing glutamates is initiated and Glutamates bind to post-synaptic AMPARs. The ion channels of AMPARs are permeable to Na+ and K+, but Na+ influx is strong and K+ efflux is weak. This is why the activation of AMPAR causes depolarization of the postsynaptic neuron. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen NMDAR 1. NMDA receptor channels are permeable to Na+, K+, and Ca++. 2. When Mg++ ions bind to NMDAR, its function is blocked. 3. When the membrane is depolarized, Mg++ ions are unbound and the channels open. 4. This means NMDA receptors are functional only when the post-synaptic neuron is depolarized. (This is linked to “neurons that fire together, wire together.”) Metabotropic Glutamate receptors MGluRs are often found outside of the synaptic cleft on the postsynaptic membrane or presynaptic terminals. Large quantities of glutamate release causes “spill-over” of glutamate that activates mGluRs. GABA receptors GABA receptors are the most common inhibitory receptors in the brain. GABAAR: The most common postsynaptic inhibitory receptors. They are permeable to Cl-, are ionotropic and have fast kinetics. GABABR: Found on both postsynaptic membrane and presynaptic terminals. They are permeable to K+ (GABA indirectly activates K+ channels) and also causes hyperpolarization. They are metabotropic and have slow kinetics. GABABRs are metabotropic and they are activated by the spill-over of GABA. Chemical synapse part ll Postsynaptic potentials (PSPs): An activation of synaptic transmission evokes postsynaptic potentials: -Excitatory postsynaptic potentials (EPSP) = depolarization -Inhibitory postsynaptic potentials (IPSP) = hyperpolarization -At neuromuscular junction, it is called “end-plate potential” (EPP) - “unitary” PSP: postsynaptic potential evoked by a single presynaptic neuron Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen EPP: Neuromuscular junction A neuromuscular junction is a synapse between axon terminals of motor neurons and muscle fibers. Its neurotransmitter is Ach (excitatory) and its receptor is a nicotinic acetylcholine receptor (nAchR). The postsynaptic side is called the “end-plate region” and its postsynaptic potential is called “end- plate potential” (EPP). The figure above shows a recording of the membrane potential. The inward synaptic current is inward positive current and therefor it causes depolarization. The synaptic current was recorded while the membrane potential was kept constant (a technique called voltage clamp). When receptor channels open, the charges (ions) move and it takes time to accumulate the charge at the membrane. This is why the time course of PSP is slower than input current. This means that the synaptic current reflects the time course of the synaptic input. PSP reflects the time course over which the membrane is charged by the synaptic current. Time course of synaptic current The time course of the synaptic current determined by the conductance. The conductance is the inverse of resistance so g=1/R. The conductance says something about how easily a current can flow. So the opposite to resistance. The opening of channels means an increase of membrane conductance. Vm and Esyn are constant, therefor the time course of a synaptic input (Isyn) reflects the time course of the opening and closing of receptor channels (Gsyn). Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen EPSP You have AMPAR and NMDAR mediated EPSP. You can depolarize the postsynaptic cell and measure the effects. NMDAR are active by -45mV only. IPSP GABAAR-mediated IPSPs are mediated by Chloride (Cl-). Chloride reversal occurs during development (called “chloride switch”)! The timing of the switch during development varies among species. (mainly it depends on the properties and types of chloride pump) Effects of distance When a neural membrane shows only passive properties, so no activation of voltage-gated channels, its response to an input signal decays in distance. This means that a synaptic input at a distal dendrite evokes a smaller PSP at the cell body than a synaptic input at a proximal dendrite. EPPs recorded at a distance are weaker and IPSPs from a synapse at a distal dendrite are weaker. It also influences the synaptic inputs given at spines, for instance when a spine is longer than the other. Every time a PSP is evoked, the amplitude varies. Sometimes a synapse even fails to evoke PSPs = a “failure” of transmission. The amplitude varies because the synaptic strength changes: presynaptic side: Ca channel activities, number of transmitters/vesicle, size of vesicle, number of vesicles in exocytosis, number of active synaptic contacts, number of active zones. postsynaptic side: number of available receptors, activities of receptors. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Electrical synapse How to distinguish an electrical synapse from a chemical synapse -The distance between cell membranes are smaller in electrical synapse. -Chemical synapses contain vesicles and in electrical synapse these are not observed. -Because of the vesicles that need to release neurotransmitters there is a darker cloud at the membrane junction of chemical synapses. Electrical communication across cells is mediated through gap junction channels: Gap junctions are composed of pairs of hemichannels Each hemichannel consists of a hexameric complex of connexins (or innexins in invertebrates) In mammals, there are ~20 different types of connexins Each hemichannel can be homomeric or heteromeric Two hemichannels with the same or different connexin composition can form gap-junctions: Homotypic vs heterotypic gap junctions. Direct electrical coupling between neurons Electrical synapses are experimentally studied by simultaneous intracellular recordings from synapse- coupled neurons. If the direct subthreshold somatic current injection in one neuron changes the membrane potential in the other, the two neurons are thought to be coupled via gap-junctions. Gap- junction mediated coupling was first shown between electrically coupled inhibitory neurons. Electrical communication through the electrical synapse is rapid and has lower failure rates. (compared to the chemical synapse) The density of connexons determine the “size” and “strength” of the gap junction. The permeability of gap junctions can be regulated. Electrical synapses work as low-pass filters, because membrane capacitance takes time to charge. Note that the hyperpolarization after the action potential does not adapt – Why? Because the response due to opening of channels. Electrical coupling enables supralinear integration of synaptic inputs. Supralinear synaptic integration in relation to spike generation: If number of action potentials generated upon simultaneous subthreshold current injections in two neurons is higher than the sum of the action potentials Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen generated by current injections into individual neurons in isolation, then the synaptic integration is considered to be supralinear. This might contribute to learning and memory. Action potential is not necessary to establish correlated activity between neurons. For example, Two neurons can be simultaneously induced using an agonist of metabotropic glutamate receptors(ACPD) and the same effect in activity is observed. So synchronized oscillations do not require chemical synaptic transmission or action potentials. Oscillatory activity across electrotonically coupled neurons are spatially constrained to neurons whose soma are within ~400 micrometre. If the soma are too far away from each other it does not happen. Signals relayed through electrical synapses are amplified by voltage gated sodium channels: Voltage-gated sodium channels improve the strength of the coupling as measured by the power of the cross-correlation. Upon inactivation of the voltage-gated sodium channels the relative timing of the spike synchrony is impoverished. The strength of the electrical synapse can be modulated by high-frequency stimulus and blocked by the inactivation of metabotropic glutamate receptors (by MCPG). Just like the chemical synapse, the electrical synapse undergoes activity dependent plasticity. Electrical and chemical synapses may co- exist (heterosynaptic interaction). NMDA receptors can modulate the opening of gap-junctions Parkinson’s Disease The cause of Parkinson’s disease is thought to be a loss of dopaminergic neurons in the substantia nigra projecting to the basal ganglia. Substantia nigra produce dopamine. In parkinson’s disease a progressive loss neurons and increase of Lewy bodies are found in the brain. Lewy bodies are the intracellular plaques, with aggregation of protein alpha-synucleine. Alpa- synucleine causes toxicity in the cells leading to a decreased synaptic vesicle release. This in turn leads to a reduced energy production and induces apoptosis. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Braak’s hypothesis Braak's hypothesis states that sporadic Parkinson’s is caused by a pathogen that enters the body via the nasal cavity, and subsequently is swallowed and reaches the gut, initiating Lewy pathology (LP) in the nose and the digestive tract. It is then transported to the brain stem and substantially to the substantia nigra. Basal ganglia is important for initiation of movement, stopping unwanted movements and modulation of upper motor neuron activity. Two pathways Focused selection: Direct pathway: Initiate goal or sensory driven motor programs. Indirect pathway: suppress competing motor programs. Direct pathway: D= Dopamine The frontal cortex is where the motor cortex get activated. The direct pathway is excitation of the motor cortex by disinhibition. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Indirect pathway: It suppresses movements. It fires Dopamine 2 receptors and inhibits subthalamic nucleus. People with tremors are often treated at this area. In Parkinson’s disease the D1 and D2 receptors are degenerated. Figure shows pathological oscillations in parkinsonian measurements. There is more rhythmic STN activity found in parkinson’s disease. When STN region is stimulated so that it mimics healthy oscillations, the tremors symptoms stop. With Deep Brain Stimulation (DBS) these pathological activity is lowered (reversible). The target areas are downstream of the substantia nigra in the basal ganglia, typically the subthalamic nucleus (STN) is stimulated. Neural activity changes in Parkinson disease Healthy patients have: irregular firing and gamma-oscillations. Parkinson’s Disease: Synchronized firing, beta/theta oscillations (at a lower frequency than gamma) and bursting. Mechanism for the emergence of synchrony in Parkinson’s disease 1. Possible existence of gap junctions in Globus pallidus. 2. Basal ganglia dynamics in presence of pallidal gap junctions. Hypotheses about parkinsons disease: Gap junctions in external globus pallidus (GPe) exist and their conductance and number is increased in PD. They can shape synchrony in the basal ganglia by interacting with chemical synapses. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Experiment: Detection of gap junction protein Cx36 in post-mortem brain tissue. Results: elevated levels of Cx36 found in putamen, GPe and GPi of PD compared to healthy patients. So remodelling of electrical synapses Cx36 occurs in PD. Cx36 in GPe and GPi suggests gap junctional coupling Gap junctional coupling may be increased in PD GPe gap junctions can disturb desynchronization of the basal ganglia by inhibitory synapses. Synaptic plasticity part l Synaptic strength may change reflecting the history of the activities = “plasticity”. “Plasticity”= The quality of being easily shaped or modelled and the adaptability of an organism to changes in its environmental conditions. Synaptic strength changes is thought to be the memory and learning mechanism. How can a synaptic strength change? At the presynaptic side by changes in: Ca++ channel activities, Ca++ concentration, number of transmitters/vesicle, size of vesicle, number of vesicles in exocytosis and number of active zones. At the postsynaptic side: activities of receptors, number of receptors, postsynaptic membrane and properties. Morphology changes: shape of spines, number of spines (contact numbers), length & diameter of dendrites. There are different kinds of plasticity: Short term plasticity -Short-term facilitation -Short-term depression Long-term plasticity -Long-term potentiation (LTP) -Long-term depression (LTD) Spike-timing dependent plasticity (STDP) Homeostatic plasticity Structural plasticity Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Long term potentiation (LTP) If the activity of cell A coincides with the activity of cell B, the strength of the synaptic connection (efficiency of synaptic transmission) increases i.e. “fire together, wire together” (coincidence detection mechanism) This is called “Hebbian synapse”. NMDAR’s are important in this process: 1. At resting membrane potential, Mg2+ ions bind to NMDAR and the function of NMDR is blocked. 2. When the membrane is depolarized because of binding of glutamate to AMPA receptors, Mg2+ ions are removed. 3. When the depolarization is combined with a glutamate binding of NMDAR , the channel opens. 4. It causes Ca²+ influx. 5. Ca2+ causes changes in the synaptic properties. NMDA receptor dependent LTP: glutamate is released from the presynaptic neuron and binds to postsynaptic AMPA receptors, depolarizing the post-synaptic neuron (causing it to fire) and opening a channel in the post-synaptic NMDA receptors. The opening of the NMDA channel results in an influx of calcium ions into the post-synaptic neuron, which, in turn, induces a molecular cascade of phosphorylation of AMPA receptors. This increases the conductance of the receptors already in the synapse, and triggers the insertion of additional AMPA receptors into synapse. Persistent-LTP is when Dopamine comes to play and it requires gene transcription and protein synthesis in the postsynaptic cell by activating the dopamine receptor. The postsynaptic neuron has to be depolarized at the same time when glutamates are released from the presynaptic terminal. So ‘’Neurons that fire together, wire together’’ = Both pre- and post-synaptic neurons have to be active at the same time. Only then, NMDARs are functional and Ca2+ influx occurs. The Ca2+ influx causes strengthening of synaptic connection. (note that NMDAR-dependent LTP is not the only form of LTP) LTP can be researched by tetanic stimulation. So enter a stimulating electrode in the hippocampus near the axons of the neurons you want to study and enter a recording electrode near the cell bodies (extracellular recording). You can activate the axon by giving a high frequency electrical signal (tetanus) from the stimulating electrode to induce LTP. The data will look like: In the data the EPSP response is a negative curve because the recording was extracellular, so the polarity of the response is flipped upside down. After tetanic stim again the response of a single stimulation is recorded and you see that the response is larger and lasts long ~90 min. This response is LTP. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Why are EPSPs larger after LTP induction? Three main changes in synaptic properties: 1. Increased activities of AMPA receptor channels (postsynaptic effect) phosphorylation of AMPARs: NMDAR activation triggers a cascade of processes in the downstream signal pathways. It causes a phosphorylation of AMPARs. This results in an increased activity of AMPAR channels => increased. 2. Increased number of AMPARs (postsynaptic effect) AMPARs are moved from internally stored locations, AMPARs are moved from areas outside of the synaptic cleft to postsynaptic membrane or AMPARs are newly synthesized. Like a ‘’silent’’ synapse: Before LTP occurs, EPSPs are silent because without depolarization/at membrane resting potential the NMDARs are not active. When internally stored AMPAs are added to the membrane because NMDARs are activated, EPSPs starts (active membrane). increased number of AMPARs => increased EPSP size. 3. Increased release of glutamate (presynaptic effect) Mechanism 1 (independent of postsynaptic activity) -> Non Hebbian mechanism. Tetanus causes large Ca++ influx in presynaptic terminals. This leads to a phosphorylation of presynaptic vesicle proteins. It results in an enhancement of glutamate release => increased EPSP size. Mechanism 2 (dependent on postsynaptic activity) -> NMDAR dependent Messengers released from postsynaptic cell (as a result of NMDAR activation). They reach to presynaptic terminal. Processes of glutamate release are modified => increased EPSP size. Long-term depression (LTD) When the activities of two neurons are not associated anymore, the synaptic strength weakens. Hence, downregulation mechanisms are also necessary to maintain homeostasis. It is induced my prolonged low frequency stimulation and there is a decrease of AMPARs. There are several different forms of LTD: Dephosphorylation of AMPARs (postsynaptic effect) Decreased number of AMPARs (postsynaptic effect) Decreased glutamate release (presynaptic effect) Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Spike-timing dependent plasticity (STDP) The differential effect of LTP and LTD can occur based on the timing of action potentials in pre- and post- synaptic cells. (you can stimulate this) This is a more natural induction of LTP/LTD than the induction by tetanus. LTP can be induced by a spike-pairing protocol: A synaptic transmission is evoked in the presynaptic cell before an action potential in the postsynaptic cell = causal => LTP. LTD can also be induced by a spike-pairing protocol. A synaptic transmission is evoked in the presynaptic after an action potential in the postsynaptic cell = acausal => LTD. STDP indicates that plasticity in the neural system is sensitive to causality STDP elaborates the Hebb’s idea of “fire together”: The pre- and post- synaptic firings have to follow a precise temporal sequence. A “causality” (sequence of events), not just a “coincidence” (overlap of timing), is what is detected. LTP in inhibitory synapse LTP and LTD can be induced in GABAergic inhibitory synapses. Mechanisms for inhibitory synaptic plasticity: Presynaptic: -Retrograde signals -Presynaptic metabotropic receptors Postsynaptic: -Phosphorylation of GABAARs -Insertion of GABAARs Homeostatic plasticity Homeostatic plasticity= A slow and large scale plasticity to maintain the range of activity level of a network. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Why is it necessary?  LTP and LTD tend to destabilize activities in a neural circuit (activity becomes too high or too low)  Homeostatic plasticity adjusts synaptic strength to stabilize a neural circuit The neural network tries to keep the activity within a set range by homeostatic plasticity. It controls the activity range, while keeping LTP/LTD effects on individual synapses. The overall synaptic strength is lowered, but the effect of LTP is maintained (the right synapse is stronger than the left synapse). While plasticity is essential for neural functions, neural activity has to be kept in a proper physiological range. Homeostatic plasticity adjusts overall synaptic strength. It maintains neural activity within a range. It is crucial for memory storage and development of the neural system. Morphological changes by plasticity Spines keep changing in numbers, sizes and shapes. There are constant increases (as a results of LTP) and decreases (as a result of LTD) of spines. Induction of plasticity causes morphological changes in spines (“structural plasticity”). LTP as a possible memory storage mechanism. -Cooperativity: Near-simultaneous activation of a large number of afferent inputs. This stems from that release of Mg2+ block of the NMDAr requires a large depolarization, which is achieved only when the postsynaptic cell receives input from a large number of presynaptic cells. -> Events of a high degree of coherence, like these, result in memory storage. -Associativity: A weak presynaptic input normally does not produce enough depolarization to induce LTP. However, if the weak input is paired with a strong input that produces a suprathreshold depolarization, the Mg2+ block is released and LTP occurs. -> An event that has little significance is endowed with a higher degree of meaning if that event occurs with another more significant even (analogous to associative Pavlovian conditioning). -Specificity: If a particular synapse is not activated during a period of strong synaptic stimulation, the NMDAr at that site will not be able to bind glutamate and thus will not be activated despite the strong postsynaptic depolarization. The synapse will not undergo LTP -> Information not related to a particular event will not be stored. Hence, LTP shows the key properties required as an underlying mechanism of memory. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Synaptic plasticity part ll During LTP functional and structural plasticity both occur There is a correlation between structure and function of the spines. If you block the insertion of AMPAR no LTP occurs, but also if you block spine growth no LTP occurs. Rho GTPase are molecular ‘’switches’’ that cycle between Rho GTP (ON-stage) and Rho GDP (OFF- stage) and are the main pathway for actin phosphorylation. It can be regulated by GAP and GEF. Actin is required for spine stability. CaMKll (Calcium/calmodulin-dependent protein kinase II) : when It is not bound to Ca2+/calmodulin it is inactive. CaMKll is a kinase. When it is bound to Ca2+/calmodulin it gets active and starts a cascade of events where cam autophosphorylates and stays active. It is inactive because of autoinhibitory domains. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Transcriptional regulators are important to sustain the effects of LTP. In the figure above you see that also synapse growth proteins are translated. Experiments: -2-photon glutamate uncaging can be used to study LTP at single synapse level. - Förster resonance energy transfer (FRET) signalling can be used to study CaMKll or RhoGTPase activities. By adding two fluorescents at the CaMKll sites you can see that when its inactive -> FRET signal is possible, but when its active -> No FRET signal is detected. -You can add GFP to AMPAR and make it pH7-sensitive. So that only when AMPARs are at the surface, the GFP glows. (Just outside the cell/ at the surface the pH is 7-7.5) Short term plasticity When multiple action potentials are evoked in a presynaptic cell in a short interval (a few hundred milisec or less), the size of PSPs varies systematically. This is called “short-term plasticity”. Short-term plasticity is either Short-term facilitation or Short-term depression:  The properties of short-term plasticity are cell-type specific (the majority of synapses show short term depression, but there are some synapse that have short-term facilitation)  The majority of synapses show short-term depression  Synaptic inputs to SST+ cells are one of the exceptions (they show short-term facilitation) Cell-type specificity of short-term plasticity means: Even a single cell making synapses onto two different cell types show different properties of short-term plasticity. So its dependent on the postsynaptic side. The figure below shows short-term facilitation on the left (with red circle) and short-term depression on the right. mechanisms underlying short-term plasticity Short-term plasticity depends on (in the presynaptic terminal): - Density and distributions of Ca2+ channels - Ca2+ buffering - Availability of vesicles - The mechanisms are postsynaptic-cell-type specific Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Short-term facilitation: - It starts with low Ca2+ activity in the presynaptic terminal => many “failures”. - With repetitive inputs, Ca2+ accumulates and synaptic transmission becomes stronger. short-term depression: - It starts with high Ca2+ activity in the presynaptic terminal => strong 1st PSP. - With repetitive inputs, vesicle depletion occurs and synaptic transmission becomes weaker. Short-term plasticity affects the response pattern of the postsynaptic cell. It influences how signals are transferred from one neuron to another. The signal transfer is history dependent. Patterns of network activity are constantly changing due to the short-term plasticity, reflecting the context of sensory inputs in time (history). Neural Circuit & Signal integration Neurons are connected by synapses, forming a neural circuit. Depending on the firing pattern of individual neurons, short-term plasticity of involved synapses, and the structure of the circuit, a neural circuit functions differently. The term “recurrent connection” can be used for: a single neuron (an axon projects back to its origin) or a group of neurons (interconnections within a group). Inhibitory neurons often make extensive divergent connections. See figure below. Although the number of inhibitory neurons is much less (10~15% of all neurons) than the number of excitatory neurons, inhibitory neurons can still influence dynamics of the neural network significantly because of the divergent connections. Inhibitory input to neuron C is given by an inhibitory neuron B Inhibition of neuron C is suppressed by: -circuit 1: inhibiting the excitatory neuron A that is exciting neuron B OR -circuit 2: directly inhibiting the inhibitory neuron B. In these conditions, it is said that “neuron C is disinhibited”. Summations Temporal and spatial summation: When there are two inputs from the same origin, there is a summation of response. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Summation: Linear and non-linear summation Linear= EPSP1 + EPSP2= 1+2 so 5mV + 5mV= 10mV Non-linear happens more often in biology. Non-linear summation can occur due to: - Voltage-gated ion channels that amplify or attenuate PSPs. - Non-linear effects of multiple channel opening. When the summation becomes larger then it’s a supralinear summation ( so 5mV+ 5mV= 20mV), when it is smaller it is called sublinear summation. The figure below shows that when the stimulation at two locations happen close to each other a supralinear summation occurred. If the stimulation happens in two different branches a normal linear summation occurred. This biological event is difficult to simulate with computational modelling. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen IPSP is not visible at resting condition, so when Cl- potential is the same as the membrane potential. Even when IPSP is not visible (because the membrane potential is at the Cl- equilibrium potential), GABAAR channels are open and they can influence other coinciding PSPs. This is because some portion of the depolarization current by EPSP leaks out from GABA channels. This is called GABA shunting. This is an example for non-linear summation. Layer-specific connections in neocortex Many different types of neurons in neocortex are differentially distributed in 6 layers. They are connected in a specific way. Currently, we only have limited knowledge of cell-type specific and layer specific connections. There are some efforts to develop a database of cell–type specific connections. The exact functions of microcircuits in neocortex should be elucidated in the future. Cells are differently distributed along different layers of the neocortex. (like the percentage basket cells in a layer is different in another layer) Thalamo-cortical (feedforward) projection. This is when the projection goes in the thalamus and out to deep cortical and subcortical nuclei. The majority of feedforward projections from thalamus to primary sensory cortex make synapses at layer 4. Spiny stellate cells in layer 4 receive the feedforward inputs. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Inter-areal connections. Connections between different brain areas in the neocortex. In neocortex, different brain areas are organized in a hierarchical structure. Connections between the areas (different levels in hierarchy) are called inter-areal connections. Inter-areal connections are bi- directional (feedforward and feedback). The inter-areal connections are made in a layer-specific way. General principles of inter-areal connections: In neocortex: - Inter-areal connections are made by pyramidal cells. - Feedforward projections are sent from layer II/III pyramidal cells. - Feedforward projections mainly project to layer IV of the higher area. - Spiny stellate cells in layer IV receive the feedforward projections. - Feedback projections are sent from layer V and VI pyramidal cells. - Feedback projections project to layer I and VI of the lower area. - Both excitatory and inhibitory neurons make local connections (intra-areal connections). - Inter-areal connections are made by myelinated axons (fast). - Intra-areal connections are made mostly by unmyelinated axons (slow). Molecular dynamics at the synapse Learning is an experience-dependent process of obtaining knowledge about the world. Memory is the storage of retrievable information. We think information is retained by neurons. They’re all interconnected. We think information is stored in the synapses. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Excitatory synapses have fast and long-lasting changes in response to activity. There’s a relationship between the function and the size of a synapse. If you release glutamate at a synapse a few times, the synapse grows, because AMPARs are added. LTP and LTD are the molecular correlates of memory. They are the most basic way of retaining information on a molecular level. The memory is stored by modifying the number of AMPARs, i.e., the composition of the synapse (called proteome remodelling (proteome = composition of proteins)). There are multiple ways to look at what those proteins are. One method is to isolate the proteins within the brain, and specifically the synaptic proteins, by dissecting the brain, cutting it into pieces (e.g., only cerebellum, cortex, or hippocampus), and then doing a series of centrifugations. A sucrose gradient is formed: high at the bottom, lower at the top. They optimised the protocol so that the synapses are in the middle, in between the two. Then you can do mass spectrometry to get the composition of the synapses. You can make two types of preparations:  Synaptosome: mainly presynaptic terminal.  Synaptoneurosome: to preserve both the presynaptic and postsynaptic parts. The most abundant proteins are proteins of the actin skeleton. These are important for modelling the synapse. The second biggest group are kinases and phosphatases. These are important to respond to signalling. They phosphorylate things and make sure that the whole response can be orchestrated. One molecule that is especially important is CaMKII, which is central to both LTP and LTD. mRNAs are extremely unstable (they only live for a couple of hours). The proteins live longer than mRNA, but not much longer (a couple of days). Memories are made out of proteins, but can be stored for decades. How does that work? Delivery of proteins to synapses How do synapses contain any proteins, since these are mostly formed close to the nucleus. And even more specifically, how does it contain exactly those proteins needed for that synapse to either be potentiated or depressed? There are three models: 1. Sushi belt model for intracellular molecules. Proteins are delivered to the different compartments and dendrites in the neuron. They are not delivered in a targeted fashion. Instead, there’s a big supply and the synapses can just take whatever they need at any given time. 2. Surface diffusion for transmembrane proteins. For example how AMPARs are put into the synapses. 3. Local mRNA translation/protein synthesis. For example how CaMKII is produced in/close to the synapse and brought into the synapse. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Sushi belt model When a spine is potentiated and needs more receptors, the response needs to be relatively fast (in the timeframe of seconds/minutes). It cannot be that fast if you have RNA polymerase go to the gene on demand and produce mRNA, which then travels and produces its protein. mRNA production in the nucleus takes 5-15 minutes depending on the size and the travelling takes a couple of minutes. So, things have to be ready so that a potentiated synapse can just grab whatever it needs from the dendrite. How are goods delivered to the synapse? It functions a bit like a sushi belt. Vesicles move in all directions, some vesicles stop moving for a bit and then continue moving (in the same or the other direction), and they don’t seem to be directed. When a vesicle moves away from the soma, it’s called anterograde. If it moves back towards the soma, it’s called retrograde. When they stop, it’s called pose (?). Microtubules are a type of cytoskeleton. They are present in the dendrites and act like the highway into the synapse. They allow for active transport of vesicles (it costs ATP, but it’s very fast). The motor proteins move the cargo into the different directions along the microtubules. With chymographs, you can capture the dynamics of molecules that travel along the microtubules. Potentiation will activate CaMKII. This releases the cargo from the motor protein (and thus the microtubule/highway) by phosphorylating the motor protein. The cargo is then delivered to where it needs to be. There are two types of cargos that can be delivered to the synapse:  Transmembrane vesicles containing protein. These vesicles containing receptors need to fuse to the plasma membrane so that the receptors are added to the membrane and can go to the synapse. E.g., glutamate receptors.  mRNA granules aka RNPs (ribonucleoprotein particles). These need to disassembled so that the mRNA can be translated and the protein can be produced. E.g., CaMKII. Surface diffusion for transmembrane proteins How are the receptors delivered to the synapse? The receptors arrive inside vesicles. Inside the vesicles, they are not active; they need to be established at the surface of the neuron. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen  The receptor is transported inside a vesicle via the microtubule.  At some point it pauses because there’s a lot of CaMKII activation (blue in the picture below).  The vesicle is released from the motor protein/microtubule.  Via exocytosis, it leaves the  cytoplasm and enters the plasma membrane.  Here it is mobile/can move, since it’s not anchored to anything.  It moves until it finds something it can bind to: the postsynaptic density (PSD). How does it bind to the PSD? There is an auxiliary subunit, which is a molecule that’s part of the receptor. It consists of:  An N terminal on the extracellular side of neuron.  Several transmembrane (TM) domains.  A PDZ-binding domain. It bind to the scaffolding proteins in the synapse. It’s basically the anchor.  RS-domain: consists of a lot of Serine and Arginine residues. Serine is the main residue that’s phosphorylated in the cell. They are the targets for CaMKII. They are positively charged when they are not phosphorylated, so this domain is highly positively charged. When CaMKII is activated, the receptor stops moving.  A C tail. The C tail and the RS domain together are important for the receptor to stop. However, they are far away from each other. They wanted to look at the interaction between the receptor and a PSD protein. This protein is orientated perpendicular to the membrane. Their hypothesis was that when you phosphorylate the part of the receptor at the bottom (I think the RS part?), you actually release the C-terminal domain so that it has better access to the binding site on the PSD protein. It binds with more efficiency, which increases the number of receptors that binds to the synapse. They used FRET to test this theory. They measured how long the electron stays excited. If there are more ways to deexcite (because the other fluorophore takes the energy to become excited itself), it will stay excited for less time. Thus, if the lifetime is longer, it means that FRET doesn’t happen. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Phosphorylation is adding phosphates to a serine residue, which will become negatively charged. You can also mimic phosphorylation by replacing the serine by some amino acid that is negatively charged. It’s not perfect, but it works charge-wise. They wanted to test whether it was the charge that mattered, or the fact that when it’s negatively charged, the cytoplasmic domain can go deeper into the cytoplasm and have better access to the binding domains (which would be measured as more interaction between the two fluorophores and thus a shorter lifetime). Local mRNA translation/protein synthesis Another example of something that needs to be delivered to the synapse is CaMKII. When there’s LTP, the amount of CaMKII is increased in a local fashion. CaMKII mRNA is taken up and translated so that more CaMKII proteins can be added to the potentiated synapses. This process works the same for a lot of other mRNAs that produce synaptic proteins that respond to LTP. The advantages of having proteins produced directly in the dendrite:  Cost of transport. From one mRNA molecule, multiple proteins can be produced. So, it is only the cost of one transport, but you end up with multiple copies.  Improved protein targeting. If you transport proteins, it can be lost or moved to a different compartment. Here it’s produced locally.  On demand protein synthesis. If you have to produce the proteins in the soma and then transport them, it takes a long time. If you have the mRNA that is already on the belt and grab it when you need it, it is a lot faster. Neural recording methods neural recording of single cell activity: 1. Extracellular recording 2. Intracellular recording -Sharp electrode -Patch clamp 3. Optical recording -Calcium signal recording -Voltage signal recording Large scale recording: fMRI, MEG, EEG, NIRS, Ultrasound imaging A neuron (cell) is a space enclosed by a hydrophobic lipid bilayer. In the membrane you have pumps that control the molecule gradient and channels that open and close. From the neuron there is an electrical signal leaking out of the cell for cell-cell communication. Recording: Extracellular recording is when you insert the recording right outside of the cell. The signal is low and should be amplified. Intracellular recording enters the intracellular space. Patch clamp recording is exactly at the membrane of the cell for whole cell configuration. Optical intracellular recording by a Ca2+ sensitive channel rhodopsin. Stimulation: Optogenetic stimulation makes use of light sensitive channel rhodopsin. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Micro-injection for local stimulation For example caged glutamate. Only by light stimulation the glutamate uncages and activates the neurons. Pros and cons of recording techniques Extracellular recording pros: Easy, long period recording (days), whole population recording. Cons : Signals are weak, required strong amplification, shapes of action potentials are distorted, signals are dependent on the position of the electrode, impossible to record individual events. Intracellular recording Pros: High signal-to-noise ratio, intrinsic shapes of action, individual events can be recorded. Cons: Tip needs to be small, short recording time (1hr), sensitive to motion, not for multiple cells at the same time, in vivo recording is difficult as you have to enter the electrode inside a cell. Intracellular recording gives important information that extracellular recording cannot (synaptic signals, membrane potential, shapes of action potentials) But sharp electrode intracellular recording has become obsolete (it was a common technique in the last century). It is mostly replaced by patch clamp recording and optical recording today. Patch clamp recording (detects synaptic events) Pros: High signal-to-noise ratio, in vivo recording, all of the intracellular pro’s without the sensitivity to motion, longer recording period (hours). Cons: Only a few cells (~12) can be measured. Essential intracellular macromolecules may be removed from the membrane. There are to modes of patch clamp recording: ‘Current clamp’ and ‘voltage clamp’. -The current clamp records voltages: membrane potential is recorded, EPSP/IPSP, around 10mV, a current is added, depolarization is shown as a positive curve. -The voltage clamp records currents: membrane current is recorded, EPSC/IPSC, around 50pA, voltage commands are given, inward current is negative. Why voltage clamp? - The state of voltage-gated ion channels can be controlled. - When synaptic inputs are recorded in current clamp mode, the signal involves responses by synaptic transmission, charging processes of membrane, and voltage-gated ion channels. - In voltage clamp mode, synaptic signals are isolated from the other current. Electrical stimulation techniques Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Deep brain stimulation (DBS) and Brain Computer Interface (BCI). Pros: Electric stimulation has been used since the early time of electrophysiology, Various stimulation electrodes are available, It is quick and easy, The parameters can be adjusted easily. Cons: Non-specificity of neural stimulation. Optical recordings: Pros: - Good spatial resolution - The recording is relatively easy - It is possible to record for a long time (days) - Populations can be recorded - Individual synaptic events can be recorded - Specific cell types can be selectively recorded - It can be used for in vivo recording with freely moving animals - Technologies in optics are progressing rapidly Cons: - The tissue has to be pre-processed before recording with optical indicators - Expression levels of the molecules vary between cells - “Bleaching” (gradual decay of fluorescent signal) - Temporal resolution is less than electric recording - Membrane potentials cannot be measured - Membrane potentials cannot be manipulated as in patch clamp How to load neurons with optical molecules? A. Intracellular (inserting inside a single cell) B. Extracellular (whole network loading, cells take up the molecules by diffusion. C. Genetic engineering (viral, transgenic mice) Optical indicators: Ca2+ indicator (GECI) - strong signal = high signal-to-noise ratio - slow voltage indicator - fast - reflects small changes in membrane potential - weak signal - requires high power light source Ca2+ indicators emit strong signal and easy to handle. Ultra-fast voltage indicators can record action potentials with a high temporal resolution. Voltage indicators can record small synaptic events, for example, spontaneous synaptic input is too small for depolarization. The result is only of AMPA receptors and no calcium influx takes place. This can only be measured by a voltage-indicator. Optogenetic stimulation Channel rhodopsin (ChR2) is found in many species and is involved in various functions sensitive to Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen light (phototaxis, photoreceptors in the retina, etc). Illumination of light on ChR2 causes openings of channels. This opening of channels cause depolarization of the neuron. It can be combined with fluorescent proteins to visualize the cells carrying ChR2. ChR2 can be delivered to specific target neurons. Hence, specific neurons can be selectively stimulated. (=optogenetics) Pros: specific, multiple locations of the brain can be stimulated, Activation of channel rhodopsin and most of its variants depolarize neurons nut “inhibitory opsins” can be used for hyperpolarization. There are more optogentic molecules. ChETA and ChrimsonR are faster than Chr2 for instance. Summary: With rapidly progressing technologies in optics and genetic engineering, optical physiology provides robust approaches in neuroscience, allowing us to record and/or stimulate large populations of specific neurons with high spatial and temporal resolutions. Can “all-optical physiology” replace the electrophysiology entirely? The answer is no, so far. It is still the best to record electric signals by electrical approaches (for example, actual values of membrane potential cannot be measured by optical recording). Optical physiology and electrophysiology work in a complementary manner. Fill in the blanks below to practice: Extracellular recording Intracellular Optical recording Pros cons Network properties of neuronal tissue Dissociated neuronal cultures: dissociated soma plated on day of birth, a mixture of neurons and astrocytes are plated. Eventually the neurons will form synapses wherever axons and dendrites cross. Neurons start to be active after 1 week and matured neurons after 3 weeks are measured (between 3-4 weeks). Inactive synapses are removed. The neuronal spontaneous bursting is measured. Network burst= synchronous firing of neurons in a network. Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen Oxygen is important for ATP function. During hypoxia is found that the spontaneous firing rates goes down as well as the synchronicity. This is found in patients as well as in the cultured dish in which hypoxia is induced. Electrophysiology: -Firing rate -Synchronicity -Stimulus responses -Functional connectivity -Network excitability Immunostaining (optical measurement) -endo/exocytosis, number of synapses and other proteins Effect of low oxygen on network functioning: ischemic penumbra, hypoxic ischemic encephalopathy. Memory is said to form through systems consolidation. Ischemic penumbra: around the core, there is still some blood supply after brain damage by a blood clot in the arteries of the brain, but way lower than before the stroke. The brain damage cannot be seen, but there is no brain activity. Ischemic penumbra can be recovered or it gets part of the core. Why is unknown. Dissociated cortical cultures under controlled oxygen/nitrogen levels are used to investigate the mechanism behind this. Question: Do we see a difference in spontaneous firing rate if we mimic ischemic penumbra.? Activity drops during hypoxia, but it is initially reversible. Most neurons survive. So the reason behind this cannot be cell death (as cells can be recovered). So it can’t be cell death that occurs. Question: is it due to synaptic failure? By adding glutamate to hypoxia, we do see action potential/activity -> this means that the loss of function is at the presynaptic synapse, Where exocytosis and endocytosis happen. With FM dye you can investigate this as FM is taken up by the cell through exo/endocytosis. FM is only fluorescent when it is taken up by the cells. (outside the cell it is not fluorescent, so endocytosis is measured) When taken up by the cells, you can stimulate the cells and they will lose the FM by exocytosis. (exocytosis is measured) They found that synaptic failure is the reason of activity drops during hypoxia, however, cell death does occur at the same time in wide ranges of hypoxic depths. Does decreased synaptic activity cause cell death? Network excitability goes up after 50% hypoxic depth, because the synapse sensibility is increased by overcompensation of the lack of oxygen. (activity homeostasis) Inhibitory neurons are more vulnerable to hypoxia compared to excitatory neurons, the number of inhibitory synapses decrease while excitatory synapses stay more or less the same. Question: Does synaptic inactivity cause cell death? Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen This is investigated by measuring apoptosis in hypoxic cell cultures. After 6 hours by hypoxia, apoptosis starts. After 12h of exposing cell cultures with TTX (sodium channel blocker), apoptosis occurs. Closed-loop activation: Use Channel rhodopsin to stimulate the cells with light and measure the activity. Then cause hypoxia and the activity will drop. Whenever the activity drops too far, the light is switched on to recover cell activity towards the threshold range and the light is switched off to prevent the activity to go above the threshold range. This improves activity recovery and survival. Conclusion:  Low ATP-> Synaptic failure -impeded endocytosis and exocytosis -Post-synapse remains functional  Activity homeostasis  Energy consuming, limited effectiveness  Cell death is possibly due to insufficient activity- > It could be lactate dependency or BDNF decrease but this is still hypothesized.  Activation of synapses may promote recovery/ survival (Ghrelin trial) Memory in dissociated cortical networks System consolidation: Repeated activation/replay of memories by the hippocampus to form long term memory in the neocortex. Activity of neurons depends on connectivity, but also vice versa. The activity and connectivity have to be in balance. Criteria to form a memory trace 1. Formed in an experience-dependent manner; 2. Specific to the presented cues; 3. Outlast the period of exposition to the cues; 4. Reactivate with subsequent presentation of the stimuli. Learning; new input causes connectivity changes. Input without connectivity change could be due to: insufficient stimulation, sheer coincidence or learned and already learned and memorised. Memory traces can be stored in parallel, without overwriting each other. Simple model nicely reproduces experimental findings: Activity - connectivity balance is very robust External input disturbs balance Initially large connectivity change Subsequent changes are far less significant Different inputs have similar effects Returning to 1st input has no further effect Stimulus response ‘approaches’ spontaneous patterns. Why does systems consolidation occur during slow-wave sleep? The sharp wave ripples found in sleep. The other difference in sleep and wake state is that Acetylcholine is more present during wake time compared to sleep. Synchronous firing (oscillatory) Gedownload door: ChickenNuggets010 | [email protected] € 912 per jaar Dit document is auteursrechtelijk beschermd, het verspreiden van dit document is strafbaar. extra verdienen?  Stuvia - Koop en Verkoop de Beste Samenvattingen during sleep is higher. Acetylcholine (ACh) plays an important role in memory function and has been implicated in aging-related dementia, in which the impairment of hippocampus-dependent learning strongly manifests. Cholinergic neurons densely innervate the hippocampus, mediating the formation of episodic as well as semantic memory. High cholinergic tone hinders memory consolidation. Extra notes after re-watching all the lectures: -Excitatory neurons can have inhibitory (GABA) receptors. (so they excite the post- but themselves can be inhibited by another neuron) -Disinhibition: Inhibitory neurons have inhibitory receptors. -Dale’s law: A neuron releases the same neurotransmitters at all of its synapses. Meaning one neuron can makes synaptic connections with multiple target neurons. All neurons will receive the same neurotransmitter that’s being released. -Astrocytes clean up the synaptic cleft by removal of extra neurotransmitters and recycles molecules etc. -The reversal potential of GABAergic IPSPs are cell-specific and it depends on the Cl- pumps. (reversal potential is that when the equilibrium of the membrane is higher than the Ecl- then depolarization occurs, but when the membrane equilibrium is lower than Ecl - then hyperpolarization occurs.) -Early LTP: CaMKll induced phosphorylation of AMPAR and they move towards the membrane so more AMPAR are added. - Late LTP: CREB system is used for RhoGTP to activate transcription of actin which in turn causes spine growth. -Short-term plasticity is postsynaptic cell type specific, different connections by the same cell evokes different short-term plasticity. Naoki said we should know these terms as well: Written by Stephany. 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