1. Brains are composed of separate neurons/other cells. 2. Cells are independent. 3. Neurons are polarised cells. 4. Info is transmitted from cell to cell across tiny gaps.

- graded potential in input zone - when presynaptic neuron = excited by incoming signal (neurotransmitter)

- spikes - generated in integration zone if depolarisation passes threshold - signal transmission continues towards output zone

- depolarisation of axonal terminal membrane opens Ca2+ channels; Ca2+ ions enter terminal - Ca2+ concentration increase stimulates release of neurotransmitter stored in vesicles - when vesicles fuse w/presynaptic membrane, neurotransmitter diffuses into synaptic cleft - neurotransmitter either crosses synaptic cleft; interacts w/ionotropic receptors embedded in membrane of dendrite/soma of postsynaptic neuron - neurotransmitter can also interact w/metabotropic receptors

Synapses and Networks Flashcards by Gabriela Ilewicz

HISTORY OF NEUROSCIENCE

early theories on brain organisation diffuse reticular syncytium of neural matter
GOLGI (1843-1926)
historical silver neuron stains (bathing tissue in potassium chromate/silver nitrate solution)
RAMON Y CAJAL (1853-1934)
“father of neuroscience”
bril neuroanatomist; artist; drew microscopic brain structures

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THE NEURON DOCTRINE

Brains are composed of separate neurons/other cells.
Cells are independent.
Neurons are polarised cells.
Info is transmitted from cell to cell across tiny gaps.

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SIGNAL GENERATION/TRANSMISSION DETERMINED VIA ION CHANNEL NATURES

- gen ion channel locations (in varying numbers/densities):
VOLTAGE-GATED CHANNELS
- axonal hillock (integration zone of axon)/axon (conduction zone)
LEAK CHANNELS/ION PUMPS
- entire neural membrane
VOLTAGE-GATED CA2+ CHANNELS
- axon terminals (output zone)
LIGAND-GATED CHANNELS
- dendrites/soma (input zone)

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SIGNALS TRANSMITTED OVER SYNAPSE TO NEURONS/TISSUES UNDER DIRECT NEURONAL CONTROL (MUSCLES/GLANDS)

PRESYNAPTIC CELL
- cell body -> axon -> synapse
POSTSYNAPTIC CELL
- synapse -> axon -> another neuron/muscle

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SYNAPTIC PROCESS LOCATIONS

TYPICAL 
- axo-dendritic 
- axo-somatic
- axo-axonic 
RARE 
- dendro-dendritic

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SERIAL ELECTRON MICROSCOPY RECONSTRUCTION

focused on axonal inputs (various colours) onto a small segment of apical dendrite

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GOLGI-IMPREGNATED PYRAMIDAL CELL

in hippocampal area CA1

- have soma/apical/basal dendrites

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DEPOLARISATION

graded potential in input zone

- when presynaptic neuron = excited by incoming signal (neurotransmitter)

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ACTION POTENTIALS

spikes
generated in integration zone if depolarisation passes threshold
signal transmission continues towards output zone

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SIGNAL TRANSMISSION TO NEXT NEURON

when action potentials reach output zone

- neurotransmitter released into synaptic cleft (chemical synapse)

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SIGNAL TRANSMISSION ACHIEVED

achieved if neurotransmitter leads to graded potential (depolarisation/hyperpolarisation) in input zone of postsynaptic neuron

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PRESYNAPTIC NEURON

depolarisation of axonal terminal membrane opens Ca2+ channels; Ca2+ ions enter terminal
Ca2+ concentration increase stimulates release of neurotransmitter stored in vesicles
when vesicles fuse w/presynaptic membrane, neurotransmitter diffuses into synaptic cleft
neurotransmitter either crosses synaptic cleft; interacts w/ionotropic receptors embedded in membrane of dendrite/soma of postsynaptic neuron
neurotransmitter can also interact w/metabotropic receptors

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IONOTROPIC RECEPTORS = LIGAND-GATED ION CHANNELS

ligand-gated ion channels open when bound by neurotransmitter molecules
dif types of iontropic receptors; vary in affinity for particular neurotransmitter/drug
reuptake = transmitter taken up to presynaptic cell
some neurotransmitter molecules don’t close cleft; bind to auto-receptors that inform presynaptic cell about net

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METABOTROPIC RECEPERS

slower; control ion channels indirectly
coupled to G protein (guanine nucleotide-binding) consisting of 3 subunits (therefore also known as GPCRs (G protein-coupled receptors))
when activated by conformational change of GPCR (shape change), G protein (alpha-unit bound to GTP (guanosin triphosphate)) can interact directly w/ion channel or control it via second messenger molecules release inside postsynaptic cell (ie. cAMP (cyclic adenosine monophosphate)/PIP2 (phosphatidynositol 4, 5-bisphosphate)
neurotransmitters synthesised; stored in vesicles in neuron’s output zone

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IONOTROPIC RECEPTORS

- fast/signal transmission; ie.
AMINS
- acetylcholin (nicotinic/nACh receptors/serotonin (5-HT)
AMINO ACIDS
- glutamate (NMDA/AMPA receptors)
- gamma-aminobutyric acid (GABA A)
- glycine
- aspartate

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METABOTROPIC RECEPTORS

- slow/long-lasting; more varied effects; neuronal modulation; ie. 
AMINS
- acetylcholic (muscarinic) 
- dopamine/serotonin
- norepinephrin
- octopamine
AMINO ACIDS
- glutamate 
- GABA B
- glycine
NEUROACTIVE PEPTIDES
- vasopressin (antidiuretic hormone ADH)
- oxytocin

GAP JUNCTIONS CONNECT CYTOPLASMS OF 2 NEURONS

instantaneous current flow (v fast electric signal transmission across connexons producing coupling effect ie. virtually no time delays)
gap junction (membrane gap) small as 20-40nm
found where fast responses/activity synchronisation required
fast action = commanding escape responses (crayfish/fish)
synchronised activity = inhibitory neurons in mammalian brain; eye-moving muscles

NEURAL SIGNALS OVER DISTANCE

spiking neurons w/long axons transmit signal via action potentials along axon
signal sustained effectively via voltage-gated ion channel pop in axonal membrane
non-spiking neurons w/short/thin/no axons don’t generate action potentials; signal spreads passively to output zone

SIGNAL STRENGTH OVER DISTANCE

weakens the longer it travels
longer distance = stronger attenuation
solutions to reduce signal transmission costs include:
1. long neurons have thick axons (ie. squid/invertabrates)
2. white matter = myelinisation of axons in spiking neurons (vertebrates)

NEUROLOGIA CELLS

SCHWANN CELLS/OLIODENDROCYTES
- assist signal propagation
ASTROCYTES 
- provide nutrients to neurons
MICROGLIA 
- clear debris
- mediate immune response

AXON MYELINSATION W/SALTATORY TRASMISSION GAPS

gaps for saltatory transmission of action potentials
neural membrane exposed at nodes of Ranvier for ion conductance via voltage-gated channels
saltatory conduction of action potentials increases transmission speed

CONDUCTION VELOCITY ^ W/^ OF DIAMETER/AXON MYELINISATION

myelinated neurons w/thin axons can reach similar conduction velocities as those w/unmyelinated thick axons

EACH NEURON FORMS MANY SYNAPSES

neurons collect info from few -> hundreds of others
when/which signal is picked up depends on:
1. synapse type (excitatory/inhibitory); associated neurotransmitter
2. synapse number; spatial position on dendrites/soma of input zone
3. duration/synchrony of neurotransmitter release from dif synapses

NEUROTRANSMITTER TYPE/RECEPTOR DEFINES POSTSYNAPTIC POTENTIAL TYPE

EXCITATORY 
- glutamate 
- aspartate 
- nicotinic acetylcholine (nACh)
INHIBITORY 
- GABA
- glycine
- muscarinic acetylcholine

NEUROTRANSMITTER/RECEPTOR REACTION AT POSTSYNAPTIC MEMBRANE

DEPOLARISATION - EPSP (excitatory postsynaptic potentials) HYPERPOLARISATION - IPSP (inhibitory postsynaptic potentials)

TEMPORAL SUMATION

- if neurotransmitter released for longer into synaptic cleft, postsynaptic potential = stronger

MULTIPLE SYNAPSE ACTIVATION EXAMPLE

- imagine 2 excitatory & inhibitory synapses activated simultaneously by 2 dif stimuli - spatial summation = if postsynaptic potentials arrive together in integration they're summed up - spiking neurons = if membrane at integration zone depolarised above threshold, action potential generated; the more excitatory input arrives, the stronger the output signal - at integration zone, EPSPs/IPSPs arriving simultaneously/within small window = summed up - the more inhibitory input arrives, the weaker the output signal; neuron may not even transmit output signal

NEURAL LANGUAGE VOCAB

- when neuron receives info from single neuron, it responds in binary code (ie. yes/no) - YES = signal gets transmitted to output zone; neurotransmitter released - NO = signal doesn't reach output zone SO neurotransmitter not released

NLV: WHEN NEURON RECEIVES INFO FROM MANY NEURONS

- w/synapses neurons can only use simple arithmetics: 1. sum (temporal/spatial summation) 2. multiply - summing up (local operator "and") = YES - if both EPSPs (1/2) present simultaneously & sum up to depolarisation that surpasses determined threshold THEN action potential generated/signal transmitted - summing up inhibitory input of sufficient strength (logical operator "not") = NO - if IPSPs (3/4) present simultaneously as EPSPs (1/2) THEN action potential NOT generated/signal not transmitted - whether signal transmitted/not depends on sum of relative EPSPs/IPSPs strengths that arrive at integration zone in given time window

INFO CODED IN NEURAL NETWORKS

- spatial/temporal summation at synapses determine how signal travels through network - connectivity of network determines when/where signal travels faster/slower = amplified/reduced/muted - in feedforward circuits, signal distributed to many neurons through divergent connections OR determined via collecting signals from many converging neurons - divergence = neuron broadcasts to many others - convergence = neuron listens to many others; has high sensitivity; gatekeeper/decision-maker - feedback loops (positive/negative) provide direct/indirect input influencing signals, thus info - signal amplified soon after reaching network (positive loop/excitatory feedback synapse)/reduced (negative loop/inhibitory feedback synapse)

COMPUTATIONAL NEURAL NETWORK MODELS

- simple neural language rules (summation/multiplication) & binary (0/1) code - complexity of neural language comes w/number of connections/layers/connectivity types between neurons in networks CONNECTIONIST MODEL - ANNs (artificial neural networks) for modelling/understanding cog/brain DEEP LEARNING - ANNs for solving AI problems (ie. object recognition in images/speech recognition); may not require reference to cog mechanisms/neurobiological circuits; don't aim to explain details on brain function

SUMMARY

- neurons communicate via electric signals - dif ion channel types gen located in dif neuron parts - neuron doctrine = brains composed of separate neurons & other independent cells; neurons = polarised; info transmitted cell -> cell across synapses - synapses = electric (rare/fastest)/chemical (most frequent) - ionotropic (ligand-gated ion channels)/metabotropic (coupled to G-protein) receptors - signal transmission over long distances requires modifications for loss of signal (axon diameter/myelinisation) - saltatory signal conduction in myelinised neurons