Cells {neuron} can have cell body, dendrites sending signals in, and axon carrying signals out.
shapes
Surroundings and connections cause unique neuron shapes. Large named neurons with special shapes include spinal cord anterior horn cells, cerebellum Purkinje cells, lateral vestibular Deiters cells, teleost Mauthner cells, primate cerebral cortex Betz cells, and primate cerebral cortex Meynert cells. Mauthner cells are for escape and startle reflexes.
size
Neurons are largest human cells, with average diameter 20 microns. Neuron volume is thousand times larger than bacterium volume.
firing rate
Neurons can output signals at maximum rate {rate saturation, neuron}, different for different neurons, up to 900 per second.
neuron genes
100 different neuron types express different gene sets. 80% of genes have repression, and the other 20% have expression at different levels, varying with cell conditions and transcriptional control. 20% of genes are for transcriptional control [Keller, 2000] [Stevens, 1998].
individuality
Neurons are different, because brains have hundreds of neurotransmitters and neurohormones, such as amino-acid derivatives, peptides, and small RNAs. Neurons have different membrane-receptor, membrane-polarization, myelination, microtubules, and gene-expression patterns. Neurons differ temporally, with different responses over different time scales. Interactions cause neurons to act differently [Grush and Churchland, 1995] [Shepherd, 1991].
One neuron {command neuron} can code for complex functions {single neuron doctrine}, because it responds to feature set [Calvin and Ojemann, 1994] [Kreiman et al., 2000] [Kreiman, 2001] [Kreiman et al., 2002] [Ojemann et al., 1998]. For example, after training, neurons can respond preferentially to face, hand, or object aspects.
problems
Number of brain neurons is not large enough to account for all possible objects and views. Neurons do not seem to converge on one brain area. Convergence is slow process, but pattern-representation formation is fast. Researchers have not found brain command neurons. Single cells cannot change efficiently in response to environment or body changes.
Dendrite proteins {cytoplasmic polyadenylation element binding protein} {CPEB protein} can bind to polyA regions, have active and inactive states, build other synapse proteins, and affect other protein shapes.
Neuron extracellular-potential changes have minor electrical effects {ephaptic interaction} on other neurons [Holt and Koch, 1999].
Neuron output depends on product of various inputs {gain field}. Outputs combine neuron population codes.
Neuron can output continuously variable electrical potential {graded potential} rather than impulse.
A microelectrode {neuronography} can stimulate one nerve.
Neurons have reflexes, ON-center neurons, other neuron types, ganglia, orientation columns, topographic maps, association cortex, and memory systems {neuron assembly}.
reflexes
Brain can modify reflexes. Inhibition from controllers and excitation from motivators compete to cause behavior or fading.
ON-center and OFF-center neurons
ON-center and OFF-center neurons detect points and lines. ON-center and OFF-center neuron arrays can detect point and line arrays and so textures, boundaries, and objects.
neuron types
Besides ON-center neurons, other neuron types detect color and other intensities.
orientation columns
Cortical orientation columns detect line and boundary orientations and detect angles and so detect surface orientations.
topographic maps
Ganglia and topographic maps code relations among perceptions and motions.
association cortex
Association cortex controls series and parallel perceptual and motor systems, integrating information to guide behavior.
memory systems
Cortical systems have three-dimensional registers to hold spatial perceptual information temporarily and permanently.
The main fiber {axon}| from soma is thin and smooth cylinder with neurofilaments. It is conductive. It has same-size branches at obtuse angles. It has bulbous endings and/or calyciform endings. It has small varicosities in chains. It has no ribosomes. It attracts specific dendrites.
Chemical flow in both directions supplies axon ends with molecules from cell body.
number
Most neurons have one axon. Retinal amacrine and olfactory granule cells have no axons. Dorsal root ganglion cells have multiple axons and no dendrites. Dorsal root ganglion cells have axon that bifurcates. Invertebrate cells often have one axon, with dendrites from it.
microtubule
Only axon hillock and initial segment have microtubule fascicles, have membrane undercoating, and have high sodium-channel density.
excitation or inhibition
Axon terminals are either all excitatory or all inhibitory.
regeneration
Axons can regenerate, if allowed by surrounding glial cells.
Axon initial segment {trigger zone} {axon hillock}| has many sodium channels and allows action potential to trigger.
If axon disrupts, Nissl substance changes appearance {chromatolysis} over 4 to 12 weeks.
Microtubules and neurofilaments {cytoskeleton} make cell and axon framework.
Wide filament protrusions {dendrite}| from soma have synapses for axons.
anatomy
Dendrites have microtubules. Dendrites do not myelinate or have one myelin layer.
shape
Branching dendrites provide maximal surface area for receiving input from other neurons. Larger diameter and/or shorter length make larger effects on initial segment.
Proximal and distal dendrites are different.
Widespread dendrites receive from many sources. Compact dendrites receive from one source.
Dendrites can radiate straight out in all directions with few spines, as in large ventral-horn motor cells and reticular-formation cells. They can branch with spines curving in one direction, as in cerebral-cortex pyramidal cells and secondary sense nuclei. They can have special patterns and locations. Cerebellar Purkinje cells are planar semicircles. Inferior-olive clustered cells are curved and wavy. Ventral-cochlear nucleus cells are tufted. Smaller branches are at acute angles and have thorns.
Dendrite patterns match incoming axon patterns. Branches orient along body axes, brain surfaces, and nerve bundles.
Dendrites can change shape over days.
main
Cell body typically has several dendrite origins {basal dendrite}. Cerebellar Purkinje cells have one dendrite trunk {apical dendrite}. Dorsal-root ganglion cells have no dendrites. Invertebrate cells often have one axon, with dendrites from it.
properties
Dendrites have high resistance and capacitance. At dendrite ends, membrane is relatively unexcitable.
If stereotyped behavior happens over four to five months, dendrites from several antagonistic motor neurons make a bundle {dendritic bundle}.
Dendritic protrusions {dendritic spine} have asymmetric synapses. Spines vary in shape, size, and density, even on one dendrite. They can change shape over days. One dendritic spine has only one Type 1 synapse but can also have one Type 2 synapse. Spines have alpha-tubulin, beta-tubulin, actin, and myosin filaments. They have endoplasmic reticulum. Excitation is at spine tips. Inhibition is at dendritic bases or on cell surface. More spines indicate more excitation.
All animals have synapses {electrical synapse} {gap junction, synapse} that use ion flows in one direction and are excitatory or inhibitory [Beierlein et al., 2000] [Blatow, 2003] [Gibson et al., 1999].
function
Electrical synapses make adjacent cells fire at same time. Cortex interneuron groups link by electrical synapses and can act together to inhibit.
comparison
Electrical synapses are faster but less efficient than chemical synapses, with signal one-quarter original signal. For example, if presynaptic membrane is 100 mV, post-synaptic membrane is 25 mV.
properties
Electrical synapses cannot have facilitation and do not change shape.
Sodium, potassium, calcium, and chloride have passageway {ion channel, neuron}| through membrane protein. Ion channel for receptor potential differs from ion channel for action potential [Doyle et al., 1998] [Heinemann et al., 1992] [Hille, 2001].
Synapses {Malsburg synapse} {von der Malsburg synapse} can rapidly control connectivity between cells, allowing transient cell assemblies.
Lipids {myelin}| can increase axon conduction rates and separate nerve fibers. Schwann cells in PNS, and oligodendrocytes in CNS, make myelin. Schwann cells measure neuregulin in axons and make more myelin if it is higher and less if it is lower. Myelination begins in brain lower back after birth and moves toward frontal lobes, finishing about age 25. Myelin can have up to 150 layers. Conduction is fastest when axon diameter to total diameter is 0.6.
Neuron membrane sites {neuroreceptor} bind molecules.
types
Neuroreceptors include alpha-adrenergic catecholamine such as alpha2-adreneric, AMPA, angiotensin, beta-adrenergic catecholamine, D1, D2, GABA, glycine, kainate, M, metabotropic, muscarinic ACh, N, and NMDA receptors.
hormone
Hormone binds to cell-membrane outer-surface neuroreceptor protein, which opens membrane channel for up to one second. On cell-membrane inner surface, neuroreceptor protein couples to G protein and activates adenylate cyclase, guanylate cyclase, phospholipase c, or phosphoinositidase C, which produces soluble cAMP, cGMP, or phosphoinositide second messenger, which diffuses into neuronal cytoplasm and changes local membrane potential.
Cyclic nucleotide or phosphoinositide can either stimulate or inhibit other enzymes. ADP triphosphoinositide {phosphatidylinositol 4,5 diphosphate} hydrolyzes to release water-soluble inositol triphosphate (IP3) (ITP), which releases calcium ion from intracellular storage, which initiates enzyme phosphorylation. Phosphoinositidase C hydrolysis makes diglyceride containing arachidonic acid, which, with calcium and phospholipid, activates protein kinase C.
neurotransmitter
A 10-nanometer-wide glycoprotein channel spans cell membrane and activates by neurotransmitter. Activation allows ions to flow through channel down concentration gradient. Sodium ions flow from outside to inside membranes. Potassium ions flow from inside to outside membranes. Chloride ions flow from outside to inside membranes. Channel opens for only one microsecond, because neurotransmitter rapidly dissociates or inactivates.
Ribosome clumps {Nissl body}| {Nissl substance} are in rough endoplasmic reticulum cisterns. If axon disrupts, Nissl substance changes appearance {chromatolysis, Nissl substance} over 4 to 12 weeks.
Points {Ranvier node} {node of Ranvier}| along myelinated axon have no myelin. Conduction jumps from node to node. Beside node, which has sodium channels, is paranode, which has juxtaparanode, which has potassium channels, beside it.
Neurons have cell bodies {soma} {perikaryon}.
Protein discs {postsynaptic density} (PSD) are on presynaptic and postsynaptic chemical-synapse membranes. PSDs have beta-adrenergic, glutamate, and gamma-aminobutyric acid (GABA) receptors. They have protein kinase enzymes that phosphorylate to alter synaptic structure. They contain filamentous proteins that can move and change shape, such as actin, actin/calmodulin-binding protein, fodrin or brain spectrin, and tubulin. Fodrin or brain spectrin is an actin-binding and calmodulin-binding protein.
Axon terminal synapses have hexagonal grids {presynaptic grid}, with six particles surrounding each vesicle.
Cell enzyme produces soluble cyclic nucleotide, cAMP or cGMP, or phosphoinositide {second messenger}|, which diffuses into neuronal cytoplasm and changes local membrane potential.
Axons connect to dendrites at chemical sites {synapse}|. Neuron activity, habituation, and sensitization affect synapses. With more activity, number of synapses per neuron increases, synapse density per unit volume rises, and dendrite length increases. Neuroactive compounds exert influence up to 20 nanometers within synaptic cleft or up to 2 millimeters from varicosities or unstructured release points [Gray, 1977].
In synapses, space {synaptic cleft}| between membranes is 20 nm wide and has acidic and basic glycoproteins and mucopolysaccharides, with dense line in middle, that bind membranes. Synaptic cleft is bigger in asymmetric synapses.
Cortical axons have ending arrays {arborization} {synaptic terminal}| with total diameter 0.5 millimeters, containing 2000 boutons and synapses.
Presynaptic areas have membrane sacs {synaptic vesicle}| {vesicle} with neurotransmitter molecules. Vesicles contain only one transmitter type.
types
Clear synaptic vesicles contain acetylcholine, glycine, GABA, glutamate, aspartate, or neurohormones. Vesicles with granule in middle contain dopamine, noradrenaline, adrenaline, or serotonin. Larger granular vesicles contain peptides.
transmitters
Adrenal chromaffin cells store opiate peptides and catecholamines. Sympathetic neurons and neuromuscular junctions store ATP and other transmitters. Hypothalamic magnocellular neurons store vasopressin and oxytocin. Autonomic neurons store acetylcholine and VIP or norepinephrine Y.
biology
One spike releases one packet. Vesicle containing acetylcholine has 1000 to 10,000 molecules. Vesicles contact cell membrane, because time is less than 200 microseconds between first calcium entry and first neurotransmitter in synapse.
Excitatory synapses {asymmetric synapse} {Type 1 synapse} can have postsynaptic density, round vesicles, and wide clefts and connect mainly to dendritic spines.
Inhibitory synapses {symmetric synapse} {Type 2 synapse} can have small and narrow synaptic clefts, ellipsoidal or flattened vesicles, and no postsynaptic density and connect mainly to dendritic shafts and cell bodies.
Neuron axons can have long chains of swellings {varicosity}, which are similar to synapses and release neurotransmitter from their surfaces near dendrite terminal branching regions.
Neurons attach {neuron adhesion} {neural attachment} symmetrically at zonula adhaerens, punctum adhaerens, zonula occludens, and nexus. Neurons have no macula adhaerens or desmosome.
Neurons adhesions {punctum adhaerens} attach symmetrically.
Neurons attach symmetrically at chemical synapses {zonula adhaerens}.
Neurons attach symmetrically between epithelial or endothelial cells {zonula occludens} {tight junction}.
Neurons make electrotonic synapses {nexus, synapse}.
Electrotonic synapses have membrane proteins {connexon} for ion transmission.
Neurons have physiology {neuron, physiology}.
signals: initiation
Neurotransmitter reception reduces membrane voltage. Membrane voltage reduction spreads. At axon hillock, membrane voltage can reach threshold voltage, causing depolarization spike, which initiates depolarization-spike traveling wave down axon.
Perhaps, only one dendrite-and-cell-body membrane potential distribution can cause initiation. Only one distribution can reach threshold potential. One distribution has much higher probability than others, because it can happen in the most ways. Dendrite and cell body changes can change distribution. Perhaps, neuron groups also detect only one input distribution.
signals: firing rate
Neuron input to neuron-firing rate ratio is linear or S-shaped.
conduction rate
Non-myelinated-axon conduction rate is between 0.5 and 2 meters per second, 1 millimeter per millisecond. Myelinated-fiber conduction rate is between 2 and 120 meters per second, 10 millimeters per millisecond, and is faster because signals jump from one Ranvier node to the next {saltatory conduction, myelin}.
Conduction rate in axons varies irregularly.
Faster impulse conduction became necessary as animals became larger.
conduction rate: synapse
Conduction rate across synapse is one micrometer per millisecond. Irregular synapse sizes and neurotransmitter-packet release times vary conduction rate.
Post-synaptic decay takes up to ten milliseconds.
conduction rate: synchronization
Information-flow rates along axons, synapses, and receptors change typically do not synchronize with rates on other axons, synapses, and receptors.
neuron growth
Neurons grow, differentiate, migrate, and extend axons and dendrites, at different rates. Extracellular substances, cell-membrane molecules, and cell and axon spatial arrangements affect growing axons. Cell-membrane-molecule and extracellular-substance gradients change over time. Target neurons grow and mature in coordination with axon growth.
neuron growth: direction
Adhesion-glycoprotein neurotrophins guide growing nerve processes to appropriate target neurons.
neuron growth: process
First, several axons travel over relatively short distance. After axons stop extending, they produce multiple branches, which form many connections. Branch retraction and synapse reduction then reduce connections. First nerve impulses, which are possibly synchronous, refine axon connections [Thompson, 1940] [Wolpert, 1977].
nutrition
Nerve cells need glucose and oxygen, because they have no substitute biochemical pathways.
plasticity
Neuron number, spatial arrangements, diameters, composition, lengths, types, controllers, molecules, membranes, axons, dendrites, cell bodies, receptors, channels, synapses, threshold voltages, and packet number can change. Receptor number, type, effectiveness, and position can change.
plasticity: repair
After brain damage, nearby axons invade damaged region to make new circuits, and axons try to contact nearby dendrites.
Unconditioned stimulus (UCS) releases serotonin from axon to axon synapses {axoaxonic synapse}, which increase protein kinase A, which releases more glutamate. Association is non-Hebbian.
Cutting axons {axon cutting} makes neuron die and nearby axons sprout processes to innervate neuron dendrites that used to contact dead neuron.
Proteins, lipids, and neurotransmitters travel 300 mm/day {axon transport}|, away from cell soma. Mitochondria travel 75 mm/day, away from cell soma. Actin microfilaments, glycolytic enzymes, myosin-related polypeptides, calmodulin, and clathrin travel 5 mm/day, away from cell soma. Microtubules and neurofilaments travel 1 mm/day, away from cell soma. Lysozyme breakdown products travel 250 mm/day, back to cell soma. Fast transport uses ATP and kinesin protein along microtubules.
Depolarization increases glutamate binding to NMDA receptor, which activates pathways {CREB pathway} to increase cyclic AMP, which increases CREB protein, which increases transcription of genes that make synapses larger and more efficient.
Neurotransmitter packets reaching post-synaptic cell-membrane neuron receptors cause small voltage differences, positive {excitation} {hyperpolarization} or negative {inhibition} {depolarization}|. Sodium ions diffuse into cell, and potassium ions diffuse out, causing voltage change across cell membrane. Voltage change spreads to nearby cell membrane.
Depolarization increases glutamate binding to NMDA receptor, which activates CREB pathway to increase cyclic AMP, which increases CREB protein, which increases transcription of genes that make synapses larger and more efficient.
In sympathetic autonomic ganglia, presynaptic cholinergic fibers excite one neuron class with acetylcholine and another class with LHRH-like peptide, which diffuses several micrometers to make slow excitatory postsynaptic potential {excitatory postsynaptic potential} (EPSP).
Neuron-axon back projections can cause long-term membrane depolarization {facilitation}|.
Most reflex responses decrease {habituation}| if non-threatening stimulus repeats without reinforcement. Receiving same stimulus repeatedly or continuously decreases sensation.
purpose
Habituation allows animal to ignore persisting situation or disregard irrelevant stimuli.
specific
Habituation is only to specific stimulus. Habituation ends immediately when stimulus pattern changes. Therefore, dishabituation can detect if animal perceives anything new.
behavior
Sexual behavior can have habituation.
timing
Habituation happens sooner the second time. Habituation happens sooner to weak stimuli.
time
In mammals, habituation decreases receiving-neuron post-synaptic potential for up to one hour. Because back-projection signals decrease, calcium influx is lower, sending neuron releases less transmitter, and receptor alters.
In marine snails, decreased vesicle release, from sense to motor neurons, causes habituation that persists for minutes. Repeated habituation decreases presynaptic-terminal number.
comparison
Tiredness does not cause habituation. Habituation cannot be for associative learning.
Senses have absolute intensity differences {just noticeable difference}| (JND) {difference threshold}, between two stimuli, that people can perceive. Stimulus intensity ratio typically ranges from one to three but can be up to sixty.
For fast millisecond effects, neurotransmitter receptors have ion channels {ligand-gated ion channel}. Fast neurotransmitters include acetylcholine and glutamate.
If climbing fiber depolarizes Purkinje cell, parallel fibers make nitrogen oxide, which increases cGMP in Purkinje cell, which activates protein kinase G, which makes receptors less sensitive {long-term depression} (LTD).
Dendrite spine synapses can have long-lasting changes {long-term potentiation} (LTP).
process
Presynaptic glutamate release activates N-methyl-D-aspartate (NMDA) postsynaptic receptors, causing Ca++ entry into postsynaptic neurons, which activates calcium/calmodulin protein kinase II (CaM kinase II), protein kinase C, and/or tyrosine kinase, which changes spine shape, synapse shape, or receptors. Perhaps, CaM kinase II adds AMPA receptors to postsynaptic membrane. Spine shape alteration exposes NMDA receptors and changes spine electrical properties. Short spine neck has high electrical resistance that amplifies depolarization. Lengthening neck permits increased Ca++ influx.
time
High-frequency hippocampus or cortex nerve stimulation increases synapse depolarization for hours {early LTP}, and, if repeated, up to weeks {late LTP}.
purposes
LTP aids space representation and affects spatial memory.
protein
Cell-membrane binding integrin protein maintains long-term potentiation and so aids memory.
locations
In hippocampus, Schaffer collateral pathway, from hippocampus region CA3 pyramidal cells to hippocampus region CA1, uses glutamate, is associative, and has post-synaptic NMDA receptor modulation. Hippocampus region CA3 pyramidal cells receive from dentate gyrus. Mossy fiber pathway, from dentate gyrus granule cells to hippocampus region CA3, uses glutamate, is non-associative, has norepinephrine interneuron modulation, and seems not to affect declarative memory. Dentate-gyrus granule cells receive from entorhinal cortex.
Regular low-frequency stimulation causes presynaptic bulb hypopolarization {low-frequency depression} (LFD) and decreases post-synaptic neuron output.
Acetylcholine can bind to slow neurotransmitter receptors {muscarinic ACh receptor}.
Adult bird, primate, and human brain neural stem cells divide to form neural precursors and new neural stem cells {neurogenesis}. Neurogenesis increases with brain activity.
Drugs, learning, growth, disease, accident, mutation, hormones, and chance can alter neuron properties {plasticity}|. Brains can change structure in response to stimuli and so learn [Petit and Ivy, 1988] [Robertson, 2000].
After binding a neurotransmitter packet of 1000 to 10,000 molecules, post-synaptic membrane changes 1 mV to 15 mV {post-synaptic potential} (PSP), with average of 10 mV, lasting 10 to 100 milliseconds. Initial change is rapid, and decay is slow. Potential change affects membrane up to two millimeters away. Spontaneous neurotransmitter release makes changes of 0.5 mV, lasting 20 milliseconds. Miniature end plate potentials depolarize synapse by 0.7 mV, lasting 10 milliseconds. Frequency is directly proportional to membrane depolarization. Frequency is five per second at membrane resting voltage.
Regular high-frequency stimulation causes presynaptic bulb hyperpolarization {post-tetanic potentiation} (PTP) and increases post-synaptic neuron output.
All cells in all organisms have receptor potentials and action potentials {potential gradient}, caused by sodium-ion, potassium-ion, and chloride-ion concentration gradients across cell membranes. All cells have potential changes, as ions move through membrane channels. Neurons require energy to maintain ion balance across membranes.
In excitatory axons, conditioned stimulus (CS) allows calcium to enter axon terminal and release glutamate {presynaptic facilitation}. Unconditioned stimulus (UCS) releases serotonin from axon-to-axon axoaxonic synapses, which increase protein kinase A, which releases more glutamate. Association is non-Hebbian. More UCS also activates MAP kinase and expresses genes to make more glutamate synapses.
In excitatory axons, unconditioned stimulus inhibits presynaptic bulb {presynaptic inhibition}.
Brain activity leaves trace {priming, nerve}, making path more easily excitable next time. Priming lasts tens to hundreds of milliseconds. Priming sets or sequences {context, priming} last minutes or hours.
Conscious states last 100 to 150 milliseconds {psychological refractory period}, same time it takes to make or perform decisions. Perhaps, after sending feedforward signal, brain sends no more signals for refractory period, to allow time to check first-signal results.
Inactive periods {refractory period, neuron}|, 0.75 milliseconds to 4 milliseconds, follow neuron spikes at axon positions, as membrane returns to normal voltage.
Receptor size and information processing method determine smallest size {resolution} that sense can detect. For example, eye can see 1 arc-second or 0.000001 meter, microwave size. Wavelengths longer than microwaves are not good for vision because spatial resolution is poor.
In myelinated fibers, conduction rate is between 2 and 120 meters per second or 10 millimeters per millisecond, as signal jumps {saltatory conduction, myelinated fiber}| from one Ranvier node to the next. Conduction rate along all axons varies irregularly.
Neurotransmitter binding to synapse receptors reduces membrane voltage, which spreads to axon hillock. When membrane voltage reaches threshold at axon hillock, cell membrane has large and rapid voltage change {spike, axon}| [Koch, 1999] [Salinas and Sejnowski, 2001] [Softky, 1995].
level
Spike voltage rises from -70 mV to +5 mV in 0.5 millisecond and then falls back to -70 mV in 0.5 millisecond.
time
Depolarizations have short duration, allowing precise time and time-interval coordination and comparison.
strength
Depolarizations have same strengths and time intervals. Depolarization prevents nerve-signal deterioration with distance and time, allowing axons to be long and act over long time intervals. Neurons can thus be anywhere and have any pattern.
threshold
Threshold can vary, between -50 mV and -30 mV.
Depolarization makes neurons act like switches. Threshold keeps neurons off until they switch on. Rapid recovery makes them switch off.
Computers are switching networks and can change switch thresholds.
Switches can contain messages in binary code [Adrian, 1980].
travel
Depolarization brings adjacent cell membrane to threshold, causing adjacent spike. That spike, in turn, causes adjacent cell membrane to reach threshold, causing adjacent spike. Spikes travel along axon from axon hillock to synapse.
direction
Spikes cannot go backward because cell membrane takes time to recover from spike. Ions at previous-spike cell membrane have low concentration and do not flow across membrane.
rate
Axons can sustain up to 800 spikes per second. Spikes cannot repeat faster at a cell-membrane location, because cell membrane takes 0.5-millisecond refractory period to recover from a spike.
factors
Axon hillocks do not distinguish neurotransmitters, receptors, or input patterns. All things that effect membrane voltage merely add.
Neurotransmitter synapse effects can be fast and short or slow and long {synaptic transmission}.
For fast millisecond effects, neurotransmitters, such as acetylcholine and glutamate, bind to receptors with ligand-gated ion channels.
For slow 0.1-second to 10-second effects, neurohormones, such as dopamine, acetylcholine, and neuropeptides, bind to receptor that activates GTP-binding proteins {G-protein}, which make second messengers such as cyclic AMP, diacylglycerol (DAG), or inositol triphosphate (IP3), which phosphorylate.
Stimuli can cause neuron sets to fire simultaneously {synchronization}, 40 to 100 milliseconds after stimulus. Neurons with overlapping same-type receptive fields have the most synchrony. Synchronous neuron activity is always in phase, not in opposite phase. Synchronous neuron signals do not encode information about space, objects, or time.
High-frequency electrical stimulation causes maximum nerve signaling {tetanus, nerve}|.
Cell membrane has voltage {threshold, neuron} at which it starts depolarization spike. Low threshold allows too much noise. High threshold cuts off boundary effects, shading, and small differences.
absolute
Senses have smallest detectable stimulus {absolute threshold}, which people can sense 50% of time. For vision, humans can detect light if seven photons flash in absolute darkness. For hearing, humans can detect whisper at five meters in absolute silence. For touch, humans can detect small insect wing or foot in still air. For smell, if small perfume drop is in ballroom, air is still, and no other odors are present, humans can detect perfume. For taste, humans can detect four grams of sugar in one liter of water.
Presynaptic cholinergic neurons excite peripheral sympathetic neurons {trans-synaptic enzyme induction}.
Neuron and glia molecules cause nerve growth {trophism}|, guide axon tips to final locations during development, regulate and maintain nervous-system connections, and stimulate neurotransmitters.
At receptor, catecholamine agonist causes desensitization {tolerance, regulation}, because agonist receptors have reduced affinity and subsequently decrease in number {down-regulation}. Down-regulation persists for days after transmitter concentrations have returned to normal {temporal amplification, down-regulation}.
Denervation, catecholamine depletion, or catecholamine antagonist treatment causes suprasensitivity {up-regulation}, because receptor number increases. Up-regulation persists for days after transmitter concentrations have returned to normal {temporal amplification, up-regulation}.
Motor neurons {alpha motor neuron} can initiate movement by stimulating muscles.
Brain has many small nerve cells {amacrine cell, neuron} {microneuron}, which inhibit other neurons in memory and other processes. Amacrine cells have 27 types and send to the ten inner-plexiform layers.
Cerebellum neurons {basket cell} can receive excitation from parallel fibers and laterally inhibit adjacent-column Purkinje cells. Axons go one millimeter away and form multiple synapses on cell bodies and dendrites. Basket cells make GABA for inhibition.
Brain has elongated neurons {bipolar cell, neuron} of 10 types that send to the ten inner plexiform layers.
Medial entorhinal cortex and para-subiculum have 10% cells {border cell} that fire when viewing a nearby border.
Brain neurons {chandelier cell} can inhibit pyramidal cells with multiple synapses at axon base. Chandelier cells make GABA for inhibition.
Basal nucleus of Meynert, medial septal nucleus, and brainstem nuclei neurons {cholinergic neuron} can make acetylcholine and alter in Alzheimer's disease, ALS, and spinal cord injury.
Visual-cortex cells {complex cell} can receive from simple cells and ganglion cells. Complex cells have ocular dominance or orientation tuning.
functions
They can detect stereoscopic effects, such as line-segment ends, colors, motions, and line orientations. They can mark region boundaries, such as regions with same reflectance. They discriminate and aggregate. They can detect patterns, at any location.
fields
Complex cells have different receptive-field sizes and detect different spatial frequencies and so widths. Sets can detect 8 to 30 different frequency bands and spatial scales, using spatial frequency channels, from pixel, spot, region, quadrant, or whole visual field.
time
Complex-cell sets can operate at 10 to 30 different temporal scales, from milliseconds to years.
Edge neurons {double-bouquet cell} can make GABA to inhibit other-edge-side vertical-column activity.
Simple cells can detect line or bar ends {end-stopped inhibition} {end stopping, simple cell} or detect no end. End-stopping cells increase firing rate, as bar length increases up to maximum, and then decrease firing rate, as bar gets longer.
Neurons {excitatory neuron} can excite other neurons.
types
They are either bursting {bursting cell} or non-bursting. They cannot change from one type to the other.
bursting
Bursting cells respond to sustained intracellular current with two to four spikes, followed by hyperpolarization, followed by burst, followed by hyperpolarization, and so on, with 0.2 to 10 cycles per second.
Bursting cells are large. Apical dendrites extend to layer 1 to contact many cells. Axons project to ipsilateral superior colliculus.
Bursting neurons accumulate calcium more efficiently in axon terminals than cells that have isolated spikes.
Layer-5 bursting neurons induce synaptic plasticity in neurons outside cortex. Spike bursts turn on short-term memory, which then decays over several seconds.
non-bursting
Non-bursting neurons, such as pyramidal or spiny stellate neurons, have one spike or sustained output. They do not have spike cycles.
Interneurons {GABA neuron} {GABA+ neuron} can have no dendrite spines {smooth neuron, GABA}. They affect epilepsy and Huntington's disease [Koch, 1999] [Lytton and Sejnowski, 1991] [McBain and Fisahn, 2001].
Motor neurons {gamma motor neuron} can control muscle length/tension relationships, by exciting nearby alpha motor neurons and stimulating muscle sensors.
Retinal output neurons {retinal ganglion cell} {ganglion cell, retina output}| generate action potentials and have axons in optic tract to brain [Enroth-Cugell and Robson, 1984] [Meister, 1996] [Niremberg et al., 2001] [Warland et al., 1997].
biology: types
Ganglion cells are magnocellular M, parvocellular P, and mixed W, which process signals separately and send separate information streams to lateral geniculate nucleus (LGN).
Retinal ganglion cells {X-cell} {beta retinal ganglion cell} can sum linearly across receptive fields. X cells have large dendritic fields. X-cells resolve finer visual patterns with higher spatial frequencies. X cells make tonic and sustained signals, with slow conduction, for detecting details and spatial orientation. More X cells are in fovea. X cell axons go to simple cells.
Magnocellular cells respond better to motion, respond better to transient stimulation, respond better to small intensity differences, are larger, have larger receptive fields, have thicker axon with faster signals, have firing rate that plateaus only at high intensity, and signal scene changes.
Retinal ganglion cells {Y-cell} {alpha retinal ganglion cell} can sum non-linearly. Y cells have small dendritic fields. Y-cells are larger and have thicker and faster conducting axons. Y cells make phasic and transient signals, with fast conduction, for stimulus size and temporal motion. More Y cells are in periphery. Y cell axons go to complex cells.
Parvocellular cells have several types, have better spatial resolution, detect color, detect higher contrast, detect more detail, are more numerous, and have more linear responses.
Both X-cells and Y-cells have ON-center and OFF-center neurons. X-cells and Y-cells have different receptive field sizes, stimulus velocity sensitivities, and spatial frequencies.
Retinal ganglion cells {W cell} can be small and direction sensitive, with slow conduction speed. W cells mix M and P cell properties and are rarest.
biology: neuron shapes
Ganglion cells {bistratified neuron} {small bistratified neuron} can have two dendrite layers. Cells {shrub neuron} can have dendrite bushes.
Ganglion cells look like auditory nerve cells, Purkinje cells, olfactory bulb cells, olfactory cortex cells, and hippocampal cells.
biology: input
Small central-retina midget ganglion cells have small dendrite clump, to collect signals from one midget bipolar cell. Midget cells respond mostly to contrast.
Parasol ganglion cells can receive from diffuse bipolar cells with bigger dendrite trees and can have dendrite umbrella, to collect signals from wide area. Parasol cells respond mostly to change.
biology: output
Ganglion cells send to LGN and then to cortical hypercolumn.
functions
Visual-receptor cells take illumination logarithm and hyperpolarize 0 mV to 4 mV from resting level 10 mV to 30 mV [Dowling, 1987] [Enroth-Cugell and Robson, 1984] [Wandell, 1995]. Retinal ganglion cells sum bipolar, horizontal, and amacrine retinal-neuron activities. Retinal ganglion cells have low spontaneous-firing rate. Ganglion cells typically respond quickly and then turn off.
Retinal ganglion cells make action potential after cyclic GMP reduces, decreasing sodium conductance through cell membrane.
functions: spots
Retinal ON-center ganglion cells can respond when light intensity above background level falls on center of their receptive field. See Figure 1. Light falling on annulus surrounding receptive-field center inhibits cell.
When light smaller than center falls on center, ON-center neuron fires rapidly and then slowly. After removing light, ON-center neuron continues low firing rate. When light smaller than annulus falls on annulus, ON-center neuron does not fire. After removing light, ON-center neuron fires rapidly and then slowly.
ON-center neurons have four types, depending on excitation and inhibition. One has high firing rate at onset and zero rate at offset. One has high rate, then zero, then high, and then zero. One has high rate at onset, goes to zero, and then rises to constant level. One has high rate at onset and then goes to zero.
Other ganglion cells {OFF-center neuron} respond when light intensity below background level falls on receptive-field center. OFF-center neurons increase output when light intensity decreases in receptive-field center. Light falling on annulus around receptive-field center excites OFF-center cells.
When light smaller than center falls on center, OFF-center neuron does not fire. After removing light, OFF-center neuron fires rapidly and then slowly. When light smaller than annulus falls on annulus, OFF-center neuron fires rapidly and then slowly. After removing light, OFF-center neuron continues low firing rate.
Bipolar cells excite ON-center and OFF-center neurons. ON-center and OFF-center neurons compare light intensity falling on receptive-field center with that falling on annulus.
functions: bars
Band, bar, stripe, grating, or edge excites ON-center neuron in different ways.
If grating has wide stripes, ON-center neuron has only spontaneous firing, because one bright band affects both center and surround, exciting and inhibiting. See Figure 2.
If grating has narrow stripes, ON-center neuron has only spontaneous firing, because several bright bands affect both center and surround, exciting and inhibiting. See Figure 3.
If grating-stripe width lands on center exactly, ON-center neuron fires rapidly and then slowly. See Figure 4.
For wide single long bar, ON-center neuron has only spontaneous firing, because bright band affects both center and surround. See Figure 2.
For narrow single long bar, ON-center neuron has some firing, because bright band affects mainly center but is small. See Figure 5.
If long bar lands exactly on center, ON-center neuron fires rapidly and then slowly. See Figure 4.
For long bar with end beyond center, ON-center neuron has only spontaneous firing, because bright band affects both center and surround. See Figure 5.
For short bar with end not yet at center, ON-center neuron has only spontaneous firing, because bright band does not reach center. See Figure 6.
For bar with end on center {end stopping, neuron}, ON-center neuron fires rapidly and then slowly. See Figure 7.
For bright edge over center, ON-center neuron fires rapidly and then slowly. See Figure 8.
For bright edge not yet at center, ON-center neuron has no firing. See Figure 9.
For bright edge at middle, ON-center neuron fires some. See Figure 10.
If grating-stripe width lands on center exactly, ON-center neuron fires rapidly and then slowly. See Figure 4.
If grating-stripe width shifts to half on and half off, ON-center neuron fires some. See Figure 10.
If grating-stripe width shifts to all off, ON-center neuron does not fire. See Figure 9.
For bright or dim regions, ON-center neurons have only spontaneous firing, because bright light affects both center and surround.
For bright or dim regions, OFF-center neurons have only spontaneous firing, because bright light affects both center and surround. Relative brightness depends on lateral-inhibition patterns.
functions: movement
Ganglion cells {ON-OFF-center neuron} can detect movement. ON-OFF-center neurons use time derivative of ON-center neurons to find general direction and position. Amacrine cells also excite transient ON-OFF-center neurons.
functions: color
Retinal ganglion cells can be cone-shaped cells for color detection. The three types compare light intensities in frequency range. Type is for brightness, adds green and yellow-green, and has both on-center and off-center neurons. Another type has center for one cone color and surround for another color, to compare colors. Third type, with no surround, adds green and yellow-green for excitation and subtracts blue for inhibition, to compare blue to yellow.
Cerebellum has clusters {glomerulus, cerebellum} {glomeruli, cerebellum} of 20 granule cells.
Cerebellum neurons {Golgi cell} can be in granule-cell layer. Golgi cells receive input from parallel, mossy, and climbing fibers. They inhibit granule cells to provide feedback and feedforward inhibition. They change parallel fiber activity into brief burst.
Neurons {Golgi type I neuron} {local circuit neuron} can send locally with unmyelinated axons.
Neurons {Golgi type II neuron} {projection neuron} can send to other areas with myelinated axons.
Golgi cells inhibit cerebellum neurons {granule cell}. One mossy fiber excites 20-granule-cell clusters {glomerulus, granule cell} {glomeruli, granule cell}.
Medial entorhinal cortex has some cells {grid cell} that fire when body is at many spatial locations, which form a triangular grid [Sargolini et al., 2006].
Post-subiculum, retrosplenial cortex, anterior thalamic nuclei, lateral dorsal thalamic nuclei, lateral mammillary nucleus, dorsal tegmental nucleus, striatum, and entorhinal cortex have some neurons {head direction cell}, which receive from vision and vestibular systems, that fire only when head has an orientation in space [Sargolini et al., 2006].
Superficial pyramidal cells {hypercomplex cell} can detect corners, depths, and lengths.
Neurons {inhibitory neuron}| that inhibit other neurons fire faster than excitatory neurons, have few spines on dendrites {smooth neuron, inhibition}, and synapse directly on dendrites, cell bodies, or dendritic stumps, closer to axon hillock than excitatory axons. Inhibitory axons connect horizontally, only up to 100 to 200 microns away, except for basket cells.
Large spindle-shaped neurons {spindle neuron} {Korkzieher cell} are only in great-ape anterior-cingulate and frontal-area-FI lower layer 5, for output to other regions [Economo and Koskinas, 1925] [Nimchinsky et al., 1999]. Humans have them in much higher densities than other apes. They form after birth. Spindle neurons are for attention and self-reflection. Layer 6 has small spindle-like cells.
People have three million nerves {motor nerve} {motor neuron} to muscles and glands. Alpha motor neurons can initiate movement by stimulating muscles. Gamma motor neurons can control muscle length/tension relationships by exciting nearby alpha motor neurons and stimulating muscle sensors. Internuncial neurons can allow reciprocal inhibition.
input
Cerebrum, basal ganglia, brainstem, and cerebellum act on motor neurons. Impulse excitatory effect on motor neuron lasts 5 milliseconds.
regeneration
Motor nerves can regenerate connections to muscles.
Brain neurons {multipotent neural stem cell} can divide regularly to make neural stem cells and neural precursors. Half of neural precursors migrate, then mature into neurons or glia. Migration and maturation take one month.
Neurons {neural stem cell} can make five neuron types: TH+ neurons, GABA+ neurons, cholinergic neurons, astrocytes, and oligodendrocytes. Neural stem cells are mainly in ventricles and hippocampus. They can migrate to hippocampus and olfactory bulbs.
Excitatory neurons {pyramidal cell}| can connect one cortical area to another and fire in bursts. Pyramidal cells are both superficial and deep [Elston, 2000] [Elston and Rosa, 1997] [Elston and Rosa, 1998] [Elston et al., 1999].
functions
Pyramidal cells detect fast moving stimuli, such as moving edge at one orientation.
output
Cerebral cortex layer-5 pyramidal cells send to thalamic nuclei, mainly to lateral geniculate nucleus and inferior, lateral, and medial pulvinar nuclei. Half of layer-6 pyramidal cells send to lateral geniculate nucleus, and others send to claustrum, hippocampal system, and anterior cingulate sulcus motor-system higher planning levels. Pyramidal cells with short dendrites, not reaching into layer 1, send to other cerebral cortex regions. Excitatory extrinsic axons come from pyramidal cells. Pyramidal cells also send to local neurons using axon collaterals.
Pyramidal cells inhibit stellate cells. Pyramidal-cell to stellate-cell ratio is two to one.
processing
Pyramidal cells have high spontaneous activity and large receptive fields. Sustained intracellular current causes high-frequency action potentials {regular spiking cell}, which decrease within 50 to 100 milliseconds.
All vertebrate sense cells {sensory cell} developed from ectothelial cell type.
Visual-cortex cells {simple cell} can receive from lateral geniculate nucleus (LGN) ON-center and OFF-center neurons. Simple cells receive from both eyes but process one eye faster and so have ocular dominance. Simple cells have more precise tuning than LGN or retinal cells. See Figure 1.
lines
Cells that compare ON-center and OFF-center neuron superpositions can find boundaries and heighten contrasts. Simple cells can detect lines, edges, stripes, or gratings. Simple cells can detect 12 to 30 line, edge, stripe, or grating orientations {orientation tuning, cell}. Simple cells have different receptive field sizes and detect different line, edge, stripe, or grating spatial frequencies and widths.
color
Some simple cells detect color.
movement
Some simple cells detect movement.
arrays
Simple cells have arrays in topographic maps. See Figure 2.
Neurons {smooth stellate cell} can send only to superficial and deep pyramidal cells.
Neurons {spiny stellate cell} can send only to superficial and deep pyramidal cells. Spiny-stellate-cell inhibitory axons connect horizontally only up to 100 to 200 microns away.
Cortical layer-4 cells {stellate cell} can detect bars, slits, and edges, in static pictures, at 20 orientations. Stellate cells have small receptive fields and low spontaneous activity. Smaller stellate cells excite easier. Cerebellum stellate cells receive excitation from parallel fibers and laterally inhibit adjacent-column Purkinje cells.
Neurons {superficial pyramidal neuron} can send unmyelinated collaterals, with no terminal branches, sideways to tight terminal clusters. Neurons repeat this for many millimeters: every 0.43 mm in primary visual cortex, every 0.65 mm in secondary visual areas, every 0.73 mm in sensory strip, and every 0.85 mm in monkey motor cortex. Macrocolumns of similar emphasis connect by synchronizing excitation.
Brain output neurons {tufted cell} can be secondary.
Neurons {TH+ neuron} can make serotonin and affect Parkinson's disease.
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Date Modified: 2022.0225