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.
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Date Modified: 2022.0225