Dendrites are the branched projections of a neuron that act to propagate the electrochemical
stimulation received from other neural cells to the cell body, or soma, of the neuron from
which the dendrites project.
Electrical stimulation is transmitted onto dendrites by upstream neurons via synapses
which are located at various points throughout the dendritic tree.
Dendrites play a critical role in integrating these synaptic inputs and in determining the
extent to which action potentials are produced by the neuron.
Long outgrowths on immune system dendritic cells are also called dendrites.
These are not to be confused with dendrites on a neuron.
Dendritic cells are antigen-presenting cells in the mammalian immune system.
Their dendrites do not process electrical signals.
Dendrites are one of two types of protoplasmic protrusions that extrude from the cell body
of a neuron, the other type being an axon.
Axons can be distinguished from dendrites by several features including shape, length,
and function.
Dendrites often taper off in shape and are shorter, while axons tend to maintain a constant
radius and be relatively long.
Typically, axons transmit electrochemical signals and dendrites receive the electrochemical
signals.
Although, some types of neurons in certain species lack axons and simply transmit signals
via their dendrites.
Dendrites provide an enlarged surface area to receive signals from the terminal buttons
of other axons allowing for a chemical signal to pass simultaneously to many target cells.
Synapses involving dendrites can be axodendritic, involving an axon signaling to a dendrite,
or dendrodendritic, involving signaling between dendrites.
Dendritic branching is also called "dendritic arborization" and "dendritic ramification."
When an electrochemical signal stimulates a neuron it causes changes in the electrical
potential across the neuron’s plasma membrane.
This change in the membrane potential will passively spread across the dendrite but becomes
weaker with distance without an action potential.
The action potential propagates the electrical activity along the membrane of the dendrite
to the cell body and then afferently down the axon to the terminal buttons where is
crosses the synapse.
Certain classes of dendrites contain small projections referred to as dendritic spines
that increase receptive properties of dendrites to isolate signal specificity.
Increased neural activity and the establishment of long-term potentiation at dendritic spines
change the size, shape, and conduction.
This ability for dendritic growth is thought to play a role in learning and memory formation.
There are approximately 200,000 spines per cell, each of which serves as a postsynaptic
process for individual presynaptic axons.
Dendritic branching can be extensive and in some cases is sufficient enough to receive
as many as 100,000 inputs to a single neuron.
There are three main types of neurons; multipolar, bipolar, and unipolar.
Multipolar neurons, such as the one shown in the image, are composed of one axon and
many dendritic trees.
Pyramidal cells are multipolar cortical neurons with pyramid shaped cell bodies and large
dendrites called apical dendrites that extend to the surface of the cortex.
Bipolar neurons have one axon and one dendritic tree at opposing ends of the cell body.
Unipolar neurons have a stalk that extends from the cell body that separates into two
branches with one containing the dendrites and the other with the terminal buttons.
Unipolar dendrites used to detect sensory stimulus such as touch or temperature.
The morphology of dendrites such as branch density and grouping patterns are highly correlated
to the function of the neuron.
Malformation of dendrites is also tightly correlated to impaired nervous system function.
History Some of the first intracellular recordings
in a nervous system were made in the late 1930s by Kenneth S. Cole and Howard J. Curtis.
German anatomist Otto Friedrich Karl Deiters is generally credited with the discovery of
the axon by distinguishing it from the dendrites.
Swiss Rüdolf Albert von Kölliker and German Robert Remak were the first to identify and
characterize the axon initial segment.
Alan Hodgkin and Andrew Huxley also employed the squid giant axon and by 1952 they had
obtained a full quantitative description of the ionic basis of the action potential, leading
the formulation of the Hodgkin–Huxley model.
Hodgkin and Huxley were awarded jointly the Nobel Prize for this work in 1963.
The formulas detailing axonal conductance were extended to vertebrates in the Frankenhaeuser–Huxley
equations.
Louis-Antoine Ranvier was the first to describe the gaps or nodes found on axons and for this
contribution these axonal features are now commonly referred to as the Nodes of Ranvier.
Santiago Ramón y Cajal, a Spanish anatomist, proposed that axons were the output components
of neurons.
He also proposed that neurons were discrete cells that communicated with each other via
specialized junctions, or spaces, between cells, now known as a synapse.
Ramón y Cajal improved a silver staining process known as Golgi's method, which had
been developed by his rival, Camillo Golgi.
Dendrite Development
During the development of dendrites, several factors can influence differentiation.
These include modulation of sensory input, environmental pollutants, body temperature,
and drug use.
For example, rats raised in dark environments were found to have a reduced number of spines
in pyramidal cells located in the primary visual cortex and a marked change in distribution
of dendrite branching in layer 4 stellate cells.
Experiments done in vitro and in vivo have shown that the presence of afferents and input
activity per se can modulate the patterns in which dendrites differentiate.
Little is known about the process by which dendrites orient themselves in vivo and are
compelled to create the intricate branching pattern unique to each specific neuronal class.
One theory on the mechanism of dendritic arbor development is the Synaptotropic Hypothesis.
The synaptotropic hypothesis proposes that input from a presynaptic to a postsynaptic
cell eventually can change the course of synapse formation at dendritic and axonal arbors.
This synapse formation is required for the development of neuronal structure in the functioning
brain.
A balance between metabolic costs of dendritic elaboration and the need to cover receptive
field presumably determine the size and shape of dendrites.
A complex array of extracellular and intracellular cues modulates dendrite development including
transcription factors, receptor-ligand interactions, various signaling pathways, local translational
machinery, cytoskeletal elements, Golgi outposts and endosomes.
These contribute to the organization of the dendrites on individual cell bodies and the
placement of these dendrites in the neuronal circuitry.
For example, it was shown that β-actin zipcode binding protein 1 contributes to proper dendritic
branching.
Other important transcription factors involved in the morphology of dendrites include CUT,
Abrupt, Collier, Spineless, ACJ6/drifter, CREST, NEUROD1, CREB, NEUROG2 etc.
Secreted proteins and cell surface receptors includes neurotrophins and tyrosine kinase
receptors, BMP7, Wnt/dishevelled, EPHB 1-3, Semaphorin/plexin-neuropilin, slit-robo, netrin-frazzled,
reelin.
Rac, CDC42 and RhoA serve as cytoskeletal regulators and the motor protein includes
KIF5, dynein, LIS1.
Important secretory and endocytic pathways controlling the dendritic development include
DAR3 /SAR1, DAR2/Sec23, DAR6/Rab1 etc.
All these molecules interplay with each other in controlling dendritic morphogenesis including
the acquisition of type specific dendritic arborization, the regulation of dendrite size
and the organization of dendrites emanating from different neurons.
Electrical Properties The structure and branching of a neuron's
dendrites, as well as the availability and variation of voltage-gated ion conductance,
strongly influences how the neuron integrates the input from other neurons.
This integration is both temporal, involving the summation of stimuli that arrive in rapid
succession, as well as spatial, entailing the aggregation of excitatory and inhibitory
inputs from separate branches.
Dendrites were once thought to merely convey electrical stimulation passively.
This passive transmission means that voltage changes measured at the cell body are the
result of activation of distal synapses propagating the electric signal towards the cell body
without the aid of voltage-gated ion channels.
Passive cable theory describes how voltage changes at a particular location on a dendrite
transmit this electrical signal through a system of converging dendrite segments of
different diameters, lengths, and electrical properties.
Based on passive cable theory one can track how changes in a neuron’s dendritic morphology
impacts the membrane voltage at the cell body, and thus how variation in dendrite architectures
affects the overall output characteristics of the neuron.
Electrochemical signals are propagated by action potentials that utilize intermembrane
voltage-gated ion channels to transport sodium ions, calcium ions, and potassium ions.
Each ion species has its own corresponding protein channel located in the lipid bilayer
of the cell membrane.
The cell membrane of neurons covers the axons, cell body, dendrites, etc.
The protein channels can differ between chemical species in the amount of required activation
voltage and the activation duration.
Action potentials in animal cells are generated by either sodium-gated or calcium-gated ion
channels in the plasma membrane.
These channels are closed when the membrane potential is near to, or at, the resting potential
of the cell.
The channels will start to open if the membrane potential increases, allowing for sodium or
calcium ions to flow into the cell.
As more ions enter the cell, the membrane potential continues to rise.
The process continues until all of the ion channels are open, causing a rapid increase
in the membrane potential that then triggers the decrease in the membrane potential.
The depolarizing is caused by the closing of the ion channels that prevent sodium ions
from entering the neuron, and they are then actively transported out of the cell.
Potassium channels are then activated, and there is an outward flow of potassium ions,
returning the electrochemical gradient to the resting potential.
After an action potential has occurred, there is a transient negative shift, called the
afterhyperpolarization or refractory period, due to additional potassium currents.
This is the mechanism that prevents an action potential from traveling back the way it just
came.
Another important feature of dendrites, endowed by their active voltage gated conductance,
is their ability to send action potentials back into the dendritic arbor.
Known as back-propagating action potentials, these signals depolarize the dendritic arbor
and provide a crucial component toward synapse modulation and long-term potentiation.
Furthermore, a train of back-propagating action potentials artificially generated at the soma
can induce a calcium action potential at the dendritic initiation zone in certain types
of neurons.
Plasticity Dendrites themselves appear to be capable
of plastic changes during the adult life of animals, including invertebrates.
Neuronal dendrites have various compartments known as functional units that are able to
compute incoming stimuli.
These functional units are involved in processing input and are composed of the subdomains of
dendrites such as spines, branches, or groupings of branches.
Therefore, plasticity that leads to changes in the dendrite structure will affect communication
and processing in the cell.
During development dendrite morphology is shaped by intrinsic programs within the cell's
genome and extrinsic factors such as signals from other cells.
But in adult life, extrinsic signals become more influential and cause more significant
changes in dendrite structure compared to extrinsic signals during development.
In females, the dendritic structure can changes as a result of physiological conditions induced
by hormones during periods such as pregnancy, lactation, and following the estrous cycle.
This is particularly visible in pyramidal cells of the CA1 region of the hippocampus,
where the density of dendrites can vary up to 30%.
References
External links Histology at OU 3_09 - "Slide 3 Spinal cord"
Dendritic Tree - Cell Centered Database Stereo images of dendritic trees in Kryptopterus
electroreceptor organs