Dendrite

Dendrites (from Greek δένδρον déndron, “tree”) are the branched projections of the neuron that transmit post-synaptic potentials to the soma of the neuron. They have high variability in the branching pattern and extent (characteristic for individual neuronal types): different numbers of axonal contacts (up to approximately 100 000) and different types of contacts (axo-shaft, axo-spine, dendro-dendritic). The dendrites contain dendritic organelles: neurofilaments, neurotubules, endoplasmic reticulum, mitochondria, ribosomes (metabolic autonomy). There are also special dendritic organelles: dendritic spines, dendritic swellings.

Dendrite's Special functions

 * 1) Enlarged surface area to receive signals from axons of other nerve cells:
 * 2) * The size of the dendritic tree limits how many synaptic inputs the neuron can receive.
 * 3) * The orientation of the dendritic tree determines the types and number of sources from which it can receive synaptic connections.
 * 4) Transmission of received signals:
 * 5) * Because dendrites are long, narrow, branching structures, the synaptic signal produced in the dendrites is significantly attenuated (due to increased resistance) by the time it reaches the soma. Thus they cannot propagate the action potentials.
 * 6) * A dendrite may be considered to be an electrically leaky cable having a relatively low-resistance cytoplasm surrounded by a membrane consisting of resistive and capacitive elements in parallel. Therefore, the signal is conducted by electrotonic conduction: If a steady signal is applied to the end of a dendrite, the attenuation of the signal with distance will critically depend on the specific membrane resistance of the dendrite (the membrane potential will decline exponentially – decremental conduction).
 * 7) * The farther the origin of excitation from the soma (cell body), the greater the degree of the decrement until the current reaches the cell body.
 * 8) * Measurements made in vivo suggest, that neuron's cable properties are not fixed quantities. It appears more efficient than the mathematical models indicate. The most probable explanation is based on the presence of accumulations of the voltage-gated channels in various location of the dendritic membrane (at the heads and necks of the dendritic spines, at the dendritic branching points, and even at the whole dendritic segments).
 * 9) * These accumulations (hot-spots) may recover the declining membrane potential and dramatically increase the effectiveness of conductance: pseudo-saltatory transmission by dendritic spines.
 * 10) * Voltage-Gated-Channels at the dendritic membrane are selectively permeable to either Na+ or Ca2+. At the peripheral branches, the Ca2+ channels are more numerous, meanwhile the Na+ channels are present at more distal segments. As the Ca2+ channels operate in lower speed than the Na+ channels, the transmitting signal travels along the dendrite with a different speed.
 * 11) Elementary integrative function: summation of excitatory + inhibitory potentials before the final current reaches the cell body.
 * 12) Possible contribution to memory: facilitation is possible due to plasticity of dendritic spines.
 * 13) Possible source of neuromodulators and tissue factors

=Dendritic Spines= Dendritic spines are small protrusions of the cell membrane on the dendrite. This is where a single synapse with an axon typically takes place.

Anatomy of the dendritic spines
Dendritic spines occur at a density of up to 20 spines/10 µm stretch of dendrite. A dendritic spine is a small membranous protrusion from a neuron's dendrite that typically receives input from a single synapse of an axon. Dendritic spines serve as a storage site for synaptic strength and help transmit electrical signals to the neuron's cell body. Most spines have a bulbous head (the spine head), and a thin neck that connects the head of the spine to the shaft of the dendrite. The dendrites of a single neuron can contain from thousands up to a few hundred thousand spines.

In addition to spines providing an anatomical substrate for memory storage and synaptic transmission, they may also serve to increase the number of possible contacts between neurons. Dendritic spines are small (2μm length max) with spine head volumes ranging 0.01 µm3 to 0.8 µm3. Spines with strong synaptic contacts typically have a large spine head, which connect to the dendrite via a membranous neck. The most notable classes of spine shape are: The variable spine shape and volume is thought to be correlated with the strength and maturity of each spine-synapse.
 * 1) Thin
 * 2) Stubby
 * 3) Mushroom
 * 4) Branched

Electrical properties
The conduction is done by electrotonic conduction (passive conduction of current). A dendritic spine has high input resistance, the resistance increases with smallness of head size and narrowness of stem size. The capacitance of the membranes of spines is relatively small with the result that synaptic potentials can be relatively fast. The capacitance of the whole dendrite however becomes higher as the number of spines increases. Because there is an impedance mismatch between the dendritic spine and the dendrite, it is necessary with active signal boosting. The impedance mismatch also causes the spine to follow the potential of the parent dendrite.

Morphological changes - manifestations of plasticity
Dendritic spines are very "plastic", that is, spines change significantly in shape, volume, and number in small time courses. Because spines have a primarily actin cytoskeleton, they are dynamic, and the majority of spines change their shape within seconds to minutes because of the dynamicity of actin remodeling. Furthermore, spine number is very variable and spines come and go; in a matter of hours, 10-20% of spines can spontaneously appear or disappear on the pyramidal cells of the cerebral cortex, although the larger "mushroom"-shaped spines are the most stable. Spine maintenance and plasticity can be activity-dependent (ribosomal accumulation at the spine’s base) or activity-independent.