Dendrite
A dendrite (from
Dendrites play a critical role in integrating these synaptic inputs and in determining the extent to which action potentials are produced by the neuron.[1]
Structure and function
Dendrites are one of two types of protoplasmic protrusions that extrude from the cell body of a
Synaptic activity causes local changes in the electrical potential across the plasma membrane of the dendrite. This change in membrane potential will passively spread along the dendrite, but becomes weaker with distance without an action potential. To generate an action potential, many excitatory synapses have to be active at the same time, leading to strong depolarization of the dendrite and the cell body (soma). The action potential, which typically starts at the axon hillock, propagates down the length of the axon to the axon terminals where it triggers the release of neurotransmitters, but also backwards into the dendrite (retrograde propagation), providing an important signal for spike-timing-dependent plasticity (STDP).[4]
Most synapses are axodendritic, involving an axon signaling to a dendrite. There are also dendrodendritic synapses, signaling from one dendrite to another.[6] An autapse is a synapse in which the axon of one neuron transmits signals to its own dendrite.
The general structure of the dendrite is used to classify neurons into
History
The term dendrites was first used in 1889 by
Some of the first intracellular recordings in a nervous system were made in the late 1930s by
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.[12] 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.[13] 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.[14]
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 (and maturation of excitatory synaptic inputs) eventually can change the course of synapse formation at dendritic and axonal arbors.[15]
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 (ZBP1) 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.[1][16]
Types of dendritic patterns
Dendritic arborization, also known as dendritic branching, is a multi-step biological process by which neurons form new dendritic trees and branches to create new synapses.[1] Dendrites in many organisms assume different morphological patterns of branching. 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.[14]
Branching morphologies may assume an adendritic structure (not having a branching structure, or not tree-like), or a tree-like radiation structure. Tree-like arborization patterns can be spindled (where two dendrites radiate from opposite poles of a cell body with few branches, see bipolar neurons ), spherical (where dendrites radiate in a part or in all directions from a cell body, see cerebellar granule cells), laminar (where dendrites can either radiate planarly, offset from cell body by one or more stems, or multi-planarly, see retinal horizontal cells, retinal ganglion cells, retinal amacrine cells respectively), cylindrical (where dendrites radiate in all directions in a cylinder, disk-like fashion, see pallidal neurons), conical (dendrites radiate like a cone away from cell body, see pyramidal cells), or fanned (where dendrites radiate like a flat fan as in Purkinje cells).
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.[17]
Dendrites were once thought to merely convey electrical stimulation passively. This passive transmission means that
Action potentials initiated at the axon hillock propagate back into the dendritic arbor. These back-propagating action potentials depolarize the dendritic membrane and provide a crucial signal for synapse modulation and long-term potentiation. Back-propagation is not completely passive, but modulated by the presence of dendritic voltage-gated potassium channels. Furthermore, in certain types of neurons, a train of back-propagating action potentials can induce a calcium action potential (a dendritic spike) at dendritic initiation zones.[20][21]
Plasticity
Dendrites themselves appear to be capable of plastic changes during the adult life of animals, including invertebrates.[22] 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 intrinsic signals during development. In females, the dendritic structure can change 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%.[14]
Recent experimental observations suggest that adaptation is performed in the neuronal dendritic trees, where the timescale of adaptation was observed to be as low as several seconds only.[23][24] Certain machine learning architectures based on dendritic trees have shown to simplify the learning algorithm without affecting performance.[25]
References
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The nerve cell with its uninterrupted processes was described by Otto Friedrich Karl Deiters (1834-1863) in a work that was completed by Max Schultze (1825-1874) in 1865, two years after Deiters died of typhoid fever. This work portrayed the cell body with a single chief "axis cylinder" and a number of smaller "protoplasmic processes" (see figure 3.19). The latter would become known as "dendrites", a term coined by Wilhelm His (1831-1904) in 1889.
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- ^ Michmizos D, Koutsouraki E, Asprodini E, Baloyannis S. 2011. Synaptic Plasticity: A Unified Model to Address Some Persisting Questions. International Journal of Neuroscience, 121(6): 289-304. https://www.tandfonline.com/doi/abs/10.3109/00207454.2011.556283
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External links
- Histology image: 3_09 at the University of Oklahoma Health Sciences Center - "Slide 3 Spinal cord"
- Dendritic Tree - Cell Centered Database
- Stereo images of dendritic trees in Kryptopterus electroreceptor organs