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The mysterious tree structure: How do neuron dendrites work?
The word dendrite (English: dendrite) originates from the Greek word meaning "tree". It is a branching cytoplasmic process of neurons. Its main function is to transmit electrochemical stimuli from other nerve cells and transfer these signals. The cell body passed to the nerve cell is also called the "cell body". Those electrical stimuli are typically transmitted to the dendrites through synapses, which are distributed throughout the entire tree-like structure of the dendrites. Dendrites play a key role in integrating these synaptic inputs and determining the extent to which the neuron generates action potentials.
Dendrites are not simply signal receivers. Their structure and function make them the core of neuronal message processing.
Structure and function of dendrites
Dendrites are one of two cytoplasmic processes that project outward from the cell body of a neuron. The other is the axon. Dendrites are distinguished from axons by several characteristics including shape, length, and function. Typically, dendrites are tapered and shorter in shape, whereas axons maintain a constant diameter and can be very long. The main function of dendrites is to receive signals from the axon terminals of other neurons and to provide a larger surface area to receive these signals.
It is estimated that the dendrites of a large cone cell can receive signals from approximately 30,000 presynaptic neurons. Excitatory synapses terminate on dendritic spines, which are small projections of dendrites that contain a high density of neurotransmitter receptors. Most inhibitory synapses make direct contact with the main trunk of the dendrite. Synaptic activity induces local changes in dendritic membrane potential that become progressively attenuated over distance.
To generate an action potential, many excitatory synapses must be active simultaneously, causing strong depolarization of the dendrites and their cell bodies.
Historical evolution of dendrites
The term dendrite was first used by Wilhelm His in 1889 to describe the many smaller "protoplasmic processes" that connect nerve cells. German anatomist Otto Deiters is generally credited as the discoverer of axons by distinguishing them from dendrites.
The first intracellular recordings in the nervous system were made in the 1930s by Kenneth S. Cole and Howard J. Curtis. Rüdolf Albert von Kölliker in Switzerland and Robert Remak in Germany were the first to identify and describe the initial segment of the axon. Later, Alan Hodgkin and Andrew Huxley used the giant axons of squid to provide a complete quantitative description of action potentials, which also earned them the 1963 Nobel Prize. award.
Development of dendrites
During the development of dendrites, multiple factors can influence their differentiation, including modulation of sensory input, environmental pollutants, body temperature, and drug use. For example, it was once found that the number of dendritic ridges of cone cells in the primary visual cortex of mice raised in a dark environment was significantly reduced, and the dendritic branch distribution of stellate cells also changed significantly.
The complex tree-like structure of dendrites is formed by the interaction of multiple external and internal signals.
Diversity of dendrites
Dendrites form many different morphological patterns in different organisms, and the morphology of these branches (such as branch density and distribution pattern) is closely related to the function of neurons. Dendrites can vary widely in number, sometimes being able to receive as many as 100,000 different inputs. Dendrite morphology errors are closely related to impairment of nervous system function.
The morphology of dendrites can be a branchless structure or a radiating structure like a tree. These dendritic branching patterns may be spindle-shaped, spherical, or take on a multiplanar shape, such as in the Purkinje cells of the cerebellum.
Electrical properties and plasticity of dendrites
Changes in the dendritic structure, branches and voltage-dependent ion conductance of neurons will profoundly affect how neurons integrate input from other neurons. Dendrites are thought to be more than passive transmitters of electrical stimulation, but are capable of plastic structural adjustments in adult life. The divisions made up of dendrites are called functional units, and they are able to calculate and process incoming stimuli.
Recent experimental observations show that dendritic adaptation can occur within seconds, and the impact of such structural changes on neuronal function can be significant. The composition of dendrites can also change significantly with changes in the external environment. For example, under the influence of pregnancy or hormonal cycles, dendrite structure can change by up to 30%.
All this makes us wonder, is there a deeper connection between the evolution of dendrites and learning ability?
