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The axon is the long projection of a?nerve that can reach a length of tenths of centimeters, that conveys electrical impulses from the dendrites/soma of the neuron to the next neuron. They have a high variability of branching pattern and extent (characteristic for individual neuronal types):
Projecting neurons?? long axon with terminal branching (several mm in length up to about 1 m); usually larger cell bodies + numerous cytoplasmic organelles.
Interneurons?(neurons of the local circuits) ? short axons with side branches in the vicinity of the cell body (up to several mm); usually smaller cell bodies, lower amount of cytoplasm, less organelles.
The axon arises from the soma (or sometimes from a dendrite) in a specialized region called the axon hillock. It passes into initial segment, which differs from the next part of axon in the special functional properties of its membrane (low threshold ? high excitability due to densely packed Na+-VGCs). Most of the projecting neurons send axon collaterals into the local neuronal circuits, some of the neurons of local circuits send axons collateral toward more distant structures ? no distinct functional difference between the two classes of neurons.
Dendrites (in Greek it means "tree") are branched protrusions from the neuron's soma that transmit post-synaptic potentials to it. 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
Enlarged surface area to receive signals from axons of other nerve cells:
The size of the dendritic tree limits how many synaptic inputs the neuron can receive.
The orientation of the dendritic tree determines the types and number of sources from which it can receive synaptic connections.
Transmission of received signals:
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.
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).
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.
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).
These accumulations (hot-spots) may recover the declining membrane potential and dramatically increase the effectiveness of conductance: pseudo-saltatory transmission by dendritic spines.
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.
Elementary integrative function: summation of excitatory + inhibitory potentials before the final current reaches the cell body.
Possible contribution to memory: facilitation is possible due to plasticity of dendritic spines.
Possible source of neuromodulators and tissue factors.
Projecting neurons?? long axon with terminal branching (several mm in length up to about 1 m); usually larger cell bodies + numerous cytoplasmic organelles.
Interneurons?(neurons of the local circuits) ? short axons with side branches in the vicinity of the cell body (up to several mm); usually smaller cell bodies, lower amount of cytoplasm, less organelles.
The axon arises from the soma (or sometimes from a dendrite) in a specialized region called the axon hillock. It passes into initial segment, which differs from the next part of axon in the special functional properties of its membrane (low threshold ? high excitability due to densely packed Na+-VGCs). Most of the projecting neurons send axon collaterals into the local neuronal circuits, some of the neurons of local circuits send axons collateral toward more distant structures ? no distinct functional difference between the two classes of neurons.
Dendrites (in Greek it means "tree") are branched protrusions from the neuron's soma that transmit post-synaptic potentials to it. 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
Enlarged surface area to receive signals from axons of other nerve cells:
The size of the dendritic tree limits how many synaptic inputs the neuron can receive.
The orientation of the dendritic tree determines the types and number of sources from which it can receive synaptic connections.
Transmission of received signals:
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.
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).
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.
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).
These accumulations (hot-spots) may recover the declining membrane potential and dramatically increase the effectiveness of conductance: pseudo-saltatory transmission by dendritic spines.
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.
Elementary integrative function: summation of excitatory + inhibitory potentials before the final current reaches the cell body.
Possible contribution to memory: facilitation is possible due to plasticity of dendritic spines.
Possible source of neuromodulators and tissue factors.
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