NeuronResearch
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Neural Network Research
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Contents
Meaning of Neuron
- Neuron is biological cell of complex structure and form
- Neuron is building block of Central Nervous System (CNS) and Peripheral Nervous System (PNS)
- Each neuron performs relay, calculation and memory functions
- Neurons are of different types which allow neural regions to play various roles
- Neuron is max of 0.1 mm width and its axon can be up to 1m length
- Neurons are surrounded by glia cells, which fed them and participate in neuron functions
Neuron Development
- neuron is kind of cell
- neurons are divided only inside core tube in embrion before birth and do not divide (i.e created) after birth
- up to adulthood number of neurons decreased by 2
- going to end of life, neurons are decreazed in size by 2
- DNA forms genes, combined in chromosomes
- genes construct proteins
- proteins include hormones (carry messages between cells) and neurotransmitters (carry messages between neurons) and enzymes that synthesize and degrade those messengers
Neuron Structure and Sequence of Signal Transmission
- neuron consists of several dendrites, one soma and one axon
- dorsal root ganglion cells, primary afferent sensory neurons of spinal cord, do not have dendrites; instead, they have bifurcating axon that ends in sense organs in periphery and in synapses within spinal cord or caudal medulla on upstream neurons that are involved in motor responses or in transmission of sensory information to higher levels of nervous system
- dendrites can be of different compartment - apical, distal, proximal
- soma relays and produces neurotransmitters, which allow neurons to build neural networks
- soma produces spike - electrical wave, holding so called action potential which travels through axon
- axon contains starting segment, trunk, branches
- starting axon segment can have connections from another neuron
- trunk is mielinated - behaves like isolated electrical wire
- at certain point, where mielination ends, axon is split into many branches, branches - into subbranches and so on
- at the end of each branch there is axon terminal, which if located near another cell, makes connection
- signal goes to every branch to every axon terminal, where via neurotransmitter affects dendrite, soma or axon of another neuron
- duration of signal is greater when diameter of branch is smaller (see soliton concept)
- dendtite has spines which is connection of dendrite to specific axon terminal
- neurotransmitter substance excited by action potential makes certain effect on dendrite, by activating neurotransmitter-dependent receptors
- chemical reaction in receptor affects membrane of soma
- effect can be of 3 types - excitatory, inhibitory and modulatory, which defined by neurotransmitter
- excitatory effect, e.g. from GLU, increases total membrane potential, which causes neuron to fire when potential is above specific threshold, it lasts about 20ms
- inhibitory effect, e.g. from GABA, blocks neuron from firing within 150ms
- modulatory effect is excitatory but much slower
Signal propagation in Axon branches
- see diameter-based metric - GR=(c1d^3/2^+ c2d^3/2^)/pd^3/2^, signals spreads two-way if GR=1
- collateral axon or axon collateral is branch off of main axon that usually feeds back onto soma
- axon collateral can form synapses with nearby inhibitory neurons, it serves as a regulation system. Excitatory cells excite nearby inhibitory cells that prevent them from firing quite as often
- axon collaterals bifurcate from primary axon segments at unmyelinated regions
- thalamic neurons in retinogeniculate system lose approximately ten inputs for every one input that is maintained soon after birth
