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Neurons communicate with other cells via synapses, which are specialized connections that commonly use minute amounts of chemical neurotransmitters to pass the electric signal from the presynaptic neuron to the target cell through the synaptic gap. Neurons are the main components of nervous tissue in all animals except sponges and placozoans.
In the human brain, approximately 20–30% of the neurons in the neocortex are interneurons, and the remaining majority of neurons are pyramidal. [7] Investigations into the molecular diversity of neurons is impeded by the inability to isolate cell populations born at different times for gene expression analysis.
They can only function in collaboration with other neurons and interneurons in a neural circuit. [1] There are an estimated 100 billion neurons in the human brain. [1] Neurons are polarised cells that are specialised for the conduction of action potentials also called nerve impulses. [1] They can also synthesise membrane and protein.
The Human Cell Atlas project, which started in 2016, had as one of its goals to "catalog all cell types (for example, immune cells or brain cells) and sub-types in the human body". [13] By 2018, the Human Cell Atlas description based the project on the assumption that "our characterization of the hundreds of types and subtypes of cells in the ...
The function of neurons depends upon cell polarity. The distinctive structure of nerve cells allows action potentials to travel directionally (from dendrites to cell body down the axon), and for these signals to then be received and carried on by post-synaptic neurons or received by effector cells. Nerve cells have long been used as models for ...
Examples of bipolar neurons include most invertebrate sensory neurons and bipolar cells of the vertebrate retina. Some vertebrate sensory neurons are classified as pseudounipolar. Pseudounipolar neurons initially develop as bipolar cells, but at some point the two processes that extend from the cell body fuse to form a single neurite. [4]
For example, motor neurons, which travel from the spinal cord to the muscle, can have axons up to a meter in length in humans. The longest axon in the human body belongs to the Sciatic Nerve and runs from the great toe to the base of the spinal cord. These are archetypal examples of neural pathways. [citation needed]
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. [1] This process is called sensory transduction. The cell bodies of the sensory neurons are located in the dorsal root ganglia of the spinal cord ...