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Neurons)
Neurons (also called nerve cells) are the primary cells of the nervous system.
Location
In vertebrates, they are found in the brain, the spinal cord and in the nerves and ganglia of the peripheral nervous system.
Classes
There are three classes of neurons; afferent neurons, efferent neurons, and interneurons.
Anatomy
Many highly specialized types of neurons exist, and these differ widely in appearance.
Characteristically, neurons are highly asymmetric in shape.
Neurons consist of:
- The soma, a relatively fat central cell.
- The axon, a much finer, cable-like projection which may extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length.
- The dendrite, a short, branching arbor of cellular extensions.
Axon and dendrites alike are typically only about a micrometer thick, while the soma is usually about 25 micrometers in diameter and not much larger than than the cell nucleus it contains. An axon of a human motoneuron, meanwhile, can be a meter long.
Connectivity
Neurons join to one another and to other cells through synapses, which connect the axon tip of one cell to a dendrite of another, or less commonly to its axon or soma. Neurons of the cortex in mammals, such as the Purkinje cells, have over 1000 dendrites apiece, enabling connections to tens of thousands of other cells.
Types of signalling
Neurons stimulate one another across synapses chemically by rapid secretion of neurotransmitter molecules. They are known most, however, for their ability to undergo electrical excitation and to transmit this excitation along their axons as an impulse, called an "action potential." Arrival of an action potential at the tip of an axon triggers the release of neurotransmitter into the synaptic gap. Arriving neurotransmitters then either stimulate or suppress an action potential in the target cell, depending on the neurotransmitter and its receptor.
Signals are sent in a series of pulses of action potentials. Stronger signals, corresponding to larger stimuli, are sent with a higher frequency of pulses, rather than larger pulses.
Another kind of signalling is direct influence by the changes in electric field potential. Field signalling is as yet quite incompletely known and notices about it are found in the WWW by searching through the words electroneurobiology and relativistic brain biophysics.
Adaptations to carrying action potentials
The narrow cross-section of axons and dendrites lessens the metabolic expense of carrying action potentials, although fatter axons convey the impulses more rapidly, generally speaking.
Many neurons have insulating sheaths of myelin around their axons, which enable their action potentials to travel faster than in unmyelinated axons of the same diameter. Formed by glial cells, the myelin sheathing normally runs along the axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier. Multiple sclerosis is a neurological disorder which results from abnormal demyelination of peripheral nerves. Neurons with demyelinated axons do not conduct electrical signals properly.
Neurons and glia make up the two chief cell types of the nervous system. There are far more glial cells than neurons, though glia are not currently thought to be directly involved in electrical signaling.
Neurons of the brain
The nematode worm (Caenorhabditis elegans) has 302 neurons. Scientists have mapped all of the nematode's neurons.
The human (Homo sapiens) brain has about 100 billion neurons and 100 trillions of connections (synapses) between them.
See also
External links
- NeuroWiki (http://purl.net/net/neurowiki), a wiki website for Neuroscience related topics. All content (unless explicitly proclaimed otherwise) is published to the public domain thus can be relocated to the Wikipedia.
de:Nervenzelle fr:Neurone nl:zenuwcelja:ニューロン pl:Neuron sv:Neuron
zh:神经元
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