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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.
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
There are about 100 billion neurons in the human brain.
See also
External links
- 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.
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