Neurons (also called nerve cells) are the primary cells of the nervous system. In vertebrates, they are found in the brain, the spinal cord and in the nerves and ganglia of the peripheral nervous system.
There are three classes of neurons: afferent neurons, efferent neurons, and interneurons.
Anatomy and histology
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, the relatively fat central part of the cell between the dendrites and the axon.
- 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.
Nerve cell bodies stained with basophilic dyes will show numerous microscopic clumps of Nissl substance (named after German psychiatrist and neuropathologist Franz Nissl, 1860–1919), which consists of rough endoplasmic reticulum and associated ribosomes. The prominence of the Nissl substance can be explained by the fact that nerve cells are metabolically very active, and hence are involved in large numbers of protein synthesis.
The cell body of a neuron is supported by a complex meshwork of structural proteins called neurofilaments, which are assembled into larger neurofibrils. Some neurons also contain pigment granules, such as neuromelanin (a brownish-black pigment, byproduct of synthesis of catecholamines) and lipofuscin (yellowish-brown pigment that accumulates with age).
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.
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 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 brain has about 100 billion neurons and 100 trillion connections (synapses) between them.
- NeuroWiki (http://purl.net/net/neurowiki), a wiki website for Neuroscience related topics.