Navigation Links
Neuron


Neurons (also spelled neurones or 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.

Contents

Classes

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, or cell-body, the relatively large 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. This is the structure which carries nerve signals away from the neuron. Each neuron has only one axon, but this axon may undergo extensive branching and thereby enable communication with many target cells.
  • The dendrite, a short, branching arbor of cellular extensions. Each neuron has very many dendrites with profuse dendritic branches. These structures form the main information receiving network for the neuron.

Axon and dendrites alike are typically only about a micrometre thick, while the soma is usually about 25 micrometres in diameter and not much larger than than the cell nucleus it contains. The axon of a human motoneuron can be over a metre long, reaching from the base of the spine to the toes.

Connectivity

Neurons communicate with one another and to other cells through synapses, where the axon tip of one cell impinges upon a dendrite or soma of another, or less commonly to an axon. Neurons of the cortex in mammals, such as the Purkinje cells , have over 1000 dendrites apiece, enabling connections with tens of thousands of other cells.

Types of signalling

Neurons communicate with one another across synapses. This communication is usually chemically mediated by rapid secretion of neurotransmitter molecules. Pre-synaptic neurons (i.e.the neurons which release the neurotransmitter) may produce in the post-synaptic neurons (i.e. the neurons being affected by the neurotransmitter) an electrical stimulation (an electrical excitation) which will spread to the axon hillock generating an action potential which then travels as a wave of electrical excitation along the axon. Arrival of an action potential at the tip of an axon triggers the release of neurotransmitter at a synaptic gap. Neurotransmitters can either stimulate or suppress (inhibit) the electrical excitability of a target cell. An action potential will only be triggered in the target cell if neurotransmitter molecules acting on their post-synaptic receptors cause the cell to reach its threshold potential.

Adaptations to carrying action potentials

The narrow cross-section of axons and dendrites lessens the metabolic expense of carrying action potentials, although thicker 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 in the central nervous system and Schwann cells in the peripheral nervous system. The myelin sheath in peripheral nerves 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 central nervous system. There are far more glial cells than neurons, and recent experimental results have suggested that glial cells play a vital role in information processing among neurons.

Histology and internal structure

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 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 (1011) neurons and 100 trillion (1014) connections (synapses) between them.

See also

External links


'"/>


(Date:8/27/2014)... getting effective doses of the HIV drug maraviroc, a ... studies, completed before the drug was licensed in 2007, ... is key to removing maraviroc from the body. The ... the protein including nearly half of African-Americans ... to those who lack the protein even when given ...
(Date:8/27/2014)... grass species frequently found in forests has created a ... American toads, a new University of Georgia study has ... the U.S. in the early 1900s, is one of ... more than a dozen states in the past century, ... in forests, it can survive in widely diverse ecosystems ...
(Date:8/27/2014)... what is described by the Volunteer Wildfire Services of ... Cape provincial government warned residents in certain parts of ... veld fires. A high veld fire danger rating ... coast in the Great Kei and Mnquma area. ... thunderstorms. The thunderstorms bring lightning strikes and subsequent ...
Breaking Biology News(10 mins):Dosage of HIV drug may be ineffective for half of African-Americans 2More wolf spiders feasting on American toads due to invasive grass, UGA study shows 2More wolf spiders feasting on American toads due to invasive grass, UGA study shows 3
... great mysteries of life is how it began. What physical process ... a living cell? For more than a century, scientists have ... life. Until recently, their focus has been trained on how the ... early Earth, or perhaps in space. But because it happened so ...
... firing in a brain reward circuit instantly rendered mice ... a study supported by the National Institutes of Health ... mimic the pattern, previously resilient mice instantly succumbed to ... they avoided other animals and lost their sweet ...
... of Notre Dame biochemist Anthony S. Serianni is providing ... and treating diabetes. Serianni points out that biological ... human body from the natural breakdown of the simple ... in diabetic patients because glucose concentrations in the blood ...
Cached Biology News:ASU researchers propose new way to look at the dawn of life 2ASU researchers propose new way to look at the dawn of life 3Stress-resilience/susceptibility traced to neurons in reward circuit 2Stress-resilience/susceptibility traced to neurons in reward circuit 3Stress-resilience/susceptibility traced to neurons in reward circuit 4Notre Dame research may have important implications for combating diabetes 2
Other biology definitionOther Tags