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  Neurons (IPA: /njˈɒɹɒns/, also known as neurones and nerve cells) are electrically excitable cells in the nervous system that process and transmit information. Neurons are the core components of the brain, and spinal cord in vertebrates and ventral nerve cord in invertebrates, and peripheral nerves.

Additional recommended knowledge



Neurons are usually considered permanently amitotic[1] however, recent research shows that they do indeed undergo adult neurogenesis.[2][3][4][5][6][7] Neurons are typically composed of a soma, or cell body, a dendritic tree and an axon. The majority of vertebrate neurons receive input on the cell body and dendritic tree, and transmit output via the axon. However, there is great heterogeneity throughout the nervous system and the animal kingdom, in the size, shape and function of neurons.



The neuron's place as the primary functional unit of the nervous system was first recognized in the early 20th century through the work of the Spanish anatomist Santiago Ramón y Cajal.[8] Cajal proposed that neurons were discrete cells that communicated with each other via specialized junctions, or spaces, between cells.[8] This became known as the neuron doctrine, one of the central tenets of modern neuroscience.[8] To observe the structure of individual neurons, Cajal used a silver staining method developed by his rival, Camillo Golgi.[8] The Golgi stain is an extremely useful method for neuroanatomical investigations because, for reasons unknown, it stains a very small percentage of cells in a tissue, so one is able to see the complete microstructure of individual neurons without much overlap from other cells in the densely packed brain.[9]

Anatomy and histology



  • The axon terminal contains synapses, specialized structures where neurotransmitter chemicals are released in order to communicate with target neurons.






Structural classification


Most neurons can be anatomically characterized as:

  • Unipolar or pseudounipolar: dendrite and axon emerging from same process.
  • Bipolar: axon and single dendrite on opposite ends of the soma.
  • Multipolar: more than two dendrites:
    • Golgi I: neurons with long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells.
    • Golgi II: neurons whose axonal process projects locally; the best example are the granule cells.



Functional classification




Action on other neurons

  • Excitatory neurons excite their target neurons. Excitatory neurons in the central nervous system, including the brain, are often glutamatergic. Neurons of the peripheral nervous system, such as spinal motoneurons that synapse onto muscle cells, often use acetylcholine as their excitatory neurotransmitter. However, this is just a general tendency that may not always be true. It is not the neurotransmitter that decides excitatory or inhibitory action, but rather it is the postsynaptic receptor that is responsible for the action of the neurotransmitter.

Discharge patterns

Neurons can be classified according to their electrophysiological characteristics:


Neurotransmitter released

Some examples are

  • cholinergic neurons
  • GABAergic neurons
  • glutamatergic neurons
  • dopaminergic neurons
  • 5-hydroxytryptamine neurons (5-HT; serotonin)


Main article: Synapse




Mechanisms for propagating action potentials





Histology and internal structure





The neuron doctrine

The neuron doctrine is the now fundamental idea that neurons are the basic structural and functional units of the nervous system. The theory was put forward by Santiago Ramón y Cajal in the late 19th century. It held that neurons are discrete cells (not connected in a meshwork), acting as metabolically distinct units. Cajal further extended this to the Law of Dynamic Polarization, which states that neural transmission goes only in one direction, from axons toward dendrites.[15] As with all doctrines, there are some exceptions. For example glial cells may also play a role in information processing.[16] Also, electrical synapses are more common than previously thought,[17] meaning that there are direct-cytoplasmic connections between neurons. In fact, there are examples of neurons forming even tighter coupling; the squid giant axon arises from the fusion of multiple neurons that retain individual cell bodies and the crayfish giant axon consists of a series of neurons with high conductance septate junctions. The Law of Dynamic Polarization also has important exceptions; dendrites can serve as synaptic output sites of neurons[18] and axons can receive synaptic inputs.

Neurons in the brain

The number of neurons in the brain varies dramatically from species to species.[19] One estimate puts the human brain at about 100 billion (1011) neurons and 100 trillion (1014) synapses.[19] By contrast, the nematode worm Caenorhabditis elegans has just 302 neurons making it an ideal experimental subject as scientists have been able to map all of the organism's neurons. By contrast, the fruit fly Drosophila melanogaster has around 300,000 neurons (which do spike) and exhibits many complex behaviors. Many properties of neurons, from the type of neurotransmitters used to ion channel composition, are maintained across species, allowing scientists to study processes occurring in more complex organisms in much simpler experimental systems.

Neurologic Diseases

Alzheimer's disease: Alzheimer's disease (AD), also known simply as Alzheimer's, is a neurodegenerative disease characterized by progressive cognitive deterioration together with declining activities of daily living and neuropsychiatric symptoms or behavioral changes. The most striking early symptom is loss of short-term memory (amnesia), which usually manifests as minor forgetfulness that becomes steadily more pronounced with illness progression, with relative preservation of older memories. As the disorder progresses, cognitive (intellectual) impairment extends to the domains of language (aphasia), skilled movements (apraxia), recognition (agnosia), and functions such as decision-making and planning get impaired.

Parkinson's disease: Parkinson's disease (also known as Parkinson disease or PD) is a degenerative disorder of the central nervous system that often impairs the sufferer's motor skills and speech. Parkinson's disease belongs to a group of conditions called movement disorders. It is characterized by muscle rigidity, tremor, a slowing of physical movement (bradykinesia), and in extreme cases, a loss of physical movement (akinesia). The primary symptoms are the results of decreased stimulation of the motor cortex by the basal ganglia, normally caused by the insufficient formation and action of dopamine, which is produced in the dopaminergic neurons of the brain. Secondary symptoms may include high level cognitive dysfunction and subtle language problems. PD is both chronic and progressive.

Myasthenia Gravis: Myasthenia gravis is a neuromuscular disease leading to fluctuating muscle weakness and fatigability. Weakness is typically caused by circulating antibodies that block acetylcholine receptors at the post-synaptic neuromuscular junction, inhibiting the stimulative effect of the neurotransmitter acetylcholine. Myasthenia is treated with immunosuppressants, cholinesterase inhibitors and, in selected cases, thymectomy.


Demyelination is the act of demyelinating, or the loss of the myelin sheath insulating the nerves. When myelin degrades, conduction of signals along the nerve can be impaired or lost, and the nerve eventually withers. This leads to certain neurodegenerative disorders like multiple sclerosis, chronic inflammatory demyelinating polyneuropathy.

Axonal Degeneration

Although most injury responses include a calcium influx signaling to promote resealing of severed parts, axonal injuries initially lead to acute axonal degeneration (AAD), which is rapid separation of the proximal and distal ends within 30 minutes of injury. Degeneration follows with swelling of the axolemma, and eventually leads to bead like formation. Granular disintegration of the axonal cytoskeleton and inner organelles occurs after axolemma degradation. Early changes include accumulation of mitochondria in the paranodal regions at the site of injury. Endoplasmic reticulum degrades and mitochondria swell up and eventually disintegrate. The disintegration is dependent on Ubiquitin and Calpain proteases (caused by influx of calcium ion), suggesting that axonal degeneration is an active process. Thus the axon undergoes complete fragmentation. The process takes about roughly 24 hrs in the PNS, and longer in the CNS. The signaling pathways leading to axolemma degeneration are currently unknown.


  1. ^ Nature Reviews Neuroscience 8, 368-378 (May 2007) | doi:10.1038/nrn2124
  2. ^ [1]
  3. ^ [2]
  4. ^ [3]
  5. ^ [4]
  6. ^ [5]
  7. ^ [6]
  10. ^ The Neuron: Size Comparison
  11. ^ Brain Facts and Figures
  13. ^ Milestones in Neuroscience Research
  14. ^ Electrical activity of nerves


This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Neuron". A list of authors is available in Wikipedia.