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History The neuron's role as the primary functional unit of the nervous system was first recognised in the early 20th century work of the Spanish anatomist Santiago Ramón y Cajal. Cajal proposed that neurons were discrete cells which communicated with each other via specialized junctions, or spaces, between cells. This became known as the neuron doctrine, one of the central tenets of modern neuroscience. To observe the structure of individual neurons, Cajal used a silver staining method developed by his rival, Camillo Golgi. When the Golgi stain is applied to neurons, it binds the cells' microtubules and gives them a black outline when illuminated. Anatomy and histology
Classes of pyramidal neurons in mouse cerebral cortex expressing green fluorescent protein. The red staining indicates GABA|GABAergic interneurons. Source PLoS Biology http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0040029 Structural classification Most neurons can be anatomically characterized as: Some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are basket, Betz, medium spiny, Purkinje, pyramidal and Renshaw cells. Functional classification Afferent and efferent can also refer to neurons which convey information from one region of the brain to another. Classification by action on other neurons Classification by discharge patterns Neurons can be classified according to their electrophysiological characteristics: Classification by neurotransmitter released Some examples are cholinergic, GABA-ergic, glutamatergic and dopaminergic neurons. Connectivity Neurons communicate with one another via synapses, where the axon terminal of one cell impinges upon a dendrite or soma of another (or less commonly to an axon). Neurons such as Purkinje cells in the cerebellum can have over 1000 dendritic branches, making connections with tens of thousands of other cells; other neurons, such as the magnocellular neurons of the supraoptic nucleus, have only one or two dendrites, each of which receives thousands of synapses. Synapses can be excitatory or inhibitory and will either increase or decrease activity in the target neuron. Some neurons also communicate via electrical synapses, which are direct, electrically-conductive junctions between cells. In a chemical synapse, the process of synaptic transmission is as follows: when an action potential reaches the axon terminal, it opens voltage-gated calcium channels, allowing calcium ions to enter the terminal. Calcium causes synaptic vesicles filled with neurotransmitter molecules to fuse with the membrane, releasing their contents into the synaptic cleft. The neurotransmitters diffuse across the synaptic cleft and activate receptors on the postsynaptic neuron. The human brain has a huge number of synapses. Each of 100 billion neurons has on average 7,000 synaptic connections to other neurons. Most authorities estimate that the brain of a three-year-old child has about 1,000 trillion synapses. This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 100 to 500 trillion synapses. Adaptations to carrying action potentials The cell membrane in the axon and soma contain voltage-gated ion channels which allow the neuron to generate and propagate an electrical impulse (an action potential). Substantial early knowledge of neuron electrical activity came from experiments with squid giant axons. In 1937, John Zachary Young suggested that the giant squid axon can be used to study neuronal electrical properties. As they are much larger than human neurons, but similar in nature, it was easier to study them with the technology of that time. By inserting electrodes into the giant squid axons, accurate measurements could be made of the membrane potential. Electrical activity can be produced in neurons by a number of stimuli. Pressure, stretch, chemical transmitters, and electrical current passing across the nerve membrane as a result of a difference in voltage can all initiate nerve activity. The narrow cross-section of axons lessens the metabolic expense of carrying action potentials, but thicker axons convey impulses more rapidly. To minimize metabolic expense while maintaining rapid conduction, many neurons have insulating sheaths of myelin around their axons. The sheaths are formed by glial cells: oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. The sheath enables action potentials to travel faster than in unmyelinated axons of the same diameter, whilst using less energy. The myelin sheath in peripheral nerves normally runs along the axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier which contain a high density of voltage-gated ion channels. Multiple sclerosis is a neurological disorder that results from abnormal demyelination of peripheral nerves. Neurons with demyelinated axons do not conduct electrical signals properly. Some neurons do not generate action potentials, but instead generate a graded electrical signal, which in turn causes graded neurotransmitter release. Such nonspiking neurons tend to be sensory neurons or interneurons, because they cannot carry signals long distances. Histology and internal structure
Challenges to the neuron doctrine The neuron doctrine is a central tenet of modern neuroscience, but recent studies suggest that this doctrine needs to be revised. First, electrical synapses are more common in the central nervous system than previously thought. Thus, rather than functioning as individual units, in some parts of the brain large ensembles of neurons may be active simultaneously to process neural information. Second, dendrites, like axons, also have voltage-gated ion channels and can generate electrical potentials that carry information to and from the soma. This challenges the view that dendrites are simply passive recipients of information and axons the sole transmitters. It also suggests that the neuron is not simply active as a single element, but that complex computations can occur within a single neuron. . Third, the role of glia in processing neural information has begun to be appreciated. Neurons and glia make up the two chief cell types of the central nervous system. There are far more glial cells than neurons: glia outnumber neurons by as many as 10:1. Recent experimental results have suggested that glia play a vital role in information processing. Finally, recent research has challenged the historical view that neurogenesis, or the generation of new neurons, does not occur in adult mammalian brains. It is now known that the adult brain continuously creates new neurons in the hippocampus and in an area contributing to the olfactory bulb. This research has shown that neurogenesis is environment-dependent (eg. exercise, diet, interactive surroundings), age-related, upregulated by a number of growth factors, and halted by survival-type stress factors. . Of particularly compelling interest, Charles Gross and Elizabeth Gould provided evidence suggestive that neurogenesis occurred in neocortex after birth, in areas of the brain known to be important for cognitive function. Strong challenges to this work have come from more well-controlled studies by Pasko Rakic and others which support Rakic's original hypothesis that neurogenesis after birth is restricted to the olfactory bulb and hippocampus. Rakic argues that the Princeton group's work has not been substantiated by multiple other groups. Neurons in the brain The number of neurons in the brain varies dramatically from species to species. The human brain has about 100 billion () neurons and 100 trillion () synapses. 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. 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. See also Sources | |||||||||||||
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