Neuron

Drawing of neurons in the pigeon cerebellum by Santiago Ramón y Cajal, the Spanish anatomist who first recognized the neuron’s role as the primary functional unit of the nervous system.

Neurons (also known as neurones, nerve cells, and nerve fibers) are electrically excitable cells in the nervous system that function to process and transmit information from internal and external environments. In vertebrate animals, neurons are the core components of the brain, spinal cord and peripheral nerves. The output of the nervous system is a function of the interconnectivity of neurons (that is, the strength and configuration of the connections between neurons).

The basic function of a neuron is to communicate information, which it does via chemical or electric impulses (synapses). The fundamental process that triggers these impulses is the action potential, a propagating electrical signal that is generated by exploiting the electrically excitable membrane of the neuron.

While there is great heterogeneity across the nervous system and across species in the size, shape and function of neurons, they can be classified based on the following functions:

• Sensory neurons have specialized receptors to convert diverse stimuli from the environment (such as light, touch, pressure) into electric signals. These signals are then converted into chemical signals that are passed along to other cells. A sensory neuron transmits impulses from a receptor, such as those in the eye or ear, to a more central location in the nervous system, such as the spinal cord or brain.
• Motor neurons transmit impulses from a central area of the nervous system to an effector, such as a muscle. They regulate the contraction of muscles; other neurons stimulate other types of cell, such as glands.
• Interneurons convert chemical information back to electric signals. Also known as relay neurons, interneurons provide connections between sensory and motor neurons, as well as between themselves.

Neurons are typically composed of four main components: a soma, or cell body, which contains the nucleus; one or more dendritic trees that receive input; an axon that carries an electric impulse; and an axon terminal that functions to transmit signals to other cells.

The number of neurons in a given organism varies dramatically from species to species. The human brain has approximately 100 billion ($10^{11}$) neurons and 100 trillion ($10^{14}$) synapses (or connections between neurons). By contrast, the nematode worm (Caenorhabditis elegans) has just 302 neurons. Many properties of neurons, from the type of neurotransmitters used to ion channel composition, are maintained across species; this interconnectedness of life allows scientists to study processes occurring in more complex organisms in much simpler experimental systems.

The structure of a neuron

The structure of a typical neuron includes four main components (from left to right): dendrites, cell body (or soma), axon, and axon terminal.

Depending on their function, neurons exist in a wide variety of structures, sizes, and electrochemical properties. However, most neurons contain the following four regions:

• The cell body, or the soma, is the central part of the neuron. It contains the nucleus of the cell; therefore, it is the site of most protein synthesis in the neuron.
• The axon is a finer, cable-like projection which can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. The axon is specialized for the conduction of a particular electric impulse, called the action potential, away from the cell body and down the axon (the action potential is a series of sudden changes in the electric potential across the plasma membrane). Many neurons have only one axon, but this axon may - and usually will - undergo extensive branching, enabling communication with many target cells. The junction of the axon and the cell body is called the axon hillock, which has the greatest density of voltage-dependent sodium channels. This makes it the most easily excited part of the neuron.
• The axon terminal refers to the small branches of the axon that form the synapses, or connections with other cells.
• The dendrites of a neuron are cellular extensions with many branches, where the majority of input to the neuron occurs. Thus, the overall shape and structure of a neuron's dendrites is called its dendritic tree. Most neurons have multiple dendrites; dendrites extend outward from the soma and are specialized to receive chemical signals from the axon termini of other neurons. Dendrites convert these signals into small electric impulses and transmit them to the soma.

Although the canonical view of the neuron attributes dedicated functions to its various anatomical components, dendrites and axons often act in ways contrary to their so-called main function. While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons. Information outflow (i.e. from dendrites to other neurons) can also occur.

Axons and dendrites in the central nervous system of vertebrates are typically only about a micrometer thick, while some in the peripheral nervous system are much thicker. The soma is usually about 10–25 micrometers in diameter and often is not much larger than the cell nucleus it contains. The longest axon of a human motor neuron can be over a meter long, reaching from the base of the spine to the toes. Sensory neurons have axons that run from the toes to the dorsal columns, over 1.5 meters in adults. Giraffes have single axons several meters in length running along the entire length of their necks. Much of what is known about the function of axons comes from studying the squid giant axon, an ideal experimental preparation because of its relatively immense size (0.5–1 millimeters thick, several centimeters long).

The transmission of an impulse

Neurons communicate with one another via synapses, junctions where neurons pass signals to target cells, which may be other neurons, muscle cells, or gland cells. Neurons such as Purkinje cells in the cerebellum may 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, possess only one or two dendrites, each of which receives thousands of synapses.

Synapses generally conduct signals in one direction. Synapses can be excitatory or inhibitory; that is, they will either increase or decrease activity in the target neuron. While chemical synapses are common, some neurons also communicate via electrical synapses, which are direct, electrically-conductive junctions between cells.

Chemical synapses

Chemical synapses are specialized junctions through which the cells of the nervous system signal to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow the neurons of the central nervous system to form interconnected neural circuits. They are thus crucial to the biological computations that underlie perception and thought. They provide the means through which the nervous system connects to and controls the other systems of the body.

In a chemical synapse, the process of synaptic transmission is as follows:

1. 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.
2. The neurotransmitters diffuse across the synaptic cleft and activate receptors on the postsynaptic neuron (the neuron receiving the signal).

Electric synapses

An electrical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. At gap junctions, cells approach within about 3.5 nm of each other, a much shorter distance than the 20 to 40 nm distance that separates cells at chemical synapses. As opposed to chemical synapses, the postsynaptic potential in electrical synapses is not caused by the opening of ion channels by chemical transmitters, but by direct electrical coupling between both neurons. Electrical synapses are therefore faster and more reliable than chemical synapses.

While chemical synapses are generally more common, there are large number of electric synapses in many cold-blooded fishes, which suggests that electrical synapses may be an adaptation to low temps, as the lowered rate of cellular metabolism in the cold reduces the rate of impulse transmission across chemical synapses.

The action potential

Need something on threshold potential: a certain voltage to which the membrane at the axon hillock is depolarized; generating action potential is an all-or-nothing endeavor

Each neuron averages all the electric disturbances on its membrane and makes a decision whether to trigger an action potential and conduct it down the axon —the frequency w/ which aps are generated in a particular neuron is the important parameter of its ability to signal other cells

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. 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. MS characterized by patchy loss of myelin in areas of the brain and spinal cord

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.

Myelination increases rate of impulse conduction Myelin, a stack of specialized plasma membrane sheets produced by a glial cell that wraps itself around the axon

The interconnectivity of neurons

The neuron's role 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. Cajal proposed that neurons were discrete cells that 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. 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.

Cajal used a histological staining technique developed by his contemporary Camillo Golgi. Golgi found that by treating brain tissue with a silver chromate solution, a relatively small number of neurons in the brain were darkly stained. This allowed Golgi to resolve in detail the structure of individual neurons and led him to conclude that nervous tissue was a continuous reticulum (or web) of interconnected cells much like those in the circulatory system.

Using Golgi's method, Ramón y Cajal reached a very different conclusion. He postulated that the nervous system is made up of billions of separate neurons and that these cells are polarized. Rather than forming a continuous web, Cajal suggested that neurons communicate with each other via specialized junctions called "synapses", a term that was coined by Sherrington in 1897. This hypothesis became the basis of the neuron doctrine, which states that the individual unit of the nervous system is a single neuron. Electron microscopy later showed that a plasma membrane completely enclosed each neuron, supporting Cajal's theory, and weakening Golgi's reticular theory.

However, with the discovery of electrical synapses (gap junctions: direct junctions between nerve cells), some have argued that Golgi was at least partially correct. For this work Ramón y Cajal and Golgi shared the Nobel Prize in Physiology or Medicine in 1906.

he 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); that neurons are genetically and metabolically distinct units; that they have cell bodies, axons, and dendrites; and that neural transmission goes only in one direction, from dendrites toward axons.

Before the neuron doctrine was accepted, it was widely believed that the nervous system was a reticulum, or a connected meshwork, rather than a system made up of discrete cells. This theory, the reticular theory, held that neurons' somata mainly provided nourishment for the system. Even after the cell theory was postulated in the 1830s, most scientists did not believe the theory applied to the brain or nerves.

The neuron doctrine is a central tenet of modern neuroscience, but recent studies suggest that this doctrine may need 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.

While the neuron doctrine has remained a central tenet of modern neuroscience, recent studies challenging this view have suggested that the narrow confines of this doctrine need to be expanded. Among the most serious challenges to the neuron doctrine is the fact that electrical synapses are more common in the central nervous system than previously thought. This means that rather than functioning as individual units, in some parts of the brain large ensembles of neurons may be active together in order to process neural information. A second challenge comes from the fact that dendrites, like axons, also have voltage gated ion channels and can generate electrical potentials which convey 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. Finally, the role of glia in processing neural information has begun to be appreciated more. Neurons and glia make up the two chief cell types of the central nervous system. There are far more glial cells than neurons, it has been estimated that glial cells outnumber neurons by as many as 50:1. Recent experimental results have suggested that glial cells play a vital role in information processing among neurons, indicating that neurons may not be the sole information processing cells in the nervous system.

Classes of neurons

Image of pyramidal neurons in mouse cerebral cortex expressing green fluorescent protein. The red staining indicates GABAergic interneurons. Source PLoS Biology [1]

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Structural classification

Most neurons can be anatomically characterized as:

• Unipolar or Pseudounipolar: dendrite and axon emerging from same process.
• Bipolar: single axon and single dendrite on opposite ends of the soma.
• Multipolar: more than two dendrites
  • Golgi I: neurons with long-projecting axonal processes.
  • Golgi II: neurons whose axonal process projects locally.

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 neurons convey information from tissues and organs into the central nervous system.
• Efferent neurons transmit signals from the central nervous system to the effector cells and are sometimes called motor neurons.
• Interneurons connect neurons within specific regions of the central nervous system.

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

• Excitatory neurons evoke excitation of their target neurons. Excitatory neurons in the brain are often glutamatergic. Spinal motoneurons use acetylcholine as their neurotransmitter.
• Inhibitory neurons evoke inhibition of their target neurons. Inhibitory neurons are often interneurons. The output of some brain structures (neostriatum, globus pallidus, cerebellum) are inhibitory. The primary inhibitory neurotransmitters are GABA and glycine.
• Modulatory neurons evoke more complex effects termed neuromodulation. These neurons use such neurotransmitters as dopamine, acetylcholine, serotonin and others.

Classification by discharge patterns
Neurons can be classified according to their electrophysiological characteristics:

• Tonic or regular spiking. Some neurons are typically constantly (or tonically) active. Example: interneurons in neurostriatum.
• Phasic or bursting. Neurons that fire in bursts are called phasic.
• Fast spiking. Some neurons are notable for their fast firing rates, for example some types of cortical inhibitory interneurons, cells in globus pallidus.
• Thin-spike. Action potentials of some neurons are more narrow compared to the others. For example, interneurons in prefrontal cortex are thin-spike neurons.

Classification by neurotransmitter released

Some examples are cholinergic, GABA-ergic, glutamatergic and dopaminergic neurons.

References

ISBN links support NWE through referral fees

• Bullock, T.H., Bennett, M.V.L., Johnston, D., Josephson, R., Marder, E., and R.D. Fields. 2005. The Neuron Doctrine, Redux, Science, 310:791-793.
• Kandel E.R., Schwartz, J.H., and T.M. Jessell. 2000. Principles of Neural Science, 4th edition. New York, NY: McGraw-Hill.
• Lodish, H., Baltimore, D., Berk, A., Zipursky, S.L., Matsudaira, P., and J. Darnell. 1995. Molecular Cell Biology, 3rd edition. New York, NY: Scientific American Books.
• Peters, A., Palay, S.L., and H.D. Webster. 1991. The Fine Structure of the Nervous System, 3rd edition. New York, NY: Oxford University Press.
• Ramón y Cajal, S. 1933. Histology, 10th edition. Baltimore, MD: Wood.
• Roberts A. and B.M.H. Bush. 1981. Neurones Without Impulses. New York: Cambridge University Press.

External links

• Cell Centered Database UC San Diego images of neurons.
• High Resolution Neuroanatomical Images of Primate and Non-Primate Brains.

Histology: nervous tissue

Neurons (gray matter): soma, axon (axon hillock, axoplasm, axolemma, neurofibril/neurofilament), dendrite (Nissl body, dendritic spine)
types (bipolar, pseudounipolar, multipolar, pyramidal, Purkinje, granule)

Afferent nerve/Sensory nerve/Sensory neuron (GSA, GVA, Type Ia sensory fiber), Efferent nerve/Motor nerve/Motor neuron (GSE, GVE, Alpha motor neuron, Gamma motoneurons, Upper motor neuron, Lower motor neuron), Interneuron (Renshaw)

Synapses: neuropil, boutons, synaptic vesicle, neuromuscular junction, electrical synapse

Sensory receptors: Free nerve ending, Meissner's corpuscle, Merkel nerve ending, Muscle spindle, Pacinian corpuscle, Ruffini ending, Olfactory receptor neuron, Photoreceptor, Hair cell, Taste bud

Glial cells: astrocyte, ependymal cells, microglia, radial glia

Myelination (white matter): Schwann cell, oligodendrocyte, nodes of Ranvier, internode, Schmidt-Lanterman incisures, neurolemma

closely related Connective tissue: epineurium, perineurium, endoneurium, nerve fascicle, meninges

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