Neuron

Neurons are a major class of cells in the nervous system. Neurons are sometimes called nerve cells, though this term is technically imprecise, as many neurons do not form nerves. In vertebrates, neurons are found in the brain, the spinal cord and in the nerves and ganglia of the peripheral nervous system. Their main role is to process and transmit information. Neurons have excitable membranes, which allow them to generate and propagate electrical impulses.

History

The concept of a neuron as the primary computational unit of the nervous system was devised by the Spanish anatomist Santiago Ramón y Cajal in the early 20th century. Cajal proposed that neurons were discrete cells which communicated with each other via specialized junctions. This became known as the Neuron Doctrine, one of the central tenets of modern neuroscience. However, Cajal would not have been able to observe the structure of individual neurons if his rival, Camillo Golgi, (for whom the Golgi Apparatus is named) had not developed his silver staining method. When the Golgi Stain is applied to neurons, it binds the cell's microtubules and gives stained cells a black outline when light is shone through them.

Anatomy and histology

Many neurons are highly specialized, and they differ widely in appearance. Neurons have cellular extensions known as processes which they use to send and receive information. Neurons are typically 4 to 100 micrometres in diameter, the size varies depending on the type of neuron and the species it is from. ^*

The soma, or 'cell body', is the central part of the cell, where the nucleus is located and where most protein synthesis occurs.

The dendrite is a branching arbor of cellular extensions. Most neurons have several dendrites with profuse dendritic branches. The overall shape and structure of a neuron's dendrites is called its dendritic tree, and is traditionally thought to be the main information receiving network for the neuron. However, information outflow (i.e. from dendrites to other neurons) can also occur.

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 carries nerve signals away from the soma (and carry some types of information in the other direction also). Many neurons have only one axon, but this axon may - and usually will - undergo extensive branching, enabling communication with many target cells. The part of the axon where it emerges from the soma is called the 'axon hillock'. Besides being an anatomical structure, the axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium channels. Thus it has the most hyperpolarized action potential threshold of any part of the neuron. In other words, it is the most easily-excited part of the neuron, and thus serves as the spike initiation zone for the axon. While the axon and axon hillock are generally considered places of information outflow, this region can receive input from other neurons as well.

The axon terminal a specialized structure at the end of the axon that is used to release neurotransmitter and communicate with target neurons.

Although the canonical view of the neuron attributes dedicated functions to its various anatomical components, dendrites and axons very often act contrary to their so-called main function.

Axons and dendrites in the central nervous system 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 motoneuron can be over a meter long, reaching from the base of the spine to the toes, while giraffes have single axons running along the whole length of their necks, several meters in length. Much of what we know about axonal function 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).

Classes

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.

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.

Connectivity

Main article: synapse

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 might be used to better understand neurons ^*. 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.

Histology and internal structure

Nerve cell bodies stained with basophilic dyes 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 amounts 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).

There are different internal structural characteristics between axons and dendrites. Axons typically almost never contain ribosomes, except some in the initial segment. Dendrites contain granular endoplasmic reticulum or ribosomes, with diminishing amounts with distance from the cell body.

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. [http://www.princeton.edu/pr/news/99/q4/1014-brain.htm

Neurons in the brain

The number of neurons in the brain varies dramatically from species to species. The human brain has about 100 billion (

10^{11}

) neurons and 100 trillion (

10^{14}

) synapses. By contrast, the nematode worm (Caenorhabditis elegans) has 302 neurons. Scientists have mapped all of the nematode's neurons. As a result, such worms are ideal candidates for neurobiological experiments and tests. 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.

Sources

Kandel E.R., Schwartz, J.H., Jessell, T.M. 2000. Principles of Neural Science, 4th ed., McGraw-Hill, New York.
Bullock, T.H., Bennett, M.V.L., Johnston, D., Josephson, R., Marder, E., Fields R.D. 2005. The Neuron Doctrine, Redux, Science, V.310, p. 791-793.
Ramón y Cajal, S. 1933 Histology, 10th ed., Wood, Baltimore.

Peters, A., Palay, S.L., Webster, H, D., 1991 The Fine Structure of the Nervous System, 3rd ed., Oxford, New York.

External links

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

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