Neuron
Neurons are a major class of cells in the nervous system. Neurons are sometimes called nerve cells, though this term is technically imprecise since many neurons do not form nerves. In vertebrates, they are found in the brain, the spinal cord and in the nerves and ganglia of the peripheral nervous system, and their primary role is to process and transmit neural information. One important characteristic of neurons is that they have excitable membranes which allow them to generate and propagate electrical signals.
The concept of a neuron as the primary computational unit of the nervous system was devised by 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, it is important to note that Cajal would not have been able to observe the structure of individulal neurons and their processes, and in turn devise the Neuron Doctrine, if his rival, Camillo Golgi, (for whom the Golgi Apparatus is named after) had not developed his highly specific 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 highly-specialized types of neurons exist, and these differ widely in appearance. Neurons have cellular extensions known as processes which they use to send and receive information. Neurons are highly asymmetric in shape, and consist of:
- The soma, or cell-body, is the central part of the cell between the dendrites and the axon. It is where the nucleus is located and is where most protein synthesis occurs.
- The dendrite, a branching arbor of cellular extensions. Most neurons have multiple dendrites with profuse dendritic branches. The overall shape and structure of a neuron's dendrites is called its dendritic tree. The dendritic tree form has traditionally been 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, 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 that carries nerve signals away from the neuron (and can carry in the other direction also). 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-excitable 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 is to assign strictly defined and dedicated functions to its various anatomical components, the fact that dendrites and axons very often act contrary to their so-called main function is but one small glimpse into the complex integrative capacity of every nerve cell. Nervous systems bear little resemblance to simple feed-forward Input/Output circuits, and this understanding begins by appreciating the global signaling capacity of individual neurons.
Axons and dendrites in the central nervous system are typically only about a micrometer thick, while some of those in the peripheral nervous system are much thicker. The soma is usually about 10–25 micrometers in diameter and 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 currently know about axonal function comes from studying the squid giant axon, an ideal experimental preparation for research due to its relatively immense size (0.5–1 millimeters thick, several centimeters long).
Classes
Functional classification There are three functional classes of neurons: afferent neurons, efferent neurons, and interneurons.
- 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.
- 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 into one of three categories:
- 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
Neurons communicate with one another and to other cells through synapses. Synapses are specialized junctions through which cells of the nervous system signal to one another and to non-neuronal cells such as muscles or glands. A synapse between a motor neuron and a muscle cell is called a neuromuscular junction.
Neurons such as the Purkinje cells in the cerebellum, can have over 1000 dendrites each, enabling connections with tens of thousands of other cells. Synapses can either be excitatory or inhibitory and will either respectively increase or decrease activity in the target neuron. Neurons can also communicate via electrical synapses, which are direct, electrically-conductive junctions between cells.
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 also provide the means through which the nervous system connects to and controls the other systems of the body.
The human brain has a gigantic number of synapses. Each of 100 billion neurons has on average 7,000 synaptic connections to other neurons. Most authorities estimate total number of synapses at 1,000 trillion for a three-year-old child. This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 100 to 500 trillion synapses. [1]
The word "synapse" comes from "synaptein" which Sir Charles Scott Sherrington and his colleagues coined from the Greek "syn-" meaning "together" and "haptein" meaning "to clasp".
Anatomy
At a prototypical synapse, such as those found at dendritic spines, a mushroom-shaped bud projects from each of two cells and the caps of these buds press flat against one another. At this interface, the membranes of the two cells flank each other across a slender gap, the narrowness of which enables signalling molecules known as neurotransmitters to pass rapidly from one cell to the other by diffusion. This gap, which is about 20 nm wide, is known as the synaptic cleft.
Such synapses are asymmetric both in structure and in how they operate. Only the so-called pre-synaptic neuron secretes the neurotransmitter, which binds to receptors facing into the synapse from the post-synaptic cell. The pre-synaptic nerve terminal (also called the synaptic button or bouton) generally buds from the tip of an axon, while the post-synaptic target surface typically appears on a dendrite, a cell body, or another part of a cell. The parts of synapses where neurotransmitter is released are called the active zones. At active zones the membranes of the two adjacent cells are held in close contact by cell adhesion proteins. Immediately behind the post-synaptic membrane is an elaborate complex of interlinked proteins called the postsynaptic density. Proteins in the postsynaptic density serve a myriad of roles, from anchoring and trafficking neurotransmitter receptors into the plasma membrane, to anchoring various proteins which modulate the activity of the receptors. The postsynaptic cell need not be a neuron. Postsynaptic cells can also be gland or muscle cells.
There also exists a less elaborate form of junction called an electrical synapse, in which neurons are electrically coupled to each other via protein complexes called gap junctions.
Signaling across chemical synapses
The release of neurotransmitter is triggered by the arrival of a nerve impulse (or action potential) and occurs through an unusually rapid process of cellular secretion: Within the pre-synaptic nerve terminal, vesicles containing neurotransmitter sit "docked" and ready at the synaptic membrane. The arriving action potential produces an influx of calcium ions through voltage-dependent, calcium-selective ion channels. Calcium ions then trigger a biochemical cascade which results in vesicles fusing with the presynaptic-membrane and release their contents to the synaptic cleft. Receptors on the opposite side of the synaptic gap bind neurotransmitter molecules and respond by opening nearby ion channels in the post-synaptic cell membrane, causing ions to rush in or out and changing the local transmembrane potential of the cell. The resulting change in voltage is called a postsynaptic potential. The result is excitatory, in the case of depolarizing currents, or inhibitory in the case of hyperpolarizing currents. Whether a synapse is excitatory or inhibitory depends on what type(s) of ion channel conduct the post-synaptic current, which in turn is a function of the type of receptors and neurotransmitter employed at the synapse.
Integration of synaptic inputs
Generally, if an excitatory synapse is strong, an action potential in the pre-synaptic neuron will trigger another in the post-synaptic cell; whereas at a weak synapse the excitatory post-synaptic potential ("EPSP") will not reach the threshold for action potential initiation. In the brain, however, each neuron typically connects or synapses to many others, and likewise each receives synaptic inputs from many others. When action potentials fire simultaneously in several neurons that weakly synapse on a single cell, they may initiate an impulse in that cell even though the synapses are weak. On the other hand, a pre-synaptic neuron releasing an inhibitory neurotransmitter such as GABA can cause inhibitory postsynaptic potential in the post-synaptic neuron, decreasing its excitability and therefore decreasing the neuron's likelihood to fire an action potential. In this way the output of a neuron may depend on the input of many others, each of which may have a different degree of influence, depending on the strength of its synapse with that neuron. John Carew Eccles performed some of the important early experiments on synaptic integration, for which he received the Nobel Prize for Physiology or Medicine in 1963. Complex input/output relationships form the basis of transistor-based computations in computers, and are thought to figure similarly in neural circuits.
Detailed properties and regulation
Following fusion of the synaptic vesicles and release of transmitter molecules into the synaptic cleft, the neurotransmitter is rapidly cleared from the space for recycling by specialized membrane proteins in the pre-synaptic or post-synaptic membrane. This "re-uptake" prevents "desensitization" of the post-synaptic receptors and ensures that succeeding action potentials will elicit the same size EPSP. The necessity of re-uptake and the phenomenon of desensitization in receptors and ion channels means that the strength of a synapse may in effect diminish as a train of action potentials arrive in rapid succession—a phenomenon that gives rise to the so-called frequency dependence of synapses. The nervous system exploits this property for computational purposes, and can tune its synapses through such means as phosphorylation of the proteins involved. The size, number and replenishment rate of vesicles also are subject to regulation, as are many other elements of synaptic transmission. For example, a class of drugs known as selective serotonin re-uptake inhibitors or SSRIs affect certain synapses by inhibiting the re-uptake of the neurotransmitter serotonin. In contrast, one important excitatory neurotransmitter, acetylcholine, does not undergo re-uptake, but instead is removed from the synapse by the action of the enzyme acetylcholinesterase.
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 known as 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 nerve cells [2]. Since they are much larger than human neurons, but similar in nature, it was easier to study them with less advanced technology at 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 potential difference in voltage all can initiate nerve activity [3].
The narrow cross-section of axons lessens the metabolic expense of carrying action potentials, however thicker axons convey the impulses more rapidly. In order to minimize metabolic expense yet maintain a rapid conduction velocity, 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 the action potentials to travel faster than in unmyelinated axons of the same diameter whilst simultaneously spending less energy to "recharge" the action potential after. 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 which 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 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 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).
Challenges to the neuron doctrine
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.
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: It has been estimated that glial cells outnumber neurons by as many as 10:1. 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.
Recent research has challenged the view that neurogenesis, or the generation of new neurons, does not occur in adult primate brains. This research has shown that neurogenesis can be environment-dependent in addition to being age-related and is halted by survival-type stress factors. [4] [5]
Neurons in the brain
The number of neurons contained within the brain varies dramatically across species. For example the human brain has about 100 billion () neurons and 100 trillion () connections (synapses) between them. In 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, ranging from the type of neurotransmitter used to ion channel composition are maintained across species, allowing scientists to study processes occurring in more complex organisms in much simpler experimental systems.
ReferencesISBN links support NWE through referral fees
- 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.
- M.F. Bear, B.W. Connors, and M.A. Paradiso. 2001. Neuroscience: Exploring the Brain. Baltimore: Lippincott. ISBN 0781739446
- Kandel ER, Schwartz JH, Jessell TM. Principles of Neural Science, 4th ed. McGraw-Hill, New York (2000). ISBN 0838577016
- J.G. Nicholls, A.R. Martin, B.G. Wallace and P.A. Fuchs. "From Neuron to Brain". 4th ed. Sinauer Associates, Sunderland, MA. ISBN 0878924391
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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