Structure of a typical neuron

Neuron
Dendrite Soma Axon Nucleus Node of Ranvier Axon Terminal Schwann cell Myelin sheath

An axon or nerve fiber is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma. Dendrites (from Greek δένδρον déndron, “tree” are the branched projections of a Neuron that act to conduct the electrochemical The soma, or cyton or perikaryon, is the bulbous end of a Neuron, containing the Cell nucleus. In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed Nodes of Ranvier are known as the gaps (about 1 micrometer in diameter formed between myelin sheath cells along axons or nerve fibers Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands Named after the German physiologist Theodor Schwann, Schwann cells (also referred to as neurolemmocytes) are a variety of Glial cell that mainly Myelin is an electrically-insulating Dielectric Phospholipid layer that surrounds only the Axons of many Neurons It is an outgrowth Neurons (ˈnjuːɹɒn also known as neurones and nerve cells) are responsive cells in the Nervous system that process and transmit information In Neurophysiology, the action potential is a self-regenerating Wave of Electrochemical activity that allows Nerve cells to carry a signal The soma, or cyton or perikaryon, is the bulbous end of a Neuron, containing the Cell nucleus.

Anatomy

Axons are in effect the primary transmission lines of the nervous system, and as bundles they help make up nerves. The nervous system is a Network of specialized cells that communicate information about an animal's surroundings and itself A nerve is an enclosed cable-like bundle of peripheral Axons (the long slender projections of Neurons. Individual axons are microscopic in diameter (typically about 1μm across), but may be up to multiple feet long. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre The longest axons in the human body, for example, are those of the sciatic nerve, which run from the base of the spine to the big toe of each foot. The sciatic nerve (also known as the ischiatic nerve) is a large Nerve that starts in the lower back and runs through the buttock and down the Lower limb In Human anatomy, the vertebral column ( backbone or spine) is a column of 34 Vertebrae the Sacrum, Intervertebral These single-cell fibers of the sciatic nerve may extend a meter or even longer.

In vertebrates, only the axons of many neurons are sheathed in myelin, which is formed by either of two types of glial cells: Schwann cells ensheathing peripheral neurons and oligodendrocytes insulating those of the central nervous system. Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes Myelin is an electrically-insulating Dielectric Phospholipid layer that surrounds only the Axons of many Neurons It is an outgrowth Glial cells, commonly called neuroglia or simply glia (Greek for "glue" are non- Neuronal cells that provide support and nutrition Named after the German physiologist Theodor Schwann, Schwann cells (also referred to as neurolemmocytes) are a variety of Glial cell that mainly Origin Oligodendroglia arise during development from an Oligodendrocyte precursor cell, which can be identified by its expression of a number of Antigens, including In Vertebrates the central nervous system ( CNS) is the part of the Nervous system which is enclosed in the Meninges. Along myelinated nerve fibers, gaps in the sheath known as nodes of Ranvier occur at evenly-spaced intervals, enabling an especially rapid mode of electrical impulse propagation called saltation. Nodes of Ranvier are known as the gaps (about 1 micrometer in diameter formed between myelin sheath cells along axons or nerve fibers For Saltation definition and other use disambiguation see Saltation Saltatory conduction (from the Latin saltare, to hop or leap is The demyelination of axons is what causes the multitude of neurological symptoms found in the disease Multiple Sclerosis. Multiple sclerosis (abbreviated MS also known as disseminated sclerosis or encephalomyelitis disseminata) is an autoimmune condition in which the The axons of some neurons branch to form axon collaterals, that can be divided into a number of smaller branches called telodendria. Along these the bifurcated impulse travels simultaneously to signal more than one other cell.

Physiology

The physiology can be described by the Hodgkin-Huxley Model, extended to vertebrates in Frankenhaeuser-Huxley equations. Physiology (from Greek grc φύσις physis, "nature origin" and grc -λογία -logia) is the study of the mechanical physical The Hodgkin–Huxley model is a Scientific model that describes how Action potentials in Neurons are initiated and propagated

Types

Peripheral nerve fibers can be classified based on axonal conduction velocity, mylenation, fiber size etc. For example, there are slow-conducting unmyelinated C fibers and faster-conducting myelinated Aδ fibers. Structure and Anatomy Location C fibers are found in the peripheral nerves of the somatic sensory system. A delta fibers, or Aδ fibers are a Type of sensory fiber. They are associated with cold and pressure and as Nociceptors they convey fast Pain information More complex mathematical modeling continues to be done today.

There are several types of sensory- as well as motorfibers. Other fibers not mentioned in table are e. g. fibers of the autonomic nervous system

Motor

Lower motor neurons have two kind of fibers:

Sensory

Different sensory receptors are innervated by different types of nerve fibers. &trade The autonomic nervous system ( ANS) (or visceral nervous system) is the part of the Peripheral nervous system that acts as a Control Lower motor neurons ( LMNs) are the Motor neurons connecting the Brainstem and Spinal cord to Muscle fibers, bringing the nerve Skeletal muscle is a type of Striated muscle, which usually attaches to tendons Alpha motor neurons ( α-MNs) are large Lower motor neurons of the Brainstem and Spinal cord. Extrafusal muscle fibers are a class of Muscle fiber innervated by Alpha motor neurons They generate tension do mechanical work and allow for movement by contracting Gamma motoneurons ( γ-motoneurons) also called gamma motor neurons, are a component of the Fusimotor system, the system by which the Central Intrafusal fibers are Muscle fibers that comprise the Muscle spindle and are innervated by the gamma motor neurons ( Gamma motoneuron) In a Sensory system, a sensory receptor is a structure that recognizes a stimulus in the internal or external environment of an Organism. Muscles and associated sensory receptors are innvervated by type I and II sensory fibers, while cutaneous receptors are innervated by Aβ, Aδ and C fibers. A cutaneous receptor is a type of Sensory receptor found in the dermis or epidermis

Growth and development

Growing axons move through their environment via the growth cone, which is at the tip of the axon. A nociceptor is a Sensory receptor that sends signals that cause the perception of Pain in response to a potentially damaging stimulus Nociception (synonym nociperception is defined as "the neural processes of encoding and processing Noxious stimuli. A thermoreceptor is a Sensory receptor, or more accurately the receptive portion of a sensory neuron that codes absolute and relative changes in temperature primarily A growth cone is a dynamic Actin -supported extension of a developing Axon seeking its synaptic target The growth cone has a broad sheet like extension called lamellipodia which contain protrusions called filopodia. The lamellipodium (pl Lamellipodia) is a cytoskeletal Actin projection on the mobile edge of the The filopodia (also microspikes are slender cytoplasmic projections similar to Lamellipodia, which extend from the leading edge of migrating cells The filopodia are the mechanism by which the entire process adheres to surfaces and explores the surrounding environment. Actin plays a major role in the mobility of this system. Actin is a globular roughly 42-kDa Protein found in all eukaryotic cells (except for Nematode sperm where it may be present at concentrations of Environments with high levels of cell adhesion molecules or CAM's create an ideal environment for axonal growth. Cell Adhesion Molecules ( CAM s are Proteins located on the cell surface involved with the Binding with other cells or with the Extracellular This seems to provide a "sticky" surface for axons to grow along. Examples of CAM's specific to neural systems include N-CAM, neuroglial CAM or NgCAM, TAG-1, MAG, and DCC, all of which are part of the immunoglobulin superfamily. Neural Cell Adhesion Molecule (NCAM also the Cluster of differentiation CD56) is a homophilic binding glycoprotein expressed on the surface of Neurons, Dicyclohexylcarbodiimide ( DCC) is an Organic compound with chemical formula C13H22N2 whose primary use is to couple Amino Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily Another set of molecules called extracellular matrix adhesion molecules also provide a sticky substrate for axons to grow along. Examples of these molecules include laminin, fibronectin, tenascin, and perlecan. Laminin is the major non-collagenous component of the Basal lamina, such as those on which cells of an Epithelium sit Fibronectin is a high-molecular-weight extracellular matrix Glycoprotein containing about 5% Carbohydrate that binds to membrane spanning Receptor proteins Tenascins are Extracellular matrix Glycoproteins. They are abundant in the extracellular matrix of developing Vertebrate Embryos and they reappear Heparan sulfate proteoglycan 2 ( HSPG2) is a human Gene which encodes the perlecan Protein. Some of these are surface bound to cells and thus act as short range attractants or repellents. Others are difusible ligands and thus can have long range effects.

Cells called guidepost cells assist in the guidance of neuronal axon growth. These cells are typically other, sometimes immature, neurons.

History

Some of the first intracellular recordings in a nervous system were made in the late 1930's by K. Cole and H. Curtis. Alan Hodgkin and Andrew Huxley also employed the squid giant axon (1939) and by 1952 they had obtained a full quantitative description of the ionic basis of the action potential, leading the formulation of the Hodgkin-Huxley Model. Sir Alan Lloyd Hodgkin, OM, KBE, FRS (5 February 1914 Banbury, Oxfordshire, England – 20 December 1998 Cambridge Sir Andrew Fielding Huxley, OM, FRS (born 22 November 1917, Hampstead, London) is an English physiologist The squid giant axon is the very large (up to 1 mm in diameter typically around 0 In Neurophysiology, the action potential is a self-regenerating Wave of Electrochemical activity that allows Nerve cells to carry a signal The Hodgkin–Huxley model is a Scientific model that describes how Action potentials in Neurons are initiated and propagated Hodgkin and Huxley were awarded jointly the Nobel Prize for this work in 1963. The Nobel Prize in Physiology or Medicine (Nobelpriset i fysiologi eller medicin is awarded once a year by the Swedish Karolinska Institute. Year 1963 ( MCMLXIII) was a Common year starting on Tuesday (link will display full calendar of the Gregorian calendar. The formulas detailing axonal conductance were extended to vertebrates in the Frankenhaeuser-Huxley equations. Erlanger and Gasser later developed the classification system for peripheral nerve fibers, based on axonal conduction velocity, mylenation, fiber size etc. Even recently our understanding of the biochemical basis for action potential propagation has advanced, and now includes many details about individual ion channels. Ion channels are pore-forming Proteins that help establish and control the small Voltage Gradient across the Plasma membrane of all living

Concussion

Concussion is considered a mild form of diffuse axonal injury ^[3].

See also

• Neuron
• Dendrite
• Synapse
• Axon guidance
• Electrophysiology

References

External links

• Histology at OU 3_09 - "Slide 3 Spinal cord"