Nervous tissue is one of four major classes of vertebrate

Vertebrate

Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 tissue

Tissue (biology)

Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

.

Nervous tissue is the main component of the nervous system - the brain, spinal cord, and nerves-which regulates and controls body functions. It is composed of neuron

Neuron

A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

s, which transmit impulses, and the neuroglia cells, which assist propagation of the nerve

Nerve

A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...

 impulse as well as provide nutrients to the neuron.

Nervous tissue is made of nerve cells that come in many varieties, all of which are distinctly characteristic by the axon or long stem like part of the cell that sends action potential

Action potential

In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

 signals to the next cell.

Functions of the nervous system are sensory input, integration, controls of muscles and glands, homeostasis, and mental activity.

All living cells have the ability to react to stimuli. Nervous tissue is specialized to react to stimuli and to conduct impulses to various organs in the body which bring about a response to the stimulus. Nerve tissue (as in the brain, spinal cord and peripheral nerves that branch throughout the body) are all made up of specialized nerve cells called neurons. Neurons are easily stimulated and transmit impulses very rapidly. A nerve is made up of many nerve cell fibers (neurons) bound together by connective tissue. A sheath of dense connective tissue, the epineurium

Epineurium

The epineurium is the outermost layer of connective tissue surrounding a peripheral nerve. It is made of dense irregular connective tissue and usually contains multiple nerve fascicles as well as blood vessels which supply the nerve...

surrounds the nerve. This sheath penetrates the nerve to form the perineurium

Perineurium

In the peripheral nervous system, nerve fibers are each wrapped in a protective sheath known as the endoneurium. These are bundled together into groups known as fascicles, each surrounded by a protective sheath known as the perineurium. Several fascicles may be in turn bundled together with a blood...

which surrounds bundles of nerve fibers. Blood vessels of various sizes can be seen in the epineurium. The endoneurium, which consists of a thin layer of loose connective tissue, surrounds the individual nerve fibers.

The cell body is enclosed by a cell (plasma) membrane and has a central nucleus. Granules called Nissl bodies are found in the cytoplasm of the cell body. Within the cell body, extremely fine neurofibrils extend from the dendrites into the axon

Axon

An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

. The axon is surrounded by the myelin sheath, which forms a whitish, non-cellular, fatty layer around the axon. Outside the myelin sheath is a cellular layer called the neurilemma or sheath of Schwann cell

Schwann cell

Schwann cells or neurolemmocytes are the principal glia of the peripheral nervous system . Glial cells function to support neurons and in the PNS, also include satellite cells, olfactory ensheathing cells, enteric glia and glia that reside at sensory nerve endings, such as the Pacinian corpuscle...

s. The myelin sheath together with the neurilemma is also known as the medullary sheath. This medullary sheath is interrupted at intervals by the nodes of Ranvier

Nodes of Ranvier

Myelin sheath gaps or nodes of Ranvier are the gaps formed between the myelin sheaths generated by different cells. A myelin sheath is a many-layered coating, largely composed of a fatty substance called myelin, that wraps around the axon of a neuron and very efficiently insulates it...

.

Neuronal Communication

Nerve cells are functionally made to each other at a junction known as a synapse

Synapse

In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

, where the terminal branches of an axon and the dendrites of another neuron lie close to each other but normally without direct contact. Information is transmitted across the gap by chemical secretions called neurotransmitter

Neurotransmitter

Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...

s. It causes activation in the post-synaptic cell. All cells possess the ability to respond to stimuli.
The messages carried by the nervous system are electrical signals called impulses.

Classification of Neurons

Neurons are classified both structurally and functionally.

Structural Classification Neurons are grouped structurally according to the number of processes extending from their cell body. Three major neuron groups make up this classification: multipolar (polar = end, pole), bipolar and unipolar neurons.

Multipolar Neurons (3+ processes) : These are the most common neuron type in humans (more than 99% of neurons belong to this class) and the major neuron type in the CNS
Bipolar Neurons: Bipolar neurons are spindle-shaped, with a dendrite at one end and an axon at the other. An example can be found in the light-sensitive retina of the eye.
Unipolar Neurons: Sensory neurons have only a single process or fibre which divides close to the cell body into two main branches (axon and dendrite). Because of their structure they are often referred to as unipolar neurons.

Cancer

Tumor

Tumor

A tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...

s in nervous tissue include:

• Glioma
  Glioma
  A glioma is a type of tumor that starts in the brain or spine. It is called a glioma because it arises from glial cells. The most common site of gliomas is the brain.-By type of cell:...
  
  s (glial cell
  Glial cell
  Glial cells, sometimes called neuroglia or simply glia , are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the brain, and for neurons in other parts of the nervous system such as in the autonomous nervous system...
  
   tumors)

Gliomatosis cerebri

Gliomatosis cerebri

Gliomatosis cerebri is a rare primary brain tumor. It is commonly characterized by diffuse infiltration of the brain with neoplastic glial cells that affect various areas of the cerebral lobes...

, Oligoastrocytoma

Oligoastrocytoma

Oligoastrocytomas are a subset of brain tumors that present with an appearance of mixed glial cell origin, astrocytoma and oligodendroglioma. These types of glial cells that become cancerous are involved with insulating and regulating the activity of neuron cells in the central nervous system...

, Choroid plexus papilloma

Choroid plexus papilloma

A Choroid plexus papilloma is a rare, slow-growing, histologically benign intracranial neoplasm or tumor that is commonly located in the ventricular system of the choroid plexus. It may obstruct the cerebrospinal fluid flow, causing increased intracranial pressure.-Pathophysiology:The tumor is...

, Ependymoma

Ependymoma

Ependymoma is a tumor that arises from the ependyma, a tissue of the central nervous system. Usually, in pediatric cases the location is intracranial, while in adults it is spinal. The common location of intracranial ependymoma is the fourth ventricle...

, Astrocytoma

Astrocytoma

Astrocytomas are a type of neoplasm of the brain. They originate in a particular kind of glial-cells, star-shaped brain cells in the cerebrum called astrocytes. This type of tumor does not usually spread outside the brain and spinal cord and it does not usually affect other organs...

 (Pilocytic astrocytoma

Pilocytic astrocytoma

Pilocytic astrocytoma or juvenile pilocytic astrocytoma or cystic cerebellar astrocytoma is a neoplasm of the brain that occurs more often in children and young adults...

, Glioblastoma multiforme

Glioblastoma multiforme

Glioblastoma multiforme is the most common and most aggressive malignant primary brain tumor in humans, involving glial cells and accounting for 52% of all functional tissue brain tumor cases and 20% of all intracranial tumors. Despite being the most prevalent form of primary brain tumor, GBMs...

), Dysembryoplastic neuroepithelial tumour

Dysembryoplastic neuroepithelial tumour

Dysembryoplastic neuroepithelial tumour, commonly abbreviated DNT or DNET, is a type of brain tumour.It appears similar to oligodendroglioma, but with visible neurons....

, Oligodendroglioma

Oligodendroglioma

Oligodendrogliomas are a type of glioma that are believed to originate from the oligodendrocytes of the brain or from a glial precursor cell. They occur primarily in adults but are also found in children...

, Medulloblastoma

Medulloblastoma

Medulloblastoma is a highly malignant primary brain tumor that originates in the cerebellum or posterior fossa.Previously, medulloblastomas were thought to represent a subset of primitive neuroectodermal tumor of the posterior fossa...

, Primitive neuroectodermal tumor

Primitive neuroectodermal tumor

Primitive neuroectodermal tumor is a neural crest tumor. It is a rare tumor, usually occurring in children and young adults under 25 years of age...

• Neuroepitheliomatous tumors

Ganglioneuroma

Ganglioneuroma

Ganglioneuroma is a tumor of the sympathetic nerve fibers arising from neural crest cells.For example, it can be found also in the eye , or in the medulla of adrenal glands. Some may be a progressive form of a neuroblastoma....

, Neuroblastoma

Neuroblastoma

Neuroblastoma is the most common extracranial solid cancer in childhood and the most common cancer in infancy, with an annual incidence of about 650 cases per year in the US , and 100 cases per year in the UK . Close to 50 percent of neuroblastoma cases occur in children younger than two years old...

, Atypical teratoid rhabdoid tumor

Atypical teratoid rhabdoid tumor

Atypical teratoid rhabdoid tumor is a rare tumor usually diagnosed in childhood. Although usually a brain tumor, AT/RT can occur anywhere in the central nervous system including the spinal cord. About 60% will be in the posterior cranial fossa...

, Retinoblastoma

Retinoblastoma

Retinoblastoma is a rapidly developing cancer that develops in the cells of retina, the light-detecting tissue of the eye. In the developed world, Rb has one of the best cure rates of all childhood cancers , with more than nine out of every ten sufferers surviving into...

, Esthesioneuroblastoma

Esthesioneuroblastoma

Esthesioneuroblastoma is a rare form of cancer involving nasal cavity and believed to arise from the olfactory epithelium. It can cause loss of vision, sight and taste....

• Nerve sheath tumor
  Nerve sheath tumor
  A nerve sheath tumor is a type of tumor of the nervous system which is made up primarily of the myelin surrounding nerves.A peripheral nerve sheath tumor is a nerve sheath tumor in the peripheral nervous system....
  
  s

Neurofibroma

Neurofibroma

A neurofibroma is a benign nerve sheath tumor in the peripheral nervous system. Usually found in individuals with neurofibromatosis type I , an autosomal dominant genetically-inherited disease, they can result in a range of symptoms from physical disfiguration and pain to cognitive disability...

 (Neurofibrosarcoma, Neurofibromatosis

Neurofibromatosis

Neurofibromatosis is a genetically-inherited disorder in which the nerve tissue grows tumors that may be benign or may cause serious damage by compressing nerves and other tissues...

), Schwannoma

Schwannoma

A schwannoma is a benign nerve sheath tumor composed of Schwann cells, which normally produce the insulating myelin sheath covering peripheral nerves....

, Neurinoma, Acoustic neuroma

Acoustic neuroma

A vestibular schwannoma, often called an acoustic neuroma, is a benign primary intracranial tumor of the myelin-forming cells of the vestibulocochlear nerve . The term "vestibular schwannoma" involves the vestibular portion of the 8th cranial nerve and arises from Schwann cells, which are...

, Neuroma

Neuroma

A neuroma is a growth or tumor of nerve tissue. Just as the Latin word for swelling is now restricted to neoplasias, the equivalent Greek suffix -oma has shared in that fate. Thus, the typical modern usage of neuroma is for nerve tumors...

See also

• Epineurium
  Epineurium
  The epineurium is the outermost layer of connective tissue surrounding a peripheral nerve. It is made of dense irregular connective tissue and usually contains multiple nerve fascicles as well as blood vessels which supply the nerve...
  
  
• Perineurium
  Perineurium
  In the peripheral nervous system, nerve fibers are each wrapped in a protective sheath known as the endoneurium. These are bundled together into groups known as fascicles, each surrounded by a protective sheath known as the perineurium. Several fascicles may be in turn bundled together with a blood...
  
  
• Endoneurium
  Endoneurium
  The endoneurium, also referred to as an endoneurial channel, sheath or tube, is a layer of delicate connective tissue made up of endoneurial cells that encloses the myelin sheath of a spinal cord nerve fiber. These are bundled up into groups called nerve fascicles, which have a protective sheath...
  
  
• Nerve fascicle
  Nerve fascicle
  A small bundle of nerve fibers, enclosed by the perineurium, is called a funiculus; if the nerve is of small size, it may consist only of a single funiculus; but if large, the funiculi are collected together into larger bundles or nerve fascicles, which are bound together in a common membranous...
  
  
• Connective tissue in the peripheral nervous system
  Connective tissue in the peripheral nervous system
  A peripheral nerve contains two types of tissue: nerve fibers, and connective tissue. Dendrites and axons with schwann cells and myelin sheath are surrounded by connective tissue. A nerve fiber in the peripheral nervous system consists of an axon or long dendrite, myelin sheath and their schwann...