Nervous tissue

Nervous tissue,also calledneural tissue,is the maintissuecomponent of thenervous system.The nervous system regulates and controls body functions and activity. It consists of two parts: thecentral nervous system(CNS) comprising thebrainandspinal cord,and theperipheral nervous system(PNS) comprising the branching peripheralnerves.It is composed ofneurons,also known as nerve cells, which receive and transmit impulses to and from it, andneuroglia,also known as glial cells or glia, which assist the propagation of thenerve impulseas well as providenutrientsto the neurons.

Nervous tissue
Cells of nervous tissue
Identifiers
MeSH	D009417
Anatomical terminology

Nervous tissue is made up of different types of neurons, all of which have anaxon.An axon is the long stem-like part of thecellthat sendsaction potentialsto the next cell.Bundles of axonsmake up thenervesin the PNS andtractsin the CNS.

Functions of the nervous system aresensory input,integration, control ofmusclesandglands,homeostasis,andmental activity.

Structure

See also:Brain cell

Histologic example of nervous tissue

Nervous tissue is composed ofneurons,also called nerve cells, andneuroglial cells.Four types of neuroglia found in the CNS areastrocytes,microglial cells,ependymal cells,andoligodendrocytes.Two types of neuroglia found in the PNS aresatellite glial cellsandSchwann cells.In thecentral nervous system(CNS), the tissue types found aregrey matterandwhite matter.The tissue is categorized by its neuronal and neuroglial components.

Components

Neurons are cells with specialized features that allow them to receive and facilitate nerve impulses, oraction potentials,across their membrane to the next neuron.They possess a large cell body (soma), with cell projections calleddendritesand anaxon.Dendrites are thin, branching projections that receiveelectrochemicalsignaling (neurotransmitters) to create a change in voltage in the cell. Axons are long projections that carry the action potential away from the cell body toward the next neuron. The bulb-like end of the axon, called theaxon terminal,is separated from the dendrite of the following neuron by a small gap called asynaptic cleft.When the action potential travels to the axon terminal, neurotransmitters are released across the synapse and bind to thepost-synaptic receptors,continuing the nerve impulse.

Neurons are classified both functionally and structurally.

Functional classification:

• Sensory neurons(afferent): Relay sensory information in the form of anaction potential(nerve impulse) from the PNS to the CNS
• Motor neurons(efferent): Relay an action potential out of the CNS to the proper effector (muscles, glands)
• Interneurons:Cells that form connections between neurons and whose processes are limited to a single local area in the brain or spinal cord

Structural classification:

• Multipolar neurons:Have 3 or more processes coming off thesoma(cell body). They are the major neuron type in the CNS and include interneurons and motor neurons.
• Bipolar neurons:Sensory neurons that have two processes coming off the soma, one dendrite and one axon
• Pseudounipolar neurons:Sensory neurons that have one process that splits into two branches, forming the axon and dendrite
• Unipolar brush cells:Areexcitatoryglutamatergicinterneuronsthat have a single short dendrite terminating in a brush-like tuft of dendrioles. These are found in the granular layer of thecerebellum.

Neuroglia encompasses the non-neural cells in nervous tissue that provide various crucial supportive functions for neurons. They are smaller than neurons, and vary in structure according to their function.

Neuroglial cells are classified as follows:

• Microglial cells:Microglia aremacrophagecells that make up the primary immune system for the CNS.They are the smallest neuroglial cell.
• Astrocytes:Star-shapedmacroglialcells with many processes found in the CNS. They are the most abundant cell type in the brain, and are intrinsic to a healthy CNS.
• Oligodendrocytes:CNS cells with very few processes. They formmyelin sheathson the axons of a neuron, which are lipid-based insulation that increases the speed at which the action potential, can travel down the axon.
• NG2 glia:CNS cells that are distinct from astrocytes, oligodendrocytes, and microglia. They serve as the developmental precursors of oligodendrocytes.
• Schwann cells:The PNS equivalent of oligodendrocytes, they help maintain axons and form myelin sheaths in the PNS.
• Satellite glial cell:Line the surface of neuron cell bodies inganglia(groups of nerve body cells bundled or connected together in the PNS)
• Enteric glia: Found in theenteric nervous system,within the gastrointestinal tract.

Classification of tissue

In thecentral nervous system:

• Grey matteris composed of cell bodies, dendrites, unmyelinated axons, protoplasmic astrocytes (astrocyte subtype), satellite oligodendrocytes (non-myelinating oligodendrocyte subtype), microglia, and very few myelinated axons.
• White matteris composed of myelinated axons, fibrous astrocytes, myelinating oligodendrocytes, and microglia.

In theperipheral nervous system:

• Gangliontissue is composed of cell bodies, dendrites, and satellite glial cells.
• Nervesare composed of myelinated and unmyelinated axons, Schwann cells surrounded byconnective tissue.

The three layers of connective tissue surrounding each nerve are:

• Endoneurium.Each nerve axon, or fiber is surrounded by theendoneurium,which is also called the endoneurial tube, channel or sheath. This is a thin, delicate, protective layer of connective tissue.
• Perineurium.Eachnerve fasciclecontaining one or more axons, is enclosed by the perineurium, a connective tissue having a lamellar arrangement in seven or eight concentric layers. This plays a very important role in the protection and support of the nerve fibers and also serves to prevent the passage of large molecules from the epineurium into a fascicle.
• Epineurium.The epineurium is the outermost layer of dense connective tissue enclosing the (peripheral) nerve.

Function

Myelinated axons (right) conduct impulses faster than unmyelinated axons.

The function of nervous tissue is to form the communication network of thenervous systemby conducting electric signals across tissue.In the CNS, grey matter, which contains thesynapses,is important for information processing. White matter, containing myelinated axons, connects and facilitates nerve impulse between grey matter areas in the CNS. In the PNS, the ganglion tissue, containing the cell bodies and dendrites, contain relay points fornerve tissueimpulses. The nerve tissue, containing myelinated axons bundles, carry action potential nerve impulses.

Clinical significance

Tumours

Neoplasms(tumours) in nervous tissue include:

• Gliomas(glial celltumors)

Oligoastrocytoma,Choroid plexus papilloma,Ependymoma,Astrocytoma(Pilocytic astrocytoma,Glioblastoma multiforme),Dysembryoplastic neuroepithelial tumour,Oligodendroglioma,Medulloblastoma,Primitive neuroectodermal tumor

• Neuroepitheliomatoustumors

Ganglioneuroma,Neuroblastoma,Atypical teratoid rhabdoid tumor,Retinoblastoma,Esthesioneuroblastoma

• Nerve sheath tumors

Neurofibroma(Neurofibrosarcoma,Neurofibromatosis),Schwannoma,Neurinoma,Acoustic neuroma,Neuroma