The Nervous System Vol. 3

2STRUCTURE OFNERVOUS TISSUEThe nervous system is composed primarily of nervous tissue. Nervous tissue isone of the four basic tissue types that we examined previously in Volume 1 ofWonders of the Human Body.Nervous tissue consists of two primary types of cells: neurons and neuroglia.Light micrograph showing the Golgi apparatusin neurons of dorsal root ganglion.

11STRUCTURE OF NERVOUS TISSUETissue TypesEpithelial TissueEpithelial tissue (or epithelium)lines body cavities or coverssurfaces. For example, the outerlayer of skin is epithelium. Thesheet of cells that line the stomachand intestines, as well as the cellsthat line the heart, blood vessels,and the lungs, is epithelial tissue.Connective TissueConnective tissue helps providea framework for the body. It alsohelps connect and support otherorgans in the body. Further, it helpsinsulate the body, and it even helpstransport substances throughoutthe body. This tissue can be hardor soft. Some connective tissuestretches. One type is even fluid.Connective tissue is comprisedof three parts: cells, fibers, andground substance.Nervous TissueNervous tissue is the primarycomponent of the nervous system.The nervous system regulates andcontrols bodily functions.Nerve cells are incredible. Theyare able to receive signals or inputfrom other cells, generate a nerveimpulse, and transmit a signal toother nerve cells or organs.Muscle TissueMuscle tissue is responsible formovement. There are three typesof muscle tissue: skeletal muscle,smooth muscle, and cardiacmuscle.Neurons are the excitable nerve cells thattransmit electrical signals.What starts such an electrical signal? Sometype of change in the environment acts asthe stimulus that excites a neuron, triggeringan electrical signal called an actionpotential. The electrical signal transmittedby a neuron is also called an impulse. Animpulse travels like a wave along the nervecell membrane from one end of the neuronto another. We will soon study this in depth.The other cells in nervous tissue arecalled neuroglia. There are several typesof neuroglia cells. They help protect andsupport the neurons.Let’s examine the neuron in greater detail.NeuronsThe neuron is often called a nerve cellbecause it is the cell type that does theprimary work of the nervous system. Youhave neurons in your brain, in your spinalcord, in your peripheral nervous system, andeven in specialized sensory organs like youreye, nose, and ear.A neuron doesn’t look like a typical cell.If you have seen sketches of “typical” cellsbefore, you will notice that, while the neuronstill has a cell membrane, cytoplasm, anda nucleus, it has an unusual shape. Theneuron is a very specialized type of cell thatis designed to transmit electrical impulses(nerve impulses) rapidly to various parts ofthe body.The neuron is composed of three parts: thecell body, dendrites, and the axon.

12The cell body contains the typical organelles wediscussed at length in Volume 1 of Wonders of theHuman Body. The cell body contains a nucleussurrounded by cytoplasm. The cytoplasm containsplenty of protein-building organelles like roughendoplasmic reticulum dotted with ribosomesand free ribosomes. An extensive Golgi apparatusprocesses the proteins made by these ribosomes.Neurons require a lot of energy to build thesubstances they require, so lots of energy-generatingmitochondria are also found in the cell body. Energyprovided by these mitochondria fuels the building ofthe substances neurons need to do their job. Someof the most important substances synthesized in theneuron’s cell body are neurotransmitters. As we willsoon see, neurotransmitters are the chemicals thattransmit an electrical impulse from one neuron tothe next.Some dendrites resemble the branches of a tree.Others have more thread-like branches, and somehave branches covered with tiny spines. The reasonfor this branching design is simple. Remember,dendrites are the parts of neurons that receive inputs(signals). The branching pattern covers an extensivearea, allowing the neuron to receive an enormousnumber of inputs. When an input is received bya dendrite, an electrical signal is generated andtransmitted toward the cell body.The axon is the portion of the neuron that carriesa nerve impulse away from the cell body. The axonbegins at a coneshaped axon hillockon the cell body.The hillock narrowsto form the morethread-like axon.The axon can bevery short or up toseveral feet long.The axon of a motornerve to the musclethat enables youto curl your big toehas to travel a longway, all the wayAxonfrom your spinalcord to your foot.TAKING A CLOSER LOOKNeuronCell bodyNucleusDendriteNode of RanvierSchwann cellExtending from the cell body are numerousprojections, or processes. Neuron cell bodies havetwo kinds of processes protruding from them,dendrites and axons. Dendrites are designed toreceive signals. Axons are designed to carry signalsaway.Axon terminal

13STRUCTURE OF NERVOUS TISSUEA neuron can have multiple dendrites but onlyone axon. Axons end in small branches called axonterminals. At the axon terminal, neurotransmittersare released to carry the neuron’s signal on to thenext cell in line. You will learn more about thisshortly.Neurons — The LowdownThere are hundreds of millions of neurons in thehuman body. And that’s a really good thing. Why?Unlike most cell types in your body, neurons cannotbe routinely replaced. Once neurons mature, withonly rare exceptions, they are no longer able todivide. The neurons you have, once your nervoussystem matures, are all the neurons you will everhave.So when neurons are damaged by drugs, disease,or injury, the loss of function is often permanent.Neurons are designed to last a lifetime, but weneed to take care of them. For instance, we must bevigilant about what we put into our bodies, as manyillicit drugs destroy these precious messengers. Alifetime of poor eating habits and lack of exercisecan increase the risk of a stroke in later life, whichcan destroy many neurons in the brain. Riding yourbicycle without a helmet puts the irreplaceableneurons in your brain at risk right now. Followingthe rules for safety in contact sports may preventa tragic accident that could leave you paralyzed.Operating power tools unsafely may lead topermanent loss of peripheral nerve function in aninjured body part, even if you do not lose the bodypart itself. Habitually exposing your ears to loudmusic or explosive noise without ear protection maydestroy the specialized neural structures in yourears and impair your hearing. Looking directly at thesun can permanently damage your retina, the veryspecialized extension of your brain that enables youto see.God only gave you one body, and there are nodo-overs when it comes to neuron damage. Whilemany diseases and conditions that damage neuronsin this sin-cursed world are not preventable, youshould take care to avoid those that are.Further, neurons require lots of oxygen and glucoseto function properly. Neuron cells can be quicklydamaged by lack of these essentials. Loss of oxygenfor as little as four minutes can permanently damageneurons. For this reason, many people take coursesin basic CPR and water safety, so that they will beable to help others avoid permanent damage or lossof life.Performing CPR (cardiopulmonaryresuscitation) on someone who hasstopped breathing.

14Types of NeuronsThere are several types of neurons. We can classifythem according to how they look or according to howthey work. Each type of classification can help usunderstand how the nervous system works.One method of classifying neurons is based on thenumber of processes they have. Remember, processesare dendrites and axons, the projections sticking outfrom the cell body.Most neurons have one axon and multiple dendrites.These are called multipolar neurons. This is by farthe most common type of neuron in the body.Bipolar neurons have only two processes: one axonand one dendrite. These are only found in specialsensory organs, such as the eye, ear, and nose.Unipolar neurons have a more unusual configuration.They have only one process extending from the cellbody. This process looks like a “T.” The dendrite andthe axon form the arms of this “T.”TAKING A CLOSER LOOKTypes of NeuronsMultipolar neuronMotor neuronPyramidal neuronDendritesAxonBipolar neuronRetinal neuronOlfactory neuronDendritesPurkinje cellNeurons are also classifiedaccording to the direction theycarry nerve impulses. Someneurons carry instructionsfrom the central nervoussystem, and others bringinformation to the centralnervous system.DendritesAxonUnipolar neuron(touch and pain sensory neuron)DendritesAxonAnaxonic neuron(Amacrine cell)AxonDendritesNeurons that transmitimpulses away from thecentral nervous system arecalled motor or efferent(remember “carrying away” or“carrying outward”) neurons.These impulses containinstructions to muscles or toglands in the body. Most motorneurons are multipolar.Sensory or afferent (remember“bringing toward”) neuronscarry impulses triggered bysensory receptors toward thecentral nervous system. Mostsensory neurons are unipolar.

15STRUCTURE OF NERVOUS TISSUEYet one other class of neurons carries impulses fromone neuron to another within the central nervoussystem. These connectors are called interneurons,a word that obviously means “between neurons.”Interneurons make up the vast majority of theneurons in the body. Some estimates are as high as99 percent. Interneurons are located in the brain andspinal cord, forming connections between sensoryand motor neurons. Signals from sensory neurons aredelivered to the interneurons. The interneurons passthe impulse on to the appropriate motor neurons.If you recall the basic functions of the nervoussystem, this is the integration step we discussed, astep in which inputs are processed and passed on togenerate suitable output.NeurogliaNeurons are not usually alone. They are generallysurrounded by several types of smaller cells in thenervous system. These other cells are known asneuroglia, or glial cells. Neuroglia are found bothin the central nervous system and the peripheralnervous system. Neuroglia have various functionsdepending on their cell type and location.We will first examine the neuroglia in the CNS.Astrocytes are the most numerous of the neuroglialcells in the CNS. Astro means “star,” and cytes means“cells.” Astrocytes are therefore glial cells withTAKING A CLOSER LOOKTypes of NeurogliaOligodendrocytesMicrogliaEpendymal cellsAstrocytesSchwann cells

16many star-shaped processes. These cells anchor andsupport the neurons associated with them. They helpthe neurons pass on impulses efficiently. Astrocytesalso protect their neurons. They monitor nearbycapillaries, ensuring that harmful substances inthe blood do not reach the neuron. Astrocytes helpmaintain the correct level of ions, such as potassium(K ), and other nutrients around the neurons.They contain a readily available supply of glucosethat they supply to neurons when lots of energy isneeded. They even help recycle neurotransmittersreleased from their neurons.Satellite cells surround the cell bodies of neurons inthe PNS. They provide structural support and alsocontrol the extracellular environment around the cellbodies. Thus, the satellite cells function in the PNSmuch in the way astrocytes do in the CNS.Schwann cells form the myelin sheaths around axonsin the PNS. Therefore, Schwann cells function in thePNS the way oligodendrocytes do in the CNS. Let’sexplore myelination in more detail next.Microglia are small cells with long slender processes.(Micro means “small,” so this is a good name.)Microglial cells “keep watch” over neurons intheir vicinity. If they detect damage to a neuron orinvading bacteria, they transform into a cell that canremove damaged nerve tissue or engulf and destroythe bacteria.Ependymal cells line the ventricles of the brain andthe spinal canal. The ventricles in the brain, like thecanal surrounding the spinal cord, are filled withcerebrospinal fluid. Ependymal cells produce muchof the cerebrospinal fluid that fills these cavities.Cerebrospinal fluid doesn’t just sit still; it circulatesthrough these fluid-filled spaces in the CNS. Cilia onthe ependymal cells help move this fluid around.Oligodendrocytes resemble astrocytes, but they aresmaller. Oligodendrocytes produce and maintaina special covering (called a myelin sheath) aroundneuronal axons. This myelin sheath is made of lipidsand protein. We will be learning much more aboutmyelinated axons shortly.Okay, now you know there are four types of glial cellsin the central nervous system—astrocytes, microglialcells, ependymal cells, and oligodendrocytes. Thereare two types of neuroglial cells in the peripheralnervous system, satellite cells and Schwann cells.This image shows the four different types of glial cells foundin the central nervous system: Ependymal cells (light pink),Astrocytes (green), Microglial cells (red), and Oligodendrocytes(functionally similar to Schwann cells in the PNS) (light blue).

17STRUCTURE OF NERVOUS TISSUEMyelinationMyelination is a process in which long axons arecovered by a myelin sheath. The myelin sheath is aspiral wrapping of the modified cell membranes ofthe Schwann cells or oligodendrocytes responsiblefor forming the myelin. Axons having this myelincovering are said to be myelinated. Axons not havingthis covering are called nonmyelinated.The myelin sheath provides electrical insulation forthe axon. It also increases the speed a nerve signalcan travel.In the PNS, myelination is carried out by Schwanncells. These cells initially indent to receive the axon,and then wrap themselves repeatedly around theaxon. Ultimately, this wrapping has the appearanceof tape wrapped around a wire or gauze wrappedaround a finger. At the end of the wrapping process,there may be several dozen layers of wrapping to thesheath.Each of the Schwann cells wraps only a small lengthof a single axon. Other Schwann cells wrap theremaining length of the axon, like so many hot dogsin buns laid end to end. However, Schwann cells donot touch each other. There are small gaps betweenadjacent Schwann cells. These gaps are called nodesof Ranvier. (They were discovered by—you guessedit!—French anatomist Louis-Antoine Ranvier inthe 19th century, and his name is pronouncedron’- vee-ay.)TAKING A CLOSER LOOKMyelinationNucleusSchwanncellsNode of RanvierOligodendrocyteNucleusCentral nervoussystem (CNS)Peripheral nervoussystem (PNS)

18It should be pointed out here that a Schwann cellcan enclose a dozen or more axons without wrappingthem. These axons are nonmyelinated even thoughthey are in contact with a Schwann cell.In the CNS, it is the oligodendrocyte thatis responsible for myelination. Because anoligodendrocyte has many processes, it can wraparound numerous axons rather that only one, as inthe case of the Schwann cell.The amount of myelin in the body is very low at birthand increases as the body develops and matures.Thus the number of myelinated axons increasesfrom birth throughout childhood until adulthood.Myelination increases the speed of nerve impulseconduction through the axon. Faster conductionMultiple SclerosisMultiple Sclerosis (MS) is an autoimmune disease that results in the destruction of myelin sheaths in the centralnervous system. (In autoimmune diseases, the body’s immune system turns against its owner’s own tissues.) Inmultiple sclerosis, the body’s immune system attacks myelin proteins, creating hardened lesions called scleroses.These lesions commonly occur in the optic nerve, the brain stem, and the spinal cord.As the myelin loss increases, conduction of nerve impulses becomes progressively slower. Short circuits developand interfere with the proper functioning of the neurons. That this disease is so debilitating shows the importanceof the myelination of nerve fibers to properfunctioning of the nervous system.MS primarily occurs in people under 50 years ofage. Symptoms include double vision, weakness,loss of coordination, and paralysis.One form of MS is characterized by periodsSchwanncellsof active disease alternating with periods ofminimal symptoms. Another form of MS is slowlyDamagedmyelinprogressive, without the symptom-free periods.Although in recent years much progress has beenmade in our understanding of multiple sclerosis,at present there is still no cure.Node of RanvierExposed fiberAxon

19STRUCTURE OF NERVOUS TISSUEmakes those nerves work better, more efficiently, asan individual matures.Think of a newborn baby. It has very little controlof its body in the beginning. It cannot hold itshead up or sit up or walk. As more axons becomemyelinated, it has more and better control of itsmuscles. Compare this to a teenager. After yearsof development, the teenager has much bettercontrol and coordination of the body. Much of thisTAKING A CLOSER LOOKimprovement of due to increased myelination bothin the central and peripheral nervous systems.NervesWhat are nerves? They not the same thing asneurons.A neuron is a nerve cell. Neurons have dendrites andaxons. The neuron is the cell that transmits electricalimpulses in the nervous system. Thus, the neuron,not the nerve, is the basicunit of nervous tissue.Anatomy of a NerveEpineuriumAxonBlood ineuriumAxon fasciclesEpineuriumPerineuriumSo what is a nerve?

the nervous system controls the complex activities of the human body. Overview of the Nervous System We will begin our tour of the nervous system by taking a broad look at its two major divisions, the central nervous system (CNS) and the peripheral nervous system (PNS). Even though these parts work together as a highly efficient, integrated unit,