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Chapter 2

Chapter 2 - Neurons and Glia.docx

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Janelle Leboutillier

Neurons and Glia o The brain is an organ no need to spate mind from brain; focus on different types of cell in the nervous system: neurons and glia Although there are many neurons in the human brain (about 100 billion) the glia outnumber neurons by tenfold; however neurons are most important cell sfor the unique functions of the brain It is the neurons that sense changes in the environment and communicate these changes to other neurons and command the bodys responses to these sensations. Glia or glial cells, are thought to contribute to brain function mainly by insulating, supporting, and nourishing neighboring neurons; keep the brain from running out of our ears o Neurons size is at or beyond the limit of what can be seen by the naked eye. Thus microscopes were the scientists way of observing neurons, it was necessary to make very thin slices. But brain tissue has a consistency like a bowl of jelly: not firm enough to make slices. Thus scientists needed a method to harden the tissue without disturbing the structure fixing tissues by immersing them in formaldehyde and developed special device called a microtome to make very thin slices Technical advances spawned the field of histology, the microscopic study of the structure of tissues l Faced another problem since freshly prepared brain had a uniform, cream-colored appearance under the microscope; the tissue has no differences in pigmentation to enable histologists to resolve individual cells. Final breakthrough in neurohistology was the introduction of stains that could selectively color some, but not all, parts of the cells in brain tissue o Franz Nissl, German neurologist, introduce one stain and showed that a class of basic dyes would stain the nuclei of all cells and also stain clumps of material surrounding the nuclei of neurons Clumps are called the Nissl bodies, and the stain is known as the Nissl stain extremely useful for two reasons: a. It distinguishes neurons and glia from one another b. It enables histologists to study the arrangement, or cytoarchitecture is an arrangement of neuronal cell bodies in various parts of the brain, of neurons in different parts of the brain o Camillo Golgi, Italian histologies, discovered that by soaking brain tissue in a silver chromate solution, now called the Golgi stain, a small percentage of neurons became darkly colored in their entirety This revealed that the neuronal cell body, the region of the neuron around the nucleus The gain in the brain is mainly in the stain Golgi stain shows that the neurons have at least two distinguishable parts: 1. A central region that contains the central region 2. The swollen region containing the cell nucleus; can be called cell body, soma, and perikaryon The thin tubes that radiate away from the soma are called neuritis and are of two types: i. Axons is of uniform diameter throughout its length, and if it branches, the branches generally extend at right angles; can travel over great distances in body, act like wires that carry the output of the neurons ii. Dendrites rarely extend more than 2mm in length, extend from the cell body and generally taper to a fine point. Dendrites come in contact with many axons, they must act as the antennae of the neuron to receive incoming signals, and input o Santiago Ramon y Cajal, Spanish skilled histologist and artist, who learned about Golgis method in 1888 and make a remarkable series of publications over the next 25 years, Cajal used the Golgi stain to work out the circuitry of many regions of the brain. Ironically concluded different conclusions from Golgi Golgi championed the view that the neuritis of different cells are fused together to form a continuous reticulum, or network, similar to the arteries and veins of the circulatory system Cajal, on the other hand, argued forcefully that the neurites of different neurons are not continuous with one another and must communicate by contact, not continuity This idea that the neuron adhered to the cell theory came to be known as the neuron doctrine. Invention of electron microscope in 1950s with increased resolving power, it was finally possible to show that the neuritis of different neurons are not continuous with one another o Then neuron also called the nerve cell consists of several parts: the soma, the dendrites, and the axon. The inside of the neuron is separated from the outside by the limiting skin, the neuronal membrane, which lies like a circus tent on an intricate internal scaffolding, giving each part of the cell its special 3D appearance. o The soma, the roughly spherical central part of the neuron. The cell body of the typical neuron is about 20 m in diameter. The watery fluid inside the cell, called the cytosol, is a salty, potassium-rich solution that is separated from the outside by the neuronal membrane Within the soma are a number of membrane-enclosed structures called the organelles, the cell body of the neuron contains the same organelles that are found in all animal cells The important organelles include the nucleus, the rough endoplasmic reticulum, the smooth endoplasmic reticulum, the Golgi apparatus, and the mitochondria Everything contained within the confines of the cell membrane, including the organelles but excluding the nucleus is referred to collectively as the cytoplasm o The nucleus of the cell is spherical, centrally located, and about 5-10 m across. It is contained within a double membrane called the nuclear envelope which contains perforated pores. Within the nucleus are the chromosomes, which contain genetic material, DNA (deoxyribonucleic acid). The DNA in each of your neurons is the same, and it is the same as the DNA in the cells of your liver and kidney. Difference distinguished by the specific parts of the DNA that are used to assemble the cell. These segments are called genes The reading of the DNA is known as gene expression. The final product of gene expression is the synthesis of molecules called proteins, which exist in a wide variety of shapes and size, and perform many different functions, and bestow upon neurons virtually all of their unique characteristics Protein synthesis, the assembly of protein molecules, occurs in the cytoplasm. Messenger ribonucleic acid or mRNA carries the genetic message to the sites of protein synthesis; it consists of four different nucleic acids The process of assembling a piece of mRNA that contains the information of a gene is called transcription, and the resulting mRNA is called the transcript
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