Chapter 2: Neurons and Glia
* Although there are many neurons in the human brain (about 100 billion), glia outnumbers neurons by tenfold.
* Neurons are the most important cells for the unique functions of the brain. * Neurons sense changes in the environment, communicate these changes to other neurons, and command the body’s responses to these situations. * Glia, or glial cells, are thought to contribute to brain function mainly by insulating, supporting, and neighboring neurons.
2. The Neuron Doctrine
* Histology: the microscopic study of the structure of tissues. * Breakthrough in histology: introduction of stains that could selectively color some, but not all, parts of the cells in brain tissue. * Franz Nissl: 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 (Nissl Stain) * Useful for two reasons: Distinguishes neurons and glia from one another and enables histologists to study the arrangement, or cytoarchitecture, of neurons in different parts of the brain. * Camillo Golgi: 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. * The Golgi stain shows that neurons have at least two distinguishable parts: a central region that contains the cell nucleus, and numerous thin tubes that radiate away from the central region.
* Swollen region containing the cell nucleus has several names: cell body, soma, and perikaryon. * The thin tubes that radiate away from the soma are called neuritis and are of two types: axons and dendrites. * Cell body usually gives rise to a single axon. Axons act like wires that carry the output of the neurons. Dendrites act as the antennae o the neuron to receive incoming signals or input. * Camillo Golgi versus Santiago Ramon y Cajal: Golgi championed the view that the neuritis of different cells are fused together to form a continuous reticulum, or network, similar to the arteries ad veins of the circulatory system. According to this reticular theory, the brain is an exception to the cell theory. Cajal argued that the neuritis of different neurons are not continuous with one another and must communicate by contact, not continuity (neuron doctrine).
3. The Prototypical Neuron
* The neuron (also called a nerve cell) consists of several parts: soma, dendrites, and axon. * The inside of the neuron is separated from the outside by the limiting skin, the neuronal membrane.
B) The Soma
* The roughly spherical central part of the neuron (also called cell body) 20 µm 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 organelles. * The cell body of the neuron contains the same organelles that are found in all animal cells: nucleus, rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi apparatus, and mitochondria. * Everything contained within the confines of the cell membrane, including the organelles, but excluding the nucleus, is referred to collectively as the cytoplasm.
* Spherical, centrally located, 5– 10 µm across, contained within a double membrane called the nuclear envelope (perforated by pores that measure about .1µm) * Within the nucleus are chromosomes, which contain DNA. * Genes: A unit of heredity; sequence of DNA that encodes a single polypeptide or protein. * Gene Expression: The process of transcribing the information from a gene into messenger RNA. Final product of gene synthesis proteins * Protein synthesis, the assembly of protein molecules, occurs in the cytoplasm. * Messenger RNA (mRNA) A molecule constructed from...
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