Neurons And Glia Notes
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19.1 Methods of study of the nervous system Core: basic histological techniques (Nissl, Golgi, Weigert); different types of microscopy
-The first important step was the discovery of fixatives- formaldehyde that allowed thin slices of brain tissue to be made so that it could be looked at under the microscope.
-Stains were then needed to enable individual cells to be distinguished. Nissl Stain: uses basic dies such as cresyl violet that colour nuclei and clumps of material around the nucleus now known as Nissl bodies.
-This stain allows neurons to be distinguished from glia and study of the cytoarchitecture in different regions of the brain (eg. allowed Brodmann to define his areas). Golgi stain: uses silver nitrate and potassium dichromate, which cause silver chromate to precipitate out within the cytoplasm of the cells.
-It stains a relatively small proportion of cells, which can make it easier to see detail, and stains entire cells so that all their processes can be traced.
-It is not possible, however, to predict which cells within the population as a whole will take up the stain. weigert stain stains myelinated fibres and is useful for studying and comparing the arrangement of grey and white matter.
-The development of EM was also important, as synapses were seen for the first time. Core: CT, MRI, Functional MRI, PET, EEG: the information each can provide
-CT and MRI can show gross anatomical features.
-Positron Emission Tomography scanning gives information about the activity of the brain, so can be used to study which areas are actively involved during different stimuli, tasks, conditions etc.
-A radioactive tracer conjugated to a biologically active substance is given. This emits positrons, which can be detected.
-In neurology, the tracer is often added to glucose or oxygen, which are taken up in greater quantities by more active brain regions, so are more radioactive.
-There are also radiotracers that are specific for receptors in the brain, so are taken up by specific regions.
-Functional MRI is an adaptation of MRI that shows changes in the blood flow in the brain. Increased blood flow is taken as a sign of increased activity in that area, although this is not necessarily the case.
-Electoencephalography is a measure of electrical activity in the brain, as recorded by electrodes placed on the scalp. One of its main clinical uses is for epileptic patients.
Extension: reporter gene-expressing transgenic mice (eGFP, LacZ); tracttracing techniques; magnetocephalography
19.2 Structure and function of neurons and glia The PNS arises from neural crest stem cells which originate from the edge of the neural tube and migrate away to peripheral organs and ganglia outside the CNS. The neural crest stem cells specify, migrate and terminally differentiate to have functional specification. Wilhelm von Waldeyer-Hartz consolidated the 'neuron theory' in 1891, and invented the term 'neuron'. He used the work of Golgi and Cajal who had used silver nitrate to stain nerve tissue (stains nerve fibres and endings black; other tissues brown) to study the processes and interconnections of nerve cells. Neurons are excitable cells and are the basic cells of the nervous system. They are post mitotic, polarised. All neurons whether central or peripheral include 3 main parts: 1) Cell body (perikaryon) contains the nucleus and other cell organelles, including abundant mitochondria and RER (due to high protein synthesis). Free ribosomes and RER can be seen as Nissl bodies (basophilic granular areas) in the cytoplasm and are responsible for the production of proteins including neurotransmitters. Basic dyes- Cresyl violet stains nuclei and ribosomes surrounding it. As there are large amounts of RER, Nissl stains are useful in studying organisation of CNS. 2) Axon a long, cylindrical process extending from the cell body, consisting of axoplasm surrounded by an axolemma (plasma membrane). The axon hillock arises from the cell body as a pyramidal shaped region from which the axon projects. Axons transmit impulses away from the cell body and synapse with other cells via terminal boutons. May be myelinated or unmyelinated. They are microscopic in diameter (1µ-1mm) and may be over a metre in length. 'output' of cells - release NTs. 3) Dendrites multiple shorter processes from the cell body which become thinner as they divide. They are specialised for receiving external stimuli from other cells; they synapse with axons of other neurons and transmit impulses towards the cell body. They are the signal reception and processing site of the nerve. 'input' of cells - have receptors for NTs.
-MRNA is also found in dendrites- suggesting that protein synthesis also happens here
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