#14101 - Neurons And Glia - Neuroscience 1

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); tract-tracing 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

Conduction takes place in the direction of dendrites to the soma to the end of the axon

Cytoskeleton: The shape of nerve cells are maintained by the cytoskeleton

Microtubules: Are stabalised by other proteins that bind to them-tau proteins. Important for structural stability of axons and allows for transport to distal parts of the neuron via motor proteins kinesin and dynein

-Kinesins move cargo along microtubules from cell body to distal tip. Kinesins move things anterograde, away from soma along axons and dendrites. Dyenins move things retrograde ,back to soma. Motor proteins are energy dependent and require ATP.

-To show transport was happening down axon, Silk is tied around the width of an axon and it was shown that there was an accumulation of vesicles behind the site of blockage.

-Axon transport system is traced by labelling proteins with radioactive proline.

-Anti-mitotic drugs: interfere with the exchange of tubulin subunits between microtubules and cytoplasmic pool of tubulin

-Abundant neurofilaments: (intermediate filaments, Main structural proteins of the cytoskeleton and maintain the cell shape-come in 3 sizes light, medium, heavy) protein is thought to act as an internal scaffold to maintain the shape of the axon and cell body.

-Selectively stain with silver salts- Bodian stain. Mutatant with deletion of Neurofilament Medium tail region, cross links with microtubules in axon structure are disrputed causing a reduction in axon diameter.

-Actin: Important in development and continuing adult plasticity at synapses. Growing nerve fibres are responsible for generating an axon and determining where it goes -growth cone contains actin microfilament. Experiment: Loss of use of neuron, actin cytoskeleton shrinks. Activity of actin cytoskeleton is controlled by local interactions especially neurotrophin release- block Neurotrophic factor TrkB-IgG actin microfilament doesn’t grow but if you block TrK IgG actin microfilament- branch more

- In synaptic boutons the actin cytoskeleton is responsible for synaptic terminal.

Classification of neurones

Neurons can be classified by shape, biochemistry (transmitter conten, excitatory/inhibitory), function: projection neurons/interneurons

There are three major axonal shapes, determined by the number of cell processes formed by the dendrites and axons:

Multipolar neurones more than two cell processes – one axon and more than one dendrite. Eg. motor neurons.

Bipolar neurones one axon and one dendrite (which can still branch to form a number of synapses). Found only in specialised sensory areas such as the olfactory region and the retina.

Pseudo unipolar neurones a single process close the cell body, which divides into two branches, one towards the CNS and one towards peripheral endings. The cell body forms a ‘T-shape’ with the dendritic processes and the axon ‘bypassing’ the cell body. Impulses bypass the cell body, passing directly from dendrites to axons.

Core: characteristic appearance and functions of projection neurons and interneurons; excitatory and inhibitory neurons

A) PROJECTION NEURONS

-Neurons can be classified according to shape, their neurotransmitters (specifically or broadly into excitatory and inhibitory) or function.

-Projection neurons are cells that conduct impulses over long distances between different regions of the nervous system or between the nervous system and a peripheral effector eg muscle or receptor.

-They are usually myelinated and fast-conducting, so will stain with stains such as Weigert.

-They also have large cell bodies because they must support very long axons.

-Examples include pyramidal cells...