Composition of the Nervous System

Transcription

1 Composition of the Nervous System Brain -100 billion neurons and synapses per neuron -20% of body s oxygen (94% used by grey matter) % of total blood supply -Composed of 10-12% lipids and 8% proteins -60% white matter and 40% grey matter Nervous system = an organised association of neurons and supporting cells (glia) with its own blood supply (endothelial cells) 1) Neurons =Main signalling units in the nervous system (the excitable cells) -Several morphologies bipolar, unipolar, multipolar 2) Glia -Macroglial cells (astrocytes and oligodendrocytes) -Pericytes = immune cells and contractile cells (around blood vessels) -Microglial cells = cells of the immune system invaders from the bone marrow 3) Endothelial cells =Line blood vessels front line of the blood supply to the nervous system =Form the blood brain barrier Neurons Neurons are structural and functional unit responsible for transfer of information via electrical (ionic movement) and chemical communication. Neurons are excitable cells that are capable of transmitting signals along cell membrane by action potentials to other excitable cells (other neurons or muscle cells) in one direction (dendrite cell body axon) via synapses. Transmission of the signal between cells at the synapse is chemically mediated these chemicals are known collectively as neurotransmitters. Unidirectional signal of neurons 1) Sensory systems (afferent) towards the CNS direction of signal is from the periphery towards the brain -Trigger may be from outside the body (e.g. visual world) or internal receptors (e.g. joints, via visceral receptors, chemoreceptors) -Transduced by a special sense organ/structure -Transmitted to the brain for interpretation/integration 2) Motor systems (efferent) away from the CNS direction of signal is to the periphery -Voluntary (to skeletal muscle) -Involuntary signals (to smooth muscle contraction, glandular action) -Direction of signal passage through a neuron is always the same: dendrite cell body axon -Dendrites collect electrical signals. Cell body contains nucleus and organelles. Axon passes electrical signals on to dendrites of another neuron or to an effector cell (e.g. muscle) The structural components of the neuron: 1) Cell body/ Soma/ Perikaryon Nucleus prominent nucleolus containing chromosomal DNA Prominent Golgi apparatus for processing of proteins Nissl substance = Rough endoplasmic reticulum = Most abundant organelle in neurons -Reflects their very high level of protein synthesis -Made up of membrane stacks dotted with ribosomes -Contain mrna and are sites of protein synthesis (translation)

2 -Elaborate endoplasmic reticulum and abundant free ribosomes Abundant mitochondria site of oxidative metabolism (neurons undergo high metabolic activity) Cytoskeleton = For structural support (dendritic shape for memory) and movement of organelles and proteins (directs + delivery to terminal bouton) through extremely long neurons -Microtubules -Neurofilament -Actin 2) Dendrites = shorter fibers extending from the cell body -Receives signals from other neurons. -Membrane at postsynaptic cleft includes specialised receptor proteins that detect neurotransmitters. 3) Axon = Cytoplasmic extension of neuron = single long fibre extending from the cell body -Extends from axon hillock to axon terminal (terminal bouton) -May branch at terminals -Contains no rough ER or ribosomes -Axon terminal contains large numbers of vesicles and proteins necessary for the release of transmitter -Carries information from the soma to another neuron or effector cell Axon hillock = junction between soma and axon -Plays essential role in integration and transmission of signals -The action potential usually generated in the axon hillock -Plasma membrane forms transmembrane protein barriers with actin filaments to block free diffusion of proteins from soma to axon -Region of summation of excitatory and inhibitory activity -Has no synapses Terminal bouton 4) Membrane -Phospholipid bilayer & special embedded proteins -The phospholipid bilayer isolates the cytosol from the extracellular fluid -The inserted proteins provide a means for the diffusion or active transport of molecules and ions across the cell membrane -Other proteins act as receptors at synapse -Ion channels and carrier proteins ( pumps ) -Preserves the highly-controlled internal environment and intercellular contact necessary for excitation -The composition of the membrane is different in the different parts of the neuron. 5) Synapse = Specialised junctions between neurons -Where neurons communicate through chemical messengers neurotransmitters -May also be electrical synapse Neuronal transmission 1) Ion segregation is essential for neuronal transmission -Neurons maintain a potential difference across the neuronal membrane; + outside and inside -Neuronal voltage is measured by inside vs outside: negative voltage means more negative inside the cell Ratio Outside : Inside [K + ] = 1:20 [Na + ] = 10:1 [Ca 2+ ] = 10,000:1 [Cl - ] = 11.5:1 2) Ion segregation is supported by the neuronal membrane -Hydrophilic portions facing intracellular and extracellular watery environment -Hydrophobic, non-polar lipid layer sandwiched between hydrophilic portions

3 Neuronal membrane regulation of permeability and intracellular environment 1) Passive segregation lipid solubility -O, N, CO 2 and alcohols are lipid soluble. They freely diffuse through the cell membrane -Even though H 2 O is lipid insoluble, H 2 O diffuses through (it s small and has high kinetic energy) -Ions cannot diffuse through the cell membrane because: -Ions become hydrated n the watery environment and become too large -Polarities of the phosphate head of the phospholipid repel charged particles 2) Active segregation a) Ion pumps Carrier proteins/ Active pumps -Require energy/ ATP for transport of ions against the gradient (low to high concentration) -Permeability determined by diameter of the pore and nature of subunits that line it (especially charge) -Segregation of ions creates potential = voltage difference -Allows excitability and propagation of a current -Na + /K + pumps maintain resting potential (negative inside cell) -Ca 2+ pumps keep Ca 2+ concentration very low inside cell Ca 2+ is important in initiating an action potential when a ligand/ neurotransmitter binds -The transport of the ions is driven by the energy released from the hydrolysis of ATP b) Ion channels/ Ion-selective pores Passive channels -Require no energy for ions to pass through (channels require energy for production, removal maintenance and insertion into the membrane) -When open, channels let selected ions diffuse rapidly down electrical and concentration gradients -Family of structurally and functionally similar proteins which forms a pore to allow ions to move across membrane -Selective for particular ions by size and/or charge Passive non-gated ion channels (leaky) are always open and permitted ions flow down concentration gradient Gated ion channels = change in conformation state of the channel protein -Voltage-gated =conformational change with change in voltage -Voltage-gated ion channels mediate changes in membrane permeability to propagate the action potential within the neuron (explosive reaction) -Inactivating voltage-gated Na + channel -Delayed (rectifying) voltage-gated K + channel Na + and K + ions have equal charge but different sizes when hydrated. Partially hydrated K + ion is bigger. This allows Na + pore to be selective and only let Na + ion pass through. -Ligand-gated = conformational change with binding of effector molecule i.e. neurotransmitters Neuronal signalling: 1) Generation of action potential along axon -Requires a membrane potential Resting phase: membrane potential at around -70 mv -Na + and K + channels are closed at -80mV to -65mV -Inputs alter membrane potential at axon hillock partial depolarisation triggers opening of Na + channels -Na + channel begins opening between -65mV and -40 mv and allows influx of Na + into cell -Depolarises and more channels open (fully open at -15mV) Rising phase: inside of cell becomes more positive Overshoot: the membrane potential becomes positive -K + channels open at +30 to 35mV and K + flows out of the cell -Na + channels close at about +30mV Falling phase: inside of cell becomes less positive and returns to negative Hyperpolarisation: there is little sodium permeability and great potassium permeability Refractory period: Sodium channels inactivate when the membrane becomes strongly depolarised and they cannot be activated again

4 Membrane depolarises until it re-establishes resting charge potential - K + channels close -High density of ligand-gated channels at the input end of the cell membranes of dendrites and soma. These channels open with binding of neurotransmitter (chemical input) -Highest density of voltage-gated channels at the axon hillock and axon allows generation and conduction of action potential down the axon -Density of voltage-gated channels determines sensitivity to depolarization -Different types of neurons have differences in both type and density of channels expressed in their membranes Neuron accomplishes a unidirectional signal Conduction speed -Myelin (oligodendrocyte membrane) insulates the axon -Reduces ion leakage fewer ions are pumped -There is a cluster of voltage-gated channels in Nodes of Ranvier -Saltatory propagation: Allows faster conduction of action potential Distinctive concentrations of ions inside and outside establish a potential difference across the cell membrane - the membrane potential. The membrane becomes depolarised (decreasing the potential difference) when a neuron is excited and hyperpolarised (increasing the potential difference) when a neurone is inhibited, respectively. 2) Signalling to other cells Chemical synaptic transmission -Neurotransmitters stimulate or inhibit receptors to alter potential in postsynaptic cells Electrical synapses -Flow of current (ions) through gap junctions Neurohistology Macroscopic 1m-1mm Nerve Root Spinal nerve Peripheral Nerve Light Microscope 1mm-1μm Nerve Cell Myelin Sheath Paccinian Corpuscle Electron Microscope 1μm-1nm Membrane Details Synaptic Vesicles Mitochondria Molecular Ion Channels

5 Cells of the nervous system Neurons Supporting Cells: -Schwann: found surrounding some peripheral nerve cell processes where they form the axonal myelin sheath, which improves the conduction velocity of impulses -Satellite: surround nerve cell bodies located in the ganglia of the peripheral nervous system and have functions similar to astroglia -Perineural cells: found in peripheral nerves where they form the perineurium. The perineurium defines the fascicular substructure of peripheral nerves -Astroglia: prominent in the grey matter (protoplasmic) but are also found in white matter (fibrous). Their processes are found near blood vessels, pia, ependyma and neurons including near synapses; mechanical support, metabolic, maintain homeostasis of extracellular fluid, form scar tissue -Oligodendroglia: prominent in white matter, processes surround axons; myelination -Microglia: enter and leave the CNS, widely distributed; immunological, phagocytic Other Cells -Ependymal: line the ventricles and along with the pia and capillaries form the choroid plexus, which secretes cerebrospinal fluid (CSF). -Meninges (Pia, Arachnoid, Dura): membranes, which surround the brain and protect it. They are composed of varying proportions of flattened epithelial like cells and connective tissue -Blood vessels including capillaries -The structure of a typical neuronal cell is comprised of a body, many branching dendrites and a single branching axon. The proximal part of the axon is called the axon hillock. -Schematic neurons: Particularly when drawing circuits neurons can represented in a schematic way by a circle (cell body and dendritic tree) a line of varying length (axon) with a small V at the end (the synapse). This notation can be varied to indicate various neuronal shapes and ways of connecting with other neurons. -Structural variation: There are many ways by which the variation in neuronal cells is classified, described or characterized. Some are structural (morphological) others functional (behavioural, physiological) others biochemical or pharmacological (transmitter). The morphological classification or description of neurons is based upon their external appearance and or their location within the nervous system. And is usually based on what is observed in the light microscope. Describing the structure of typical neurons and their variations: (CNS is heterogeneous neuronal structure and function identification and characterisation of neuron) -Name -Microscope level (light/ electron) -Stain -Cell body size -Cell type -Sensory -Motor -Interneuron -Cell shape -Pyramidal -Stellate -Cell body, dendritic tree -Nucleus, size, shape, location, chromaticity (euchromatic), nucleolus (prominent) -General location -Central (fibres in brain, spinal cord) intermediary/ integrative -Peripheral (fibres on the outside) -Particular / Topographical location, region -Axon length -Golgi 1 neuron/ Projection neuron Long axon convey message in long distance -Golgi 2 neuron/ Interneuron Short axon -Polarity/ Number of processes from cell body -1 process = Unipolar neuron somatic sensory neurons -2 processes = Bipolar neuron some cells in the retina, olfactory cells

6 -Other -3+ processes = Multipolar neuron interneurons, principal neurons, lower motor neurons -At the boundaries of this system are sensory cells, which through the process of transduction collect the information about the environment (external and internal) and motor neurons that via excitation contraction coupling and muscles and glands act upon the environment. In between are the intermediary neurons represented at various phylogenetic levels as nets, ganglia or the brain. In humans the boundary neurons form the peripheral nervous system and the intermediary neurons the central nervous system. This leaves the enteric nervous system either as part of the peripheral system or a second system in its own right. Grey and white matter Grey and white matter are so named because of their color when viewed macroscopically (with the unaided eye) in a section of the central nervous system without the use of any staining techniques. In macroscopic and particularly microscopic sections where stains are used the grey and white matter may be coloured differently. Composition of grey matter Neuronal cell bodies Projection/ Golgi 1 Interneuron/ Golgi 2 Dendrites (short processes hence stay with cell bodies in Projection grey matter) Axons Synapses Astrocytes (protoplasmic) Capillaries many rich blood supply (Mainly interneurons) Interneuron Projection, proximal, distal, intransit Interneuron complete Grey matter in general is defined by the presence of nerve cell bodies and further characterized by the presence of the dendrites of projection neurons and small interneurons, axons of interneurons, proximal axons of projection neurons, distal axons of projection neurons from elsewhere, projection neuron axons in transit or passing through, synapses and astrocytes (protoplasmic). Grey matter also has a good blood supply and contains many capillaries. It is a site of information processing. The organization of neuronal processes and their connections within the grey matter is termed the microcircuitry. Composition of white matter (Mainly projection neurons) -Axons (myelinated and unmyelinated) -Oligodendroglia -Astrocytes (fibrous) -Capillaries some White matter is in general characterized by the axons of projection neurons, which may be myelinated or unmyelinated. White matter also contains oligodendrocytes which provide the myelination. It is a site of information transfer. Connections between several groups of grey matter, with some functional link are termed the macrocircuitry. Common stains used for visualising neurons Haematoxylin and Eosin -Grey matter distribution can be defined but not connections -Cell nuclei the size, numbers, density and distribution of neurons can be approximated and seen -Glial and other cells are also stained (glia in particular cannot be readily differentiated)

7 Nissl - toluidine blue and cresyl violet (basophilic stain) -Nissl stains colour acidic components (DNA, RNA) and thus the nucleus and rer. All cell types of the brain (microglia, macroglia, endothelial cells) contain them and are stained but neuronal somata are MORE prominent due to the rich rer. -Grey matter - organisation -More detail on cell body (particularly nucleus, nucleolus, Nissl substance), location, density -Differentiate axon from dendrite and cell body -Distinguishing neurons from other cell types (glia, and blood vessel-related cells) -Measuring their number, size and visualizing their organization (nuclei or layers, for example) Reduced silver -Grey and white matter -Clearly show axons, dendrites, chase pathways/ connection -The extent to which axons can be traced is limited by the section thickness and process density

8 Golgi -Stains isolated (not obscured by dense fibres) cells completely -Labels an entire cell to enable morphological comparison of different cell types -Enables thick sections to be looked at and gives a clear picture of the cell body shape but not the intracellular structures -Particularly shows the dendritic tree (and sometimes individual cell connections) -Can show synaptic spines and glial cells Fibre -White matter -useful for indicating fibre tracts and white matter in general but not cell bodies Degenerating fibre Stains which detect degeneration in fibres (axons) are useful if pathways can be interrupted experimentally at a known point and the degeneration (after a certain period of time) detected elsewhere. This helps trace the extent of pathways and also the localization of particular pathways within larger regions of white matter such as in the funiculi of the spinal cord. Transition Electron Microscope The electron microscope demonstrates particularly well membranes and intracellular structure. For neurons this includes microtubules, neurofilaments, microfilaments and synaptic vesicles, synapses and relationships with glia such as myelin sheaths. The electron microscope has a limited ability to trace the extent of any particular cell process. Uptake markers One way to reveal particular groups of cells and their connections is to make use of the fact that neurons take up some molecules from their environment and transport them within the cell via the intracellular transport mechanism. Some markers are taken up in the axons and transported to the cell body (retrograde) others by the cell body and transported to the axonal synapse region (anterograde). Using a combination of the two processes it is sometimes possible to show where some cells in a region project to and what cells project to that region (eg the reciprocal pathways of the thalamus and cerebral cortex). Microinjection By using micropipettes marker substances can be injected into individual neurons. The marker travels throughout the cytoplasm and can reveal the appearance of the cell in particular the cell processes much in the way the Golgi stain does. Microinjection is more selective and prior to staining the cells can be recorded from or stimulated and a correlation made between cell shape, location and behaviour. Immunostaining and Histochemistry

9 Neurons and glial cells contain proteins such as structural and receptor proteins, enzymes, nucleic acids and transmitter substances (e.g. synaptotagmin, microtubules, neurofilaments, substance P receptors, glial fibril acid protein) that can act as antigens or take part in chemical reactions. Some of these proteins and molecules are unique to particular groups of cells. They can be visualized by reacting them with antibodies or in chemical reactions and attaching other molecules (e.g. ones which fluoresce) to them which allow them to be seen under the microscope. Several antibodies and reactions can be used on the one preparation so that subpopulations and correlations between populations can be made. Glial stains As mentioned stains such as haemotoxylin and eosin, Nissl stains and reduced silver stains will show glia, in particular their nuclei. Glial cell bodies and processes are either not shown or not well shown. Golgi techniques will show glia in the same sporadic way that it shows neurons. In order to stain glia specialised general stains are required such as phosphotungstic and phosphomolybdic stains and some heavy metal stains involving gold, silver and mercury. Other ways to reveal glial cells involve more recent techniques which use immunological stains and flouresence to detect proteins unique to particular cell types such as glial fibrillary acid protein (GFAP) in astrocytes. Neurotransmitter/peptide labelling -Groups of cells which carry out the same sort of task in a particular system or location tend to have similar profiles of enzymes, neurotransmitters and neuropeptides. These can be visualized using histochemistry or by using antibodies to the molecules of interest. Synapses Neuronal connections Grey matter: agglomerations of neurons, often called nuclei or laminae White matter: myelinated fibres (axons) Structural units of the CNS: Neurons 1) Local neurons, interneurons 2) Projection neurons Neurons: Excitable cells with large nuclei

10 1) Cell bodies (perikarya = region close to nucleus) 2) Neurite = any projection from the cell body -Dendrites never myelinated; receive signal; excitation process towards nucleus -Axons (conduct signals) myelinated; out of cell body Groups of neurons (nuclei) send groups of fibres (axons) projecting toward other neurons: -These are called fibre tracts, often following specific identifiable paths, like highways through brain -Tracts are sometimes called bundles (median forebrain bundle) or fasciculi (fasciculus retroflexus)* -Tracts can project to a nucleus on the same side (ipsilaterally) or cross towards a contralateral target (on the other side of the brain, crossing the midline). The crossing is usually referred to as decussation -Specific types of information (somatosensory, motor, auditory, visual) are usually sent through a series of tracts: pathways 1. Synapses as regions specializing in transfer of signals from one neuron to another Synapses are specialized regions where information passes from one neuron to another. Synapses are most often between: a) Axon terminals (boutons terminal, nerve endings) and dendrites: axodendritic b) Axon terminals and perikarya (perikaryon = neuronal cell body) c) Axon to axon ( axo-axonic, e.g. in case of presynaptic inhibition)

11 2. Definition of excitatory and inhibitory synapses Type of synapses 1) Electrical: gap junctions where current can pass directly from one cell to another -Very fast -Found in retina, developing brain -Transmission may be bidirectional -At an electrical synapse or gap junction there is a free flow of ions between cells such that information may pass in either direction -While gap junctions are uncommon in the CNS, there is an elaborate network of coupled neurons in the retina 2) Chemical: signal is transmitted by a synaptic transmitter ( neurotransmitter ) -At a chemical synapse a neurotransmitter is released from a specific location - presynaptic terminal of one cell and taken up by specific receptors at the postsynaptic specialization located on another cell. -Information passes in one direction, from the pre-synaptic to the post-synaptic cell. These are by far the most common type of synapse in the CNS. Neurotransmitters: -small molecules: amino acids, biogenic amines (but also NO) -larger molecules: peptides ( neuropeptides )

12 Two types of synapses 1) Grey Type 1 (RA) -Round -Asymmetrical: synaptic density is found mostly on the postsynaptic side -Excitatory = depolarising 2) Grey Type 2 (FS) -Flat -Symmetrical: synaptic densities are distributed on both sides of the synaptic cleft -Inhibitory = hyperpolarising -Electron microscopy shows specially impregnated structures ( electron dense ) in dead tissue 3. Synapses have specific functional components 1) Release -Synaptic vesicles come to the nerve ending, fuse with the presynaptic membrane and are ejected into the synaptic cleft through exocytosis -Release from synaptic vesicles -coupled to the presynaptic stimulus -Ca2+ dependent -regulated by many proteins -Synaptic vesicles are recycled and refilled with neurotransmitter -Vesicles are constituted by many vesicle-specific proteins 2) Receptors -Neurotransmitters are recognised by specific receptors -Receptors at postsynaptic membrane translate chemical structure into membrane polarity Ionotropic

13 -Ionic (Transmitter gated) channels open/close to let ions move across the membrane -Very fast change in conformation of protein/ns -Inserted into cell membranes -Ionotropic receptors consist of subunits metabotropic -G-protein operated - either activate G-protein gated ion channels and/or activate second messenger systems inside the postsynaptic cell -Very sensitive 1 molecule can trigger many amplifiers synaptic: within the synaptic cleft presynaptic: on the presynaptic terminal regulate release of NT extrasynaptic (not in synaptic cleft): activated by spilled-over neurotransmitter (NT that leave the presynaptic space and enter the extracellular space without being inactivated) volume transmission (exciting/inhibiting volume of postsynaptic neurons) 3) Inactivation -Released transmitter must be cleared from the synaptic cleft to prevent desensitization of the receptors, or other serious side effects like excitotoxicity -Mechanisms of inactivation of neurotransmitters: -Diffusion away from the cleft -Binding to a specific site on a transporter molecule (active transport) -Re-uptake by the transporter into the presynaptic terminal -Uptake and recycling by glia expressing specific transporter proteins and metabolic enzymes 4. Integration of synaptic inputs into one all or nothing response -Convergence of many axons (and terminals) on one neuron -Integration of synaptic signals: Many excitations and inhibitions will result either in firing or not firing all or nothing response -Excitatory synaptic signals > Inhibitory signals Fire action potential -Inhibitory signals > Excitatory signals NO action potential Glia = Glue -More than 50% of the brain is made up of glia -There are up to 10x more glia than neurons -Each astrocyte may enclose > 2 million synapses -Oligodendrocytes wrap billions of axons -Neuronal energy supply relies on astrocytic interface with capillaries -Blood flow through muscular vessels (arterioles) and non-muscular capillaries is modulated by astrocytes and pericytes -White matter is highly enriched in lipids oligodendrocyte membranes compromise most of this volume