Download Composition of the Nervous System

Composition of the Nervous System
▪Brain
-100 billion neurons and 1000-10000 synapses per neuron
-20% of body’s oxygen (94% used by grey matter)
-15-25% 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)
-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
[Ca2+] = 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
•Neuronal membrane regulation of permeability and intracellular environment
1) Passive segregation – lipid solubility
-O, N, CO2 and alcohols are lipid soluble. They freely diffuse through the cell membrane
-Even though H2O is lipid insoluble, H2O 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)
-Ca2+ pumps keep Ca2+ concentration very low inside cell – Ca2+ 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
▪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
Light Microscope
1mm-1μm
Electron Microscope
1μm-1nm
Molecular
Nerve Root
Spinal nerve
Peripheral Nerve
Nerve Cell
Myelin Sheath
Paccinian Corpuscle
Membrane Details
Synaptic Vesicles
Mitochondria
Ion Channels
•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
-3+ processes = Multipolar neuron – interneurons, principal neurons, lower motor neurons
-Other
-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
Dendrites (short processes hence stay with cell bodies in
grey matter)
Axons
Projection/ Golgi 1
Interneuron/ Golgi 2
Projection
Interneuron
Projection, proximal, distal, intransit
Interneuron complete
Synapses
Astrocytes (protoplasmic)
Capillaries many – rich blood supply
(Mainly interneurons)
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)
▪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
▪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
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
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)
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”)
▪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
-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