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- Neuron = functional unit of nervous system
- Excitable - respond to stimuli
- Transmit information from one part of the neuron to the other
- Soma (cell body)
- Dendrites = processes from the soma, receptive part, for synapses
- Axon = thicker at the beginning than these other processes, and doesn't branch. Can be 1.5m long. Surrounded by myelin sheath, which is segmented. When the axon (conducting part) reaches its target, it breaks up into branches (telodendria), and the swellings on the ends of those are axon terminals (that make synaptic contact with another neuron)
- Dendrites = increase SA to receive lots of impulses. There are dendritic spines projecting from dendrites to increase surface area.
Morphological types of neurons
- Unipolar neuron
- Bipolar neuron
- Multipolar neuron
- Pseudounipolar neuron
- The cells that carry information about a) hearing and balance b) visual are bipolar neurons.
- Pseudounipolar neuron = modified bipolar neuron; the axon going to the soma is a double conducting pathway
- Cell body of pseudounipolar neuron is in DRG
- Most axons are covered by myelin sheath
White v grey matter
- Grey matter = cell bodies.
- White matter = axons.
- Cerebral cortex = 2-3mm of grey matter, beneath which is lots of white matter.
- Staining for myelin --> white matter stains black and grey matter stays unstained
- Nucleus = group of cells in the grey matter, and have similar function
- Tract = groups of fibres clumped together
- Neurons have a lot of endoplasmic reticulum because of their protein synthesis, which is usually arranged on laminae, which is very dense (Nissl substance, which characterises neurons)
- Glial cells surround the neuron, and support its metabolism
- Nissl stain locates nuclei and extends out into proximal parts of the dendrites - doesn't tell us much about the shape of the neuron, just its size
- Golgi staining shows dendritic morphology, which varies depending on the types and functions of the neurons
- Top right = retinal ganglion cell, which is multipolar with radiating dendrites
- B = purkinje cells in the cerebellum, which has a large dendritic tree in 2D, because they receive many synapses
- Cerebral cortex = pyramidal cells. Dendrites come out from sides of cell bodies, and have an axon that always goes up and is always perpendicular to the brain surface. These are the output cells of the cerebral cortex, important in the motor system.
- Phospholipid bilayer
- Ion channels embedded in that layer, to allow ions to move across
- Usually a centrally-located nucleus
- It has a centrally-located nucleolus to produce transcription proteins
- Chromatin is scattered throughout the nucleus
- rER, throughout the cell and beginnings of the dendrites as well - lots of proteins produced, using lots of energy
- mitochondria extend throughout the axons and down to axon terminal as well
- golgi = package proteins to transport to other parts of the neuron
- dendritic spines
- Neurons are very large cells, and have a well developed cytoskeleton, formed by neurotubules and neurofilaments
- Microtubule = 20nm, made of tubulin
- Neurofilament = 10nm
- Microfilament = 5nm, made of actin, help with growth of neurons
- Neurofilaments are important for axoplasmic transport
- Neurotubules form railway tracks for axoplasmic transport; involving kinesin and dynein
- Anterograde - away from the soma e.g. neurotransmitter proteins; towards axon terminals
- Retrograde - towards the soma e.g. metabolic byproducts
- Some move slowly (2mm/day), and neurotransmitters move quickly (400mm/day)
- Use axon transport to work out where neurons start and finish (inject a tracer that is transported back to the cell body, and label the cell body), and around the other way. This is important for working out how structures interconnect in the nervous system
Section through dendrite
- Few organelles, lots of neurotubules, microtubules (transport) and neurofilaments (skeleton)
Myelin sheath - PNS
- In PNS, myelin sheath is produced by Schwann cells (a type of glia - supporting cell in NS)
- Outer membrane, as it develops, wraps around and around the neuron, and its membrane forms the myelin sheath
- The myelin sheaths are separated by Nodes of Ranvier
- The area between two nodes is called an internode
- In the PNS, one Schwann cell forms a single internode
- Myelin sheath acts as an insulator to facilitate conduction along the neuron
- Bigger axon = thicker myelin sheath = faster conduction velocity (e.g. fastest = motor neuron). Range of conduction velocities = 0.1 m/sec to 80 m/s; reflected by thickness of myelin sheath; thickness used in EM to identify axons
- Cytoplasm of Schwann cell surrounds the sheath
Myelin sheath - CNS
- Sheath is produced by oligodendrocyte
- Sheath produced looks similar to Schwann cell sheath
- The sheath of the oligodendrocyte has many branches, so can form sheaths for multiple axons from a single oligodendrocyte
- Cut up a PNS nerve, then the Schwann cells can regrow myelin sheaths and form the nerve again. But in the CNS, the axons can't recover because the oligodendrocyte cannot reform the sheath.
- Some axons are unmyelinated - usually very small and are slow (no myelin insulation). Axons become embedded within the cytoplasm of an oligodendrocyte or Schwann cell - they have 1-2 layers of membrane surrounding them. They are very slow (mostly pain fibres)
- Nearly all synapses are chemical
- At terminal of axon, there is a swelling, packed with synaptic vesicles (neurotransmitters) and mitochondria and other organelles. There is also a thickening on presynaptic surface
- If the action potential is strong enough to generate an impulse, it causes changes (Ca channels open etc), and the synaptic vesicles are exocytosed. There is a thickening on the presynaptic membrane and the vesicle is released into synaptic cleft.
- On the opposite side of the synapse there is a postsynaptic thickening with postsynaptic receptors (on postsyn. dendrite).
- This will either cause excitation or inhibition of the postsynaptic neuron
- Most common neurotransmitters:
- GABA - inhibitory
- ACh and glutamate - excitatory
- NB: the thing that determines whether it excites or inhibits is the receptor, not the neurotransmitter. Therefore the same neurotransmitter can either inhibit or excite
Types of chemical synapses
- Most occur on the dendrites, which are called axodendritic synapses (aka axospinous)
- They may also go elsewhere (e.g. directly on cell body), or onto other axons
- There is an area at the beginning of the axon (axon hillock) has no organelles and is the trigger zone of the action potential (no Nissl substance).
- If stimulus is great enough, it will trigger the AP from there, and it will travel down the axon.
- Look for thickenings -- all the thickenings represent chemical synapses
- Axon is packed with synaptic vesicles and mitochondria
- Looking closely, you can see synaptic space
- Gap junctions = electrical synapses: two cells are electrically coupled (these are very rare in mammalian NS)
Glia - astrocytes
- 90% of the cells in the NS are glial cells. There are about 100 billion neurons
- Glia also includes Schwann cells and oligodendrocytes
- Astrocytes - lots of processes from surfaces (star shaped). Wrap themselves around neurons and blood vessels. Nourish neurons, maintain electrolyte balance, repair damaged tissue (scarring in brain damage), contribute to BBB, aid neuronal growth and synapse formation in developing neural tissue. (Most places in brain, capillaries are not fenestrated - selective active transport of some substances)
- BBB stops any change in brain internal environment (don't want to affect brain).
- Develop from the neural tube, like oligodendrocytes
Glia - microglia
- Migrated into NS with blood vessels (not from neural tube)
- Phagocytes to engulf and remove invading organisms, scattered throughout NS
Glia - ependymal cells
- Developed from neural tube
- Line the central hollow portions of the CNS (ventricles of the brain, central canal of spinal cord. Cilia aid circulation of cerebral spinal fluid in the cavities).
- Epithelial-type lining. Separates brain tissue from ventricles. Fused by tight junctions. To cross this barrier, need active transport (selective transport of substances into brain.