Cells of the nervous system Flashcards
Neuron organisation: recall the definition, structure, and function of a neuron (including nucleus, soma, dendrites, axon, synapses, lamina, synapses and fibre tract) Neuronal cytoskeleton: recognise the key components of the neuronal cytoskeleton Synapses: explain the distinguishing features of axo-dendritic, axo-somatic and axo-axonic synapses Neuroglia: list the main classes of neuroglia and explain their functions in the nervous system
What are the main functions of a neurone? (x3)
Information processing unit. Responsible for generation and conduction of electrical signals. Communication between areas of the body and between neurones.
What are the features of a neuronal cell? (x6 – why for x3+1)
CYTOPLASM IS VERY PACKED – indicates complexity of the neurone. Prominent nucleus – presumably because of amount of protein synthesis that occurs? Prominent nucleolus. Abundant RER – because neurone is larger than most cells because of all its processes (extensions). Well-developed Golgi (very large – indicated by the photo) – because neurones are secretory cells and so proteins need to be packaged and sent to the relevant areas of the cell. Abundant mitochondria – because highly metabolically active. Highly organised cytoskeleton.
What do dendrites do? Structure? (x2 points)
Major area of reception of incoming information. Spread from cell body and branch frequently. To increase surface area of the neuron so can receive more inputs. Often covered in protrusions called spines. Dendritic spines receive the majority of synapses.
What is a dendritic spine?
A small membranous protrusion (lump) from a dendrite that receives input from a single axon (through a synapse). Each spine can receive several inputs.
What is the basic structure of a dendritic tree in a pyramidal cell?
Pyramidal cells are a type of neurone found in the brain. Primary dendrites are the three thick dendrites that leave from the poles (the one that emerges from the top is called the apical dendrite. The others are called basal dendrites and there can be more than two). Each branch into secondary dendrites. Then tertiary dendrites. An axon also leaves the pyramidal cell from the bottom also. They all originate from the soma (cell body) – indicated by the green triangle.
What is plasticity? What area of the CNS is most plastic? How is this?
The ability of nervous system to be functionally modified as a result of repetitive activation. It describes how adaptive something is. Dendritic spines are the most plastic elements of the nervous system. We can withdraw or produce spines depending on functionality, and they can also be destroyed from too much alcohol, for example.
What is the function of the axon?
Conduct impulses away from soma.
What is the structure of an axon?
USUALLY only one per cell. Emerges at the bottom of the pyramid. May branch (into axon collaterals) after leaving the cell body and at the target. Emerge at an area called the AXON HILLOCK. Very prominent micro-tubules and neurofilaments.
How do axons differ structurally from dendrites? (x3)
Axons smaller. Axons do not branch extensively. Axons do not vary in diameter – dendrites get thinner the further away from the soma.
What are the areas of exposure in a myelinated axon called?
Nodes of Ranvier.
What is meant by ‘axons have CABLE PROPERTIES’?
They need to maintain the same speed of conduction. Hence, they are the same diameter. Without this, they would not be able to conduct impulses as quickly.
What is the structure of the node of Ranvier?
Myelinated region of axon just before node contains potassium channels (function not really known) = juxtaparanode. Area where the end loops of the myelin (produced by the oligodendrocytes) are attached to the axon = paranode. This precedes and succeeds the node of Ranvier.
What are the two types of axon terminal?
BOUTON – occurs at the end of an axon as we would usually expect. VARICOSITIE – axon swells along the neurone and neurone affects target as impulse passes through it e.g. in smooth muscle.
What is the synaptic density?
Synaptic density is dense on the electron microscope because of increased number of proteins in the area. Examples of proteins: proteins responsible for machinery involved in exocytosis, receptors, protein channels, ion pumps. Found on the post-synaptic membrane across the area concerned with the synapse.
How are the synaptic vessels processed and used in a synapse?
Packaged in the Golgi and transported by fast anterograde transport. Synaptic vessels become associated with the plasma membrane ready for release by specialised mechanisms. LOTS of mitochondria for ion pumping and synaptic transmission. They are therefore very sensitive to O2 deprivation because of high metabolic activity.
What is the mechanism by which a single output is created for the axon from multiple inputs from dendrites?
NEURONAL INTEGRATION. Soma receives lots of inputs from a variety of chemical transmitters (excitatory, inhibitory…). Competing inputs are integrated in the postsynaptic neurone.
What are the types of synapse? (x3) (x2 points for each)
Axo-dendritic: usually EXCITATORY and most common. The synapse occurs between an axon and dendrite. Axo-somatic: usually INHIBITORY. Synapse occurs between an axon and the cell body of another neurone. Axo-axonic: usually MODULATORY. Synapse between two axons.
What is the neuronal cytoskeleton composed of?
Microfilaments, intermediate filaments and microtubules.
What is the function of intermediate filaments in the neurone? Alternative name?
ALSO CALLED NEUROFILAMENTS. Maintain tensile strength of the neurone. Required to maintain precise structure of same diameter. Tight bundles of fibres are crosslinked to maintain the structure. It also dictates the axon caliber (thickness) which determines speed of transmission.
What is the function of microfilaments in the neurone? (x2)
Plays some role in providing mechanical strength for the cell. Main function is controlling processes like exo/endocytosis.
What is the function of microtubules in the neurone?
Enable migration of organelles and components down the membrane e.g. mitochondria at axon terminals for synapses.
How are proteins synthesised in the soma of a neurone transported to the relevant location in the cell?
Proteins are packaged into single vesicles. These, or organelles, are attached to microtubules by motor proteins, which walk down the microtubule. They are transported based on polarity: some travel from the positive to negative end of the microtubule, and some in the opposite direction.
What are the two types of neuronal transport? How do they differ? (x2)
Retrograde: proteins transported from the periphery of a cell (e.g. axon terminal), back to the soma (e.g. for degradation because they’ve come to the end of their life). Anterograde: proteins are transported away from the soma (e.g. synaptic vessels containing neurotransmitters are synthesised in the soma and transported to the axon terminal). The two ways of movement are morphologically and biochemically distinct from each other.
What are the morphological subtypes of neurones? (x4) Example for each?
- Pseudounipolar: e.g. sensory neurons in skin: have two fused processes (extensions) which are axonal in structure i.e. a pseudounipolar neuron has one axon with two branches: central and peripheral. These axonal branches should not be confused with dendrites. These sensory neurons are an exception to the typical neuron, in that they do not have separate dendrites and an axonal process, but rather one branched process that serves both functions.
- Bipolar: e.g. retinal bipolar cells: Have two processes that extend from one soma.
- Golgi type I multipolar: e.g. pyramidal cells of the cerebral cortex: not named after the Golgi apparatus. Highly branched dendritic trees, and the axons extend across long distances.
- Golgi type II multipolar: e.g. stellate cells of the cerebral cortex: highly branched dendritic trees but with SHORT axons. They modulate the main output neurones.
What are interneurons?
Link other neurones. Found in the CNS. Modify, coordinate, integrate, facilitate and inhibit sensory input. Basically like memory neurones.
What are neuroglia and name types? (x7)
Support cells of the nervous system. Astroglia, oligodendroglia, microglia, immature progenitors, ependymal cells, Schwann cells, satellite glia.
What is the structure of astroglia? (x3)
Much like ASTRONOMY – they are star shaped and are the most numerous cell type. Also called an astrocyte. Contain lots of intermediate filaments in cytoplasm (in FIBROUS astroglia aka. GFAP) – related to function (remember that IFs are for mechanical strength). Have gap junctions between adjacent astroglia cells which suggests that they signal to one another – they are electrically active but do not produce action potentials.
What are the functions of astroglia? (x9) SBTSNFSPS
- Scaffold for neuronal migration and axon growth during development. 2. Form part of blood-brain barrier. 3. Transport substances between blood and neurones (linked to above). 4. Found in between neurones at the synapse to control the process. 5. Involved in removal of neurotransmitters (linked to above). 6. Synthesis of neurotrophic factors which help neurones. 7. Neuronal-glial and glial signalling (via gap junctions) – obviously. Usually for spread of signal. 8. Potassium ion buffering. 9. Glial scar formation. SBTSNFSPS
What is the structure of oligodendroglia? (x4)
Small spherical nuclei. Few thin processes. Prominent ER and Golgi. Look very dark.
What are the functions of oligodendroglia? (x1)
Production and maintenance of myelin sheath which insulates neurones.
What does myelin do?
Lipid-rich insulating membrane. Seen as dark and light bands in an EM. Loss of oligodendroglia and myelin has disastrous consequences. Slow transmission and pink-looking brain. Diseases include Multiple Sclerosis (MS).
How do microglia develop?
Derived from the bone marrow in early development. Unlike everything else which is derived from the brain in development.
What is the function of microglia?
They are the resident macrophage population for the CNS which are involved in immune surveillance. They present antigens to invading immune cells. They also play a role in tissue modelling by removing various processes and unnecessary synapses.
How do microglia exercise their role of immune surveillance?
They survey using their processes. When it detects something, it becomes activated: soma enlarges, and processes shorten. It becomes phagocytic when it needs to clean up dead matter and debris: here, it loses its processes, and soma is large.
What is the function of Schwann cells?
Carries out role of astrocytes. They also carry out the role of oligodendrocytes and produce myelin. They are phylogenetically more simple (haven’t evolved as fast) – each Schwann cell produces only one myelin sheath.
Where are Schwann cells found?
They are found in the peripheral nervous system. In the central nervous system, the role of the Schwann cell is instead carried out by oligodendrocytes and astrocytes.
What would be the consequence if the CNS was myelinated by Schwann cells?
Because Schwann cells produce only one myelin sheath, many cells would be needed in the brain, which we simply don’t have room for. So, our CNS would essentially by very large.
How are axons packaged into nerves?
Spinal nerves contain both sensory and motor neurones (and therefore afferent and efferent axons). Individual axons are wrapped in myelin (though not all e.g. nociceptive (pain) neurones) and endoneurium (connective tissue around the myelin sheath). These nerves are bundled into fascicles surrounded by perineurium. The whole nerve is in a tough epineurium capsule, containing blood vessels also. When the nerve reaches the spinal cord, it splits off to a dorsal and ventral root.
