Neurons & the Neuroglia Flashcards
What are the 3 main functions of a neuron?
- Receiving information
- Conducting information
- Transmitting information
Describe the path of a signal through a ‘typical’ neuron
- Received via the dendrites from the presynaptic nerve fibres
- conducted through the axon
- transmitted via synaptic boutons
What is contained within the cell body of a neuron?
- Extensive RER as many proteins need to be made, called the ‘Nissl substance’
- Large nucleolus as neurons are secretory and hence carry out a lot of transcription
Describe the dendrite structure
- Branch repeatedly giving a large surface area for receiving/transmitting information
- Contain RER and SER at the base, as well as mitochondria, free ribosomes, microtubules and microfilaments
- Also contain small spines on which synapses form, that have ribosomes and ER at their base
What is the number of dendritic protrusions influenced by?
Disease and input
Describe the structure of the axon hillock
- Lacks ribosomes but contains bundles of microtubules and neurofilaments
- Contains voltage-gated Na+ channels and other transmembrane proteins
- Ends at the start of myelination
What is contained within the axon process?
Normal organelles but has no ribosomes, can be myelinated
Describe the structure of synaptic boutons
- Store neurotransmitter in secretory vesicles
- Have mitochondria but no microtubules
- Have pre-synaptic and post-synaptic side
- Connecting membrane contains a large ammount of protein called the post synaptic density (PSD)
What are the most/least common types of synapses?
Most common: axons & somas, 2 axons, axons & dendrites
Least common: dendrites & soma, 2 dendrites
What is the role of growth cones?
- Ability to be very motile and detect environmental cues
- Signals then used to determine rate and direction of neuron growth
Describe the structure of growth cones
- Flattened tip with thin fan-shaped sheet (lamellipodia) and many spikes radiating forward (filopodia)
3 domains:
1. Peripheral (P) domain -> lamellipodia and filopodia
2. Transitional (T) domain -> interface of the P & C domain
3. Central (C) domain -> thicker region containing organelles and vesicles
What organelles are contained in the growth cones?
- P domain contains actin filaments (devoid in C domain)
- C domain contains mitochondria (energy), endosomes (recycling), vesicles, poly-ribosomes (local protein synthesis)
- neurofilaments extend as far as the axon
- microtubules extend further into the lamellae
What are the 3 stages of axon growth?
- Protrusion: rapid extensions of filopodia / thin lamellae containing actin
- Engorgement: microtubules invade protrusions bringing vesicles and organelles
- Engorgement: microtubules invade protrusions bringing vesicles and organelles
How are proteins transported through long neurons?
- Transport retrograde
- Local synthesis
How do growth cones travel over long distances?
- Guided by a succession of positive and negative cues
- Up to 50% of neurons die after reaching their goal (competition?)
What can act as an attractant/repellant signal?
chemical - growth factors - substrates - integrins/cadherin physical - tactical - electrical - photonic
How are neurons classified
- number of neurites
- bipolar/unipolar/multipolar
- types of connections (sensory/motor/interneurons)
- type of neurotransmitter
- axonal length
Name the 8 major neurotransmitters
- Acetylcholine (Ach)
- Noradrenaline
- Adrenaline
- Dopamine
- 5-HT, serotonin
- Glutamic acid
- Gamma
- Glycine
What are the 2 largest neurotransmitter receptor families?
- Ligand-gated ion channels (ionotropic receptors)
2. G-protein-linked receptors (metabotropic receptors)
How does the neuromuscular junction (reflex) ensure efficient transmission?
- Junctional folds on post-synaptic side of the NMJ increase the number of Ach receptors exposed to the transmitter
- Large action potential on the motor end-plate
What is myasthenia gravis?
- autoimmune disorder causing a lack of enzymatic degradation of Ach by acetylcholinesterase
- Ammount of Ach released gradually declines leading to muscle weakness and fatigue due to lack of muscle action potential and contraction
What type of cells form 90% of the nervous system?
The neuroglia
What are the 3 main types of neuroglia?
- Astrocytes
- Microglia
- Oligodendrocytes/Schwann cells
Describe the structure of the astrocytes and where are they found?
- Highly branched with the white matter being called fibrous and the grey matter termed protoplasmic
- Have gap junctions between themselves to propagate their own signals
- Specialised intermediate filaments containing specialised protein markers
- Make up 20-50% of the CNS
What are the 4 functions of the astrocytes?
- Separate nerve cells and oligodendrocytes maintaining the integrity of the nervous system
- ‘end-feet’ covering most free space around synapses and ‘cuffs’ around nodes of ranvier
- Buffer against extracellular K+, other ions, and pH fluctuations
- Provide homeostasis via the detoxification of the nervous system
How do astrocytes affect synaptic transfer?
Play key role in glutamate recovery
- Glutamate converted to glutamine via action of glutamate synthase and ammonia
- Glutamine taken up by neurons and converted back into glutamate
How do astrocytes affect the development and growth of neurons?
- Produce cytokines and growth factors affecting the proliferation, differentiation and survival of neurons
- Produce cell adhesion molecules which guide the development of neurons
How does the role of astrocytes change from the embryo to in adults?
Embryo - astrocytes develop from the radial glial which span the brain parenchyma forming a framework for neuronal migration.
Adult - radial glial develop to form the progenitors of
astrocytes. Some cells left and are called Bergmann glial and Muller cells
What can result from proliferate hypertrophic and
hyperplastic ‘astrogliosis?
Astrocytic scars which can form a physical barrier for neuron cell growth
What role do astrocytes have in the blood-brain barrier?
Responsible for inducing and maintaining the tight junctions in the endothelial cells that form the barrier
Describe the structure of the microglia
- Have rod-like somas and multiple processes
- Present in both grey and white matter
- Do not seem to form networks (such as in astrocytes)
How is it thought that the microglia develop?
Proposed that they are made in the bone marrow and migrate to the brain where they are trapped by the blood-brain barrier during early development
How do microglia respond to disease?
- Proliferate and migrate to the injury site, here they change morphology and have ticker processes and larger cell bodies
- Express MHCI and MHCII to T cells, produce cytokines (Interleukin I and tumor necrosis factor alpha)
- Act as macrophages, phagocytose cell debris
What is the function of an oligodendrocyte (CNS) and where are they found?
- Found mostly in white matter
- Can myelinate several axons
What are the functions of Schwann cells (peripheral nervous system)?
- Myelination (1=1)
- separate neurons to isolate them from the extracellular environment
- secrete laminin, fibronectin and collagen: components of the basal lamina and extracellular matrix
- produce growth factors and cytokines
When does myelination occur during development?
late embryonic gestation and post-natal, with CNS caudal to rostral and after PNS.
What is the name given to a segment of the myelin sheath?
Internode (separated by the nodes of ranvier)
What are myelinated cells in the PNS bundled by?
The perinurieum
Describe the process by which the myelin sheath forms
- Schwann cell/oligodendrocyte makes contact with the axon
- Process forms a cup around the point of contact whose two lips advance around the axon until they meet (mesaxon)
- One then passes underneath the other to become the inner leaf which then rotates around the axon to form the multiple layers
- Compaction where cytoplasm is squeezed out
- Myelin sheath extends lengthways to form an internode
What is the function of the structure of the myelin sheath?
To produce a region of high resistance where ions cannot leak out (for saltatory conduction)
