Neurones and Astrocytes Flashcards
What is the difference between the CNS and PNS?
The difference between the PNS and CNS is about where the cell bodies are.
• Peripheral neurones have their cell bodies outside the brain or spinal chord
• Central neurones have their cell bodies within the brain and spinal chord.
What are the characteristics of neurones?
Features of Neurones
• Large nucleus (the biggest neurones probably have the largest nuclei in the body)
• Prominent nucleolus
• Abundant RER
• Well developed Golgi, in keeping with high amounts of protein trafficking via the secretory pathway.
• Abundant mitochondria
• Highly organised cytoskeleton
• Highly organised metabolically active cell.
What are the ways neurones can be classified morphologically?
Can be classed based on the number of neurites:
• Unipolar are extremely rare in vertebrates, and are mainly found in insects.
• Multipolar neurones (have multiple dendrites) are the most common in the brain and spinal chord.
• Bipolar neurones are typically sensory neurones.
○ Retinal bipolar cells, sensory cells of the cochlea, and vestibular ganglia are included in this category.
• Pseudo-unipolar neurones (1) have one axon that branches into two - a peripheral and central branch. They are typically sensory neurones that detect pressure and pain. The axon closest to the receptor is the trigger zone for the neurone. The cell body lies within the dorsal root ganglion. The central axon passes into the dorsal (posterior) horn and synapses with another neurone, while the peripheral axon travels through the spinal nerve until it reaches the skin, muscle or joint.
Neurones can also be classed based on their axon length:
• Golgi type I neurones are (multipolar) neurones that connect one part of the brain to another, and so have axons that extend long distances. Examples of these include:
○ Pyramidal cells of the cerebral cortex
○ Purkinje cells of the cerebellum
○ Anterior horn cells of the spinal chord
• Golgi type II neurones are (multipolar) neurones that connect as part of a local circuit, and so have a short axon. Examples include:
○ Stellate cells of the cerebral cortex and cerebellum.
What are the different ways neuronal circuits can be organised?
- Divergence: spreading stimulation to multiple neuronal pools in the CNS
- Convergence: a mechanism providing input to a single neurone from multiple sources
- Serial processing: when neurones or pools work together in a sequential manner.
- Parallel processing: when multiple neurones or pools of neurones process information simultaneously
- Reverberation: a feedback mechanism that may be excitatory or inhibitory
What is meant by saying neurones are polarised cells?
Neurones are polarised cells. This does not refer to the electrical polarity, but to the fact that there are molecularly distinct compartments.
How is the polarity of a neurone determined?
Neurones initially extend several short neurites (term used to describe both axons and dendrites). The one that starts growing faster becomes the axon, while the others develop into dendrites. The axonal compartment is kept separate from the somato-dendritic compartment. This is important as certain proteins, complexes and cargos must be directed to particular regions of the neurone. For example, the machinery for neuro-exocytosis must be directed to the synapse, sodium channels to the Nodes of Ranvier, and receptors involved in axon growth and guidance must be directed to the growth cone etc.
Give two examples of somato-dendritic specific components
Examples of somato-dendritic specific components include:
• Microtubule stabilising protein MAP2B.
• All the neurotransmitter receptors, postsynaptic density (PSD) scaffolding and signalling proteins required at the post-synaptic area.
Describe the organisation of the neuronal receptive/input field
Dendrites are the major area of reception of incoming information. They spread from the cell body and branch frequently, greatly increasing the surface area of the neurone. Primary dendrites are those directly connected to the cell body, while secondary dendrites are those attached to primary dendrites.
They are often covered in protrusions called spines. The formation of these spines depends on external signalling.
What is the most plastic part of the CNS?
Dendritic spines
What are the different morphologies dendritic spines can assume?
- Stubby
- Thin
- Mushroom
- Filopodium
Give three examples of axon specific components
Axon-specific components include:
• Neurofilaments are only present in the axon where they are important for strength.
• Microtubule stabilising protein tau
• Cell adhesion molecules L1 (NgCAM), TAG-1.
Describe the features of an axon
Axons conduct impulses away from the cell body, emerging away from the axon hillock. There is usually only one per cell, but may branch after leaving cell body into axon collaterals.
They have prominent microtubules and neurofilaments. They can be myelinated or unmeylinated. Has a large number of mitochondria to maintain membrane potential.
What are the domains of a Node of Ranvier?
In order to maintain action potential condiction at high speed, the axonal membrane is organised into specific domains: juxtaparanode, paranode and node (of Ranvier)
What proteins are important in maintaining the Node of Ranviers
There is an array of proteins important for maintaining this system:
• Na+ channels are achored to actin filaments by various proteins such as Ankorin B.
• Cell adhesion molecules bind loops of oligodendrocytes to the axon - to form a tight structure - this is to 1) stop leakage of current and 2) stop proteins moving from node of Ranvier to myelinated axon.
○ Animal models without this tight adhesion can lead to severe neurological disorders
○ These adhesions can be lifted in MS
• K+ channels in the paranode?
What are the two types of synapses?
Axons often branch extensively close to their target, forming a terminal arbour. They form synaptic terminals with target. These synapses can either be boutons or varicosities.
