HTHSCI 1DT3 Lecture Notes - Lecture 6: Olfactory Ensheathing Glia, Endothelium, Capillary
23 Jun 2018
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After the neurons migrate to their final positions, they become polarized and extend an axon
towards their target tissues and form synapses (see later lectures). They also develop dendrites at
this point.
Neurons are considered post-mitotic, and once they differentiate into neuronal morphology,
they cannot re-enter mitosis. Therefore, neurogenesis does not continue throughout life (unlike
most other cells of the body).
Therefore, neurons have to last a lifetime, and death/damage to neurons have severe
consequences. They therefore need to be resilient, adapt to longevity and be supported by other
cells.
Neuron Theory: Neurons are discrete elongated cells that communicate with each other via
synapses (Suggested by Santiago Ramon y Cajal, proved Golgi was wrong – who suggested that nerves
formed a syncytium network which was fused together).
Morphology:
Elongated morphology, and very large volume and surface area
Broadly specialized regions:
Soma – (cell body) containing nucleus
Dendrites – receives incoming synaptic transmission from other neurons
Axon – transmits action potentials to target tissue or neurons
The distal tip of a neuron will differ depending on whether the neuron is mature or
developing. If it is developing, the tip of the axon will be a growth cone, rather than a pre-
synaptic terminal (and growth cones can change their shapes according to guidance cues)
Further specialized regions include:
Axon hillock (aka – initial segment)
Myelin sheath
Nodes of Ranvier
Pre-Synaptic terminals
Post-Synaptic specialisations (on dendrites, e.g. dendritic spines)
Myelin
Myelin is formed by oligodendrocytes (CNS) and Schwann Cells (PNS), which wrap around
the axon.
Myelin is rich in specialized lipids, giving it its white appearance (hence ‘white matter’)
Myelin has two main roles:
Protection – of axons (e.g. from toxins)
Fast Conductance – (main role), allows fast transmission of action potentials
Only axons are myelinated, but not all axons are myelinated (e.g. cerebellar granule neurons,
and nociceptive sensory neurons are unmyelinated).
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Neuronal Diversity: Neurons can be classified in multiple ways
Degree of myelination: myelinated (e.g. motor neurons) vs. unmyelinated (e.g. cerebellar
granule neuron)
Direction of signal / Type of target:
Interneuron – in the CNS synapsing with another neuron (e.g. cerebellar granule
neuron)
Efferent CNS Neuron – transmitting action potentials AWAY from the CNS (e.g. motor
neuron)
Afferent PNS Neuron – transmitting action potentials TOWARDS the CNS (e.g. sensory
neuron)
Location of Cell Body:
CNS neurons with cell bodies in the brain or spinal cord (e.g. motor neuron)
PNS neurons with cell bodies outside brain or spinal cord (e.g. sensory neurons in
dorsal root ganglion)
Morphology / Number of Projections from Cell Body:
Unipolar (e.g. sensory neuron) with only a single projection from the cell body
Bipolar (with a single axon and single dendrite – e.g. bipolar retinal neurons)
Multipolar (single axon and multiple dendrites – e.g. pyramidal neuron, Purkinje
neuron, motor neuron)
Synaptic Function
The function of a neuron is to transmit and receive action potentials, or stimulate electrical
activity in a target tissue such as skeletal or cardiac muscle.
When action potentials reach the synapse, neurotransmitters released by a presynaptic terminal
bind to postsynaptic receptors of the neuron/muscle it is synapsing with.
Some neurotransmitters reduce the likelihood of action potential firing by its target
(postsynaptic neuron), which are called inhibitory neurotransmitters which decrease
depolarization (e.g. GABA, dopamine)
Other neurotransmitters increase the likelihood of potential firing by its target neuron, and
these are called excitatory neurotransmitters (increase depolarization) (e.g. glutamate,
acetylcholine)
Glutamergic and GABAergic synapses differ morphologically (See diagram below)
Synapses can form on dendritic shafts, cell bodies or on dendritic spines (though not all
neurons have dendritic spines)
Synapses are important structures, with a distinctive mushroom-like appearance. Note how
weaker synapses are present in mentally retarded patients (as shown below).
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Each neuron will receive many synaptic inputs and form many connections with target cells.
A single neuron may have receptors for multiple neurotransmitters – both inhibitory and
excitatory.
The sum of all depolarizing and hyperpolarizing signals received from activated receptors will
determine whether or not the neuron fires an action potential.
Voltage activated sodium channels (essential for propagating the action potential), are only
presented in the non-myelinated regions of the axons and therefore are clustered at the axon
hillock and the Nodes of Ranvier.
The concentration of ion channels at the axon hillock is important for determining WHEN an
action potential will be fired.
The action potential will be initiated at the axon hillock based on the SUM of all inhibitory and
excitatory input from the other neurons received in the post-synaptic regions on the dendrites.
Clustering of voltage activated sodium channels at the Nodes of Ranvier allows the action
potential to jump from node to node: Saltatory Conduction.
Myelination is important for projection neurons that need to transmit potentials over long
distances.
Intracellular Architecture of Neurons – Similarities with traditional cell structures
The essential cellular contents of a neuron are much the same as other cell types (nucleus,
cytoskeleton, endoplasmic reticulum, golgi, mitochondria etc.)
However, neurons tend to have less cytoplasm that other cells (e.g. astrocytes), and often
smaller cell bodies.
Intracellular Architecture of Neurons – Differences from traditional cell structures
One of the major differences between neurons and other cells is their elongated shape.
However, this is a particular problem for:
Intracellular transport
Protecting the potentially fragile axon
Therefore, efficient transport mechanisms (below), along the axon is essential
Anterograde (towards growth cone/synapse) and…
Retrograde (away from growth cone/synapse)
The microtubules are pivotal for:
Retrograde transport of signaling information from the growth cone to the nucleus
Anterograde transport from the Golg, of large quantities of lipids and proteins required
for axon extension, neurofilaments, neurotransmitters etc.
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Document Summary
After the neurons migrate to their final positions, they become polarized and extend an axon towards their target tissues and form synapses (see later lectures). Neurons are considered post-mitotic, and once they differentiate into neuronal morphology, they cannot re-enter mitosis. Therefore, neurogenesis does not continue throughout life (unlike most other cells of the body). Therefore, neurons have to last a lifetime, and death/damage to neurons have severe consequences. They therefore need to be resilient, adapt to longevity and be supported by other cells. Neuron theory: neurons are discrete elongated cells that communicate with each other via synapses (suggested by santiago ramon y cajal, proved golgi was wrong who suggested that nerves formed a syncytium network which was fused together). Elongated morphology, and very large volume and surface area. Dendrites receives incoming synaptic transmission from other neurons. Axon transmits action potentials to target tissue or neurons.
