23.1 Neural Circuit Construction Relies on Basic Mechanisms of Cell Polarity
Learning goal: Is familiar with neuronal polarization.
Answer: Neuronal polarization is the process by which a postmitotic neuroblast breaks its initial
symmetry to establish distinct axonal and dendritic domains. It is a critical first step in neural circuit
formation. After neurogenesis, the immature neuron extends multiple neurites—initially
undifferentiated processes.
Local intracellular cues and external signals lead to one neurite acquiring axonal identity, while the
others develop into dendrites. This polarization enables the neuron to differentiate functionally
and structurally into compartments for signal reception (dendrites) and transmission (axon).
Learning goal: Describe the roles of cytoskeletal organization and intracellular trafficking in neuronal polarization.
Cytoskeletal organization:
The cytoskeleton, composed primarily of microtubules, actin filaments, and intermediate filaments,
provides the structural framework for establishing and maintaining neuronal polarity.
Microtubules are aligned along the axon and dendrites, serving as tracks for directed cargo transport. In
axons, they are oriented with their plus ends outward, supporting anterograde transport. Actin filaments are
concentrated at the growth cones and branching points, playing a critical role in pathfinding and neurite
extension.
Intermediate filaments, especially neurofilaments, provide structural stability, particularly in long axons.
PAR proteins (e.g., Par-3) are crucial for establishing polarity. They localize to the future axon and coordinate
cytoskeletal remodeling. Disruption of PAR proteins leads to failure in specifying a single axon, resulting in
multiple processes acquiring axonal traits.
Intracellular trafficking:
Intracellular trafficking delivers proteins, lipids, organelles, and mRNAs from the cell body to axons and
dendrites. Kinesin motor proteins mediate anterograde transport along microtubules toward the axon terminal, carrying materials like
mitochondria and synaptic vesicles.
Dynein motors mediate retrograde transport, moving endocytic vesicles and degradation products back to the soma. Myosin motors interact
with actin to support local transport in growth cones, dendrites, and dendritic spines. Dendrites have unique Golgi outposts and satellites that
locally support protein trafficking and postsynaptic site assembly.
This trafficking is energy-intensive and vital for growth, polarization, and synapse formation. Disruption can lead to neurological disorders like
epilepsy and intellectual disability.
23.2 Neural Circuit Construction Relies on Basic Mechanisms of Cell Polarity
Learning goal: Can describe the process of axon outgrowth and know the associated molecules.
Process of axon outgrowth:
• Initiation: Once an axon is specified, it extends from the neuronal cell body, navigating complex terrain to reach specific targets.
• Growth Cone Role: The growth cone, a motile structure at the axon's tip, leads axonal extension by exploring the extracellular
environment and directing growth.
• Extension: As the growth cone advances, new cellular material (membrane, cytoskeleton) is added, elongating the axon.
Associated molecules and structures:
• Cytoskeletal components:
o Actin: Found in lamellipodia and filopodia; regulates shape and directional movement.
o Tubulin: Forms microtubules that support axon elongation and intracellular transport.?
• Binding proteins:
o Actin-binding proteins: Modify actin dynamics; help anchor actin to membrane and promote vesicle fusion.
o Microtubule-binding proteins: Stabilize axonal microtubules via tubulin modification.
• Motor proteins: Transport vesicles and proteins along the axon.
• Signaling molecules: cAMP, cGMP, and intracellular Ca²⁺ regulate cytoskeletal rearrangements via second messengers.
• Receptors and ion channels: Concentrated in lamellipodia and filopodia to detect environmental cues.
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,Learning goal: Knows the different structures of the growth cone and the molecular basis of growth cone motility
Structures of the growth cone:
• Lamellipodium: A broad, sheetlike extension at the tip of the axon. Contains
both filamentous and non-filamentous actin.
• Filopodia: Thin, finger-like projections from the lamellipodium. Rich in F-actin,
used to "probe" the environment.
• Axon shaft: Contains stable microtubules (acetylated tubulin) that provide
structure and transport.
Molecular basis of motility:
• Actin dynamics:
o Polymerization of G-actin into F-actin at the leading edge drives protrusion.
o Depolymerization in response to repulsive cues allows retraction.
o Actin-binding proteins regulate this balance and help localize receptors.
• Microtubule dynamics:
o Dynamic (tyrosinated) microtubules extend into the lamellipodium.
o Stabilization of direction occurs via polymerization of tubulin in the axon shaft.
• Calcium signaling:
o Ca²⁺ influx via voltage-gated or TRP channels influences cytoskeletal dynamics.
• Vesicle transport:
o Vesicles containing membrane components travel via microtubules and associate with actin in filopodia to support membrane
expansion.
Learning goal: Knows 6 different cues that growth cones can navigate to the correct location: attractive and repulsive diffusible molecules,
extracellular matrix adhesion, fasciculation, cell surface adhesion and contact inhibition.
Answer: Growth cones interpret environmental cues to determine their path. These include:
• Attractive diffusible molecules: Secreted factors that attract growth cones, often by inducing actin polymerization at the leading edge.
• Repulsive diffusible molecules: Trigger cytoskeletal collapse or actin depolymerization, steering the growth cone away.
• Extracellular matrix (ECM) adhesion: ECM molecules (laminin) interact with receptors on growth cone, promoting adhesion and guidance.
• Fasciculation: Growth cones follow axons that have already formed a path (pioneer axons), adhering via cell adhesion molecules.
• Cell-surface adhesion: Contact with cells expressing specific surface proteins promote directional growth through adhesive interactions.
• Contact inhibition: Contact with certain cell types or molecules causes the growth cone to collapse or change direction, often to prevent
incorrect pathway entry.
23.3 Neuronal Growth and Synapse Formation Depend on Signalling Molecules
Learning goal: Can explain how commissural axons cross the floorplate in the developing nervous
system.
Answer: Commissural axons in the developing spinal cord are guided across the midline
(floorplate) by a carefully orchestrated sequence of chemoattractive and chemorepulsive cues,
whose responsiveness is tightly regulated in time and space.
Initially, these axons are attracted to the midline by netrin-1, a diffusible chemoattractant secreted
by the floorplate. Netrin binds to receptors such as DCC and neogenin on the growth cones of
commissural axons. Binding initiates intracellular signaling cascades that reorganize the actin
cytoskeleton, steering axons toward the midline.
Once axons reach and cross the floorplate, their behavior changes to prevent them from crossing back. This directional switch is ensured by:
• Downregulation or inactivation of DCC, for example via proteolytic cleavage, reducing the axons’ sensitivity to netrin.
• Upregulation of Slit-Robo signaling: Slit, another guidance cue secreted by the floorplate, binds to Robo receptors on commissural
axons. This binding triggers repulsive signaling, mediated in part by Rho GTPases, which reorganize the cytoskeleton to repel the axon
from the midline and guide it laterally.
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, Learning goal: Knows the cell adhesion molecules that are involved in
synapse formation (protocadherins, DSCAM (invertebrates), Neurexin,
neuroligin and neuregulin).
Answer: Synapse formation and specificity rely heavily on cell
adhesion molecules that help neurons identify appropriate partners
and stabilize their connections. These molecules contribute to
processes such as dendritic self-avoidance, axon tiling, synapse
initiation, and synapse maturation. The key molecules include:
• Protocadherins (a subclass of cadherins):
o These are Ca²⁺-dependent homophilic adhesion molecules
that provide each neuron with a unique molecular identity
through stochastic expression of large gene clusters.
o They are important for axon tiling, preventing redundant innervation by neighboring neurons, and contribute to synapse specificity.
o In some cases, they influence gene expression via β-catenin signaling, linking adhesion to intracellular transcriptional changes.
• DSCAM (Down Syndrome Cell Adhesion Molecule, especially in invertebrates like Drosophila):
o Involved in dendritic self-avoidance by enabling neurons to distinguish self from non-self through thousands of alternative splicing
isoforms.
o This ensures that branches of the same neuron do not form synapses with one another, which is critical for proper circuit wiring.
o In vertebrates, DSCAM also contributes to dendritic field organization, e.g., in the retina.
• Neurexin (presynaptic) and Neuroligin (postsynaptic):
o These molecules form a trans-synaptic adhesion complex that is essential for the initiation, stabilization, and maturation of
synapses.
o Neurexins on the presynaptic terminal bind to neuroligins on the postsynaptic side, helping align pre- and postsynaptic specializations.
o Mutations in these genes are linked to autism spectrum disorders, highlighting their importance in cognitive and behavioral function.
• Neuregulin 1 (Nrg1):
o A secreted signaling molecule that binds to ErbB receptors on the postsynaptic membrane.
o It regulates the expression of postsynaptic receptor proteins and plays a crucial role in synapse differentiation, particularly at the
neuromuscular junction.
o Alterations in Nrg1-ErbB signalling have been implicated in schizophrenia.
Learning goal: Knows how neurotrophic factors can lead to survival of motoneuron pools in the chick.
Answer: During the development of the chick nervous system, more motoneurons are initially generated than are ultimately needed. This surplus
is reduced through programmed cell death (apoptosis) unless motoneurons receive sufficient neurotrophic support from their peripheral
target tissues, such as developing muscles.
Motoneurons compete for limited amounts of neurotrophic factors—including classical neurotrophins such as NGF (nerve growth factor),
BDNF (brain-derived neurotrophic factor), and NT-3—secreted by their target tissues. Only those motoneurons that successfully reach and
form stable connections with their targets receive adequate survival signalling via Trk receptors. This selective survival mechanism ensures a
precise numerical match between motoneurons and the size and innervation needs of their targets, allowing efficient motor control.
In addition to these classical neurotrophins, other trophic factors have also been shown to play a role in chick motoneuron survival:
• Hepatocyte Growth Factor (HGF): Secreted by mesenchymal cells in limb and craniofacial muscle regions, HGF serves a dual role as a
chemoattractant guiding motor axons and as a survival-promoting trophic factor mimicking the activity of target tissues.
• Vascular Endothelial Growth Factor (VEGF): Although best known for its role in angiogenesis, VEGF also acts as a chemoattractant and
trophic factor for motor and sensory neurons. It is secreted by spinal cord floorplate cells and may help attract axons while supporting
survival once proper connections are formed.
These findings reinforce the principle of target-derived neurotrophic support, where only neurons that reach and properly engage their targets
are "rewarded" with survival signals. This process not only sculpts the final size of motoneuron pools but also ensures that functional
connectivity is established between the central nervous system and peripheral effectors.
3|Page
Learning goal: Is familiar with neuronal polarization.
Answer: Neuronal polarization is the process by which a postmitotic neuroblast breaks its initial
symmetry to establish distinct axonal and dendritic domains. It is a critical first step in neural circuit
formation. After neurogenesis, the immature neuron extends multiple neurites—initially
undifferentiated processes.
Local intracellular cues and external signals lead to one neurite acquiring axonal identity, while the
others develop into dendrites. This polarization enables the neuron to differentiate functionally
and structurally into compartments for signal reception (dendrites) and transmission (axon).
Learning goal: Describe the roles of cytoskeletal organization and intracellular trafficking in neuronal polarization.
Cytoskeletal organization:
The cytoskeleton, composed primarily of microtubules, actin filaments, and intermediate filaments,
provides the structural framework for establishing and maintaining neuronal polarity.
Microtubules are aligned along the axon and dendrites, serving as tracks for directed cargo transport. In
axons, they are oriented with their plus ends outward, supporting anterograde transport. Actin filaments are
concentrated at the growth cones and branching points, playing a critical role in pathfinding and neurite
extension.
Intermediate filaments, especially neurofilaments, provide structural stability, particularly in long axons.
PAR proteins (e.g., Par-3) are crucial for establishing polarity. They localize to the future axon and coordinate
cytoskeletal remodeling. Disruption of PAR proteins leads to failure in specifying a single axon, resulting in
multiple processes acquiring axonal traits.
Intracellular trafficking:
Intracellular trafficking delivers proteins, lipids, organelles, and mRNAs from the cell body to axons and
dendrites. Kinesin motor proteins mediate anterograde transport along microtubules toward the axon terminal, carrying materials like
mitochondria and synaptic vesicles.
Dynein motors mediate retrograde transport, moving endocytic vesicles and degradation products back to the soma. Myosin motors interact
with actin to support local transport in growth cones, dendrites, and dendritic spines. Dendrites have unique Golgi outposts and satellites that
locally support protein trafficking and postsynaptic site assembly.
This trafficking is energy-intensive and vital for growth, polarization, and synapse formation. Disruption can lead to neurological disorders like
epilepsy and intellectual disability.
23.2 Neural Circuit Construction Relies on Basic Mechanisms of Cell Polarity
Learning goal: Can describe the process of axon outgrowth and know the associated molecules.
Process of axon outgrowth:
• Initiation: Once an axon is specified, it extends from the neuronal cell body, navigating complex terrain to reach specific targets.
• Growth Cone Role: The growth cone, a motile structure at the axon's tip, leads axonal extension by exploring the extracellular
environment and directing growth.
• Extension: As the growth cone advances, new cellular material (membrane, cytoskeleton) is added, elongating the axon.
Associated molecules and structures:
• Cytoskeletal components:
o Actin: Found in lamellipodia and filopodia; regulates shape and directional movement.
o Tubulin: Forms microtubules that support axon elongation and intracellular transport.?
• Binding proteins:
o Actin-binding proteins: Modify actin dynamics; help anchor actin to membrane and promote vesicle fusion.
o Microtubule-binding proteins: Stabilize axonal microtubules via tubulin modification.
• Motor proteins: Transport vesicles and proteins along the axon.
• Signaling molecules: cAMP, cGMP, and intracellular Ca²⁺ regulate cytoskeletal rearrangements via second messengers.
• Receptors and ion channels: Concentrated in lamellipodia and filopodia to detect environmental cues.
1|Page
,Learning goal: Knows the different structures of the growth cone and the molecular basis of growth cone motility
Structures of the growth cone:
• Lamellipodium: A broad, sheetlike extension at the tip of the axon. Contains
both filamentous and non-filamentous actin.
• Filopodia: Thin, finger-like projections from the lamellipodium. Rich in F-actin,
used to "probe" the environment.
• Axon shaft: Contains stable microtubules (acetylated tubulin) that provide
structure and transport.
Molecular basis of motility:
• Actin dynamics:
o Polymerization of G-actin into F-actin at the leading edge drives protrusion.
o Depolymerization in response to repulsive cues allows retraction.
o Actin-binding proteins regulate this balance and help localize receptors.
• Microtubule dynamics:
o Dynamic (tyrosinated) microtubules extend into the lamellipodium.
o Stabilization of direction occurs via polymerization of tubulin in the axon shaft.
• Calcium signaling:
o Ca²⁺ influx via voltage-gated or TRP channels influences cytoskeletal dynamics.
• Vesicle transport:
o Vesicles containing membrane components travel via microtubules and associate with actin in filopodia to support membrane
expansion.
Learning goal: Knows 6 different cues that growth cones can navigate to the correct location: attractive and repulsive diffusible molecules,
extracellular matrix adhesion, fasciculation, cell surface adhesion and contact inhibition.
Answer: Growth cones interpret environmental cues to determine their path. These include:
• Attractive diffusible molecules: Secreted factors that attract growth cones, often by inducing actin polymerization at the leading edge.
• Repulsive diffusible molecules: Trigger cytoskeletal collapse or actin depolymerization, steering the growth cone away.
• Extracellular matrix (ECM) adhesion: ECM molecules (laminin) interact with receptors on growth cone, promoting adhesion and guidance.
• Fasciculation: Growth cones follow axons that have already formed a path (pioneer axons), adhering via cell adhesion molecules.
• Cell-surface adhesion: Contact with cells expressing specific surface proteins promote directional growth through adhesive interactions.
• Contact inhibition: Contact with certain cell types or molecules causes the growth cone to collapse or change direction, often to prevent
incorrect pathway entry.
23.3 Neuronal Growth and Synapse Formation Depend on Signalling Molecules
Learning goal: Can explain how commissural axons cross the floorplate in the developing nervous
system.
Answer: Commissural axons in the developing spinal cord are guided across the midline
(floorplate) by a carefully orchestrated sequence of chemoattractive and chemorepulsive cues,
whose responsiveness is tightly regulated in time and space.
Initially, these axons are attracted to the midline by netrin-1, a diffusible chemoattractant secreted
by the floorplate. Netrin binds to receptors such as DCC and neogenin on the growth cones of
commissural axons. Binding initiates intracellular signaling cascades that reorganize the actin
cytoskeleton, steering axons toward the midline.
Once axons reach and cross the floorplate, their behavior changes to prevent them from crossing back. This directional switch is ensured by:
• Downregulation or inactivation of DCC, for example via proteolytic cleavage, reducing the axons’ sensitivity to netrin.
• Upregulation of Slit-Robo signaling: Slit, another guidance cue secreted by the floorplate, binds to Robo receptors on commissural
axons. This binding triggers repulsive signaling, mediated in part by Rho GTPases, which reorganize the cytoskeleton to repel the axon
from the midline and guide it laterally.
2|Page
, Learning goal: Knows the cell adhesion molecules that are involved in
synapse formation (protocadherins, DSCAM (invertebrates), Neurexin,
neuroligin and neuregulin).
Answer: Synapse formation and specificity rely heavily on cell
adhesion molecules that help neurons identify appropriate partners
and stabilize their connections. These molecules contribute to
processes such as dendritic self-avoidance, axon tiling, synapse
initiation, and synapse maturation. The key molecules include:
• Protocadherins (a subclass of cadherins):
o These are Ca²⁺-dependent homophilic adhesion molecules
that provide each neuron with a unique molecular identity
through stochastic expression of large gene clusters.
o They are important for axon tiling, preventing redundant innervation by neighboring neurons, and contribute to synapse specificity.
o In some cases, they influence gene expression via β-catenin signaling, linking adhesion to intracellular transcriptional changes.
• DSCAM (Down Syndrome Cell Adhesion Molecule, especially in invertebrates like Drosophila):
o Involved in dendritic self-avoidance by enabling neurons to distinguish self from non-self through thousands of alternative splicing
isoforms.
o This ensures that branches of the same neuron do not form synapses with one another, which is critical for proper circuit wiring.
o In vertebrates, DSCAM also contributes to dendritic field organization, e.g., in the retina.
• Neurexin (presynaptic) and Neuroligin (postsynaptic):
o These molecules form a trans-synaptic adhesion complex that is essential for the initiation, stabilization, and maturation of
synapses.
o Neurexins on the presynaptic terminal bind to neuroligins on the postsynaptic side, helping align pre- and postsynaptic specializations.
o Mutations in these genes are linked to autism spectrum disorders, highlighting their importance in cognitive and behavioral function.
• Neuregulin 1 (Nrg1):
o A secreted signaling molecule that binds to ErbB receptors on the postsynaptic membrane.
o It regulates the expression of postsynaptic receptor proteins and plays a crucial role in synapse differentiation, particularly at the
neuromuscular junction.
o Alterations in Nrg1-ErbB signalling have been implicated in schizophrenia.
Learning goal: Knows how neurotrophic factors can lead to survival of motoneuron pools in the chick.
Answer: During the development of the chick nervous system, more motoneurons are initially generated than are ultimately needed. This surplus
is reduced through programmed cell death (apoptosis) unless motoneurons receive sufficient neurotrophic support from their peripheral
target tissues, such as developing muscles.
Motoneurons compete for limited amounts of neurotrophic factors—including classical neurotrophins such as NGF (nerve growth factor),
BDNF (brain-derived neurotrophic factor), and NT-3—secreted by their target tissues. Only those motoneurons that successfully reach and
form stable connections with their targets receive adequate survival signalling via Trk receptors. This selective survival mechanism ensures a
precise numerical match between motoneurons and the size and innervation needs of their targets, allowing efficient motor control.
In addition to these classical neurotrophins, other trophic factors have also been shown to play a role in chick motoneuron survival:
• Hepatocyte Growth Factor (HGF): Secreted by mesenchymal cells in limb and craniofacial muscle regions, HGF serves a dual role as a
chemoattractant guiding motor axons and as a survival-promoting trophic factor mimicking the activity of target tissues.
• Vascular Endothelial Growth Factor (VEGF): Although best known for its role in angiogenesis, VEGF also acts as a chemoattractant and
trophic factor for motor and sensory neurons. It is secreted by spinal cord floorplate cells and may help attract axons while supporting
survival once proper connections are formed.
These findings reinforce the principle of target-derived neurotrophic support, where only neurons that reach and properly engage their targets
are "rewarded" with survival signals. This process not only sculpts the final size of motoneuron pools but also ensures that functional
connectivity is established between the central nervous system and peripheral effectors.
3|Page
